Need help designing keyway in gear

[Matt Adams]

Originally Posted by sanddrag

Will the oversize (1/8") keyway in the picture below be okay to make? I want to go oversize because I'm worried that a 3/32 key is too small for where this gear is in our box. It needs to be able to handle some torque, maybe somewhere around 13 Newton-meters.

Originally Posted by Max Lobovsky

19 Nm = 9.6 ft-lb
9.6 * 12 = 115 in-lb
115 / ((3/8)/2) = 614 lb of force shearing the key

You need one more piece of information: Length of the gear and hub. This gear has a face width of .50 and hub of .46, for a total of .96 inches.

You will have an overall width of either .125 or .09375.

Steel has a tensile strength of 40,000 lbs / in², and a pure shear strength, (using Mohr's circle), of roughly half that. This is conservative, because most keys will be of a higher grade material with a higher strength.

You also have 614 lbs of shear, correctly calculated earlier.

Area = .96 * .09375 = .09 in²

614 lbs / .09 in² = 6,822 lbs / in²

Giving you a factor of safety around 2.93 with your 3/32" keyway.

If your feel pretty comfortable (or even within 100% of) the expected torque, I would say that your 3/32 key should be acceptable.

Matt