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I'm working on a project that uses a strong stepper motor to power a wheel that needs to be able to "coast" freely at times. The motor is too stiff when not engaged, and along with my gearing ratio, makes it impossible to get the free-spinning coast** that I want from the wheel when the motor is not powered. I only need the wheel to turn in a single direction, so I've been playing around with a design (see image) where the motor slides just a bit to engage via a small set of gear tracks to pull itself into the other gear. I was thinking that I would simply just reverse direction when I wanted to pull the motor away to enable free-spinning. Unfortunately, it doesn't work so well. The gear does engage, but the little bit of play it has when it slides causes it to make contact with the very last tooth of the tracks at the same time. This causes it to make a grinding noise, and certainly isn't efficient.

Does anyone know how I could get this to do what I want? I've been searching online, but haven't found anything and I am not sure of how to technically describe what I'm trying to do in a few search terms. So, I'd be grateful for any links to similar designs as well.

** Sorry, I forgot to add an important detail that will explain why I need to find a way to disengage the motor... The motor will turn the wheel forward, but when disengaged, the wheel needs to be able to turn freely in either direction.

image

So, it's two days later, and this is what I'm going to try next:

image

It consists of:

A drive gear (in red) that turns counterclockwise, with a one-way clutch bearing mounted to it. The clutch bearing turns freely clockwise.

There is an arm with an intermediate gear that will make contact with the large gear when engaged.

There is a spring attached to the arm. The spring pulls the arm down, to engage the gear. The important thing is that the spring is NOT strong enough to overcome the resistance of the idle stepper motor. So the arm will not move unless assisted by the stepper motor.

So here's how I hope this will work, assuming we start with it not engaged:

The stepper motor turns on, counterclockwise. The movement, in addition to the spring, engages the intermediate gear. The clutch bearing prevents the arm from dis-engaging the intermediate gear, yet allows the stepper motor to turn the gears freely.

To disengage, we simply reverse direction of the stepper motor just a few degrees. The clutch bearing will lock, pulling the arm away.

I have yet to prototype this, as my 3D printer is down for a few days. But I would be interested in feedback and whether anyone thinks this will work.

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  • $\begingroup$ you want a one-way clutch or a freewheel? $\endgroup$
    – ratchet freak
    May 26, 2015 at 11:49
  • $\begingroup$ Did you ever build this. Do you have the 3d design. $\endgroup$
    – Tommie Jones
    Apr 27, 2019 at 13:13

5 Answers 5

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You can drive a lightly resisted gear on a swivel that rotates on the same axis as the stepper axle. Similar to a sun&planet configuration but the planet can rotate around its own axle.

When the stepper is turning it will turn the planet into the wheel where the gear on the end meshes with it. To disengage you turn the stepper in the other direction for a bit to clear the planet off the wheel.

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  • $\begingroup$ That's exactly the approach I decided to take, except in order to pull it off, it will require a one-way bearing and a spring tensioner. I will not be able to try it out until I get my 3-D printer working again. $\endgroup$
    – nonlinearmind
    May 28, 2015 at 18:25
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This solution seems a bit complicated. Since you say it only has to go in one direction, have you considered a ratchet mechanism as an alternative, such as those used on the pedals of a bike? This would only kick in if the motor speed exceeded that of the gear and would otherwise just coast along.

I'm not 100% clear on your idea, but perhaps it would work better if it was friction rather than contact based (e.g. made contact with a rubber slider) and was designed to work only as long as there would be no contact, having no impact when it would be engaged.

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  • $\begingroup$ Sorry, I forgot to add an important detail that will explain why I need to find a way to disengage the motor... The motor will turn the wheel forward, but when disengaged, the wheel needs to be able to turn freely in either direction. $\endgroup$
    – nonlinearmind
    May 26, 2015 at 15:26
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    $\begingroup$ I guess you're best of using an clutch actuated by a servo to achieve connection. I'd leave the additional gears. $\endgroup$
    – BeyondLego
    May 27, 2015 at 12:48
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Stepper motors have significant cogging. If that is getting in the way, then a stepper is probably not appropriate here.

Take a look at using a DC motor with a shaft encoder, or some way of knowing position. Then it's a matter of a servo control loop to drive the DC motor to get the desired position. If you specify a low-cogging motor, the mechanical resistance when left open circuit can be quite small.

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  • $\begingroup$ I need the stepper to move with precise increments and speeds. $\endgroup$
    – nonlinearmind
    May 26, 2015 at 15:29
  • $\begingroup$ @non: So? A DC motor inside a control loop and with the right kind of sensor can do exactly that. $\endgroup$
    – Olin Lathrop
    May 26, 2015 at 17:57
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You need a variable-reluctance stepper motor. They have no permanent magnet and virtually no cogging torque. When unenergized, they will rotate freely in either direction. Available from 24 steps/revolution to 200 steps/revolution.

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You can use a SPRAG type clutch from Altramotion.com

or a backstop that would spin the opposite direction which could act like a engagement gear.

But i still believe you can get a all in one unit from them for a SPRAG clutch that will be useful for you.

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