View Full Version : Why use pulleys on axis motors

03-05-2004, 11:02 PM
Hello all,
I am going to attempt to build my first desktop CNC machine from scratch. I have seen several pictures of home-made machines with gear reduction of their servo/stepper motors by way of belts and pulleys. Is this probably done just to increase the torque thats coming out of the motor or are there other reasons for this? I will be using 214oz/in steppers and was planning on just using direct drive.

03-06-2004, 09:30 AM
The are generally two reasons to use some kind of transfer system between motor and shaft:
1. torque/speed trade-off - the reason you cited.
2. mechanical fit - sometimes a configuration will not allow a motor to sit at the end of the driven shaft.

robotic regards,

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"I find Paul appealing and Peale appalling."
- - Adlai Stevenson (refering to Saint Paul and Norman Vincent Peale)

03-06-2004, 11:07 AM
Hi, One thing often overlooked is the inertia ratio between the motor and the load this is what will govern how much acceleration your system will have, and usually most designs should have a inertia ratio of motor to load of not more than 10:1. Fortunately the inertia value is reduced by the square of the reduction ratio, so even a reduction of 2:1 will have a significant effect. The down side with steppers is with their inherent low torque at top rpm, you do not want to go to a higher reduction than necessary.
There are many motion control manufacturers that offer free system design software, where you can literally drag-and-drop all kinds of mechanical systems to simulate what you have, and give a table of results as to what performance your system might have. They have plug in tables where you can find out the mass/inertia of a coupling for instance.
One typical program is Motioneering from Kolmorgen look at www.motionvilliage.com www.electromate.com www.motionshop.com/catalog/danaher.shtml ,there is something offered by most of the stepper servo manufacturers, and also the reduction gear box manuf.

High Seas
03-06-2004, 12:33 PM
Another is -
Some designs/builders use the belt drive to help prevent any "racking" or twist in a moving gantry setup. They drive each leg of the gantry with seperate screws from a single motor.
:cheers: Jim