‘Rolling Ring’ Linear Motion: A Mechanical Alternative
Bob Jeter, Applications Engineer, Amacoil, Inc.

Painting, grinding, measuring, scanning, stirring, cutting: machines designed to perform these and similar

Figure 1: In an electronically controlled winding system, a motion controller and other components are needed to synchronize the linear movement of the linear drive with the rotational movement of the spool.

processes incorporate a linear drive assembly to move the drive head back and forth. In these instances, a primary design goal is smooth, reciprocating linear motion.
Equipment offering traversing or reciprocating linear motion often features automatic reversal of the drive head and the ability to adjust linear speed. Almost invariably, this involves multi-speed, direct-braked reversible motors, sensors, valves and solenoids, gear head assemblies, PLCs and other useful and innovative devices.

Whether you design, operate or maintain this type of machinery, you’re probably familiar with these components, and you’re likely aware that they can increase the complexity and cost of machine design, operation and maintenance. However, depending on the linear motion requirements indicated by a specific production process, it is frequently possible to simplify the linear motion system by using mechanical methods to meet motion control needs.

An automated spray painting process, for example, may not need sophisticated electronic controls to move the spray head back and forth. Electronic controls can mean investing in costly operator training. Additionally, repairs may require the services of electronics experts. While training and repairs are pretty much accepted as a fact of life in linear motion systems, it makes sense to examine the application to see if a mechanical system is a viable alternative.

Mechanical linear drives reduce or eliminate concerns about programming and electronics.
Set-up for mechanical systems is largely intuitive, and operating mechanical components requires little to no training. Maintenance on mechanical linear motion systems is basic, with the devices being relatively easy to fix and thereby reducing downtime and helping to sustain consistent product rates.

To examine how a mechanical system can simplify linear motion automation, let us look at a winding system. Figures 1 & 2 illustrate how a winding system (using a reciprocating drive) may be simplified by substituting a mechanical traverse drive as opposed to a motion-controller-based system. The mechanical system here is a “rolling ring” drive. A series of specially machined bearings are compressed within a housing. The payload (a guide roller in this case) is attached to the housing. When the shaft turns, the friction against the bearings’ inner race causes the drive to roll along the length of the shaft. The rolling ring bearings are always in point contact with the shaft, even during reversal, so there is no backlash during linear motion.

The speed and travel direction of the drive is not dependent on the rotational speed or rotational direction of the motor. Instead, speed and direction are controlled by the angle of the rolling ring bearing assembly on the shaft. The angle is adjustable, enabling on-the-fly changes to linear speed. Since there is no requirement for gearing down, changing the rotation direction of the motor or adjusting any controls, this variable speed mechanical system may be driven by a relatively inexpensive, single-speed, unidirectional motor.
 Manipulating the angle of the rolling ring assembly on the shaft enables the modification of linear motion to better meet production needs. Ramping down or up and dwelling in place are examples. A rolling ring drive may be fitted with hardware that mechanically pivots the ring bearing assembly to achieve the desired changes in linear motion. Since modifying the linear motion is achieved mechanically, without changing the motor speed or shaft rotation direction, this means a variable speed system may be free of clutches, cams, gears and so forth.

When using a rolling ring system, linear speed and linear pitch should not be confused. Adjusting the pitch setting on a rolling ring drive affects the linear travel speed of the drive. Changing the rotational speed of the motor also will affect the linear speed, but it will not change the linear pitch. Envision a rolling ring drive that travels one inch per shaft revolution at a shaft rotational speed of 10 rpm. If the shaft speed is doubled, the drive will travel the one inch at a faster rate, but it will still travel only one inch per shaft revolution.
Correctly exploiting rolling ring bearing performance characteristics permits the use of a rolling ring linear drive to meet a variety of other linear speed and motion requirements normally associated with more complex systems that may include stepper and servo motors, controllers and sensors. Unlike linear motion systems that rely on these devices to achieve reciprocating linear motion, a rolling ring linear drive readily enables all changes to linear motion through purely mechanical means.

Reversal of travel direction occurs when the spring-actuated reversal mechanism on a rolling ring drive is triggered by contacting an end stop. When the end stop is contacted, the ring bearing assembly flips to its “mirror” position on the shaft. Controlling the degree to which the bearing assembly flips permits slowdown and ramp-up. This is done mechanically, making the process totally independent of the drive motor or other controls.

At no time does the rolling ring bearing lose contact with the drive shaft. This is how rolling ring linear drive assemblies prevent backlash, because there is no play between shaft and bearing. Furthermore, the shaft on which a rolling ring bearing operates is not threaded, which means dirt and debris cannot be trapped and cause clogging or jamming.

“Ramping” down or up is usually desirable to lessen the effects of jarring or jerking on the payload attached to the linear drive. For example, suppose a finishing operation involved the attachment of a buffer on the end of an extension arm. The extension arm, mounted to the drive head, travels back and forth when the system is in operation. If reversal isn’t gentle, the inertia and whip-like action of the arm could wrench or torque the system, possibly damaging valuable components.

Normally, meeting motion requirements for ramping up or down during the reversal process means designing in clutches, gearboxes and control systems. With a rolling ring system, however, controlled changes to linear speed are achieved with relatively inexpensive hardware modifications to the auto-reverse mechanism on the bottom of the rolling ring drive unit.

The linear speed of the drive head may be changed while the drive is operating simply by adjusting the pitch control, which changes the angle of the rolling ring bearing assembly on the drive shaft. This increases or decreases the drive head’s travel distance relative to each revolution of the shaft. This translates into an increase or decrease of linear speed, even if the drive motor speed and rotational direction remain unchanged.

For some applications, rolling ring drive systems do require additional controls to meet linear motion requirements.

Figure 2: When the rolling ring drive shaft is belted to the drive shaft, linear movement is automatically synchronized with rotational movement, without clutches, cams or gears.

For example, a metrology machine moving measuring probes or scanners forward and backward requires the instruments to be accurately positioned. Accurate positioning requires two-way shaft rotation and sensors or some other device to tell the drive how far to travel. In this case, the rolling ring system would need a control system similar to a screw-based set-up possibly utilizing a PLC controller.

Rolling ring drives are friction drives. Their sole function is to provide side thrust. Any load placed on the drives (weight or overturning moments of force) must be relieved. This is usually accomplished with a linear bearing slide load carrier.

In summary, to control costs and reduce complexity, it is sometimes desirable to reduce the need for electronics and programming in variable speed linear motion systems. Besides simplifying design and operation, mechanical linear motion such as a rolling ring drive helps streamline maintenance. There is no need to spend time on tasks such as replacing bent piston rods, replacing leaky seals, programming controls and other procedures.

Travel direction and linear speed are mechanically controlled and independent of the drive motor speed and rotation, thus eliminating the need for electronic control systems. For designers, rolling ring engineering simplifies machine design and helps control costs. For production personnel, it reduces both operator learning curve and the time spent adjusting controls during operation.

Rolling ring drive assemblies are designed to perform a variety of automated linear motion processes. When an application calls for involving repetitive, reciprocating motion such as slitting or spraying, rolling ring drive assemblies pose a practical alternative to traditional linear motion systems.

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