Como funcionam os sistemas de cremalheira e pinhão

How rack and pinion systems work

rack and roll

A rack and pinion system is a variation on the traditional rack and pinion that replaces the toothed pinion (gear) with a cage pinion that incorporates rollers supported by bearings.

pinion and roller

Each pinion roller is supported by two needle bearings, which eliminates the sliding friction that occurs in traditional rack and pinion systems. This significantly reduces heat and wear on the pinion, with an efficiency of up to 99%.

The tooth profile of the rack is also unique. It is created by placing evenly spaced points around a circle and, as the circle rolls over a flat surface, following these points until it reaches the surface. The paths created by these points are cycloidal curves that form a tooth-like profile. Next, rollers, which function as pinion teeth, are placed at each point on the rim. As the circle rolls once again over a flat surface, the rollers modify the curves to create the actual gear teeth. The result is a tooth profile that allows the pinion rollers to approach the teeth tangentially so that they roll smoothly along the tooth face. Not only does this significantly reduce noise, but the tooth design can be modified to allow two or more rolls to be loaded in opposition (preloaded), virtually eliminating backlash.

pinion and roller

Equidistant points around a circle form cycloidal curves as the circle rolls on a flat surface. Rollers positioned at these same points change curves as the circle returns to the flat surface. This creates the rack tooth profile, which is further modified to allow multiple pinion rollers to engage in opposition, creating preload.
Image credit: Nexen Group, Inc.

When installed and used correctly, some rack and pinion models can achieve a positioning accuracy of ±30 μm, and the positioning accuracy does not deteriorate until the needle bearings reach the end of their service life. Needle roller bearings are permanently lubricated and sealed, so they do not require lubrication.

For corrosion resistance, rack and pinion bodies can be made of stainless steel, hard chrome steel or nickel plated steel. And some models can work without lubricating the rack or pinion, as long as the speed does not exceed 0.5 m/s. Racks can also be made from thermoplastic material, which is self-lubricating and resists corrosion from many types of contaminants and chemicals.

Mounting and operating considerations

To function properly and provide optimal system life, roller rack and pinion systems require constant preload over their entire length. This means that the rack and linear guide system (e.g. profile rail) must be mounted parallel to each other. If the rack and guide converge (move toward each other) at any point, the preload on the pinion will be excessive, which can cause noise and binding, resulting in shorter pinion bearing life. If the rack and guide diverge (move away from each other) at any point, pinion preload will be reduced or lost, resulting in system play, noise, and reduced system life.

pinion and roller

The alignment between the rack and the linear guide is essential to ensure correct maintenance of the pinion preload.

Applications and uses

Rack and roller pinions can often achieve better positioning accuracy and repeatability than traditional rack and pinion systems (without additional components such as separate pinions or dual pinions). They can be supplied in much longer lengths than ball screws, with speed capabilities of up to 11 m/s regardless of length.

rack and pinion

Linear motors offer the long length and high speed capabilities of rack and pinion systems, but iron core motors are often required for dynamic applications with high loads, which means that tooth forces must be mitigated and cooling can be necessary. Belt drives can also provide long strokes and high speeds, but with much lower stiffness and load capacity.

Compared to these alternatives, the “sweet spot” for a rack and pinion system could be described as an application that requires long stroke, high travel speed, and good thrust force capability, all while maintaining good positioning accuracy and repeatability. Examples are robot transfer units, gantry robots, and large machining or cutting equipment such as waterjet cutters. And an application with any combination of these requirements, occurring in a cleanroom environment or requiring very high corrosion resistance, would also be suitable for a rack and pinion system.

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