Issue with synchronized movement

[DedongC]

I have an extendable hook DC model. To make it extend its length without distortion, it seems to have to have it modeled in two pieces, one is a bar and the other is just hook part. My DC seems work, but the movement is not as clean as expected and it shows some gap between the bar and hook part during the course of the extendable hook moving back to it's original position.
Anything wrong with my formula or any other better way to make the two parts move together seamlessly?
Attached is my DC.

Thanks in advance

DedongC

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hook_test.skp

[prokrastinator]

This method seems to run smoothly on my system:

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hook_test V2.skp

[DedongC]

thanks very much for your help! I think it solved the issue with your solution.