Screws create motion differently than linear motors. Photo courtesy Nook Industries
Ball screws continue to be the drive of choice when the loads are high and variable.
Not so fast linear motors. You won’t be replacing ball screws anytime soon. Screws create motion differently than linear motors. Linear motors are magnetically direct acting devices, while a power transmission screw offers a mechanical advantage.
"While linear motors have their place," says Tom Solon, P.E. an applications engineer with Haydon Kerk Motion Solutions, "improved technology and manufacturing processes may increase the range of appropriate uses, but [linear motors] are not interchangeable with screws."
According to John Skaltsas with Steinmeyer, linear motors continue to be chosen for high speed and acceleration and perform best when the load is relatively light and constant. Skaltsas states that linear motors are "preferred in special environments such as high vacuum and/or clean rooms, because there is no metal-to-metal contact — and wear — as there is with ball screws."
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However, ball screws continue to be the drive of choice in machine tools where the loads are high and variable.
In general, ball screws out-perform lead screws in terms of speed, accuracy, load and life ratings, but at a higher cost, according to Skaltsas.
Lead screws are easier to integrate because they are more compact and are preferred in applications in which cost is a primary factor; the screw will be run completely dry, or back-driving is not permitted.
Greater Integration, Reduced Maintenance As ball and lead screws evolve, improved designs will include higher speed and acceleration ratings as well as reduced maintenance as wipers improve and new materials and lubrication reservoirs are developed.
Skalsas predicts the combination of a lead and ball screw inside of a motor will produce a more compact actuator.
"Miniaturization will be a principal growth path," says Solon. "Screw-based solutions will become more common as programmable motors and drives expand the use of intelligent automation. Advanced material technologies will additionally provide new performance options."
"Many linear bearing designs offer self or permanent lubrication options and it is likely to become more prevalent in the ball and lead screw markets," says John McLoughlin, vice president of marketing at Nook Industries.
According to Solon, it is critical that friction and wear be managed through the proper material combinations, but today’s technology enables screws to be offered in designs that users never have to lubricate over the life of the application.
Minimizing Backlash
The quality of materials and manufacturing are the best ways to eliminate backlash — the free-play that leads to lost motion.
"Backlash is best controlled by a nut that self-adjusts to compensate, with minimum pre-load, without relying on a spring to resist deflection," says Solon. "The successful implantation is dependent on having a high quality screw with very uniform thread pitch."
According to Solon, the least desirable approach relies on a heavy compression spring to bias the nut, because it creates high friction, which leads to erratic movement and accelerated wear.
Many anti-backlash designs exist for both ball and lead screw products. The best designs are the ones that self-compensate as wear occurs as long as the design is not too complex.
As McLoughlin suggests, the greater the number of parts per assembly, the greater the number of parts that can fail.