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How to size a DC motor? Answered

I want to rotate a stainless steel shaft that is in a vertical position having a 40 mm diameter and 300 mm length. The motor will be attached to the bottom of the shaft, so when the motor is turned, the said shaft will rotate. The shaft is not driving anything, it just turns on its own. The shaft's mass is 3kg(according to my calculations).  I want the shaft to rotate at an angular speed of 45 degrees per second. Having this information, how can i size the motor that will do the job. What kind of torque would i need, what kind of wattage. Please show me some calculations so I can know how motor sizing is correctly done. 

Very much appreciated,





Best Answer 5 years ago

steveastrouk is right, you will not need a very powerful motor.

If you want to turn the shaft with 45°/s, that is 8s for one revolution. A normal DC motor will rotate much faster than this. You can either use a stepper motor or a speed reduction by either a belt drive or some gears.

Using a speed reduction will also help with getting the weight of the shaft away from the bearing of the motor. They won't like the 3kg on them for long.

Consider the only load on this is the drag on your bearings.

You don't say you will have any other than the motors, and the sort of tiny motor that will run this load won't like having 3kg sat on it.

You really need only a very small, 1W motor, if that.

If it does nothing, why is it turning ?

The shaft has 4 lights attached to it at the top, each positioned at 90 degrees from the other. So basically like a cross with the motor at the middle and 4 colors of lighting at each end(one of them points in the direction i want). I don't want continuous rotation, i just want to switch which light is in the direction i want. So i want to turn 90 degree increments every once in a while(example: ill turn 90, then wait for 10 seconds, then turn another 90 degrees.) I was thinking of using a servo motor with a 360 degree rotation span. I am shouldering the weight of the shaft and mechanism on an external bearing. Knowing this, can i use just any servo motor to turn my shaft? Don't i need certain wattage or torque to overcome the weight of the shaft and to get that acceleration going to make 90 degrees? The 45 degree per second isn't really an exact figure, I just want something not very fast since i want to stop my rotation at exact 90 degree increments. Thanks!

Now you have clarified your spec, we can put some real numbers on it, yes, now there IS a required torque and power to accelerate the substantial shaft and stop it, but its still not very high. You can't use a 360 deg servo, because they have no internal feedback to tell them where to stop. Is there some precision needed to the final position of the lights.

Yes there is a precision needed. With a quick search i found this servo motor http://www.pololu.com/catalog/product/522

It provides the 360 degree span with the position feedback as the normal servos.

You are saying there IS a torque requirement, 2 questions:
1.How do i quantify it? How would it change if the shaft+lights weight changes from 5kg to lets say 25kg?
2. In your previous post you said the only load was the drag on the bearings. What has changed? Is the torque i need equal to the torque produced by the friction at the external bearing?

Thanks again, I very much appreciate all your answers!

That servo does NOT do what you want.

"These servos do not have physical end stops, so commanding them past their limits will cause them to rotate continuously until you change the commanded position back to something within its actual range. In our tests, this did not damage the servo, but we strongly recommend against it as there is no guarantee this will not damage the servo’s feedback potentiometer."

What you've ADDED is that the load will be stopped and started, and with some precision. accelerating the load to speed, and slowing it down takes a certain torque, doing it at a certain rate allows us to calculate power and torque. 

They do not have physical stops but as long as my pulse positive is between 1 and 2 ms, I will achieve the result I want right?

If not, is stepper my best option? They give me the positional control I need and their speed can be controlled with the supply voltage.


A stepper is more complicated than you need here, and the control harder.

Also is it possible to control speed on a servo motor, or just the position?

What you need here is only an indexing mechanism, like a Maltese Cross.


7.5 RPM isn't very fast.

If you mount the shaft on a bearing so the motor doesn't have to support the weight then as Steve says a very small geared motor will turn it easily.

I would try out a cordless drill motor as a starting point - they have very high torque at a low speed.

360/45 = 8 So 45 s=deg per sec = 1 full rev every 8 seconds or 7.5 per minute?

Or did I get something wrong ?

Lets see 1/8 revolution per sec
is 1 revolution every 8 seconds .
That's 60 ÷ 8 = 7.5 RPM  !!

WOW, I did it wrong before *$@!!

Thanks for calling me on it :-)

Rotation of 45° per second is a seven point five RPM,
  • If you want continuous rotation - - steveastrouk is right the smallest DC motor that can overcome the thrust bearing drag that supports your 3Kg shaft.
  • If you want to turn 45° from dead-stop in one second - - the DC motor will need be a fractional horsepower or a gear_motor.
  • If you want to start and stop 45° in a second - - you will want to use a microprocessor controlled stepper motor, or a gear_motor with integral break or a (Big_Servo_motor).


Please refer to my reply at Steveastrouks post! Assuming an external bearing shoulders all the weight, will a small servo motor do it? Will the weight of the shaft even matter?