Motion control: How to choose an actuator of 4 ft. or less
In its conversion of motor torque to linear thrust, the single axis ball screw actuator is king. Here are 8 factors to consider when selecting one.
By Control Engineering Staff -- Control Engineering, 4/9/2009
By Naoki Yamaguchi, NB Corporation of America
Whether for packaging, life sciences, or factory automation, smooth motion, fast accelerations, and a high degree of accuracy are hallmark requirements for linear movement actuators. Increasingly, miniaturization has added the additional requirement of compactness. One system that can meet all of the above requirements in its conversion of motor torque to linear thrust is the single axis ball screw actuator. Ball screws convert rotary motion to linear motion, or torque to thrust, and vice versa. Here are eight factors to consider when selecting an actuator for your next project.
A ball screw actuator is a combination of a ball screw on which a slide block or nut’s movement is guided by recirculating steel balls that roll between the block and raceways of guide rails. This configuration eliminates backlash.
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System solution vs. custom built
Traditionally, many equipment manufacturers designed and assembled their own custom ball screw actuators. In fact, many semiconductor manufacturers and medical equipment manufacturers still do. To design custom stages, engineers have to design the ball screw size—meaning they have to find the right three main components: ball screw, carriage, and guide rails. They also need supports for the ball screw on both ends, and motor brackets. After mounting all the components, adjustments have to be made repeatedly.
Many companies have recognized the advantages of purchasing off-the-shelf. Ball screw actuator systems typically come in at least five different stage sizes. They have several travel length options, and there are other options already designed. Once the engineer identifies the optimum system, he or she just has to get the right motor.
However, to choose the right system for an application, actuator systems must be analyzed carefully. Comparisons of the design and sizing of components such as the slide block, raceway, bearing, guide rail, ball-screw, nut, and housing materials are critical. They all factor into actuator performance.
2. Compactness
Choosing to assemble ball screw actuator components into a custom housing usually creates a larger unit overall than using a pre-designed system. For one thing, a pre-designed 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, if you were to build an actuator from assorted components, you would need 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.
3. Choice of actuators
To choose the most effective actuator for a particular application, critical information must first be ascertained. Factors such as load capacity, operation speed, stroke length, environment, orientation, and positional accuracy have to be identified and quantified. Once these factors are known, we can consider the effects of component design differences on the operation of ball screw actuators in the 4 ft. and under class.
4. Load capacity
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, which does not increase the overall size of the actuator, is to increase the 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 of the block doubles the load capacity of the actuator. Since we are discussing actuators with about 4 ft. of rail travel, an example of maximum load for a standard 1,380 mm rail length with two ball circuits is 37 kilonewtons. With four ball circuits it is 74 kilonewtons.
On the low end of the actuator size range, an actuator with a 100 mm length rail with two ball circuits can be expected to have a load capacity of 3.945 kilonewtons, whereas with four ball circuits it’s load is 7.89 kilonewtons.
5. Precision
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.” “Precision Grade” is the highest. 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 specs 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 in the raceways the balls recirculate. At travel lengths of 4 ft. 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 itself. Furthermore, to ensure positional accuracy, the balls within the ball grooves of the raceways 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 at four points. 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 (called a Gothic arch) eliminates any clearance that could lead to deflection. Therefore, the four-point design is best suited for applications requiring maximum precision.
6. Rigidity
Ball screw actuator rigidity is affected, primarily, by the composition of the guide rail. As the outer structure of the system, this is the actuator’s support. Its rigidity determines how consistently true the grooves of the raceways are. 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. 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, this design even allows one-end supported applications because there's less deformation and better accuracy. Another advantage to guide rails positioned lower than the ball screw center is greater compactness. Moreover, the number of ball circuits also affects rigidity. Four ball circuits provide greater rigidity than two ball circuits, all things being equal (ball, guide rail, block, and ball screw).
7. Actuator sizing
Depending on the application, the speed at which the actuator must travel helps determine the length of the ball screw lead. 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.
There is a direct correlation of speed to length. For example, assuming the revolution of the motor is 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. But to achieve the same speed, the motor must be bigger if the lead is shorter.
8. Liquids and particulate matter in the environment
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 that are designed to be impervious to fluids and particulate matter.
Naoki Yamaguchi is assistant technical manager for NB Corporation of America in Hanover Park, IL. Reach him at naoki@nbcorporation.com
- Edited by Renee Robbins, senior editor Register here and scroll down to select your choice of eNewsletters free.
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