[Joe Johnson]

Are you locking out the slip clutch inside that gearbox?

This is a very bad idea, imho.

I have not done the math, but I am pretty sure you are pushing on a rope on this one.

The gearbox you are discussing has a slip clutch inside of it for a reason: The loads are huge if you allow the full stall torque of the motor ripple through the gearbox.

If you have not locked out the clutches, I am surprised that you are able to damage the gearbox at all.

Basically, you can do the math yourself if you want to.

Figure out the load on the tooth (assume one tooth carries the load).

Put the load at the tip (for safety factor purposes).

Approximate the tooth as a rectangular beam.

The max. stress is then

Max Stress = Mc/I

M = Force on Tooth X Tooth Height

c = (Thickness of Tooth at Base) / 2

I = (1/3) (Face Width) (Thickness of Tooth at Base)^3

Do all your calculation in lbs and inches and you will get a number for stress in psi:

Good steel, unhardened has a yeild stress of 80,000 - 90,000 psi
Aluminum 30,000 psi or so (it can be better, but again, be safe).
Hardened steel can be 100,000 - 120,000 psi or so (harder if really need it, but things get pretty brittle up in those regions).

Good luck.

Joe J.