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  #1  
Old 06-24-2021, 07:42 AM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 10,268
Default Diagnosing A Plane Power Bearing Failure

Symptom: Post-flight, engine still hot, moved the prop and heard a faint scraping noise. The noise was much diminished, almost non-existent when the engine was cold. Put another way, it wasn't notable if/when the prop was moved during a preflight.

Here's the interesting part. Removed cowl, removed the belt from the alternator pulley, spun the alternator by hand without any noise. Pushing and pulling the shaft axially made no difference. Otherwise wiggling the pulley detected no abnormality.

Hmmm.

Removed the alternator from the engine. Padded the jaws on my big shop vice with some soft aluminum, and clamped the pulley in the vice so the shaft pointed straight up, brush end on top. Now the problem was evident. With the pulley firmly fixed, the brush end of the alternator case could be moved roughly a millimeter total in any axis.

The problem was an enlarged bearing bore in the aluminum rear cover. The point here is that simply spinning the rotor isn't an inspection which will detect the problem. The bearing itself had not failed, so it rotated smoothly. The lever ratio (distance from the front bearing to the rear bearing/distance from the pulley centerline to the front bearing) is so large that hand-manipulating the pulley won't generate enough force to detect the problem. The weight of the rotor when horizontal also plays a role.

Scraping when hot, but not cold? I assume thermal expansion of the big Lycoming ring gear carrier, which would significantly tighten the belt in operation, apparently providing enough force to pull the rear end of the rotor into fleeting contact with the stator.

There is one other clue worth noting. This alternator had a bit more black dust in the vicinity of the rear cover. I assume it was brush dust. With the rear bearing floating around in an enlarged bearing bore, the brushes would have been subjected to more than normal movement in their holders.

Plane Power knows there is a problem. Their service rep cheerfully admits to it, but says the cause is too much belt tension. Some of us think that's unlikely, but I'm not going there with this thread. Nothing we say will fix the issue; that's up to Hartzell now. Until it is fixed, best if users learn how to safely live with it, i.e. catch during maintenance. And yes, we already ran a poll; B&C units appear to be far more reliable. In fairness, this tip would apply to any alternator, including B&C, and perhaps I'm just the last guy to have learned it the hard way.
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Last edited by DanH : 06-24-2021 at 08:30 AM.
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  #2  
Old 06-24-2021, 08:47 AM
Ralph Inkster Ralph Inkster is offline
 
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Location: Calgary, Alberta
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Years ago I went scrounging at a local car junk yard & gathered up 6 Nippon Denso style alternators off various makes of cars. Took them all apart to see what I could see. I would assume that they all had at least 100K miles on each of them. I was suprised to note that the front bearing was noisy on only one of them, brushes were nearly worn down to the holder on all of them, and 2/3 of them had the rear bearing knocked out of them. I would assume results would have been the same if I chose different manufacturers to look at.

Alternators ARE a maintenance item after all! No wonder there are so many rebuild/reman auto alternator sources out there! So how many 'miles' are on your alternators & do they need looking at for normal wear & tear?
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  #3  
Old 06-24-2021, 09:07 AM
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rv8ch rv8ch is offline
 
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Quote:
Originally Posted by DanH View Post
... Until it is fixed, best if users learn how to safely live with it, i.e. catch during maintenance. ...
Thanks for flagging this, Dan. Were there any other indicators, like aluminum dust? Otherwise, short of removing the alternator, I guess move the prop a bit when the engine is hot and listen?
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  #4  
Old 06-24-2021, 09:53 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by rv8ch View Post
Were there any other indicators, like aluminum dust?
Not that I noticed externally. I did not personally dismantle this particular alternator, as it was subject to warranty, and most vendors buck when you hand 'em a component which looks like it has been opened. The service rep showed me the removed rear cover w/enlarged bearing bore the following day. The service center is local to me, so I was visiting in person.

Quote:
Otherwise, short of removing the alternator, I guess move the prop a bit when the engine is hot and listen?
Yes, that's what tipped me to go looking. I think the longer term answer will be to add alternator removal to the annual condition inspection checklist. This particular problem was very evident with the alternator vertical and the pulley in a vice.
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  #5  
Old 06-24-2021, 10:21 AM
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Low Pass Low Pass is offline
 
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Good info. I'll add the listening to the alternator to my preflight inspection. Never know what else might be rubbing or clanking around...
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  #6  
Old 06-24-2021, 10:25 AM
BillL BillL is offline
 
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Location: Central IL
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Quote:
Originally Posted by Ralph Inkster View Post
Years ago I went scrounging at a local car junk yard & gathered up 6 Nippon Denso style alternators off various makes of cars. Took them all apart to see what I could see. I would assume that they all had at least 100K miles on each of them. I was suprised to note that the front bearing was noisy on only one of them, brushes were nearly worn down to the holder on all of them, and 2/3 of them had the rear bearing knocked out of them. I would assume results would have been the same if I chose different manufacturers to look at.

Alternators ARE a maintenance item after all! No wonder there are so many rebuild/reman auto alternator sources out there! So how many 'miles' are on your alternators & do they need looking at for normal wear & tear?
My 540/Bosch went 157k and was still quiet, M5/Bosch 155k still running, Volvo/Bosch still ran, rotated quiet at 140k, Sable/Motorcraft 100k had dry bearings, making noise and brushes nearly out. I am assuming the still running really would need overhaul.

Having a PP with less than 200 hrs (7500 mi at 37 MPH) and wallowed bearing housing does not compete with automotive.

The SRE/rear bearings are problematic. Good ball bearing design has to allow one race to float for differential expansion of housing and rotating element. Some designs use a needle bearing aft for this reason. Kind hard to assemble a pressed inner and outer race for both bearings.

At one time PP/Unipoint/XYZ had the issue addressed with a clever "expansion control ring". See photo for earlier PP bearing selection. Cheap and cheerful market?
Click image for larger version

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  #7  
Old 06-24-2021, 10:25 AM
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riseric riseric is offline
 
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Default How many hours ??

Dan, nice catch !!!


How many hours on the alternator ??
That could give a clue to the inspection intervals.
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  #8  
Old 06-24-2021, 10:39 AM
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skylor skylor is offline
 
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Quote:
Originally Posted by DanH View Post
Symptom: Post-flight, engine still hot, moved the prop and heard a faint scraping noise. The noise was much diminished, almost non-existent when the engine was cold. Put another way, it wasn't notable if/when the prop was moved during a preflight.

Here's the interesting part. Removed cowl, removed the belt from the alternator pulley, spun the alternator by hand without any noise. Pushing and pulling the shaft axially made no difference. Otherwise wiggling the pulley detected no abnormality.

Hmmm.

Removed the alternator from the engine. Padded the jaws on my big shop vice with some soft aluminum, and clamped the pulley in the vice so the shaft pointed straight up, brush end on top. Now the problem was evident. With the pulley firmly fixed, the brush end of the alternator case could be moved roughly a millimeter total in any axis.

The problem was an enlarged bearing bore in the aluminum rear cover. The point here is that simply spinning the rotor isn't an inspection which will detect the problem. The bearing itself had not failed, so it rotated smoothly. The lever ratio (distance from the front bearing to the rear bearing/distance from the pulley centerline to the front bearing) is so large that hand-manipulating the pulley won't generate enough force to detect the problem. The weight of the rotor when horizontal also plays a role.

Scraping when hot, but not cold? I assume thermal expansion of the big Lycoming ring gear carrier, which would significantly tighten the belt in operation, apparently providing enough force to pull the rear end of the rotor into fleeting contact with the stator.

There is one other clue worth noting. This alternator had a bit more black dust in the vicinity of the rear cover. I assume it was brush dust. With the rear bearing floating around in an enlarged bearing bore, the brushes would have been subjected to more than normal movement in their holders.

Plane Power knows there is a problem. Their service rep cheerfully admits to it, but says the cause is too much belt tension. Some of us think that's unlikely, but I'm not going there with this thread. Nothing we say will fix the issue; that's up to Hartzell now. Until it is fixed, best if users learn how to safely live with it, i.e. catch during maintenance. And yes, we already ran a poll; B&C units appear to be far more reliable. In fairness, this tip would apply to any alternator, including B&C, and perhaps I'm just the last guy to have learned it the hard way.
My first PP purchased in 2006 or 2007 (when they were pretty new to market) failed in this manner with about 350 hours on it. I took off for a flight and the field breaker tripped and wouldn't reset. I landed and it reset on the ground, so I tried again and it tripped off again very early in the takeoff roll. When I took the alternator off and disassembled it, I found evidence of the rotor rubbing the stator and it was clear that the rear bearing was very loose in the housing. The explanation that improper belt tension causes this is BS. This would impact the pulley end bearing more than the brush end. This is most likely caused by the bearing bore being machined too loose.

Skylor
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  #9  
Old 06-24-2021, 10:44 AM
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rv6ejguy rv6ejguy is offline
 
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My experiences mirror Bill's in automotive use. Seen multiple Denso, Bosch and Hitachi alternators go 5000-7500 hours with the bearings still fine. Brushes worn down to nubs in something like 4000 hours on the Denso and Bosch units in high revving applications, 7500 hours on the Hitachi brushes in one of my old cars before they finally hit the springs.

Never seen spun bearings in the housing on any auto alternators while I had my auto repair business or to date.

Not enough press fit would seem to be the cause, which is a lack of QC in the machining process or a wide OD variation on bearings (crappy bearing QC).

Doesn't seem to happen in the auto world any more in my experience with a lot of cars owned and worked on over 4 decades- mostly Japanese and German stuff.

Last time I replaced alternator bearings was back in the '80s on some ancient Delco Corvair units which had the small needle bearings at the rear.
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  #10  
Old 06-24-2021, 11:22 AM
scsmith scsmith is offline
 
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Were you able to identify any process which enlarged the bearing bore in the housing? Did it become enlarged from some kind of deformation of the housing? Was it machined too big to start with? Given the high number of mass-produced alternator housings, that would seem to be a design/set-up problem at the manufacturer. As Bill points out, this bearing may be intended to have some end float? I would think a better design would be to have the bearing tight in the case, and let the steel shaft float in the inner race.

I wonder if the bearing is set in some kind of retention compound that failed? Loctite makes a liquid that is intended to set bearing races.

My PP alternator may provide a useful test case - I run with the belt looser than most would. 670 hrs and still OK - I think.
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