Common Ball Screw Terminology You Should Know

Ball screws are found in many machines that move in straight lines. They help convert rotary motion into smooth, accurate linear motion. This makes them important in CNC tools, factory robots, medical equipment, and more. This guide walks you through the key ball screw terms.

What Is Ball Screw?

A ball screw is a mechanical part that changes rotary motion into linear motion. It includes a threaded shaft and a ball nut filled with small steel balls. These balls roll along the threads and carry the load, which makes the motion smooth and efficient.

Because the balls roll instead of slide, ball screws have much less friction than standard screws. This helps reduce wear, improve speed, and increase positioning accuracy. Ball screws are used in CNC and 3D machines, factory automation systems, robotics, medical tools, packaging equipment and so on.

Understanding how ball screws work means knowing the parts and how they’re measured. That’s what we’ll cover next.

Common Ball Screw Terms

Below are the most used terms when working with ball screws. These help you compare options, read specs, and design better systems.

Lead

This is the distance the nut moves when the screw makes one full turn. A higher lead means faster travel. For example, if a screw has a 10 mm lead, the nut moves 10 mm forward each time the screw turns once.

Pitch

Pitch is the distance between threads, measured along the shaft. In single-start screws, pitch and lead are the same. But in multi-start screws, the pitch is smaller than the lead. This is because there are multiple thread paths working at once.

Multiple Start Screw

These screws have more than one thread starting at different points around the shaft. For example, a 2-start screw has two threads wrapped around the shaft. This design increases the lead, which makes the nut move faster, but can lower load capacity slightly.

Pitch Diameter

This is the diameter where the rolling balls make contact with the screw and nut. It lies between the outer (major) diameter and the root. It’s important for calculating strength and speed limits.

Root Diameter

This is the smallest diameter of the screw shaft, measured at the bottom of the threads. A larger root diameter makes the screw stronger and helps prevent bending under heavy loads.

Circuit

A circuit is the complete path the balls travel through inside the nut. Some nuts have one circuit, while others have multiple. More circuits spread the load better and increase the ball screw’s life.

Turns

This refers to the number of thread loops inside the ball nut that the balls roll through. More turns can improve smoothness and control. It also affects how far the balls travel in one full circuit.

Deadband

Deadband is the small space or delay before motion begins when the screw starts turning. It’s caused by a slight gap between parts. A lower deadband improves response and positioning accuracy.

Dynamic Load

This is the highest force the ball screw can handle while moving. It tells you how much weight the screw can support under normal operating conditions. This value helps predict the ball screw’s working life.

Static Load

This is the maximum force the screw can support when it’s not moving. It matters when the system must hold a load in place without motion, such as in vertical lifting setups.

Tension Load

This is the amount of pulling force a ball screw can withstand along its length. If a system puts force on both ends of the screw—like in vertical machines—tension load becomes important.

Rigidity

Rigidity is the stiffness of the ball screw system. Higher rigidity means less stretch or deflection when under load. It helps keep the motion accurate, especially in high-force applications.

Critical Speed

The critical speed is the fastest a screw can spin before it starts to vibrate or whip. Going beyond this speed may cause noise, wear, or even failure. Choosing the right screw length and support helps prevent this.

Thread Direction

Ball screws can be right-hand or left-hand threaded. A right-hand thread moves the nut forward when turned clockwise. Most systems use right-hand threads, but left-hand options are available for special needs.

Ball Circle Diameter (BCD)

This is the diameter of the path that the balls roll along inside the nut. It helps define the fit and match between the screw and nut. Designers use BCD to ensure the parts work together correctly.

Working with ball screws means knowing how they move, what they can handle, and how to describe their parts. The terms in this guide are a solid foundation for anyone working with linear motion systems. And you can check the types of ball screw, what causes ball screw failure to know more about the ball screw. If you’re looking for a linear motion components manufacturer you can count on, reach out to our team. We are the leading manufacturer for linear motors, linear modules, direct drive motors and marble-based linear motor stages and more.