Ball Screws: The Method of Choice in Linear Actuation Applications
by Jenny Knodell, IQS Editor
When I think of a screw, the first thing that comes to mind, of course, is a threaded fastener that holds two objects together. Those simple little things sure do come in handy. In fact, I used a couple yesterday to re-attach my closet organizer to the wall after it had collapsed under the weight of all my clothes. Standard screws are composed of a threaded shaft (the main component, with the spiral grooves running horizontally) and a nut, the small piece with internal threading that runs up and down the shaft. Pretty straightforward, right? Well, not in all cases. They aren’t well known, but there are power screws out there that do much more than hold objects securely together. One of those types is a ball screw. While they are essentially composed of the same parts, these screws are more complicated and vital components in many technically advanced applications.
Photos courtesy of GTEN Ball Screw Technology, Co., LTD.
Ball screws are components of heavy machinery like assembly equipment, robots, aircrafts and machine tools—all systems that require constant, continuous motion from a power source, which are usually driven by a cylinder. The ball screws play a very important role in all of these systems. When you need a back-and-forth motion, but it’s derived from a revolving-motioned power source, what can you do? Ball screws solve this problem—they convert movement from rotary (circling around) to linear (back and forth). The ball screw assembly is more complicated than other motion convertors, but it also offers the highest efficiency—usually around 90%. They are used in automobile engines, on planes to control the wing positions, and in semiconductor wafer transport systems. Ball screws are composed of a screw and a nut, as well as multiple metal balls that roll between the thread grooves. The threading running up the screw acts as raceways for the balls. As the screw rotates, the balls roll down the threads, creating a rolling friction. When the screw turns, the nut moves in a linear direction, and vice versa.
Although there are a couple alternatives to achieving linear motion from rotary other than ball screws, they remain the first choice for several reasons. The rolling friction is much more efficient than line friction—3 times that of Acme screws, allowing lower system power requirements and the use of smaller gears, clutches and motors. They offer quieter operation and lower cost than hydraulic or pneumatic systems, and don’t require extra components like pumps, hoses or fluids. Belt, cable or chain drives are a cheaper solution than ball screws, but they are also much less precise and reliable, which can lead to inaccurate positioning. If they are properly taken care of, ball screws offer a long lifespan of motion control. They require consistent lubrication to help with friction, as well as a bellow made of rubber or plastic that protects the screw from dirt or abrasive particles, which could contaminate the operation and lead to a decrease in efficiency. If they are well maintained, ball screws are highly reliable, efficient motion control systems.
Photo courtesy of Universal Thread Grinding Company.