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High-load ball screws – The motion control solution with higher load density

01 April 2021

Motion engineering quiz: if you need to actuate a 500kN axial load along a 1500mm stroke, do you use roller screw or a ball screw?

If you reflectively said roller screw, you may not be familiar with high-load ball screws as an economical and simplified alternative. Roller screws have been promoted as the only technology of choice for handling large loads when size is a constraint. In actuality, advances in ball screw technology make special versions viable candidates for high-load applications as well, and this is important because a high-load ball screw is typically less than half the cost of a comparable roller screw at equivalent performance points.

What’s the difference?

A ball screw consists of a threaded metal shaft with a nut that rides along its length as a motor rotates the shaft. The nut connects to a table, an arm or other load, as determined by the industrial process that is being automated. Recirculating ball bearings, packaged within the nut, contact the threads and provide the load bearing path. The coefficient of friction between the mating components is very low, which provides a system efficiency that is typically greater than 90 percent. The load capacity of the ball screw is then a function of the ball bearing diameter, number of ball bearings, and the surface contact area. The combination of these parameters then defines the load capacity of the assembly and therefore the resultant life of the components. 

For higher loads, design engineers have specified roller screws in many applications. In a roller screw, the load bearing mechanism is a set of recirculating rollers, instead of balls. The rollers have a greater total surface contact area than balls, which increases the load carrying capacity and can also increase service life. 

These benefits come at a premium. Compared to the relatively simple, time-tested ball system, the roller screws require extensive upfront precision machining and more complex assembly. This contributes to the overall cost of the roller screw, as well as a larger footprint for final installation.

Read the full article in the April issue of DPA

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