Roller Pinion Systems
The precision ring drive is a relatively new type of rotary positioning system featuring a unique roller pinion/toothed rack combination that delivers high accuracy positioning with zero backlash and virtually eliminates cumulative error.
Precision ring drives, at first glance, look similar to ring and pinion sets but, instead of spur gear teeth, bearings supported rollers engage the ring teeth. The rollers engage a tooth profile designed to match the pinion’s path, providing friction-free meshing that allows the pinion to be pre-loaded into the ring, eliminating mechanical clearance. The rollers approach the tooth face on a tangent path and then smoothly roll down the face. Each tooth is precisely measured relative to the first, eradicating cumulative error and maintaining high positional accuracy. The resulting smooth rolling friction provides 99 percent efficient rotary motion. Due to the smooth way the rollers engage the rack teeth, the new approach generates very low noise and vibration. The system is whisper quiet at low speeds and produces less than 75 db noise at full speed. Drives are offered with ratios ranging from 64:1 to 220:1. Peak torque goes from 563 to 1,936 Nm. Accuracy ranges from ±11 to ±35 arc-sec, with repeatability of ±4.2 to ±1.2 arc sec.
Unlike traditional cam-drive systems, the precision ring drive can start and stop at any incremental position. Users can change the motion profile simply by loading a new servo drive program. The roller pinion system driving the precision ring drive also allows the application of maximum acceleration or deceleration at any point without risking damage. The precision ring drive is capable of speeds up to 300 rpm and can handle peak torque inputs at any time, resulting in index speeds up to two times faster than other types of positioning systems.
A given size product in a premium model can support a maximum dynamic load (N) of 1,000. The drive mounts on a table supported by cross-roller bearings rated for 1,575-kN loads. The roller pinion system requires little maintenance. The pinion consists of 10 or 12 needle-bearing supported rollers that are sealed and lubricated for life. The ring is lubricated with a high-performance light grease at installation and then every 6 months or 2 million pinion revolutions. No messy oil baths are required. Pinion life is rated at 60 million revolutions and the pinion gear can usually be replaced numerous times before the ring gear needs replacement. The ring drive has a large open center that allows users to easily mount equipment and cabling in the center of the rotating plate. (See Figure 4)
Rotary motion technologies, such as belt drives, cam indexers, planetary gearheads, direct drive systems, and roller pinion systems each offer their own unique mix of advantages and disadvantages. In order to apply the correct type of rotary motion technology in a particular application, the design engineer should carefully consider the specific capabilities of each alternative. Selecting the right technology can improve performance, ensure long life, and reduce the overall cost of the assembly.
This article was written by David R. Bickert, Regional Sales Manager, Nexen Group, Inc., Vadnais Heights, MN. For more information, Click Here .