Ball screw preload

When a linear motion application requires high thrust force and rigidity along with good positioning accuracy, a preloaded ball screw assembly is often the best choice. Manufacturers offer ball screw preload amounts ranging from just 1 or 2 percent for applications where backlash would be detrimental, up to 10 percent or greater for applications that require very high stiffness.

While preload is beneficial for some applications, it produces additional loads that must be accounted for when sizing the screw assembly, motor, and other related components.

Methods for achieving ball screw preload

There are several ways to preload a ball screw, but the three most common methods use oversized balls in the ball nut, an adjustable-diameter ball nut, or a double nut system. Creating preload by using oversized balls allows the manufacturer to set the preload to a precise amount, although this technique is generally used for preload amounts of 5 percent or less. However, this is the most common method for preloading because it can be used with a variety of nut styles and is relatively low-cost.

Axial play, or backlash, is caused by clearance between the balls and raceways. Using oversized balls in the ball nut can reduce or eliminate backlash.

One way to produce preload in a ball screw assembly is to use a double ball nut, with either a spacer or spring between the two nuts.

Ball screw preload reduces axial play and increases rigidity

In many applications, the primary reason for using a preloaded ball screw is to reduce internal clearance between the raceways of the nut and screw, which causes axial play (backlash). Reducing or eliminating backlash in turns improves the repeatability of the screw assembly, since there is no “lost motion” when the direction of travel is reversed. (Note that preload does not affect lead error or positioning accuracy.)

Preload reduces or eliminates axial play (backlash) between the nut and the screw shaft, which improves repeatability and rigidity .

Higher preload also correlates with higher nut rigidity, which improves the rigidity of the complete screw assembly. But because the ball nut is only one component in the ball screw assembly, and its rigidity is relatively high compared to the other components, increasing the preload — from 3 percent to 5 percent, for example — has a minimal effect on the rigidity of the assembly. The most noticeable increase in rigidity of a ball screw assembly occurs when a non-preloaded ball nut is replaced with a preloaded version.

The ball nut rigidity (RN) is one component of the total rigidity of the screw assembly, along with the screw shaft, end bearings, and housing.

Preload also increases motor drive torque and decreases life

Another effect of ball screw preload is that it induces an internal load on the ball nut, which needs to be considered during sizing to ensure the motor can produce sufficient torque for the application. Preload is specified as a percentage of the ball nut’s dynamic load capacity, so in order to determine the internal load due to preload, the preload amount (0.02 for 2 percent, or 0.07 for 7 percent, for example) is multiplied by the dynamic load capacity.

The load due to preload must also be accounted for in the applied load (F) component of the bearing life equation, as shown below. Because of the cubic nature of the equation, even a small internal load due to preload can have a significant negative effect on the life of the ball screw. (Note: When the external load meets or exceeds 2.8 times the internal load due to preload, it counteracts the internal load, and the internal load can be disregarded.)

When calculating the applied force (F) for the bearing life equation, the internal load on the ball nut due to preload should be considered.