Rack-and-pinion sets can be used in vertical applications. Two things limit practical speed are the pinion-driving device (for example, the servomotor) and the process of keeping the rack and pinion lubricated.

Linear motion is indispensable to moving machines; it transports tools and products efficiently and controllably. The mechanisms that generate linear motion are generally ranked by their axial velocity and acceleration, axial forces versus structural volume, life, rigidity, and positioning accuracy.

Two common linear systems are linear motors and ballscrew drives. Rack-and-pinion drives are often overlooked as past-generation technology with limited positioning accuracy. However, this assumption is invalid.

Precision-ground mounting surfaces to tight tolerances, wear-resistant surface treatments, individually deburred gear teeth, and compact, low-mass designs are boosting performance. In fact, rack-and-pinion drives compare favorably to linear motors as well as roller or ground-thread ballscrews.

New-generation rack-and-pinion systems offer high dynamic performance and unlimited travel distance. Some include premium servogears and actuators with backlash less than 1 arc-min., efficiency to 98.5%, and far more compact sizes than standard servomotor-gear combinations. Some preassembled gear-pinion units can even run true to 10 µm, for safety and smooth motion.

Typical rack-and-pinion applications include gantry, transport, and packaging machines that carry from a few pounds up to several tons. Next-generation rack-and-pinion sets are also used in woodworking, high-speed metal cutting, and assembly machines.

Geometry and surface details

Rack-and-pinion performance has improved with general technological advances. For example, state-of-the-art machining and grinding have greatly advanced rack-and-pinion precision.

More specifically, some premium rack pieces are laser etched for cumulative pitch error ±12 µm over a 500 mm length, which allows for hand selection of target accuracy. This is useful for matching rack pieces in parallel, for dual-drive gantry applications. In fact, that level of precision allows several kinds of machines to run without external feedback devices; in contrast, other linear systems require expensive external feedback devices for commutation and positioning.

A helical rack with an optimized helix angle is preferred for quieter running at higher speeds and a higher load carrying capacity due to the higher tooth contact ratio. Single-pitch error between helical teeth can reach 3 µm. A pinion profile shift or addendum modification prevents undercut; it also balances bending stresses, for higher load capacity. Helical gearing engages smoothly and quietly — which helps improve surface finish, for example, when machining tight-tolerance parts.

Lubrication is key

Rack-and-pinion sets last longest when properly lubricated. Appropriately greased sets are also most capable of reaching highest rated speed. For many rack and pinion systems, the most common method is an automatic lubrication kit or greasing device. These devices come in various sizes or volumes, and are controlled electronically.

Different settings can be selected to control the amount of grease that flows over time — dependent on the motion cycle of the rack and pinion. A charged canister maintains pressure when not in use; closing a two-wire switch activates flow.

The grease travels through a hose into a hollow greasing pinion, a felt gear with radial holes where the grease is applied to either the rack or the pinion through the holes. Here, the design determines which half of the set is actively greased: For example, lubricating the rack for a high-speed application can prevent grease from being flung away.