1
$\begingroup$

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?

$\endgroup$
4
  • $\begingroup$ they increase the torque ... you answered your own question $\endgroup$
    – jsotola
    Jun 1 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? $\endgroup$ Jun 2 at 4:58
  • $\begingroup$ those are good questions ... you could ask the they ... or you could analyze a design $\endgroup$
    – jsotola
    Jun 2 at 5:02
  • $\begingroup$ You are right. I should have asked them first. Thanks. $\endgroup$ Jun 2 at 5:08
2
$\begingroup$

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.

$\endgroup$
2
  • $\begingroup$ 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. $\endgroup$ Jun 3 at 5:18
  • $\begingroup$ If the answer worked for you, a green check mark would be appreciated. $\endgroup$
    – r-bryan
    Jun 4 at 1:44
1
$\begingroup$

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.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.