BALLSCREW ACTUATORS DELIVER MINIARIZATION, COMPACTNESS

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 ballscrew actuator.

Ballscrews convert rotary motion to linear motion, or torque to thrust, and vice versa. The ballscrew actuator is a combination of a ballscrew 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 design and assembled their own, custom, ballscrew actuators. Today, however, many companies have recognized the advantages of purchasing an offthe- shelf system solution. Typically, ballscrew actuator systems 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, all that is left to do is to get the right motor.

However, to choose the right system for an application, actuator systems must be carefully analyzed.

Comparisons of the design and sizing of components such as the slide block, raceway, bearing, guide rail, ballscrew, nut and housing materials are critical.

Choosing to assemble your own system usually creates a larger unit overall, than using a pre-designed system. For one, 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 ballscrew nut incorporated into it. Generally, if you were to build an actuator from assorted components, you would have to have a housing to put a ballscrew 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.

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.

Assuming these factors have been ascertained, we are now going to consider the affects of component design differences on the operation of ballscrew actuators in the 4ft and under class.

In addition to ballscrew 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 ballscrew and guide rails.

Another way to increase load capacity, which does not increase the overall size of the actuator, is to increase the ball circuits. Most standard ballscrew 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 of up to about four foot rail travel, an example of maximum load for a standard 1,380mm rail length with two ball circuits is 37 kilonewton (kN).

With four ball circuits it is 74kN.

Actuator systems are generally offered in two or three grades – or levels of accuracy. "Commercial grade" is the lowest. The next is grade is "high" or "standard grade." Finally, "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 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 raceways are and how smoothly in the block and raceways the balls recirculate. As the travels discussed are 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 ballscrew itself. Furthermore, to insure 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 is, because of its shape, called a gothic arch. The gothic arch eliminates any clearance that could lead to deflection and is, therefore, best suited for applications requiring maximum precision.

Ballscrew actuator rigidity is affected, primarily, by the composition of the guide rail. As this is the outer structure of the system, it is the actuator's support and its rigidity that 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 ballscrew 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, because there is less deformation and better accuracy, this design even allows oneend supported applications. Another advantage to guide rails positioned lower than the ballscrew 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.

Depending on the application, the speed at which the actuator must travel is a determinant of the length of the ballscrew 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 was 50rps, with a 20mm lead, the speed would be 1,000mm/s, and with a 2mm lead it would be 100mm/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, if the lead is shorter, the motor must be bigger.

NB Corporation of America
Hanover Park, IL
nbcorporation.com