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This may be a noob question. I am sorry if it is but I am new to this field. I am working on a design of SCARA Robot and I am using Stepper Motors to drive it. In many youtube videos, I have seen that they increase the torque using timing belts. Now I understand that in most cases timing belts are used for keeping motors away from the joints to reduce inertia. But I don't understand why they use ratio in belt drive. As far as I know, Stepper motors provide high torque at low speeds and vice versa. Then Stepper Motor should provide that much torque at reduced speed by default, right? Or I am missing something?

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  • $\begingroup$ they increase the torque ... you answered your own question $\endgroup$
    – jsotola
    Commented Jun 1, 2021 at 16:21
  • $\begingroup$ Thanks for the reply. But I think they would provide more torque at low speeds by themselves instead of using a belt drive and then turning the Stepper motor at high speeds (thus producing low torque) and then increasing it using a timing belt. Why would they use a timing belt to do something that the motor itself can do? or is it that the torque that they want is outside the stepper motor's torque-speed characteristics? and can not achieve by reduced speed of stepper motor? $\endgroup$
    – Nirav Joshi
    Commented Jun 2, 2021 at 4:58
  • $\begingroup$ those are good questions ... you could ask the they ... or you could analyze a design $\endgroup$
    – jsotola
    Commented Jun 2, 2021 at 5:02
  • $\begingroup$ You are right. I should have asked them first. Thanks. $\endgroup$
    – Nirav Joshi
    Commented Jun 2, 2021 at 5:08

2 Answers 2

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Even at zero speed, stepper motors have a max torque, called the holding torque. If your design needs even higher torque than that, you could use a bigger (heavier, more expensive) motor. Alternatively, use a ratio in the belt drive.

To be concrete: a 1:2 ratio will double the available torque, but at the expense of making things 2x slower. It will also double the resolution by making each stepper step half-size.

A design with a very high ratio would start looking like a gearbox, which would suggest revisiting the approach.

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  • $\begingroup$ Thanks for the reply. Increasing the resolution makes much sense. I think that might be the reason. Commonly they move 1.8 degrees and that would make big difference in the final end-effector position if it is attached to a 500mm long arm. $\endgroup$
    – Nirav Joshi
    Commented Jun 3, 2021 at 5:18
  • $\begingroup$ If the answer worked for you, a green check mark would be appreciated. $\endgroup$
    – r-bryan
    Commented Jun 4, 2021 at 1:44
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As r-bryan answered they do increase the effective torque. However that also increases the effective resolution giving you better repeatability. As an example a direct drive motor that provided a 0.1 degree repeatability using a 4 to 1 ratio belt drive would give you a 0.025 degree resolution. The trade off is a lower speed.

As secondary effect is that in case of a collision, the belt will absorb a certain amount of the impact possibly preventing damage to the motor.

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