# Thread: What size servo motor do I need.

1. Here's a quick script I've made to calculate the effective inertia of each axis on a machine, from which for a given acceleration and cutting force the required motor torque rating is deduced. The script accounts for the inertia of the ballscrews and pulleys, in addition to the linearly moving axis. It also accounts for the efficiency of the ballscrews by the usual co-efficient in addition to calculating the pre-load torque for the ballscrews. Set the latter to zero unless you're preloading the ballnuts.

The script runs in MATLAB, or GNU-octave which is free/open source and can be downloaded here.

The comments in the script (lines 15-25) should make it fairly obvious which values to change to model your machine, however if you're not sure what to do feel free to post in this thread and I'll show you or run it for you. The following specifications for the machine are required for each axis:

Mass of carriage
Maximum cutting force
Acceleration
Ballscrew pitch, length, diameter, pre-load force (if applicable)
Motor rotor inertia - generally given by motor manufacturer, but facility to estimate included.

Example output:

servo_motor_sizing_forumexample.txt
Last edited by Jonathan; 31-03-2014 at 12:48 AM. Reason: Pedantic errors

2. Hi Jonathan,

Useful thread thank you. When I saw the title I thought this was something special relating to servo motors, but upon reading it is about motors in general so I wonder if 'what size stepper do I need'. would be more helpful to people? Good work anyway.

The torque output from the calculation is at a certain acceleration and feedrate. I there a way someone deciding on a motor can estimate the how the static torque quoted in spec sheets drops at the required speed above so they can compare to the torque required calculation? Or is it sufficient for someone to just apply a safety factor, say 2 or 3?
Last edited by routercnc; 31-03-2014 at 08:18 PM.

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