I'm trying to make a lifting mechanism that will support variable weights, similar to this product: <http://www.humanscale.com/products/product.cfm?group=quickstand>

On the quickstand there is a screw that adjusts the spring force between 4 and 11 pounds, which appears to be constant throughout its 17 inch stroke.

What kind of spring are they using? I've searched for a long time but there doesn't seem to be any off-the-shelf solution to accomplish this effect. There are adjustable gas springs, but that's done with gas bleeding which is obviously a one time adjustment.