A ball screw, like a lead screw, converts rotary motion into linear motion. The device consists of a threaded shaft and a ball nut. The latter device rides on the screw, supported by a series of ball bearings that provide a rolling surface rather than the sliding surface of a lead screw. The balls roll between the nut and shaft. Because there is no sliding motion, ball screws run more efficiently than lead screws. This is their great advantage. The efficiency of ballscrews is relatively constant and is typically better than 90%.
Ballscrews are often a first choice for linear-motion applications because the use of recirculating ball bearings provides high efficiency, load capacity, and positioning accuracy. Furthermore, ballscrews generally provide equal or better load capacity than leadscrews, and so are a better choice when load requirements exceed leadscrew capabilities.
One drawback to ballscrews is that they require high levels of lubrication. Ballscrews should always be properly lubricated with a proper formulation to prevent corrosion, reduce friction, ensure efficient operation, and extend operating life. Backlash, that little bit of play between several mechanical components, can be eliminated with preloading.
A few ballscrew terms, such as circuits, turns, lead, pitch, and starts, are widely used – and misused – terms that quantify various aspects of ball screw assemblies. Although these terms are related, each has a unique meaning and significance to ball-screw design and performance.
Lead and pitch are related but different specifications. Lead refers to the linear distance traveled for each complete turn of the screw, while pitch is the distance between screw threads. These terms are often used interchangeably, and for single-start screws lead and pitch are equivalent. However, lead and pitch are not equal for screws with multiple starts. Ballscrews are commonly available in medium leads between 0.200 to 0.500 in./rev, although high-helix products exist.
Considering the geometry of a screw assembly, it makes sense that as the lead of the screw becomes larger, the number of tracks inside the ball nut becomes smaller, so fewer balls are carrying the load. While larger lead screws offer longer travel per revolution and higher speeds, their ability to provide a high load capacity is compromised. In theory, the number of ball tracks could be increased by making the ball nut longer, but manufacturing constraints and limits on ball nut length make this an impractical solution.
Circuits and turns are also related concepts. A ball circuit is a closed path of recirculating balls. “Turns” refers to the number of trips the balls make around the screw shaft before being recirculated. The relationship between circuits and turns is influenced by the recirculation method. Ball returns that use the deflector or thread-to-thread (aka cross-over) method recirculate each turn of balls individually. Therefore, the number of turns is equal to the number of circuits.
When balls are returned by an internal channel or an external tube, the recirculating balls can cross several threads, so one circuit can have multiple ball turns. That is, the balls make several trips around the screw shaft before being recirculated. Multi-start ball screw assemblies typically use the internal channel method of recirculation (pictured). These can be designed for multiple circuits, by incorporating more than one internal recirculation channel in the nut body.