What are the preload methods of cold-rolled ball screws?
The preload methods of cold-rolled ball screws mainly focuses on eliminating axial backlash, increasing stiffness and enhancing repeatability. core objective is to keep the sphere in close contact with the runway by preloading, reducing idle travel during reverse motion. According to the structure design and application scenarios, there are three kinds of the preload methods of cold-rolled ball screws, each of which is different in principle, effect and application scenario:
I. Single Nut Preload: a low-cost solution for low precision applications.
Principle: By optimizing the internal structure of the nut (such as increasing the ball diameter or adjusting the curvature of the raceway), the ball can form a one-way contact with the raceway under the action of a one-way force, and a preload can be generated by elastic deformation. This method requires no extra parts, but the preload is limited and there is a slight bounce (usually between 0.01mm and 0.03mm).
Features:
Simple structure: just one nut, low cost, easy to install.
Limited Precision: The pretensioning force is limited by the elastic limits of the material, making it difficult to achieve high stiffness (typically 30 to 50 per cent of the stiffness of ground twin nuts).
Applicable Scenarios: Transmission scenarios with low precision requirements, such as: Ordinary conveying machinery (e.g. belt conveyors tensioner), low speed manual lifting platforms (e.g. warehouse racks lift), cost sensitive equipment (e.g. non-critical axle of simple CNC machine tools).
II. Double Nut Preload: Mainstream High Precision Solutions to the Core Choice for Eliminating Backlash
Principle: Through the relative displacement or structural deformation of the two nuts, the ball of the two nuts is compressed in the raceway of the two nuts at the same time, forming a bidirectional preload. This method completely eliminates axial backlash and greatly improves the stiffness of the system (stiffness can be 2-3 times that of a single nut).
Subtype:
Shim-type Double Nut Preload:
Structure: Insert A preload between the two nuts and control the preload by adjusting the thickness of the gasket.
Strengths: Simple structure, adjustable preload, suitable for large lead screws.
Weaknesses: Need to manually adjust shim thickness, resulting in low installation efficiency, long-term use of the gasket may become loose due to vibration.
Application range: low speed, medium precision applications, such as feed axes of ordinary CNC machine tools.
Variable Lead Type Double Nut Preload:
Structure: The threaded joints of the two nuts are slightly different (e.g. Rotating nuts will preload the sphere under the influence of lead difference.
Advantages: Automatic preload compensation, no additional adjustment, good rigidity, long service life.
Weaknesses: Complex structure and high cost, requiring special tools for installation.
Uses: High-speed, high-precision applications, such as robot joints, precision measuring instruments, etc..
Spring Type Double Nut Preload:
Structure: Install a spring (such as a disk spring or helical spring) between the two nuts to provide pretension through spring force.
Advantages: Automatic preload compensation (if gap increases due to temperature change or wear), good operating stability.
Weaknesses: Spring is prone to fatigue failure, requires regular maintenance, and spring stiffness limits pretension.
Applications: Scenarios requiring long-term, high-precision operations, such as semiconductor equipment and optical focusing mechanisms.
III. Special Preload Methods: Customized designs for Extreme Conditions
Ultra-precision preload (e.g., hydraulic preload):
How it works: High pressure oil is injected into the nuts through a hydraulic system to even the contact between the ball and the runway. The preload can be precisely controlled (error ≤ 0.1 micron).
Applications: Ultra-precision machine tools (e.g., nanometer-level positioning platforms), spacecraft attitude control mechanisms.
Magnetic preload:
How it works: Permanent magnet is embedded in a nut or sphere and uses magnetism to provide preload. No mechanical contact, no wear and tear.
Applications: Vacuum environments or scenarios (such as semiconductor manufacturing equipment) that require extreme cleanliness.
IV. INTRODUCTION Criteria for selection of Preload Methods: balance of accuracy, stiffness and cost.