Axial Clearance

In standard single-nut ball screws, a small axial clearance typically exists between the screw shaft and the nut.

Under axial loading, this inherent clearance combines with elastic deformation caused by the applied load, resulting in increased backlash.

To eliminate backlash, the axial clearance must be set to a negative value — that is, elastic deformation is intentionally induced between the screw shaft and nut in advance through preloading.

The combinations of axial clearance and accuracy grades for DINGS' ball screws are shown in the table below, along with the clearance symbols and allowable axial clearance values.

Effect of Preload

Preload is applied to eliminate axial clearance and minimize axial displacement under load, thereby increasing the rigidity of the ball screw.

The graph below shows the elastic displacement curves with and without preload.

As illustrated, applying preload reduces axial elastic displacement by approximately half compared to a clearance-type configuration, resulting in significantly improved stiffness.

Elastic Displacement Curves of 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 the following effects:

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

Therefore, the preload amount should be set as low as possible while meeting performance requirements.

Preload Methods

In ball screw preloading, the double-nut preload method is commonly used, where a shim (spacer) is inserted between two nuts to apply preload force.

DINGS' ball screws, however, take full advantage of the structural characteristics of miniature ball screws by adopting an "oversized ball preload" method, in which steel balls with a diameter slightly larger than the clearance between the screw shaft and nut are used to apply preload. This method eliminates axial clearance with a single nut, enabling a more compact design.

In addition, "spacer balls" — slightly smaller in diameter than the preload balls — are placed alternately between preload balls in the raceway, effectively preventing degradation of motion performance due to excessive preload.

Preload Management

Since the preload amount in a ball screw cannot be measured directly, it is common engineering practice to convert it into a preload running torque, which is then measured to indirectly monitor the preload condition.

The nominal value of preload running torque is specified in the specification drawing, with the actual value to be agreed upon between the supplier and the customer. To ensure effective management of the preload amount (i.e., zero axial clearance), preload running torque must be measured under strictly consistent conditions.

It should be noted that lubrication conditions and operating environments vary across different machines, and the actual measured running torque may deviate accordingly. Furthermore, the starting torque of a ball screw — the torque required to initiate motion — is generally slightly higher than the preload running torque, and this should also be taken into consideration.

Preload Torque Variation Curve

Note: The torque variation illustrated in this figure has been 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; however, 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 suppresses temperature rise due to friction, loss of mechanical efficiency, and accuracy degradation caused by wear. Ball screws can be lubricated with grease or oil.

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

Selecting the appropriate lubricant according to the application is particularly important. For miniature ball screws, grease churning resistance may increase torque. DINGS' provides proprietary greases optimized for ball screw performance:

• 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

Recommended Lubricants

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 the lubricant quantity and contamination, and replenish as necessary. When adding new grease, remove old and discolored grease as thoroughly as possible.