Precision and Design Features

Axial Clearance

In general, a standard single-nut ball screw has a small axial clearance between the screw shaft and the nut.

When an axial load is applied, the sum of this axial clearance and the elastic displacement caused by the load increases the clearance, resulting in backlash.

To eliminate such backlash, the axial clearance of the ball screw must be made negative by applying elastic deformation in advance between the screw shaft and the nut. This method is referred to as preload.

The combinations of axial clearance and accuracy grades for DINGS' ball screws are shown in the table below.

Effect of Preload

Applying preload not only eliminates axial clearance in ball screws, but also reduces axial displacement caused by axial loads, thereby increasing stiffness.

The figure below illustrates the difference in elastic displacement under axial load between a clearance-type ball screw and a preloaded zero-clearance ball screw.

Appropriate Preload Amount

The preload amount should be determined based on the required stiffness or allowable backlash. However, applying preload may result in:

1. Increased dynamic torque
2. Reduced positioning accuracy due to heat generation and temperature rise
3. Shortened service life

For that reason, preload should be set as low as possible while still meeting performance requirements.

Preload Methods

Ball screws are generally preloaded using the double-nut preload method, in which spacers are inserted between two nuts. DINGS' ball screws also use a large-ball preload method, where steel balls slightly larger than the clearance between the screw shaft and the nut are inserted.

This method eliminates clearance using only a single nut while maintaining a compact structure. Spacer balls slightly smaller than the preload balls are alternately used to help prevent deterioration of motion performance.

Preload Management

Direct measurement and control of ball screw preload are difficult. Therefore, preload is typically managed by converting it to preload running torque and controlling it through torque measurement.

To ensure proper preload, running torque is measured under defined conditions. Differences in lubrication and operating conditions may cause variations in measured torque. Starting torque is slightly higher than running torque.

Note: The torque variation illustrated is intentionally exaggerated for explanatory purposes.

Rust Prevention

DINGS' ball screws are coated with rust-preventive oil for long-term storage. Before use, remove the oil with clean refined kerosene and apply lubricating oil or grease. Grease can be applied prior to shipment upon request, although long-term storage with grease may cause rust.

Note: The rust-preventive oil is for corrosion protection only and provides no lubrication. Using the ball screw without removing this oil may reduce service life and cause increased torque or abnormal heat generation.

Lubrication

Lubrication is essential when using ball screws. Insufficient lubrication may cause increased torque and shortened service life. Proper lubrication helps suppress temperature rise due to friction, efficiency loss, and accuracy degradation caused by wear.

• Grease lubrication: Lithium soap-based grease is generally recommended.
• Oil lubrication: ISO VG 32–68 turbine oil is recommended.

Selecting the appropriate lubricant for the application is especially important. For miniature ball screws, grease churning resistance may increase torque.

• MSG No.1 (NLGI No.1): For low-speed positioning applications requiring smooth motion
• MSG No.2 (NLGI No.2): For high-speed and general-purpose applications

Inspection and Replenishment

When using grease lubrication, inspections should be conducted every 2–3 months. When using oil lubrication, inspections should be conducted weekly. During inspection, check lubricant quantity and contamination, and replenish as necessary.