Weld a tween on a tween

So, i would like to know how to weld a tween on a tween. I want to make a crane stabilizer system like this:


I have made the arm part to come out and i welded on it the piston that comes down. The piston moves with the arm, but it can’t move downwards (probably because of the weld).

Is there a way to weld the piston to the arm but move it freely when it is coming down?