A ball screw nut is the moving component that contains recirculating steel balls and matching helical raceways. Together with the screw shaft, it converts rotary motion into precise linear movement with much less sliding friction than a conventional lead screw and nut.
The nut is not simply a threaded sleeve. Its internal raceway, ball size, return system, preload and dimensional relationship with the screw shaft all affect load capacity, backlash, running smoothness and service life.
How Does a Ball Screw Nut Work?
The screw shaft and nut contain matching curved raceways. Steel balls run between these raceways and carry the load while rolling through the contact zone.
When the screw rotates and the nut is prevented from rotating, the nut moves linearly by one lead for every complete revolution. For example, a 5 mm lead produces approximately 5 mm of theoretical linear travel per screw revolution.
After the balls reach the end of the loaded raceway, a return component redirects them to the beginning of the circuit. This continuous recirculation allows the nut to travel along the screw without the balls leaving the assembly.
Main Parts Inside a Ball Screw Nut
| Part | Function |
|---|---|
| Nut body | Provides the internal raceway and mounting structure |
| Steel balls | Roll between the screw and nut raceways and transmit axial load |
| Ball return system | Returns the balls to the beginning of each circulation circuit |
| Flange or mounting surface | Connects the nut to the machine table, nut seat or moving structure |
| Seals or wipers | Help retain lubricant and reduce contaminant entry |
| Lubrication port | Allows grease or oil to reach the internal rolling contacts |
DLY standard ball screw nuts use 20CrMo alloy steel. Material selection must be combined with heat treatment and raceway processing because the loaded surfaces require suitable hardness, wear resistance and dimensional stability.
How Do the Balls Recirculate?
Ball nuts can use different return structures. The circulation method affects nut dimensions, suitable lead, speed capability, noise and manufacturing structure.
Internal Circulation
An internal return component redirects the balls across the screw thread into the adjacent circuit. This structure can produce a relatively compact nut body and is commonly used for precision feed applications.
External Circulation
The balls leave the loaded raceway through an external return tube or return path and then re-enter the circuit. The external structure is visible on some nut designs and can accommodate different circuit arrangements.
End-Cap Circulation
End caps guide the balls through a return passage running along the nut body. This design is commonly associated with high-lead nuts and applications requiring smooth circulation at higher travel speeds.
The circulation method cannot be identified only from the screw diameter and lead. The nut series and external structure must also be confirmed.
Single Nut, Double Nut and Rotating Nut
Single Ball Nut
A single nut has one main nut body and is widely used in automation equipment, CNC routers, positioning units and general linear motion systems. Clearance can vary according to its ball matching, manufacturing accuracy and preload arrangement.
Double Ball Nut
A double nut uses two nut sections with an axial preload arrangement between them. It is selected when reduced axial clearance and higher rigidity are more important than compact size and lower running resistance.
Adding preload does not automatically improve every machine. Excessive preload raises internal load, heat and driving torque and may reduce fatigue life.
Rotating Ball Nut
In a conventional assembly, the screw rotates and the nut travels. In a rotating ball nut system, the screw shaft remains stationary while the nut rotates and moves along it.
This arrangement still produces linear motion. Its main advantage is that a long screw does not need to rotate at high speed, helping reduce the risk of screw whip and critical-speed limitations in long-stroke applications.
What Does Preload Do in a Ball Nut?
Preload applies a controlled internal force between the screw, balls and nut raceways. It is used to reduce axial clearance and improve rigidity and reversal response.
Preload may be created through ball-size selection, an offset-pitch structure or a double-nut arrangement. The appropriate method depends on the nut design and required accuracy.
Preload should not be confused with the accuracy grade of the screw. Accuracy describes lead and positioning error, while preload primarily affects axial clearance, rigidity, friction and torque.
Can a Ball Screw Nut Be Replaced Separately?
A ball nut can sometimes be purchased as a replacement part, but matching only the nominal shaft diameter is not enough. A "16 mm ball nut," for example, may have different leads, flange dimensions, circulation structures and internal raceway geometry.
Before ordering a replacement nut, confirm:
- Nut series and complete model number
- Screw shaft diameter
- Screw lead
- Flange shape and mounting-hole positions
- Nut body length and outside diameter
- Circulation structure
- Required clearance or preload
- Lubrication-port position
Even when two nuts have similar external dimensions, their internal raceways may not match the existing screw. If the original model cannot be confirmed reliably, ordering a matched screw-and-nut assembly is generally safer than forcing an unmatched nut onto the shaft.
Why Should a Nut Not Be Removed Directly?
If a ball nut is pushed directly off the screw without a transfer sleeve or mandrel, the balls may leave their circulation tracks and fall out. Incorrect reassembly can change ball distribution, preload and running smoothness.
When removal is necessary, place a correctly sized transfer sleeve against the screw end and slowly rotate the nut onto the sleeve. Keep the nut aligned throughout the process.
Do not add arbitrary replacement balls. Ball diameter and grouping affect internal clearance and preload, so the correct specification must be confirmed before rebuilding a nut.
How to Select the Correct Ball Screw Nut
Start with the screw diameter and lead, then evaluate load, required travel speed, mounting space, flange shape, backlash, rigidity and lubrication arrangement.
- Load: Confirm dynamic load, static load and impact conditions.
- Lead: Select according to travel per revolution, speed and motor requirements.
- Accuracy: Match the screw accuracy to the machine's positioning needs.
- Backlash: Decide whether normal clearance, preload or a double nut is required.
- Mounting: Check the flange, nut seat and available installation space.
- Environment: Consider contamination, temperature and lubrication access.
DLY supplies standard and custom ball screw nuts, including SFU, SFA, SFE, SFY, SFNU, SFS and double-nut structures. Nut-only replacement and matched ball screw assemblies can be checked according to the existing model, drawing or sample.
For applications requiring higher lead accuracy, see the available precision ground ball screw options.
Need to identify or replace a ball screw nut?
Send DLY the complete model, screw diameter, lead, flange dimensions and clear photos. If available, include the old screw and nut drawing.
Email: export@dlybearing.com
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