Whitepaper 35 : Ball Screw Grading : Determining Accuracy for Your Application ?

From speed, to material, to lead time, there’s a lot to keep in mind when choosing a ball screw for your application. One aspect that is often critical to manufacturers is accuracy. Whether or not you need a ball screw suitable for a highly precise application, manufacturers rely on a ball screw grading scale to determine which type of screw is right for their usage.

The Ball Screw Grading Scale

The ball screw grading scale ranges from zero to ten, with each level corresponding to a different degree of accuracy. The lower the ball screw grading, the tighter the lead accuracy tolerance. A majority of ball screws fall in the three to seven grade range, but certain custom applications can require a tighter lead tolerance. The decision of which level of ball screw grading is appropriate is determined on an individual level by each manufacturer’s application.

While choosing the wrong level of ball screw grading will not result in catastrophic failure, it will slow down your operation. The application may begin to operate inefficiently, or fail altogether. Manufacturers must then go back to the drawing board to determine which level of ball screw accuracy is right for their application, wasting time and money.

Ball Screw Manufacturing

The ball screw grading chosen for an application will often dictate which process is used to manufacture the screws. Ball screw manufacturing consists of two major methods: thread grinding and thread rolling. Thread rolling rotates a hardened steel die, pressing it against the surface of a cylindrical blank. Thread grinding, on the other hand, utilizes a CNC machine with an abrasive grinding wheel to form threads.

Thread Grinding

Manufacturers concerned with maintaining a high level of ball screw accuracy (grading levels between zero and three) will often use ball screws manufactured by thread grinding. Thread grinding is a more stable method of ball screw manufacturing, as it is typically performed in the heat-treated state. The grinding process also allows for more precision. However, it is more expensive, and manufacturers that do not require as precise a level of accuracy can use a more cost-effective method of production.

Thread Rolling

Ball screws with higher grading are typically produced by thread rolling. Thread rolling is less accurate than thread grinding. With thread rolling, heat treatment follows the rolling process, which typically results in a less accurate ballscrew, compared to thread grinding.  For certain applications where accuracy is not as critical, thread rolling is capable of creating a ball screw that is appropriate for the application.

Ball Screw Grading in Action

Each application is different, and may require different grading in order for ball screw accuracy to be maintained. However specific industries do tend to favour certain levels of grading. Industrial applications often find higher grade ball screws appropriate for their usage. However, high precision industries, such as aerospace and medical, often require lower grade ball screws in order to ensure the accuracy that they need in their applications. Every decision is made on a case-by-case basis depending on the needs of the customer, but these generalizations provide a starting point.

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