Ball Screws Information

Last revised: January 7, 2025

Reviewed by: Scott Orlosky, consulting engineer

Ball screws convert rotary motion to linear motion, or torque to thrust, and vice versa. They are primarily a power screw with a train of ball bearings

riding between the screw and the nut in a recirculating track. The screw has a rounded shape to conform to the balls. Ball screws have a predictable service life and low wear rate. ACME screws and other lead screws transmit torque into linear motion through direct sliding friction. A lead screw device is very similar to a regular nut and bolt combination. Typically, the assembly comes complete with a screw, nut, and bearing support. For low speed and high accuracy applications acme screws are cost effective.

Manufacturing Process

Choices for manufacturing process for ball screws, lead screws, and ACME screws include:

Rolled screws are cold rolled from a blank. Typically, this produces a screw with a .004 inch per foot accuracy.
Milled screws are made on a milling machine using a cutter with an Acme form. Typically, this produces a screw with a .002 inch per foot accuracy.
Ground screws are made on a grinding wheel with an Acme form. Typically, this produces a screw with .0005 inch per foot lead accuracy.

Physical Specifications

Important physical specifications to consider when searching for ball screws, lead screws, and ACME screws include:

The design arrangement can be English or metric. Some suppliers specify both.
The length of the screw is the most important dimension to consider.
The outer screw diameter is the largest diameter value of the screw.
Screw lead is the axial distance a screw travels during one revolution, measured in inch/turn. Screw lead accuracy is represented as a percentage.
The dynamic load rating is the load that can be sustained while the assembly is moving.
The maximum static capacity is the load that can be sustained, measured while the load is stationary.

Design Features

Important design features to consider when specifying ball screws, lead screws, and ACME screws include zero backlash design, provisions for screw lubrication, right hand thread, left hand thread, and twin leads.

Backlash is the axial free motion between the nut and screw. It is a measure of the stiffness of the assembly.
Right hand thread specifies that the direction of the threads on the screw shaft causing the ball nut to travel away from the end viewed when rotated is clockwise.
Left hand thread specifies that the direction of the threads on the screw shaft causing the ball nut to travel away from the end viewed when rotated is counter-clockwise.
Lead is the axial distance a screw travels during one revolution.
Twin leads are useful in opposing dual motion using a single drive system. This design features both the left and right hand threads.
Common nut materials for ACME screws include plastic and bronze.

Standards

BSI - BS ISO 3408-1— establishes the vocabulary for ball screws and specifies their designation.

BS 6101-2 — applies to re-circulating ball screw assemblies having the shaft and nuts manufactured from hardened steel.

ISO 3408-2 — specifies the nominal diameters and nominal leads metric series for ball screws.

Ball Screws FAQs

How do the physical specifications of ball screws impact their performance in different applications?

Here are some key aspects to consider:

Sizing and Load Capacity

Proper sizing is necessary to avoid overloading, which in turn reduces the lifespan of a ball screw. Factors affecting sizing include maximum load, travel rate or speed, critical ball speed, column load, duty cycle, end fixity, and drive torque. Additionally, the operating environment, system mounting, stroke length, and load guidance are important considerations.

Unit Type and Dimensions

Ball screws are specified by unit type (English or metric), length, and outer diameter. These dimensions must align with the application's requirements to ensure proper fit and function.

Lead and Lead Accuracy

The lead or axial distance traveled per screw rotation is a critical specification. Lead accuracy, which measures the permissible travel variation, is essential for applications requiring precise positioning. Ball screws are available in different tolerance levels, such as transport (T-Grade) and positioning (P-Grade), which cater to varying accuracy needs.

Dynamic and Static Load Ratings

The dynamic load rating is the load that a ball screw can handle while maintaining a specific operational life. The maximum static load capacity defines the load that the screw can support without permanent deformation. These ratings help determine the suitability of a ball screw for specific applications.

Environmental Considerations

Environmental conditions, such as temperature and exposure to contaminants, can affect ball screw performance. Proper sealing and material selection are necessary to ensure reliability in challenging environments.

What are the differences between T-Grade and P-Grade ball screws?

The differences between T-Grade and P-Grade ball screws primarily relate to their lead accuracy and intended applications.

Lead Accuracy

T-Grade (Transport Grade): This is the lower tolerance level, with precision typically around ±50 µm/300 mm or higher. T-Grade ball screws are generally used for applications that require simple translation of a load from one point to another, where high precision is not critical.

P-Grade (Positioning Grade): When a higher precision is needed and the ball screw is intended to be operated without any feedback mechanism, then the P-Grade should be specified. The standard repeatability for a P-Grade ball screw is ±23 µm/300 mm or better.

These grades help in selecting the appropriate ball screw based on the precision requirements of the application.

How is lead accuracy (precision) defined for ball screw applications?

Definition of Lead Accuracy

Lead accuracy refers to the permissible travel variation of a ball screw, which is the difference between the actual position and the theoretical position per unit length. It is a measure of how accurately the ball screw can translate rotational motion into linear motion. Lead accuracy is directly related to tolerance levels. Lower accuracy is T-Grade (Transportation) while higher accuracy is P-Grade (Precision)

Impact on Precision

High lead accuracy ensures that the ball screw can achieve precise positioning, which is essential in applications where exact movement is required. This is particularly important in industries such as robotics, CNC machinery, and aerospace, where even small deviations can lead to significant errors.

Cost Considerations

Increasing lead accuracy can be expensive. Therefore, it is important to balance the need for precision with cost considerations. Specifying more accuracy than necessary can lead to increased costs without proportional benefits.

Lead accuracy defines the precision of ball screw applications. Selecting the appropriate tolerance level based on the application's precision helps optimize performance. This may take one or more design iterations to hit the right balance for a particular application.

What is the impact of environmental conditions on ball screw performance?

Here are some key considerations:

Temperature

Extreme temperatures can affect the material properties of ball screws, causing individual parts to expand or contract. This impacts precision and performance. High temperatures may also degrade lubricants, increasing friction and wear.

Contaminants

Exposure to dust, dirt, and other contaminants can lead to increased wear and tear on ball screws. Proper sealing and regular maintenance are essential to prevent contaminants from entering the ball screw assembly and affecting its operation.

Humidity and Corrosion

High humidity levels can lead to corrosion, especially if the ball screws are made from materials susceptible to rust. Using corrosion-resistant materials or coatings can help mitigate this risk.

Vibration and Shock

Environmental vibrations and shocks can affect the alignment and smooth operation of ball screws. Designing a robust mounting and support can help minimize these effects.

Sealing and Material Selection

Proper sealing helps protect the ball screw from environmental factors, while material selection can enhance resistance to specific conditions.

These factors must be considered during the design and selection process to ensure that the ball screw performs optimally in its intended environment.

What are the best practices for maintaining ball screws in harsh environments?

Proper Sealing: Use appropriate seals to prevent contaminants such as dust, dirt, and moisture from entering the ball screw assembly. This helps reduce wear and tear and prevents corrosion.

Material Selection: Choose materials that are resistant to the specific environmental conditions. For example, corrosion-resistant materials or coatings can be beneficial in humid or corrosive environments.

Lubrication: Regular lubrication is essential to minimize friction and wear. Ensure that the lubricant used is suitable for the environmental conditions, such as high temperatures or exposure to chemicals.

Temperature Management: Monitor and manage the operating temperature to prevent material expansion or contraction that could affect precision. High temperatures can also degrade lubricants, so selecting temperature-resistant lubricants is important.

Regular Maintenance: Implement a regular maintenance schedule to inspect and clean the ball screws. This includes checking for signs of wear, ensuring proper alignment, and replacing worn components as needed.

Vibration and Shock Mitigation: Ensure proper mounting and support to minimize the effects of environmental vibrations and shocks, which can affect the alignment and smooth operation of ball screws.