How Straight Should a Ball Screw Be? Straightness and Runout Tolerances

There is no single acceptable straightness tolerance for every ball screw. The allowable condition depends on the screw diameter, unsupported length, rotational speed, accuracy grade, bearing arrangement and machine requirements. In engineering inspection, a ball screw is therefore usually evaluated by its overall radial runout relative to the bearing journals, rather than by visual straightness alone.

A slight visible deflection in a long, slender screw does not automatically mean that the screw is defective. However, excessive runout after the shaft is correctly supported can cause vibration, uneven nut movement, noise, premature bearing wear and reduced positioning stability.

Straightness and Radial Runout Are Not Exactly the Same

The terms are often used interchangeably, but they describe different inspection concepts:

• Shaft straightness describes how much the physical axis of the screw deviates from an ideal straight line.
• Radial runout is the change shown by a dial indicator while the supported screw is rotated.
• Journal concentricity concerns the relationship between the machined bearing journals and the screw shaft axis.
• Installed alignment concerns the relationship among the screw, support bearings, nut housing and machine guideways.

A screw can appear slightly bowed when it is placed freely on a bench but still rotate within the specified runout after correct support. Conversely, a shaft that looks straight may show excessive runout if its bearing journals were machined off-center.

What Is an Acceptable Ball Screw Tolerance?

The correct acceptance value should come from the ball screw drawing, the manufacturer's inspection specification or an applicable standard such as ISO 3408 or JIS B 1192. It should not be selected from one general "millimeters per meter" rule.

This is why a purchase drawing should state the measurement reference, support points, inspection length and maximum permissible indicator reading. Specifying only "the screw must be straight" is not enough for consistent inspection.

How to Measure Ball Screw Runout

A practical inspection can be performed with two precision V-blocks, a surface plate and a dial indicator:

1. Clean the screw shaft and bearing journals to remove oil, dust and burrs that could affect the reading.
2. Support the specified bearing journals or designated reference surfaces on two V-blocks.
3. Place the dial indicator against the required measurement surface.
4. Rotate the shaft slowly through one complete revolution without applying side force.
5. Record the difference between the highest and lowest indicator readings.
6. Repeat the measurement at several axial positions if overall screw-shaft runout must be checked.

The support method must be recorded with the result. Measuring a long screw on an uneven floor, on packaging timber or at arbitrary shaft positions can produce misleading readings.

What Determines the Required Straightness?

1. Screw diameter and length
A longer and smaller-diameter shaft has lower bending rigidity. Two screws with the same nominal diameter may therefore require different acceptance criteria if their overall lengths are different.

2. Rotational speed
Runout becomes more important as rotational speed increases. A long shaft operating near its critical speed may whip or vibrate even when the lead accuracy itself is acceptable.

3. Bearing support arrangement
Fixed-supported and fixed-fixed arrangements behave differently. Bearing selection, preload, journal machining and support-unit alignment all influence the final rotating condition. DLY supplies matching ball screw support units for common BK/BF, FK/FF and EK/EF arrangements.

4. Accuracy and application
A general automation axis and a precision inspection machine should not use the same acceptance logic. Applications requiring stable positioning, low backlash and controlled mounting accuracy may require a precision ground ball screw with drawing-defined runout and journal tolerances.

5. Installation alignment
A correctly manufactured screw can still bind after installation if the bearing supports and nut housing are not coaxial. Before rejecting the screw, loosen the nut housing, rotate the shaft manually and check whether the resistance changes after realignment.

When Should a Ball Screw Be Rejected?

Further inspection or replacement should be considered when:

• The measured runout exceeds the value stated on the approved drawing or inspection standard.
• The screw produces repeated vibration after the bearings and nut housing have been correctly aligned.
• The nut becomes tight at the same shaft position during each revolution.
• The shaft has a visible local bend caused by impact, incorrect lifting or transport damage.
• The bearing journals show excessive radial runout relative to the screw axis.
• Straightening would risk damaging the hardened raceway or changing the preload condition.

Do not attempt to straighten a precision ball screw with a hammer or uncontrolled pressing. Local force can damage the raceway even when no crack is visible. The screw should be evaluated by the manufacturer or an experienced repair facility before corrective work is attempted.

Information to Provide When Ordering a Long Ball Screw

To determine whether a proposed straightness or runout requirement is practical, provide:

• Nominal screw diameter and lead
• Overall length and effective travel
• Required accuracy grade
• Maximum rotational speed
• Fixed and supported-end arrangement
• Shaft-end machining drawing
• Required runout value and measurement reference
• Installation direction and application

DLY supplies rolled and ground ball screws for CNC and automation equipment, with custom shaft length and end machining available according to drawings. For a long or high-speed screw, the diameter, support spacing, rotational speed and specified runout should be reviewed together before production.

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