What Is the Relationship Between Ball Screw Preload and Axial Clearance? Why Higher Preload Is Not Always Better
Axial Clearance Affects Reverse Motion Response
When a ball screw changes its direction of movement, axial clearance between the nut and screw raceway can cause a delay between motor reversal and actual table movement. This delay is known as reverse backlash error.
For general material handling equipment, a certain amount of clearance may be acceptable. However, applications such as:
CNC machine tools
Precision measuring platforms
Bidirectional positioning systems
usually require much smaller axial displacement during reversing motion.
How Preload Improves Ball Screw Rigidity
Preload means applying a controlled internal load between the balls and raceways before external working loads are applied.
The purpose of preload is to:
Reduce or eliminate axial clearance
Increase axial stiffness
Improve positioning response under load
Common preload methods include:
Double nut preload using spacer adjustment
Lead offset preload
Controlled ball size selection
The actual preload structure depends on the specific ball screw design and manufacturer's product series.
Why Higher Preload Is Not Always Better
Increasing preload increases the internal contact force between:
Balls
Raceways
Nut components
As a result, it also increases:
Running torque
Friction heat
Motor load
Excessive preload may lead to:
Reduced fatigue life
Faster lubricant deterioration
Increased load on motors and couplings
Higher operating temperature
Therefore, preload should be selected according to:
Maximum axial load
Required rigidity
Operating speed
Heat dissipation conditions
Required service life
The objective is not to maximize preload, but to achieve the correct balance between stiffness, accuracy, and reliability.
Preload Torque Is an Important Performance Indicator
A preloaded ball screw requires a certain amount of torque to rotate under no external load. This dynamic preload torque is an important inspection parameter.
Torque value and fluctuation can indicate:
Ball circulation condition
Raceway consistency
Assembly quality
If the actual running torque after installation is significantly higher than expected, the cause may not be excessive preload alone.
Possible causes include:
Misalignment between fixed and floating supports
Incorrect nut mounting
Seal resistance
Insufficient or improper lubrication
A complete system inspection is required before changing preload settings.
Preload Cannot Replace Structural Rigidity and Proper Assembly
Ball screw nut rigidity is only one part of the complete motion system.
Overall system stiffness is also affected by:
Fixed-end bearings
Nut housing structure
Machine table
Linear guide system
Coupling rigidity
Machine frame
If the bearing support has excessive clearance or the nut mounting structure is weak, increasing preload may only create additional heat without significantly improving positioning performance.
How to Determine Whether Preload Is Required
The first step is to define:
Allowable reverse error
Actual working load
Motion direction
Positioning requirements
For applications with mainly one-direction loading, such as:
Conveying systems
Vertical lifting axes
the external load may naturally maintain contact in one direction, and zero-clearance performance may not be necessary.
For applications involving:
Frequent bidirectional positioning
Contouring machining
Precision force control
preload should be carefully evaluated.
For high-speed, long-duration operation, designers must balance:
Rigidity
Accuracy
Temperature rise
Service life
Information Required When Ordering a Ball Screw
For proper selection, customers should provide:
Maximum and average axial load
Operating speed
Acceleration
Motion direction
Positioning accuracy requirement
Repeatability requirement
Allowable temperature rise
Installation method
Motor specifications
If a specific preload class is required, also provide:
Applicable product standard
Original model number
Manufacturer preload specification
Different manufacturers may use different preload classifications, so grades should not be directly compared without checking the actual definition.
Selecting the Correct Preload for Reliable Precision Motion
The purpose of preload is to achieve sufficient rigidity and improve reverse motion performance - not to create the highest possible internal load.
A properly selected preload level provides the best balance between:
Positioning accuracy
Mechanical efficiency
Thermal performance
Long-term service life
Tianjin OuNaiDa provides professional support for ball screw selection, including evaluation of:
Axial clearance requirements
Preload selection
Shaft-end machining
Bearing support configuration
Based on actual equipment conditions, Tianjin OuNaiDa helps customers develop reliable and optimized precision motion solutions.
Precision Motion Components | Empowering Global Intelligent Manufacturing
