You can see there some kind of “arm going up and down” and an object which moves with it on the left. The problem is that it doesn’t move accurate with it, it should move as if it is fixed with a screw! I animated it like the arm as well and set-up keyframes at every 25 and 50 frames. You’ll see.
Hope there’s someone who can explain my fault to me. (and a solution as well )
It looks like you’ve tried to keyframe the motion of the thing on the end of the arm by hand, which would be quite painful and tedious to get accurate. But blender has tools to help with this.
The first is parenting - if instead of the animation on the object you select it, then shift-select the arm and press Ctrl+P, then it will automatically follow it. However it will rotate as well, which looking at that scene you don’t want it to do.
So what I’d do is add an Empty at the screw point (an empty is an invisible object used to specify a particular point in space) and parent that to the arm, so it follows the screw hole. Then select the object on the end of the arm, and go into Object buttons (F7). In the Constraints box click Add Constraint -> Copy Location and type in the name of the Empty (probably “Empty”).
The object should then follow the end of the arm, but stay with the same rotation.
There’s nothing more to say - thanks! Seems to be the best and easiest solution, and is very accurate. Now Ill take some time to read about constraints in the blender guide, is there also another chapter that would be useful to read?