Rack & pinion: the other precision linear drive

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Rack-and-pinion drives, in concept, are one of the oldest and most basic power transmission technologies. They consist simply of a pinion gear meshing with a toothed bar or “rack.” Although either component may be fixed, it’s usually the pinion, which when rotating, forces the rack to move laterally in the direction of rotation. Depending on how they are manufactured, rack-and-pinion drives can deliver very precise motion, rivaling or exceeding that of ballscrews and even linear motors. When combined with high-end servo actuators, precision rackand- pinion drives can hold less than 1 arc-min of backlash, true running of 10 μm, and 98.5% efficiency. Such drives are employed in many precision applications, including high-speed metal cutting, laser cutting, grinding machines, and woodworking machines. Other applications include automatic assembly equipment and gantries.

Questions & answers

Q: How is accuracy maintained?
A: The accuracy of a precision rack-and-pinion system is around 12 μm per 500-mm rack. By matching sections — each section is checked and marked — lengths of any size can be created with no more than 40 μm of position error.

Q: What qualifies as “high precision?”
A: Tight tolerances, wearresistant treatments, and individually deburred gear teeth are good signs when looking for a high-precision rack-and-pinion drive. Tooth-to-tooth error, though a factor, is not as important as total cumulative pitch error. With rack-and-pinion drives, it is possible to have relatively high tooth-to-tooth error and still achieve the desired cumulative accuracy.

Q: Is mounting difficult?
A: In the case of precision drives, tight tolerances tend to simplify the mounting process. The first step is to prepare the racks and machine bed, paying close attention to parallelism. No shims or adjustments should be necessary. Subsequent rack lengths may be added with the help of an assembly jig. Mounting a section of rack this way takes no more than 20 minutes.

Information for this month’s Motion Basics column provided by Klaus Bauer, product manager, and Mike Anselmo, applications engineer, Alpha Gear Drives Inc., Elk Grove Village, Ill.