Motion System Design

Top 10 tips: Working with rack & pinion drives

Choosing the right rack and pinion drive depends on three main considerations — application parameters, required accuracy, and budget. Following are some expert tips and tactics to help you get it right the first time.

1. Take the time to size wisely. Match the quality level of the rack to the application: A soft rack for light loads and medium accuracy; a quenched and tempered rack for medium loads and medium accuracy; an induction-hardened rack for heavy loads and low accuracy; and a hardened and ground rack for heavy loads and high accuracy. For high-speed applications requiring smooth and quiet operation, use a helical rack and pinion drive. This type of drive also offers a higher contact ratio (the number of teeth engaged) than a straight (spur) drive, which increases strength and load-carrying capacity. Size the rack and pinion drive based on peak cycle forces, which usually occur during acceleration or when high external forces, such as cutting forces, are present.

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Design for easy parallel alignment of the rack to the linear guide, or use an integrated rack, which mounts directly to a linear guide and eliminates the need for separate alignment. To remove all rack and pinion backlash and produce higher quality end products, use a preloaded system consisting of a hardened and ground rack, split pinion, and servo worm reducer.

2. Know your loading and accuracy requirements upfront. To avoid premature failure, a rack and pinion drive must be selected based on peak cycle loading, which usually occurs when accelerating the mass being moved. That said, peak cycle loading could also occur on a machine with high cutting forces.

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