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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.

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    $\begingroup$ it's probably adjusting the friction brake $\endgroup$ – ratchet freak Mar 23 '15 at 22:54
  • $\begingroup$ 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/… $\endgroup$ – Jack000 Mar 24 '15 at 1:49
  • $\begingroup$ 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). $\endgroup$ – Wasabi Jan 27 '16 at 10:03
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Took something like that apart today. It uses cables and pulleys to support the monitor and a tension spring to oppose the monitors weight. A screw at the other end of the spring can increase tension as required to over come the monitors weightand friction in the mechanism holds the monitor in place at the desired height.

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I would guess that it's using a variant of an anglepoise lamp type mechanism, which relies on a zero free-length spring and some geometry. By adjusting the offset of the spring from the lever arm different loads can be balanced. The clever part will be in making the whole assembly compact and invisible to the user.

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The only "spring" i know with constant force is the magspring, used in several industrial applications to overcome gravity.

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