Rack-and-pinion actuators often have acceleration rates and peak speeds nearly as good as those of linear motors. In many cases, the machine frame and structure — not the actuator — limit peak speeds from rackand- pinion and linear-motor systems.

Ballscrews can run up significant cumulative errors over total travel length. For example, deviation over four meters of travel for a rolled screw drive may vary between 300 and 1,700 µm. Even ground-thread ballscrew deviation over four meters ranges between 30 and 110 µm. With two paired rack-and-pinion systems, cumulative error for the same travel length is only 12 to 40 µm. This makes rack-and-pinion sets suitable for even gantry drives.

For applications with long travel lengths, ballscrews have high mass moments of inertia that limit critical speed and axial load capacity; even preloaded ballscrew efficiency only reaches 90% or so. Such long-stroke applications benefit from a switch to rack-and-pinion sets — with efficiency to 97%.

Adjoining parts such as bearings influence ballscrew rigidity, housing bores, or nut housings, making it difficult to ensure stable system behavior under dynamics. Deviation of spindle stiffness depending on nut position over the spindle length compounds this problem.

In contrast, rack-and-pinion drives offer constant stiffness over the complete travel length plus good system behavior — for superior control system behavior. Finally, unlike rack-and-pinion systems, ballscrews only allow one carrier per linear axis and are not suitable for short-stroke applications. Why? Greasing demand dictates that only some balls circulate through the nut.

Rack-and-pinion versus linear motor

Compared to linear motors, rack and pinion systems can offer similar performance but at far less cost. They are smaller, allowing a more compact, less complex machine design. The absence of magnetic forces vastly decreases the need for support structures to absorb high normal forces, so standard guide rails can be used. Linear motors have overall efficiency to 90% — though sometimes it's considerably lower. Because of this inherent inefficiency, linear motors often require water cooling.

Stepper motors are a viable driving option for rack-and-pinion sets, but servomotors are more commonly paired with the mechanisms.

In comparison, rack and pinions need no cover; the guidance system can be exposed to metallic particles, and safety restrictions are minimal. Better rack-and-pinion sets do not require expensive linear scales and external brakes, either; standard motor feedback devices and brakes are enough.

In many cases, linear motors require complete machine redesign — partly because huge normal forces from the attraction between the primary and secondary have far-reaching consequences. An easier option, ready-to-mount rack-and-pinion systems facilitate blind assembly for additional cost savings — and cut assembly time to roughly 10 minutes per meter travel length.

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Quick history lesson

Mechanical linear motion devices — upon which rack-and-pinion sets are based — date back to the invention of the wheel in ancient Mesopotamia. About 1100 BC, Assyrians began using rolling log platforms to make moving objects more practical. After the Dark Ages, during the Scientific Revolution of 1600s, the rules and practices of the ancient worlds — like those of Assyria and its linear motion systems — were studied and sometimes adopted. This phase led the way to the Industrial Revolution of the 1700s and 1800s, during which the first, most basic rack-and-pinion devices came to prominence.

One major application that spurred rack and pinion innovation was rail transit. More specifically, in the 1800s, cogged railways were put into use in the United States and Europe's steeper landscapes. These railways make use of cars fitted with powered pinions that engage a toothed rack installed between a railroad's tracks. It's a power transmission mechanism that's particularly useful for climbing applications. The first cog railway in the world — still in operation — is the Mount Washington Railway, New Hampshire, first operated in 1868. Another cog railway, the Vitznau-Rigi-Bahn in Switzerland opened a few years later.

Today, modern materials, treatments, and optimized manufacturing make the latest rack-and-pinion sets perform just as well and often better than electromechanical and other linear components in a myriad of demanding industrial applications.