A

albertaflyer

Active Member
What alternator comes from vans with an o360 purchase. Just got back from a flight stopped to fuel up, went to restart engine and was surprised to hear a squealing sound (like a belt slipping). Could tell starter was struggling to start it. Roll it to hangar take cowling off only to find alternator bearings on both ends done. At least I was on the ground! How tight should the belt be? It's kind of hard to check because its so short. It only has 220 hrs on it.
 
I have info at the hangar but Lycoming has a spec for new and in-service belts. You put a torque wrench on the alternator while holding the prop; there is a minimum torque seting where the belt should not slip, and a max torque setting where the belt should slip.
 
Gorilla?

I think if the belt was so tight that the bearings failed after 220 hours, then you had one **** of a gorilla putting it together; he is now qualified to be a automotive mechanic.;) I question if the bearing failure was caused by an over tightened belt or some other reason, like fine dust from the build, insufficient grease, etc.
 
I agree, but I guess all it takes is one to start going before it knocks the other out. I just did my annual and checked the belt tension and it seemed good to me. Ill have to add checking the alternator to my list. I did research this farther and found lycoming recommends 11 to 13 ft. Lbs with a torque wrench on the alternator nut and trying to make it slip on belt. Good info to know... I learn something new everyday. Thanks for the input.
 
This is interesting. We seem to have some strange alternator failures, with low service time. I seriously doubt that being in an experimental aircraft is any more demanding than some of our automotive applications. Brush, regulators, bearings, etc, cant be under any more load than their automotive cousins.
Wonder if someone can really investigate this, and come up with a solution. Just sayin--
Tom
 
Lycoming.. http://djholman.csde.washington.edu/Yankee/Lycoming/LycomingLimitsTorque.pdf


I used... but the below is just my opinion and informational purposes only..

ALTERNATOR BELT TENSIONING (Two Methods)

Slip Torque Method:
This method consists of installing a torque wrench on the pulley-retaining nut and measuring the amount of torque required to make the pulley slip. Torque the wrench in a clockwise direction, as viewed from the pulley end and adjust belt tension accordingly.

Note: Too much tension can cause excessive pressure on alternator bearings. Too little tension can cause the belt to slip thus not turn proper RPM.

BELT WIDTH - SLIP TORQUE NEW BELT - SLIP TORQUE USED BELT
3/8 INCH ---- 11 TO 13 Ft. Lbs. ---------- 7 to 9 Ft. Lbs.
½ INCH ------ 13 TO 15 Ft. Lbs. ---------- 9 to 11 FT. Lbs.

(OR)

Belt Tension Method: This method requires a little more effort than adjusting the belt on your old 57 Chevy. You will need a Borrough’s Belt Tension Meter #BT-33-73F or equivalent. Install the belt tension meter on the alternator belt at mid point of the longest unsupported section of the belt and adjust the alternator belt tension to obtain the specification listed below. The specification is for a used belt or a new belt after 1 hour of operation.

BELT TENSION
75 Lbs.

Note: Too much tension can cause excessive pressure on alternator bearings. Too little tension can cause the belt to slip thus not turn proper RPM.
 
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Just Spitballin Here

TS Flightlines said:
This is interesting. We seem to have some strange alternator failures, with low service time. I seriously doubt that being in an experimental aircraft is any more demanding than some of our automotive applications. Brush, regulators, bearings, etc, cant be under any more load than their automotive cousins.
Wonder if someone can really investigate this, and come up with a solution. Just sayin--
Tom
Click to expand...

When you consider the size of the prop hub to the alternator pulley, I suspect we're turning at much higher RPMs than in an automobile application. If that's the case, these weren't really designed with planes in mind. I'd probably also want to check alignment in addition to load - that might also be a killer on the bearings. On the other hand, I'm just guessing at this.
Terry, CFI
RV9A N323TP
 
Both bearings failing (apparently) simultaneously is fishy - if I found the right bearings, they should carry 900 lbs ok. Maybe inferior bearings, or highly preloaded from end to end. Regardless of speculation, disassembly of the bearings and inspection will be a good start. Please post your findings. Speculation is endless, facts are exciting!
 
I have a couple of alternator troubleshooting rules of thumb - if it's a bearing failure, I look at the mount, alignment, belt tension, etc. If the windings or internal reg are failing, I look first at cooling.

For double bearing failures, I'd take a look at the mount - or just bad quality on the original build/rebuild of the alternator.
 
BillL said:
Both bearings failing (apparently) simultaneously is fishy - if I found the right bearings, they should carry 900 lbs ok. Maybe inferior bearings, or highly preloaded from end to end. Regardless of speculation, disassembly of the bearings and inspection will be a good start. Please post your findings. Speculation is endless, facts are exciting!
Click to expand...
Hard to imagine anything short of failed (old) or dry bearings from original assembly that would cause simultaneous failure. But these wouldn't have made 200+ hrs. Maybe extreme heat event? Extreme thrust load (not sure how you'd get that much on a belt driven device). Think this thing might have been failing for some time and just now locked up? Ever check (spin and feel the bearings) at annual??
 
Low Pass said:
Ever check (spin and feel the bearings) at annual??
Click to expand...

That is a check I had overlooked until last year. I had the bearings go bad in an alternator but fortunately I caught it before the alternator failed (had pulled the alternator to check the diodes). But it reminded me that the belt should be dropped off the pulley and it spun by hand to check for noisy or loose bearings. It's on my condition inspection list now.
 
I have copied this from the Aeroelectric server list.
It is a recent comment from Bob Nuckolls.
It is food for thought.


Yeah. Some years back we discovered root cause of
poor bearing life on starter-generators. It seems
that the steel shaft down through the center, while
thought to be in an area of "zero net flux" was
"excited" with microvolts of induced EMF shorted out
by micro-ohms of starter frame through the bearings
at each end. In retrospect, I'm not sure that the
frame of the starter-gen didn't see similar inducements.

A mysterious stress on bearing life went away with
incorporation of ceramic bearings or bearing
liners. Just because some stimulus is 'tiny' does
not necessarily negate its effects.


Bob . . .
 
Hey guys, sorry for the slow response. There was no rebuilding the alternator. The bearings actually spun in the housing. I did re-route the blast tube to blow air in front of the alternator. I'm going to keep an eye on this. I did adjust belt tension to the 12-13 lbs. recommended earlier. To me it seems about as tight as what it was when they failed. We will see. Thanks for all the informative responses.
 
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