ultralightweight caving, on dyneema, etc (long post)
Lev Bishop
Myself and a couple of friends have been pondering the possibility of using
modern fibres such as Dyneema/Spectra or Vectran for vertical work. I was
wondering if anyone has tried this before, or has any comments about my
ideas.
Firstly, a little bit about the fibres:
Dyneema/Spectra: are trade names for High Modulus Polyethylene (HMPE), and
other than the name are more or less identical.
*It doesn't take dye (so the white stitches on spectra climbing slings are
the HMPE stitches (the coloured stitches are nylon, I think)).
*It has a very high strength, such that a 5.5mm diameter rope should be
strong enough to use for vertical work.
*It is *very* static - you really don't want to fall onto an HMPE rope,
because even if the rope doesn't break, you will.
*It is very slippery (this is one of the reasons for the high strength).
This slipperiness means that you don't get things made only out of HMPE -
they would just unravel and fall apart. For cords, manufacturers use HMPE
for the core and nylon for the sheath, and have special techniques to reduce
the amount of slippage. Even so, they often recommend you to use a
triple-fishermans when joining the ends to make a sling and tighten it well
before use. The slipperiness is the reason they always mix the HMPE with
nylon in making tape. It's also why you can't buy spectra tape
on-the-reel -- water knots wouldn't hold, so you have to have sewn slings. I
presume the impossibility of having knotted slings is the reason why most
cavers still stick with nylon tape.
*It absorbs very little water.
*It has quite a low melting point - this is a problem (see below).
*It has good resistance to UV and chemical attack.
*It has good abrasion resistance.
Kevlar: is a trade name for an aramid fibre. It has many properties similar
to HMPE, but has the disadvantage that the abrasion resistance is very poor,
which makes it unsuitable for most caving uses.
Vectran: is a trade name for a liquid crystal fibre, the only one
commercially available, last I checked. It has many properties similar to
HMPE except that it has a higher melting point and exceptionally low creep.
The low creep is not especially important to cavers. It's also more
expensive than dyneema.
It would be nice to use dyneema for caving, because with 5.5mm rope you
could carry a hell of a lot of rope, nearly 4 times as much as you can carry
of 10mm nylon rope. Also, since it absorbs no water, it would always be
light, even after you've rigged it in a waterfall. The staticness means
there will be no annoying bouncing when you prussik, which will be nice. You
will also be able to use smaller, lighter krabs (since the loading will be
applied closer to the spine of the krab, due to the reduced diameter) and
you'll be able to tie in directly to hangers without using a krab or
maillon. All this will greatly reduce the amount of tackle you need to take
with you, which can only be a Good Thing.
However, there are several problems to be overcome before anyone can
actually take this stuff caving:
The first problem is that there are no ascenders or descenders available for
use on dyneema, so you would have to make you own. Ascenders should be
fairly easy - just think of a normal petzl or whatever and scale it down to
work on 5.5mm ropes. Descenders will be much more of a problem. This is
because dyneema has a much lower melting point than nylon (150 degrees C)
and starts to become soft and lose strength at an even lower temperature
still (66 degrees C). It doesn't absorb any water, so you can't rely on
evaporating water to prevent the rope from overheating and melting.
Therefore, the usual descender arrangement, where you have friction between
the rope and some arrangement of stationary pieces of metal won't work
because you'll melt through the rope and plummet. Perhaps you might get away
with abseiling really slowly and having a lot of surface area to dissipate
heat, but I wouldn't count on that being enough. My best idea for how to get
around this problem is to use a descender where the rope wraps a couple of
times around a rotatable drum and doesn't slip relative to the drum, so
there is no friction there. Instead, the drum turns an axle and at the other
end of the axle is some kind of braking arrangement which you control with a
handle to adjust your rate of descent. The heat gets generated at the brakes
and not at the rope, so the rope doesn't melt. The difficult thing will be
to build a descender like this which is cave-proof and can handle mud, water
and all the rest of the abuse to which we subject caving gear. Any ideas?
The second problem is to do with the staticness of the rope. If you fall
onto it, even a very short distance you will generate huge forces and
something in the system will fail, most likely you. Even just a deviation
popping or one half of a Y-hang failing could have disastrous consequences.
There are two solutions to this. The first is what the eastern europeans
used to do when they used wire-climbing techniques, and just make sure that
you simply can't fall onto the rope. This means that you attach the
rope/wire at the top of the pitch to something very solid, and don't have
any rebelays or deviations or anything else which might fail and lead to a
shock-load. This is all very well if you're using 4mm steel cable since
where it touches the rock, the rock will come off the worse from that
encounter, but while dyneema is abrasion-resistant, it's not *that* abrasion
resistant, and you need a different solution. The second alternative is to
incorporate some kind of energy-absorbing device into the system, either a
"stitch-ripping" webbing like the petzl energyca or the Yates screamer, or a
"controlled slipping" device like the ones people use for via ferratas, eg
the petzl zyper. The advantage of the stitch rippers is that they are not
affected by water, mud, etc. The advantage of the zyper-type devices is that
they have much better energy absorption and can be reset and reused (unlike
the stitch-rippers which must be discarded after a shock-loading). I have a
few interesting ideas about how best to incorporate a shock-absorbing device
into an SRT set-up but this post is already far too long, so I'll stop now
and ask for comments....
Disclaimer: Those are my ideas. I'm not thinking of trying any of this out
until I've thought it through *very* carefully and done lots of practise and
testing under controlled circumstances (at the climbing wall). I do NOT
recommend that anyone goes out and buys some dyneema and tries to do SRT on
it before they work out how to deal with the shock-loading and melting
problems - they will probably come to a sticky end.
Anyway, that's what I was thinking. I'd appreciate any ideas or input from
anyone who knows about ropes and so on, or who has ideas how to deal with
the problems I've pointed out above. If you think I'm crazy and it will
never work, please let me know *why* since looking at the information I have
available it seems perfectly feasible, once the problems are sorted out...
Lev
Nigel Dyson-Hudson
http://mgmtsys.com/Store/Manufacturers/mfgnames-home.html
www.neropes.com
[posted and mailed]
bis...@remove-this-part.teaching.physics.ox.ac.uk (Lev Bishop) wrote in
<8fd2c7$d43$1...@news.ox.ac.uk>:
>*Don't try this at home*
>
>Myself and a couple of friends have been pondering the possibility of
>using modern fibres such as Dyneema/Spectra or Vectran for vertical
>work. I was wondering if anyone has tried this before, or has any
>comments about my ideas.
>
>Firstly, a little bit about the fibres:
rest deleted ....
Scott McCrea
have also pondered many ways to lighten the load on vertical trips. But it
sounds like you have a bunch of issues to work out if you want to use the
5.5mm Dyneema. You are right that there would need to some sort of shock
absorber in the system somewhere. That is the beauty of nylon, the absorber
is built in. I guess you will have to figure out if it more trouble to
carry the larger rope or the shock absorbing system. Lots of Mexican
expeditions these days are using polyester ropes (10mm and 8.8mm, I think)
with great success. Polyester has many of the same properties as HMPE, low
stretch, abrasion, UV, chemical resistance, doesn't absorb water. Cancord
is the manufacturer I am familiar with. You can also contact Gonzo Guano
Gear, they sell it and use it. Plus you can existing gear, rapping and
climbing. And it is probably cheaper.
I don't think your ideas are crazy, just very challenging. I think the
"frictionless" rappell device is going to very difficult and the smaller,
scaled down ascender is going to be harder to come by than you think. It
would be a custom created piece of gear, $$$.
I am a big fan of ultralightweight caving, but am a bigger fan of
simplification. The more simple your gear is the safer it is, less things
to go wrong, less links in the chain and the lighter it is going to be.
Think about getting rid of all but the absolutely essential gear, ie, does
that biner need to be there or can I tie my cows tail directly into my
screwlink? You get the idea. Once you get it simplified, then think about
making the components lighter. Now let me climb down off my soap box.
Here's some links that may be helpful.
Cancord, makers of polyester rope: http://www.cancord.com/index.html
GGG, make light gear and use Cancord rope: http://www.gonzoguanogear.com/
Vertical Devices, every piece of vert gear known to man and then some,
pictures and review, Gary Storrick is very knowledgable and I'm sure will
give you his thoughts on your plans...
http://storrick.cnchost.com/VerticalDevicesPage/VerticalHome.shtml
Good luck, keep us posted
Light is right, Simple is better.
--
Scott McCrea
Flittermouse Grotto, NC
nss 40839
ca...@simplymaps.com
----------
>From: "Lev Bishop" <bis...@remove-this-part.teaching.physics.ox.ac.uk>
>Newsgroups: uk.rec.caving,alt.caving
>Subject: ultralightweight caving, on dyneema, etc (long post)
>Date: Wed, May 10, 2000, 9:28 PM
>
> *Don't try this at home*
>
> Myself and a couple of friends have been pondering the possibility of using
> modern fibres such as Dyneema/Spectra or Vectran for vertical work. I was
> wondering if anyone has tried this before, or has any comments about my
> ideas.
>
> Firstly, a little bit about the fibres:......
>
Andy Foster
Spectra has been used by climbers for some years as staic cord for chocks. I
think I've seen it attatched to Wild Country "Rocks". It was tied using a
double fishermans knot but, and here is a key point, the ends sealed over
with shrink wrap.
The reason for this is "knotability". Spectra does not take kindly to knots.
The tight weave makes tight knots almost immpossible, especially with cold,
wet clammy caving hands. Also, the small diameter makes the radius of any
knots very tight, which could also be a problem due to reduction of strength
over the bend.
IMHO these points would rule out spectra type cord for caving use. I don't
know if the other materials mentioned suffer the same.
Andy Foster
Keyhole Caving Club
Carol Haynes
I
| think I've seen it attatched to Wild Country "Rocks". It was tied using a
| double fishermans knot but, and here is a key point, the ends sealed over
| with shrink wrap.
Used as standard on Rockcentrics. Sewn though I think.
Carol
Cris Cooper
> I
> | think I've seen it attatched to Wild Country "Rocks". It was tied using a
> | double fishermans knot but, and here is a key point, the ends sealed over
> | with shrink wrap.
I have one in front of me and it appears to be tied with a triple F. I
have heard this is safe when making your own things out of the stuff. The
ends are sealed but I'm not sure that does much except stop them catching.
The dyneema on rockcentrics is tape.
- Crispin Cooper
---------------------------------------------------------
http://get.to/climbbetter
"A journey of a thousand miles begins with a single step"
Lev Bishop
news:dyyS4.3045$XO1.1...@bgtnsc06-news.ops.worldnet.att.net...
> You are right that there would need to some sort of shock
> absorber in the system somewhere. That is the beauty of nylon, the
absorber
> is built in. I guess you will have to figure out if it more trouble to
> carry the larger rope or the shock absorbing system.
One way to implement the shock absorbing system would be that each
individual caver makes all his attachments to the rope via the
shock-absorber. There are some issues to be solved if we do things this way,
like how to attach without excessive slack being introduced by the finite
length of the shock-absorber. A possible solution is to use a nylon maillon
rapide as a "weak link", to break in case of overloading, and have that
backed up with the shock absorber (I am currently trying to find out about
the properties of nylon maillons (you really can buy them!)). This would
also remove the problem that a zyper-type shock absorber might gradually
slip through during normal use and therefore have no more travel available
when you actually needed it. You may need to be a little bit careful when
rigging traverse lines, but other than that everything should be ok.
Assuming a deep cave (say 500 to 1500m) and a small team (say 3 people),
then the dyneema would win every time (since there is no way it could be
done with normal nylon ropes - cordelette technique would be the only other
possibility, but that has all sorts of disadvantages to it). For a less
vertical cave or with larger groups, perhaps it would be better to use
normal ropes.
> Lots of Mexican
> expeditions these days are using polyester ropes (10mm and 8.8mm, I think)
> with great success. Polyester has many of the same properties as HMPE,
low
> stretch, abrasion, UV, chemical resistance, doesn't absorb water. Cancord
> is the manufacturer I am familiar with. You can also contact Gonzo Guano
> Gear, they sell it and use it. Plus you can existing gear, rapping and
> climbing. And it is probably cheaper.
Spectra is a bit more than twice the cost per metre of nylon rope. How
expensive it really is depends on how long it lasts for, of course.
> I don't think your ideas are crazy, just very challenging. I think the
> "frictionless" rappell device is going to very difficult and the smaller,
> scaled down ascender is going to be harder to come by than you think. It
> would be a custom created piece of gear, $$$.
I was anticipating that I would put it together in a workshop myself and it
wouldn't be too expensive (but would be scary - I'd need to do some tests to
failure on prototypes).
> I am a big fan of ultralightweight caving, but am a bigger fan of
> simplification. The more simple your gear is the safer it is, less things
> to go wrong, less links in the chain and the lighter it is going to be.
> Think about getting rid of all but the absolutely essential gear, ie, does
> that biner need to be there or can I tie my cows tail directly into my
> screwlink? You get the idea. Once you get it simplified, then think
about
> making the components lighter. Now let me climb down off my soap box.
I agree with you entirely on this point. It's very easy to carry lots of
extra stuff which doesn't add to the safety but does add to the weight and
complication.
> Here's some links that may be helpful.
>
> Cancord, makers of polyester rope: http://www.cancord.com/index.html
>
> GGG, make light gear and use Cancord rope:
http://www.gonzoguanogear.com/
I'll check these out, thanks.
> Vertical Devices, every piece of vert gear known to man and then some,
> pictures and review, Gary Storrick is very knowledgable and I'm sure will
> give you his thoughts on your plans...
I already dropped him a note, but he's quite busy at the moment and hasn't
had a chance to respond, yet.
Thanks for the input,
Lev
Lev Bishop
"Andy Foster" <an...@xtek.co.uk> wrote in message
news:8fejor$qg2$1...@news5.svr.pol.co.uk...
> Thanks for a very interesting post.
No problem.
> Spectra has been used by climbers for some years as staic cord for chocks.
I
> think I've seen it attatched to Wild Country "Rocks".
I have seen some very impressive promotional video footage where they had
some climbing chocks on spectra cord wedged in a crack in a sound piece of
rock and used a hydraulic rig to try to pull it out. Neither the rock nor
the spectra failed, and the chock didn't pop out of the crack. Instead the
spectra gradually sliced through the metal of the chock like cheesewire
through cheddar. It was very impressive.
> The reason for this is "knotability". Spectra does not take kindly to
knots.
> The tight weave makes tight knots almost immpossible, especially with
cold,
> wet clammy caving hands.
Do you mean that it is very difficult to *undo* tight knots, especially
underground? I can easily see this, but it doesn't bother me too much as I
can always save the untying of knots until I reach the surface and sort out
the gear there. I can't see why it would be hard to tie the knots in the
first place, but then I haven't really experimented with spectra that much
to know.
> Also, the small diameter makes the radius of any
> knots very tight, which could also be a problem due to reduction of
strength
> over the bend.
I've been told that this isn't too much of a problem. The usual problem with
tight bends is that the part of the rope on the outside of the bend takes
all the load and the rope on the inside of the bend is left unloaded.
Spectra is less affected by this because it is so slippery that the strands
slide over each other and the loading is better equalised. I didn't hear
this from any kind of official source so it could be cobblers, but it seems
reasonable to me.
> IMHO these points would rule out spectra type cord for caving use. I don't
> know if the other materials mentioned suffer the same.
I know that some French cavers have been successfully using lightweight
techniques involving 8mm nylon ropes and tying directly to hangers for some
time and they have been using spectra cord to knit together their belays.
They think it is quite a mature technique now and not too dangerous if
practised correctly. The thing I remember them saying about the spectra was
that you had to be very careful not to melt through it with your carbide
flame as you got off the rope at the pitch head.
Lev
Nigel Robertson
"Cris Cooper" <ch...@camNOTSPAM.ac.uk> wrote in message
news:Pine.LNX.4.21.000511...@door.acad.cai.cam.ac.uk...
> > | Spectra has been used by climbers for some years as staic cord for
chocks.
> > I
using a
> > | double fishermans knot but, and here is a key point, the ends sealed
over
> > | with shrink wrap.
>
> I have one in front of me and it appears to be tied with a triple F. I
> have heard this is safe when making your own things out of the stuff. The
> ends are sealed but I'm not sure that does much except stop them catching.
The sealed end may also be there to stop the end of the cord being pushed
back into the knot (when it catches), thereby slackening the knot and
eventually allowing it to undo.
As an aside, about 20 years ago, Whernside Manor had some Kevlar cored rope
(nylon or polyester sheath) that it was testing. I can remember that its
use wasn't favoured but I can't remember the exact reasons why.
Nigel
Nigel Robertson
"Lev Bishop" <bis...@remove-this-part.teaching.physics.ox.ac.uk> wrote in
message news:8fffka$m7j$1...@news.ox.ac.uk...
> "Scott McCrea" <sc...@simplymaps.com> wrote in message
> news:dyyS4.3045$XO1.1...@bgtnsc06-news.ops.worldnet.att.net...
>
> > You are right that there would need to some sort of shock
> > absorber in the system somewhere. That is the beauty of nylon, the
> absorber
> > is built in. I guess you will have to figure out if it more trouble to
> > carry the larger rope or the shock absorbing system.
>
> One way to implement the shock absorbing system would be that each
> individual caver makes all his attachments to the rope via the
> shock-absorber.
What about inserting the shock absorbency at the head of the pitch and
appropriate rebelays? Perhaps a bit like a Y-hang with one leg being super
stretchy and enough slack in the Spectra side to allow the stretchy side to
work. The stretchy side would need to be low stretch at normal forces
(prusiking/abseil bounce) but become stretchy at an appropriate threshold.
Y-hangs in general shouldn't induce too much extra force if one side fails
as the rope should swing rather than fall to its new position. Perhaps the
physics needs to be checked and so, for instance, you know that you can't
tie a Y at greater than 45 degrees separation.
It's an interesting thread - about time ;)
Nigel
dlk
Lev Bishop wrote in message <8fd2c7$d43$1...@news.ox.ac.uk>...
[fascinating post snipped]
>.... My best idea for how to get
>around this problem is to use a descender where the rope wraps a couple of
>times around a rotatable drum and doesn't slip relative to the drum, so
>there is no friction there. Instead, the drum turns an axle and at the
other
>end of the axle is some kind of braking arrangement which you control with
a
>handle to adjust your rate of descent. The heat gets generated at the
brakes
>and not at the rope, so the rope doesn't melt. The difficult thing will be
>to build a descender like this which is cave-proof and can handle mud,
water
>and all the rest of the abuse to which we subject caving gear. Any ideas?
>
What you describe is known as a capstan -same thingie as one finds aboard
sailing craft. Usually equipped with a one-way (selectable as to rotation
direction) clutch in marine usage. A variant (brakes instead of a clutch) of
the
device is used in industrial applications (cranes, drag lines, etc) as a
line
tension control device. So the technology is there already, replete with
engineering analysis of the device.
But that's not your problem :) Getting somebody to build it _and sell it_
is the problem. As a caver I like the concept. As an engineer, and as a
manufacturer of equipment (non-caving related) I wouldn't touch it with the
proverbial 10-footer: the (in the USA, at least) liability insurance
premium
would be out of sight, assuming one could even find an underwriter
willing to issue.
Now I know somebody is gonna pop up and say that US manufacturers
already deal with this issue. True. But for _very simple design_ devices.
And they pay dearly for that.
A capstan will have:
Frame
2 drums (line and brake)
2 or 3 bearings subject to loading (radial and axial)
Axle
Shaft seals
Brake shoes
Shoe adjustment mechanism (lever, pivot, and springs)
Plus "environmental protection" (shrouds, covers, etc)
Plus miscellaneous hardware (nuts, bolts, clips, etc.)
Way too many parts. Way to many for a single small manufacturer to
"cover" (quality control). Some of those parts are critical, like the
bearings, shoes, drums, axle, and the drum-to-axle fasteners. Keep
in mind there are three failure modes: immediate life-catastrophic
(friction fails, free fall), life threatening (device lockup, hung on
the pitch) and injurious (unable to achieve proper friction level,
descent velocity to high).
I agree such a device is intriguing, but I don't think any US manufacturer
is gonna leap on it any time soon, this will have to come from someplace
where product liability issues are resolved with common sense instead
of "it's always somebody else's fault, never the victim's" litigation.
Dave Keever
Paul Montgomery
dlk <dlke...@hotmail.com> wrote in message
news:qsPS4.82$3j5....@ratbert.tds.net...
Dave, I agree with you on the liability problem. I make "things" for
industry and had a real problem with getting someone to underwrite.
Strangely enough I found the farming industry had underwriters willing.
As to the capstan, I can envision a device that has only four or five parts
for the brake and uses no springs. It would be pretty mud and waterproof
without alot of shielding. I think it could be done. I've always wondered
why it hasn't been done for regular rope.
One idea I had was to build a "rack" type device that is hollow. Fill the
rack with water just before using it and excess heat would be carried off as
steam. The rack would be limited to temperatures of 100 C or just a bit
above at the friction points.
Ideas, just ideas.
Paul Montgomery
Scott McCrea
In article <N7SS4.19923$701.3...@news4.giganews.com>, "Paul Montgomery"
<Pa...@tcia.net> wrote:
> One idea I had was to build a "rack" type device that is hollow. Fill the
> rack with water just before using it and excess heat would be carried off as
> steam. The rack would be limited to temperatures of 100 C or just a bit
> above at the friction points.
How about this Paul?
http://storrick.cnchost.com/VerticalDevicesPage/Rappel/SpoolPages/VSpool425.
html
You could fill it with ice at the top and have a nice refreshing drink at
the bottom of your rappel:)
Ralph Hartley
> > The reason for this is "knotability". Spectra does not take kindly to
> knots.
> > The tight weave makes tight knots almost immpossible, especially with
> cold,
> > wet clammy caving hands.
>
> Do you mean that it is very difficult to *undo* tight knots, especially
> underground? I can easily see this, but it doesn't bother me too much as I
> can always save the untying of knots until I reach the surface and sort out
> the gear there. I can't see why it would be hard to tie the knots in the
> first place, but then I haven't really experimented with spectra that much
> to know.
If my understanding is correct, the problem is exactly the opposite.
Spectra is very slippery. Knots are easy to untie. In fact they
practically untie themselves.
Two other people said:
> > One way to implement the shock absorbing system would be that each
> > individual caver makes all his attachments to the rope via the
> > shock-absorber.
>
> What about inserting the shock absorbency at the head of the pitch and
> appropriate rebelays?
No, you want the stretchy part at the bottom, otherwise the whole rope
moves as the caver ascends. This would cause abrasion. Of course if
you use the European never-let-rope-touch-rock approach this is not a
problem, but one of the advantages of spectra is its very high
abrasion resistance (5 times nylon) so why would you do that?
It is quite likely that we will be using the new high strength fibers
eventually, but the whole system will have to be re thought out. Using
an ascending rig designed for thick nylon on spectra cord may not get
you anywhere. Doing the same thing on rappel may get you somewhere -
fast. The key is not to add complexity, for each new part you add to
the system you need to remove one.
Ralph Hartley
dlk
Paul Montgomery wrote in message ...
>Dave, I agree with you on the liability problem. I make "things" for
>industry and had a real problem with getting someone to underwrite.
>Strangely enough I found the farming industry had underwriters willing.
They were probably tickled to death that a nice, low risk case like
an industrial system/component maker showed up on their doorstep.
Farming is one of the highest risk businesses in the USA. ;)
>
>As to the capstan, I can envision a device that has only four or five parts
>for the brake and uses no springs. It would be pretty mud and waterproof
>without alot of shielding. I think it could be done. I've always wondered
>why it hasn't been done for regular rope.
Oh, hell yes it's a "doable" concept. Just *I* can't afford it. Nor could
any "small" outfit such as yours or mine. Side effect of an out of
control product liability process: kills off innovation at the "garage"
level, limits it to the big firms that may take an idea, decide via
spreadsheet there's no market, so it never happens. When put
that way, makes you kinda wonder what we *all* have missed out
on.
>
>One idea I had was to build a "rack" type device that is hollow. Fill the
>rack with water just before using it and excess heat would be carried off
as
>steam. The rack would be limited to temperatures of 100 C or just a bit
>above at the friction points.
There's some interesting alloys "out there" and some clever casting
processes, also. Be nice is somebody more metallurgically inclined
than I were to develop bars that could use air for efficient convective
cooling.
From my "just an idea" dept:
Be nice if someone 'married' Peltier coolers to descending device
friction elements. But they are a bit power-hungry, so it'd be nicer
still if same someone were to figure out how to convert the kinetic
energy of descent into the electrical watts needed for the peltier
coolers...
I like the original poster's concept: a nice _long_ rope that's
lightweight and lower volume in packing. +100m of wet nylon
12mm gets damn heavy after just a little bit! For reasons unknown to
modern physics, this phemonema is worse when going uphill ;)
Dave Keever
Paul Montgomery
off the Vspool425 and then from that page click next and select vspool425news:y1TS4.61044$WF.34...@bgtnsc04-news.ops.worldnet.att.net...
>
> In article <N7SS4.19923$701.3...@news4.giganews.com>, "Paul Montgomery"
> <Pa...@tcia.net> wrote:
>
the
> > rack with water just before using it and excess heat would be carried
off as
> > steam. The rack would be limited to temperatures of 100 C or just a bit
> > above at the friction points.
>
Lev Bishop
Hash: SHA1
"Ralph Hartley" <har...@aic.nrl.navy.mil> wrote in message
news:391C1A17...@aic.nrl.navy.mil...
> > What about inserting the shock absorbency at the head of the
> > pitch and appropriate rebelays?
>
> No, you want the stretchy part at the bottom, otherwise the whole
> rope moves as the caver ascends. This would cause abrasion. Of
> course if you use the European never-let-rope-touch-rock approach
> this is not a problem, but one of the advantages of spectra is its
> very high abrasion resistance (5 times nylon) so why would you do
> that?
Well my idea was to use shock absorbers like the yates screamer or
the petzl zyper. These things aren't stretchy in the sense that you
will bounce - the way they work is that nothing happens until you
reach a critical force (which is more than anything you will generate
in normal caving) and at this point either the stitches start to tear
(screamer) or the rope starts to slip (zyper), absorbing the force of
the fall. There are other reasons, though, why I think it would be
better to have the energy-absorbing element at the caver rather than
at the belays - the original post to which you responded hasn't
reached my server yet, but when it does I'll explain my reasons.
Spectra has high abrasion resistance and also the lack of bounce
should lead to reduced abrasion, but I still think you'll have to be
reasonably careful about rigging to avoid touching the rock too much
simply because with a 5.5mm rope there really isn't all that much
material to wear away. We won't know exactly how much rubbing is
acceptable until we actually take the stuff underground and try it
out.
Lev
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Nigel Robertson
>
> If my understanding is correct, the problem is exactly the opposite.
> Spectra is very slippery. Knots are easy to untie. In fact they
> practically untie themselves.
>
That would be my understanding, too.
> Two other people said:
> > > One way to implement the shock absorbing system would be that each
> > > individual caver makes all his attachments to the rope via the
> > > shock-absorber.
> >
> > What about inserting the shock absorbency at the head of the pitch and
> > appropriate rebelays?
>
> No, you want the stretchy part at the bottom,
Sorry, but my brain must have stopped. What good would it do at the bottom?
The most dangerous place to be, with respect to shock loading, is close to
the top of a rope. Semi-static or low stretch ropes have some give under
body weight but this increases as greater forces are applied. The aim would
be to design a rope/connector that has very low stretch under normal loading
but would have considerable stretchiness when shock loaded. This is
something that I think could probably be achieved using current rope making
technology.
Nigel
Lev Bishop
Hash: SHA1
"dlk" <dlke...@hotmail.com> wrote in message
news:TaVS4.96$3j5.1...@ratbert.tds.net...
> Oh, hell yes it's a "doable" concept. Just *I* can't afford it. Nor
> could any "small" outfit such as yours or mine. Side effect of an
> out of control product liability process: kills off innovation at
> the "garage" level, limits it to the big firms that may take an
> idea, decide via spreadsheet there's no market, so it never
> happens. When put that way, makes you kinda wonder what we *all*
> have missed out on.
I don't think we're likely to see any major innovations of this
magnitude coming from any large company. Of course they'll come up
with 101 incremental improvements on ascenders and descenders or
whatever, but this is different - it's a complete overhaul of
vertical technique. The likes of Petzl, etc, won't bother getting
into making gear for spectra until there is a critical mass of cavers
already using spectra and calling out for better equipment. In the
meantime it's up to brave independent persons to knock up their own
gear and for small companies to find ways around the liability
problems. (In the USSR caving used to be state-subsidised and very
popular, but there were few companies making vertical gear, so every
caver had his own unique set of equipment made by himself, leading to
a lot of clever innovation but also a lot of problems due to lack of
standardisation).
I'm sure if we left it to the big companies to the innovation then
we'd still be using ladders. Of course they'd be ultralightweight
ladders made from amazing new alloys, but...
> From my "just an idea" dept:
> Be nice if someone 'married' Peltier coolers to descending device
> friction elements. But they are a bit power-hungry, so it'd be
> nicer still if same someone were to figure out how to convert the
> kinetic energy of descent into the electrical watts needed for the
> peltier coolers...
I'd been thinking of it the other way around, using the peltier
device on your descender to _generate_ electricity, to charge your
lamp/drill with or whatever.
> I like the original poster's concept: a nice _long_ rope that's
> lightweight and lower volume in packing. +100m of wet nylon
> 12mm gets damn heavy after just a little bit! For reasons unknown
> to modern physics, this phemonema is worse when going uphill ;)
You know that you don't have to use 12mm? I use 9mm nylon rope
without any problems regularly -- no need for nonstandard
equipment...
(8mm works too but you have to be very careful with the rigging, and
may need extra friction on your descender).
Lev
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Tony Brocklebank
using a device very similar to the one you describe, with a big engine
attached and a really ugly bloke pressing switches and levers.
Try http://ourworld.compuserve.com/homepages/bjes/#BPCevents
Tony
--
__________________________________
Email: to...@alpine.fsnet.co.uk
Web site: www.alpine.fsnet.co.uk
__________________________________
dlk <dlke...@hotmail.com> wrote in message
news:qsPS4.82$3j5....@ratbert.tds.net...
>
> Lev Bishop wrote in message <8fd2c7$d43$1...@news.ox.ac.uk>...
> [fascinating post snipped]
>
> >.... My best idea for how to get
> >around this problem is to use a descender where the rope wraps a couple
of
> >times around a rotatable drum and doesn't slip relative to the drum, so
> >there is no friction there. Instead, the drum turns an axle and at the
> other
> >end of the axle is some kind of braking arrangement which you control
with
> a
> >handle to adjust your rate of descent. The heat gets generated at the
> brakes
> >and not at the rope, so the rope doesn't melt. The difficult thing will
be
> >to build a descender like this which is cave-proof and can handle mud,
> water
> >and all the rest of the abuse to which we subject caving gear. Any ideas?
> >
>
>
> What you describe is known as a capstan -same thingie as one finds aboard
> sailing craft. Usually equipped with a one-way (selectable as to rotation
> direction) clutch in marine usage. A variant (brakes instead of a clutch)
of
> the
> device is used in industrial applications (cranes, drag lines, etc) as a
> line
> tension control device. So the technology is there already, replete with
> engineering analysis of the device.
>
> But that's not your problem :) Getting somebody to build it _and sell it_
> is the problem. As a caver I like the concept. As an engineer, and as a
Lev Bishop
"Nigel Robertson" <ni...@dont-spam-me-im-vegetarian.easegill.screaming.net>
wrote in message news:391c...@news.server.worldonline.co.uk...
> > > > One way to implement the shock absorbing system would be that each
> > > > individual caver makes all his attachments to the rope via the
> > > > shock-absorber.
> > >
> > > What about inserting the shock absorbency at the head of the pitch and
> > > appropriate rebelays?
> >
> > No, you want the stretchy part at the bottom,
>
> Sorry, but my brain must have stopped. What good would it do at the
bottom?
> The most dangerous place to be, with respect to shock loading, is close to
> the top of a rope.
I still haven't seen the post suggesting to put the shock absorber at the
top of the pitch, so I'll answer here. Nigel, what you say about the top of
the rope being the most dangerous place is true for traditional
"low-stretch" caving rope. This is because the shock-absorption is done by
the rope and if you have half as much rope between you and the belay then
you have half as much shock absorption. However, spectra is much more static
than this (think steel chain) and it doesn't really matter that much how
much of it you have between you and the belay, since all of the force of the
fall will need to be absorbed by your body, assuming you have no
shock-absorber in the system. If you do put a shock absorber in the system
then in the ideal situation it makes no difference whether the shock
absorber is at the belay-rope interface or at the caver-rope interface, from
the point of view of energy absorption. However, I think there are a number
of reasons why it is preferable to have it at the caver and not the belay:
1) You will need fewer shock absorbers if they are at the cavers. Since the
shock absorbers are likely to be both heavy and expensive, this is a good
thing;
2) If the shock absorber is activated (due to failure of some part of the
system) then it will be possible for the caver to "reset" it (pull the rope
back through for a zyper, swap in a replacement if it is a screamer) before
continuing, if it is at the caver. If the shock absorber is at the belay
then after a fall the caver must prussik up to the belay before being able
to reset it, and must not cause another failure in the process since he will
not have any energy absorption in the system at this point;
3) Consider the case that we have a rebelay failure and that the rebelay was
holding the rope away from wall of the pitch (the usual case). When the
rebelay pings, the slack comes out of the rope and the rope gets pulled into
the wall, also the caver builds up some speed. When the rope finally goes
taut, in the case that the shock absorber is at the belay it is possible
that there is so much friction from the rope against the wall that the shock
absorber at the main belay doesn't "see" much force and is therefore unable
to absorb much energy. In a worst case scenario we may imagine that the knot
from the failed rebelay gets jammed in a crack and none of the force reaches
the shock absorber whatsoever. The caver still suffers the full force,
however, and regrets it. This problem doesn't occur if the shock absorber is
attached to the caver;
4) If the shock absorber introduces any "bounce" into the system it would be
better to introduce this at the caver end, since that way the rope as a
whole moves only as much as its intrinsic stretch allows (ie not much),
which means it abrades much less if it touches the rock, as Ralph pointed
out;
5) An extra element between the knot and the rebelay is likely to make the
rebelay very tricky to pass. If you have ever tried to pass a rebelay from a
thread in an overhang where someone has used a wire or tape which is far too
long, then you'll know what I mean.
I'll admit that (4) and (5) are minor points, but I think the others are
quite important. I honestly can't think of _any_ good reason why you'd want
to put the shock-absorber at the belay, other than the fact that it is not
completely obvious how to incorporate the shock absorber into the SRT system
if it is at the caver. My current plan, as I described before, involves
using plastic maillons. If a I could find a shock absorber with a very
short "unstretched length" then the plastic maillon would be unnecessary,
but I haven't found such a thing (even though it should be technically
feasible to make a stitch ripper like this).
> Semi-static or low stretch ropes have some give under
> body weight but this increases as greater forces are applied. The aim
would
> be to design a rope/connector that has very low stretch under normal
loading
> but would have considerable stretchiness when shock loaded. This is
> something that I think could probably be achieved using current rope
making
> technology.
It already has been acheived. There are stitch-ripping devices like the
yates screamer. These work by having a loop of tape which is shortened by
lots of stitches. When you get to a critical force the stitches start to
tear, elongating the loop. See:
http://www.yatesgear.com/climbing/screamer/index.htm .
There are also controlled-slippage devices like the petzl zyper
(http://www.petzl.com/FRENG/frbelaydevice/zyper.html) which work by only
allowing the rope to pass through the metal part once a certain load has
been reached.
Both types of device have the properties you describe - they stretch very
little under normal loading, but will start to have extremely high stretch
as soon as the force reaches a critical value.
Stitch rippers are used in industry for a variety of purposes when something
must be decelerated, including for fall protection of humans (industry
people might use 8mm steel cable as their safety line, so this is a definite
requirement). In the case that they are used for fall protection they are
usually termed "shock absorbing lanyards" and are available from many
different companies.
At the moment I'm of the opinion that the stitch rippers are probably a
better choice, on the basis that they're used in industry for this purpose,
and they will not be affected adversely by mud, water, and all the other
nasty things we have underground. On the other hand, the zypers can absorb a
larger energy.
According to an email from yates, their longest stitch ripper, the zipper,
can absorb 1200 foot-pounds (in static situations like ours - in the case
that you are using one with a climbing rope the figures are different). This
equates to 1.6kJ of energy. The petzl catalogue states that the zyper can
handle a 5m fall, and if you assume the usual 80kg climber, then this
equates to 3.9kJ, over twice as much. With the stitch ripper and assuming an
80kg climber you would only be able safely to fall 2 and a bit metres.
Perhaps the thing to do is to remove the possibility of any failure which
might result in a larger fall than 2 metres, so if you have an extreme
deviation or a pendule rebelay then you will have to make sure it simply
cannot fail (using multiple anchors, Y-hangs, etc). For failures which
result in short falls - deviation failure, failure of one half of Y-hang,
etc - you let the shock absorber do its work.
Lev
Paul Montgomery
This has been an interesting thread. There has been so many things that I
wanted to comment on in different post that I think it is easer to just
start fresh.
Lev, I was thinking the same thing about making electricty, or at least
using this type of device for braking. I can brake a dc motor by shorting
out the leads. It's very common to do this thru a resistor. This would
transfer the heat away from the rope. I think the best idea for a custom
built decender discussed here is the pressure plate brake.
I agree that it is better to have the shock absorber system at the caver.
I'm trying to invision something that would be short and small but also
resetable. I'm not quite there yet.
I used spectra as thread for sewing a chest harness. It is slick but seemed
to knot well, but I don't know how strong the knot was. This stuff is tuff.
I bought it as fishing line under the name Spider Wire. With no streach
setting a hook is easy. I sawed it across a rock and got very little
abrasion to the line. It will cut right through you flesh if you are not
careful. Holds up to guns well too. (That's for Nigel)
I'm assuming that the main line will be nylon coated. Asenders may be more
of a problem that first thought.
I'm tired. Going to bed.
Lev Bishop
Hash: SHA1
"Paul Montgomery" <Pa...@tcia.net> wrote in message
news:xN5T4.77690$VR.15...@news5.giganews.com...
> I agree that it is better to have the shock absorber system at the
> caver. I'm trying to invision something that would be short and
> small but also resetable. I'm not quite there yet.
Obviously it would nicer if it were resettable, but since a
shock-loading is (hopefully) an infrequent event, it wouldn't be too
bad if it weren't, and you just had to carry a spare one with you in
case you trigger your first one.
Lev
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Mark McVey
friend about how to descend with the least amount of friction and he
suggested free fall!
Anyway, I am working on it and will probably have plans drawn up in a
couple of months. Hope you can wait that long!
Later,
Mark McVey
David Gibson
<uk.rec.caving>, on Sat, 13 May 2000
Paul Montgomery <Pa...@tcia.net> writes
>Lev, I was thinking the same thing about making electricty, or at least
>using this type of device for braking. I can brake a dc motor by shorting
>out the leads. It's very common to do this thru a resistor.
I described a system like this some time ago. You use a motor with a
nicely defined characteristic, like a DC brush servomotor, and you use
it as a generator. You dont use a resistor as a load but a device that
'clamps' the generated voltage to a fixed, known value. Something like
a lead-acid battery, for example. The result is that the contraption
moves down the rope at a fixed speed which depends on the battery
voltage. The potential energy of the descending caver is converted into
electricity - until something fails and you plummet.
The correct sense for the connection of the motor is so that under no
load it actually moves *up* the rope under the influence of the
connected battery.
The battery has to cope with a large 'fast charge'. A 70kg caver
abseiling 180m would release enough energy (at 100% efficiency) to
recharge an FX2 (which you normally take 16 hours to charge), in 10
minutes or so. (A power conversion rate of over 200W - no wonder things
get hot).
--
David Gibson
Spam-cloaking: To reply: change 'nospam' to 'mcrosolv' in domain name, i.e.
mailto:dav...@mcrosolv.demon.co.uk
Paul Montgomery
David Gibson <dav...@nospam.demon.co.uk> wrote in message
news:1u9HzAAe...@mcrosolv.demon.co.uk...
You could use the charge in the battery to send it back up the rope, or aid
a climb.
Before anyone starts picking this apart, I think David and I both know the
shortcomings of a system like this. Weight, complexity, failure mode, all
stop this from being practical. I did build something similar to this. I
have a garage with 14 ft ceilings. I happen to have a DC gearmotor lying
around, so I got the idea of building a continuous loop rope system that I
could climb while it lowered me. I calculated the motor as just being large
enough to handle the load using a 2" pipe for a drum for the rope. I
calculated about 240 lbs of lifting force would be generated at 100%
efficiency. I figured that 25% would be more loss than the system had. When
I got it put together, it would barely lift me, and quickly overheated.
Shouldn't have assumed. Someday I might rebuild it with a smaller drum or a
bigger motor. For now it lifts a bike rack so that I can hang my kids bikes
out of the way.
Lev Bishop
Hash: SHA1
"David Gibson" <dav...@nospam.demon.co.uk> wrote in message
news:1u9HzAAe...@mcrosolv.demon.co.uk...
> The correct sense for the connection of the motor is so that under
> no load it actually moves *up* the rope under the influence of the
> connected battery.
I'm pretty certain you've got this the wrong way around, and you want
the motor connected so that it wants to go *down* the rope - the
point being that it wants to go down the rope at a certain fixed
speed, which doesn't change with the addition of the caver's weight.
(The actual level of speed regulation would depend on the I-V
characteristic of the battery and the I-V-rpm characteristics of the
motor, of course).
As I remember you also got this wrong last time we discussed this :-)
Think of it as a "back emf" if you like - for an ideal motor under
zero load there will be no power transfer from battery to motor. If
there is positive load then energy will flow from battery to motor,
and vice versa.
Lev
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Lev Bishop
Hash: SHA1
"Lev Bishop" <bis...@remove-this-part.teaching.physics.ox.ac.uk>
wrote in message news:8fou57$3ql$1...@news.ox.ac.uk...
> As I remember you also got this wrong last time we discussed this
> :-)
In fact, I've been through the archives and found your admission:
David Gibson wrote:
>In article "Self-Recharging Flashlight" in <!speleonics>, on Wed, 28
>Oct 1998 David Gibson writes
>>you arrange the sense of the connections so that,
>>left on its own, the motor will climb *up* the rope - that bit is
>>important.
>
>What I meant - as described in my original article in the CREG
>journal - is that the sense of the connection should be such that
>the motor climbs *down* the rope. THAT is the important bit,
>because it is counter- intuitive! (As I have demonstrated with my
>failing memory :-)
:-)
Lev
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Paul Montgomery
I though this was backwards. I work a lot with DC motors. Regenerative
drives actually reverse polarity of the current being supplied to a spinning
motor to provide more breaking for the motor (controlled to prevent motor
damage). Resistive braking, particularly on something that has much inertia
and little gear reduction, takes too long to stop a machine. The braking
effect decrease in inverse to the speed.
David's right, it is counter-intuitive when you think in terms of braking a
load going down the rope, by connecting a battery to a motor that also wants
to go down the rope.
Thinking about David's plan to charge a battery, it would be interesting to
charge a dead battery on the way down and then see how far it would take you
up by running the motor. The results would be a test of system efficiency
that took everything into consideration. You would have to have a really
good battery to take the currents that would be generated by a caver.
Lev Bishop <bis...@remove-this-part.teaching.physics.ox.ac.uk> wrote in
message news:8foud4$49p$1...@news.ox.ac.uk...
Carol Haynes
Its a bit like reading something from Star Trek.
Beam me up Scotty!!
(A somewhat bemused) Carol
"Paul Montgomery" <Pa...@tcia.net> wrote in message
news:9DUT4.84418$VR.16...@news5.giganews.com...
Paul Montgomery
new thread. I keep forgetting that we are cross posting also. It's not like
either group has been covered up with messages anyway. You could just not
read it.:]
Carol Haynes <j_carol_ha...@hotmail.com> wrote in message
news:FIUT4.3423$m%1.6...@news6-win.server.ntlworld.com...
Hoyt McKagen
> it as a generator. You dont use a resistor as a load but a device that
> 'clamps' the generated voltage to a fixed, known value. Something like
> a lead-acid battery, for example.
Maximum braking effect is from dead shorted motor terms. I use a dc servo
motor as brake for my bicycle exerciser. I started with rheostat but soom
found out zero resistance made the best load. Why carry a battery along
for the ride when it won't work so well?
Anyway the problem is best solved w/o even a motor. Simpler brakes can be
devised that weigh ounces and don't heat up at all. A model airplane
motor propeller siz inches in diameter at 16,000 rpm can dissipate 4-500
watts directly into air.
> minutes or so. (A power conversion rate of over 200W - no wonder things
> get hot).
Any brake built into the reel will also heat it up past the safety point
for these exotics.
Regards,
Hoyt McKagen
Belfab CNC - http://www.freeyellow.com/members/belfab/belfab.html
Best MC Repair - http://www.freeyellow.com/members/batwings/best.html
Camping/Caving - http://www.freeyellow.com/members/batwings/caving.html
It rubs the lotion on its body; it places the lotion in the
basket
Hoyt McKagen
> > The correct sense for the connection of the motor is so that under
> > no load it actually moves *up* the rope under the influence of the
> > connected battery.
>
> I'm pretty certain you've got this the wrong way around, and you want
> the motor connected so that it wants to go *down* the rope -
Lev is correct.
>the
> point being that it wants to go down the rope at a certain fixed
> speed, which doesn't change with the addition of the caver's weight.
Lev is not correct. The weight must drive the motor past synchronous
speed to make a back EMF.
--
sp...@sucks.net
> Anyway the problem is best solved w/o even a motor. Simpler brakes can be
> devised that weigh ounces and don't heat up at all. A model airplane
> motor propeller siz inches in diameter at 16,000 rpm can dissipate 4-500
> watts directly into air.
Oh good. Fluid mechanics instead of all this foolish
talk of electricity. Electric cavers be gone!!!
--
Steve LePera /\v/\ VPI#351 Reacting Flow Lab
leperas at vt dot edu /\v/\ NSS#40805 Blacksburg VA, USA
Mechanical Engineering Dept. at Virgina Tech (540) 231-5882
David Gibson
Paul Montgomery <Pa...@tcia.net> writes
>>Haven't all these DC motors and braking effect drifted away from the
>>thread.
>Well yes they have, but is that a bad thing?
Its not nearly so irritating as all these "top posters" who dont edit
their replies considerately.
David Gibson
<uk.rec.caving>, on Mon, 15 May 2000
>I'm pretty certain you've got this the wrong way around
...
>As I remember you also got this wrong last time we discussed this :-)
...
>In fact, I've been through the archives and found your admission:
Sorry.
Im 83, you know.
David Gibson
<uk.rec.caving>, on Mon, 15 May 2000
>Maximum braking effect is from dead shorted motor terms.
That's not the full picture.
Maximum _opposing_torque_ is proportional to _motor_current_. So...
>Why carry a battery along
>for the ride when it won't work so well?
1. It does. With the added benefit that it 'clamps' the shaft speed. As
soon as the motor starts to generate a back-emf above this value, the
very low circuit resistance (c.f. your 'dead shorted terminals')
gives a counter torque that acts to bring the speed back down to the
clamp value.
2. You can charge your battery
3. Im not talking about *real* caving! I wouldnt trust my life to a
device like this.
>A model airplane
>motor propeller siz inches in diameter at 16,000 rpm can dissipate 4-500
>watts directly into air.
...but I wouldnt like to try jumping off a pitch, clutching a propellor.
[Lev Bishop]
>> point being that it wants to go down the rope at a certain fixed
>> speed, which doesn't change with the addition of the caver's weight.
>
>Lev is not correct.
I think you are both right :-) For a theoretical system with no circuit
resistance, Lev _is_ correct; the solution to the equations of motion
shows this. But...
...in a practical system, the torque that the caver applies does have
some bearing on the length of time taken to reach terminal velocity and
the resultant velocity - this is because of the circuit resistance. (Im
talking about dc brush servos with nicely-defined torque & speed
constants, not a motor from an old vacuum cleaner :-)
Carol Haynes
Carol Haynes
Postings to uk.rec.caving :
David Gibson 141
Carol Haynes 32
Who's "top poster" now?
See ya
Carol
Nigel Robertson
Anyway, it's the first thread for a while that's been about caving (just not
as we know it, Captain) and thread seems rather appropriate in relation to
the bootlace Lev wants to slide down.
Nigel
"Carol Haynes" <j_carol_ha...@hotmail.com> wrote in message
news:GVYT4.3933$m%1.7...@news6-win.server.ntlworld.com...
> Is that better?
>
>
PS, sorry Dave, it's at the top!
Paul Montgomery
> In article "ultralightweight caving, on dyneema, etc (long post)" in
> <uk.rec.caving>, on Mon, 15 May 2000
>
> soon as the motor starts to generate a back-emf above this value, the
> very low circuit resistance (c.f. your 'dead shorted terminals')
> gives a counter torque that acts to bring the speed back down to the
> clamp value.
>
effect is from dead shorted motor terms". I think he is thinking in terms of
practicality.
> 2. You can charge your battery
>
> 3. Im not talking about *real* caving! I wouldnt trust my life to a
> device like this.
Refer to my reply to item one.
>
> >A model airplane
> >motor propeller siz inches in diameter at 16,000 rpm can dissipate 4-500
> >watts directly into air.
>
> ...but I wouldnt like to try jumping off a pitch, clutching a propellor.
But his idea hold merit. If you gear up the prop speed and make it large
enough it might work. Wonder how big you would have to make it. Steve, this
is your area. Start caculating:]
>
>
> [Lev Bishop]
> >> point being that it wants to go down the rope at a certain fixed
> >> speed, which doesn't change with the addition of the caver's weight.
> >
> >Lev is not correct.
>
> I think you are both right :-) For a theoretical system with no circuit
> resistance, Lev _is_ correct; the solution to the equations of motion
> shows this. But...
>
> ...in a practical system, the torque that the caver applies does have
> some bearing on the length of time taken to reach terminal velocity and
> the resultant velocity - this is because of the circuit resistance. (Im
> talking about dc brush servos with nicely-defined torque & speed
> constants, not a motor from an old vacuum cleaner :-)
Don't forget the effective battery resistance. In fact I think that a caver
would go much faster down the rope using a battery over a dead short:] (got
to use plenty of smile things so I don't piss off anyone unintentionally)
Sorry Carol!!!!!
Carol Haynes
"Nigel Robertson" <ni...@dont-spam-me-im-vegetarian.easegill.screaming.net>
wrote in message news:3920...@news.server.worldonline.co.uk...
| Watch it Carol, you'll be in Dave's expansive killfile soon ;)
|
| Anyway, it's the first thread for a while that's been about caving (just
not
| as we know it, Captain) and thread seems rather appropriate in relation
to
| the bootlace Lev wants to slide down.
|
I'm sorry if I'm ignorant and/or stupid but I really found this thread very
interesting but can't quite follow where an extended discussion on how to
wire up DC motors was relevant???
Unfortunately I did want to read the thread (being ignorant and/or stupid I
had no contribution to make directly - just sit back a gaze in wonder at the
amount of thought that went into a lot of the ideas).
Keep it up chaps, I'm learning a lot - just a little less on DC motors and
their sticky out bits might help to remove the glazed expressions from my
face.
Ta Ta
Carol
Carol Haynes
can commincate with people who do!
Cheers
C
Lev Bishop
"Nigel Robertson" <ni...@dont-spam-me-im-vegetarian.easegill.screaming.net>
wrote in message news:3920...@news.server.worldonline.co.uk...
> the bootlace Lev wants to slide down.
I think it will be rather nervewracking to do SRT on something so incredibly
thin. Though I guess you'd get used to it eventually. After all, it's not so
long ago that 9mm seemed very thin - In fact, I have a quote (from Beneath
the Mountains - http://vidos.zoo.ox.ac.uk/oucc/btm/beneath.htm) to prove it:
-
The pitch was rigged with 'lightweight' rope - 9 mm in diameter instead of
the usual 11 mm. It was tied to a huge projection which jutted out over the
drop, and to get onto the rope meant squeezing down between the edge of the
shaft and the rock lump whilst tied on; the unnerving 62 metres of
nothingness directly below. I remembered Ben Lyon's words as we bought the
rope off him. He folded our cheque up and said, "Now don't be going and
killing yourselves will you?"
As I loaded the rope, about the thickness of my little finger, it visibly
narrowed. I am always scared that the rope will break. It's not enough to
know that its specifications probe it would support the weight of several
London taxis. What would I think about in the moments before hitting the
floor? Would they be seconds of peace or panic?
-
Lev
Nigel Robertson
>
>
> I'm sorry if I'm ignorant and/or stupid but I really found this thread
very
> interesting but can't quite follow where an extended discussion on how to
> wire up DC motors was relevant???
>
I can't understand it either but it makes a change from guns.
Nigel (non-physicist - fluid, mechanical, electrical or theoretical)
Lev Bishop
"Hoyt McKagen" <batw...@i-plus.net> wrote in message
news:392060...@i-plus.net...
> Anyway the problem is best solved w/o even a motor. Simpler brakes can be
> devised that weigh ounces and don't heat up at all. A model airplane
> motor propeller siz inches in diameter at 16,000 rpm can dissipate 4-500
> watts directly into air.
I like the idea but I think it's really about equally as practical as the
motor idea. You will need some kind of gearing (about 50:1) in order to get
the propeller spinning fast enough. Also, while the propeller will do a good
job of limiting your speed, you'll never be able to use it to come to a
complete stop, so you will still need some kind of braking arrangement,
anyway (although the requirements for the brake are no longer quite so
stringent - it can afford to give less sensitive control and will not have
to be able to dissipate heat continuously). The requirement for the gearing
and the brake mean that this will not really be much simpler than the motor
idea.
You also have to worry that the amount of resistance provided by the
propeller depends on it having unobstructed airflow - you'd abseil faster
when you're close to the wall than on a freehang (in the same way that you
can make a rowing machine easier by moving it closer to the wall).
If the propeller blade were to catch on something and snap, then you'd
plummet.
An idea which might work (but probably not) would be to use a similar setup
but using a viscous liquid rather than air for the friction. You could then
have the device completely enclosed and do without the gearing. You'd be
reintroducing the heat-generation problem, however (perhaps you could use
the liquid as a coolant, with the turbine causing it to circulate to a
heat-exchanger). The difficulty would be in finding a liquid which maintains
its properties over time and temperature.
Lev
Lev Bishop
have been doing some brainstorming of my own and I have come up with a novel
"solution" to the descender problem:
You could use a "normal" piece of nylon/polyester caving rope for the
descent, with a normal rack, stop, etc as your abseil device. As you abseil
you rig the 5.5mm line for your ascent. After each pitch you pull down the
thick rope after you, ready to use on the next pitch. This means that you
don't need to use any new equipment for the descent, but you still only have
to carry a single length of thick rope, equal to the length of your longest
single hang in the trip.
I know this sounds a lot like cordelette technique, but it has many
advantages over cordelette:
There is not the "one mistake and you're screwed" aspect of cordelette - if
you have a tangle or something snags, you can always change over to ascent
and sort it out, or at the very least escape from the cave;
Although the group is still forced to descend the cave together, they are
able to exit the cave independently (unlike with cordelette). Since the
ascent part is always going to be much slower than the descent, this is a
big advantage;
You can leave the cave rigged when you exit, so if necessary it can be
rigged and derigged in several stages, something which is impossible with
cordelette.
I think it has no disadvantages compared with cordelette.
It even has some advantages over "pure" dyneema technique (ie the technique
you would use in the hypothetical case that a descender suitable for dyneema
existed):
Since you don't have to leave any slack for people to rig their descenders
you can rig rebelays quite tightly, limiting the amount of shock loading
that is possible - given the static nature of dyneema this is clearly
desirable;
During rigging is when you're most likely to take long falls. With this
technique you will be attached to a relatively dynamic rope instead of the
highly static dyneema cord while you rig.
Of course it has some disadvantages, too:
You have to carry the extra weight of a piece of rope;
You'll have two things hanging down the pitch when you're rigging, and three
things as the last man comes down carrying the piece of cord which you're
going to use to pull down the rope. You'll have to be quite careful to avoid
tangles;
For the descent of the cave, you can only travel in as many independent
groups as you have lengths of rope - in general this will slow down the
descent slightly.
What do people think? At the moment, having only just come up with the idea,
I think it's definitely the way to go for the moment. It's a good way to
evaluate the use of dyneema for caving, without having to wait too long. If
it works and enough people start doing things this way then petzl or one of
the other big companies will see the potential market and is bound to solve
the descender problem properly.
Lev
Lev Bishop
"Mark McVey" <mmc...@uswest.net> wrote in message
news:ggmshs8q9cssoukth...@4ax.com...
> Well, I need to stop hanging around with smart alecks!! I asked a
> friend about how to descend with the least amount of friction and he
> suggested free fall!
> Anyway, I am working on it and will probably have plans drawn up in a
> couple of months. Hope you can wait that long!
Go for it! Keep us all informed about how you're getting along....
Lev
Paul Brooks
> Interesting statistic:
>
> Postings to uk.rec.caving :
>
> David Gibson 141
> Carol Haynes 32
>
over time. David was posting long before I arrived on the scene - in fact I
think he started the newsgroup!
I think the 'top posters' comment may have had a touch of irony and referred
back to a thread about quick-fire responses where people just hit the reply
button, type in some throw away comment and the message grows and grows!
--
Paul Brooks
Paul....@dial.pipex.com
Paul Brooks
> If the propeller blade were to catch on something and snap, then you'd
> plummet.
>
Sorry about that, but I'm far too slow to keep up with the rest of this
thread - interesting to lurk and watch though.
--
Paul Brooks
Paul....@dial.pipex.com
Swaledale Outdoor Club
http://website.lineone.net/~s-o-c
Carol Haynes
| plummet.
???? Like the rope ????
Hey I think I'm getting the hang of this (at last)
If we want to make caving an engineering experience how about some of those
flying packs on display at the LA Olympics?
They would have made Titan Shaft exploration somewhat easier.
Carol
Carol Haynes
Matthew Ryan
> I think the 'top posters' comment may have had a touch of irony and
referred
> back to a thread about quick-fire responses where people just hit the
reply
> button, type in some throw away comment and the message grows and
grows!
And surprisingly things seemed to improve somewhat after all that
ranting. Cheers to all those who made the effort.
Does this mean we can talk about CICs again now as well (!)
-Matt
Matthew Ryan
> I think the 'top posters' comment may have had a touch of irony and
referred
> back to a thread about quick-fire responses where people just hit the
reply
> button, type in some throw away comment and the message grows and
grows!
I'll blame a shitty newsreader for that shitty formatting. It's just the
sort of thing I hate to see from others !
Apologies to those just reading alt.caving - these last few messages
won't mean much I don't suppose.
-Matt
David Gibson
Paul Montgomery <Pa...@tcia.net> writes
[David Gibson, on using a motor as a generator...]
>it 'clamps' the shaft speed. As
>> soon as the motor starts to generate a back-emf above this value, the
>> very low circuit resistance (c.f. your 'dead shorted terminals')
>> gives a counter torque that acts to bring the speed back down to the
>> clamp value.
>>
>Yes but at a faster speed, so Hoyt is correct in saying "Maximum braking
>effect is from dead shorted motor terms".
Well, perhaps 'maximum braking effect' is too vague a term. Braking is
the opposite of acceleration, so 'maximum braking' should mean 'that
which slows you down to a _constant_speed_ most quickly'. If you use
this 'maximum opposing torque' definition then the criterion is minimal
circuit resistance. This is not _necessarily_ the same as shorting the
battery terminals, since a zero-resistance battery circuit will do just
as well.
>I think he [Hoyt] is thinking in terms of practicality.
Is *any* of this practical?!!
But, yes, if you just want to brake, then a rheostat is possible. Your
terminal velocity then depends on the torque you apply to the motor
(i.e. your weight) and the circuit resistance. True zero circuit
resistance means you wouldnt move at all.
The difference with a battery clamp is that, to a rough approximation,
your terminal velocity now doesnt depend on the torque you apply. (i.e.
it doesnt depend on your weight), nor does it depend (much) on the
circuit resistance [technical reasons snipped to avoid more complaints
about off-topic postings].
>In fact I think that a caver
>would go much faster down the rope using a battery over a dead short
Yes, a true 'dead short' means you would not move at all. You cant
compare the two situations. Without a battery clamp your terminal
velocity depends on unknowns, like circuit resistance and caver weight.
With a battery clamp the main factor is a *known* parameter - the
battery voltage.
The steady-state term in the solution to the differential equation that
governs the motion is, as Im sure those of you who know about motors can
work out, :-) [fixed-pitch font required]
Tc.R Vb w - the terminal angular velocity of the motor
w = ----- + -- R - circuit resistance
Kt.Kv Kv Tc - torque applied by the dangling `caver
Kt, Kv - motor torque and voltage constants
Vb - clamping battery voltage
I think this equation sums up all the discussion so far ;-)
David Gibson
sp...@sucks.net writes
>In alt.caving Hoyt McKagen <batw...@i-plus.net> wrote:
>
>> Anyway the problem is best solved w/o even a motor. Simpler brakes can be
>> devised that weigh ounces and don't heat up at all. A model airplane
>> motor propeller siz inches in diameter at 16,000 rpm can dissipate 4-500
>> watts directly into air.
>
>Oh good. Fluid mechanics instead of all this foolish
>talk of electricity. Electric cavers be gone!!!
>
Reminds me of a book, "The Inventions of Daedalus - a compendium of
plausible schemes" by David E.H. Jones (1982, ISBN 0 7167 1413 2).
(Its a compendium of Daedalus's New Scientist articles, with added
theory and calculations).
Look up "Spiders, aerial descent, theory of" in the index, and be
referred to a chapter about the frictional effect of gossamer threads.
The author calculates [fluid mechanics bit snipped] that 2kg of
gossamer-thin glass fibre threads would make a good alternative to a
parachute - slowing a 70kg person to under 6m/s (equivalent to jumping
off a 2m wall.).
The only drawback is that you have to have 10,000 fibres all 1km long.
So: only good for the *long* pitches.
David Gibson
Paul Brooks <Paul....@dial.pipex.com> writes
>I think the 'top posters' comment may have had a touch of irony
:-)
I should have hyphenated it.
Top-posters are the opposite of bottom-posters.
Carol Haynes
There's no way I'm posting my bottom!
Carol
Tony Brocklebank
plugs would pack up on the petrol powered ascender, at the same time as the
battery on the electric backup winch shorted out and the brakes failed. I
think I'll stick with the Petzl manual versions.
Tony :-)
--
__________________________________
Email: to...@alpine.fsnet.co.uk
Web site: www.alpine.fsnet.co.uk
__________________________________
Paul Montgomery <Pa...@tcia.net> wrote in message
news:mAVT4.36347$701.5...@news4.giganews.com...
> Well yes they have, but is that a bad thing? I guess that we could start a
> new thread. I keep forgetting that we are cross posting also. It's not
like
> either group has been covered up with messages anyway. You could just not
> read it.:]
> Carol Haynes <j_carol_ha...@hotmail.com> wrote in message
> news:FIUT4.3423$m%1.6...@news6-win.server.ntlworld.com...
> > Haven't all these DC motors and braking effect drifted away from the
> thread.
> >
> > Its a bit like reading something from Star Trek.
> >
> > Beam me up Scotty!!
> >
> > (A somewhat bemused) Carol
> >
> > "Paul Montgomery" <Pa...@tcia.net> wrote in message
> > news:9DUT4.84418$VR.16...@news5.giganews.com...
> > | Thanks Lev,
> > | I though this was backwards. I work a lot with DC motors. Regenerative
> > | drives actually reverse polarity of the current being supplied to a
> > spinning
> > | motor to provide more breaking for the motor (controlled to prevent
> motor
> > | damage). Resistive braking, particularly on something that has much
> > inertia
> > | and little gear reduction, takes too long to stop a machine. The
braking
> > | effect decrease in inverse to the speed.
> > |
> > | David's right, it is counter-intuitive when you think in terms of
> braking
> > a
> > | load going down the rope, by connecting a battery to a motor that also
> > wants
> > | to go down the rope.
> > |
> > | Thinking about David's plan to charge a battery, it would be
interesting
> > to
> > | charge a dead battery on the way down and then see how far it would
take
> > you
> > | up by running the motor. The results would be a test of system
> efficiency
> > | that took everything into consideration. You would have to have a
really
> > | good battery to take the currents that would be generated by a caver.
> > |
> > | Lev Bishop <bis...@remove-this-part.teaching.physics.ox.ac.uk> wrote
in
> > | message news:8foud4$49p$1...@news.ox.ac.uk...
> > | > -----BEGIN PGP SIGNED MESSAGE-----
> > | > Hash: SHA1
> > | >
> > | > "Lev Bishop" <bis...@remove-this-part.teaching.physics.ox.ac.uk>
> > | > wrote in message news:8fou57$3ql$1...@news.ox.ac.uk...
> > | > > As I remember you also got this wrong last time we discussed this
> > | > > :-)
> > | >
> > | > In fact, I've been through the archives and found your admission:
> > | >
> > | > David Gibson wrote:
> > | > >In article "Self-Recharging Flashlight" in <!speleonics>, on Wed,
28
> > | > >Oct 1998 David Gibson writes
> > | > >>you arrange the sense of the connections so that,
> > | > >>left on its own, the motor will climb *up* the rope - that bit is
> > | > >>important.
> > | > >
> > | > >What I meant - as described in my original article in the CREG
> > | > >journal - is that the sense of the connection should be such that
> > | > >the motor climbs *down* the rope. THAT is the important bit,
> > | > >because it is counter- intuitive! (As I have demonstrated with my
> > | > >failing memory :-)
> > | >
> > | > :-)
> > | >
> > | > Lev
> > | >
> > | > -----BEGIN PGP SIGNATURE-----
> > | > Version: PGPfreeware 6.5.1 for non-commercial use
<http://www.pgp.com>
> > | >
> > | > iQA/AwUBOR/84/B/d+LuL5ZXEQLCEgCgkHTU+6ARZ0cpMP8MefJeR6lVZZkAn1pz
> > | > +Ea4DXsJrF3WW9O4uY3Q+3YU
> > | > =V46Z
> > | > -----END PGP SIGNATURE-----
> > | >
> > | >
> > | >
> > |
> > |
> >
> >
>
>
Hoyt McKagen
>
> Oh good. Fluid mechanics instead of all this foolish
> talk of electricity. Electric cavers be gone!!!
long and foolishly extended trip involving rope, waterfall, grand chill
factors and bolting-climb. The latter used an electric drill, no?
BTW, have you any info on trips to Ballard's? I'm still curious about who
was in there beyond the pond but mysteriously hidden from us.
Regards,
Hoyt McKagen
Belfab CNC - http://www.freeyellow.com/members/belfab/belfab.html
Best MC Repair - http://www.freeyellow.com/members/batwings/best.html
Camping/Caving - http://www.freeyellow.com/members/batwings/caving.html
It rubs the lotion on its body; it places the lotion in the basket
Hoyt McKagen
> That's not the full picture.
> Maximum _opposing_torque_ is proportional to _motor_current_. So...
Yeah, I realized what they meant after I re-read the items.
> 1. It does. With the added benefit that it 'clamps' the shaft speed. As
> soon as the motor starts to generate a back-emf above this value, the
> very low circuit resistance (c.f. your 'dead shorted terminals')
> gives a counter torque that acts to bring the speed back down to the
> clamp value.
True!! Now you've added in a control system. Not to labor the point but
if this fails you're in free fall.
> ...but I wouldnt like to try jumping off a pitch, clutching a propellor.
You'd need a device to speed the action up, quite a bit in fact. Small
planetary gearsets for motor speed reducers can handle very large ratios
and are compact (they're available for any standard NEMA motor including
#23 and #42) and efficient. I wouldn't use propeller anyhow, a small
ducted fan on end of reducer is much better way.
Hoyt McKagen
> motor idea. You will need some kind of gearing (about 50:1) in order to get
> the propeller spinning fast enough.
I covered this in another post. Speed reducers often have over 100:1
ratios and can be 2.5" in dia and also that long, with slow shaft torque
ratings of several hundred ft-lbs.
> job of limiting your speed, you'll never be able to use it to come to
a
> complete stop, so you will still need some kind of braking arrangement,
> anyway (although the requirements for the brake are no longer quite so
> stringent
All you need to do in this case is put thumbwheel on the high-speed
shaft; torque is going to be extremely low there. I've stopped model
motors of about Hp rating by putting friction on the spinner with my
hand.
> and the brake mean that this will not really be much simpler than the motor
> idea.
But much much lighter.
>
> You also have to worry that the amount of resistance provided by the
> propeller depends on it having unobstructed airflow - you'd abseil faster
> when you're close to the wall than on a freehang
I don't see any problem exhausting air straight up or down the rope.
> If the propeller blade were to catch on something and snap, then you'd
> plummet.
Guard or ducted fan. DF dissipate even more than propellers for given
dia.
> but using a viscous liquid rather than air for the friction. You could then
> have the device completely enclosed and do without the gearing.
Have to be pretty big or still geared up.
Tony Brocklebank
Paul Montgomery
>
> >I think he [Hoyt] is thinking in terms of practicality.
>
> Is *any* of this practical?!!
Practical in "build one and see what it actually does"
>
>True zero circuit resistance means you wouldnt move at all.
Formulas aside, I hope you know that this doesn't happen, even if you have
true zero resistance in the entire circuit. I'm assuming that your generator
is a permanent magnet servo motor, although a field wound motor would do the
same. Zero velocity(w) means zero motor torque(Kt). Since caver torque(Tc)
is not zero, the motor has to move some for any motor torque(Kt) to appear.
Now if the motor and short were superconductive, the motor would have to
move only an infinitely small amount to produce enough motor torque(Kt) but
it does have to move. Your formula breaks down even in theory at zero. It is
the old what came first, the chicken or the egg. You are cheating a bit
when you say you are right, because the formula says this will happen. Until
we get room temp. superconductors and perfect batteries you can never
produce those results. People that learned by doing, see what you are saying
as bunk, not because you are wrong, but because you can never produce those
results in the real world.
I'm cheating in the sense that there is no difference between an infinitely
small amount and zero.... or is there?
>
> The difference with a battery clamp is that, to a rough approximation,
> your terminal velocity now doesnt depend on the torque you apply. (i.e.
> it doesnt depend on your weight), nor does it depend (much) on the
> circuit resistance [technical reasons snipped to avoid more complaints
> about off-topic postings].
There is something wrong here. Torque on the motor produces
current/voltage(dc wattage). Voltage in theory is constant. The braking
force generated is proportional to the resistance of the circuit connected
to the motor leads because it limits the current and thus the dc wattage. By
replacing the resistor with a battery, you have just shifted the voltage.
The motor will now turn at whatever speed that balances the equation. In
theory, discounting friction, the motor will now draw zero current from the
battery. If you try to force the motor to move slower (load) then the motor
draws current. If you try to force the motor to move faster it produces
current which charges the battery. A battery has an effective resistance
which can be thought of as it's maximum charge rate. This is the same as a
resistor in the circuit. If this wasn't true, then you could put infinite
load(caver torque) on the motor and the battery would charge at an infinite
rate.
>
> >In fact I think that a caver
> >would go much faster down the rope using a battery over a dead short
>
> Yes, a true 'dead short' means you would not move at all. You cant
> compare the two situations. Without a battery clamp your terminal
> velocity depends on unknowns, like circuit resistance and caver weight.
> With a battery clamp the main factor is a *known* parameter - the
> battery voltage.
As before, there is something wrong here. The only difference I can see is
while using a battery, you make Kv have more weight in the equation than R,
since Kv is zero using a short. One, or both of us is looking at this wrong.
Maybe we are both right and approaching it from different perspectives.
After rereading you last statement, I see our different perspectives. You
said "With a battery clamp the main factor is a *known* parameter - the
battery voltage.". This is not true. The main factor is the current that the
battery can produce, or on the other side, the current charge that it will
take. This is what I was calling the "effective resistance" of the battery.
This resistance is much greater than the circuit resistance, and is just as
much an unknown as circuit resistance and affects the equation in the
battery scenario as the circuit resistance does in a dead short.
It is getting silly.
> The steady-state term in the solution to the differential equation that
> governs the motion is, as Im sure those of you who know about motors can
> work out, :-) [fixed-pitch font required]
>
> Tc.R Vb w - the terminal angular velocity of the motor
> w = ----- + -- R - circuit resistance
> Kt.Kv Kv Tc - torque applied by the dangling `caver
> Kt, Kv - motor torque and voltage constants
> Vb - clamping battery voltage
>
In deference to Carol and the others who may really be getting tired of
this. I'll be glad to take any additional discussions off this ng.
Paul Montgomery
> | If the propeller blade were to catch on something and snap, then you'd
> | plummet.
>
>
> Hey I think I'm getting the hang of this (at last)
>
> If we want to make caving an engineering experience how about some of
those
> flying packs on display at the LA Olympics?
>
> They would have made Titan Shaft exploration somewhat easier.
>
> Carol
>
other post. Tony says I'm like a bulldog. I just hang and wear you down.
Paul Montgomery
Tony Brocklebank <tonybro...@tesco.net> wrote in message
news:8fq4h8$5mg$1...@barcode.tesco.net...
> I'm with Carol (that should get her worried??). Knowing my luck the spark
> plugs would pack up on the petrol powered ascender, at the same time as
the
> battery on the electric backup winch shorted out and the brakes failed. I
> think I'll stick with the Petzl manual versions.
>
> Tony :-)
>
Paul Brooks
news:8fr4kl$rl8$1...@quince.news.easynet.net...
> ---
> Apologies to those just reading alt.caving - these last few messages
> won't mean much I don't suppose.
>
David Gibson
<uk.rec.caving>, on Tue, 16 May 2000
>>True zero circuit resistance means you wouldnt move at all.
>Formulas aside, I hope you know that this doesn't happen
I think you know perfectly well what I meant ;-)
>Until we get room temp. superconductors and perfect batteries you can
>never produce those results.
That's obvious, isnt it?
Im sorry, you seem to be reading rather too much into what is really
just a fun little "thought experiment".
Most of the refutations in your last e-mail seem to have arisen out of a
too-literal interpretation of what I was saying. I think you are in
danger of missing my point. For example, you said..
>You
>said "With a battery clamp the main factor is a *known* parameter - the
>battery voltage.". This is not true. The main factor is the
>[Snip] "effective resistance" of the battery.
>This resistance is much greater than the circuit resistance, and is just as
>much an unknown as circuit resistance and affects the equation in the
>battery scenario as the circuit resistance does in a dead short.
Well, yes, I *know* that! I was assuming that it was obvious that
'circuit resistance' meant the *total* resistance in the circuit. And
with that proviso, what I said is still true. (But the point about being
able to fast-charge a battery is that its resistance is low). Put some
values into the equation and see!
>I'll be glad to take any additional discussions off this ng.
I think we should call it a day here. I dont feel any need to discuss it
privately either, but if you want to come up with a more descriptive
equation than the one I gave, please feel free.... :-)
Carol Haynes
Carol
Tony Brocklebank
we're saying!
--
__________________________________
Email: to...@alpine.fsnet.co.uk
Web site: www.alpine.fsnet.co.uk
__________________________________
Carol Haynes <j_carol_ha...@hotmail.com> wrote in message
news:d_%T4.3323$XW.8...@news2-win.server.ntlworld.com...
> 'Tis okay - didn't understand a word of it - just filled with pride that I
> can commincate with people who do!
>
> Cheers
>
> C
>
>
Lev Bishop
> Speed reducers often have over 100:1
> ratios and can be 2.5" in dia and also that long, with slow shaft torque
> ratings of several hundred ft-lbs.
Interesting. How cave proof do you think they are (or could be made)? Do you
think it'd be possible to make one of them sufficiently reliable that you'd
trust your life to it, or do you think additional safety systems will always
be necessary?
> All you need to do in this case is put thumbwheel on the high-speed
> shaft; torque is going to be extremely low there. I've stopped model
> motors of about Hp rating by putting friction on the spinner with my
> hand.
I guess this would probably work, though you might want to wear gloves.
I don't know anything about these gearboxes, but if they're easy, light,
reliable, caveproof, etc, then the ducted fan airbrake seems like a fairly
sensible idea. I like the idea of cavers whirring merrily as they descend,
blowing all the dust around when they touch down - like a helicopter
landing....
Lev
David Gibson
<uk.rec.caving>, on Tue, 16 May 2000
Paul Montgomery <Pa...@tcia.net> writes
>I'm cheating in the sense that there is no difference between an infinitely
>small amount and zero.... or is there?
Youre not cheating, but there's more to it than I said previously, so
perhaps I mislead you.
I said that the formula I gave was the _steady-state_ term in the
solution. It gives you the terminal velocity, but the salient point is
that this assumes you _do_ reach a terminal velocity. There is also a
dynamic term, which I didnt give.
>Your formula breaks down even in theory at zero.
It shows that with R=0 the response due to a step-change in applied
torque (caver jumping on to rope) is a zero terminal velocity. That is
correct. The full equation shows that the system behaves like a spring,
with an oscillating disturbance, but no net movement. (Or, if you like,
a motor with a very large detent torque). So there *is* movement, but
you dont make progress - I only gave the 'making progress' bit of the
formula.
But even in a non-superconducting system the formula I gave suggests a
"believable" result down to all practical values of 'dead short'. (0.1
ohm, 0.01 ohm?). If people expect 'dead short' to mean 'completely zero'
that's *their* problem :-)
Sorry - I *was* going to call it a day.
Paul Montgomery
That is interesting (to me anyway). Can you give me a reference to the
dynamic formula? I would like to read up on this. I've actually seen
something similar to this happen with two AC generators when they are put
into parallel.
I still ask the question, "What is the value of half of infinity"
Viewers should be reminded that this is a mental experiment only! The people
you have seen doing this are professionally demented. Do not try this at
home!
David Gibson <dav...@nospam.demon.co.uk> wrote in message
news:QwquEVA+...@mcrosolv.demon.co.uk...
Hoyt McKagen
>
> Interesting. How cave proof do you think they are (or could be made)?
They're enclosed in metal case and either have or can be fitted with
O-rings and seals; these often get used in sloppy or dirty places anyhow
in industry.
> Do you
> think it'd be possible to make one of them sufficiently reliable that you'd
> trust your life to it, or do you think additional safety systems will always
> be necessary?
They usually have rating for load/life at 500 hours. If the slow shaft
torque is 100 ft/lbs the device should go the whole 500 hours at that
value just fine. That's a lot of rope.
You would always be a fool to not us a safety, like an ascender or
prussik following your main rig.
>
> I guess this would probably work, though you might want to wear gloves.
Not much power left over while the fan is working. You'd wear gloves
anyhow on descent, no?
> blowing all the dust around when they touch down - like a helicopter
> landing....
Not likely, first you'd point it up to avoid that. Second the fan blast
spreads out and dissipates rapidly.
Ralph Hartley
energy from a descent.
How about stirring water? When you get to the bottom you can have a
cup of nice hot coffee. Of course this has a few disadvantages of its
own.
Some cavers I know have brains that do not operate until caffeine is
added. It is generally considered good practice, from a safety
perspective, to turn the brain on before dropping the pit instead of
after.
This could exacerbate the other problem, which is if the design of the
descender assumes it is full of water - it had better be! If the caver
(who's brain is still off) notices half way down that his coffee is
hot, and (being impatient as such people often are) drinks it, his
descender would only stir air. This could ruin his entire day.
Ralph Hartley
Lev Bishop
> > blowing all the dust around when they touch down - like a helicopter
> > landing....
>
> Not likely, first you'd point it up to avoid that.
I don't know. Seems to me if you had it pointing upwards you might end up
sandblasting your face and blowing grit in your eyes, though I suppose you
could make the ducting longer or mount the whole thing higher so this
wouldn't happen.
Have you got a URL where I can find out about these wonderful planetary
gearboxes, or a supplier to contact? All the ones I could find in my
catalogues and websearches with about the right specs weighed at least
6kg...
Lev
Paul Montgomery
Carol Haynes
interested ..... actually forgot that the conversation is entirely men
talking about boys toys ... nuf said.
Carol
"Paul Montgomery" <Pa...@tcia.net> wrote in message
news:WSBU4.95241$VR.18...@news5.giganews.com...
Tony Brocklebank
--
__________________________________
Email: to...@alpine.fsnet.co.uk
Web site: www.alpine.fsnet.co.uk
__________________________________
Carol Haynes <j_carol_ha...@hotmail.com> wrote in message
news:WFiU4.1326$86.1...@news2-win.server.ntlworld.com...
Tony Brocklebank
>
> I still ask the question, "What is the value of half of infinity"
>
Other acceptable answers include "Who gives a s****"
:-)
Tony Brocklebank
--
__________________________________
Email: to...@alpine.fsnet.co.uk
Web site: www.alpine.fsnet.co.uk
__________________________________
Ralph Hartley <har...@aic.nrl.navy.mil> wrote in message
news:3922A0C1...@aic.nrl.navy.mil...
David Gibson
Paul Montgomery <Pa...@tcia.net> writes
>That is interesting (to me anyway). Can you give me a reference to the
>dynamic formula? I would like to read up on this.
I cant give you a specific reference, because I worked it out myself.
(Its a straightforward mechanics problem :-). I'll e-mail it to you.
However, you should be able to read about the dynamic modelling of
motors in any "good book" on control systems, especially one that covers
stepper motors or servo motors.
Hoyt McKagen
> Have you got a URL where I can find out about these wonderful planetary
> gearboxes, or a supplier to contact?
I don't have much at hand but you might try 'planetary spped reducers' as
your search term. These are about the same dia as the NEMA frames they
fit and about at most 2/3 of the length. I don't see how one could weigh
six Kg for a #34 frame.
Mark McVey
had posted that they were having a hard time finding a gearset small
enough to be practical. I am having the opposite problem! I work on
cameras and they have a reducing gearset that would be perfect if it was
scaled up a bit!
Looks like I am going to have to find someone willing to make one!
Later,
Mark McVey
On Tue, 16 May 2000 10:14:04 -0700, Hoyt McKagen <batw...@i-plus.net>
wrote:
>David Gibson wrote:
>> That's not the full picture.
>> Maximum _opposing_torque_ is proportional to _motor_current_. So...
>
>Yeah, I realized what they meant after I re-read the items.
>
>> 1. It does. With the added benefit that it 'clamps' the shaft speed. As
>> soon as the motor starts to generate a back-emf above this value, the
>> very low circuit resistance (c.f. your 'dead shorted terminals')
>> gives a counter torque that acts to bring the speed back down to the
>> clamp value.
>
>True!! Now you've added in a control system. Not to labor the point but
>if this fails you're in free fall.
>
>> ...but I wouldnt like to try jumping off a pitch, clutching a propellor.
>
>You'd need a device to speed the action up, quite a bit in fact. Small
>planetary gearsets for motor speed reducers can handle very large ratios
>and are compact (they're available for any standard NEMA motor including
>#23 and #42) and efficient. I wouldn't use propeller anyhow, a small
>ducted fan on end of reducer is much better way.
>
Wookey
<URL:mailto:bis...@remove-this-part.teaching.physics.ox.ac.uk> wrote:
>
> "Hoyt McKagen" <batw...@i-plus.net> wrote in message
>
> > Speed reducers often have over 100:1
> > ratios and can be 2.5" in dia and also that long, with slow shaft torque
> > ratings of several hundred ft-lbs.
>
> that you'd trust your life to it, or do you think additional safety
> systems will always be necessary?
I can't see that there is that much to go wrong here that would result in
plumetting death. Gearboxes are very reliable - when did your car or bike
gearbox stop working? So long as it doesn't strip all its teeth of the
propeller come off the shaft (make it with woodruff keys) then it ought
to be farily reliable. I can see various ways it might jam up, but that
just leaves you stuck rather than flat. Changeover and prusik out. If you
cover up/bung up the vent does that make it go faster or slower? I can
see that might be a problem if the answer is 'significantly faster', but
if there is a manual brake for control it should be OK.
I can imagine that this thing maight be a marvelous scheme for spinning
furiously on all descents - might need some careful design to get the
exhaust direction right.
Wookey
--
Aleph One Ltd, Bottisham, CAMBRIDGE, CB5 9BA, UK Tel (00 44) 1223 811679
work: http://www.aleph1.co.uk/ play: http://www.chaos.org.uk/~wookey/
Paul Montgomery
that has a 625:1 reduction and is 4"x3.5"x2.25". That is their smallest
servo gearbox. They have hundreds of sizes. They don't give the weight on
the servo box. They have speed reducers at over 1000/1 ratio weighing 2 oz
for the smallest and go up from there. Here is their number:
W. M. Berg Inc.
499 Ocean Ave
East Rockaway
Tel 516 596 1700
http://www.wmberg.com/
You owe me big time for taking the time to look all this up!
Mark McVey <mmc...@uswest.net> wrote in message
news:65c8is40u4uphr9mi...@4ax.com...
Paul Montgomery
Then use it on a drop with dead weight several times first.
Wookey <woo...@godIhatespam.aleph1.co.uk> wrote in message
news:ant18135...@chewy.aleph1.co.uk...
Paul Montgomery
Try thinking deep thoughts Tony. It will be difficult and painful, but
try.:-}
You have never been puzzled at why you can multiply or divide infinity and
still have the same value. I doesn't make sense. It just DOESN'T!!! I can't
stand it. It drives me crazy!!!!
<Inside joke folks)
Nigel Robertson
> Viewers should be reminded that this is a mental experiment only! The
people
> you have seen doing this are professionally demented. Do not try this at
> home!
>
Sh*t. I've just gone out and bought all the parts. I was going to try it
out in a very deep pothole first though, before attempting it at home ;]
Nigel Robertson
> Genuinely meant ... really. I didn't know there were so many people
> interested ..... actually forgot that the conversation is entirely men
> talking about boys toys ... nuf said.
>
Men used to cave with hemp and wood but then it all got too heavy for the
modern generations ;]
Bring some girls toys into the conversation if you want to redress the
balance!
Nigel
Nigel Robertson
"Lev Bishop" <bis...@remove-this-part.teaching.physics.ox.ac.uk> wrote in
message news:8ftfhj$cn3$1...@news.ox.ac.uk...
>
> I don't know anything about these gearboxes, but if they're easy, light,
> reliable, caveproof, etc, then the ducted fan airbrake seems like a fairly
> sensible idea. I like the idea of cavers whirring merrily as they descend,
> blowing all the dust around when they touch down - like a helicopter
> landing....
And you're buggered if you're trying to find where the draught disappears to
halfway down a pitch.
Nigel
Lev Bishop
> I can't see that there is that much to go wrong here that would result in
> plumetting death. Gearboxes are very reliable - when did your car or bike
> gearbox stop working?
Erm... My bike gearbox (Sturmey-Archer (sp?)) has given me some problems. If
I pedal too hard then it goes into neutral and I have to pick myself off the
crossbar and put it back into gear. If that happened to the descender then
you'd plummet. I'm pretty certain we've managed to knacker the gearbox on
the club van at least once, too. I'll admit that these are perhaps not very
fair examples - my bike is not the finest piece of engineering (the gears
are the least of the troubles: you can't steer and pedal at once, and the
seat has a fun habit of tipping you off when you're least expecting it) and
you may have heard about OUCC transport
(http://vidos.zoo.ox.ac.uk/oucc/procs/proc11/transit.htm).
I am sure there's a lot less to go wrong with a simple, fixed-ratio gearbox
like we need here, compared to a bike or car gearbox, though.
> If you
> cover up/bung up the vent does that make it go faster or slower?
Faster. It relies on turning your potential energy into air movement. Think
how the note of your hoover changes if you block it.
Lev
Carol Haynes
Being a mathematician I know all about infinity (well I did 20 years ago -
but I'm sure it'll come back with prompting)
Just some thoughts for you to mull over:
0.5 x infinity = infinity
but how big is your infinity is more important??
Did you know for example that there are the same number of whole numbers
(1,2,3,4,5...etc) as there are rational numbers (i.e. fractions)? But not
the same number as irrational numbers (such as pi, e, etc.)
Before I go into further detail, can someone recommend a good formula
editor, and how the hell do you get the Hebrew alphabet on this thing???
Carol
David G Cooke
<Pa...@tcia.net> writes
>
>Tony Brocklebank <tonybro...@tesco.net> wrote in message
>news:8fv602$ej9$1...@epos.tesco.net...
>> Infinity - as is twice infinity or infinity times ten.
>>
>> >
>> > I still ask the question, "What is the value of half of infinity"
>> >
>>
>> Other acceptable answers include "Who gives a s****"
>>
>> :-)
>>
>
>Try thinking deep thoughts Tony. It will be difficult and painful, but
>try.:-}
>
>You have never been puzzled at why you can multiply or divide infinity and
>still have the same value. I doesn't make sense. It just DOESN'T!!! I can't
>stand it. It drives me crazy!!!!
><Inside joke folks)
>
>
OK. Here's a Deep Thought (wasn't he in Hitch Hikers?).
What is infinity divided by infinity?
--
Cookie
Paul Montgomery
David G Cooke <da...@alchemy.co.uk> wrote in message
news:Ax4WMBAz...@alchemy.co.uk...
That would be one (1).
Paul Montgomery
As In Hitch Hikers Guide to the Galaxy? I don't think so. Do you mean "he"
as in Deep Thought? Am I confused?
If I have to ask that question, I guess I am.
This is starting to get way off topic.
Carol Haynes
See my other entry below.
Carol
"Paul Montgomery" <Pa...@tcia.net> wrote in message
news:EzgV4.56560$701.7...@news4.giganews.com...
|
| David G Cooke <da...@alchemy.co.uk> wrote in message
| news:Ax4WMBAz...@alchemy.co.uk...
| > In article <SF0V4.52160$701.7...@news4.giganews.com>, Paul Montgomery
| > <Pa...@tcia.net> writes
| > >
| > >Tony Brocklebank <tonybro...@tesco.net> wrote in message
| > >news:8fv602$ej9$1...@epos.tesco.net...
| > >> Infinity - as is twice infinity or infinity times ten.
| > >>
| > >> >
| > >> > I still ask the question, "What is the value of half of infinity"
| > >> >
| > >>
| > >> Other acceptable answers include "Who gives a s****"
| > >>
| > >> :-)
| > >>
| > >Well it's obvious that I do.
| > >
| > >Try thinking deep thoughts Tony. It will be difficult and painful, but
| > >try.:-}
| > >
| > >You have never been puzzled at why you can multiply or divide infinity
| and
| > >still have the same value. I doesn't make sense. It just DOESN'T!!! I
| can't
| > >stand it. It drives me crazy!!!!
| > ><Inside joke folks)
| > >
| > >
| >
| > OK. Here's a Deep Thought (wasn't he in Hitch Hikers?).
| >
Paul Montgomery
> Depends on which size infinity you are talking about!
>
> See my other entry below.
>
> Carol
>
Paul
Nigel Robertson
"Paul Montgomery" <Pa...@tcia.net> wrote in message
news:EzgV4.56560$701.7...@news4.giganews.com...
>
> David G Cooke <da...@alchemy.co.uk> wrote in message
> news:Ax4WMBAz...@alchemy.co.uk...
> >
> > OK. Here's a Deep Thought (wasn't he in Hitch Hikers?).
IT (not he) was in hhgttg
> >
> > What is infinity divided by infinity?
>
> That would be one (1).
>
And what if it was infinity minus one divided by infinity? Or infinity
squared divided by infinity?
Mark McVey
whiskey, fine wine.... Or did you have some other form of payment in
mind?!!! :<)
Anyway, It looks like I am going to have to dig around and find my copy
of Auto CAd, so I can look at the pictures! (i'm a visual sort of
person!)
Thanks,
Mark McVey
On Thu, 18 May 2000 15:38:40 -0400, "Paul Montgomery" <Pa...@tcia.net>
wrote: