Yes, ball screws can have backlash. A non-preloaded screw-and-nut assembly may have axial clearance that produces lost motion when the direction of travel or axial load reverses. A correctly selected preloaded assembly can eliminate clearance-related backlash within its specified operating conditions, but the complete machine axis can still show reversal error.
There is no single backlash value that applies to every ball screw. The result depends on the screw-and-nut configuration, preload, condition, mounting arrangement and measurement method.
For OEMs, the practical questions are: where does the measured movement originate, how much reversal error can the application tolerate, and what must the supplier verify before a model is approved?
Do Ball Screws Have Backlash?
A ball screw transfers axial force through recirculating balls running between the screw shaft and nut raceways. If the assembly has axial clearance, reversing the load can require the contacting components to take up that clearance before force is transmitted in the opposite direction. Thomson identifies this internal clearance as a source of ball screw backlash and explains that factory ball selection or suitable preload arrangements can reduce or eliminate it.
However, movement measured at a machine table does not isolate the screw-and-nut assembly. Several interfaces can contribute:
Screw-and-nut assembly: internal axial clearance, wear or a change in preload.
Support bearings: axial play, bearing displacement or movement in the shaft retention arrangement.
Coupling and drive connections: clearance, slipping connections or torsional deformation.
Nut housing and mounting structure: loose interfaces or elastic deflection.
Complete axis: guide friction, structural compliance and control response can influence the observed reversal behavior.
A “zero-backlash” ball screw specification is therefore not a guarantee of zero reversal error at the working point. Component clearance and installed-axis performance need separate acceptance criteria.
Backlash, Lost Motion and Positioning Accuracy Are Different
These terms describe related aspects of motion, but they should not be used interchangeably in a purchase specification. For this article, backlash refers to clearance-related movement; lost motion describes the broader response observed during reversal.
Term | Meaning | What is measured | Common sources | What to confirm when purchasing |
|---|---|---|---|---|
Backlash | Clearance-related movement before force transfers in the opposite direction | Relative motion across a component or interface under a defined reversal test | Screw-and-nut clearance, drive connection clearance or support-end play | Maximum axial clearance, component boundary, test force and method |
Lost motion | Input motion that does not immediately produce the corresponding output motion during reversal | Relationship between drive input and actual axis output | Backlash, elastic deformation, friction and other reversal effects | Measurement point, load, reversal sequence and compensation status |
Positioning accuracy | Agreement between actual position and the specified target over the working travel | Position error at defined locations | Travel error, thermal displacement, compliance, alignment and feedback arrangement | Required accuracy over the stated stroke and acceptance procedure |
Repeatability | Consistency when returning to the same target under defined conditions | Spread of repeated position measurements | Mechanical variation, friction, thermal changes and control behavior | Unidirectional or bidirectional test, repetitions and operating conditions |
THK treats lead accuracy, axial clearance, feed-system rigidity, thermal displacement and guide-system orientation changes as separate contributors to positioning error. This supports specifying the complete axis requirement alongside the ball screw’s component specifications.
What Is Ball Screw Lead, and How Does It Relate to Error?
Ball screw lead is the nominal axial travel produced by one relative revolution between the screw and nut. Lead accuracy describes how closely actual travel follows the specified travel relationship.
Lead error and backlash answer different questions:
Lead error concerns travel accuracy as rotation produces linear motion.
Backlash concerns clearance-related behavior when motion or load reverses.
A low-backlash assembly may still have travel error over its stroke. Likewise, selecting a more accurate lead grade does not automatically correct a loose coupling or support-bearing problem. Specify lead accuracy and reversal performance separately.
A Practical Diagnosis Sequence
The following sequence is a practical framework for locating the source of reversal error. Use the machine manufacturer’s service procedure where available, and agree on the measurement method before comparing results.
1. Establish Repeatable Test Conditions
Record the axis position, payload, applied force, temperature, lubrication condition and warm-up state. Define the approach direction, reversal movement, speed and settling time.
Use an independent displacement instrument suitable for the required resolution. A motor encoder alone cannot establish whether the machine table has followed the commanded movement.
For comparable measurements, keep the same load, approach direction, reversal sequence and measurement point. When relocating the instrument to investigate another interface, document the new reference clearly.
Secure vertical axes before releasing a drive or brake, and isolate power for hands-on inspection.
2. Check the Support End
Investigate whether the screw shaft moves axially relative to the bearing housing during the defined reversal test. Then check whether the bearing housing moves relative to its mounting structure.
Review the bearing arrangement, shaft retention, locknut condition, housing attachment and assembly records. Interpret movement against the approved design: a support intended to accommodate thermal expansion should not be assessed as though it were the locating end.
3. Check the Coupling and Drive Connections
Inspect the motor-to-coupling and coupling-to-screw connections. Look for evidence of relative slip, loose clamping interfaces or clearance in the connection.
Also distinguish clearance from torsional flexibility. A coupling can transmit motion without mechanical play while still twisting under torque. Its suitability depends on the axis stiffness requirement as well as its nominal backlash rating.
4. Check the Nut Housing and Mounting Interfaces
Measure relative movement between the nut flange, nut housing, carriage and supporting structure as appropriate.
HIWIN’s troubleshooting instructions use measurements at the shaft end, bearing housing and nut mounting interfaces to help locate excessive backlash. The diagnostic principle is to identify which interface moves before attributing the full reading to the ball screw itself.
5. Evaluate the Screw-and-Nut Assembly
After investigating external interfaces, evaluate relative axial movement between the screw and nut using the supplier’s specified fixture, restraint and test force.
A force-reversal measurement can include elastic deformation as well as clearance. Report the applied force and measured displacement together; do not label every measured displacement as backlash.
Where practical, repeat the procedure at several positions along the working stroke. Position-dependent results warrant investigation of both the screw assembly and the surrounding axis.
6. Verify the Installed Axis After Correction
Repeat the original test after correcting the identified cause. If the application requires it, also verify bidirectional positioning and repeatability at the working point.
There is no universal pass/fail threshold for this sequence. Acceptance must follow the application requirement, approved drawing, supplier specification and agreed test conditions.
When Preload Helps and What It Costs
Preload establishes internal loading in the screw-and-nut contacts to remove axial clearance and increase nut rigidity. It can help when internal clearance or contact compliance limits reversal performance. THK also cautions that excessive preload increases heat generation and can adversely affect service life.
Preload will not correct loose mounting interfaces, coupling slip or inadequate structural stiffness.
Compare the Configuration Options
The following are established configuration types described in manufacturer literature. Availability and suitability must be confirmed for the specific diameter, lead, nut design and application.
Configuration | Potential benefit | Main considerations | Supplier confirmation needed |
|---|---|---|---|
Non-preloaded assembly with specified clearance | May suit applications whose reversal requirements allow clearance | Clearance-related reversal movement remains | Maximum clearance and measurement method |
Single nut with selected oversized balls | Compact arrangement that can reduce clearance or establish preload | Requires compatible raceway geometry and controlled ball selection | Matched assembly requirements, preload and running torque |
Single nut with internal lead offset | Preload within one nut body | Suitability depends on the particular design | Available configuration, stiffness and torque characteristics |
Double nut with fixed preload | Opposing loading can remove clearance and increase rigidity | Larger installation envelope and controlled preload setting | Nut length, preload method and operating limits |
Double nut with spring preload | Provides opposing nut loading through a spring arrangement | Spring behavior and external loading affect performance | Usable load range, stiffness and adjustment procedure |
These options are design alternatives, not a list of confirmed offerings from a particular supplier.
Evaluate Friction, Heat and Life Together
A preload decision should address more than the initial reversal measurement.
Additional internal loading affects running torque and motor sizing. Heat generated during operation can change screw temperature and therefore its length. THK identifies thermal expansion as a contributor to positioning error and recommends managing heat generation as part of accuracy control.
Preload also needs to be included in the supplier’s life calculation. THK explicitly notes that a preloaded nut already carries internal load, which must be considered when evaluating service life.
Ask the supplier to evaluate the proposed preload against the actual load spectrum, speed, duty cycle, lubrication and thermal conditions. Avoid selecting preload solely by requesting the highest available level.
Changes to ball size, spacers or preload adjustment should follow the manufacturer’s procedure. A configuration that feels tight by hand is not evidence of acceptable torque, temperature or life over the full stroke.
OEM RFQ Checklist
A useful RFQ defines both the component and the conditions under which its performance will be accepted.
Specification item | Information to provide |
|---|---|
Application and orientation | Machine function, horizontal or vertical installation, and reason for the reversal requirement |
Screw diameter and lead | Required nominal dimensions or design constraints if selection is still open |
Travel and length | Usable stroke, effective threaded length, overall length and installation envelope |
Shaft ends | Dimensioned bearing seats, shoulders, threads, coupling interfaces and tolerances |
Support arrangement | Locating and floating functions, bearing arrangement and relevant support drawings |
Nut form | Flanged or cylindrical form, mounting dimensions, orientation and space restrictions |
Loads | Axial loads in both directions, peak loads, shocks, acceleration and duty profile |
Speed | Linear speed, screw rotational speed, acceleration and operating cycle |
Target reversal performance | Maximum permitted lost motion, measurement point, applied load and test sequence |
Component axial clearance | Required clearance or preload condition, stated separately from installed-axis lost motion |
Accuracy and repeatability | Required travel accuracy and unidirectional or bidirectional repeatability over the working stroke |
Preload proposal | Supplier recommendation, preload method and application assumptions |
Torque requirements | Permitted starting and running torque, torque variation and measurement conditions |
Operating environment | Temperature, contamination, coolant exposure, corrosion risks and protection requirements |
Lubrication and life | Lubricant, maintenance interval and target operating life |
Quantity | Prototype quantity, production batch size and expected annual demand |
Drawing control | Drawing number, revision, units and referenced specification editions |
Inspection documents | Required dimensional, travel-accuracy, clearance or torque reports |
Require a Traceable Model Proposal
Ask the supplier to return an exact part number or controlled custom assembly designation linked to the submitted drawing revision.
The proposal should identify the screw-and-nut configuration, clearance or preload specification, operating assumptions and inspection method. Any deviation from the RFQ should be stated explicitly.
When submitting an inquiry to LILY Bearing, include the completed checklist, current drawing and any existing measurement report. Request confirmation of a suitable model and its supported performance before approving the order. Available preload structures, dimensions and delivery specifications should be verified for the quoted assembly.
The procurement goal is a traceable specification connecting component clearance, installed-axis lost motion and application conditions. That gives engineering and purchasing a sound basis for comparing proposals and accepting the finished assembly.
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