Roller screw actuators appear to be winning market share over their ball screw, pneumatic and hydraulic counterparts, thanks to their robustness, speed and accuracy. Ball screws are generally limited to around 2,000rpm, but a roller screw can operate at 5,000rpm or more, which translates into higher linear travel rates. Moreover, the number of contact points in a ball screw is limited by the ball size, whereas planetary roller screw designs provide many more contact points, increasing the load carrying capacity and improving system stiffness.
When an electromechanical as opposed to more conventional pneumatic or hydraulic actuators is a viable option, Olsen Engineering believes that the performance of roller screw drives places them high on the list of buyers seeking servo controlled linear actuators. Not only do they take up considerably less space for a given load rating, they are also claimed to outlive a ball screw driven actuator by a factor of 15.
Olsen supplies Exlar Corporation's Tritex range of electric linear actuators in the UK. Tritex, with its novel inverted roller screw mechanism is illustrative of the capabilities of servo driven, roller screw actuators. This fully integrated package, comprising a servomotor, amplifier and position controller in a single IP54 sealed enclosure, can be directly programmed to perform specific tasks requiring motion sequencing and force control.
For example, the integral motion control software features drop-down boxes, which enable sequences to be created in which pre-programmed forces trigger actions such as stopping or retraction. This functionality allows the units to be used in press fitting or assembly applications, or as part of a dynamic test rig where accuracy, repeatability and reliability are of paramount importance.
A 75mm frame size Tritex actuator can deliver a stroke of up to 455mm and a peak thrust of 10kN (5.5kN continuous) - sufficient for applications that might previously have required a pneumatic or hydraulic actuator. Indeed, at the larger end of the range, 200mm frame actuators are available that provide peak thrusts of 288kN, linear velocities of up to 1.5m/s, zero backlash and a positioning accuracy of three microns.