How to select a stepper motor and gear ratio?

[yahastu]

Hello, I could use some help learning how to properly size stepper motor.

Suppose that I want to use a stepper motor to move a load against the force of gravity. It must be able to start, stop, and hold it's position against the static load. I know the maximum torque of the load. In addition, I want it to be able to rotate with rotational speed greater than some minimum. How should I select an appropriately sized motor?

My thought so far, correct me if I'm wrong:

I see that stepper motors have a characteristic called "holding torque" which is in Newton*meters/Amp. So, I'm thinking that I just need to multiply this by the operating current to get an actual torque of the motor. Then, if my motor torque is greater than or equal to the maximum torque of my static load due to gravity, I assume that means the motor is adequately sized to start/stop/hold this load against the force of gravity. Is this thinking correct?

I am fee to play with gear ratios as well. Therefore, for any given stepper motor, if the native torque is not enough, I could just determine the multiple that I need for torque, and this determines the needed gear ratio. If slowing down the rotational speed by this gear ratio does not achieve necessary RPM, then the motor is not adequate for the job.

Comparing between two different configurations of stepper motor/gears that are both adequate of meeting these two constraints, it would then just be a question of cost and/or space/complexity of the required gear ratios.

Is this a good approach?

 

[Baluncore]

You must define the system first.
Identify the mass and static load.

Specify the time constraints, speed and therefore acceleration required.

From that compute forces and torque required.

 

[jrmichler]

When you have done that, you can start looking at stepping motors. Stepping motors have a torque vs speed curve that you get from the data sheet. As an example, here is the torque curve for a particular stepping motor rated at 1288 oz-in holding torque:

The selection of a stepping motor is not just selecting a motor. It is selecting the motor, drive, and gear reducer system. Study the stepping motor catalog and data sheets carefully, The catalog for this motor states that you need a 100% safety factor on motor torque. Because of the number of variables, the process is iterative. Stepping motor catalogs normally have a lot of good information on the selection process. Plan to spend several hours, or more, reading stepping motor catalogs after you have defined accelerations and speeds as indicated in the post by @Baluncore.