Rolling Ring Linear Drives

An alternative to motion controller-based linear motion systems, rolling ring linear drives meet a variety of positioning and reciprocating linear motion applications. A purely mechanical device, rolling ring drives convert the rotary motion of a smooth, threadless shaft into linear output at fixed or variable linear pitch.

How it works
The inner race of a rolling ring bearing is machined to form a ridge, which runs around the centerline of the inner race surface of the bearing. When mounted on a shaft, a rolling ring bearing contacts the shaft only on the apex of this ridge. The bearing may be pivoted left or right on the shaft and still maintain point contact with the shaft.

In a rolling ring linear drive, an assembly of three or four rolling ring bearings is fixed and compressed within a housing. A smooth, threadless shaft is inserted through the housing, and through the rolling ring bearing assembly. When the rolling ring bearing assembly is angled on the smooth shaft, and the shaft is rotated, compression against the central ridge surface causes the bearings to roll along the length of the shaft carrying along the load-bearing housing.

The rotary input provided by the motor-driven shaft is thereby converted to linear output. The adjustable angle of the rolling ring bearing assembly, relative to the shaft, determines travel direction and pitch (speed).

No Backlash
Backlash is inherent in many types of linear motion devices. For example, in ballscrews, backlash results from the amount of clearance between the ball bearing and the threads in the screw. Backlash may also be caused by clearance or play between contacting surfaces in gears, belt drives, and other linear motion drive systems.

To prevent backlash in ballscrews, users typically preload the nut to keep the balls under steady pressure. Other methods for fighting backlash include active cam mechanisms, shims, and other devices, which load the geartrain. These methods add cost to the linear motion system and their performance level deteriorates over time due to wear and tear.

Rolling ring drives require no preload to eliminate backlash. The drives are engineered to assure continuous point-contact between the rolling ring bearing surface and the shaft. There is no play and virtually zero backlash – even during reversal. Additionally, rolling ring drives have built-in overload protection. If the system is overloaded, the nut will slip, not jam, because there are no threads where material can trap or clog.

Backlash may also be caused by clearance or play between contacting surfaces in gears,
belt drives, and other linear motion drive systems.

Linear Pitch
Linear pitch is the linear distance the nut travels per shaft revolution. Most linear motion devices, like screws, have a fixed pitch.  Some types of rolling ring drives, however, offer variable pitch. This is accomplished without changing gears or relying on clutches, cams, and control systems.

In a rolling ring drive, the angle of the rolling ring bearing assembly on the shaft is manually adjustable on-the-fly by moving a lever on the rolling ring drive unit. Adjusting the angle changes the drive’s pitch. Therefore, a variable pitch (linear speed) system is possible without programming or complex control system.

Modifying Speed, Travel
Controlled adjustment of the angle of the rolling ring bearings on the shaft lets rolling ring drive users make precise changes to linear speed and travel direction without any adjustments to the motor speed or shaft rotation direction. For example, if a rolling ring bearing assembly is pivoted so that it is perfectly perpendicular to the shaft, the pitch is essentially zero. So even though the shaft-ring contact is still perfectly intact (zero backlash), and although the shaft is still rotating at a constant speed, the nut is essentially in neutral and will not move. Re-actuating the rolling ring assembly causes the nut to move again and linear movement resumes. If the rolling ring bearing assembly is pivoted only part way – stopped before it becomes perpendicular on the shaft – then a range of changes to travel direction and linear speed is possible regardless of the rotational direction or speed of the drive motor.

This purely mechanical control over travel direction and linear speed simplifies design, setup, and operation. This is especially true when compared to systems that require designing-in clutches, brakes, gearboxes, servomotors, and complex control systems. With rolling ring systems, typically, a comparatively inexpensive, uni-directional drive motor may be used and this further minimizes operating and maintenance costs. Since rolling ring systems operate continuously, even when changing direction or pitch, production rates and throughput improves.

Right for You?
rolling ring linear drive is a mechanical power transmission device offering an alternative to screw-based systems and other types of linear motion set-ups. Rolling ring drive systems are cost-effective because they typically do not require an investment in preloads, programming, or additional encoders and motors.

Fixed pitch rolling ring drives might require controls similar to a ballscrew system. However, for repetitive, back-and-forth linear motion applications, variable pitch rolling ring linear drives can save time and money by eliminating the need for clutches, cams, gears, and other external controls.

Rolling ring drive suppliers will typically supply the drives within a production framework that is dropped into manufacturing equipment. Most suppliers also provide the drive units separately. If the drive is in a production framework, adjustable end stops are used to control stroke length. When the linear drive traverses to the point where the mechanical reversal mechanism contacts an end stop, the reversal mechanism is turned and the drive reverses itself automatically without reversing the drive motor.

Using specific application data, rolling ring drive suppliers can correctly size a rolling ring drive system. Additionally a rolling ring drive rep can recommend useful options to meet unique linear motion application needs.

To see if your linear motion application might benefit from a rolling ring drive system, contact your linear motion component supplier.

Amacoil Inc.
Aston, PA
amacoil.com