To measure a ball screw diameter correctly, first determine which diameter you need: the nominal screw diameter, the outside diameter of the threaded shaft, the groove root diameter, or a machined shaft-end diameter. These dimensions serve different purposes and cannot always be measured with the same tool.
For quick identification, a vernier or digital caliper can be used to check the outside diameter across the thread crests. For dimensional inspection or replacement confirmation, the result should be checked against the ball screw model, lead, manufacturer drawing and other installation dimensions.
Which Ball Screw Diameter Do You Need to Measure?
The term "ball screw diameter" may refer to several different dimensions. Identifying the required dimension before measuring helps prevent incorrect model selection.
| Dimension | Meaning | Recommended Method |
|---|---|---|
| Nominal screw diameter | The diameter used in the model designation | Confirm from the model code or drawing |
| Thread outside diameter | Diameter measured across the screw-thread crests | Caliper or suitable outside micrometer |
| Groove root diameter | Diameter at the bottom of the ball raceway | Profile measurement, CMM or manufacturer drawing |
| Ball center diameter | Design diameter passing through the centers of the loaded balls | Confirm from technical drawings or specialized inspection |
| Shaft-end diameter | Diameter of a machined bearing seat, coupling seat or other journal | Outside micrometer or caliper |
The nut outside diameter and flange diameter are nut dimensions. They should not be recorded as the ball screw shaft diameter.
What Does the Diameter in a Ball Screw Model Mean?
In many common ball screw model codes, the first group of numbers indicates the nominal screw shaft diameter and the second group indicates the lead.
Example: In SFU1605, "16" normally identifies a nominal screw diameter of 16 mm, while "05" identifies a lead of 5 mm per revolution.
This does not mean every surface measured on an SFU1605 assembly will be exactly 16 mm. The groove root, ball center diameter and machined journals are different dimensions. The measured outside diameter may also differ slightly from the nominal value because of the thread design and manufacturing tolerance.
You can review DLY's available diameters, leads and nut structures on the ball screw product page.
Tools for Measuring a Ball Screw Diameter
Digital or Vernier Caliper
A caliper is convenient for quick size identification and general inspection. It can measure the outside diameter across the thread crests and the diameters of accessible shaft-end sections.
However, the helical grooves can make jaw contact unstable. A caliper reading should therefore not be treated as an accurate measurement of the groove root diameter or ball center diameter.
Outside Micrometer
An outside micrometer is generally more suitable for checking smooth cylindrical journals, bearing seats and coupling seats. It may also be used for a thread outside-diameter check when its measuring faces can contact the crests consistently.
The actual measurement uncertainty depends on the instrument range, resolution, calibration condition, contact force, temperature and operator technique. A fixed accuracy value should not be assumed for every caliper or micrometer.
Profile Measuring Machine, CMM or Optical Comparator
Specialized inspection equipment is required when the groove geometry, root diameter or complete thread profile must be verified. A coordinate measuring machine is a dimensional inspection system and should not automatically be described as an optical instrument; its probing method depends on the equipment configuration.
How to Measure the Outside Diameter of a Ball Screw
- Clean the screw. Remove oil, dust and loose contamination from the measurement area without scratching the raceway.
- Allow temperature to stabilize. Avoid measuring immediately after machining, transport or high-speed operation.
- Check the measuring tool. Clean the measuring faces, check the zero position and confirm that the instrument is within calibration.
- Select the threaded section. Do not measure a machined shaft end, damaged crest or visibly worn area when identifying the screw diameter.
- Position the jaws correctly. Keep the caliper or micrometer perpendicular to the screw axis and contact the opposing thread crests.
- Apply light, consistent force. Excessive force can tilt the instrument or produce an inconsistent reading.
- Measure in more than one direction. Repeat the measurement after rotating the tool or screw to check whether contact with the helical crests changes the result.
- Check several axial locations. Record readings from multiple positions along the threaded length, excluding damaged or unfinished sections.
- Record the conditions. Note the tool, measurement locations, maximum and minimum readings, and temperature when inspection traceability is required.
This procedure provides an outside-diameter check. It does not directly determine the groove root diameter or confirm the dimensional accuracy of the complete ball raceway.
Why a Caliper Cannot Reliably Measure the Groove Root Diameter
A ball screw raceway is a helical, curved contact surface designed to work with recirculating balls. Standard flat caliper jaws cannot consistently reach the two opposing groove bottoms at the same cross-section.
If the groove root diameter is required for engineering verification, use the manufacturer's dimension drawing or an approved inspection method involving appropriate probes, balls, profile equipment or a CMM. Do not estimate the root diameter by forcing caliper tips into the grooves.
How to Measure a Machined Ball Screw Shaft End
Ball screw shaft ends can include several different diameters for bearings, support units, couplings, threads and retaining features. Measure every smooth cylindrical section separately.
- Measure bearing journals with an outside micrometer where possible.
- Take readings in at least two angular directions to check for out-of-roundness.
- Measure at more than one axial position on longer journals.
- Record journal lengths, shoulders, threads, keyways and retaining-ring grooves.
- Compare all dimensions with the support-bearing and coupling requirements.
The shaft-end dimensions may be completely different from the nominal threaded-shaft diameter. They should be specified separately when ordering a custom-machined ball screw.
How to Identify an Unknown Replacement Ball Screw
Outside diameter alone is not enough to identify a replacement. Record the following information before requesting a quotation:
- Existing model number, if visible
- Observed thread outside diameter
- Lead, measured as nut travel per screw revolution
- Right-hand or left-hand thread direction
- Nut type, flange shape and mounting-hole dimensions
- Overall screw length and threaded length
- All machined shaft-end dimensions
- Required axial clearance, preload and accuracy grade, if known
- Clear photos and an installation drawing
For example, two ball screws may have the same nominal diameter but different leads, nut dimensions, circulation structures or shaft-end machining. They are not necessarily interchangeable.
Common Measurement Mistakes
- Assuming the first caliper reading is the exact nominal diameter
- Measuring a machined shaft end instead of the threaded shaft
- Confusing the screw diameter with the nut or flange diameter
- Trying to measure the groove root with standard flat caliper jaws
- Identifying a ball screw without checking its lead
- Measuring across dirt, grease deposits, corrosion or damaged thread crests
- Using excessive measuring force
- Ignoring calibration and temperature conditions
Does a Larger Ball Screw Diameter Carry More Load?
Increasing the screw shaft diameter generally improves shaft stiffness and can help with buckling resistance and critical-speed performance. However, the allowable load of a complete ball screw cannot be determined from shaft diameter alone.
Load capacity also depends on the ball diameter, nut structure, number of loaded circuits, material, heat treatment, accuracy, preload, unsupported length and end-support arrangement. Always use the product load ratings and application calculations when selecting a ball screw.
Frequently Asked Questions
Can I identify a ball screw using only a caliper?
A caliper can help estimate the threaded-shaft outside diameter, but it cannot confirm the complete model. You must also check the lead, nut type, overall length, shaft ends and accuracy requirements.
Is the measured outside diameter always equal to the nominal diameter?
Not necessarily. The model code normally states a nominal shaft diameter, while the actual crest measurement is influenced by thread geometry, manufacturing specifications and measurement contact.
Can I use a micrometer on a ball screw thread?
It may be used for an outside-diameter check if the measuring faces contact the thread crests consistently. It is better suited to smooth shaft-end journals and does not directly measure the groove root diameter.
Are Acme screws a type of ball screw?
No. An Acme screw is a sliding screw with a trapezoidal thread form. A ball screw uses recirculating balls between the screw shaft and nut, so its groove geometry and measurement requirements are different.
Conclusion
The correct way to measure a ball screw diameter depends on the dimension you need. A caliper is useful for a quick outside-diameter check, while a micrometer is preferred for smooth machined journals. Groove root and ball center dimensions should be confirmed from technical drawings or measured with specialized inspection equipment.
When identifying a replacement ball screw, record the diameter together with the lead, nut dimensions, shaft length, end machining and model information. This provides a much more reliable basis for product matching than one diameter reading alone.
Need help identifying a ball screw?
Send DLY the model number, measured dimensions, photos and shaft-end drawing. Our team can help confirm the required specification before quotation.
