A ball screw cannot operate continuously unless its rolling balls are returned from the end of the loaded raceway to the beginning. The ball recirculation method is the mechanism that guides this return movement inside or around the ball nut.
The three commonly encountered arrangements are the return-tube (external circulation), deflector or return-piece (internal circulation), and end-cap circulation methods. Each creates a continuous ball path, but the return route, nut dimensions and typical operating characteristics differ.
Why Do the Balls Need to Recirculate?
The screw shaft and ball nut contain matching helical grooves. Together, these grooves form loaded raceways. When the screw or nut rotates, the balls roll through the raceway and transmit axial force while the nut and shaft move relative to one another.
Without a return path, the balls would reach the end of the nut and motion would stop. A recirculation component lifts or redirects each ball out of the loaded raceway, carries it back to an earlier groove, and feeds it into the loaded zone again. This process repeats continuously during operation.
| Method | Return path | Typical structural characteristic | Common selection reason |
|---|---|---|---|
| Return tube | Balls leave the loaded groove and pass through a tube or return component mounted on the nut. | The return component is visible outside the main nut body and may increase the required installation envelope. | A widely used, established arrangement available in many shaft, lead and nut configurations. |
| Deflector / return piece | A deflector transfers balls across the screw shaft circumference into an adjacent groove. | The return route is contained within the nut, allowing a compact outside diameter. | Useful where the available space around the ball nut is limited. |
| End cap | End caps pick up the balls and direct them through longitudinal passages in the nut body. | The nut contains internal return holes connected by components at both ends. | Frequently associated with large-lead and fast-feed designs, subject to the manufacturer's rated speed. |
1. Return-Tube or External Ball Recirculation
In a return-tube design, pickup sections guide the balls out of the helical raceway. The balls then travel through a tube or return piece attached to the outside of the nut and re-enter the raceway several turns earlier.
This is one of the most widely used recirculation structures and can be supplied in many nut configurations. Its external component, however, must be considered when designing the nut housing, installation clearance and protection against impact or contamination.
A return tube does not by itself establish the ball screw's load capacity. Dynamic load rating depends on the complete design, including shaft diameter, ball diameter, number of loaded turns or circuits, raceway geometry and material condition.
2. Deflector or Internal Ball Recirculation
A deflector, sometimes called a return piece, redirects a ball from one groove to an adjacent groove by guiding it across the screw shaft. Each deflector normally forms a short circulation circuit, and several deflectors may be distributed around the nut.
Because there is no projecting return tube, this arrangement can reduce the nut's outside envelope. It is therefore useful for compact nut designs. The exact permissible lead, rotational speed and circuit arrangement still depend on the individual product series.
Internal circulation should not automatically be interpreted as higher positioning accuracy. Accuracy grade is determined mainly by lead accuracy and manufacturing control, while rigidity and axial clearance are influenced by the nut structure and preload.
3. End-Cap Ball Recirculation
In an end-cap design, a pickup feature in the end cap removes balls from the loaded raceway. The balls pass through a longitudinal return hole inside the nut and are guided back into the raceway by the cap at the opposite end.
This structure is commonly used for large-lead ball screws and fast linear feed. A larger lead creates more linear travel per revolution, but the final speed limit must still be checked against the ball screw manufacturer's permissible rotational speed, DN value, shaft critical speed and installation conditions.
Example of a DLY large-lead ball screw assembly. Confirm the circulation structure from the exact model specification.
The nut exterior alone may not reveal every internal return passage or circuit detail.
Does the Recirculation Method Determine Performance?
The circulation method affects the return path, nut size and ball transition behavior, but it should not be used as a shortcut for judging the entire ball screw. Two ball screws using the same circulation method can have very different ratings and operating characteristics.
Evaluate these factors together:
- required axial load and calculated service life;
- shaft diameter, lead and number of ball circuits;
- maximum linear speed, acceleration and duty cycle;
- permissible rotational speed and shaft critical speed;
- required lead accuracy, axial clearance and preload;
- available nut envelope and mounting interface;
- lubrication, sealing, contamination and operating temperature.
Noise and vibration also depend on ball transition geometry, manufacturing quality, lubrication, installation alignment and operating speed. It is therefore inaccurate to assume that every internal-circulation ball screw will be quieter than every return-tube design.
How to Select the Correct Ball Nut
Start with the machine requirements rather than choosing a circulation label in isolation. Define the travel, load, speed, acceleration, mounting space, positioning requirement and environment. The supplier can then match these conditions to an available nut series and verify the relevant ratings.
For a replacement, provide the complete model number whenever possible. Ball screws with the same nominal diameter and lead may still differ in nut dimensions, flange pattern, circuit count, seals, preload and circulation components. A dimensional drawing is especially important when the replacement must fit an existing housing.
You can review DLY's available configurations in the ball screw product range. Final structure and performance should always be confirmed against the selected model and drawing.
Information to Provide with an Inquiry
- screw shaft diameter, lead and total length;
- required stroke and shaft-end machining drawing;
- axial load, speed, acceleration and operating cycle;
- accuracy grade and allowable axial clearance;
- nut style, flange dimensions and available installation space;
- preload requirement, if applicable;
- working environment, lubrication and sealing needs;
- quantity and existing model number for replacement projects.
Frequently Asked Questions
Are there only internal and external circulation methods?
No. "Internal" and "external" are broad descriptions. In product selection, return-tube, deflector or return-piece, and end-cap structures provide a more useful classification.
Is external circulation always better for heavy loads?
No. Load capacity must be checked from the rated load and life data of the exact model. The circulation method alone does not determine how much axial load a ball screw can carry.
Is end-cap circulation always the fastest?
End-cap circulation is often used in large-lead, fast-feed designs, but the allowable speed remains model-specific. Nut circulation, shaft critical speed, supports, alignment and lubrication all need to be verified.
Can the circulation method be identified from a photograph?
A projecting return tube may be visible, but internal passages and some return pieces cannot always be confirmed from an exterior photograph. Use the model specification, sectional drawing or manufacturer confirmation.
Contact DLY for Ball Screw Selection
Send us your diameter, lead, length, load, speed, accuracy requirement and installation drawing. DLY can review the available ball nut structure and provide a configuration matched to your application.
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