# Gear Design criteria for Momentary Overload

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QUESTION
I have to size a gear that is sufficient for momentary overload, for example a max stall torque.

According to AGMA, the gear material yield stress (Say) should be used to determine the max allowable stress instead of the fatigue bending strength (Sat).

1. By how much is one allowed to exceed the yield stress of the gear material when sizing for stall torque?

2)Is it an absolute must the applied stress is less that the yield stress in gears or is there some leeway?

3)Is there a way of calculating the number of cycles we may get out of a gear that is exposed to greater stresses than yield (Plasticity)? I haven’t found anything in AGMA.
Surely since the applied stress is much localised at the root fillet that it may not cause global deformation.

I am specifically referring to the differential gears in a standard open diff. I currently need to size these for a stall torque. I will aim to stay below yield since I believe I have some design scope.
Can anyone source the RMC approach? Is this mentioned in AGMA? I don’t think I want to use this unless I can reference it against some source etc.

Also as a side note, I was trying to ‘physically’ see how a standard open differential ‘splits’ the torque to each half shaft. Does anyone understand this or is it counter intuitive and perhaps math is required to see why it splits?
I mean it appears that all standard open diff’s still splits the torque 50:50 regardless of the pinion / gear ratio / geometry inside the diff.

REPLIES

tbuelna
If you want to design for an acceptable stall torque load with your (straight bevel?) diff gearset, then you should use a simple static Hertzian contact approach, since there will be no EHL oil film effects to consider.

Roark’s will give you the necessary equations to establish the brinnell (elastic) limits of your gear tooth contacts, as long as you know the relative gear tooth profile’s radius of curvature, face widths, contact ratio, and the metallurgical properties of the gear tooth contact surfaces.

A brinnell failure is generally considered a loading condition that produces a permanent strain of >0.0001D or more in the contact surface.