Screw drive train selection made simple

To help simplify specification, grasping the fundamentals of the various technologies can prove a real advantage in quickly and easily arriving at the best solution. There are three principal screw drive train technologies to consider – ball screw, lead screw and roller screw – and all produce linear thrust by converting rotary torque inputs taken from a motor.

Ball screws are arguably the most common type due to their widespread commercial availability, impressive efficiency and load-life attributes. Comprising a screw and nut, with ball bearings deployed as the interface between threads; when designed and manufactured correctly, ball screws prove to be an optimised solution for applications featuring elevated duty cycles and high thrust, similar to applications that would usually demand a pneumatic, or perhaps even hydraulic cylinder.

Turning to lead screws, principal applications normally centre on low precision, low duty cycle systems, albeit for a reduced cost. Mechanically, the metal screw mates with a plastic or bronze nut, and therein lies the issue. The use of these softer materials means that wear will occur fairly easily. As a result, lead screws are normally only advisable for systems where low duty cycle adjustments are required.

Finally, roller screws, which are like a ball screw in that they utilise a screw and interfacing nut. In contrast, however, small rotating rollers facilitate the nut/screw interface, not ball bearings. The line contact achieved as a result, means roller screw technology is considered superior to ball screws in regard to shock load and total stiffness. On the flip side, this type of screw drive train is typically noisier in motion than ball screws or lead screws due to the larger interface contact area. They are also more expensive as a result of tighter machining tolerances, while lead times are longer. As such, roller screws are normally only specified for high-load applications that demand long operational life.

With lead screws, most design engineers will be able to determine fairly easily whether or not these are appropriate for a specific system, so the remainder of this article will centre on situations where there is a selection dilemma between ball screw and roller screw electric actuators.

For example, consider a high duty cycle application that requires 450 kilograms force (kgf) of continuous thrust over a 250 millimetre (mm) stroke. The required speed is 300 millimetres per second (mm/s) at 100% duty cycle, and the aim is to ensure maximum actuator life.

For the purposes of this application, consider two screw drive trains with similar attributes: a ball screw with a 95 mm square frame; and a roller screw with a 102 mm square frame.

In a load against life comparison, it can be shown that the ball screw actuator, despite being slightly smaller, displays a longer anticipated life than the roller screw. The reason can be attributed to the optimised packaging of the ball screw within the body of the actuator. When factoring in the lower cost of the ball screw, selection can be deemed reasonably unequivocal. In reality, however, there is more to consider.

Performance metrics

Although relevant performance metrics can provide help in the selection process, this has to be based on more than just reading the label. In real applications, always consider what is most important to achieving the desired motion, life, delivery or price point. Here, a simple three-step process can be deployed.

The first step is to identify the relevant performance metrics, namely quantifiable outputs that measure how appropriate one actuator type might be over another. Beneficial metrics, such as actuator life for example, should be maximised, while metrics that burden the application, like cost, should be minimised.

The second step is to apply a specially devised actuator performance equation to calculate the relative performance of each solution. Here, characteristics that are ideally maximised, such as actuator life, are input in the numerator, with the denominator represented by the maximum across all actuator options. Conversely, attributes that the design engineer wants to minimise, such as cost, are input in the denominator, with the minimum across all actuator solutions input as the numerator.

The same calculation can also be used to weigh one metric higher than another. Using this model, the higher the performance score, the more optimised the actuator will be in application. Thus, scoring the actuators is the third and final step in the selection process.