What Ball Nut Sizes Are Available? Screw Diameter, Lead & Nut Dimensions

Ball nut size cannot be described by one number alone. When someone asks for a "16 mm ball nut" or "25 mm ball nut," that number usually refers to the nominal diameter of the matching ball screw shaft, not the outside diameter of the nut itself.

To determine whether a ball nut will fit a machine, engineers normally need to confirm at least the screw diameter, lead, nut outside diameter, flange dimensions, nut length and mounting-hole layout.

For example, an SFU1605 ball nut is designed for a 16 mm nominal-diameter ball screw with a 5 mm lead. However, the nut body itself is larger than 16 mm, and different SFU1605 configurations can even have different overall lengths.

What Does "Ball Nut Size" Actually Mean?

Ball nut size is commonly confused with ball screw diameter. They are related, but they are not the same dimension.

A ball nut drawing normally includes several important dimensions:

What Size Range of Ball Nuts Does DLY Supply?

DLY supplies multiple ball screw nut series for standard assemblies, replacement projects and drawing-based custom requirements.

Across the general DLY ball nut range, published dimensions include approximately:

These values should be treated as an overall product range rather than dimensions for one single nut family. An SFU nut, SFE high-lead nut and compact internal-circulation nut can have very different external dimensions even when they are designed around a similar screw diameter.

You can view the available structures on the DLY Ball Nut product page.

How Do You Read a Ball Nut Model Number?

The SFU series provides a simple example.

• SFU = standard flanged single-nut series;
• 20 = 20 mm nominal screw diameter;
• 05 = 5 mm lead per screw revolution;
• 4 = four ball circulation circuits.

Therefore, SFU2005 does not mean that the complete nut measures 20 × 5 mm. It means that the nut is designed for a 20 mm diameter, 5 mm lead ball screw.

Its actual outside diameter, flange size and length must still be checked from the dimensional drawing.

Common SFU Ball Nut Size Examples

The following examples show why screw diameter and actual nut dimensions should not be confused.

The table shows a clear example: an SFU2505 uses a 25 mm screw shaft, but the nut body outside diameter is approximately 40 mm and the flange dimension is approximately 63 mm.

For complete SFU dimensional tables, see the DLY SFU Ball Screw specifications.

Can Two Ball Nuts with the Same Diameter and Lead Have Different Sizes?

Yes.

This is particularly important for replacement orders.

For example, DLY lists both:

• SFU1605-3: 16 mm diameter, 5 mm lead, nut length approximately 43 mm;
• SFU1605-4: 16 mm diameter, 5 mm lead, nut length approximately 50 mm.

Both work around the same nominal screw diameter and lead, but their circulation configurations and nut lengths are different.

The same issue appears in other sizes. An SFU2005-3 and SFU2005-4, for example, also have different nut lengths and different load ratings.

Why Does the Number of Ball Circuits Affect Nut Size?

Inside the nut, steel balls travel through loaded raceways and then return through a recirculation path.

Increasing the number of effective circulation circuits can increase the loaded contact area and may improve load capacity and rigidity. However, it may also require a longer nut structure.

This is why two nuts with the same screw diameter and lead are not automatically dimensionally interchangeable.

When replacing a nut, the following should be checked together:

• nominal screw diameter;
• lead;
• ball diameter;
• number of circuits;
• nut body diameter;
• nut length;
• flange dimensions;
• mounting-hole pattern.

Does a Larger Ball Nut Always Carry More Load?

A larger ball screw and nut system will often be designed for greater load and rigidity, but size alone is not enough to determine load capacity.

Ball nut load ratings also depend on:

• ball diameter;
• number of loaded circuits;
• raceway geometry;
• nut structure;
• material and heat treatment;
• preload or axial clearance;
• working load direction and duty cycle.

For example, two SFU nuts using the same nominal screw diameter may have different dynamic and static load ratings because the number of circuits or ball diameter is different.

Therefore, engineers should compare the manufacturer's dynamic load rating, static load rating and rigidity data rather than assuming that the physically larger-looking nut is automatically the better choice.

How Does Lead Affect Ball Nut Size?

Lead is the linear distance travelled by the nut for one revolution of the screw.

For example:

• 1605 = 16 mm screw diameter with 5 mm lead;
• 1610 = 16 mm screw diameter with 10 mm lead;
• 2510 = 25 mm screw diameter with 10 mm lead;
• SFE2525 = 25 mm screw diameter with 25 mm lead.

Changing lead can change the internal ball circulation design, ball diameter, nut length and other dimensions. Therefore, a 1605 nut should not be assumed to have the same dimensions as a 1610 nut simply because both use a 16 mm nominal screw diameter.

Lead also affects machine performance. A larger lead produces more linear travel per revolution, while a smaller lead produces less travel per revolution and generally requires more motor revolutions for the same linear distance.

Different Ball Nut Series Have Different Installation Envelopes

Ball nut dimensions also depend strongly on the nut structure.

DLY supplies several nut configurations, including:

As a result, comparing only the screw diameter does not tell you which nut occupies less space. A compact internal-circulation nut may have a different installation envelope from a standard flange nut or double nut of a similar screw size.

Which Dimensions Matter Most for a New Machine Design?

For a new ball screw system, nut size should be considered together with the surrounding mechanical structure.

The main dimensions to reserve in the design are:

• Nut body clearance: enough radial space around the nut body;
• Flange clearance: enough room for the flange and fastening bolts;
• Nut length: sufficient axial space throughout the complete travel;
• Mounting-hole position: compatible with the carriage or nut housing;
• Lubrication access: space for grease or oil maintenance where applicable;
• Travel clearance: the nut and carriage must not collide with support units or machine structures at either end of travel.

This is particularly important in compact linear modules where a few millimetres of extra flange width or nut length may require a redesign of the carriage.

How Do You Select a Ball Nut for Replacement?

Replacement is usually more difficult than selecting a nut for a completely new design because the existing machine already fixes the installation space.

For a replacement ball nut, check the following in this order:

1. Read the complete model number from the existing nut if it is still visible.
2. Confirm the screw nominal diameter.
3. Confirm the screw lead.
4. Measure the nut body outside diameter.
5. Measure the total nut length.
6. Measure the flange size and thickness.
7. Measure the mounting-hole diameter and centre distance.
8. Check lubrication-hole position.
9. Confirm whether the existing nut is a single nut, double nut or another structure.
10. Provide photos or a dimensional drawing when the model cannot be confirmed.

Do not assume that a nut from another series will fit simply because the screw is labelled 1605, 2005 or 2505.

Can a Ball Nut Be Replaced Without Replacing the Screw?

Sometimes, but it should be confirmed carefully.

A replacement nut must match the screw raceway geometry, nominal diameter, lead, ball size and circulation design. Wear of the existing screw shaft should also be considered.

For new equipment or when compatibility is uncertain, supplying the screw and nut as a matched assembly is generally safer than selecting a nut only from an approximate external dimension.

DLY can supply ball nuts separately or together with a matched ball screw assembly according to the required model, drawing and installation conditions.

What Information Should You Send When Asking for a Ball Nut?

Instead of asking only for a "20 mm ball nut," send as much of the following information as possible:

• complete ball nut model;
• nominal screw diameter;
• lead;
• nut series;
• number of circuits, if known;
• nut body outside diameter;
• nut overall length;
• flange dimensions;
• mounting-hole dimensions;
• required preload or axial clearance;
• old nut photos;
• machine or assembly drawing.

For new ball screw assemblies, also provide the required screw length, travel, working load, speed, accuracy grade and shaft-end machining requirements.

Conclusion

There is no single dimension that defines the size of a ball nut.

The number in a model such as 1605, 2005 or 2505 mainly identifies the matching screw diameter and lead. To determine whether the nut physically fits the machine, the actual nut body diameter, flange size, nut length and mounting-hole dimensions must also be checked.

Different nut series - and even different circuit configurations within the same diameter and lead - can have different external dimensions and load ratings.

For this reason, the safest selection method is:

rather than selecting a ball nut from nominal screw diameter alone.

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