Cobra top hinge plate bolt failure

On May 17, 5:37*pm, sisu1a wrote:
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.

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/20...l/Cat11065.pdf

-paul

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

On 5/17/2011 11:38 AM, Andy wrote:

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)

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

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)

On May 18, 11:44*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

Why not use the proper metric size?

Peter

On May 18, 11:43*am, vontresc wrote:
On May 18, 11:44*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

Why not use the proper metric size?

Peter

Because they break.

GW

Hi Guys - Spindelberger is looking into this.
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...

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

On May 21, 6:25*pm, bumper 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.
Ref: http://www.engineeringtoolbox.com/li...ents-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

I have an older (early 1992) Cobra with a fiberglass top. I haven't
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