How to calculate motor torque?

(Torque x Speed)/5252 = HP

Torque= 7.0432 x (Watts/Speed)

This should get you started

To size the motor correctly, measure the torque required to move the Z axis with a torque wrench or other torque-measuring device.

The next thing to do is to choose a motor with that rated torque at the speeds you wish to move the Z axis. Almost every stepper motor manufacturer publishes torque-speed curves for each model of stepper motor.

Keep in mind that single-stepping or micro-stepping torques are not explicit in speed-torque curves and should be given in the motor specifications, and those are somewhat dependant upon the stepper motor driver used.

It is certainly possible to calculate the needed torque at a given RPM but unless you are able to set the correct parameters (carriage mass, desired carriage acceleration, drag due to grinding force, etc.) it's a shot in the dark. A method I have used in retrofitting new drives to equipment is determine the output parameters of the one that is on it now and replace it with a new one of similar output plus a margin. Are you replacing just the motor or the motor and the reduction train? The equations to handle this are straightforward but probably require experience to apply them correctly. The application engineer from the company you are buying the new drive from should be able to help you.

The easy way to do this is to calculate the torque at the rated speed of the original motor (3 lb-ft/HP @ 1750 RPM is close enough), then check that this point is within the envelope of the torque-speed curve for the motor and drive you're evaluating. This will work but you may wind up choosing more motor than you really need. Replacing the original motor's rated torque and speed with the actual torque-speed curve of the load may let you choose a smaller motor, but it would be a good idea to allow 20% to 30% margin to ensure reliable operation.

Don