# What kind of spring exerts a constant force but is also adjustable?

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.

• it's probably adjusting the friction brake – ratchet freak Mar 23 '15 at 22:54
• hm.. it's possible, but the install instructions makes it pretty clear that spring tension is being adjusted to balance the weight (page 8): humanscale.com/UserFiles/File/… – Jack000 Mar 24 '15 at 1:49
• This clearly doesn't have much to do with the actual question at hand, but it's worth noting that springs don't exert a constant force, since $F=Kx$, where $K$ is the spring's stiffness and $x$ is the displacement from the spring's point of equilibrium. A mechanism which exerts truly constant force is a simple weight (duh) or a vacuum cylinder (such as the ARED used for exercising in space, as described in this video). – Wasabi Jan 27 '16 at 10:03