What is Preloaded Ball Screws?

March 6, 2025

When linear motion applications require high thrust, stiffness, and good positioning accuracy, preloaded ball screw assemblies are often the best choice. The preload of ball screws provided by manufacturers ranges from 1% or 2% to 10% or more.

1. What is Preloaded Ball Screws?

Preload refers to the process of applying a certain force during the design and manufacturing process of the ball screw to eliminate the gap between the ball and the screw, thereby improving the accuracy and stability of the motion.

2. Advantages

2.1 Improved Accuracy

Preloaded ball screw can reduce the tooth gap of the entire system and improve the transmission accuracy and stability of the ball screw. If the ball screw is not preloaded, a large tooth gap will be generated, affecting the transmission accuracy of the entire system.

2.2 Improved Rigidity

After the ball screw is preloaded, since its internal components have been pre-tightened, the rigidity of the entire system can be increased and the stability of the system can be improved. If it is not preloaded, the rigidity of the system will decrease due to the looseness between the components.

2.3 Reduce Noise

After the ball screw is preloaded, the internal components are easy to balance due to the reduction of the gap, and the noise generated during movement is smaller and quieter.

3. Preload Level

According to the degree of preload, the preloaded ball screw level is usually divided into four levels: P0, P1, P2 and P3. Each level of preload design has its specific application scenarios and performance characteristics.

Preload Level	Features
P0	Stock standard product preload accuracy, axial gap
P1	Gapless light preload (zero gap)
P2	Gapless medium preload assembly, suitable for high-precision equipment
P3	Heavy preload assembly, used for ultra-precision equipment, only limited to grinding C5/C3 level selection

4. Ways to Achieve Preloaded Ball Screw

During the working process of the ball screw, double nut preload or single nut (large ball, large lead) method is usually used to control the elastic deformation to a minimum, so as to reduce or eliminate the axial gap and improve the stiffness of the ball screw pair.

4.1 Double Nut Gasket Preload Principle

Depending on the thickness of the gasket, it is divided into two forms. When the gasket is thicker, "pre-tension stress" is generated, and when the gasket is thinner, "pre-compression stress" is generated to eliminate the axial clearance.
Features: Simple structure, easy loading and unloading, high rigidity, reliable operation, and the most widely used. However, it is inconvenient to adjust and cannot be preloaded at any time.

4.2 Double Nut Thread Preload Principle

By adjusting the round nut at the end, the right nut of the screw moves axially to the right, generating tensile preload, thereby eliminating the gap.
Features: Compact structure, easy adjustment, reliable operation, wide application, but the preload amount is not easy to control.

4.3 Single Nut Preload Principle (Increasing Ball Diameter Method)

In order to compensate for the gap of the raceway, the size of the ball is appropriately increased during design so that it contacts at 4 points to generate preload force. In order to improve working performance, spacer steel balls can be added between the load-bearing balls.

4.4 Single Nut Preload Principle (Offset Lead Method)

The offset lead method only increases the lead of the nut by a preload amount Δ in the middle of the nut to achieve the purpose of preload.

5. Points to Note for Preloaded Ball Screws

The axial clearance of the currently manufactured single-nut ball screw pair is 0.05mm, while the double-nut type can basically eliminate the axial clearance after adding preload adjustment.

The preload force must be of appropriate size. Too small cannot ensure gapless transmission; too large will increase the driving torque, reduce efficiency and shorten life. The preload force should not exceed 1/3 of the maximum axial load.
Pay special attention to reducing the gap between the screw mounting part and the drive part. These gaps cannot be eliminated by preloading, and they have a direct impact on the transmission accuracy.

Summary

The selection of preload level is crucial to the performance and application of ball screws. According to specific application requirements, selecting the appropriate preloaded ball screw level can effectively improve the performance and reliability of the equipment.