From aerospace to machine tooling, glass cutting, medical and more, manufacturing processes depend on reliable motion control. Delivering the speed and precision required by these applications are various servo-controlled linear drive systems.
One common setup combines servo controls with a traditional involute rack and pinion. The latter can require clearance between rack and gear teeth to prevent jams and excessive wear, or else environmental changes (such as a 10° temperature shift) can lock up the system as gear teeth expand. On the other hand, clearance results in backlash, which is equivalent to error.
Clearance issues in twin and split pinions
For precision applications, one typical clearance-issue fix is to add a second pinion that pulls in the other direction — against the first system, to act as a control.
One iteration of this idea is to use a split pinion. Here, a pinion is essentially cut down the lateral middle, with a spring positioned between the two halves. As the split pinion moves along a rack, the first half of the pinion pushes on one side of a rack tooth and the other half on the next rack tooth. In this way, a split-pinion setup eliminates backlash and error.
Here, because only half of the pinion performs work — while the other half acts as the control — torque capacity is limited. In addition, because the drive dynamics must overcome the spring's force, motion loss occurs, decreasing overall efficiency. While moving under acceleration, the spring can also give slightly, degrading motion accuracy. Finally, when the pinion is stopped to perform an operation, such as drilling, the spring system in the pinion can flex slightly, instead of remaining rigid.
Another clearance fix consists of a twin-pinion system. In this arrangement, two separate pinions move along the same rack. The pinions act in a master/slave fashion, with the leading (master) pinion performing the positioning, and the second (slave) pinion counteracting backlash. Typically, pinions are electronically controlled, so accuracy is maintained and control settings can be adjusted to compensate for system wear.
What's the catch? Twin-pinion systems can be costly, because designers must usually purchase a second motor, pinion, and gearbox. The design footprint must also be increased: A second motor necessitates more length to execute the driving. For example, if a user needs the motion control system to reciprocate back and forth one meter, a rack length of 1.2 or 1.3 m is required to accommodate the second pinion, which rides 200 to 300 mm behind the first. Finally, the cost of powering two motors is substantial over a typical five to 10-year design life cycle.
Another option: Roller pinions
Roller pinion technology includes a pinion comprised of bearing-supported rollers that engage a rack with a customized tooth profile. Two or more rollers connect with rack teeth in opposition at all times, to deliver higher accuracy than split pinion and pinion drive systems: In short, each roller approaches each tooth face in a tangent path, and then rolls down the face for low-friction operation with more than 99% efficiency in converting rotary to linear motion.
The design has no spring to collapse and degrade accuracy, either, and no efficiency is lost in overcoming a spring force. In addition, the roller action requires no clearance, so eliminates backlash and error. In contrast, for a traditional rack and pinion system, one pinion tooth must push off of one side of a rack tooth and instantly move on to the next side of the tooth.
A roller pinion flanks different teeth simultaneously, straddling one side of one tooth and allotting clearance with another. No second pinion is needed to counteract the first; one pinion accurately transmits the necessary torque capacity.
Roller-pinion-based designs also extend life and reduce maintenance. In slower applications, the system can run without lubrication. Traditional racks wear over time and require compensation for positional accuracy and torque, but roller pinions maintain accuracy. The pinions of both designs require periodic replacement, but at least in comparison with twin pinions, overall replacement costs for a roller pinion are lower.
Consider the production of large aircraft fuselage panels. This application can require a long travel length and high precision across gantry-style machines. Roller-pinion drives deliver accurate linear positioning over these long distances.
In contrast, traditional rack and pinion positional accuracy may be insufficient due to clearance requirements; minimal clearance maintains accuracy over short travel lengths, but the design can be expensive to manufacture and install over long distances. A twin-pinion system (with two pinions preloaded against each other) may also be implemented, but is costly and typically doesn't allow for the varying clearance that occurs over long distances, either.
Another common use of a twin-pinion system is in positioning a cutting head in a fiberglass routing machine. While the twin-pinion drive may initially work well in this application, the combination of fiberglass dust and constant sliding friction created by the opposing pinion can cause premature wear. By using a roller-pinion system, which utilizes rolling as opposed to sliding, life expectancy can be increased by 300% or more.
A rotary version of the roller-pinion system can also be used to perform multi-axis positioning. Here, multiple pinions (all moving independently) are mounted to one gear. The design uses less space than the twin-pinion drives sometimes used in these applications.
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