Banebots 42mm gearbox: Recommendations for use

[Joe Johnson]

Originally Posted by ZZII 527

Bad news: our tempered (HRC 49.5) 42mm carrier failed in the gearbox by fast fracture. More specifically, three of the pins fractured just below the plane of the plate. This occurred at almost no load.

To confirm that it was not just the pins (and to let out frustration) we subjected the carrier itself to the "hit it hard with a hammer" test and sure enough it fractured as well (picture attached).

Conclusion: It was too brittle, by far, at this tempering. I can try it again with a higher temperature temper and longer time in the oven to get it a little softer and less brittle, but I will need to get my hands on more and I don't think I want to buy whole gearboxes. I'm now less optimistic about the tempering solution since it seemed to fracture so easily, but there may still be hope.

You are beautiful. I am sorry about your failure but you give such great details that I love to read about it...

...I think you should dial down your targets a bit.

#1. Are you sure you need the extra strength? If you can make due with 350in-lbs I would highly recommend that you just leave them alone. Note that the 350in-lbs that I am predicting is not a single load failure it will take 100's of these cycles before the gearbox will fail to function.

#2 If you need that extra bit of safety margin, I recommend that the hardness of the shaft is a good point to start. Unless you were going to harden both, you will just push your failure to the shaft. So, I would say, push out the pins, harden and then temper back to RC 40-41. I think you will find that value a good compromise between brittle and ductile failure.

Good luck.

Joe J.