Ball screws

Ball screws are mechanical devices that convert rotational motion to linear motion with a minimal amount of friction. There are several components to a ball screw including a nut and a screw with helical grooves and balls that roll between the nut, the screw and the grooves, while the screw or nut is rotating. Balls are routed into the ball return system of the nut and travel in a continuous path to the ball nut’s opposite end.

Ball screws are used in an array of applications ranging from low accuracy transport guides to exceptionally high accuracy precision grade screws. They’re also used in robots, machine tools and precision assembly devices.

Ball screws are usually classified according to factors such as axial play and preload, lead accuracy and life/load relationship. Axial play is the degree to which a ball nut can be moved in the screw axis direction without any rotation of either nut or screw, while preload is usually considered as the negative axial play. The process of preloading removes backlash and increases stiffness.

Lead accuracy refers to the degree to which the shaft’s rotational movements are translated into the run’s proportional linear movement. With lead accuracy and axial play primarily determined by the manufacturing method of the ball screw shaft, high lead accuracy and zero or negative axial play is generally associated with relatively higher cost and precision ground ball screws, while lower lead accuracy and positive axial play with lower cost rolled ball screws. Fabricated by rolling or other means, ball screw shafts yield a less precise but mechanically efficient and less expensive ball screw.

Perhaps the biggest benefit of a ball screw in general is that it has high efficiencies that can be well over 90%. There are also minimum thermal effects and they can be easily preloaded to eliminate backlash. They also offer smoother movement over the full travel range.

Compared to other alternatives, a ball screw’s low friction generates high mechanical efficiency. Unlike comparable Acme lead screws, which are only 50 percent efficient, a typical ball screw offers 90 percent efficiency. The higher cost of ball screws can be offset by decreased power requirements for similar net performance.

One drawback to ball screws is that they require higher levels of lubrication. Ball screws should always be properly lubricated. Lubrication prevents corrosion, reduces friction, extends operating life and leads to more efficient operation.

However, contaminated lubrication can increase friction. Ball screws can fail if the balls travel over metal chips or dirt in the ball thread raceway. Using lubricants recommended by machine tool manufacturers can help prevent this. Also, consider using telescopic covers or bellows if ball screws are used in environments with a lot of contaminants. Aside from being bulkier, ball screws are more prone to damage when installed compared to conventional lead screws.

When selecting a ball screw, a few parameters are essential. Knowing the expected load, the operating speed needed, as well as the positional accuracy are a good start to sizing the right ball screw for the application. This information can help determine the ball screw diameter and lead and help make further decisions based on expected life, any special mounting configurations and assembly considerations as well as environmental conditions.