Ball screw backlash is the axial lost motion that appears when the direction of movement reverses. The screw or motor may have already changed direction, but the nut, table or moving component does not respond immediately because mechanical clearance must first be taken up.
A small amount of clearance may be acceptable in general automation equipment. For CNC machines, measuring equipment and applications with frequent direction changes, however, excessive backlash can reduce positioning accuracy and repeatability.
What Is Backlash in a Ball Screw?
A ball screw contains a screw shaft, a ball nut and recirculating steel balls. The balls roll between matching raceways in the shaft and nut to convert rotary motion into linear movement.
If axial clearance exists between the balls and raceways, the driving side must move through that clearance when rotation reverses before the load is fully transmitted in the opposite direction. This produces backlash or lost motion.
Ball screw backlash should not be confused with lead accuracy. Lead accuracy describes the difference between theoretical and actual travel over a specified distance. Backlash describes lost motion during direction reversal. A higher accuracy grade does not automatically mean that the assembly has no axial clearance.
What Causes Ball Screw Backlash?
Manufacturing and Assembly Clearance
Some standard ball screw nuts are designed with controlled internal clearance to maintain smooth running and reasonable torque. This does not necessarily indicate a manufacturing defect.
When lower backlash is required, ball size, raceway geometry and preload must be properly matched during production and assembly. Randomly installing larger balls is not a safe adjustment method because excessive interference can increase torque, heat and raceway wear.
Wear of Balls and Raceways
Repeated operation gradually wears the balls and raceways. Wear can develop more quickly under excessive load, impact, contamination, inadequate lubrication or poor alignment. As the contact surfaces wear, internal axial clearance may increase.
If backlash appears only in one section of the screw stroke, local raceway wear or damage should be considered. Replacing only the nut may not correct the problem if the screw shaft is also worn.
Support-Bearing Movement
Not all measured reversal error comes from the ball nut. Loose locknuts, incorrect bearing preload or worn fixed-end support bearings can allow the entire screw shaft to move axially.
Before replacing the nut, check whether the screw end moves when axial force is applied. If the complete screw moves inside the support arrangement, the bearing assembly may be responsible for the error.
Loose Couplings or Nut Mounts
A loose motor coupling, nut-seat bolt or bearing housing can also create lost movement during reversal. The measured value at the machine table may therefore be greater than the actual clearance inside the nut.
Incorrect Installation
Misalignment between the screw shaft, nut housing and support bearings does not always create backlash immediately. However, it produces uneven loading, higher running resistance and local wear that may increase clearance over time.
How Does Backlash Affect a Linear Motion System?
Positioning Error
When an axis reverses, the motor or screw may move before the table responds. The commanded position and actual position can therefore differ, especially when the target is approached from alternating directions.
Reduced Repeatability
A machine may return consistently to a position when approaching from one direction but produce a different result when approaching from the opposite direction. This is a common sign of reversal-related lost motion.
Machining and Surface Errors
In CNC equipment, backlash may cause dimensional deviation, visible marks during contour changes or unstable cutting when an axis reverses. The severity depends on the machine structure, cutting load and control system.
Noise and Vibration
Clearance can allow a component to re-engage suddenly after reversal, producing impact, vibration or noise. However, abnormal noise alone does not prove that the ball nut has excessive backlash. Lubrication, alignment, support bearings and other drive components should also be inspected.
How Can Ball Screw Backlash Be Reduced?
Use a Properly Matched Preloaded Nut
Preload applies a controlled internal force between the balls and raceways. It helps reduce axial clearance and improves rigidity and reversal response.
Preload must match the required load, speed, motor torque and duty cycle. Excessive preload can increase friction, operating temperature and wear, so higher preload is not automatically better.
Consider a Double-Nut Structure
A double ball nut uses two nut sections preloaded against each other. This structure can provide better backlash control and higher axial rigidity than a standard-clearance single nut, but it requires more installation space and generally produces higher running resistance.
For a detailed comparison of these two structures, see SFU vs DFU Ball Screw: Backlash, Load and Precision Differences.
Correct Installation Errors
Check the alignment of the screw, nut housing and support units. Confirm that the bearing locknut, coupling, nut flange and mounting bolts are correctly secured.
If the error comes from a loose support bearing or coupling, replacing the ball nut will not solve the problem.
Maintain Proper Lubrication
Lubrication does not directly remove existing mechanical clearance, but it reduces friction and helps slow wear of the balls and raceways. The lubricant type and maintenance interval should match the speed, load, stroke and working environment.
Replace Worn Components
If the nut, screw raceway or support bearings are already worn, adjustment may provide only a temporary improvement. The worn components should be inspected to determine whether nut replacement or a complete matched screw-and-nut assembly is more appropriate.
More detailed correction methods are explained in How to Eliminate Backlash in Ball Screws.
Can Backlash Be Completely Eliminated?
A properly selected preloaded ball screw can reduce measurable axial clearance to a very low level. However, describing the complete machine axis as having "absolute zero backlash" is usually misleading.
The final reversal error can still be affected by support-bearing movement, coupling deformation, nut-seat rigidity, machine-frame deformation, thermal expansion and control-system settings.
The practical goal is therefore to control total lost motion within the positioning requirement of the machine, rather than pursuing the highest possible preload without considering torque, heat and service life.
What Should Be Checked Before Replacing a Ball Nut?
Before ordering a replacement, confirm:
- Complete screw and nut model
- Screw diameter and lead
- Single-nut or double-nut structure
- Original clearance or preload requirement
- Whether backlash occurs along the full stroke or only locally
- Condition of the screw raceway and nut circulation
- Fixed-end support-bearing movement
- Coupling, nut seat and mounting-bolt condition
One of the most common replacement mistakes is ordering a nut only by nominal diameter and lead. Two nuts marked with similar basic sizes may still differ in flange dimensions, circulation structure, internal matching and preload.
For more information about nut replacement and matched assemblies, see Can an Anti Backlash Ball Screw Nut Really Remove Backlash?.
DLY Ball Screw and Nut Matching
DLY supplies C7 rolled ball screws for general automation and C5 ground ball screws for applications requiring higher lead accuracy. Ball screw shafts use S55C steel, while DLY ball nuts use 20CrMo alloy steel.
When backlash control is required, the screw diameter, lead, nut structure, preload, support arrangement and installation conditions should be confirmed together. For replacement projects, clear photos, the complete model and a drawing are more reliable than diameter and lead alone.
Need help checking ball screw backlash?
Send DLY the screw model, measured reversal error, nut structure, support-bearing arrangement and application requirements for checking.
Email: export@dlybearing.com
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