The basics of ball & roller screws

There are a number of screw actuation methods providinglinear motion. These include basic lead screws to other types includingball screws and roller screws. Here’s how the latter two compare.

BALL SCREWS

Ball screws consist of a screw with helical grooves, a nut, and balls that roll between the nut, screw and the grooves when either the screw or nut rotates. The balls, made of steel, ceramic, or a hard plastic material, are routed into a ball return system of the nut and travel in a continuous path to the ball nut’s opposite end.

Compared to other screw actuation methods, ball screws have a significant advantage with their high efficiency ratings that can be well over 90%. By way of comparison, a typical acme lead screw has efficiency ratings of 50% or less. Even though ball screws typically cost more than other screw types, this is usually offset by decreased power requirements for a similar net performance.

A number of factors play a role in assessing ball screw performance. These include lead accuracy, axial play, preload, and life/load relationships. For starters, lead accuracy refers to the degree to which the shaft’s rotational movements are translated into linear movements. Axial play is the degree to which a ball nut can be moved in the screw axis direction without any rotation of either nut or screw. Preload is typically applied to eliminate axial play, which also has the effect of removing backlash and increasing stiffness.

Lead accuracy and axial play are determined by the manufacturing method of the ball screw shaft and the assembly of the nut. For instance, high lead accuracy and zero axial play is usually associated with relatively higher-cost precision ground ball screws, while lower lead accuracy and some axial play is associated with lower cost rolled ball screws.

Other important performance parameters are precision and repeatability, which can be impacted by ball recirculation inside the ball nut. This is why ball nuts are available with a range of preload options to reduce or remove the axial play as they rotate around the screw. Minimal axial play allows better accuracy because no motion is lost from the clearance in the balls as they reengage.

ROLLER SCREWS

In contrast, a roller screw replaces the recirculating balls with threaded rollers. The ends of the rollers are toothed to mesh with geared rings at each end of the nut. The rollers both spin on their axes and orbit around the nut, in a planetary configuration. (This is why roller screws are also referred to as planetary roller screws.)

The geometry of a roller screw provides significantly more contact points than are possible with a ball screw. This means that roller screws typically have higher dynamic load capacities and rigidity than similarly sized ball screws. And the fine threads (pitch) provide a higher mechanical advantage, meaning that less input torque is required for a given load.

Because their load-carrying rollers don’t contact each other, roller screws can typically travel at higher speeds than ball screws, which have to deal with the forces and heat generated by the balls colliding with each other and with the recirculation end caps.

Although their load carrying mechanism differs, many attributes of planetary roller screws are similar to ball screws. They can be preloaded to reduce or eliminate backlash, by using either a split nut (in which the nut is cut transversely and a precision spacer is inserted between the front and back halves), a double nut, or oversized rollers. Roller screws are also manufactured to the same precision classes as ball screws, so lead deviation and positioning accuracy are easy to determine. And they experience the same critical speed and buckling limitations that are found with ball screws.

Despite the load and speed advantages of roller screws over ball screws, where roller screw actuators really make sense are cases where an electromechanical actuator is replacing a hydraulic actuator. In many instances, the desire is to move away from hydraulics to eliminate the use of hydraulic fluid, or to simplify the overall system. Roller screw actuators are the best option because they provide load capacities and stiffness characteristics similar to hydraulics.

For high-load applications, roller screws may be a better choice than a ball screw. In a roller screw, the load bearing mechanism is a set of recirculating rollers instead of balls. The rollers have more total surface contact area than balls, which increases the load carrying capacity and can also increase service life. However, due to the precision machining and complex assembly required, roller screws are generally more expensive than ball screws. Image courtesy of Thomson Linear.