Picking the Right Stepper Motor

[howard.t.sun]

Hi all,

I am not too familiar with motors and I am trying to pick a motor for my current application which requires the rotation of a mechanical arm. After going through the different motors types I realize I would probably need a bi-polar stepper motor (for its precision in position) attached with a planetary gearhead with a high reduction ratio (for high torque). I have calculated that I would require at least 130 oz-in or 950 mNm of torque in order to smoothly rotate the arm but I am stuck on transferring that number to something I can read and compare on the spec sheet.

For example, on the spec sheet of one of the motors it says "Holding Torque (at nominal current in both phases) = 22 mNm" which I take as the torque it would take to move the motor out of its stationary position given that the coils are charged. How do I know how much torque the motor would actually generate? I am also attaching it to a gearhead with a 809:1 reduction ratio at 60% efficiency, does that mean that 22 mNm value now turns into (22 mNm)*(809)*(0.6) = 10679 mNm? Yet on the gearhead's spec sheet there are two items, one for continuous torque at 600 mNm and the other for intermittent torque at 1000 mNm. Which number should I go by?

Thanks

 

[barry]

Usually stepper motor specs include a speed/torque curve or a curve for a motor/driver combination.

 

[howard.t.sun]

Usually stepper motor specs include a speed/torque curve or a curve for a motor/driver combination.

Thanks for the reply. So all I have to do is to find the speed I am driving it at and find the speed I wish to operate at and look at the corresponding torque? So let's say I wish to drive the motor at a maximum 5000 steps/s and according to the graph the torque for that is 9mNm in current mode operation. How does the gearbox factor in? For a 809:1 reduction ratio do I get (9mNm)(60% efficiency)(809) = 4,369 mNm as the maximum torque then? I am also assuming the 9mNm is if I drive it to the nominal current rating? So if I drive the windings with 60% of the current we will get about (4369 mNm)(0.6) = 2621 mNm as the maximum? What about the continuous (600 mNm) and intermittent torque (1000 mNm) specs from the gearbox spreadsheet?

 

[barry]

You also have to consider start-up torque. I'm not a mechanical guy, so I'm reluctant to answer those gearbox questions. But it sounds like the motor can produce more torque than your gearbox can handle, but if your load is less than the gearbox rating, I think you should be ok.

 

[FvM]

What about the continuous (600 mNm) and intermittent torque (1000 mNm) specs from the gearbox spreadsheet?

It tells that the stepper motor can easily break the gear when the driven shaft is stalled. It also suggests that the requested 950 Nm are probably beyond a reasonable load for this gear.

 

[howard.t.sun]

It tells that the stepper motor can easily break the gear when the driven shaft is stalled. It also suggests that the requested 950 Nm are probably beyond a reasonable load for this gear.

Thanks. So basically I would either have to find a way to lower my torque requirement to under 600mNm and make sure I never drive it past that, or find a new gearbox that could handle the 950mNm requirement?

 

[FvM]

Depends on the nature of the load.
The point is that a high ratio gear often can't be protected by limiting the motor torque. So a certain overload margin should be designed-in.