Counterweight

The first heavy thing I found was a concrete block which just happened to weigh exactly the right amount.  I put a couple of garage door pulleys under my bench top, tied one end of a cable to the block, and tied the other end of the cable to a cross member on the swing arm.  Result:  smooth up/down movement.

 

See also

Balancing Act

To balance the weight of the swing arm, swing box, and router, I decided to hang a counterweight under my bench.

pulleys for counterweight