Shear Key Failure Surface  
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The figure shows the front view of a gear wheel mounted on a shaft with a shear key. The load applied at the right end of the gear breaks the shear key. The failure surface of the shear key is shown atop the image.
 

Suppose we apply more and more force to the gear until the shear key actually breaks. What would the failure surface in the shear key look like?

 

Finished