Grinding when turning prop

[Manchu16]

After a bit of troubleshooting, I figured it out, but I thought I should post it here in case others ever experienced this.

Symptom: when rotating the prop by hand, I would feel a distinct grinding feeling. This would only happen immediately after shutdown when rotating the prop to put the towbar on and after a day or two when it was time to preflight. It would only happen at these two times, and only for the first 90°. Think sharpening a pencil type grinding.

My first thought is, I cracked a ring or there was some debris stuck in a cylinder. scoping ruled that out.

In order to remove the cowl, I would have to move the prop and by the time everything was off, there was no more grinding. Running the engine on the ground did not generate enough heat without the cowl to create the symptom.

I drained the oil expecting to see metal. Everything was normal.

There was no leaking around the prop seal nor anywhere else that would indicate some kind of issue.

Solve: I just happened to run across a few posts about alternators and bearings failing. I pulled my alternator and lo and behold there was about 1/16 of an inch of play in the pulley. The rear bearing had spun in the housing. Just shy of 300 hours on the alternator. Still putting out solid power when pulled. Internals looked like new.

 

[Mark Dickens]

After a bit of troubleshooting, I figured it out, but I thought I should post it here in case others ever experienced this.

Symptom: when rotating the prop by hand, I would feel a distinct grinding feeling. This would only happen immediately after shutdown when rotating the prop to put the towbar on and after a day or two when it was time to preflight. It would only happen at these two times, and only for the first 90°. Think sharpening a pencil type grinding.

My first thought is, I cracked a ring or there was some debris stuck in a cylinder. scoping ruled that out.

In order to remove the cowl, I would have to move the prop and by the time everything was off, there was no more grinding. Running the engine on the ground did not generate enough heat without the cowl to create the symptom.

I drained the oil expecting to see metal. Everything was normal.

There was no leaking around the prop seal nor anywhere else that would indicate some kind of issue.

Solve: I just happened to run across a few posts about alternators and bearings failing. I pulled my alternator and lo and behold there was about 1/16 of an inch of play in the pulley. The rear bearing had spun in the housing. Just shy of 300 hours on the alternator. Still putting out solid power when pulled. Internals looked like new.

What brand alternator?

 

[DanH]

What brand alternator?

Plane Power. It's a known problem. Originally the engineers I spoke with here at the alternator HQ blamed it on improper belt tension. Reality is a design detail; the rear bearing must be a slip fit, but there is no provision to accommodate thermal dimensional change. When hot, the aluminum housing grows more than the steel bearing, developing a clearance. Any relative movement rapidly wears the bore in the rear casing.

Given there are thousands in the field, every annual should include removing the alternator for a bearing check. Simply clamp the pulley in a vice with the slip ring end pointed up. Now rock the alternator case back and forth. Any freeplay you can feel is cause to replace or repair the alternator.



One solution would be the substitution of a bearing which includes an anti-rotation feature, like the plastic rings on the bearing shown below (courtesy BillL).



Proposed, but not yet proven, would be the addition of cooling at the rear bearing cavity. Although lots of folks have a blast tube on their alternator, most do it wrong. Here's an example. The blast tube is pointed at the general vicinity of the case center, which is bass ackwards. The fans move air in the ends and out at the center. An effective blast tube would pipe air to a spigot on the rear case cover, providing captured cooling to the regulator assembly and rear bearing bore.

 

[Manchu16]

What brand alternator?

Mine was an NK that looks just like the plane power. It was Dan’s post that got me pointed in the right direction.