How can I power a wheel but let it spin freely when not under power?

I saw the question How can I modify a low cost hobby servo to run 'freely'? but I'm more interested in knowing if there is some sort of gearbox that disengages ( moves to 'neutral' ) when no torque is being applied to it.

Two ideas that come to mind are:

  1. A drive gear on a spring loaded arm with a nominal amount of resistance before engaging. Perhaps when under power it would first use power to move in one direction, then it would engage with another gear, but without power the spring would return it to a non-engaged position

  2. A centrifugal clutch - although I'd like something that works at low RPMs as well

The idea is to create a small bot that can move up and down a track, but if someone interacts with it when not under power it will just roll and won't damage the gearbox.

  • $\begingroup$ Gas ("nitro") powered radio-controlled cars need something like this too. They usually use a centrifugal clutch mounted directly on the engine's crankshaft. The clutch bell then has the first drive gear mounted on it. This means higher RPMs for the clutch than if it were downstream, which is probably an advantage for the clutch to work well...it should be the same case with an electric motor. $\endgroup$ Commented Dec 23, 2014 at 3:52
  • $\begingroup$ Thanks. I have also found some Electro-Magnetic Clutches via eBay that may work for this purpose. They look to be used in laser copiers. But I think there must be other solutions, too $\endgroup$
    – cwd
    Commented Dec 23, 2014 at 3:55
  • $\begingroup$ Another idea is a regular slipper clutch (also used in gas R/C), to limit the torque that can feed back through the gearbox if someone pushes the robot. $\endgroup$ Commented Dec 23, 2014 at 4:28
  • $\begingroup$ One way bearing, used often for helicopter drive gears so the rotor can spin/coast when power is not applied. $\endgroup$
    – Spiked3
    Commented Dec 23, 2014 at 5:50
  • $\begingroup$ Thanks - both of those are good ideas. Post as an answer and I can upvote / mark as correct. Still open to other options if they exist. $\endgroup$
    – cwd
    Commented Dec 23, 2014 at 7:59

1 Answer 1


You can use a magnetic clutch. When the strain on the horn passes a particular mark, it overpowers the pairs of magnets holding the clutch plates causing it to slip without damaging the gears. You can easily make them, and here is a company selling them http://robosavvy.com/store/dagu-magnetic-servo-clutches-for-miniature-servos.html

If you want to use an off the shelf one there are clutches designed for r/c cars that you maybe able to use. There is a description of one here http://nitrorc.com/articles/clutch.html and here is a company that sells them http://www.bukupower.com/clutch.aspx .These are designed for use with gas vehicles so some modification to your design maybe required.

It may also not be too hard to make a gear train disengagement mechanism you can control using a servo.

  • $\begingroup$ thanks! still open to other answers but this is certainly a good option $\endgroup$
    – cwd
    Commented Mar 24, 2015 at 19:12

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