I
took the re-drive off today for examination of the damper plate and
rivets. Here are my findings and observations. Somewhat detailed for the
average user, but could be useful for Tracy.
- The ‘Front’ side
of the plate (the side toward the prop) is shown in the first photo. This
side is easily seen when the drive is on. Everything looks fine.
- The ‘Back’ side
(toward the flywheel/ring-gear) is shown in the second photo. Here
the streaks from the rivets are easily seen. These are the rivet heads.
- Of course you’ll also
note all the streaks coming out from the spline. Does that mean the spline
is wearing? No, I’d say that’s just grease that lubed the spline,
and what was in the idler bearing hole at the end of the shaft. It
is likely that some portion of the streaks from the rivets could just be
oily dirt.
- Note that the elastomer snubber
bushings are all thinner on the ‘clockwise’ side than on the
other. They have taken a permanent set in the direction of the applied
load. It is more apparent on the front side of the plate where there is a
slight bevel around the inner steel bushing (third photo). These snubbers
have about 125 hours on them; and suggest there is a ‘service
life’ to these, and they should be replaced at some interval. These snubbers
may run hotter in my pusher where they are at cowl exit temp – 125-150F.
- Almost all of the rivets
could be rotated in the holes with a pair of pliers; some were rather
tight, others just snug; just one or two that rotated with reasonable
ease. I could not detect any movement between the spline flange and the
plate when holding with my hands. Whatever movement I had previously noted
with my ‘sensitive fingertip’ test when jogging the prop on
the plane I’d say was very small, like .001”. (apparently I
have very sensitive fingertipsJ)
- I noted when turning the
rivets that when rotated 90 degrees they were tight. This suggested that
they were wearing only in the direction of the applied load. I further
verified this after removing a couple rivets and noting the wear on one
side where it was in the spine flange (see shiny area in blurry photo
‘Rivet1’), and when rotated 180 the shiny area that was in the
plate (blurry photo Rivet2). Sorry, couldn’t seem to get
the camera to focus that close in, but you get the picture.
- I conclude that the wear on
the rivets is due to the somewhat cyclical loading from the engine to the
prop, and would be expected to be worse on the 20B, or 13B run at
significant boost just due to the greater HP.
So; did I over-react in canceling my
trip to Nevada? Yeah, I think so. This would have operated safely for many more
hours. However; at some point these rivets needed replacing, and
over-reacting where safety is involved is not a bad idea.
Further; in regard to replacing with
bolts, the holes mic at .190-.191” and ‘close tolerance bolts are
nominally .190”, so that’s a good fit. Regular AN bolts are
.002.-003 smaller dia. I measured the combined thickness of the flange and the
plate at .350”; the grip length on an AN173-6A is nominally .375, but
measures more like .400 so is good with an AN-960 washer.
Al G