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Grinding or rubbing noise from plane power alternator

S

SteveR

Active Member
When I tension my alternator belt so that I get 10 to 12 ft-lbs of torque before belt slip (per Lycoming s. i. 1129A), the alternator makes a "grinding" noise. Roughly 5 years and 450 hours on this alternator. I'm assuming the bearing is failing and the unit needs to be replaced. Anyone ever experienced this?

For what it's worth, when I loosen the belt to get 6 to 8 ft-lbs before slipping, the noise is gone. Using the "old" rules of thumb on belt tension (deflection, difficulty to twist 90 degrees, etc.), the belt is "tight enough", and it doesn't slip under load.
 
If it's grinding, then most likely you are right, there is a problem with the bearing(s). I've also heard other strange noises that were due to the alignment of the alternator pulley to the flywheel pulley - might be worth a check to ensure that it's really perfectly aligned.
 
I suspect it is either the rear bearing or a loose stator that only hits other material under more tension, as that is the most problematic area for that alternator. I would run at the lower tension as you figure out what you're going to replace it with, as it will get worse. Suggest skipping the PP on the next go.
 
For clarity, per Lycoming Service Instruction No. 1129D, only new belts should be torqued to 11 to 13 ft.-lbs., Used belts should be torqued to 7 to 9 ft.-lbs.

Over tensioning alternator belts is a good recipe for premature bearing failure.

Alternator Pulley Tension.jpg
 
Almost guaranteed to be an enlarged bearing cavity at the slip ring end. I've written about how to diagnose without disassembly by clamping the pulley in a solid bench vice. Search "bearing" with user as DanH.

If the serial number says manufacture was prior to a certain date, you may get a new alternator by asking. Call Hartzell in Montgomery.
 
drill_and_buck said:
Over tensioning alternator belts is a good recipe for premature bearing failure.
Click to expand...

Over tensioning was Hartzell's original whipping boy. However, note the failures are not actually bearing failures, nor are they at the bearing behind the pulley. The issue has been enlargement of the bearing cavity in the brush end cover, which by design is a slip fit at the bearing OD.

I've seen the same failure with both tight and loose belts.
 
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