In addition to all of the very relevant and detailed replies above, I'd like to add the following:
1. You said "...We tend to buy a handful of shafts when we run low". Where do you buy them from? Are they genuine Lear Siegler parts or PMA items? Do they come with the correct paper-work?
2. Do you use external power (GPU) for engine start, particularly the first start of the day? If you do and it's not by using a bank of large batteries what is the starting current limiter set to? If the limiter is set too high it will result in excessive 'wrenching' torque when you initially press the starter. GPU's with a 'soft-start' function whereby there is a quick but gradual build-up of starting current when the starter is pressed are ideal.