### Author Topic: Stepper motor torque  (Read 557 times)

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#### drinu

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• Posts: 61
##### Stepper motor torque
« on: February 20, 2014, 05:31:59 PM »
Hi,

I was following a tutorial of how to calculate the torque required to drive the Y-axis of a CNC machine. I came into the conclusion that to drive a 35kg load with a 2010 ballscrew (1.3m long), a 0.12Nm stepper motor would be enough. Isn't that too low for driving that load?

Any help will be appreciated and sorry for my bad English.

#### bdeuell

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• Posts: 93
##### Re: Stepper motor torque
« Reply #1 on: February 20, 2014, 06:03:20 PM »
You say Y-axis does this imply in a horizontal plane?

What kind of acceleration do you plan to have?

What are you accounting for in your calculation (friction, inertia, static load, damping ...)?

My calculations give 75 newtons as the axial force along the ballscrew with those dimensions and a .12Nm torque, neglecting any losses.

keep in mind the torque output of a stepper motor is not constant over the speed range.

#### drinu

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• Posts: 61
##### Re: Stepper motor torque
« Reply #2 on: February 20, 2014, 06:27:52 PM »
Hi,

Thanks a lot for the reply. I was following the tutorial in the link below, but I think it is over-complicated for my level.

http://www.mycncuk.com/forums/machine-building-faqs-problems-solutions/1524-what-size-stepper-motor-do-i-need.html

Do you know of any easy-to-follow tutorial to determine the torque please? There is no need for such detail. I will then add a safety margin to be on the safe side.

#### drinu

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• Posts: 61
##### Re: Stepper motor torque
« Reply #3 on: February 20, 2014, 06:58:40 PM »
By you calculation, you determined that the torque must be .12Nm torque. So does that mean a 1Nm stepper motor (such as the one in the link below) is more than enough?

http://www.stepperonline.com/nema-23-cnc-stepper-motor-30a-095nm135ozin-23hs223008s-p-69.html

#### bdeuell

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• Posts: 93
##### Re: Stepper motor torque
« Reply #4 on: February 22, 2014, 01:59:30 AM »
First you need to know what the forces you need to act against/move are. You should answer all of the fallowing questions in order to define the problem:

Is the motion in a horizontal plane?

What is the maximum acceleration?

What speed do you plan to move at and what is the torque your motor can supply at that speed?

Will you have any type of feedback? (in an open loop stepper motor system it is possible to drive the motor at too high of a speed for the required torque which would result in the motor missing steps, for this reason open loop stepper motor drives are often designed with larger motor to ensure there is sufficient torque over the entire speed range.)

Are any other forces that will be applied to your drive, such as the cutting forces described in your link?

My calculation simply took the .12Nm torque you provided worked backward to determine the axial load that could provide using the ballscrew you specified. I used the formula: torque=(force*pitch)/(2*pi). *** this calculation does not in any way say that the theoretical 75N output (with zero losses) of your drive will be sufficient for your application.***

Frictional losses in the drive screw can be approximated with an efficiency which makes the math much simpler. 90% might be a good estimate but I would look for come references. This value can be affected by many things such as lubrication, surface finish, preload in your ball-nut. and normal load applied to the screw.