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Alternator Shear Coupling Failures

GreggC

I'm New Here
Within the last ~250 hours I've had two alternator shear couplings fail on my RV-6. The first failed after ~248 hours of flying, the second failed at less than two. The alternator is a Plane Power FS1-14B (30A, internally regulated, bolted to the accessory pad) and the airplane is wired using Bob Nuckoll's Z-11 diagram in the Aero Electric connection. This is the only alternator on the plane.

The airplane is a full-up Dynon system (10" Skyview, ADS-B, transponder, wx rcvr), a Garmin GTR200 radio, landing light, strobes, nav light. None of the lights were turned on between repairing the first failure and the second one.

The going failure theory is an intermittent connection in the field circuit that induces a heavy alternator load by turning it on and off but there isn't anything in the 1Hz data traces from the Dynon that indicates this is the case. Nor is there evidence of any loose connections or nicked wires in the field circuit.

The image shows the volt and amp trace for the last 1.7 hour flight. The failure occurred around count #6184. The noise on the front end is startup and taxi. The blip around 2291 was plugging in an iPad.

Any ideas on the failure mode?
 

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Sounds Like

Sounds like a bad bearing. I can't imagine an electrical load will be sufficient to break a mechanical shear coupling. I am using a B&C pad alternator, but not flying yet. I would also check the accessory gears for damage. If it was a bearing failure/jam, you could have shock loaded the accessory gears and caused damage.
 
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Serious Stuff

A shear coupler failure is very serious. Something needs to be done differently besides just changing the coupler. I would be inclined to change alternator's manufacturer unless I found a smoking gun that wasnt an alternator failure. Serious stuff
 
Within the last ~250 hours I've had two alternator shear couplings fail on my RV-6. The first failed after ~248 hours of flying, the second failed at less than two. The alternator is a Plane Power FS1-14B (30A, internally regulated, bolted to the accessory pad) and the airplane is wired using Bob Nuckoll's Z-11 diagram in the Aero Electric connection. This is the only alternator on the plane.

The airplane is a full-up Dynon system (10" Skyview, ADS-B, transponder, wx rcvr), a Garmin GTR200 radio, landing light, strobes, nav light. None of the lights were turned on between repairing the first failure and the second one.

The going failure theory is an intermittent connection in the field circuit that induces a heavy alternator load by turning it on and off but there isn't anything in the 1Hz data traces from the Dynon that indicates this is the case. Nor is there evidence of any loose connections or nicked wires in the field circuit.

The image shows the volt and amp trace for the last 1.7 hour flight. The failure occurred around count #6184. The noise on the front end is startup and taxi. The blip around 2291 was plugging in an iPad.

Any ideas on the failure mode?

Which engine and prop and ignition system do you have?
And how long (hours & years) have you had a vacuum pump flange-mounted alternator?
 
What sheer coupling are you using? Try the ones from B&C. I think they are made a little differently than the typical vaccum pump ones.

I assume you cleaned out the drive assembly from the first failure so there are not any little parts left over that could be creating some drag?

Vic
 
Not serious at all.

A shear coupler failure is very serious. Something needs to be done differently besides just changing the coupler. I would be inclined to change alternator's manufacturer unless I found a smoking gun that wasnt an alternator failure. Serious stuff

What specifically makes a sheared coupling "very serious?"

Is it annoying, absolutely. VERY SERIOUS, not in the least bit.

The reason for the coupling is that it performs as the weak link by preventing any damage to the engine and accessory case.

The worst possible outcome is loss of alternator which in my case gives me about an hour of useful battery to land. Again, very annoying but not serious at all. If you arent operating in vfr and are IFR, you will or should have redundancy so no issues there either.

Im not saying that the problem shouldnt get sorted out, buy saying its VERY SERIOUS is a bit of a stretch and overly dramatic.
 
Thanks for the responses guys.

The engine is a lycoming O-320, Catto prop, bendix mags, wired iaw Bob Nuckolls diagram Z-11 less the voltage regulator, which is contained in the Plane Power alternator. 630 hrs on the Hobbs, 560 Tach time since major overhaul, which is when I went with this configuration.

I've tried the Tempest and RAPCO shear couplings from Spruce. Each time I made sure everything was clean and turning freely.

Just had another one fail on the last flight I took. It was the Tempest coupling.

I've looked at data since the last failures and have found noise on the ammeter channel, to include negative readings. The alternator B-lead goes direct to the ammeter shunt. Bad diodes in the alternator maybe? If so, would this put more stress on the coupling?

The chart shows amps, volts, and normalized engine rpm (rpm/150) for the last flight. The failure occurs around 22:20:40.
 

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Try one of mine

You're welcome to try one of my generators, less than half the weight, more compact, same current and I haven't had any shear couplings fail. I have a feature that starts the device slowly over 100 ms as a measure to prevent it from breaking the shear coupling.

Bill
monkworkz.com
 
Update:
Troubleshooting after the last post showed two things. First, the coupling was replaced with the B&C coupling. Second, I found an intermittent connection at the alternator field terminal that plugs into the B&C alternator, which might have accounted for the noisy electrical performance noted above - replaced the alternator field harness/terminal with a new one. The airplane now has 779 hours on the tach and 876 hours on the Hobbs and I've not had any more problems. Hoping this might help some of you in the future with similar issues.
 
Just a thought, borrowed from an alternator issue on a different activity.

Wonder if there may be an internal, torsional vibration factor causing this?

If the B&C is stronger, it may last longer but the TV might be related to pulley ratios ????. Maybe if it happens again, change the smaller pulley diameter slightly?
 
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