I have been building an automatic curtain opening system using an arduino, motor controller and dc motor. You can see my main thread on the project here.
Basically, the curtains open with a cord by hand, I will attach the cord to the motor on a 50mm pulley.
The motor I originally tried didn't have enough torque to open the curtains, and when attaching the motor to the curtains resulted in the motor stalling immediately. Turns out the torque required needs to be a lot greater than I thought.
I have been googling how to calculate the torque requirements and I am really struggling to understand how to do it: Here is a sketch of my setup and my calculations I've tried:
To convert the force from kilograms to newtons, I used the conversion factor:
1 kg = 9.8 N
Force = 4 kg * 9.8 N/kg
Force = 39.2 N
So if my motor was directly attached to the rope (no pulley / drum) it would need a torque rating of 39.2N
Question 1:
Is this calculation correct?
Question 2:
The motor supplier provides the torque of the motor in kg/cm, how can I calculate the correct motor size If the motor shaft is 6mm, do I use this in the calculation?
Question 3:
I will be adding a 50mm diameter pulley on the shaft so how does this affect the choice of the motor?
The non-standard unit of torque is kg-cm, (kgf * cm) although lots of people, including some manufacturers, mistakenly use "/" as the separator instead.
Note that 4 kg x 2.5 cm = 10 kg-cm. Choose a gearmotor with at least double that torque, so it does not stall.
You might want to choose a motor that gives you at least 10 kg*cm, with a healthy margin for error. The load on the cable varies by the position of the curtain too, so check the force needed in both the fully open and fully closed positions.
It seems this motor will need to operate daily, multiple times a day. Best to go with the rated torque rather than the maximum torque figure. After all, the manufacturer is telling you that this what they have rated it for. And I expect that the maximum torque is at stall current.
Another consideration with the table of motors above is the rpm delivered for the given torque. One revolution of your pulley will give a cable movement of ~15 cm. Is the no-load rpm adequate for your needs? The on-load rpm will be lower.
@jremington is exactly correct. You would not purchase a vehicle where the engine was at full max just to move the car along the road.
Also consider speed. All those motors you found should have a torque speed curves. I would think you want a robust gear motor that goes kinda slow but has a good amount of torque.
Keep in mind that all the numbers you read could well be "hopeful" specifications.
Thanks very much for the great feedback, considering all comments I'm going to give the 15rpm motor a try according to sheet, the its rated torque is 13.5 kg-cm, and stall torque is 36 kg-cm.
That is a rather firm statement, since we don't even have a formal measurement of the required torque.
It could as easily be 3x, since we also don't know the accuracy of the reported motor output or the gear train efficiency.
Do you have a reference to how that applies to this situation? Otherwise, you might as well pull 2.5x or 3.5x out of a hat and say that is what is the correct margin in this case.
As I said, we just don't know enough to make definitive statements in the setting of a hobby load being powered by a hobby grade motor.
