Rolling Ring Drives in Linear Motion

Bob Eisele, Amacoil, Inc.

In linear motion systems, compression and friction on drivetrain components are normally necessary to assure efficient power transmission. However, compression and friction generate heat and wear, which ultimately have adverse affects on drive performance and longevity.

To counter these effects, compression and friction must be managed through basic maintenance. For example, the threads in a screw-based linear drive system wear over time. Lubricants help retard the degrading effects of compression and friction.

A linear drive technology that takes advantage of compression and friction—rather than forcing the user to minimize it—is rolling ring linear motion. A rolling ring system uses a specially designed bearing which relies on compression and friction to convert the rotary motion input of a smooth (threadless) shaft into linear motion output.

Rolling ring drive linear motion employs compression and friction for efficient function at little cost to system longevity. As seen in Figure 1, the inner race of a rolling ring bearing is machined to form a sort of "ridge." The apex of the ridge is in constant point contact with the drive shaft.

Figure 1

In a rolling ring drive, a series of bearings are fixed and compressed within a housing (Fig. 2). The payload is attached to the housing. When the shaft turns, the friction against the ridge causes the rolling ring bearing to roll along the length of the shaft.

Figure 2

Rolling ring bearings are always in point contact with the shaft—even during reversal—so there is no backlash during linear motion.

The ridge on the inner race of rolling ring bearings enables the angling of the bearing assembly relative to the shaft. The angle of the bearings determines the rolling ring drive's travel direction and linear pitch (speed).

Pivoting the bearings at the end of each stroke causes the drive nut to change direction (Fig. 3)—without requiring reversal of the rotational direction of the motor. Thus, a variable speed, bi-directional drive system can be powered with a relatively inexpensive single-direction motor. No electronic controls or programming are needed.

Figure 3

In repetitive, back-and-forth linear motion applications, this simplifies design, setup and operation and also reduces cost. Spraying, slitting, coating and other applications where a nozzle, guide or other tool must be moved back and forth may employ a rolling ring drive system in this manner.

Read about rolling ring drives in Winding and Spooling Applications

Rolling ring systems are attractive from the standpoint of minimizing system dependency on controls and reducing costs. However, there are applications that require special attention if a rolling ring drive is to be used.

Like all machined parts, rolling bearings have tolerances. Two rolling ring bearings having the same part number can have slight differences in the contour of the inner race. This gives each bearing slightly different linear motion performance characteristics.

Consequently, if two of the same model rolling ring drives are placed on shafts and the shafts are driven by a common motor, it is possible that the travel distance of the two identical drive units will differ by minute amounts.

Over time this difference accumulates, which means that applications requiring rolling ring drives to run in tandem must have separate motors. Furthermore, a motion controller coordinating the two motors—or switches signaling the drive's position—are needed.

In this case the system sacrifices the advantages of mechanical control. A control system, similar to a screw-based system, is required.

Figure 4 shows an x-y positioning table using two identical rolling ring drives to move each axis. Proximity switches are used to signal the position of each drive nut. A motion controller coordinates the switch information and controls the motion of each motor accordingly.

Figure 4

Because the rolling ring linear motion drive is a friction drive, it can slip if it's overloaded. Overload in a rolling ring system is usually caused by:

• Selecting a rolling ring drive with insufficient axial thrust to move the payload at the desired speed

• Worn bearings

• Using the wrong lubricant (which eliminates the friction necessary to give the drive thrust)

• Mounting a payload that is either too heavy or that places overturning moments on the drive housing

Additionally, rolling ring drives will slip if something interferes with the pivoting action of the rolling ring bearings. For instance, if the screws or bolts used to mount the payload are too long, they can extend into the housing and interfere with the pivoting of the rolling ring bearings.

Rolling ring drive suppliers can provide options which easily overcome the abovementioned issues. The rolling ring drive provider can also assure correct selection of a rolling ring system. More than a matter of simply selecting a drive with enough axial thrust, correct selection of a rolling ring linear drive is based on numerous application requirements, including:

• Payload weight
• Drive shaft RPM
• Travel distance
• Estimated resistance to movement
• Rate of linear speed
• Manner in which payload is attached to nut

A properly sized rolling ring system can last ten years or more. Conversely, once a rolling ring drive is correctly selected, changes to application requirements can cause an otherwise properly sized rolling ring system to wear out in a year or less.

Rolling ring drive providers can offer assembly design options to meet specific linear motion requirements. Instead of adjusting linear speed with electronic controls and programming, rolling ring systems use basic hardware fittings which change the angle of the rings while the drive is in transit. The result is a mechanical solution to controlling linear motion.

For example, Figure 5 shows how simple, V-shaped cams may be used to pivot the rings in a rolling ring drive. In this case, a spray gun is mounted to the drive. The V cams at each end of the stroke ramp down the drive as it nears the end of its travel, then ramp it up after reversal.

Figure 5

This is an inexpensive alternative to clutches and gears. Design engineers desiring to eliminate sudden stops and starts from reciprocating motion processes will find rolling ring drives a simple solution which does not complicate design.

Rolling ring technology is an elegant design concept that can frequently result in simpler linear motion systems. For best results, discuss your application with the rolling ring drive provider.