Actuators with Fewer Controls Add
Efficiency to Linear Motion Applications

by John Scavitto, Product Line Manager, Amacoil, Inc., Aston, PA

Rolling ring actuator designer for reciprocating linear motion

For certain applications, using rolling ring linear actuators in designing linear motion systems can provide three benefits: the elimination of complex, electronic controls that keep overall costs of the linear motion system within customer budget; a minimimum of end-user investment in operator training and maintenance; and help in ensuring non-stop production. To meet such system goals as automatic reversal and rapid adjustments to pitch, most linear motion systems offer no alternative but to rely on a complex array of clutches, cams, gears, and other drive components and control systems. These systems require significant capital investments and highly skilled design, operations and maintenance personnel. And, the more devices in the system, the greater the potential for downtime. For some applications, such complexity might be warranted. Examples of these applications include batch control processes and precision machine tool applications. But for other procedures, especially reciprocating motion procedures such as spooling or winding, a simpler-designed system will be more advantageous.

The costs of complexity

The costs of designing and building a complex linear motion system are complex in themselves.

Spooling operation using screw-based system with many external controls

An OEM designer can consume considerable design time using conventional technology to configure complex controls for a routine linear motion system. The various components that go into a control system, for example, must be selected, compared, matched and tested. Customers may have accepted the fact that there is no other way to design a linear motion system without including high-priced and hard-to-maintain components such as multi-speed direct-braked motors, valves and solenoids, gear head assemblies, PLCs and more. However, designing a less-expensive, yet more-efficient system can provide more value. For example, a ball screw has to be stopped to reverse the traversing direction of the traversing nut. Clutches, gears and other accessories are needed to provide effective reciprocating movement. Additionally, the threads on a screw often require the purchase or construction of a bellows assembly to protect them from debris that could clog the screw and cause the system to jam -- resulting in damage to controls and equipment. Similarly, timing belts require servo/stepping motors, switches, encoders, sensors, slide tables, PLCs and other dollar-heavy investments in controls. Pneumatic systems pose design space limitations -- twice the stroke distance is required per piston. Hydraulic systems entail complicated mechanics and electronics as well as multi-speed, direct braked motors, pumps and solenoids.

Same operation with rolling ring actuator system

There are rolling ring linear actuators that are not dependent on such complex, expensive controls. This reduces design time and costs and therefore enhances profitability. At the same time, the customer enjoys a solution that provides significant savings on operation and maintenance expenses.

Production and profitability also suffer because traditional linear motion systems (e.g. screw-based, hydraulic, pneumatic, timing belt, etc.) often demand the downtime of valuable production machinery while performing routine tasks, such as changing gear ratios, cleaning threads, or adjusting pitch.

Rolling ring technology

The constant speed, unidirectional rotation of a smooth, unthreaded shaft in a rolling ring actuator is converted into linear output. Within the actuator housings, several ball-bearing-based rolling rings (the number dependent on the system design requirements) with specially contoured inner-race surfaces maintain continuous contact with the drive shaft. As the rings bear against the shaft, linear output is generated from the motor-driven rotary input.

In the type of rolling ring actuator shown, the operator-adjustable angle at which the rings contact the drive shaft determine the pitch, or the distance of linear output, for each rotation of the shaft. When reciprocating motion is required, adjustable end-stops are used to set the motion limits. Upon reaching the end-stops, the rings are automatically "flipped" internally to a mirror image orientation, instantly reversing the direction of the actuator assembly.

Rolling ring engineering is a very simple concept that combines compression and friction to produce linear output. These linear actuators operate using very few moving parts, and can deliver up to 800 lbs. of axial thrust. Travel length is up to 16 feet at speeds of up to 13 feet per second. Accuracy is typically within ±0.005 in. -- some rolling ring designs will deliver ±0.0004 in. accuracy. The intrinsic dynamic shaft/bearing interface of the rolling ring linear actuator is virtually backlash-free. Play has been eliminated at the reversal points, preventing bunching, tangling, and backlash that could result in mistakes in the manufacturing process resulting in a flawed final product that may be unusable.

Benefits

In many instances, rolling ring linear actuator technology reduces design time because it bypasses the need for complex, costly
controls.

Rolling ring linear actuators can be configured to handle a variety of linear motion applications including positioning, cutting, spooling/winding, spraying, slitting and packaging. Certain models feature automatic reversal and variable pitch, and are suited to reciprocating motion applications. Rolling ring actuators permit changes to pitch and traversing direction without adjusting motor speed or the rotational direction of the drive shaft. Additionally, these actuators operate without electrical switches, cams, gears and clutches, allowing the system to operate continuously without stopping the system to change the rotational direction of the shaft, but still permitting the necessary pitch/speed adjustments, thereby ensuring production throughput rates stay on or very close to predictable levels. Many styles perform the desired tasks using a simple, single speed, unidirectional, non-braked motor, resulting in simpler, less costly, less time consuming design and a higher level of customer satisfaction. Training expense and time are also reduced, and freedom from a complex and costly control system also results in lower maintenance expenditures.

These actuator units remain virtually clog-free because of the absence of threads on rolling ring system drive shafts, eliminating the need to build a bellows assembly. In the event of overload, rolling ring linear actuators "slip," instead of jamming. This affords operators and technicians time to remedy an overload situation before "churning" occurs (as with threaded systems) causing problems for expensive system components.

Processes dependent on linear actuator systems may sometimes require manual positioning of a load-bearing, traversing nut. To achieve this, most systems need to be started and stopped in succession to "jog" the linear actuator to a desired position. This tedious procedure consumes valuable production time. Rolling ring linear actuators eliminate this work because of a manual "free movement" override feature. This is usually a lever on the actuator housing which, when toggled, permits manual or pneumatically actuated positioning of the traversing housing without starting-up the system. This feature, in certain procedures -- such as changing-out spools or rolls -- saves time and expedites production.

Conclusion

Designing with certain styles of rolling ring linear actuators or ones having features similar to those described above can save OEM designers valuable design time. Bypassing complex controls results in faster, simpler, less expensive design.