View Full Version : Rack & pinion

11-23-2004, 01:15 AM
What is the benefits and draw backs of using a rack and pinion to move the axises? The Cad Cut site says that "Dual pinion / rack drive on X axis - No torque of gantry during cutting operations". Can one use the rack & pinion in the X, Y, and Z axis? How does this compare to the Ball Screw method in terms of accuracy?


11-23-2004, 03:04 AM
With dual leadscrews or timing belts you could similarly avoid the torque. I think they're just noting the fact that there is not a single rack which could cause the torque.

Notice in the design that they use screws on the Y and Z axis. The problem with using screws in the 8 foot X axis is that even the slightest bend in the screw (and ALL screws have SOME bend) would cause it to "whip" like a jump rope if rotated too quickly. That's why racks are used - screws wouldn't give the speeds desired. If you dig around the web you'll find documentation on the max speed at which a screw of a given diameter can rotate.

Screws are much easier to implement properly and less expensive. The screws will be much more accurate as well, which is not to say that racks aren't accurate, just that they take a bit of fiddling to remove the backlash.

11-23-2004, 12:40 PM
Rack and pinion is less expensive and more practical for larger tables. Backlash is easy to take out by mounting the pion shaft so that it is spring loaded against the rack. as far as accuracy its not as good as ballscrews..but you have to put things in perspective if your doing a large 8 ft table then chances are you aren't intrested in .001in accuracy.

For some pictures of a rack system I think the Shopbot website has some pictures in there Pdf. manuals.


11-30-2004, 11:57 AM
Thanks for those answers guys.......

I coulndt have answered them better myself ! :cheers: