How Does a Recirculating Ball Screw Work?

A recirculating ball screw converts rotary motion into linear movement by using steel balls that roll between matching helical grooves in the screw shaft and ball nut. After passing through the load-carrying section, the balls return through a recirculation path inside or around the nut and re-enter the loaded raceway.

Continuous ball circulation allows the nut to travel along the screw without the balls reaching the end of the assembly. Rolling contact also produces much lower friction than the sliding contact used in a conventional lead screw.

Main Components of a Recirculating Ball Screw

A complete ball screw pair consists of several interacting components:

• Screw shaft: Contains a precision helical raceway along which the balls roll.
• Ball nut: Contains a matching internal raceway and surrounds the screw shaft.
• Steel balls: Transfer force between the screw and nut through rolling contact.
• Recirculation components: Redirect the balls from the end of one loaded path back to its beginning.
• Seals or wipers: Help retain lubricant and reduce the entry of external contamination.
• Lubricant: Reduces contact stress, friction, heat and wear at the balls and raceways.

The nut body contains the working raceway and ball-return structure required for continuous circulation.

The Working Cycle Step by Step

1. The Screw or Nut Rotates

In a conventional ball screw axis, the motor rotates the screw while the ball nut is prevented from rotating. The nut then moves along the screw and carries the connected machine table or carriage.

The arrangement can also be reversed. In a rotating-nut system, the screw is held against rotation while the nut rotates and produces relative linear movement. The same ball-and-raceway principle applies in both arrangements.

2. Balls Enter the Loaded Raceway

The helical grooves in the screw and nut form a rolling path. As relative rotation begins, the balls roll through the contact zone and transfer axial force between the two components.

The exact contact geometry depends on the raceway design. Load is carried through small contact areas between each ball and the opposing raceways, making hardness, groove geometry, lubrication and surface quality important.

3. Rotation Produces Linear Travel

The helical raceway forces the rotating and non-rotating components to move axially relative to one another. The theoretical linear travel is determined by the ball screw lead:

Linear travel = Number of revolutions × Ball screw lead

For example, a ball screw with a 5 mm lead produces 5 mm of theoretical linear movement per revolution. At 1,000 rpm, its theoretical linear speed is:

5 mm/rev × 1,000 rev/min = 5,000 mm/min

Actual positioning accuracy still depends on lead error, axial clearance, preload, mounting accuracy, thermal expansion and the control system.

4. Balls Leave the Loaded Section

When the balls reach the end of a loaded circuit, they cannot continue indefinitely along the nut raceway. A return component lifts or redirects them away from the working groove.

At this point, the balls enter an unloaded return path. They continue moving because the following balls and the recirculation geometry guide them through the circuit.

5. Balls Return to the Beginning

The return path carries the balls back to the beginning of the loaded raceway. They then re-enter the contact zone and repeat the cycle.

This circulation takes place within the nut assembly. The balls do not travel to the end of the complete screw shaft and return from there.

Common Ball Recirculation Structures

Different nut designs use different methods to return the balls. The main structures include:

External Return Tube

A return tube mounted on the outside of the nut redirects the balls across one or more thread turns. This structure is easy to recognize because the return component is visible on the nut body.

External return tubes can support multiple loaded turns, but they increase the outside dimensions of the nut and require protection from impact during handling and installation.

Internal Deflector

An internal deflector transfers each ball over the screw thread ridge and returns it to the adjacent groove. The ball circuit is compact and contained mainly within the nut body.

Because the balls are redirected over a short distance, the deflector geometry must be manufactured and assembled accurately to avoid impact, jamming or irregular circulation.

End-Cap Recirculation

End-cap systems guide the balls through return passages from one end of the nut to the other. This structure is often used in large-lead or high-speed designs where smooth circulation is particularly important.

The permissible speed and noise level still depend on ball size, nut geometry, lubricant, manufacturing accuracy and operating conditions. An end-cap design should not automatically be described as silent or suitable for every high-speed application.

What Does the Number of Circuits Mean?

A ball circuit is one complete loaded-and-return path inside the nut. A nut may contain several independent circuits or a specified number of turns and rows.

Increasing the number of effective loaded balls can increase load capacity and rigidity, but performance does not depend on circuit count alone. Ball diameter, raceway geometry, contact angle, nut length and manufacturing accuracy also affect the result.

Circuit descriptions are not always written in the same format by every manufacturer. When replacing a nut, confirm the complete drawing and load ratings rather than comparing only the stated number of circuits.

Why Rolling Contact Reduces Friction

A conventional lead screw relies mainly on sliding contact between the screw and nut threads. A ball screw replaces much of this sliding motion with rolling balls.

Lower friction offers several practical benefits:

• Higher mechanical efficiency
• Lower driving torque for the same axial load
• Less frictional heat than a comparable sliding screw
• Smoother motion at properly selected preload
• Ability to support repeated positioning movement

Low friction also means that many ball screws can be back-driven. An axial force on the nut may cause the screw to rotate. A vertical axis therefore requires a motor brake, counterbalance or another safety mechanism; the ball screw should not be assumed to be self-locking.

How Preload Changes the Contact Condition

A clearance-type nut allows a small amount of axial movement before the balls contact the opposite sides of the raceways during direction reversal.

Preload establishes opposing contact forces inside the ball screw pair. This can reduce axial clearance and increase rigidity, but it also increases friction torque and internal contact load.

A double-nut design does not simply mean that there are "two sets of balls for twice the load." Two nuts are commonly adjusted against each other to create preload and control axial clearance. The final load rating and service life must be taken from the actual specification.

What Determines Ball Screw Accuracy?

Recirculation enables continuous movement, but it does not by itself determine positioning accuracy. Accuracy depends on:

• Lead accuracy of the screw shaft
• Axial clearance or preload
• Support-bearing rigidity
• Alignment between the screw and linear guides
• Thermal expansion
• Coupling and drive-system rigidity
• Control resolution and feedback

Rolled and ground ball screws can both use recirculating balls. Ground production is generally selected for higher lead-accuracy requirements, while rolled screws are widely used in general automation and cost-sensitive applications.

What Can Interrupt Ball Circulation?

The balls must move smoothly through both the loaded raceway and the return path. Common problems include:

• Insufficient or unsuitable lubrication
• Metal chips, dust or other contamination
• Damage to a return tube, deflector or end cap
• Incorrect ball size or missing balls
• Raceway dents caused by impact
• Misalignment between the screw, nut and support bearings
• Operating speed above the permissible circulation capability

Symptoms may include irregular torque, clicking, rough movement, abnormal heat, noise or local sticking. Continuing to operate a damaged ball-return system can damage the raceways and cause the nut to jam.

Handling and Maintenance

A ball nut should not be removed from the screw without a suitable transfer sleeve. Removing it incorrectly can allow the balls to fall out or become displaced from their circuits.

During operation:

• Maintain the specified grease or oil supply
• Protect the screw from chips, dust, water and cutting fluid
• Check for abnormal noise, torque and temperature
• Avoid striking the screw raceway or nut body
• Maintain alignment with the support bearings and linear guides
• Do not mix lubricants without confirming compatibility

Typical Industrial Applications

Recirculating ball screws are commonly used in equipment requiring efficient linear transmission, controlled positioning or repeated movement, including:

• CNC machine feed axes
• Automation equipment
• Linear modules and actuators
• Laser and engraving machines
• Packaging machinery
• Woodworking equipment
• Lifting and adjustment mechanisms with suitable safety devices

The specific ball screw must still be selected according to load, speed, stroke, accuracy, mounting orientation, duty cycle and operating environment.

When replacing an imported or inch-series assembly, also check the differences between metric and imperial ball screws .

Conclusion

A recirculating ball screw works by continuously moving rolling balls through a loaded helical raceway and returning them through a separate path inside or around the nut. This closed circulation converts rotary motion into linear travel with relatively low friction.

Reliable operation depends on more than the basic circulation principle. Correct lead, load capacity, preload, lubrication, return structure, installation alignment and contamination protection must all be considered.

View DLY ball screw products for available screw diameters, leads, nut structures and accuracy options.

← Back to DLY Blog