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So i want to have a reel (~4cm diameter) that will pull a ~80cm string with a torque of ~8kg-cm (edit: thanks for the clarification). To pull I want to use a gear motor. E.g. 12V DC motor like this: dc motor

The problem is that I want to be able to easily unwind the string with my hands when the motor is not pulling in the string. So I would need some kind of possibility to disengage the motor. The problem is something like a sprag clutch (or one-way bearing) would not work as I pull the string in the opposite direction of the way the motor would spin. I found something like the "freecoaster" designs for bikes, but I don't know how I could build this cheap and easy especially given the small form factor (and limited tooling: mostly a 3d-printer, and normal tools). My other idea would be to try and create some kind of clutch that i would engage using a servo motor, by sliding a clutch like this on the axle using a servo motor. But I am not sure how well that would work or if there is a better alternative.

So what would be your suggestion to create a system that allows me to engage the motor to a free spinning spool when I need it to wind the string.

This motor will only be used to pull in the string once every month or so, so it doesn't need to be able to withstand thousands of cycles. But when it is used, it should work relatively reliable. I also found this question that seems to try and solve a similar problem. But I don't know how well that would work for my usecase. Thank you for your help!

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A common solution to this which may or may not work for you is to use a centrifugal clutch.

centrifugal clutch

Off the shelf options would be too large for your application; and they wouldn't kick in until too high an rpm. But you may be able to 3D print something based on this principal.

If you make the arms very heavy, with light springs it will work at lower rpms. You could also replace the friction element with teeth or a pawl style mechanism so that once it's engaged it will never slip.

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  • $\begingroup$ There is a variant of a centrifugal clutch that is based on flexures which is easily 3D printable. It essentially looks like a big flatened ER collet that has a cross in the middle. So 2 parts. I produce these out of steel with waterjet cutting but they are also easily made in laser, plasma and mill. Also 3ad printing is easy. $\endgroup$
    – joojaa
    Commented Jul 10, 2020 at 9:46
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"torque of ~8kg/cm". Torque is force times distance so it will be 8 kg-cm or 80 N-cm.

Since you are going to switch something to change the mode why not switch the motor into constant torque mode by switching from a voltage source to a current-limited source. Motors tend to run constant speed when fed from a voltage source and tend to run constant torque when fed from a current source. It might be as simple as switching a resistor in series with the motor to keep a little tension on the string during unwind.

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  • $\begingroup$ Thank you for your clarification. But my problem is not how to produce the torque. I mainly added this to give an idea of the forces involved. My problem is how to make the reel freely spinning. Because of the gearing between motor and reel, I think spinning the motor in reverse will be very difficult. Therefore I need to disengage it, while it is not reeling in the string $\endgroup$
    – joz
    Commented Jul 7, 2020 at 9:21
  • $\begingroup$ Your question is misleading then. Please edit it to replace "motor" with "gear motor" and clarify the problem. To solve your problem you require an electro-magnetic clutch. There are some which use a magnetic powder to lock the input and output electromagnetically. $\endgroup$
    – Transistor
    Commented Jul 7, 2020 at 9:34
  • $\begingroup$ An electromagnetic clutch seems to be the "nicest" solution, but its also relatively expensive (at least thats what I found even when importing from china). Is there maybe a more fitting mechanical solution? $\endgroup$
    – joz
    Commented Jul 7, 2020 at 12:05

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