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. For a more thorough look at these processes, check out our recent blog.
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.
Ball screws with higher grading are typically produced by thread rolling. Although thread rolling is less accurate than thread grinding, the PST Group is able to adjust the sizing during thread rolling to meet customer specifications. 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 favor 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.
Sometimes, things don’t go as planned. One of our customers, a medical dispensing equipment supplier, was provided a ball screw to their specification but the accuracy was too low. The dispensing equipment utilizing the screw was not working properly, allowing the wrong amount of fluid and medicine out. After analyzing the situation, The PST Group recommended a lower grade ball screw, increasing the accuracy of the application, and solving the problem. This simple fix required no major redesign, saving the customer time and money.
Choosing a Ball Screw Grading Partner
While choosing the wrong ball screw grading won’t cause a catastrophic failure, it can cause a major headache. When you need to decide which ball screw grading is right for your application, the PST team is ready to guide you through the process. Contact us.