Disengaging or allowing a DC motor to rotate freely

Question

I have a two DC motors with smd reel mounted on the 1st one and an empty reel on 2nd one. What I want to happen is that my operation will composed of two halves: 1st half: The 1st DC motor is off or "disenganged". What I mean is, the 2nd DC motor will be pulling the SMD reel off the 1st DC motor, but only the 2nd DC motor is driven. I just want to 1st DC motor to freely rotate as the 2nd motor pulls the reel off it.

2nd half: The 2nd DC motor now contains the components. I want the "roll back" the SMD components back to original reel. I will now drive the 1st motor and I want to turn off the 2nd motor or "disengage" it such that it will be free to move.

I currently have the part: HG16-240-AB-00 and it seems to offer so much resistance when I try to manually rotate it while it's off. This motor is DC brushless geared (1:240 ratio).

1. Can anyone point me to what type of DC motor should I be searching given this requirement? Brushless? Brushed with less gear ratio? Stepper?
2. Is the reason my motor so hard to turn while it's off is because off high gear ratio?

Answer 1

For a DC motor that is easy to turn when unpowered: - you probably want a universal motor, it is like a DC motor but it uses electromagnets instead of permanent magnets. They will run on DC or AC.

But you might get a better result if you instead use a rotation detecting mechanical gearbox, you could then have a single motor that would drive one reel when run forwards and the other reel when run backwards.

Answer 2

This is more of a mechanical question than electronics one.

That motor is hard to turn by hand as you are trying to spin the internal magnets of that motor really very fast due to the gearbox. Due to friction and inertia of the motor and the gearbox it is very hard to turn. Considering what you’re trying to do, that motor is massively over spec’ed.

Assuming you are only trying to spin a light wheel at a constant speed, you don’t need much power at all, a very small cheap brushed DC motor will do the job, a small reduction gear box would make it easier to control but that will still have the issue you have with having to overcome the high friction/inertia of the gear box.

You have two options: use motors with a lot lower gear ratios or use a clutch to disengage the motors.