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I need to find a small motor (max 10W) with a high torque(max 1Nm) and low rpm(max 60rpm).

This question has been asked several time but I still have some doubts.

  1. I understand that the best option is to use a gearbox.

  2. The reason why find this motors is quite hard is explained in this question:

    You can play with all three of these variables, but there are definite limits on how much field intensity you can get with reasonable materials. Therefore, you end up needing either a long motor or a large-diameter motor, both of which require lots of extra (expensive) material to produce.

My questions refers to same applications in particular gimbal and robots. In this products is there a gearbox?

In particular the gimbal, I can't figure out how the manufacturer can put a gearbox in such small object considering also the axial and radial loads apply directly on the gears. I find also that there is a particular kind of motor called gimbal motor that should provide high torque and low rpm. Is there another type of motor or is it equal to normal BLDC motors?

The same problem is related to the robot. What kind of gearboxes do they use (if they use)?

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    \$\begingroup\$ Something like a handheld screwdriver probably fits your power and torque requirements, but its not usable for a gimbal given the backlash. That's actually a hard problem, and typically uses either fairly high power motors or exotic things like a harmonic drive for gearing. \$\endgroup\$ – Chris Stratton Jul 9 '19 at 19:37
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The commercial gimbals work like this:

They have no gearbox, they're a BLDC motor in direct drive. The motors work the same way as a regular BLDC, but they have ultra low kv. The motors are driven open loop, similar to a stepper motor, since they're spinning too slow for the back-emf method that is normally used. And here's the important part: they don't actually have much torque! They depend on the load being well balanced to reduce the torque requirement.

Edit: Deleted some math, as it was incorrect.

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  • \$\begingroup\$ Thank you for the answer, the power should be P=w*M=(2*pi*60)/60 * 1 = 6.28 W, the values for torque and speed are maximum values \$\endgroup\$ – Ugo Mela Jul 9 '19 at 20:03
  • \$\begingroup\$ Ah, it appears you are correct. My bad. I'll edit my answer, \$\endgroup\$ – Drew Jul 10 '19 at 3:21
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In particular the gimbal, I can't figure out how the manufacturer can put a gearbox in such small object considering also the axial and radial loads apply directly on the gears.

They balance the load on the gimbal motor so that the center of mass runs through the axis of the motor so that it takes zero torque to hold the gimbal at every position. The torque is only required to overcome the moment of inertia of the gimbal and accelerate the gimbal to that position quickly enough.

A regular direct-drive (ungeared) BLDC gimbal motor like you see in UAVs nowadays does not have enough torque to actually lift it's load against gravity. But it doesn't need to. It just needs enough torque to accelerate the load to the desired angular position quickly enough.

The same principle also lets you use seemingly undersized gearmotors for gimbals since they don't need to lift the weight of the payload.

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