How Much Ball Screw Backlash Is Acceptable?

Acceptable ball screw backlash depends on what the axis is expected to do. A general transport axis may tolerate a small amount of lost motion, while a CNC machining, inspection or precision positioning axis may require very low axial clearance or a preloaded ball nut.

There is no single backlash value that is correct for every machine. The allowable value must be lower than the positioning error permitted when the axis reverses direction. The measurement must also separate clearance inside the ball nut from movement in the support bearings, coupling, nut bracket and machine structure.

Ball Screw Backlash and Axial Clearance

Ball screw backlash is the lost movement observed when the direction of rotation or axial load is reversed. Axial clearance inside the ball nut is one possible source of this lost motion.

However, a dial indicator mounted on the machine may measure the combined movement of:

• The ball nut and screw raceways;
• The fixed-side support bearings;
• The motor coupling and shaft connection;
• The nut flange and mounting bracket;
• The guide system and carriage;
• The machine frame under reversing load.

For that reason, measured axis reversal error should not automatically be reported as ball nut backlash.

Typical Axial-Clearance Ranges

The following ranges are useful as an initial engineering reference. They are not universal acceptance limits for every DLY model or machine.

The correct acceptance limit should come from the machine's positioning-error budget and the selected ball screw specification. A value described as acceptable for one transport axis may be unacceptable for a machining axis.

When Is a Small Amount of Backlash Acceptable?

A small amount of axial clearance may be acceptable when:

• The axis mainly moves in one direction under a constant axial load;
• The application transports parts rather than performing precision machining;
• The positioning tolerance is substantially larger than the measured reversal error;
• The controller uses a verified and stable backlash-compensation value;
• Lower friction and lower running torque are more important than maximum rigidity;
• The ball screw is used for manual adjustment or non-critical positioning.

For example, axial clearance may have little effect while a vertical load consistently keeps the nut loaded in one direction. It becomes more important when the load direction reverses or the machine approaches the same position from opposite directions.

When Should Backlash Be Close to Zero?

Very low axial clearance or preload should be considered when the axis performs:

• Repeated bidirectional positioning;
• CNC contouring or interpolation between multiple axes;
• Grinding, measurement or inspection movement;
• Frequent direction reversal under load;
• Fine positioning without direct linear feedback;
• Machining where reversal marks affect the finished surface;
• Servo control requiring high axial rigidity.

"Zero backlash" should not be interpreted as zero elastic displacement. Even a preloaded ball screw, support bearing and machine structure deform slightly under load.

Preload Is Not the Same as Accuracy Grade

Preload reduces or removes axial clearance and increases nut rigidity. It mainly improves behavior when the load or travel direction reverses.

Preload does not change the manufactured lead accuracy of the screw shaft. A C7 screw does not become C5 or C3 because the nut is preloaded.

Excessive preload can cause:

• Higher running torque;
• Greater temperature rise;
• Higher motor-torque demand;
• Greater sensitivity to misalignment;
• Shorter life if lubrication is inadequate;
• Abnormal wear under continuous high-speed operation.

The preload level should therefore match the required rigidity, speed, load, duty cycle and thermal conditions.

Do Not Judge Backlash from the Ball Screw's Appearance

A ball screw's external appearance cannot confirm whether the nut is preloaded or how much axial clearance it has.

Nuts with different flange and body structures may also use different clearance or preload specifications.

A product photo cannot show whether the axial clearance is 0.005 mm, 0.01 mm or 0.02 mm. The value must come from the model specification, inspection record or controlled measurement.

Does a High-Lead Ball Screw Have More Backlash?

Not necessarily. A larger lead does not automatically create a larger clearance between the balls and raceways.

Backlash is more directly affected by:

• Ball and raceway matching;
• Nut preload or clearance class;
• Ball diameter variation;
• Raceway wear;
• Nut structure and ball circuits;
• Lubrication and contamination;
• Support-bearing and mounting condition.

High lead changes the transmission ratio, required motor torque, screw RPM and positioning resolution. It should not be described as an automatic cause of excessive backlash.

Do Rolled Ball Screws Always Have More Backlash?

No. Rolled and ground describe how the screw raceway is manufactured. Axial clearance or preload describes the fit between the screw, balls and nut.

A rolled ball screw can be assembled with low axial clearance, while a ground ball screw can also have measurable clearance if it is supplied without preload.

The manufacturing process mainly affects achievable lead accuracy, surface control and cost. Backlash should be confirmed separately from the screw's accuracy grade.

How to Measure Ball Screw Backlash

The measurement should apply a controlled direction reversal while preventing unrelated components from moving.

A practical machine-level method is:

1. Clean and lubricate the ball screw according to the normal operating condition.
2. Position the axis near the centre of its normal stroke.
3. Attach a dial indicator to a rigid stationary part of the machine.
4. Place the indicator tip against the moving table or nut bracket in the travel direction.
5. Apply a small controlled movement or force in one axial direction.
6. Set the indicator to zero after the system settles.
7. Reverse the force or command slowly.
8. Record the movement before the screw begins producing corresponding table travel.
9. Repeat the test at several positions along the stroke.

The test force, measurement position and machine state should remain consistent. A hand-pushed test without a defined force may not be repeatable.

Separate Nut Clearance from Support-Bearing Movement

If the complete axis shows excessive reversal error, the next step is to identify where the movement occurs.

Replacing the ball nut will not correct backlash caused by a loose support bearing, coupling or nut bracket.

Backlash Compensation Does Not Repair Mechanical Clearance

A controller can apply a stored compensation value when the axis changes direction. This may improve commanded positioning when the mechanical backlash is small, stable and repeatable.

Compensation is less effective when:

• The measured value changes along the stroke;
• The clearance changes with load;
• The support bearing or coupling moves unpredictably;
• Wear continues to increase;
• The machine performs contouring under changing force;
• The carriage rocks or deforms during reversal.

Software compensation should not be used to hide loose fasteners, damaged raceways or unstable support bearings.

When Is the Measured Backlash Too High?

The measured value is too high when it consumes an unacceptable part of the machine's total positioning tolerance or causes a visible process problem.

Warning signs include:

• The axis reaches different positions depending on approach direction;
• Circular interpolation produces a mark near quadrant changes;
• The machine requires frequent compensation updates;
• The backlash has increased compared with earlier inspection records;
• Noise or vibration appears during reversal;
• The nut or support bearing has detectable axial movement;
• Finished-part dimensions change with cutting direction;
• The measured value exceeds the specified clearance class.

What to Check When Backlash Has Increased

1. Repeat the measurement. Confirm the indicator setup, test force and direction reversal.
2. Inspect the support bearings. Check locking nuts, bearing preload and axial housing movement.
3. Inspect the coupling. Check clamping screws, keys, hubs and shaft fit.
4. Inspect the nut mounting. Check flange bolts, bracket rigidity and locating surfaces.
5. Check lubrication and contamination. Poor lubrication can accelerate raceway wear.
6. Inspect the screw raceway. Look for pitting, indentation, corrosion or uneven wear.
7. Compare several stroke positions. Local variation may indicate uneven screw wear.
8. Confirm the original nut specification. Determine whether it was supplied with clearance or preload.

Do not automatically install larger balls or increase preload without confirming the raceway condition and original nut design. Incorrect ball matching can create excessive torque and shorten service life.

Replace the Nut or the Complete Ball Screw?

Replacing only the nut may be reasonable when:

• The screw raceway remains in good condition;
• The original nut model and matching specification are known;
• The replacement nut can be matched to the existing screw;
• The support bearings and mounting structure have been ruled out;
• The required axial-clearance or preload condition can be confirmed.

A complete screw-and-nut replacement may be safer when:

• The screw raceway is worn, pitted or corroded;
• The backlash varies significantly along the stroke;
• The original ball size and preload data are unavailable;
• The nut cannot be matched reliably to the old screw;
• The machine requires restoration of a verified precision level.

What Information Should Be Sent to DLY?

To assess whether a measured backlash value is acceptable, provide:

• Complete ball screw and nut model;
• Screw diameter, lead and total length;
• Measured backlash or lost-motion value;
• Measurement method and applied test force;
• Position along the screw where the test was performed;
• Whether the value changes along the stroke;
• Required machine positioning accuracy;
• Horizontal, vertical or inclined installation;
• Normal axial load and reversing load;
• Support-bearing and coupling arrangement;
• Operating time, lubrication and maintenance history;
• Photos or video of the measurement setup.

For the complete explanation of causes and prevention, read Ball Screw Backlash: What It Is and How to Prevent It.

For nut structures designed to reduce axial clearance, review the DLY ball nut range and double-nut ball screw options.

Final Answer

Acceptable ball screw backlash is the amount of reversal error that remains within the machine's positioning requirement. As a general reference, axial-clearance classes may range from zero or preload to approximately 0.005 mm, 0.01 mm, 0.02 mm or 0.05 mm, depending on the ball screw design and intended application.

Precision bidirectional positioning normally requires very low clearance or preload. General transport and single-direction-load applications may tolerate more clearance when it remains within the machine's error budget.

Before replacing the nut, confirm whether the measured lost motion comes from the ball nut, support bearings, coupling, mounting bracket or guide system. Send DLY the model, measured value, test method, load, accuracy requirement and installation drawing for technical review.

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