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I am using servo and stepper motors for the first time, and have been searching for a solution to this basic problem:

I want to drive a shaft with a motor, whereby this shaft also has a ‘neutral’ state that can be switched on/off with a microcontroller. In this neutral state the shaft should spin freely, without any friction or resting torque from the motor.

This is any extremely low RPM application, so a precisely balanced solution is not needed, however a low weight/volume solution is preferable. I am currently using a small servo (35 x 16.9 x 32mm) with a max torque of 4.2kg/cm.

The simplest solution I could think of was to build a primitive clutch, whereby one or more solenoids bring a plate that is attached to the motor into contact with a plate that is attached to the shaft. If there is an existing off-the-shelf solution or alternative mechanism that anyone can suggest I would be very grateful to hear your thoughts.

George Ellis
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    I think, for this situation, I'd be more tempted to make a primitive [dog clutch](https://www.mathworks.com/help/physmod/sdl/ref/dogclutch.html?requestedDomain=www.mathworks.com), rather than a primitive plate clutch, like you're describing. Releases just as well, but gives a positive lock when engaging, and requires less "holding force" to maintain torque through it. – Robherc KV5ROB Apr 25 '17 at 01:15
  • A hysteresis clutch works well. http://www.magtrol.com/brakesandclutches/hysteresis_clutches.html – Trevor_G Apr 25 '17 at 02:00
  • Thanks @RobhercKV5ROB for those great suggestions and links; will check them out! – George Ellis Apr 25 '17 at 05:48
  • Thanks @Trevor for those great suggestions and links; will check them out! – George Ellis Apr 25 '17 at 05:49

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I found this website that sells small electric clutches. Industrial Cluctch

Dan_LXI
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  • Thanks very much for that link! Your suggested 'Electric clutch' description is a great search term. I also found similar items described as magnetic clutches and [link](http://www.rrfisher.com.au/products/transmission-equipment/electromagnetic-clutches-&-brakes/electro-magnetic-actuated-type-clutches-and-brakes/) 'electromagnetic clutches' Thanks again! – George Ellis Apr 25 '17 at 05:46
  • Glad I could help. – Dan_LXI Apr 26 '17 at 03:15