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27/01/2004
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A linear actuator has been developed that combines an efficient rolling action with high dynamic load carrying capacity and is backlash-free. Dean Palmer reports
A UK-based power transmission company has developed a linear actuator that has an unusually efficient rolling action, a high dynamic load carrying capacity with almost no axial backlash or wear, and which could find its way into many applications where historically hydraulic or pneumatic actuators were used.
Power Jacks' actuator is based around two key building blocks - the 'actuation' unit and the 'drive' unit. The actuation unit is based around a basic lead screw assembly. To meet the performance range two types are used - a high quality ballscrew and nut for medium duty applications and a roller screw and nut assembly for medium to high duty applications. The roller screw design is essential to meet the high duty aspect while keeping the unit compact and providing the customer with a practical life rating for the unit.
Roller screws in general have higher dynamic load carrying capability than ball screws for a given working envelope. The roller screw assembly in the Rolaram unit is Power Jacks' own 'Spiracon' design. The Spiracon design consists of a multi-start screw with an involute thread form and a number of planetary rollers with annular grooves, which engage with the screw.
The rolling action results in a high efficiency mechanism, while the line contact and hardened and ground construction achieves a high dynamic load carrying capacity along with almost no axial backlash or wear. The lead screw assembly is packaged inside a ram arrangement with internal and external guides and seals to provide a sturdy enclosure. At the drive end of this assembly is a thrust housing which carries the load from the lead screw to a mounting that allows the unit to be secured to the surrounding structure.
The 'drive' unit, which is attached to the 'actuation' unit, provides the rotary motion, which the lead screw converts to linear motion. The "drive unit" is either a helical or bevel gearbox that allows the motor to be mounted either parallel, in-line or at right angles to the actuation unit. This allows the actuator to fit into most application spaces without special modifications. The mounting options for the units are equally varied and flexible and include the classic trunnion to clevis and double clevis arrangements.
As each actuator will require a varying level of motion control for each application the units have the capability to have a wide array of feedback devices fitted. These include encoders (incremental and absolute), limit switches (mechanical or proximity sensors) and clutches.
Power Jacks' Rolaram linear actuator range provides a powerful, flexible linear motion solution, designed to satisfy medium to high-duty actuation applications with 5kN to 400kN loads, at speeds of up to 7000 mm/min as standard. Higher speeds are available as special designs.
Since their initial development, the actuator has undergone extensive tests both in-house and in industrial applications globally. The results have been successful even when the units were operated to the extreme, with some operating on a continuous cycle basis, 24 hours a day in harsh environments.
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Author
Tom Shelley
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