The bolt failures are the symptom of the problem not the cause!
The problem is the plywood stack that the tail spring sits on. During the build,
the plywood is sheathed in waterproof epoxy, but then you have to drill the
hole for the bolt to pass through, exposing the wood to the atmosphere and THAT
is the problem.
In service, moisture wicks up the bolt and into the plywood end grain, where it
sits unseen. Eventually the wood begins to rot and the structural rigidity is
lost.
At this point, the clamping force of the bolt is lost and the spring begins to
rock from side to side during taxying.
This allows the bolt shank to fret and eventually the fatigued bold will fail.
I have attached a picture of a tail spring bolt that was replaced just in time
(the ground handler noticed the tailwheel flopping from side to side during
taxying).
This shows everything; the deep pitted rust indicates that the plywood stack was
wet, the fretting has worn away the blot shaft where is passes through the end
of the tail spring, and you can see the fatigue crack developing nicely.
Simply replacing the bolt will cure the symptoms, but it will eventually fail
again
in exactly the same way.
To affect a lasting repair, the plywood should be thoroughly dried out and then
impregnated with a thinned epoxy resin to restore the structural rigidity.
To keep the bolt shank away from the wood, I over bored the mounting hole to
accept
a 5/16 bore 4041 sleeve, which was bonded permanently in place. The new bolt
was then greased and inserted through the sleeve.
I also made up a steel saddle bar that sits on top of the tail spring mount to
prevent it from rocking (just like Jons).
Nigel
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=483177#483177
Attachments:
http://forums.matronics.com//files/tail_spring_bolt_110.jpg
http://forums.matronics.com//files/tail_spring_bolt_201.jpg
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