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  #1  
Old 05-15-2022, 04:42 PM
AlexPeterson's Avatar
AlexPeterson AlexPeterson is offline
 
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Location: Ottertail, MN
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Default Smoking rivets on spinner backing plate assy

I'm doing the annual inspection on the plane, the 21st such time! Somewhere after a few hundred hours, I noticed a universal head rivet on the spinner backing plate assembly had lost its head. I drilled out the stump, re-riveted it and moved on. A few years later, the same thing happened in the same position. Again, I drilled and re-riveted it. Many years and probably 1200 additional hours went by with no more trouble. A year ago, I had the prop overhauled, and have flown around 50 hours in the past year.

Now I have, for the 3rd time, lost that same rivet. I did a very close inspection of the entire backing plate for any signs of cracks, damage, etc. None found. All four rivets in the symmetrical positions had smoking, not observed before. They are the rivets closest to the large bolts which attach it to the hub.

It must be related to exactly how the prop hub bolts are torqued. This prop has always been very smooth with no vibrations. I will likely get it dynamically balanced again, but I expect there won't be much change to the weights.

I may, symmetrically, add a few more rivets in the areas of interest. Fortunately, my squeezer reaches this area, so it's not a big deal.

Thoughts?
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  #2  
Old 05-16-2022, 04:57 PM
Marc Bourget Marc Bourget is offline
 
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Location: Stockton, California
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Default

Just ruminating here . . .

How well does the spinner track?

What is the spinner material? Fiberglass or Aluminum?

Is the wall thickness pretty much equal?

I'm thinking, if the rivet that fractured is closest to the bolt attaching the propeller, that's a stress concentration.

Spinner balance, installation etc., may be the mechanism of failure.

May check fine while stopped. Maybe a strobe while running if all else checks out?
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  #3  
Old 05-16-2022, 05:02 PM
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bjdecker bjdecker is offline
 
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Location: Georgetown, TX
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Default AN470's

The color of those rivets looks a little silverish...got me thinking that they *may* be the wrong kind; AN470A's or Monel types instead of AN470AD's

Just a thought
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  #4  
Old 05-17-2022, 10:13 AM
Ralph Inkster Ralph Inkster is offline
 
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Cad color shouldn't matter, each batch is different. Also the mfg head of the rivet heads pictured have the single dimple meaning the proper 2117T rivets.
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  #5  
Old 05-17-2022, 11:31 AM
lr172 lr172 is offline
 
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I think it is just a very high stress area with huge centrifugal forces. I still lose one screw in the spinner on my 6 every 100 or so hours; Always from the smaller, more foreward bulkhead.
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  #6  
Old 05-17-2022, 01:25 PM
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Walt Walt is offline
 
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I would suspect movement between the bulkhead and doubler, possibly from drill burrs or debris trapped between the surfaces allowing movement.
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  #7  
Old 05-17-2022, 03:14 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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Location: Hubbard Oregon
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Default

Quote:
Originally Posted by AlexPeterson View Post
I'm doing the annual inspection on the plane, the 21st such time! Somewhere after a few hundred hours, I noticed a universal head rivet on the spinner backing plate assembly had lost its head. I drilled out the stump, re-riveted it and moved on. A few years later, the same thing happened in the same position. Again, I drilled and re-riveted it. Many years and probably 1200 additional hours went by with no more trouble. A year ago, I had the prop overhauled, and have flown around 50 hours in the past year.

Now I have, for the 3rd time, lost that same rivet. I did a very close inspection of the entire backing plate for any signs of cracks, damage, etc. None found. All four rivets in the symmetrical positions had smoking, not observed before. They are the rivets closest to the large bolts which attach it to the hub.

It must be related to exactly how the prop hub bolts are torqued. This prop has always been very smooth with no vibrations. I will likely get it dynamically balanced again, but I expect there won't be much change to the weights.

I may, symmetrically, add a few more rivets in the areas of interest. Fortunately, my squeezer reaches this area, so it's not a big deal.

Thoughts?
May sound like a silly question but I have seen it before..... it's not visible in the photos.... is there a fwd. bulkhead on the front of the prop hub dome?
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  #8  
Old 05-17-2022, 08:11 PM
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AlexPeterson AlexPeterson is offline
 
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Quote:
Originally Posted by rvbuilder2002 View Post
May sound like a silly question but I have seen it before..... it's not visible in the photos.... is there a fwd. bulkhead on the front of the prop hub dome?
Scott, yes. The strange thing about this is that it went many hours without a problem, then about 50 hours after the backing plate was disassembled/reassembled to the prop hub (I had the prop overhauled). There is some connection, it would seem, to the backing plate being mounted/torqued to the hub. As I mentioned, it had gone around 1200 hours since the last time one rivet had broken.

It was mentioned the rivets have the wrong color - that is only a photo artifact.

I replaced all four rivets today. I'll pull the spinner after a dozen hours or so and report back. I again studied the entire backing plate assembly and saw no signs of anything untoward.
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  #9  
Old 05-17-2022, 08:23 PM
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AlexPeterson AlexPeterson is offline
 
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A couple additional details:

Fiberglass spinner.
Almost 1800 hours total time on the spinner assembly and installation.
Assembly per plans circa 1993.
Tracking very true.
The prop blade paint is perfect and hasn't touched the backplate.
Screws line up perfectly to the spinner.
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  #10  
Old 05-18-2022, 05:05 AM
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drill_and_buck drill_and_buck is offline
 
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Default Similar Experience

Similar experience. Smooth running prop. After 8 years, 1,000 hours, noticed two rivet heads missing during annual inspection.

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