Round Key Ways

Q

quickcut

H-M Supporter - Gold Member
H-M Supporter Gold Member
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Apr 9, 2014
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More questions !! I just wish I had some answers for a change. In a shaft key way the slot is a "square trough". Now because of the rapid change of section there is a stress concentration in the corners. If we machined the slot with a ball-nose cutter would that not alleviate the stress concentration in the corner? Thanks
 
I won't recommend a round key, as they wear rather fast in my experience in moderate load applications. For light loads however, they should be just fine.

If you're worried about cracking, anneal the shaft entirely or use a endmill with a corner radius and bevel the edges of the key. The key seat will still be rounded, but still partially square and leaving more material for the key to bear against.
 
quickcut said:
More questions !! I just wish I had some answers for a change. In a shaft key way the slot is a "square trough". Now because of the rapid change of section there is a stress concentration in the corners. If we machined the slot with a ball-nose cutter would that not alleviate the stress concentration in the corner? Thanks
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Yes. On the other hand, the round key would tend to "roll" out of the keyway. It would tend to force the hub and shaft apart when a load was applied, and would bear only on one edge of each slot.

I have stuff where the key is a setscrew in a hole drilled and tapped along the line between shaft and hub.
 
Are you sure we are not talking about a "Dutchman" here and not a key?

"Billy G"
 
sounds like he is referring to a square key but relieving the sharp corners at the bottom of the keyway, my interpretation of the question
 
A "Dutchman" is a pin put in a drilled hole to keep the part located. The hole it drilled after the parts are mated. Sometimes the hole is threaded. The hole is drilled between both parts.

"Billy G"
 
What is the application? For low torque stuff like hand wheels, you can ignore the stress riser.
If there will be torque on it, just use a keyway cutter.

Round keys are not a good idea if there will be torque on it. Like already stated, they tend to roll up and make havoc. I have seen gears split from them.
 
Thank you for the quick replies. The application is a speed reducer for about 100 hp motor for a generator. A 100mm (wide ) timing pulley has been selected and I was looking at ways to "key" it to the shaft. I think I will just buy a broaching tool. Round keys are out it looks like and I did not even think about the rolling motion. Thanks again
 
The term “Dutchman” in the woodworking trade is referring to a patch that is made to hide a problem/hole/defect/knot on the wood surface. In the metal world there is the square stock, woodruff, scotch key, set screw and a taper pin at 90° to rotation. These are all used to key/lock a shaft assembly…Good Luck, Dave.
 
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