May 2020

42 n LINEAR MOTION May 2020 www.drivesncontrols.com Realising the benefits of rodless actuators E lectromechanical rodless linear actuators can offer better load handling in smaller spaces than rod- style actuators, but achieving these benefits requires careful attention at the specification level. Whether you are replacing a failed actuator, looking for better performance, or building a new application from the ground up, success depends on how precisely you specify the application requirements. Unlike rod actuators, in which a piston extends unsupported beyond the actuator housing, rodless actuators use a carriage (also known as cart or truck) on which a thrusting mechanismmoves a load back and forth along a track. Getting the optimal efficiency, reliability and overall performance requires careful balancing of numerous variables. Whether changing actuators or specifying new ones, the basic principles are similar. Rodless actuators are distinguished from each other primarily on the basis of their lengths, widths and heights. Typical dimensions range from 40 x 40mm up to 120 x 120mm. Within that span, achieving the potential benefits requires a careful analysis of required stroke length, maximum speed, dynamic carriage loading and bearing requirements. Key components are the thrust mechanisms and guide systems, supplemented by motors, sensors and other accessories. Choosing thrust mechanisms Thrust mechanisms used widely in rodless actuators include leadscrews, ballscrews and timing belts. Leadscrews Also, known as acme screws or trapezoidal screws, these deliver rigidity and high thrust in a small package. The action between the screw and nut is sliding, has a high friction factor and is inefficient, although it sometimes can be beneficial in that it has a self-locking tendency when the system is at rest. Increasing clearance caused by wear between the screw and nut will determine the useable life of the actuator. Some suppliers fit actuators with plastic, self-lubricating nuts, which extend their lives. The pressure velocity (PV) limit is another factor affecting leadscrew load and speed ratings. Pressure equates to thrust, so as the thrust goes up, the speed goes down, and vice versa. Leadscrew actuators can have strokes of up to 3m, but are most often found in smaller, lighter-duty applications. Ballscrews Ballscrew actuators are muchmore robust than leadscrews and fit better inmost industrial applications. For example, a ballscrew actuator based on a 120mm extrusion with a 32mmdiameter and a 20mm-lead, precision- rolled ballscrew, would have a thrust capacity of 12kN and a maximum velocity of 1m/s with a 3,000 rpm input. The same actuator and screw diameter deployed with a 40mm lead, however, would have a velocity of 2m/s and a thrust capacity of 8kN. A typical precision-rolled ballscrew and ball nut could have a position repeatability of ±0.01mm or less. Rolled ballscrew actuators are cost-effective for applications requiring strokes of up to 3m. They can be chosen to optimise power density or small size per thrust. The L10 life is predictable because the ball nut is essentially a ball bearing and uses the same ISO calculation. The length of the screw affects the load rating, due to the buckling limit, and generally the speed rating, due to excessive vibration. Some suppliers, however, add screw supports that allow input speeds of up to 3,000 rpm, independent of the screw length. Rodless electromechanical actuators offer attractions including good load-handling and high in precision. Alexander Schollin, product line systems specialist for Thomson Industries in EMEA and Asia, and Max Miller, Thomson’s representative at US-based Motion Industries, offer advice on how to choose and specify these actuators. Rodless actuators are often used in packaging machines where they provide efficient, precise load-handling