Machine Design

Rolling-ring linear actuators put friction to work

Compressive forces (labeled F and F/2) keep the three inner rings in contact with the driveshaft. Force on the upper, central ring is balanced by compressive forces on two lower, flanking rings. A four-ring actuator has two upper rings instead of one. Inner rings rotate with the shaft and outer rings are fixed to the translating housing. Inner rings set at an oblique angle to the driveshaft. Friction between the inner rings and rotating driveshaft produce axial thrust to drive the rings and attached housing.

Direction of travel (and thrust) can be reversed with a simple mechanical switch. The switch flips rings to their mirror image angle at the desired reversal points. This action is triggered automatically by the motion of the actuator itself (again, through the application of friction and compression), and requires no external devices. And direction reversal is backlash-free and far simpler mechanically than in traditional actuator types.

The ability to produce smooth, precise, and accurate linear motion is key to many manufacturing and industrial processes. A variety of linear actuators can do the job including screw-type actuators (ball, Acme, or leadscrews), pneumatic and hydraulic cylinders, and timing belts. Here, good designs minimize friction and compressive loads because these factors waste power, promote wear, and generate excess heat.

But linear actuators built around a novel "rolling-ring" design from Amacoil Inc., Aston, Pa. (, use compression and friction forces to smooth rather than impede linear motion. They also substantially reduce mechanical complexity, maintenance, and overall operating costs compared with conventional linear drives.

Friction and compression
In traditional linear actuators, friction and compression waste energy and compromise smooth linear motion. For systems based on hydraulics or pneumatics, friction during movement saps available power and creates heat. It can also slow or produce jerky motion. Friction in timing-belt drives can lead to premature belt wear and heat build-up.

Screw-type actuators use a threaded nut to carry payloads and therefore must ride smoothly as the threaded shaft turns. Excessive friction between nuts and shafts promote wear and tear on threads, and in extreme cases, may seize nuts to shafts, resulting in costly repairs and downtime. Unfortunately, friction is inherent to threaded devices. Lubricants are probably the most common way to help reduce friction. Some manufacturers lower friction by using nuts made from special materials while others plate or coat threads.

Most thread-type, reciprocating-motion linear drives use preloaded nuts. Here, a spring or adjusting collar maintains compression on threads to eliminate play during direction reversals. A properly designed preloaded nut can nearly eliminate backlash, but the compressive forces inherent to the setup increase friction between threaded surfaces and accelerate wear. Alternatives to spring collars aren't without problems.

For example, it's nearly impossible to precisely match female and male threads so as to maintain complete contact. These metal-to-metal designs must have space to accommodate lubricant and prevent seizing. Self-lubricating designs, on the other hand, require tighter manufacturing tolerances to ensure complete thread-nut contact and can be costly to produce.

Rolling-ring technology
Contrast these designs with rolling-ring technology. Rolling-ring linear drives move loads with a rotating, smooth, unthreaded driveshaft. Replacing the threaded nut is a housing containing three or four shaft-mounted, metal rolling-ring bearings. The bearings mount in a pivoting ring carrier enclosed in a housing. Each bearing contains a specially machined inner race that maintains contact with the shaft and is free to rotate with it. The outer race is fixed to the pivoting ring carrier. The rings themselves mount at an oblique angle to the driveshaft.

Rotating the driveshaft applies an axial thrust (from friction and compression) to the rings and housing. Adding one extra ring (changing a three-ring into a fourring design for example) doubles axial thrust. In models that produce reciprocating motion, the same forces also trigger direction reversal. Power-transmission efficiency for the devices is greater than 90%.

Ring-to-shaft angle determines pitch or linear distance traveled per shaft rotation. Some designs are fixed pitch while others have rings that adjust to different angles. Pitch adjustment can be made as the shaft rotates, allowing a fixed-speed driveshaft to generate a wide range of linear travel rates.

Other advantages
The mechanical simplicity of rolling-ring actuators reduces maintenance costs, adjustments, and operator training. Consider maintenance due to environmental contaminants for instance.

Screw-type actuators may jam when fouled with debris and dirt requiring that threads be cleaned frequently or protected with a bellows assembly. And pneumatic and hydraulic devices typically rely on seals to exclude debris. In contrast, the unthreaded shafts of rolling-ring actuators self-clean and normally require no such protection or maintenance.

Routine operations are simpler as well. Screw-type actuators can jam when nuts seize to shafts, or when nut movement is blocked or hampered. In contrast, rolling-ring types — when inadvertently stopped — slip instead of jam, preventing costly equipment damage.

Precisely positioning the devices is another factor. Traditional actuators must briefly start and stop (jog) into position which accelerates wear and tear on mechanical and hydraulic/pneumatic assemblies. Rolling-ring devices instead use a "free-movement" lever to un-load compressive ring-shaft forces to permit effortless manual positioning. This helps cut downtime for setup and changeover.

Nonreciprocating models are ideal for indexing, materials handling, and X-Y table positioning. Reciprocating rolling-ring actuators provide backlash-free direction reversal for spooling, winding, spraying, cutting and slitting operations.

This information supplied by John Scavitto, product line manager at Amacoil Inc., Aston, Pa.

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