Choosing the Correct Ball Screw

A ball screw is a mechanical device for translating rotational motion to linear motion with little friction. A threaded shaft provides a spiral raceway for ball bearings which act as a precision screw. As well as being able to apply or withstand high thrust loads they can do so with minimum internal friction. They are made to close tolerances and are therefore suitable for use in situations in which high precision is necessary. The ball assembly acts as the nut while the threaded shaft is the screw.

There are few, if any, external clues on a ball screw that hint at its quality.  This is especially true of precision ball screws, which have an accuracy of five microns.  More often than not, the inferior quality of a ball screw remains unknown until it fails prematurely.  Thus, knowing what is required to manufacture a precision ball screw enables the user to ask the right questions of their ball screw manufacturer, and lets the user confidently select the correct one for their application, and ensures that the selected ball screw will have a long service life.


The Manufacturing Process

The first factor that influences a ball screw’s quality is the steel used in its manufacture.  Microscopic particulates, called “inclusions,” can form during the processing of steel; these inclusions lead to crack propagations and other surface fatigue that dramatically shorten a ball screw’s life.  Although steel processing technology is at the point to severely curtail the formation of these inclusions, these higher quality steels cost significantly more.  Normally, high-quality ball screws are manufactured from vacuum-degassed alloy steel, which is subjected to high vacuums during manufacturing to remove hydrogen and carbon as needed.

Most steel components are subjected to heat treating during their manufacture in the interest of increased strength.  Improper heat treating can reduce the service lift of a ball screw if the heat treatment is too shallow.  Differences in hardness between the balls and the shaft should be known before selecting a ball screw, as heavy loading conditions may cause the balls to make indentations along the shaft raceway, which will over time increase the indentation size and lead to contamination and failure.  Because these conditions are not immediately apparent, questions about the heat treatment of the ball screw should be asked prior to purchasing.  For best service life, the ball screw’s heat treatment should be equal in depth and amount across the entire length, with special attention paid to ensure that the entire threaded length of the ball screw has been heat treated.


Ball Screw Designs and Accuracy Grades

Industry-tested and application-proven ball screw designs will employ tubes or deflectors for ball recirculation.  Recently, ball screws have been manufactured that feature internal ball recirculation; this approach allows the balls to run quieter and faster, enabling higher operating speeds and lower noise levels than conventional recirculation assemblies.

Selecting the right ball screw requires extensive knowledge of the ball screw’s accuracy grade, as well as the operating environment.  Ball screw accuracy grades are given as C0 through C5; machining applications would typically use a C5 accuracy ball screw, whereas inspection equipment might use C3 or C1 grade ball screws, due to that applications higher resolution requirements.  Ball screws that are created by rolling processes (as opposed to grinding) carry a Ct grade (“t” as in “transport”). Great care should be taken to avoid confusing C and Ct grades, as Ct grades have looser tolerances than C grades.  As with most things, a lower accuracy grade ball screw may have a lower acquisition cost, but this may also result in a reduced service life, costing more to replace often than a higher-quality ball screw cost to acquire.  Users are advised to conduct trade studies to see which accuracy grade of ball screw is actually needed in their application.


Designing for Specific Applications

Specifying a ball screw for a specific application requires an intimate knowledge of how the operating environment affects the selection of ball screws.  Temperature and process materials, for instance, would affect the best ball screw for the application: woodworking applications might require special seals to keep contaminants out of the ball nut, whereas an application which requires the ball screw be installed in a location that is hazardous or difficult to reach would make ball screws with maintenance-free lubrication systems preferable.  A holistic understanding of the application in question will ensure that the correct ball screw is selected, and will save money not only by ensuring that the correct accuracy grade is chosen, but also by ensuring that the ball screw’s service life and features are sufficient to meet the needs of the user.

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