Excellent photos showing the wear, Al.

 

I agree with you, you may have had many more hours on the unit without problem.  But, as you know once things start to go, the process can escalate rapidly, one rivet fails, two fail, then three and who knows the remainder after some point may fail in cascade fashion – with the remaining rivets failing with increasingly  more ease as they carry more of the load.  So the thing could possibly “unzip” suddenly.

 

There’s always another airshow – good call in my opinion.

 

Ed

 

 

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

 

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