How can such a small motor produce so much torque?

Question

I have only recently gotten into robotics - so I'm not that familiar - please excuse me if this is a silly question.

I was looking at servo and stepper motors. I can see that a lot of servo motors basically look like this one:

This servo can produce 1.1Nm of force. I have seen it. It's very small, light and cute. It doesn't look as "professional" as the types below.

There are also much larger (stepper) motors that look like these:

This is significantly larger, heavier, yet produces only 0.4Nm of torque.

It's very shocking to me that such a small motor outperforms this larger one. Ignoring the differences between stepper and servo motors, is there any advantage to using this larger one?

Will it have better speed or reliability perhaps?

Answer 1

While there are a lot of different ways to evaluate motors, what you're getting at is likely the power output of each. Power (Watts) is defined as (force x distance)/time. So while the servo may have a higher torque output, it moves much more slowly and has a lower overall power output.

There are other factors as well such as cost & quality of the motors, but I don't believe that's the core of your question based on your framing.

Answer 2

It's very shocking to me that such a small motor outperforms this larger one. Ignoring the differences between stepper and servo motors, is there any advantage to using this larger one?

The Stepper motor (the second one) has higher holding torque by its design and operates in an open loop, hence a less complex control system, whereas Servo achieves higher torque at high speed and has poor holding torque, and operates in a closed feedback loop using an encoder setup for positional feedback. Hence, servos use complex control systems. Stepper motors because of their simplicity are priced cheaper compared to servo motors.

Eg: Cheaper 3D printers have stepper motors, whereas high-precision (and expensive) ones use servo motors.

Said that the smaller servo achieves its high torque capability at the cost of its high speed and limited range in its movement by utilizing a compound gearing setup.

The stepper is bare bone with no gears in it hence lower output torque than the servo in comparison.

@jsotola I doubted it too. So are you saying if we compared the naked motors, the servo one would have a lower torque? And if we had applied the same gearbox to each, would the stepper have a larger torque?

Yes, it's safe to say that given the same gearing ratio, the stepper will produce way more torque than the servo. The 1.1NM value on the servo motor is achieved only at the end of the gearbox, but the 0.4NM value on the Stepper motor is its own torque rating.

So your application determines which one is suited best based on their ability and capability. There is form-factor to take into account as well.

Hope this helps :)