Is this normal?
No.
You need to loosen the set screws then tighten the nut on top to force the pulley down. then you can re-tighten the set screws.
When done correctly you should not be able to move the acme screw up or down in the bearing.
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Agreed with Mark. In your photo you can not even see the top of the ACME screw from the top of the nut. Loosen the set screw, push the ACME screw up while holding the collar. Tight the set screw. With your allen wrench still in the set screw hole tighten down the nut using the allen and leverage.
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