Bulk modulus of rubber/Viton ?
Some guy
What is the bulk modulus of rubber/Viton ?
I've heard people say that hard rubber is incompressible, but that rubber
with fillers is compressible. Where is Viton on this scale ?
Thanks.
Someguy.
dlzc@aol.com (formerly)
> What is the bulk modulus of rubber/Viton ?
>
> I've heard people say that hard rubber is incompressible, but that rubber
> with fillers is compressible. Where is Viton on this scale ?
The durometer of Viton is typically 90. Which makes it pretty hard, but
still "compressible". Try DuPont, this is their product.
Your compressibility will be a function of the durometer.
David A. Smith
Bob Jones
<fYBJ6.8747$Hk4.1...@news1.rdc1.ab.home.com>:
> What is the bulk modulus of rubber/Viton ?
>
> I've heard people say that hard rubber is incompressible, but that rubber
> with fillers is compressible. Where is Viton on this scale ?
Poisson's ratio is 1/2 for rubbers. This is not the same as having an
infinite bulk modulus.
brian whatcott
wrote:
Poisson's ratio for soft vulcanized rubber can be 0.46 to 0.49
Young's modulus E = 0.1 to 0.7 E10 N.m^-2
Bulk modulus k is E/(3*[1 - 2 X Poisson's ratio] )
k can be as high as E/0.06 = 0.1E12 N.m^-2
on the strength of these numbers....
Dunno about Viton.
Brian Whatcott Altus OK
<in...@intellisys.net>
Eureka!
Some guy
"> Bulk modulus k is E/(3*[1 - 2 X Poisson's ratio] )
> k can be as high as E/0.06 = 0.1E12 N.m^-2
> on the strength of these numbers...."
Are you sure your formula is correct ?
People have been telling me that although various rubbers will elongate and
compress in one (or two) dimensions, they are NOT compressible, ie their
bulk modulus is at or near water.
Someguy
"brian whatcott" <in...@intellisys.net> wrote in message
news:3afa0a7e....@snews.intellisys.net...
brian whatcott
becomes, is the Poisson ratio for rubber ever higher than 0.49?
If it gets too close to 0.5, you can see the bulk modulus soars way
high.
Brian
On Wed, 16 May 2001 22:27:09 GMT, "Some guy" <insu...@cadvision.com>
Bob Jones
occurs at Poisson's ratio of 0.5000000000000000000000000. Use the formula
and figure out the Bulk Modulus with the values given above. (I haven't
checked them myself though.) That really is your answer, IF you use linear
elasticity. A hint is to take the trace of the constitutive equation in
tensor form.
If you want to know more, go to an engineering library at a "good" school
and look at Treloar's "Rubber Elasticity" for as much as you probably want
to know about rubber elasticity (hence the title I suppose...). You can
also look at a Continuum Mechanics book. Malvern has a nice one. I also
like Lai, Rubin & Krempl. Gurtin is good too. The list goes on and on.
Now for the next question: Do you want to know the isentropic or isothermal
bulk modulus and does it matter?
brian whatcott
then Bob? :-)
Brian
On Fri, 18 May 2001 02:22:27 GMT, bob_j...@hotmail.com (Bob Jones)
wrote:
>If the material is incompressible, then the bulk modulus is infinite. This
>occurs at Poisson's ratio of 0.5000000000000000000000000. /snip/
On Wed, 09 May 2001 04:30:01 GMT, bob_j...@hotmail.com (Bob Jones)
wrote:
/snip/
>Poisson's ratio is 1/2 for rubbers. This is not the same as having an
>infinite bulk modulus.
----------------------------------------------------------------------
Bob Jones
right, then mu = 1/2 is the same as infinite bulk modulus. Bay, I hope
there is some loophole I can squirm out of this through...
in...@intellisys.net (brian whatcott) wrote in
<3b051abe...@snews.intellisys.net>:
Bob Jones
4.1666666666666694E09 to 1.1666666666666656E11 Pa.
brian whatcott
wrote:
>Nope, no loopholes. The formula for k is right too. This gives k from
>4.1666666666666694E09 to 1.1666666666666656E11 Pa.
>
Gee, you took it in good spirit, and you didn't get defensive.
What exactly is wrong with you, anyway? :-)
Brian
Bob Jones
>What exactly is wrong with you, anyway? :-)
Thanks, and how much time do you have? Thanks for providing the answer to
the original question also. Are you a continuum mechanician? I used to
be...
Some guy
If that is the case, can anyone tell me about a <solid> that is compressible
? Someone was saying that the fillers in certain rubbers make them
compressible.
By compressible, I am looking for a "solid" that has a bulk density of about
20,000 PSI
Someguy
"Bob Jones" <bob_j...@hotmail.com> wrote in message
news:Xns90A65FAA1DAC7bo...@24.2.68.78...
dlzc@aol.com (formerly)
> If that is the case, can anyone tell me about a <solid> that is
compressible
> ? Someone was saying that the fillers in certain rubbers make them
> compressible.
Under what conditions? Over what range of compression (percent wise)?
A metal sponge (or a spring) might be your best bet.
David A. Smith
Bob Jones
>So... do we all agree that Viton is incompressible ?????
>
>If that is the case, can anyone tell me about a <solid> that is
>compressible ? Someone was saying that the fillers in certain rubbers
>make them compressible.
>
>By compressible, I am looking for a "solid" that has a bulk density of
>about 20,000 PSI
>
>Someguy
>
I thought we had agreed that nu = 1/2 is incompressible, but that Viton is
not. If you convert my figures (calculated using his figures above) you
get 600,000 to 16,000,000 PSI for the bulk modulus. That is compressible.
Rubber elasticity is usually modeled using a Mooney-Rivlin material, which
is elastic -- just not linearly elastic. I don't know off hand what the
bulk modulus is for a M-R material. Even if I did, I still wouldn't know
the material constants associated with your material.
What are you trying to do with a rubber like material that you want to be
compressible at about 20,000 PSI? It is usually best to ask the real
question, if you want a good answer. There may be things that have been
overlooked or you (in the general sense) may be asking the "wrong"
question.
Some guy
> compressible at about 20,000 PSI? It is usually best to ask the real
> question, if you want a good answer. There may be things that have been
> overlooked or you (in the general sense) may be asking the "wrong"
> question.
Ok. Good point: we've got a hydraulic rail that supplies a bunch of fast
valves. We've got two problems with the setup:
a) turning the valves on and off quickly is producing pressure pulses from
water hammer effects. We would like to minimize the amplitude of the
spikes. One way to do that is to decrease the bulk modulus of the rail.
Don't ask me to change the dimensions of the rail or slow the valves down:
it can't be done for other reasons.
b) the valves "use" hydraulic oil faster (instantaneously) than the supply,
so there is a resulting pressure drop larger than what we want. If the
supply was more compressible, the pressure drop would be smaller.
BTW: the fluid is hydraulic oil, which has a bulk modulus of 200,000 PSI.
It is up to 240F in temperature. The valves use about 1cm^3 of oil in about
1.5ms. The rail holds about 600 cm^3.
dlzc@aol.com (formerly)
> Ok. Good point: we've got a hydraulic rail that supplies a bunch of fast
> valves. We've got two problems with the setup:
I assume you mean a linear manifold...
> b) the valves "use" hydraulic oil faster (instantaneously) than the
supply,
> so there is a resulting pressure drop larger than what we want. If the
> supply was more compressible, the pressure drop would be smaller.
>
> BTW: the fluid is hydraulic oil, which has a bulk modulus of 200,000 PSI.
> It is up to 240F in temperature. The valves use about 1cm^3 of oil in
about
> 1.5ms. The rail holds about 600 cm^3.
The manifold will have two pressure ports. One has your supply on it.
Connect the other to a bladder accumulator. The pre-fill pressure on the
accumulator should be 50% of the pressure you normally run the supply at (to
prevent scuffing the bladder). The size should be 1 quart (liter, whatever)
or so. Vickers used to supply such a beast. It will kill the spikes, and
allow you to ride through pressure fluctuations.
From the temperature you are quoting, you might want to install an oil
cooler. This high a temperature will shorten the life of the
valves/seals/solenoid coils, and will increase the blowby losses for the
various components. Got any room for such a device? The lost energy for
braking motion is sometimes dissipated into these manifolds. What is the
receiver tank temperature (near the pump inlet)?
If you don't have the money/space for an accumulator, add a vertical leg of
tubing that is capped at the top end, will sometimes serve. This tube will
start out (hopefully) full of air, and will also act as an accumulator.
Unfortunately, it can either lose its "charge" over time by entrainment, or
over fill if your pump is cavitating (or the reservoir is low).
David A. Smith
brian whatcott
familiar to electrical engineers, they think of it as reflections on a
transmission line.
They start with a ('characteristic') impedance match to terminate a
line, and they try to have connections matched to the lines impedance
also.
The comparison with the terminator and the hydraulic accumulator is
evident. It does need to consume some power, not just store it.
When the end reflection is beaten, it will be time to see if the
valves in the path are kicking up much of a pule reflection.
How to tell the characteristic impedance? electrically, its the
source impedance that momentarily causes the line voltage (pressure?)
to drop to half its unloaded value when connected across the line.
Brian
Brian Whatcott Altus OK
<in...@intellisys.net>
Eureka!
Some guy
"dl...@aol.com (formerly)" <dl...@home.com> wrote in message
news:HzhP6.46136$S5.98...@news1.rdc1.az.home.com...
> Dear Some guy:
>
> > Ok. Good point: we've got a hydraulic rail that supplies a bunch of fast
> > valves. We've got two problems with the setup:
>
> I assume you mean a linear manifold...
Yes.
>
> > b) the valves "use" hydraulic oil faster (instantaneously) than the
> supply,
> > so there is a resulting pressure drop larger than what we want. If the
> > supply was more compressible, the pressure drop would be smaller.
> >
> > BTW: the fluid is hydraulic oil, which has a bulk modulus of 200,000
PSI.
> > It is up to 240F in temperature. The valves use about 1cm^3 of oil in
> about
> > 1.5ms. The rail holds about 600 cm^3.
>
> The manifold will have two pressure ports. One has your supply on it.
> Connect the other to a bladder accumulator. The pre-fill pressure on the
> accumulator should be 50% of the pressure you normally run the supply at
(to
> prevent scuffing the bladder). The size should be 1 quart (liter,
whatever)
> or so. Vickers used to supply such a beast. It will kill the spikes, and
> allow you to ride through pressure fluctuations.
Been there, done that: it makes hardly any difference in the operation. (It
does make some.)
Another post compared the problem to that of a transmission line with a
specific characteristic impedence and he is right: we can cut down on the
reflections by tying accumulators into the end. However, what we really
want to do is the magnitude of oscillations in the first place and that
requires changing the impedence of the line itself, not just the
terminations.
The best way I could think to do that was to distribute some cushion along
the length of the accumulator.
> From the temperature you are quoting, you might want to install an oil
> cooler. This high a temperature will shorten the life of the
> valves/seals/solenoid coils, and will increase the blowby losses for the
> various components.
Agreed.
Got any room for such a device?
Nope: don't worry about it.
The lost energy for
> braking motion is sometimes dissipated into these manifolds. What is the
> receiver tank temperature (near the pump inlet)?
Same.
Tom Mills
manifold, as opposed to putting it at the end of the manifold? What I'm
visualizing from your description sounds like a "noisy" dc power supply,
which should be quieted by capacitance ahead of the load.
As a point of curiosity, are the oscillations problematical for the valves,
the pump, the lines, or all of the above?
"Some guy" <insu...@cadvision.com> wrote in message
news:1tzP6.40974$Ub.4...@news1.rdc1.ab.home.com...
dlzc@aol.com (formerly)
> > The manifold will have two pressure ports. One has your supply on it.
> > Connect the other to a bladder accumulator. The pre-fill pressure on
the
> > accumulator should be 50% of the pressure you normally run the supply at
> (to
> > prevent scuffing the bladder). The size should be 1 quart (liter,
> whatever)
> > or so. Vickers used to supply such a beast. It will kill the spikes,
and
> > allow you to ride through pressure fluctuations.
>
> Been there, done that: it makes hardly any difference in the operation.
(It
> does make some.)
If you are experiencing pressure spikes even with a bladder accumulator,
then:
1) the line size between the accumulator and the manifold needs to be
decreased in length and/or increased in diameter.
2) create the "vertical leg accumulator" as many places as possible on the
pressure line near the manifold (one on inlet *and* one on outlet).
> Another post compared the problem to that of a transmission line with a
> specific characteristic impedence and he is right: we can cut down on the
> reflections by tying accumulators into the end. However, what we really
> want to do is the magnitude of oscillations in the first place and that
> requires changing the impedence of the line itself, not just the
> terminations.
The oscillations in this case are caused because there is:
1) too much impedance (small line diameter, long line length) between the
source and the manifold,
2) inadequate supply for the instantaneous demand (similar to high
resistance in a battery).
The pressure fluctuations in a hydraulic system are very similar to
electrical noise. This is typically handled by lots of small mylar/ceramic
capacitors. This provides a "low inertia" fast response coupling of the
electrical noise to the ground plane. The accumulator(s) will provide this
dampening, if the impedance between them and the manifold can be reduced.
The bladder may present too high an "inductance", which is why just a
vertical leg might perform better.
> The best way I could think to do that was to distribute some cushion along
> the length of the accumulator.
I think you meant manifold? Good idea... you may be able to tap off
additional accumulators, or buy a longer manifold and use every other valve
position for a little mini accumulator. (custom of course).
> > The lost energy for
> > braking motion is sometimes dissipated into these manifolds. What is
the
> > receiver tank temperature (near the pump inlet)?
>
> Same.
I'm afraid the bladder accumulator will not last too long at this
temperature (250F). They may have special bladders available. Viton will
be fine, they supply these for exotic fluids, usually.
David A. Smith
dlzc@aol.com (formerly)
> As a point of curiosity, are the oscillations problematical for the
valves,
> the pump, the lines, or all of the above?
From his description, the valves are very fast acting with relatively low
impedance (even a compressible) load. The pump will be fixed displacement
(like a current source) when short duration pulses are involved. The lines
will be both resistive and inductive (capacitive too, if they are hoses).
If it weren't that you could deliver 25hp with something smaller than your
head, people probably wouldn't put up with hydraulics.
David A. Smith
Some guy
"Tom Mills" <t_k_...@compuserve.com> wrote in message
news:KBBP6.233$kh4....@bgtnsc04-news.ops.worldnet.att.net...
> Would there be any value in adding an accumulator between the pump and the
> manifold, as opposed to putting it at the end of the manifold? What I'm
> visualizing from your description sounds like a "noisy" dc power supply,
> which should be quieted by capacitance ahead of the load.
>
> As a point of curiosity, are the oscillations problematical for the
valves,
> the pump, the lines, or all of the above?
>
The actual problem is this: when a valve closes, it creates a pressure wave
due to the water hammer effect. IF that pressure wave happens to be at the
mouth of a valve when it opens, it will deliver oil at a pressure higher
than what it would otherwise be. Thus, we get uneven flow from the valves,
depending upon when we open them and in what sequence.
Some guy
Comments below.
>
> If you are experiencing pressure spikes even with a bladder accumulator,
> then:
> 1) the line size between the accumulator and the manifold needs to be
> decreased in length and/or increased in diameter.
An accumulator at the END of the rail will NOT stop the formation of
pressure waves right at the valves. It will decrease their reflection, but
not their formation. The only way to decrease their formation is to
decrease the modulus of the fluid or the rail.
> 2) create the "vertical leg accumulator" as many places as possible on the
> pressure line near the manifold (one on inlet *and* one on outlet).
>
> > Another post compared the problem to that of a transmission line with a
> > specific characteristic impedence and he is right: we can cut down on
the
> > reflections by tying accumulators into the end. However, what we really
> > want to do is the magnitude of oscillations in the first place and that
> > requires changing the impedence of the line itself, not just the
> > terminations.
>
> The oscillations in this case are caused because there is:
> 1) too much impedance (small line diameter, long line length) between the
> source and the manifold,
yes, partially
> 2) inadequate supply for the instantaneous demand (similar to high
> resistance in a battery).
yes.
dlzc@aol.com (formerly)
> > If you are experiencing pressure spikes even with a bladder accumulator,
> > then:
> > 1) the line size between the accumulator and the manifold needs to be
> > decreased in length and/or increased in diameter.
>
> An accumulator at the END of the rail will NOT stop the formation of
> pressure waves right at the valves. It will decrease their reflection,
but
> not their formation. The only way to decrease their formation is to
> decrease the modulus of the fluid or the rail.
If it was "water hammer effects" in the manifold, it would.
You had indicated you were using a valve that switched on for 1.5mS. What
is the off time? What type of valve is it poppet or spool?
I am suspecting that the hammer effects you are seeing is the poppet
seating, perhaps on a nearby valve. Could you stagger the valve timing, or
have valves that close be far from those valves that are opening? Poppet
valves are very fast operating, but they tend to displace fluid in both
directions.
I await your reply, before I guess myself into embarrasment.
David A. Smith
Tom Mills
compressible inside the manifold as you described sounds like a reasonable
approach. To switch away from the electrical metaphor, this would be like
the anechoic skin on a submarine (something "rubber-like" - anybody know for
sure what the Navy uses?). If a solid material isn't sufficiently spongy,
then maybe something hollow would help (a closed-cell foam, or maybe some
kind of "pillow"). Some kind of baffling might also help, to break up the
pressure waves or direct each valve's pulses away from its neighbors (can't
quite convince myself that this would work in an incompressible environment,
but it's a technique that's used in acoustic applications).
Oh, for the back of a virtual envelope to scribble on!!
"Some guy" <insu...@cadvision.com> wrote in message
news:LPDP6.41600$Ub.5...@news1.rdc1.ab.home.com...
dlzc@aol.com (formerly)
> If the meat of the problem is within the manifold, then putting something
> compressible inside the manifold as you described sounds like a reasonable
> approach. To switch away from the electrical metaphor, this would be like
> the anechoic skin on a submarine (something "rubber-like" - anybody know
for
> sure what the Navy uses?). If a solid material isn't sufficiently spongy,
> then maybe something hollow would help (a closed-cell foam, or maybe some
> kind of "pillow"). Some kind of baffling might also help, to break up the
> pressure waves or direct each valve's pulses away from its neighbors
(can't
> quite convince myself that this would work in an incompressible
environment,
> but it's a technique that's used in acoustic applications).
If it were "water hammer" due to the valve closing and momentum effects, he
could port the manifold away from where the port touched the valve. This
would reduce the momentum, since the velocity would be lower.
I think he needs to use a rotary spool valve, which will not displace fluid
back into the manifold upon closing.
An internal damper would be nice, the fuild properties could be changed by
allowing the fluid to heat more, the fluid could have intrained air added
(both sources of catastrophic failure in other ways). I just don't know of
anything compressible enough and faster than gas. Getting rid of the
bladder might help, but without knowledge of the duration of the operation,
I don't know how long a gas pocket alone would retain its properties.
David A. Smith
David A. Smith
Bob Jones
>wave due to the water hammer effect. IF that pressure wave happens to
>be at the mouth of a valve when it opens, it will deliver oil at a
>pressure higher than what it would otherwise be. Thus, we get uneven
>flow from the valves, depending upon when we open them and in what
>sequence.
You also get a reduction in metered quantity from the variation itself. If
you take a sine wave pressure and integrate it inside the square root
(orifice equation) and even for the same mean pressure value the quantity
is reduced.
I believe that there is some patent coverage on internal line dampers in
gasoline fuel systems. I haven't checked it myself, but Delphion is still
free for a few more days (until June 1). This idea is sort of diamond
shaped internal piece filled with air inside the rail (or linear manifold).
You also may want to check out AMESim to model this system to optimize it.
There website is www.amesim.com. If you don't want to buy a license call
them and ask them who they recommend for consulting work or if they will do
it for you themselves. The have an office in the Detroit, MI area. They
are a French company.
I have to read all these suggestions more cafefully. Lots of nice advice
IMHO.