The correct ball size for a ball screw nut is determined by the screw and nut raceway geometry, ball circulation structure and required preload or axial clearance. It is not normally a separate specification that the machine designer selects after choosing the ball screw.
When purchasing a new ball screw, select the complete screw-and-nut assembly according to load, speed, lead, accuracy and service-life requirements. When repairing an existing assembly, replacement ball diameter must be confirmed through inspection and torque or clearance testing. Installing balls simply because they have a similar measured diameter can cause excessive preload, backlash, heat or circulation failure.
What Determines the Ball Diameter?
The balls roll between helical raceways in the screw shaft and nut. Their nominal diameter must correspond to the geometry of these two raceways and the ball return path.
The main design factors include:
- Screw diameter and lead: these establish the basic size and helix geometry of the ball screw.
- Raceway groove geometry: groove radius and contact geometry determine how the balls contact the screw and nut.
- Ball circulation method: internal, external and end-cap return systems impose different space and circulation requirements.
- Number of loaded circuits: this affects load distribution and nut structure.
- Required preload or clearance: small changes in ball diameter can alter internal contact and axial clearance.
- Manufacturing tolerance: the finished screw, nut and balls must be matched within controlled tolerances.
This is why two ball screw nuts with the same nominal screw diameter and lead do not necessarily use interchangeable balls. For example, different 1605 nut series may have different raceway, return and preload designs.
Does a Larger Ball Increase Load Capacity?
In a new ball screw design, ball diameter is one factor that influences contact load and rated capacity. However, this does not mean that larger balls can be installed in an existing nut to increase its load rating.
The dynamic and static load ratings belong to the complete ball screw assembly. They depend on raceway geometry, material, hardness, number of loaded circuits, ball diameter and contact conditions. Replacing the original balls with larger ones does not change the assembly into a higher-load model.
Instead, oversized balls in an existing raceway may increase preload and contact stress. If the size increase is excessive, the ball screw may develop high starting torque, abnormal heat, rapid wear or complete locking.
What Happens If the Balls Are Too Large or Too Small?
| Ball Condition | Likely Effect | Possible Symptoms |
|---|---|---|
| Balls too large | Excessive preload and contact stress | High torque, heat, tight spots, rapid wear or locking |
| Balls too small | Increased internal and axial clearance | Backlash, reduced rigidity, vibration and positioning variation |
| Mixed uncontrolled sizes | Uneven load sharing and circulation | Torque fluctuation, local wear, noise or return-path damage |
| Correct matched size | Specified clearance or preload | Stable torque, smooth circulation and controlled axial movement |
Some ball screw designs intentionally use oversized balls to create preload, but this is a controlled engineering method. The ball diameter, raceway contact and operating torque are matched during manufacturing. It should not be reproduced by adding an arbitrary amount to the original ball diameter.
Do Smaller Balls Improve Speed or Accuracy?
Smaller balls do not automatically make an existing ball screw faster, quieter or more accurate. High-speed performance also depends on lead, shaft diameter, rotational speed, circulation method, nut design, lubrication and mounting accuracy.
Likewise, positioning accuracy is mainly determined by lead accuracy, axial clearance, preload, mounting, support bearings and thermal conditions. Installing smaller balls generally increases clearance rather than improving accuracy.
Noise is also not controlled by ball diameter alone. Common causes of abnormal noise include:
- incorrect or insufficient lubrication;
- contamination inside the nut;
- damaged raceways or return components;
- excessive preload;
- misalignment between the screw and support bearings;
- operation above the permitted rotational speed.
How Should a New Ball Screw Be Selected?
For a new machine, the correct process is to select the complete ball screw specification rather than loose ball diameter.
- Calculate the axial load and required service life.
- Confirm travel, screw length and mounting arrangement.
- Select screw diameter and lead according to load and speed.
- Check permissible axial load and critical rotational speed.
- Select the nut type and circulation structure.
- Confirm accuracy grade, axial clearance and preload requirement.
- Verify lubrication, temperature and environmental conditions.
DLY supplies complete ball screw assemblies and multiple ball screw nut types. For a new order, provide the screw diameter, lead, length, load, speed, accuracy and end-machining requirements instead of specifying an assumed internal ball diameter.
What Should Be Checked During Ball Screw Reballing?
Reballing is not simply a matter of counting the old balls and buying replacements of the same approximate size. Used balls and raceways may already have measurable wear.
A proper repair assessment should check:
- the complete screw and nut model;
- original ball circuit and ball quantity;
- raceway wear, pitting, corrosion and surface damage;
- ball diameter using suitable precision measuring equipment;
- axial clearance or intended preload;
- starting and running torque after assembly;
- smooth circulation through the full usable stroke.
If the raceways are worn, installing larger balls to remove backlash may temporarily increase preload without restoring the original raceway accuracy. It can also create uneven contact and rapid heat generation. In this situation, replacing the nut alone or replacing the complete matched screw-and-nut assembly may be safer.
Do not remove a ball nut directly from the screw shaft unless a suitable transfer sleeve is used. Otherwise, balls may fall out or enter the return path incorrectly during reassembly.
The Practical Answer
For new equipment, choose the correct ball screw model and performance specifications; the manufacturer determines the compatible internal ball diameter. For repair, do not select replacement balls solely from load, speed or an approximate measurement. Confirm the raceway condition, required clearance or preload, ball grade, quantity and assembled torque before returning the screw to service.
Need to replace a ball nut or confirm a ball screw assembly?
Send DLY the complete model, screw diameter, lead, length, photos and application requirements.
Email: export@dlybearing.com
