How to Measure Ball Screw Radial Runout with a Dial Indicator

Ball screw radial runout is measured by rotating the screw and recording how much a selected cylindrical surface moves toward and away from a dial indicator. The result is normally expressed as total indicator reading, or TIR:

However, there is no single measurement point that represents every type of ball screw runout. A technician may need to check the motor-coupling journal, the threaded portion, the ball nut outside diameter or several positions along the complete screw shaft. Each measurement evaluates a different geometric relationship.

This distinction is important because radial runout, shaft straightness, journal concentricity and installation alignment are related, but they are not interchangeable. A dial indicator reading is meaningful only when the reference axis, support method and measured surface are clearly defined.

What Does Ball Screw Radial Runout Mean?

Radial runout describes the radial movement of a measured surface as the ball screw rotates around a defined reference axis. For a finished ball screw, the reference axis is normally established by the machined bearing journals because these surfaces locate the screw in its support bearings after installation.

If the motor-end journal, screw thread or nut outside diameter is not sufficiently concentric with this axis, the component appears to move from side to side during rotation. Excessive runout may contribute to vibration, cyclic torque variation, coupling stress, bearing load and unstable high-speed operation.

A runout reading can contain several geometric errors at the same time:

• Eccentricity of the measured diameter;
• Roundness error of the measured surface;
• Shaft bending or straightness deviation;
• Error in the machined bearing journals;
• Dirt, burrs or damage at the support or measurement point;
• Deflection caused by incorrect support or excessive indicator force.

For this reason, a high reading identifies a problem but does not automatically prove that the screw shaft is bent. The measurement arrangement must be checked before deciding whether the ball screw requires straightening, regrinding or replacement.

Radial Runout, Straightness and Alignment Are Different

Rotating a shaft under a dial indicator produces a runout value. It does not directly produce a pure straightness value because the result may also include eccentricity, roundness and datum errors.

Tools Required for Measuring Ball Screw Runout

A workshop inspection normally requires a precision surface plate, two clean V-blocks, a dial indicator or dial test indicator, and a rigid indicator stand. The indicator resolution should be fine enough for the specified tolerance. A 0.01 mm indicator may identify serious bending, but it is generally inadequate when the acceptance limit is only several micrometres.

Additional useful equipment includes a micrometer for checking journal diameter, lint-free cleaning materials, marking tape and a measurement record sheet. For long or slender screws, adjustable supports may be required to control sag without creating a false reference axis.

All supports and measuring equipment must be stable. Movement of the magnetic base, V-blocks or surface plate can be incorrectly recorded as screw runout.

Prepare the Ball Screw Before Measurement

Allow the ball screw and measuring equipment to stabilize in the inspection environment before making a precision measurement. A large temperature difference between the screw and surface plate can affect dimensions and cause the shaft to change shape during inspection.

Clean the bearing journals, V-blocks and measured surface carefully. Remove oil buildup, dust and small burrs without damaging the precision surfaces. Even a small particle trapped between a journal and a V-block can tilt the shaft and create a repeating indicator error.

Inspect the shaft for impact marks, rust, dents and handling damage. Do not place the indicator tip on a damaged area or directly over a lubrication hole, keyway, thread relief or other interrupted surface.

The ball nut must not be removed casually. If removal is necessary, use the correct transfer sleeve so that the balls remain inside their circulation circuits.

Method 1: Measure the Motor-End Journal Runout

This measurement checks the radial runout of the motor-coupling journal relative to the bearing journals. It is important because an eccentric coupling journal can cause coupling movement, vibration and additional load at the motor connection.

1. Place the two finished bearing journals on clean V-blocks. The V-blocks establish the reference axis used for the inspection.
2. Position the dial indicator on the cylindrical motor-coupling journal. Keep the probe approximately perpendicular to the measured surface.
3. Apply only enough indicator preload to maintain continuous contact.
4. Set the indicator to zero at a convenient angular position.
5. Rotate the screw shaft slowly through one complete revolution without applying axial or lateral force.
6. Record the highest and lowest readings.
7. Subtract the minimum value from the maximum value to obtain the total radial runout.

Repeat the test to confirm that the high point occurs at approximately the same angular position. If the result changes significantly between rotations, check the cleanliness of the V-blocks, indicator stability and manual rotation method.

Method 2: Measure Runout of the Threaded Portion

The ball screw thread does not provide a simple continuous cylindrical surface for direct contact with a conventional flat indicator tip. A probe placed carelessly on the helical groove will move axially or drop into the groove as the screw rotates, creating an invalid reading.

For a formal mounting-surface inspection, the bearing journals are supported on V-blocks and the indicator contacts the outside diameter of the ball nut. The screw shaft is then rotated through one revolution while the nut is prevented from rotating. The nut follows the screw groove axially, and its outside diameter acts as the measuring surface for evaluating the grooved portion relative to the support axis.

The nut must be supported so that it can move as required without being forced sideways. Do not clamp the nut rigidly in a position that adds bending load to the screw.

This method should be performed according to the manufacturer's inspection procedure because nut design, preload and measurement arrangement can affect the result. For a quick workshop diagnosis, measuring several smooth reference diameters or using a dedicated measuring fixture may be more practical than placing a standard probe directly on the thread crest.

Method 3: Measure Overall Radial Runout Along the Screw

Overall radial runout is checked at multiple positions along the screw shaft rather than at only one point. This helps identify whether the deviation is concentrated near a machined end or changes across the full shaft length.

1. Support the finished bearing journals on V-blocks.
2. Choose and mark several axial measuring positions, including locations near both ends and around the middle of the threaded length.
3. Place the indicator at the first valid measuring surface.
4. Rotate the screw through one complete revolution and record the total indicator reading.
5. Move the indicator to the next axial position and repeat the test.
6. Record both the runout value and angular high point at every position.

A shaft with a gradual increase in runout toward the middle may be affected by bending or insufficient support. A high reading confined to one machined journal is more likely to be related to local machining error or journal damage.

Long and slender ball screws require special care. Their own weight can produce measurable sag. Adding an intermediate support can reduce sag, but an incorrectly positioned support can also bend the shaft upward and hide the real deviation. The inspection setup must therefore match the ball screw length, diameter and applicable standard.

Method 4: Measure Ball Nut Outside-Diameter Runout

This test checks the radial runout of the nut circumference relative to the screw shaft axis. It is different from checking the threaded shaft relative to the bearing journals.

Support the threaded portion of the screw shaft on V-blocks close to the nut. Position the dial indicator against the nut outside diameter. Keep the screw shaft stationary and rotate the nut through one complete revolution. The difference between the highest and lowest indicator readings is the nut outside-diameter radial runout.

This inspection is relevant when the nut outside diameter is used as an assembly reference. For a nut mounted mainly by its flange face, flange runout and mounting-face accuracy must also be evaluated separately.

Reference Runout Values for Precision Ball Screws

The acceptable value depends on the ball screw accuracy grade, shaft diameter, measured feature and inspection standard. The following selected figures illustrate permissible radial runout of the grooved screw surface or part-mounting surface relative to the support axis. Values are shown in micrometres.

These values are reference limits taken from a precision ball screw mounting-surface accuracy table. They are not universal acceptance values for every rolled or ground ball screw. Always confirm the applicable manufacturer specification, drawing, accuracy grade and measurement definition.

The accuracy grade must not be judged from this runout table alone. Ball screw lead accuracy, travel deviation, axial clearance or preload, mounting-surface accuracy and overall shaft runout are different inspection items.

Common Measurement Errors

Using the thread crest as an ordinary smooth diameter

A standard indicator tip can move across the helical profile and produce a reading that reflects probe movement in the groove rather than true radial runout. Use the specified inspection method or a suitable fixture.

Supporting the screw on rough or unfinished surfaces

If the V-blocks contact surfaces that are not the defined bearing journals, the resulting axis may not represent the axis used after installation.

Rotating the screw by pushing it sideways

Applying lateral force while turning the shaft can lift it against one side of the V-block and change the indicator reading. Rotate slowly with minimal force.

Ignoring shaft sag

The weight of a long ball screw can produce deflection between the supports. Although constant vertical sag does not always appear as rotational runout, changes in support contact, shaft orientation or intermediate support force can distort the measurement.

Confusing indicator amplitude with TIR

If the indicator moves from −0.006 mm to +0.008 mm, the total runout is 0.014 mm, not 0.008 mm. Always calculate the difference between the maximum and minimum readings.

Measuring only one position

A single measurement near the centre cannot show whether the error comes from one shaft end, a local damaged section or overall bending. Record values at multiple defined locations.

How to Interpret the Measurement Pattern

If excessive runout is confirmed, do not immediately attempt to straighten the screw with uncontrolled force. Precision ball screws have hardened raceways, and improper straightening can cause surface cracking, local deformation or changes in lead accuracy. The complete measurement record should first be reviewed by the manufacturer or a qualified repair provider.

Does Low Runout Guarantee Good Ball Screw Accuracy?

No. Low radial runout confirms only the measured geometric relationship. It does not prove that the ball screw has the required lead accuracy, axial clearance, preload, load capacity or surface quality.

A ball screw can rotate with low radial runout but still have excessive lead deviation. Conversely, a screw with acceptable lead accuracy may vibrate at speed if the motor journal, support bearings or installed coupling have excessive runout.

For a complete inspection, radial runout should be considered together with:

• Lead accuracy and travel deviation;
• Axial clearance or preload;
• Support-end-face runout;
• Nut flange and outside-diameter accuracy;
• Shaft straightness and overall runout;
• Installed alignment and running torque.

Conclusion

To measure ball screw radial runout correctly, first define which surface and reference axis are being evaluated. For shaft-end and overall runout checks, the finished bearing journals are normally supported on clean V-blocks. A dial indicator is placed against the specified surface, the shaft is rotated slowly through one revolution, and the difference between the maximum and minimum readings is recorded as TIR.

Measure more than one axial position, control shaft support and sag, and do not place an ordinary probe directly on a helical groove without an appropriate method. Most importantly, do not treat runout, straightness, lead accuracy and machine alignment as the same measurement.

Learn more about ball screw straightness and overall runout, or explore the DLY ball screw range.

← Back to DLY Blog