Cobra top hinge plate bolt failure
Andy
both bolts securing one trailer top hinge plate failed. Of course I
checked my bolts and all four were secure at that time.
Yesterday I arrived at the club field and had a problem releasing the
left rear top latch. Sure enough the outer bolt on the left hinge
plate had failed and this allowed the top to move back slightly and
bind up the rear latch. I was able to find replacement bolts at a
local hardware store. 5/16 x 3.5 inch are very close to the
diameter the original metric bolt and provide more load bearing
unthreaded shank.
The bolts in question go through the front handles, the trailer glass
top, the trailer top frame (with an internal spacer block), a spacer
plate, and then the main hinge plate. All the parts try to misalign
under load. With the help of 5 people I was able to align all the
pieces well enough to get the new bolt in and was then able to rig and
fly the task.
Another isolated failure? No. While I was working on it GW walks up
and says he found the same bolt failed on his trailer the previous
evening. My trailer is 9 years old and GW's I think 10 years old.
Both are glass top but I don't know if that is significant. So 3
known failures of these bolts.
Do you feel lucky?
Andy (GY)
Dave Nadler
I was just about to post something on this very topic,
as I'm about to do some work related to this...
1) Spindelberger has had only one other failure of
this bolt reported to them; actually while I was with
them a few weeks ago and they were very surprised and
thinking maybe a bad batch of bolts.
2) The gas struts that open the trailer top push
the top FORWARD when closed. This puts a load on
these bolts and over time the top actually moves
forward. When this happens you will notice the
rear latches don't line up with the top and are
hard to close.
3) Do NOT remove these bolts without clamping the
trailer top down and restrained from moving
forward. If you undo the bolts without restraining
the top, even on just one side, even just one bolt,
the gas strut will deliver you a nasty surprise
(as happened to me on the highway to Hobbs).
CAREFUL !
4) Spindelberger has replacement hinge parts
with a bend to move the top aft and gave me a
set.
5) I'm designing a clamp to safely hold the top
during replacement of the hinge and bolts...
When I get this sorted I'll post something with
pictures. Not this week as we're having contest
weather.
Hope this helps,
Best Regards, Dave "YO electric"
joesimmers
Did the bolt break or shear at the location where the thread ends and
solid shank starts?
Where the new bolts you used "grade 8"?
I would not use stainless bolts as they are softer and will shear much
easier than
a "grade 8" heat treated bolt.
By the way 5/16" is exactly the same size as 8mm, difference is only .
002, less
than a hair thickness. And I would trust the heat treatment of a grade
8 5/16" bolt over a class 12.9 metric bolt.
I think I would get bolts long enough to have a solid shank all the
way thru the structure causing the misaligned stress
you decribe and only enough thread on the back side to hold the washer
and nut.
I'll be checking mine if it ever quits raining.....................
Joe Simmers
Andy
My bolt failed at the thread to plain shank intersection. I have not
yet found the nut and threaded end. The plain shank length of the
original bolt is far too short and any shear load it taken at the
intersection rather than on the full shank diameter.
I don't know what grade the replacement bolt is. At the time I was
very pleased to find there was a hardware store within 10 miles of the
glider port. I have been flying there for over 20 years and had no
idea the store was there. To then find they had socket cap bolts of
the required size and length was beyond my expectations.
For the emergency repair at the glider port I was able to insert a #2
Phillips screw driver to pin the parts and then open the trailer. I
had 4 people at the aft end holding the top up and one person pulling
down on the front of the top. That allowed the bolt to slip in. Had
the other bolt failed it could have been ugly.
GW inserted a new bolt in his solo with the help of a clamp to pull
the parts into alignment. I had people available but no clamp.
Andy (GY)
db_sonic
This happened to a friend of mine also exactly as you describe. So
make that 4 cases.
Andy
Error in the write up. The top moved forward opening a gap between
the hinge plate, spacer, and top frame. As Dave points out the gas
struts are forcing the top forward when it is closed. Of course the
failure of one bolt greatly increases the load on the other.
Andy
Dave Nadler
> Did the bolt break or shear at the location where the thread ends and
> solid shank starts?
>
>
> I think I would get bolts long enough to have a solid shank all the
> way thru the structure causing the misaligned stress
> you decribe and only enough thread on the back side to hold the washer
> and nut.
But, note that the load is not just in shear.
These 4 bolts are taking the total force of the
compressed gas springs in tension...
Steve Koerner
grade 8 or equiv. so I don't think you have to worry about that.
My bolt popped in the middle of the threaded length which apparently
is not where Andy's bolt broke. That supports my impression that it
is primarily a tension failure rather than a shear failure.
The type of clamp that I used successfully by myself is called a "Kant
Twist" 4 1/2" machinist clamp. These are sold at www.use-enco.com and
lots of other suppliers. So I'm not so sure that designing a special
clamp is needed. What does need designing is a scheme to get a
couple more bolts into that plate. The design is flawed and
dangerous.
Steve Koerner (GW)
www.wingrigger.com
Dave Nadler
> The type of clamp that I used successfully by myself is called a "Kant
> Twist" 4 1/2" machinist clamp. These are sold atwww.use-enco.comand
> lots of other suppliers. So I'm not so sure that designing a special
> clamp is needed. What does need designing is a scheme to get a
> couple more bolts into that plate. The design is flawed and
> dangerous.
>
> Steve Koerner (GW)www.wingrigger.com
Hmm - I planned to clamp with surfaces:
- bottom: grab the front inside the extrusion slot
- top: fix the angle on top of the alum frame
Nothing fancy, just a couple parts with holes
preventing clamping surfaces rotating and a bolt...
Did the kant-twist end fit inside the front's slot ?
Thanks,
Best Regards, Dave
Steve Koerner
effect that it is more likely a tension failure. However I don't
agree that the gas spring force has much to do with it. The gas
spring force is small compared to the strength of two 5/16 cap
screws. I suspect it has more to do with the inertia of the lid in
longitude with some combination of vibration and road bumping creating
the load. Heaven knows my trailer has seen some bumpy roads.
GW
Steve Koerner
Dave -- I am not really visulizing all this. Probably because I dealt
with it in the dark holding a flashlight. If you've worked through a
plan, it's probably a good one. It does seem like one of the jaws on
the clamp was fatter than I wished it were. So that is probably what
you are referring to. On my hinge plate, I was able to get the holes
to allign though even without the jaw stationed exactly where I wished
it would go. That might not be the case if the plate is positioned
just a little different on your trailer.
GW
db_sonic
Examination of the failed bolt should tell you whether it was tensile
with necking or shear with a fairly clean break.
But, main concern I would have is having a failure while rolling down
the road and doesnt sound like something one could fix by themselves
even with a spare bolt on hand. Maybe the solution is to pre-
emptively replace these bolts anyways at say 10 years?
Andy
Maybe the right solution would be for Cobra to redesign the hinge and
make a kit available to owners. The worst case failure is a nightmare
and YO was halfway there. Image what happens if all 4 bolts fail.
The front of the top could be lifted by the gas struts and at highway
speeds could tear off the trailer into the path of any vehicle
following.
There is no reason the handles have to share the same bolts as the
hinge plates. The hinge plate bolts could be made larger diameter and
the handles relocated.
Andy
Andy
Well I was up bright an early this morning intending to refit the
replacement bolt so it went through the handle. Surprise! The right
hinge outer bolt had failed on the way home.
This bolt had failed one tread pitch from the thread/shank
transition. The fracture face shows 4 distinct, almost linear, bands
running parallel to the diameter and has slight corrosion over the
area of the third band. The fourth band is clean but dull and is
about 1.25 mm wide. The only bright part is about a 1/4 turn of the
thread area. This bright area is at the end of the fourth band. The
end of that thread has been slightly curled up along the longitudinal
axis of the bolt which seem to indicate that was the last part to let
go. It appears that this bolt had been fractured about 2/3 of the
diameter for some time.
The left bolt shows also shows banding in the fracture surface but it
not as distinct as the other bolt. about one third of the fracture
surface is dull black and the clean band of the final fracture line is
about 1mm in width. On this bolt there is no obvious tearing of the
thread land. I estimate that this bolt was fractured more than 3/4
for some time before it failed.
Plans to replace all the bolts are on hold until I get a ratchet strap
to secure the top and make a clamp to pull the parts into alignment.
I have to assume the other bolts are fractured through most of their
diameter and could yield at any time so I can't risk opening the
trailer.
If you own a 10 year old Cobra trailer you probably should be
concerned about this.
Andy (GY)
Steve Koerner
Howard Banks is an expert in the field. He sent me some questions
about the appearance of the failed bolt which I couldn't answer well
for having tossed the broken bolt. Hopefully Howard will chime in
based on Andy's detailed description here. I think his suspicion was
that fatigue was a major factor which is probably supported by Andy's
description of progressive failure.
I will, at the least, change all 4 of my bolts before using my trailer
again.
GW
Howard Banks
There is a clear requirement to replace all these bolts in trailers of
some age -- or maybe trailers that have been towed over longer distances.
How old was D Nadler's trailer, how far had it been towed (lots at a
guess).
Spindelberger needs to know about this ... hope someone is forwarding
these emails.
howard banks
At 15:50 17 May 2011, Andy wrote:
>On May 17, 6:17=A0am, Andy wrote:
>> On May 16, 10:44=A0pm, db_sonic wrote:
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> > On May 16, 5:23=A0pm, Steve Koerner wrote:
>>
>> > > It looks like YO and I were both commenting at the same time to
the
>> > > effect that it is more likely a tension failure. =A0However I
don't
>> > > agree that the gas spring force has much to do with it. =A0The gas
>> > > spring force is small compared to the strength of two 5/16 cap
>> > > screws. =A0I suspect it has more to do with the inertia of the lid
>in
>> > > longitude with some combination of vibration and road bumping
>creatin=
>g
>> > > the load. =A0Heaven knows my trailer has seen some bumpy roads.
>>
>> > > GW
>>
>> > Examination of the failed bolt should tell you whether it was
tensile
>> > with necking or shear with a fairly clean break.
>> > But, main concern I would have is having a failure while rolling
down
>> > the road and doesnt sound like something one could fix by themselves
>> > even with a spare bolt on hand. =A0Maybe the solution is to pre-
>> > emptively replace these bolts anyways at say 10 years?
>>
>> Maybe the right solution would be for Cobra to redesign the hinge and
>> make a kit available to owners. =A0The worst case failure is a
nightmare
>> and YO was halfway there. =A0Image what happens if all 4 bolts fail.
>> The front of the top could be lifted by the gas struts and at highway
>> speeds could tear off the trailer into the path of any vehicle
>> following.
>>
>> There is no reason the handles have to share the same bolts as the
>> hinge plates. =A0The hinge plate bolts could be made larger diameter
JJ Sinclair
Just checked my hinge plate bolts and they appear to be OK, but one
nut was just finger tight. A loose nut could/would lead to bolt
failure. Recommend we all check to make sure our hinge plate nuts are
tight.
JJ
Andy
>
> - Show quoted text -
I checked my bolts for security last year after YO's report. Don't
remember how much I tightened them, it at all, but the damage was
probably already done. I think the only safe action is to replace the
bolts.
Given Howard's confirmation of fatigue failure I wonder how much wash
boarded dirt roads are a factor. Is this just a Western US hazard? I
didn't even know what a wash board road was until I moved here.
Andy (GY)
Dave Nadler
Pictures of the failed bolt please !
Mine had corrosion through part of the fracture,
indicating it had started to fail a year or two earlier.
Uneducated hypothesis - could these bolts have been
overstressed during trailer assembly, over-tightened
whilst trying to draw the parts into alignment ??
Meanwhile, our Duo trailer has had two of these bolts
fail in the last week; one of my partners just showed
up and left a broken bolt on my desk. Because I need
more projects...
Aaaarrrggggg....
Juanman
As Dave mentioned, I drove from Boston to Mifflin for the (cancelled)
Region 2 contest. Sunday I noticed the right handle loose and found
the outside bolt sheared at the nut. An amazing local who was hanging
out at the abandoned field ("Bob" is all I know, works at a machine
shop) took us 10 miles to Lowe's and the Tractor repairs store next
door, where we found 3/16" Grade 8 bolts (on a Sunday afternoon,
closes at 6pm). Bob went home to pick up some tools and met us at the
field. Using some large welding clamps we managed to line up the hole
and place the bolt seconds before a storm descended on us. Thanks
Bob!
Drove back to Boston last night, parking the trailer noticed that the
left handle is loose! Left outside bolt sheared inside. Went to
lunch with Dave and ordered the bolts. Went back to the trailer and
used a C-clamp to line up the hole.
Suggest replacing all bolts with grade 8 AND perhaps drilling a third
hole between the two and putting one more bolt there.
Aaaaarrrrggggg is right....
Juan (LI)
sisu1a
> fail in the last week; one of my partners just showed
> up and left a broken bolt on my desk.
Has anyone used NAS shear bolts as replacements? They're designed for
close fit, and have ~160-180,000psi in tensile and ~>95,000psi in
shear (+CAD II plating...) 'normal' bolts are as low as around
50,000psi in tensile and not even rated in shear.
http://www.aircraftspruce.com/pdf/2011Individual/Cat11065.pdf
-paul
Whiskey Delta
Interesting.
I have a 2002 Cobra trailer. Will have to check the bolts. Probably
should replace them. Not having the trailer nearby to go and look at
the relevant areas discussed above and not being that familiar with
it, it is somewhat hard to visualize and follow the discussion.
Anyone care to post a step by step "Replacing Cobra Hinge Plate Bolts
for Dummies" with specifics as to what type of clamp(s) and where to
put it(them), bolt specs, etc? Pictures would be a great help. It
would be greatly appreciated.
Thanks for the information posted thus far!
WD
Eric Greenwell
> I checked my bolts for security last year after YO's report. Don't
> remember how much I tightened them, it at all, but the damage was
> probably already done. I think the only safe action is to replace the
> bolts.
>
> Given Howard's confirmation of fatigue failure I wonder how much wash
> boarded dirt roads are a factor. Is this just a Western US hazard? I
> didn't even know what a wash board road was until I moved here.
Washboarded roads - maybe. Here's another data point: 1995 Cobra trailer
for ASH 26 E, 160,000 miles of highway driving, and the bolts are not
broken.
Some details:
*note that the ASH 26 E trailer body is 30 feet long, as the 18 meter
wing is only two pieces (tongue is another 4 feet)
*towed almost entirely by motorhomes
*I haven't actually pulled out the bolts to see if they are cracked
*Probably less than 1000 miles of dirt roads, including mostly roads
under repair or construction, and hardly any washboarded roads.
I will replace the bolts as soon as people settle on the best kind of bolt.
--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
Steve Koerner
soon as I get a chance I intend to investigate changing the bolts to
the next larger size, 3/8" grade 8. Of course that will mean drilling
the holes larger and it will also mean that the head will not be
neatly counterbored into the handle. If that plan works out, I will
post back here.
GW
Andy
I'd be interested to hear how that works out. There is a least one
spacer block inside the frame extrusion and it looks like it would
have to be extracted, drilled, and replaced. There may be two each
side. Given the limit access to the extrusion opening it could be a
bit awkward to work the inner blocks.
If I was going to larger bolts I think I'd relocate the handles and
keep them on 5/16 socket cap bolts. They wouldn't need the spacer
blocks as long as a moderate torque was used.
Andy (GY)
vontresc
Why not use the proper metric size?
Peter
JJ Sinclair
>
> > I have 3 Wing Riggers that I need to get shipped out today, but as
> > soon as I get a chance I intend to investigate changing the bolts to
> > the next larger size, 3/8" grade 8. Of course that will mean drilling
> > the holes larger and it will also mean that the head will not be
> > neatly counterbored into the handle. If that plan works out, I will
> > post back here.
>
> > GW
>
> spacer block inside the frame extrusion and it looks like it would
> have to be extracted, drilled, and replaced. There may be two each
> side. Given the limit access to the extrusion opening it could be a
> bit awkward to work the inner blocks.
>
> If I was going to larger bolts I think I'd relocate the handles and
> keep them on 5/16 socket cap bolts. They wouldn't need the spacer
> blocks as long as a moderate torque was used.
>
> Andy (GY)
The plot thickens, I moved both wings aft in order to get a good look
at the forward corner of the fiberglass top. The top has an aluminum
rail where it meets the lowar trailer and this "L" member overlaps the
cross member (where the hinge plate is bolted). There are 4 large
aluminum rivets on each side that bind these members together. Well,
all 4 rivets on the right side were sheared off and 2 on the left side
were sheared off! The forward 'shove' from the struts would try and
shear off these rather weak fastners as they are loaded in "shear".
The thing I don't understand is why the large steel bolts are shearing
off, because they are not loaded in shear?
I replaced the corner rivets with 1/8" steel rivets.
JJ
Steve Koerner
Because they break.
GW
Steve Koerner
>
> - Show quoted text -
JJ: I don't think that the big bolts are shearing off. They are
breaking due to a gradual fatigue process in tension. What I'm
thinking is that it is the fore and aft forces due to road bumps and/
or washboard acting against the inertia of the top that is putting the
cycled load on the bolts that eventually is their unduing. The
fatigue thing was definitely confirmed by Howard Banks based on the
observations on Andy's broken bolt. When you said yesterday that you
checked your bolts, did you really check them for incipient fatigue
damage?
Very interesting that there are sheared rivets going on as well. I'll
have to look for that too.
GW
JJ Sinclair
> checked your bolts, did you really check them for incipient fatigue
> damage?
I didn't remove and inspect the bolts because I was afraid the struts
would move the top, but thinking about it now, I don't think the top
would move if it was fully open and struts extended all the way. I'll
give it a try................If you hear a loud scream from no cal, it
means the top moved on me! Dave wants pictures, but some of us don't
know how to post pictures along with a whole bunch of other things
having to do with computers!
JJ
Andy
The top will move with the struts fully extended and the bolts
removed. Even with one hinge outboard bolt removed that corner of the
top rises and it look 5 of us to align the parts to get the
replacement bolt in. (people were available and clamps were not)
Please clamp the parts together so they can't move and then only
remove one bolt at a time.
Andy
JJ Sinclair
> The top will move with the struts fully extended and the bolts
> removed. Even with one hinge outboard bolt removed that corner of the
> top rises and it look 5 of us to align the parts to get the
> replacement bolt in. (people were available and clamps were not)
>
> Please clamp the parts together so they can't move and then only
> remove one bolt at a time.
>
> Andy
Yep. the inboard bolts came right out and looked fine, but the
outboard bolts didn't want to come and I wasn't inclined to force the
issue. A gap between the hinge plate and cross-member did appear as I
loosened the nut. I believe the outboard bolts fail after the bolt is
loose and repeatedly banged around (up & down + for & aft). Once the
outboard bolt gives way, the inboard isn't long for this world. I
think we're OK if the bolts are tight and kept tight.
Cheers,
JJ
Andy
>
> - Show quoted text -
I think there is significant shear loads on the bolts. The top of the
trailer mates to the lower half with a rubber seal. The top has
considerable mass and inertia. Any time the trailer floor moves up
and down, as on a washboard road, the vertical acceleration is
transferred to the top by the gap seals which will yield and then by
the forward hinges. There will therefore be a cycling shear load on
the hinge plate bolts any time the trailer moves over a rough
surface. Given that the plain shank of the bolt is not long enough to
engage in the rear wall of the front frame extrusion, or the spacer
plate, or the hinge plate, it's really not surprising the bolts fail.
Another factor is the the bolts compress the thickness of the 2 layer
grass top. Over time the top yields reducing the bolt clamping force
and allowing relative vertical motion between the parts that were
intended to be clamped together, but now are not.
I found deep compression of the glass outer shell under the handle
ends where the bolts failed. This could have be avoided by using a
solid spacer inserted into the shell.
Andy (GY)
Steve Koerner
I certainly agree that once the bolts become loose they will be seeing
shear load. I also agree that compressing the fiberglass shell over
time is a certain way for them to get loose. That sounds like a
pretty good theory. Of course, once they get loose, they'll be under
increased tension cycling too. As JJ has said, the key thing may be
to not let them get loose.
I can recall that my left rear latch had become a bit troublesome
prior to the failure. I now realize that meant that the bolt was
probably loosening in the left front.
My latest observation is that the hinge plate bolts on my trailer are
marked 8 8 which is not a high strength bolt. So I'm now thinking
that my corrective action will be to upgrade to high strength and
periodically monitor those bolts to keep them tight.
GW
Dave Nadler
It would greatly assist if you could:
1) Take close-up photographs of broken bolt ends
and email them to me.
2) Send me the broken bolts (which I will forward
on to Spindelberger):
Dave Nadler
97 Central Street
Acton, MA 01720
3) Somebody go help JJ get a picture of the rivets
he replaced and email them to me...
Thanks !
Best Regards, Dave
PS: I'm on the road so a bit hard to reach.
Going to PA for a few days, where no doubt
it will rain as they've gone and called
another contest...
JJ Sinclair
> I can recall that my left rear latch had become a bit troublesome
> prior to the failure. I now realize that meant that the bolt was
> probably loosening in the left front.
Yep, nothing new here, my ash-25 trailer was doing the same thing, it
just didn't get to the bolts before I sold it to John in Australia
(John, check your bolts). I do remember the latches had moved forward
as my present latches have, Andy is right, the forward pressuer from
the compressed struts will shear the aluminum rivets then apply
pressure under the hinge bolts which will degrade the fiberglass or
gall the aluminum top which will lead to LOOSE BOLTS over time. Loose
bolts = sheared bolts! My trailer closes and latches better, now that
I have replaced the sheared off aluminum corner rivets with 1/8" steel
rivets. BTW, I can see a slight forward bend in the right side of my
fiberglass top just outboard of the hinge plate.
Cheers,
JJ
Eric Greenwell
My trailer closes and latches better, now that
> I have replaced the sheared off aluminum corner rivets with 1/8" steel
> rivets. BTW, I can see a slight forward bend in the right side of my
> fiberglass top just outboard of the hinge plate.
My metal top Cobra trailer has had a forward bend on both sides for at
least 6 years, probably longer. I also just noticed the pivot holes for
the top are elongated fore and aft. I may add a bushing to them, which
would move the top back 1/8" or more, and make it easier to close.
MKoerner
springs or bumpy roads or inertia. I think the tension in these bolts
that leads to failure results from “bridging” the trailer across dips
in the road.
Our trailers can be viewed as two beams, the floor section and the
lid, setting one on top of the other. These beams are pinned together
at each end; at the front by the hinge pins and at the rear by the
latch pins. During normal use the lower beam, the floor section, is
loaded along its length by the sailplane parts and supported in the
center by the axel. The trailer sags at each end. The resulting
bending loads put the top edge of the lower section in tension. These
bending loads also pull down on the front and rear of the lid.
Meanwhile, contact with the lower section along the claim shell joint
pushes up on the center of the lid. As a result the lid also sees
bending loads in the same direction as the floor section. This puts
the lower edge of the lid is in compression, not tension. As a result
the front bottom edge of the lid is pulled toward the rear, away from
the mating surface of the lower section. Loads in this direction would
be transferred to the hinge brackets and not to the bolts which clamp
the brackets to the lid. Furthermore, the deflections would probably
be small relative to the play in the pins.
On the other hand, when you tow the trailer across a dip, such as
starting up a steep driveway, the trailer may form a bridge across the
dip, with the back end dragging on the pavement and the front end
supported by the tow vehicle’s hitch. This causes the trailer to sag
in the middle, opposite to the normal loading. This puts the upper
surface of the lower section in compression. The lid is still pinned
in place at the front and the back, but it doesn’t carry much load so
it does not bend nearly as much. This would be especially true with
the fiberglass tops which are probably much stiffer than the aluminum
version. The lid effectively takes a short-cut across the gap as the
lower section sags away in middle. As a result the front bottom edge
of the lid is pushed forward, past the mating surface of the lower
section. Loads in this direction would be transferred through the
bolts, putting them in tension.
Some years back a number of us, led by JJ, myself included, found
substantial cracks on our trailer tongues, opening from the bottom.
Same story: Bridging.
The odd thing about the current problem is the sudden onset of
multiple events… unless you all drove across the same ditch???
Mike Koerner
Andy
To the best of my knowledge my trailer has never been "bridged". The
rear of my trailer is high and it is towed by a vehicle with a short
overhang.
Steve may have sent you my photos that support my suggested failure
mode. If not, send me an email and I'll copy you.
Andy (GY)
Steve Koerner
rare occurence that would, if the force were sufficent, cause the
bolts to fail at once. I know my trailer had not done any "bridging"
in a very long time as I've purchased taller tires that prevent that.
I have documented my repair process in a PDF file with numerous
pictures. The document includes a design for a backing plate that
goes under the handle. The document also gives suggestions for
clamping the plate while removing the bolts. This document provides a
recommended replacement bolt and a procedure for repairing the likely
damage to the base of the handles as well. Eventually I'll post this
at my Wing Rigger site (which I can't modify at the moment because
I've replaced a computer with the web tools); for now it is posted
here:
http://www.box.net/shared/9dvnt7qgs8
Steve Koerner (GW)
db_sonic
nice write-up.
Tuno
look forward to reading it this weekend.
One thing I've learned with certainty from this thread is that Mike
and Steve fell out of the same tree!
tuN7o
Eric Greenwell
> But Mike, the bolts have failed by fatigue. What you describe is a
> rare occurence that would, if the force were sufficent, cause the
> bolts to fail at once. I know my trailer had not done any "bridging"
> in a very long time as I've purchased taller tires that prevent that.
My trailer has a 30' body,a 4' tongue, and carries 850 pounds of glider
(total trailer weight: 2450 lbs). It's towed almost exclusively by a 23'
motorhome. "Bridging" is the trailer's pastime on every trip, as it goes
in and out of gas stations and parking lots. It's bridged hundreds of
times during the 160,000 miles it's been towed, yet the bolts have not
failed. I won't know if they are damaged until I remove them; however,
they are still very tight. Or, maybe the metal top makes the trailer
more tolerant of bridging.
I doubt the tongue cracking a lot of us, including me, experienced
several years ago was due to bridging; instead, the situation and
symptoms were classic examples of fatigue failures induced by welding
stress concentrations.
In any case, the basics of the "bending" theory should be easy to check
by putting jack stands under the rear of the trailer, a floor jack on
the tongue, then jacking the tongue up while measuring the deflection of
the trailer bottom and the movement of the trailer top. My guess is the
deflections will be very small.
David Salmon
Surely when the top is up the gas struts are at the end of their travel,
and therefore are in effect a solid, weight support, somewhere near the
middle of the top. This would imply very little loading on the hinge in
that position, just whatever the unbalanced load is.
If , as it seems, the problem is fatigue in the bolts, as many last 15/20
years, they cannot be stressed much above the fatigue limit, so a small
amount of strengthening should do it. So my thought was to drill and add a
third bolt from the inside, with the nut and substantial washer inside the
handle. Once in place, the existing bolts could be replaced one at a time.
Can't remember using the handles anyway.
However Steve's is an elegant solution.
Dave
Andy
Nice write up but a disappointing solution. I was sure you would have
bored out holes in the glass top under the handles and inserted
machined spacer bosses. Those would carry the bolt clamping force
directly to the front face of the trailer frame extrusion.
I'm disappointed, not because you did it this way on yours, but
because I was hoping you'd make a second set for me ;)
Andy
Andy
If you decide on the third bolt method don't forget that each existing
bolt has a spacer block inserted inside the front frame extrusion.
Without that spacer the main function of the third bolt may be to make
the other two loose.
You are right about gas strut forces with the top open. The force is
zero and the struts just form one arm of a triangle that would like
very much to collapse. The way it would like to collapse is to move
the front of the top forward and up.
It's much easier to change the bolts before they fail than to get the
parts into alignment after the bolts fail.
Wear marks on my hinge spacer plate indicate the hinge plate had been
moving in an arc about the inner bolt until the outer bolt failed.
This suggests that only the outer bolt is likely to be subjected to
the cyclic shear loading that is assumed to have caused the fatigue
failure.
It took a while to find the nuts and bolt ends. They didn't just drop
on the trailer floor, they were projected over the top of the front
compartment dividing wall into the wing storage area.
Andy
Steve Koerner
must also have projected. It seems to me that would happen if the
bolts failed in tension but would not happen if the bolts had become
loose and were sheared.
The other thing that occurred to me yesterday afternoon is that Andy's
bolts failed at or near the juncture of the threaded and unthreaded
portion of the bolt. That would be a location about .31 inches inside
the front extrusion frame and not where the main shearing action would
be expected to occur due to the plate sliding .
So, although I had accepted Andy's theory just yesterday morning, now
I'm having doubts. I'm back to thinking that the fatigue failure must
me primarily in tension.
GW
bumper
tops, rather than aluminum, I wonder if the difference in thermal
coefficient of expansion between the two materials might be playing a
roll?
On early Stemme S10-VT motorgliders, the spoiler control rods were
aluminum in an otherwise mostly carbon fiber wing. The spoiler over-
center locks were at the fuselage end. As the temperature dropped with
altitude, the spoilers would first start to come open and then
sometimes even open all the way with no pilot input. Problem was
solved by changing the control rods to carbon fiber.
bumper
zz Minden
bildan
Bumper might be on to something.
Polyester reinforced with glass fiber has a linear coefficient of
thermal expansion of 25 (10^-6 m/m K) while structural steel is just
13. Aluminum is 22.2 which is close enough to steel there may not be
a problem.
Ref: http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html
The fiberglass top is, of course, exposed to the direct sunlight while
the steel trailer frame stays mostly in the shade.
Bill D
Chip Bearden
spent the past 19 years on the road but the trailer doesn't live in a
hangar, either. I just checked visually and all four bolts are intact
and tight, the hinge plate is tight up against the aluminum cross
member, and there is no sign of movement or looseness or distortion
anywhere. I'll remove the bolts one by one to inspect but I'm
wondering, as at least one other person did, if the cause was simply a
bad batch of bolts, or perhaps improper tightening at the time of
manufacture. For sure, once there's looseness anywhere in the bolted
system, the likelihood of failure rises significantly.
Chip Bearden
ASW 24 "JB"
USA
Andy
The bolts are always in tension with the top closed even if the
clamping force has reduced to zero. I don't think the fact that the
bolts were under tension when they failed is inconsistent with the
theory they had fatigued due to cyclic shear loading.
Maybe to fully understand the failure one would have to know the
diameters of all the holes the bolt passes through. It's possible the
bolt experienced bending rather than true shear loading. The hinge
plate outer hole is 8.8 mm. To the best of my recollection the wear
marks on the hinge plate spacer were 7mm in height at the outer end.
I didn't record that measurement but photo IMGP6653_edited-1 (email
5/20) shows both the hole and the wear marks and 7mm seems reasonable.
Since our bolts failed in different places it's possible the failure
more was different. Unfortunate that you discarded yours. It would
have been interesting to see if the fracture surface looked like mine.
GY
Andy
> On May 21, 6:25 pm, bumper <bumpe...@gmail.com> wrote:
>
> > If this problem is primarily affecting Cobra trailers with fiberglass
> > tops, rather than aluminum, I wonder if the difference in thermal
> > coefficient of expansion between the two materials might be playing a
> > roll?
>
> > On early Stemme S10-VT motorgliders, the spoiler control rods were
> > aluminum in an otherwise mostly carbon fiber wing. The spoiler over-
> > center locks were at the fuselage end. As the temperature dropped with
> > altitude, the spoilers would first start to come open and then
> > sometimes even open all the way with no pilot input. Problem was
> > solved by changing the control rods to carbon fiber.
>
> > bumper
> > zz Minden
>
> Bumper might be on to something.
>
> Polyester reinforced with glass fiber has a linear coefficient of
> thermal expansion of 25 (10^-6 m/m K) while structural steel is just
> 13. Aluminum is 22.2 which is close enough to steel there may not be
> a problem.
>
> The fiberglass top is, of course, exposed to the direct sunlight while
> the steel trailer frame stays mostly in the shade.
>
> Bill D
Steel trailer frame? Not on mine. I assume Bumper was talking about
relative expansion of the bolts and the clamped parts.
Andy
Tuno
the value-added bolts and backing plates.
bildan
I know later Cobra frames are aluminum which is why I listed its
coefficient of expansion. However, I should have said "metal" frame.
I think Bumper was thinking of the top and bottom being firmly
attached to each other at the front and back of the trailer with each
expanding at different rates producing a force on the hinge plates.
Fiberglass tops expand at twice the rate of metal frames. Working the
numbers, it's unlikely the difference could be more than a few mm.
Ken K
no additional clamping? i.e the struts are fully extended and the
'lift' vector is almost vertical.
KK
> 3) Do NOT remove these bolts without clamping the
> trailer top down and restrained from moving
> forward. If you undo the bolts without restraining
> the top, even on just one side, even just one bolt,
> the gas strut will deliver you a nasty surprise
> (as happened to me on the highway to Hobbs).
> CAREFUL !
>
> 5) I'm designing a clamp to safely hold the top
> during replacement of the hinge and bolts...
Dave Nadler
No ! No !!
To clarify #3:
Best to OPEN the top AND THEN clamp the front down...
The "lift vector" is NOT non-existent;
CG of top is AFT of strut attach point and
front of top will pivot around the strut attach
(upwards) and push forward as well...
Unless it is well clamped !
Look at the picture here:
http://www.nadler.com/public/2010_Hobbs/2010_Hobbs.html
Hope that helps, From soggy Ridge Soaring,
Best Regards, Dave "YO electric"
MKoerner
least until I got bigger tires and wheels (Eric, get bigger tires and
wheels). And like Eric, I have the more flexible aluminum top and
tight hinge bolts.
Based on the table Bill provided the thermal expansion of aluminum is
2.5% higher than glass reinforced polyester. assuming the these 30
foot trailers were assembled at 80 F, left outside on a 0 F night and
soon after dawn the top was 20 F hotter than the floor due to radiant
heating; the fiberglass top would be about 0.095” longer than the
bottom. I don’t know if the hinge and latch pins can take up that much
play. This might explain the crushed fiberglass under the handles.
Looking at a steel bolt going through 2 inches of aluminum on the
frame, under the same 80 F to 0 F conditions, the bolt would only
tighten by .001”; certainly within the elastic range of the the parts
and at loads too low to damage anything.
Another possibility is crosswind loads on the open trailer. Of course
you usually park into the wind for assembly or disassembly, but dust
devils are not too uncommon in Arizona this time of year. The air
springs won’t provide much lateral support when extended. The cross
wind loads, including those on the tail doghouse which act over the
full length of the trailer, would create a substantial load couple
across the 4-foot hinge spacing at the front of the trailer. The bolts
on the downwind side would be in tension with the outside bolt seeing
the highest load, perhaps enough to crush the fiberglass under the
handle. This would cause this bolt to lose its clamping force and
allowing the hinge to pivot around the inner bolt (if that is what you
saw).
In either case, the stronger bolts and the baseplate Steve put under
the handle should solve the problem.
Mike Koerner
Steve Koerner
good. I know that Andy and I have had to disassemble in the wind many
times and there is a tremendous lever arm working on the hinge plates
in that situation as you point out. That could be what has fatigued
the bolts in tension as well as caused the slight compression to the
fiberglass and the base of the handles. In my case the broken bolt
was discovered after a long drive back from Moriarity where my last
disassembly at Moriarity was in pretty strong wind and the trailer
was, in fact, not lined up to the wind as it should have been. So, it
fits with the patients history.
Steve Koerner (GW)
Steve Koerner
What's more, the trailer was facing north in a west wind and it was
the left side bolt that popped. That's the side that would have been
under tension. I guess I should read my own Wing Rigger FAQ document
which clearly states that the trailer should be pointed into the
wind. I had asked where to assemble and I put the trailer in the
particular spot that was recommended off the edge of the apron. I
should have moved it when it was disassembly time and the wind was
stronger but I didn't need to bother with the Wing Rigger since my son
was standing by to assist. The wind was not so strong that we
couldn't safely rotate the wing to slide into the trailer -- maybe
around 10 or 12 knots.
I think the bolt had already fatigued and that was its final straw.
The fiberglass compression and handle damage was actually the same on
both sides.
GW
Andy
What Dave said!
However the clamp does not need to be anything special. I changed my
bolts yesterday using a 2.5 inch C clamp that was quite a bit less
substantial than the one shown in Steve (GW)'s write up.
Trust us - It's far easier to clamp the parts while they are in
alignment than to try to get them back into alignment if you try
without clamping!
Andy (GY)
Ian Reekie
top). This has only been towed on reasonably smooth roads (excluding the
odd field) and only very occasionally opened in strong winds .
There is some slight compression to the glass shell at the outer mounting
holes on each handle. When checking the bolt tightness I noticed that the
nuts on the outside mounting bolts (inside the trailer on the hinge plate)
are both not square to the hinge plate as though they were fitted at a
slight angle. (approx 1mm gap is at the top of the bolt ) .
I have vague recollections they had always been like this and assumed at
the time that this was because it was not possible to square everything up
when mounting the bolts at the factory. However, I now wonder if the lid
has moved up slightly inclining the outer bolts.
I “gently” tightened all the bolts a couple of turns (using a small 6”
spanner to limit torque) so it would appear that they were working loose.
I was in a hurry when I did this and seem to remember the extra torque
reduced the gap at the top of the inclined bolts.
Few of questions
- Has anyone else noticed if any of their hinge mounting bolts were at a
slight angle when delivered ?
- Does anyone have a “method” of judging how tight to do these bolts
should be. I am loath to snap them or do more compression damage before I
get round to re-enforcing the handle area and fitting stronger bolts.
- Regarding clamping the hinge to the aluminium cross member. When the
bolts are removed what stops the aluminium cross member from ripping out
of the top shell. I assume it is glued in ?
Andy
>When the bolts are removed what stops the aluminium cross member from ripping out
>of the top shell. I assume it is glued in ?
It is riveted to the top side rails. However one US owner has found
all rivets sheared. All mine seem ok.
In any event you should only change one bolt at a time!
Andy
Ian Reekie
no structural connection from the side rails to my cross member ?
I am flying tomorrow so I will examine this further, but I wonder if they
varied the construction. Many of my aluminium support brackets are glued
to the shell with what looks like a black polyurethane glue, similar to
that used in the car industry to glue panels. I assumed this was the case
for this cross member.
I agree that It would be ideal to change one bolt at a time, but I was
thinking of implement Steve Koerners mod which I believe would require you
to remove both bolts to get the 1/4" spacer under the handle ?
Ian Reekie
no structural connection from the side rails to my cross member ?
I am flying tomorrow so I will examine this further, but I wonder if they
varied the construction. Many of my aluminium support brackets are glued
to the shell with what looks like a black polyurethane glue, similar to
that used in the car industry to glue panels. I assumed this was the case
for this cross member.
I agree that It would be ideal to change one bolt at a time, but I was
thinking of implement Steve Koerners mod which I believe would require you
Andy
>
> - Show quoted text -
Mine also has the black glue. Email me and I'll send you a photo that
shows the rivet locations.
It should be easy to change one bolt at a time even when introducing
the new plate. Just leave the first new bolt slightly loose so the
plate can be rotated into alignment after the second bolt is removed.
Even if you decide to remove the handle to work on it you can still
put bolts in place to maintain the alignment.
Having dealt with this issue once I would go to great length to ensure
that both bolts were never removed at the same time.
If you want all the bolts out then raising the top, disconnecting the
gas struts, and then doing all the hinge work with the top closed may
be the way to go. You'll need several strong friends or an overhead
hoist though.
Andy
Eric Greenwell
> Like Eric’s, my trailer has “bridged” its way around the west, at
> least until I got bigger tires and wheels (Eric, get bigger tires and
> wheels).
That would make the rear of the trailer too high for convenient rigging
with the trailer attached, something I commonly do when traveling with
the motorhome (the trailer's primary motive force). Removing it to rig
is a nuisance, especially since I need to reconnect it after rigging, so
my wife can retrieve or follow me, as needed.
A Cobra trailer dealer told me they are designed to be supported
entirely by the tail and the tongue without damage, so I don't think the
bridging causes the trailer any problems. I do have to replace the
aluminum tail skids every 100,000 miles.
--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl
- "A Guide to Self-launching Sailplane Operation Mar/2004" Much of what
you need to know tinyurl.com/yfs7tnz