Hi Jonathan,
I've had the same problems for some time - and they persist despite trying more
than 1 CKT design of spring, lining the 'hole' down the middle of the spring
with silicone tubing held in place by a combination of Red silicone and adding
a length of central locking-wire as you describe.
In an effort to try and get to the bottom of the problem I have measured the
natural frequency of the 8 springs fitted (try recording the twang emitted when
you ping the springs, with your mobile phone on the ground) - so that i can
draw up a Campbell Diagram - that compares the spread of spring resonances with
the forcing frequencies generated by the engine (i did this stuff for a living
once). Here is an example diagram that possibly illustrates the concept better
than my text: https://www.originlab.com/www/products/GraphGallery.aspx?GID=130
The idea is that the natural frequencies of these springs [like everything else
on the aircraft btw], sit waiting for excitation by forcing at the same
frequency
- in our case, by an out-of-balance (OOB) force from the engine (these OOB
forces are normal - though relatively small, in all engines btw).
Typically, when something goes into resonance, just a small level of excitation
can result in a large motion - and a commensurate increase in the amount of
strain,
and therefore stress, in a component - depending on how it is mounted and
how it is vibrating. Excessive vibration does not always result in breakage,
it may just be irritating of course.
The implication/potential remedy is that there are probably a few engine speeds
that are best avoided for e.g. cruising - if we are to avoid exciting the
exhaust
springs into resonance.
BTW, i also notice that the points between my springs and their exhaust
pipe/silencer
'hook stubs' are worn away/thinner - which hints at the failure mode.
My gut feeling is that in addition to the heat in service and possible fatigue
damage initiation caused by e.g. use of a screwdriver to fit a spring [which
like you, i never do], material thickness reduction is probably what happens to
induce a spring to 'let go'. With this in mind, I have been playing with
putting
a blob of CKT's graphite exhaust socket grease in this area - but the results
are inconclusive so far.
Damping = generally good, as it generally reduces the levels of vibration
amplitude,
though it also can 'lower' the frequency of resonance as well.
Apologies, a bit of an edict . . . More to follow.
Clive
G-YETI
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