Help needed: Trying to replace Z-motor Pulley

On my last carve,
and a couple of times before,
the pulley on the top of the Z-motor (it’s a NEMA 23) has come loose,
allowing the motor shaft to spin while the belt stays more less in one place.

this is bad as you lose Z and start to cut at incorrect depths… like this:



so that sucked.

When this last happened I observed that one of the two nuts holding the pulley in place on the motor shaft was spinning freely when I tried to tighten it down: it would not ‘lock’ when completely tightened and I inferred from this that the threads inside the pulley were stripped and the pulley would need to be replaced.

so I bought a new pulley and today I am today trying to place it on the machine.
I have loosened the Z-MOTOR so I can rock the belt off of it.
I removed the belt carefully, so as not to score it.
I then took out the little nuts holding the pulley in place.
Finally, I reach over to pluck the old pulley off of the motor shaft so I could replace it with a new one.

Because nothing is ever simple, you can guess what happened next:
now the pulley is frozen on the shaft and no matter what I do I cannot get it off.

Have tried a variety of things involving leverage and force in the right directions,
but cannot get it off.
scared to force too much as I may damage the motor, shaft, etc and with them my hopes and dreams.
I have managed to score the old pulley and knock the top lip off of it, so I guess I am committed at this point to completing the activity but not entirely sure how?

Also trying with this new information to form a new theory of why the pulley was coming loose.
Wondering if the motor was heating the shaft enough (it was > 5 hours of continuous use) for the different materials to expand differentially and cause the pulley to slide free?

if true, would suggest maybe I should get a torch and heat the pulley (?) to get it off, but wonder if driving heat down the shaft would damage the motor. Also worried about creating the smell of melted wiring.

Any thoughts and advice on this will be gratefully considered.

Happy Independence Day

The pulleys are aluminum, can you just cut them off with some small snips?

I had mine get stuck like that. I heated the pulley with a propane torche for a few seconds and it came right off.

1 Like

Thanks guys!

Before you put the new pulley on be sure your motor shaft has a flat spot for the set screw to seat against.

I had to use my dremel to grind a flat on the Nema 17. The nema 23 had a flat spot already.

funny you should mention that…

so the heat thing worked ok. Thanks for that.

the shaft seems pretty torn up:


I think when I put the last one on I FAILED to make sure the set screw was seated on the flat part of the shaft, so it was on round instead and at some point when the shaft heated it lost grip and allowed spin, resulting in some scoring.

I assume there is no way to replace the shaft on a motor with a clean one, huh?

Ouch. The motor shaft is not replaceable,

But you should be sand the shaft smooth and then grid a new flat spot. Then position the pulley either above or below the scored part.

When you grid a new flat be sure it is a large enough area and is at least as long as half the shaft

It is a NEMA 23: it has a flat spot and that seems untouched;
it is rest of the shaft that is scored.

I will sand smooth the round and mount the set screw on the flat
and we’ll see what happens…

It should be fine. Remember you can replace that tiny grub screw with a socket head M3, it is much easier to tighten.

You should also put some blue Loctite on the screw before you tighten it.

1 Like

I did not know that.
Off to home depot I go…

aaaaaand now it looks like this:


had to literally hammer the new pulley onto the drive shaft to get it down where it needed to be.
lets hope it stays there… those M3 bolts are easier to get on and tighten but if the pulley ever drops or lifts they are going to destroy the drive belt…

Hopefully it is tight enough now that the pulley will stay put. You can also get shorter M3’s if you want, I know that Home Depot does not have the best selection.