What are the preload methods for grinding ball screws? How to choose the appropriate preload?

The precompression design of grinding ball screws is the core method to improve its stiffness, eliminate axial clearance and improve dynamic response capability. Precompression method and selection should be determined comprehensively according to the precision, load and speed of the application scenario.

Positioning Preloading (fixed preload)
Principle: by adjusting the axial position of the nut or screw rod, the contact surface between the ball and the raceway will undergo initial elastic deformation, forming a a fixed preload.
Features:
The preload is constant, unaffected by load change, and rigid.
Suitable for low to mediumspeed, high precision (such as CNC machines feed axis).
Precise the preload amount control is required. Excessive preload can lead to increased friction and shorter lifespan.
Constant pressurisation (Spring Preloading)
How it works: Install springs (such as disk springs or wave springs) inside or outside the nut to provide dynamic preload through spring force.
Features:
Pre-tensioning force automatically adjusts with load and is suitable for high speed and variable load conditions (such as robot joints).
The complex structure requires regular inspection of spring fatigue.
The performance of spring directly affects the stability of the pretensioning force, so it is necessary to choose the spring material with high precision.
Double Nut Preloading
How it works: Pressure is applied to both the ball and the raceway through the raceway the relative axial displacement of two nuts (e.g. gasket adjustment, thread locking).
Features:
The pretensioning force is uniform and suitable for heavy load and high rigidity requirements (e.g. mold closing mechanism of injection molding machines).
High precision machining is required to ensure the coaxiality of the two nuts, otherwise vibrations may occur.
It is divided into "back-to-back" (with bidirectional axial loads) and "face to face" (with unidirectional load) structures.
Differential Preloading
Principle: Using different nut or screw cross section, pretension is generated by speed difference.
Features:
No additional mechanical structure is required and is suitable for miniaturization design (e.g., precision optical equipment).
The precision of lead directly affects the size of the preload and requires high high-precision machining.
The scope of application is narrow, mainly used for special customization requirements.

Load Characteristics
Constant load: Preposition prepress or double nut prepress as they provide high prepress stability.
Variable load/impact load: Constant pressurization is better because it cushions load fluctuations of springs.
Heavy Solution: Double nut pressurising disperses pressure to avoid single point overload.
Operation speed
Low speed (<1m/s): Positioning preloading meets requirements and costs less.
High Speed (≥1m/s): Constant or differential prepressure reduces prepressure loss and heat generation.
Hypervelocity (>5m/s): a combination of air bearings or magnetic levitation technology is required and a specially designed the preloading method required.
Precision Requirements
Micron Precision (e.g. semiconductor equipment): Positioning prepress or double nut prepress is required and the the preloading amount needs to be accurate to 0.1 micron.
Sub-micron precision (such as optical instruments): constant pressure prepress + temperature compensation is needed to avoid thermal deformation affecting prepress stability.
Environmental Adaptability
High temperature/corrosive environment: Double nut pressurisation (sealing structure) is preferred to avoid spring failure.
Cleanroom scenarios: positioning preload or differential preload can reduce evaporation pollution of lubricating oil.
Vacuum environment: In order to avoid gas emission spring materials, airless design (such as positioning prepressure) is required.
Life and Maintenance
Servicelife requirements: positioning prepress or twin nut prepress, simple structure, low failure rate.
Repair requirements: fixed pressure prepress can be quickly repaired by replacing springs, suitable for on-site maintenance.