Should Set Screw Press on Key or Shaft?

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rmac
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Should Set Screw Press on Key or Shaft?

Post by rmac »

Today I noticed that the drive pulley on my lathe motor had shifted axially. The pulley and the shaft are slotted to accept a square key, and the pulley has a set screw hole that's aligned with the key slot.

Depending on where the key is positioned, the set screw could press on the key itself, or it could press directly on the shaft at the bottom of the key slot. Both the key and the shaft have marks on them from the set screw, so apparently it has been set up both ways in the past.

Which is correct?

-- Russell Mac
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Harold_V
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Re: Should Set Screw Press on Key or Shaft?

Post by Harold_V »

I'd be inclined to suggest that the ultimate is to use a key and let the set screw bear on the top. My rationale for that opinion is based on the notion that a heavy load would most likely overwhelm a setscrew that penetrates the key slot, damaging the slot in which it resides. A key would not allow the movement.

H
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John Hasler
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Re: Should Set Screw Press on Key or Shaft?

Post by John Hasler »

Do you mean that the key is present but short and the pulley can be positioned so that the setscrew misses it and goes into the slot instead? That will result in the key being slightly loose in the slots. The setscrew will have to carry all the load with the result Harold describes. It will inevitiably work loose, allowing axial movement. The setscrew's job is lock the key into position. The key's job is to transfer the torque.

Install a longer key.
curtis cutter
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Re: Should Set Screw Press on Key or Shaft?

Post by curtis cutter »

Are you using a key that is as long as the slot?
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Re: Should Set Screw Press on Key or Shaft?

Post by shootnride »

The key and the keyway in the shaft should be as long as the keyway in the pulley and the set screw should bear on the key.

Ted
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rmac
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Re: Should Set Screw Press on Key or Shaft?

Post by rmac »

Thanks to all for the replies. It's good that everyone agrees!
John Hasler wrote: Mon Apr 19, 2021 7:32 am Do you mean that the key is present but short and the pulley can be positioned so that the setscrew misses it and goes into the slot instead?
Yes, except it's the key that can be moved around. (The motor pulley needs to line up with the pully it's driving.)

Regardless, it sounds like a longer key is in order, with the set screw holding it in place. I can do that.

Thanks again.

-- Russell Mac
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liveaboard
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Re: Should Set Screw Press on Key or Shaft?

Post by liveaboard »

I would have the setscrew bear on the key when possible, to avoid damage to the shaft.
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rmac
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Epilog: Should Set Screw Press on Key or Shaft?

Post by rmac »

Well, well. This problem was more interesting than it seemed at first. As the picture shows, the set screw was farther away from the end of the motor shaft than the end of the slot in the shaft for the key. So there was no way for the set screw to bear on the key. I'm guessing the lathe's history must include a motor replacement somewhere along the way.

Anyway, I made it all better by just turning the pulley around so the set screw is near the end of the shaft.

-- Russell Mac

pulley_problem.png
pete
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Re: Should Set Screw Press on Key or Shaft?

Post by pete »

That will throw your speeds completely off and I can't see how the belt would fit any more.
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Harold_V
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Re: Should Set Screw Press on Key or Shaft?

Post by Harold_V »

Shouldn't be a problem if the opposing pulley is also inverted.

H
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liveaboard
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Re: Should Set Screw Press on Key or Shaft?

Post by liveaboard »

I assume Rmac is only using one of the pulleys.
Otherwise he would have used a different solution.
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rmac
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Re: Should Set Screw Press on Key or Shaft?

Post by rmac »

What Harold said. I flipped both pulleys.

-- Russell Mac
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