Smooth motion, fast accelerations, and a high degree of accuracy are hallmark requirements for linear-movement actuators. Increasingly, compactness is an additional requirement.
One system that can meet all those requirements in its conversion of motor torque to linear thrust is the single-axis ball-screw actuator. The ball-screw actuator is a combination of a ball screw on which, to eliminate backlash, a slide block or nut’s movement is guided by recirculating steel balls that roll between the block and raceways of guide rails.
Traditionally, many equipment manufacturers designed and assembled their own custom ball-screw actuators. But, choosing to assemble ball-screw actuator components such as a linear guide and a ball screw into a custom housing usually creates a larger unit overall than using a predesigned system. A predesigned system is much more compact because the guide rail of the actuator is integrated with the structure of the actuator, and the slide block has the ball-screw nut incorporated into it. Generally, to build an actuator from assorted components, one would have to have a housing to put a ball-screw nut into. Also, there would be a separate base for linear guides. So, the entire unit would be much larger—as much as 30% larger.
Choosing the most effective actuator for a particular application requires consideration of load capacity, operation speed, stroke length, environment, orientation, and positional accuracy. These factors have to be identified and quantified. In addition to ball-screw and guide-rail size, load capacity depends on the size of the recirculating steel balls that roll between the block and the raceways in the guide rails, as well as the number of balls in contact with the raceways and the manner in which they make contact. One way to meet load capacity is by increasing the size of the ball screw and guide rails. Another way to increase load capacity, which does not increase the overall size of the actuator, is to increase the number of ball circuits. Doubling the number of ball circuits to two on either side of the block doubles the load capacity of the actuator.
Actuator systems are generally offered in two or three levels of accuracy ranging from “commercial grade,” to “high” or “standard grade,” to “precision grade.” To compare systems’ levels of accuracy, one cannot assume that all manufacturers’ lowest to highest grades have comparable accuracies. It is necessary to compare their published ranges for: positioning repeatability; positioning accuracy; running parallelism; backlash; and starting torque.
Aspects of a linear actuator that affect its precision include how true its guide rail and its raceways are and how smoothly in the block and raceways the balls recirculate. Precision-ground guide rails, slide blocks, and ball screws provide optimum accuracy.
To ensure positional accuracy, the balls within the ball grooves of the raceways must not have clearance that allows them to deflect. A slightly elliptical groove design allows the balls to make contact at two opposing points but allows a bit of clearance on the balls’ sides that are perpendicular to the contact points. The four-point contact arch design is, because of its shape, called a gothic arch. The gothic arch eliminates any clearance that could lead to deflection.
Ball-screw actuator rigidity is affected, primarily, by the composition of the guide rail, as this is the outer structure of the system. The thickness and strength of the lower edges of the guide rail are critical to its rigidity. A U-shaped outer rail provides better rigidity against moment loads.
Guide rails positioned lower than the ball-screw center also increase rail rigidity and allow the block to carry heavier loads. In combination with the more rigid U-shaped style rail, this design even allows one-end-supported applications. Another advantage to guide rails positioned lower than the ball-screw center is greater compactness. Also affecting rigidity is the number of ball circuits. Four ball circuits provide greater rigidity than two ball circuits, all else being equal (ball, guide rail, block, ball screw).
Depending on the application, the speed at which the actuator must travel is a determinant of the length of the ball-screw lead. The faster the desired travel time, the longer the lead must be. There is a direct 10:1 correlation of speed to length. However, to achieve higher accuracy, it is best to use the shortest possible lead for the job. The merit of a shorter lead is that it can move a heavier load using a smaller motor. To achieve the same speed with shorter lead, the motor must be bigger.
Ball-screw actuator systems are typically available with metal covers. However, the standard metal covers have gaps between the cover and the guide rails. This makes them unsuitable in environments where liquids or particulate matter could enter the system. Though usually a customized option, accordion-pleated bellows-type covers are available. They replace the metal cover, cover the entire guide, and are impervious to fluids and particulate matter.
Naoki Yamaguchi, Assistant Technical Manager, NB Corp., wrote this article for SAE Magazines.