Stepper motors are widely used in industrial and consumer applications ranging from packaging equipment and medical devices to 3D printers and vending machines. They have excellent torque production at (relatively) low speeds — especially for a given motor size — and offer high-resolution positioning in a simple-to-use format. These benefits make them well-suited to drive lead screws or belt and pulley systems. In fact, some of the most common stepper motor applications pair them with lead screws.
But stepper motors can also be used to drive ball screws. And although ball screw + stepper motor combinations are less prolific than lead screw + stepper motor pairings, some of the performance characteristics of ball screws allow them to make better use of stepper motors’ strengths for torque production and precision.
Efficiency, torque, and speed
Recall that lead screws rely on sliding contact between the screw shaft and the nut, and so have inherently high friction and, in turn, low efficiency. This means they require higher torque from the motor in order to turn the screw and create linear motion. And recall that stepper motor torque drops significantly at higher speeds, so lead screw + stepper motor combinations aren’t typically a good choice for high-speed applications that require high, or even moderate, torque production.
Ball screws, on the other hand, rely on rolling contact, with low friction and high efficiency, so require much less torque from the motor. This means that a ball screw + stepper motor design can make better use of the motor’s entire torque-speed curve.
Low friction without fluctuations
The friction in a lead screw assembly can also change over time, due to wear from the sliding surfaces of the nut and screw. This varying friction — and the resulting variation in required motor torque — can pose a challenge for stepper motors, which perform best in applications with stable, predictable loads. For example, if the required torque from the motor increases, the motor could have trouble producing the new, higher torque and lose steps. (This is why many stepper motors are oversized — to provide some “torque margin” for unforeseen operating conditions.) In an open-loop control system — typical of stepper motors — this loss of position can be detrimental to the process or even to the safety of workers.
The lower friction inherent in ball screw assemblies means they require less torque from the motor to move a given load with a given move profile. And the required torque doesn’t vary over the life of the ball screw, so the motor can be sized with a smaller torque margin. In other words, when paired with a ball screw, a stepper motor doesn’t need to be significantly oversized to handle uncertainties in the required torque due to varying friction in the screw.
High duty cycles
Although stepper motors often have limited duty cycles, due to heating caused by their use of relatively high current at all times during operation, some stepper motor types, — and some stepper drive types — do allow stepper motors to be used in applications with high (or even continuous) duty cycles.
But sliding friction produces significant heat, so high or continuous duty cycles can significantly reduce the life of a lead screw. (For a full explanation of how heating affects lead screws, check out this article on PV values.) For these applications, ball screws are a better match, since they have much higher efficiency and lower heat generation than lead screws.
Ball screws also provide better positioning accuracy and higher repeatability — characteristics that pair well with the high resolution of stepper motors. And when ball screws are used in closed-loop stepper systems, you get the benefit of good positioning accuracy and repeatability, together with a feedback mechanism to ensure the system reaches the intended position.
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