Cobra top hinge plate bolt failure

Andy has described clearly a fatigue failure in the bolt.
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
and
the handles relocated.

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)