Length, depth, and width are only part of the size story when choosing small actuators for an application. Knowledge of how smaller scaled components affects operation will ensure you make the right choice for your application.
Ballscrew actuators move accurately and smoothly, and accelerate quickly. Factors such as load capacity, operation speed, stroke length, environment, orientation, and positional accuracy determine the appropriate actuator for an application. Another selection factor gaining prominence is size. Increasingly, device and system specifications call for actuators that do not take up much space. But for actuators in the four-foot and under class, component design differences affect their operation in ways that may be crucial to your application.
The more ball circuits on an actuator, the greater load it can carry. Doubling the number of ball circuits to two on either side of the block doubles the load capacity of an actuator.
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, the number of balls in contact with the raceways, and the manner in which they make contact.
One way to meet load capacity is to increase the size of the ball screw and guide rails. Another way, which does not increase the overall size of the actuator, is to increase the number of ball circuits.
Most standard ball screw actuators have one set of recirculating balls on either side of the block. Doubling the number of ball circuits to two on either side doubles actuator load capacity. For a four-foot rail travel, an example of maximum load for a standard 1,380 mm rail length with two ball circuits is 37 kilonewton (kN). With four ball circuits it is 74 kN.
On the low end of the actuator size range, an actuator with a 100 mm length rail (about 4 in.) with two ball circuits can be expected to have a load capacity of 3.945 kN, whereas with four ball circuits its load is 7.89 kN.
Actuator systems are generally offered in two or three grades – or levels of accuracy. “Commercial Grade” is the lowest; next is “High” or “Standard Grade;” with “Precision Grade” the highest. To compare systems’ levels of accuracy, do not 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.
Among other aspects, precision is affected by how true its guide rail and raceways are and how smoothly in the block and raceways the balls circulate. With travels four feet and under, the slightest deflection or clearance of the recirculating balls can significantly affect accurate movement and positioning.
In this size range, for optimum accuracy, it is critical that the guide rail be precision ground. The same can be said of the slide block and ball screw. Furthermore, to ensure positional accuracy, the balls within the raceway ball grooves must not have clearance that allows them to deflect.
Of the groove designs on the market, the standard choice is between balls that make contact with the raceway grooves at two points or four points. A slightly elliptical groove design allows the balls to make contact at two opposing points with a bit of clearance on the balls’ sides that are perpendicular to the contact points. The four-point contact arch design (called a gothic arch) eliminates any clearance that could lead to deflection and is, therefore, well suited for applications requiring high precision.
Balls typically contact the raceway grooves at two or four points. With an elliptical groove design, balls make contact at two opposing points with a bit of clearance on the balls’ sides that are perpendicular to the contact points. The 4-point contact arch design eliminates any clearance that could lead to deflection. It is often the choice for applications requiring high precision.
Ball screw actuator rigidity is primarily affected by the composition of the guide rail. As this is the outer structure of the system, it is the actuator’s support. Thus, its rigidity determines how consistently true are the grooves of the raceways. The thickness and strength of the lower edges of the guide rail are critical to 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. When the recirculating balls’ grooves are closer to the bottom of the rail, the block can carry heavier loads. In combination with the more rigid U-shaped style rail, there’s less deformation and better accuracy, so one-end applications can be supported. 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 things being equal (ball, guide rail, block, ball screw).
When recirculating balls’ grooves are closer to the bottom of the rail, the block can carry heavier loads. Another advantage to guide rails positioned lower than the ball screw center is greater compactness.
Depending on the application, the length of the ball screw is a factor in actuator speed. The faster the desired travel time, the longer the lead must be. However, to achieve higher accuracy, it is best to use the shortest possible lead for the job.
A direct correlation exists between speed and length. For example, assume a motor operates at 50 rps. With a 20 mm lead, the speed would be 1,000 mm/s and with a 2 mm lead it would be 100 mm/s.
The merit of a shorter lead is that it can move a heavier load using a smaller motor. However, to achieve the same speed, if the lead is shorter, the motor must be bigger.
Ball screw actuator systems are usually available with metal covers. However standard metal covers have gaps between the cover and the guide rails. The configuration makes them unsuitable in environments where liquids or particulate matter could enter the system — a common situation in manufacturing or fabrication applications using coolant. Though usually a custom option, accordion-pleated bellows-type covers are available which are impervious to fluids and particulate matter. They replace the metal cover and cover the entire guide.
NB Corporation of America
Filed Under: Actuators, Ball screws • lead screws, Mechanical, Motion control • motor controls