Lead screws have been widely used to generate linear motion from a rotary motor. It can convert circular motion to linear motion by turning the screw with the nut fixed in place, or turning the nut while the screw remains in place. Today, we will look at two of the most popular screw types in the automation industry: ACME Screws and Ball Screws.
ACME screws use a trapezoidal shape thread to roll on to the lead screw. As the shaft rotates as the rotatory motor turns, the threads push the shaft nut forward or backward depending on the direction of the rotating motion. This transfers the circular force of the motor into linear motion on the shaft. All our actuators use the ACME screw and for this example of ACME screw linear actuators, we’ll be using one of our PA-14 Mini Linear Actuators.
The ACME screws provide many advantages for its applications. With the right material for the nut, the ACME screws can achieve self-lubrication. Our linear actuators use polymer nuts, which provide a very low frictional force. This can reduce or even eliminate the need for lubrication. On the other hand, the metallic nut can provide a higher force load, but it will also have a higher frictional force and require lubrication. ACME screw linear actuators are generally a good choice where vertical applications are concerned.
It can prevent back driving because of the inherent frictional force between the trapezoidal thread and the nut. With a preloaded nut, it can eliminate the risk of backlash entirely. This broadens the application scope of ACME screw linear actuators greatly. It is also safer for users to operate their linear actuator when the unit has a static load capacity to hold whatever weight is in place. Other benefits of the ACME screws include low operating noise and low manufacturing cost.
ACME screws may have a lot of great advantages but there are two major limitations that come from using them: low efficiency and shorter life cycles. Due to the high frictional force between the nut and ACME screw thread, most units can only achieve 20% to 40% efficiency depending on the material of the nut and lead screw. While this prevents units’ back drive, it also requires more motor torque to achieve the same force compared to its counterpart ball screws. This means a linear actuator using ACME screws will have a lower force, speed and duty cycle rating than a ball screw linear actuator using the same motor.
Ball screws use circular threads on both the screw and the nut, which provides a helical raceway for ball bearings to slide along the threads. The ball bearings roll through each helical raceway as the nut or the screw moves, then pushes either the screw or nut forward, creating linear motion.
Ball screws suffer minimum frictional loss thanks to the ball bearings. The nut and screw thread do not make contact directly but move along using ball bearings rolling between them. It is able to achieve 70% to 95% efficiency. This will allow linear actuators using ball screws to have a higher force, speed, and duty cycle rating. With lower frictional force, its nut and screw suffer less physical wear and tear, hence a longer life cycle.
The major setback for ball screw linear actuators is that it can easily back drive due to its low internal friction. This creates a safety concern for its users since it cannot hold its load in place when not in operation unless a braking system is in place. When a ball screw linear actuator is in operation, the ball bearings will constantly roll around inside the nut and screw, which cause a higher noise level than its ACME screw counterpart. The manufacturing cost for ball screws is also higher than ACME screws.