What to know when selecting a Ball Screw vs a Lead Screw

PBC Linear

What is a Ball Screw and how do Ball Screws Work?

A ball screw is a mechanical assembly composed of a screw and a nut linked by ball bearings that roll in the threads of the screw and the nut. The low rolling friction of the ball bearings makes ball screws highly efficient devices for converting rotary motion into linear motion. The ball bearings, screw threads, and nut threads are made of hardened steel which gives ball screws high load capacities. Commonly there are wipers at each end of a nut to prevent the ingress of debris. 

There must be a ball return system built into the nut. The ball return system recirculates the ball bearings in the nut or else the balls would exit the nut out the end as it traveled along the screw. 

The primary ball return system used by PBC Linear is a multiliner return system. The multiliner ball return system guides turns of balls back to the same threads creating ball circuits within the threads.

What are the advantages of ball screws vs. lead screws?

Ball screws offer higher efficiency than lead screws because ball screws use rolling friction and lead screws use sliding friction.

Ball screws require lower torque to move the same load as lead screws.

Ball screws normally have higher load ratings than lead screws because ball screws have hardened steel ball bearings running in hardened steel screw threads and nut threads. Lead screws are often made of steel but not hardened steel. The nuts of lead screws are often made of a polymer or a soft metal such as bronze.

Ball screws offer more precise positioning than lead screws. Lead screws must have some clearance between the screw threads and nut threads. Ball screws can have carefully controlled clearances between the threads and the ball bearings, or even preloaded threads and ball bearings.

Ball screws can operate at higher rotational speeds than lead screws. The sliding friction between a lead screw and its nut causes heat. Lead screw operation must not exceed a speed and loading where friction heat is generated faster than the heat can be dissipated or else the nut material will deteriorate.

What is backlash in a ball screw and how can it be reduced?

Backlash is the amount of free axial movement by the nut when the screw is held stationary. The axial movement is possible because of clearance between the nut threads, the bearing balls, and the screw threads. 

The amount of thread clearance can be reduced in a nut by installing larger diameter bearing balls, using double-nut designs, and other techniques. PBC Linear miniature ball nuts, backlash is reduced by installing larger diameter bearing balls to reduce the clearance in the threads. 

Standard backlash is <= 0.050 mm or <= 0.002 inch. 

Reduced backlash is <= 0.013mm or <= 0.0005 inch.

What does it mean that an actuator will “back drive”?

An actuator back drives when the force applied to the carriage of an actuator overcomes the resistance of the drive train and causes the carriage to move. A common situation when this can occur is an actuator operating vertically with a weight load on the carriage. When power to the driving motor is removed, the weight on the carriage may overcome the resistance of the drive train, and the carriage may drop. In such situations, a designer may choose a low efficiency drive system that will not back drive or a brake may be installed in the drive train. The brake would be used to lock the system when power is removed from the actuator’s motor.

Can ball screws be installed and used as received?

If ball screws are ordered with a specific lubricant called out, then they often can be used as received from the manufacturer. However, most ball screws ship from the manufacturer with only a rust inhibitor applied. The rust inhibitor is not a sufficient lubricant. Ball screws received with a rust inhibitor applied should be cleaned with a solvent and then a proper lubricant should be applied.