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I am trying to emulate the resistance of a hand wheel which moves a table via a series of shafts and chains which turn an acme screw. I am switching to a CNC system that will allow the user to keep the old style crank for manual positioning. The crank will instead attach to an encoder which will command the CNC system.

My problem is encoders don't have shaft resistance or enough resistance. What I am looking for is a method to produce shaft resistance that feels sticky. You could quickly accelerate it but you have to put a bit of effort behind the motion and the crank would stop dead and hold its position when let go. I tried searching for such a device but perhaps I am searching for the wrong terms. In my head I picture one or more slotted discs sealed in a housing filled with a very viscous or sticky liquid. The only thing that comes close is a viscous shaft coupling. But they are couplers. I need a through bore or keyway whose housing can be mounted to a frame.

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You're looking for a Rotary Viscous Damper like these - a device that provides viscous friction in rotary movement. Viscous friction is proportional to speed of movement.

See video of such a damper activated by a spring and a practical use - slowly falling lid.

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  • $\begingroup$ I was thinking about these. They're probably a good enough option, but won't necessarily give quite the right feel and won't do much to hold the wheel stationary. An ideal damper would provide a resistance proportional to the speed of rotation. A real machine tool will have a combination dry friction and viscous damping. A key difference is that, in the real case, you have to apply a bit of extra force to break the static fiction and actually get things going. $\endgroup$ – Dan Feb 11 '15 at 4:49
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It sounds like you're looking for either a torque limiter or a rotary damper.

A Torque limiter prevents rotational force from the driving shaft over a certain level affect the driven shaft. Kind of like audio cut off on a microphone. When the driving shaft exceeds this limit it will slip allowing it to move somewhat freely without affecting the driven shaft.

A Rotary dampner works by using paddles inside a closed container of liquid that can only pass liquid through paddles at a limited rate, thereby limiting the maximum velocity of the shaft.

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  • $\begingroup$ Your answer would be stronger if you briefly explained what those two items are along with their links. $\endgroup$ – user16 Feb 11 '15 at 1:27

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