Piper Seneca 3 has been destroying alternators. The alternator is hooked to accessory case. It is not destroying the coupler but shearing off the shaft instead. New alternator being tried. Last one lasted 97 hours.

Is this a balance issue within engine?

Bad alternators?

Any other explanations?

Do you have a heavy pulsating electrical load, such as strobes or beacon?
I don't know if it's a coincidence or not, but my C150 ate 2 alternators within 100 hours (snapped the shaft as well). My tail beacon was shorted somewhere (as in - it drew 60 amps every time it flashed). I replaced it with an LED, and have been trouble free since that *knock wood*.

Have you changed the coupler, or at least tested the slippage?

Mikko,

I like your line of thinking. Airplane had problems recently with heater breaker tripping.

Needs to be explored further.

Steve,

I am not certain. I still question why it would need to shear in the first place.

It might be valuable to have both engines balance checked and have the tech do a full spectrum vibration analysis. You can find out the gear ratio for the alternator and corresponding frequency and cross check each engine. If one has a big nasty spike in the data.... Dive deeper.

Tj

About the only thing that can shear the alternator input shaft is improper installation.

They used to ship these with a washer that had to be discarded prior to installation. Depending on who overhauled it, you might still get some sort of shipping spacer or washer that needs to be discarded.

The other thing is improper installation of the drive coupling. It has to be torqued properly or it can cause side load and shearing.

Finally, the mating surface, gasket at the studs and nuts all have to be tight and properly torqued.

Unless the accessory case is cracked, I'd be looking at the installation.

May not apply.... I flew a Cessna 210 .. Maybe a 1978 or so, new plane, it had eaten a dry pump shaft every 100-150 hours. After 3 , I think they found the case ( pad) was machined at a slight angle.

Jeff

Problem solved. Bushing missing on installation.