The working principle of a rack and pinion mechanism is as follows: a parallel-axis gear set converts rotational motion into linear motion via a motor-driven pinion gear meshing with a rack.
A variant of the rack and pinion mechanism is based on roller pinions. These ultra-quiet rack and pinion drives use bearing-supported roller pinions instead of spur gears. The rollers achieve up to 99% contact with the rack tooth flanks, with unidirectional accuracy around 2.5 microns (bidirectional accuracy is better than 5.8 microns).
Manufacturers also design the engagement geometry of the rollers so their path of motion is tangential to the tooth flanks. Consequently, the rollers smoothly slide into engagement with the rack teeth, eliminating the wear and loss of accuracy caused by sliding friction and tooth impact in some rack and pinion systems.
A theoretical center distance exists for rack and pinion systems where meshing is optimal. While backlash can be reduced by pushing the pinion into the rack, this negatively affects gear engagement, leading to excessive wear and potential binding.
Backlash inherently exists in rack and pinion drive systems due to gear meshing. However, high-precision helical rack and pinion systems can achieve pitch errors controlled at the micron level. Backlash can also be eliminated by preloading the rack and pinion drive, achievable through designs like split pinions or dual pinions.
To address these issues, engineers employ anti-backlash gears, which are designed to minimize or eliminate the clearance between meshing gear teeth. A variety of anti-backlash gears exist, such as spring-loaded gears, dual-gear systems, and precision-machined gears, each with its unique design and advantages.
An anti-backlash gear is a specially designed gear intended to minimize or eliminate the effects of backlash in mechanical systems. These gears are engineered to maintain constant contact between meshing teeth, even when the direction of rotation is reversed. By reducing backlash, anti-backlash gears enhance system accuracy, repeatability, and overall performance.
To completely eliminate backlash in a rack and pinion drive, a split pinion or dual pinion drive can be used, where one gear provides the drive and the other eliminates the backlash. The simplest zero-backlash drive system uses a split pinion consisting of two pinion halves and an axial spring pack. These two halves engage opposite flanks of the teeth on the same rack, thereby eliminating backlash.
In a split pinion design, two pinion halves engage opposite tooth flanks on the same rack. One pinion half is the driving gear, while the other is preloaded via an axial spring pack to eliminate backlash. One pinion half-shaft drives the axis, and the other half-shaft is "preloaded" to remove backlash. The preload setting for the second pinion half-shaft is fully adjustable on the machine by turning the axial spring pack at the end of the gear shaft.
A dual pinion design uses a driven pinion (also called the "master") and a preloaded pinion (also called the "slave"), driven by a gearbox and motor respectively. The preload is electronically controlled via a controller.
For rack and pinion applications requiring ultra-high precision positioning and repeatability, two pinions are used to eliminate system backlash—one to drive the axis and another to "preload" the axis to remove backlash. Positioning accuracy is maintained during acceleration, deceleration, and changes in the direction of motion.
Another method to eliminate rack and pinion backlash is to use two motor/gearbox assemblies with dual pinions operating on the same rack in a master/slave configuration. This solution eliminates all axial clearance, achieving ultra-high precision positioning and repeatability. Positioning accuracy remains stable during acceleration, deceleration, and direction changes.
By using two pinions to eliminate axial clearance—one pinion driving the axis and the other engaging the tooth flanks on the opposite side to absorb the play—a zero-clearance rack and pinion drive is created. This requires an electronic preload system where the controller actively manages the second motor/pinion (the one absorbing play), applying a constant force and optimizing preload for dynamic motion.
The electronic preload system employs two motor/gearbox assemblies, each with dual pinions, operating on the same rack in a master/slave mode. One pinion drives the axis (the "master"), and the other is "preloaded" to eliminate backlash (the "slave"). A special motor drive generates and controls the preload force. This second drive can also be set to assist the master axis operation when backlash is not critical. One of the main advantages of this system is the ease of monitoring and controlling the backlash.
To eliminate backlash in a rack and pinion drive system, a preload must be established to precisely control the torque on the output rack and pinion or pinion ring gear shaft. Consequently, two types of rack and pinion drive systems are offered, both consisting of two rack and pinion gearboxes—mechanically or electrically coupled depending on the application—with the output pinions preloaded against the rack.
