The Many Advantages of a Roller Screw

The High Efficiency of Roller Screws

There are significant operational differences when comparing a planetary roller screw and a ball screw. Within a ball screw, the steel balls contact each other and rotate such that mating surfaces are moving in opposite directions. Even when well-lubricated, the resulting friction creates significant heat and wear as rotational speeds increase. On the other hand, the planetary rollers in a roller screw are all constrained by their journals at each end of the roller. The rollers never touch each other, preventing such friction.

The Higher Speeds of Roller Screws

Why does a roller screw provide higher rotation speeds? There is no loading and unloading of balls and no sharp turns of ball return tubes. Therefore, planetary roller screws operate efficiently up to 6,000 rpm.

The Longer Life and Higher Load Capacity of Roller Screws

The long life and shock resistance of a roller screw compared to ball screws results from the larger pressure transmission area that the roller screw design exhibits. Contact is made simultaneously at each thread on every roller. On average, 200 contact points exist in a one-inch diameter, 1.5 inch long roller screw. A comparable sized ball screw will have at most only 50 balls in pressure contact at any time.

This design allows the spreading out of the forces over many contact points thus lowering the peak pressure. Spreading of the load over approximately four times the area results in a fifteen time life improvement compared to the ball screw.

Compare a similar size ball screw to Exlar's planetary roller screw design and see many more contact points on the roller screw. This results in a load carrying capacity up to 15 times greater than ball screws, and improved stiffness.

The Lower Noise of Roller Screws

Ball screw noise is generated by the ball colliding in the returns. The roller screw's noise is generated from the roller timing gears which are much higher frequency (typically 10x) and does not grow exponentially with speed. As a result, roller screws are generally quieter than ball screws.

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