Mechanical Power Transmission

Rolling ring technology drives linear actuator applications

Backlash is caused by space of "play" between contacting surfaces in ball screws, gears, belt drives, and other linear motion devices.

Mechanical power transmission devices (leadscrews, gear assemblies, couplings) usually involve joined or mated parts -- such as meshing teeth in opposing gears. And between joined parts there is always a little space. This space helps reduce wear that would result if the surfaces were in constant contact. Reducing wear improves longevity. It also permits lubricants to find their way between the parts.

While the drive is in motion, the space is not a problem. When travel direction is reversed, however, the mated parts must separate and travel in the opposite direction until they mate with the opposite part. The distance traveled, no matter how minute, causes backlash in linear motion. The greater the backlash, the more system accuracy and repeatability are compromised.

There are a variety of mechanical and electronic anti-backlash solutions. For example, purchasing preloaded nuts or active cam mechanisms. Other mechanical solutions include flexing gear systems, shims and mechanisms designed to preload the geartrain. Electronic backlash correction methods include various types of encoders and feedback devices attached to the load carrier.

The designer's goal becomes overcoming backlash without increasing overall package costs, which is difficult to do when adding extras like encoders or programmable drive motors. If a mechanical solution is used, the designer must be careful about using preloads. Adding load to the geartrain increases friction and wear.

In short, there are trade offs to consider when overcoming backlash in linear motion systems. The successful design will be the optimum marriage of functionality with cost-effectiveness.

Rolling ring linear actuator providers such as Amacoil/Uhing, offer production line-ready rolling ring linear motion assemblies for reciprocating or positioning applications.

A "rolling ring" linear drive is a mechanical power transmission device offering a different approach to overcoming backlash. One that is cost-effective because it does not require an investment in preloads, programming or additional encoders and motors.

In a rolling ring linear drive, a rolling ring bearing assembly (3 or 4 rings) is fixed within the housing (nut). The shaft is smooth and threadless. Each bearing in the assembly is held at a specific angle relative to the shaft.

Fig. B: The point contact between a rolling ring bearing and the shaft is constant -- regardless of the rotational direction of the shaft. This virtually eliminates backlash from linear motion.

The key to a rolling ring linear drive's near-zero backlash performance is the design of the rolling ring bearing. The inner race of the bearing is machined so that a central "ridge" runs around the inner race surface. When the rolling ring drive is placed on its shaft, the ridge of each bearing inside the drive forms a point of continuous contact with the drive shaft (Fig. B). When the shaft rotates, the rotary motion is immediately converted into linear output. There is never any free movement or "play" between shaft and bearing.

Rolling ring linear drives are available in fixed pitch models and variable pitch models. The fixed pitch drives are designed for incremental positioning and PLC-controlled operations. Variable pitch rolling ring linear drives are used in automatically reversing, reciprocating linear motion applications such as winding, ejecting, cutting, spraying and so forth.

Fig. A

In these types of "back-and-forth" applications, rolling ring linear drives eliminate the need for clutches, cams, gears and other external controls. Figure A shows how a rolling ring set-up can reduce the system's dependency on external controls.