03 Mar 2016

What is a Ball Screw?

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Ball screws are mechanical devices which do the job of converting revolving motion to liner motion, with a decreased level of friction. They are mainly used in various ranges of machinery such as low precision transport guides as well as high accuracy grade screws. They are also used in robots, machine tools, and precision assembly devices.

Classification of Ball screws

Ball screws are mainly classified into factors such as: axial play, pre-load, lead accuracy, life/load relationship. Axial play refers to the extent to which a ball nut can be relocated in the direction of the screw axis without the revolving motion of either the nut or screw. Preloading denotes the course of raising the stiffness of a ball screw which eliminates backlash. Lead accuracy focuses on the extent to which the shaft’s revolving movements are converted into the runs proportional liner movement. Lead accuracy and axial play are chiefly determined by the manufacturing technique used for the ball screw shaft.

Comparison of Ball screws with conventional screws

The major advantage, out of the general benefits of ball screws is the fact that they can help raise efficiencies well above the 90% mark. Secondly, there are minimal thermal effects associated with ball screws. Also, ball screws can be conveniently preloaded to eradicate the issue of backlash. Ball screws are moreover known to offer smoother movement over the full travel range.

As compared to other alternatives available, the ball screw offers low friction plus generates higher efficiencies, unlike the acme and lead screws which are only up to 50% efficient. Though the ball screw is the more expensive choice, the high cost can be offset by the decreased power requirements for similar performance.

Ball screws, however, require high levels of lubrication and they must be properly lubricated at all times to prevent corrosion and to reduce friction. Lubrication also extends the operating life, leading to a more efficient level of operation. Care must be taken though to avoid contaminated lubrication which can lead to increased friction and lower efficiency. Ball screws may fail completely if the ball travels over metal chips or the dirt present in the ball thread raceway. Using lubricants recommended by machine tool manufacturers can help in preventing these issues. Also, the use of telescopic covers and lip seals should be considered too as they provide a tight fit and thus, will keep out any contaminants in the environment. Since ball screws are bulkier ball screws are more prone to damage at the time of installation compared to other conventional screws.

Selecting a ball screw

In the process of selecting a ball screw, a few parameters must be considered. One must know the expected load and the operating speed needed as well as the positional accuracy required for a good start to sizing the right ball screw for the application. This information can help in determining the ball screw diameter and lead plus allow for the making of further decisions based on expected life, any special mounting configurations, assembly instructions or special environmental conditions.