Cobra top hinge plate bolt failure

On May 18, 12:30*pm, Steve Koerner wrote:
On May 18, 11:45*am, JJ Sinclair wrote:





On May 18, 10:23*am, Andy wrote:

On May 18, 9:44*am, Steve Koerner wrote:

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

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)

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- Hide quoted text -

- 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- Hide quoted text -

- 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)

Andy:

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

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

On 5/19/2011 7:04 AM, JJ Sinclair wrote:
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.

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

I suspect this problem has nothing to do with loads imposed by the gas
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

On May 20, 1:44*am, MKoerner wrote:
I suspect this problem has nothing to do with loads imposed by the gas
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

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)

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)