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I'm trying to rotate a lever link to gently "offload" a load using a servo. Here's a drawing:

enter image description here

Here, the red is a lever, yellow pivot and black is the load. To offload, the lever needs to be turned 90 degrees. I know I could simply put a servo where the yellow pivot is, but the problem is that the servo may not have enough torque to hold the load. The load is typically around 1-2kg and the distance from the pivot is around 15cm. The servo has a stall torque of around 10kg-cm.

Is it a good idea to drive a cable on the right hand side of the lever with a servo? Or is there a link setup that allows such a small servo to drive this lever?

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  • $\begingroup$ is there any way that you could counterbalance the load? $\endgroup$
    – jsotola
    Jan 22, 2020 at 7:04
  • $\begingroup$ @jsotola You mean counter balance the load and have the servo at the pivot? I'm trying to minimise the weight in the lever if I can. $\endgroup$
    – John M.
    Jan 22, 2020 at 7:38

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From your numbers, I believe you need something that can support at least ~30kg-cm torque at the load, unless you make other changes. A few options:

  • Counterweight - As @jsotla commented, a counterbalance might be added to the opposite side of the load on the lever. If the servo attachment point or lever can not support that extra weight though, then perhaps it could make sense to use a remote cable or lever-activation mechanism to rotate the lever from the servo. The servo would operate the cable/lever-activation mechanism to rotate the lever around the lever attachment point/pivot. This is partly dependent on how much rotation your servo can support, if you need to be able to apply force in both rotation directions, etc.

  • Gear Reduction - Another option might be to use a gear mechanism, such that extra rotation in the servo is translated into additional torque. As one example, ServoCity makes interesting gear boxes for RC type servos. Or, a custom worm gear based solution might be particularly useful due to the special properties of worm gears ...depending on your project details.

  • Pulleys - Similar to gears, a pulley system would also work. Again, depends on the rotation capabilities of your servo, and this would likely be more complex than other solutions.

In summary, you will need to find some way of offsetting the load weight, or else transforming additional movement from your servo into torque.

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  • $\begingroup$ Thanks for laying out the options. As for counterweights, if a 0.5kg is placed 10cm from the pivot on the opposite side, does that mean the servo will need a stall torque of 25kg.cm or higher? $\endgroup$
    – John M.
    Jan 29, 2020 at 5:36
  • $\begingroup$ Also, how about a torsional spring? $\endgroup$
    – John M.
    Jan 29, 2020 at 6:49

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