May 2020

LINEAR MOTION n Two other factors that contribute to the load capacity of a ballscrew-driven rodless actuator are the ball nut arrangement and the carriage length. A longer ball nut or a double ball nut will increase the thrust capacity, while a longer carriage increases the length or spacing of the bearings and increases the moment capacity. For example, Thomson Industries offers a ballscrew, ball bearing guide actuator with a single or double ball nut. The single-nut version with a 80 x 80mm profile has a 200mm-long carriage, a thrust capacity of 3.5kN and a pitch moment (My) of 180Nm. The corresponding double nut version has a 280mm-long carriage, a thrust capacity of 5kN, and a My of 300 Nm. The double nut is also more rigid and will have a slightly better repeatability and reduced axial play. Timing belt thrust mechanisms These actuators are also robust and suitable for most industrial applications. Actuators with high- tensile-strength timing belts can handle large thrust loads, velocities up to 10m/s and are almost unlimited in length. For applications requiring strokes of more than 3m, they are more cost-effective than rolled ballscrew actuators. Timing belt actuators are also clean and resist contamination. A timing belt mechanism built on the same 120mm extrusion as a ballscrew would have a thrust capacity of 5kN with a 2,308 rpm input and a velocity of 10m/s. The belt travel would be 260mm per pulley revolution, with a position repeatability of ±.05mm. Choosing guide systems Careful specification of the guide systems that support the carriage and the forces acting on the attached load as it travels is another critical factor to the success of rodless actuators. They can be specified with plain bearings, ball bearings, cam followers and/or wheels. Plain bearings These have a higher friction factor than the others, but they often do not need lubrication and can survive in contaminated or wet environments. Plain bearings also dampen vibration, run quietly and can tolerate short strokes at high cycles. Plain bearings or slide guides either run along the aluminium extrusion or are on a set of rails formed in the extrusion. They can be made of different materials, such as hardened steel, stainless steel or anodised aluminium. The plain bushings can also be made of a choice of materials, including polymers, PTFE and low- friction plastics. Ball bearing guides These guides, consisting of recirculating ball bushings attached to the carriage, run on hardened steel rails, which are usually bolted onto the extrusion or steel inserts. There may be a single profile rail for a more compact actuator or a stronger double rail. Ball guides offer high accuracy, high load ratings and medium speeds. Wheels and cam followers Simple and economical, these also run on hardened steel rails which are formed into the extrusion, and offer high load capacities, high speeds and medium accuracy. The sealed ball bearings are maintenance-free and contaminant-resistant. When choosing a rodless actuator, the service factor is a key variable. After calculating the load, determine the service factor from vendor-supplied load ratings in the FX, FY, and FZ directions. The thrust mechanism handles the X component force; the Z component force is the weight sitting on the carriage plus any force perpendicular to the top of the carriage; and the Y component force acts perpendicular to the side of the carriage. From these moment loads (My, Mx, and Mz), pitch, roll and yaw can be solved. In a typical cycle, a linear actuator accelerates, runs, and decelerates at least once during its travel in each direction. There can, however, be many stops along the length of travel. The moment load is the force needed to accelerate the driven component and its position relative to the carriage. Even driven components with their own guide systems will still have pitch and yawmoments acting on the carriage. Motor options Typical prime drivers for rodless actuators are servo and stepper motors. Standard AC and DC motors can also be used to drive linear actuators, but their duty cycles are limited to the number of starts per hour that the motor can handle. Actuator manufacturers typically offer metric and Nema motor brackets, with or without a motor attached. The torque and rpm required will ultimately determine the motor’s size. System designers considering rodless linear actuators have many options fromwhich to choose. Almost any combination of leadscrew, ballscrew or timing belt, with slide guides, ball bearing guides or wheel guides, could work. Other factors such as cost, availability, environment and service factor will eventually come into play. It is critical to enter the specification task with a thorough understanding of the intended application and objectives across the component lifecycle. n IMPROVE FOOD PRODUCTION LINE EFFICIENCY SKF Food Line hygienically designed bearing unit SKF Food Line stainless steel deep groove ball bearing with solid oil SKF Food Line stainless steel bearing units High humidity High pressure washdowns High shock loads Your authorised SKF distributor www.acorn-ind.co.uk 0800 8766 441 enquiries@acorn-ind.co.uk Call us: Email us: Check availability online: Rodless actuators offer advanced precision and a wide choice of options A motorised leadscrew can provide high torque densities and long battery lives