Cobra top hinge plate bolt failure

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.

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
he

http://www.box.net/shared/9dvnt7qgs8

Steve Koerner (GW)

On May 20, 7:59*am, Steve Koerner wrote:
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.

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
he

http://www.box.net/shared/9dvnt7qgs8

Steve Koerner *(GW)

nice write-up.

Unfortunately that box.net doesn't work from behind my firewall, but I
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

On 5/20/2011 7:59 AM, Steve Koerner wrote:
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.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

At 19:51 20 May 2011, Tuno wrote:
Unfortunately that box.net doesn't work from behind my firewall, but I
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


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

On May 20, 7:59*am, Steve Koerner wrote:
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.

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
he

http://www.box.net/shared/9dvnt7qgs8

Steve Koerner *(GW)

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

On May 21, 12:10*am, David Salmon wrote:
At 19:51 20 May 2011, Tuno wrote:

Unfortunately that box.net doesn't work from behind my firewall, but I
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

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

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

Come to think of it, my bolt ends were under the wing area too. Mine
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

On May 21, 10:43*am, Steve Koerner wrote:
Come to think of it, my bolt ends were under the wing area too. * Mine
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

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

Nice writeup Steve. You should fabricate and sell repair kits, with
the value-added bolts and backing plates.