what I did:
I created a hammer, inserted two keyframes to rotate it a bit, set the curve to linear and extrapolated it linearly. Than I duplicated the hammer, the result was two hammers that spins synchronous.
What I want to achieve:
I would like the second hammer to spin with only half the speed or I would like to create a few hammers and each one spins some degrees ahead of the other. But I only have one curve in the graph editor for both.