Preload Technique Boosts Bearing Performance and Durability

Imagine a high-speed precision machine tool where even the slightest vibration in the spindle can directly impact machining accuracy. Or consider an automobile differential where excessive noise significantly reduces driving comfort. These seemingly different applications share a common critical factor: bearing preload.

Preload, as a precise bearing installation technique, involves applying a controlled negative clearance to significantly enhance bearing system stiffness, reduce noise, improve guidance accuracy, and even extend bearing service life. This article explores the principles, methods, influencing factors, and application considerations of bearing preload to provide comprehensive guidance for engineers and technicians.

Bearing preload is not required in all applications, but it plays a crucial role in specific scenarios. The main advantages include:

• Enhanced Stiffness: Preload significantly increases bearing stiffness, reducing deformation under load and improving system precision and stability, particularly important in machine tool spindles and precision instruments.
• Noise Reduction: Preload eliminates or minimizes internal clearance, keeping rolling elements under constant force to reduce noise levels, critical for automotive differentials and electric motors.
• Improved Shaft Guidance: Preload enhances shaft guidance accuracy, maintaining stable position and direction under load, essential for applications requiring precise motion control like robotic joints and precision transmission devices.
• Compensation for Wear and Deformation: Preload compensates for increased clearance due to initial wear and microscopic deformation of mating surfaces.
• Extended Bearing Life: Proper preload improves internal load distribution, reducing individual rolling element loads to extend service life. However, excessive preload causes overload, increasing friction and heat generation.

Bearing preload is primarily achieved through two methods:

This method uses springs to apply preload force, offering simplicity, low cost, and automatic compensation for wear and deformation. Typically used in light-load, low-speed applications like small motors and fans, spring selection requires calculation and testing to ensure appropriate preload range.

This method precisely controls preload force through bearing position adjustment or shims/nuts, suitable for various loads and speeds. Common in high-precision applications like machine tool spindles, implementation methods include:

• Nut Adjustment: Simple but requires precise torque control.
• Shim Adjustment: Higher precision but requires pre-measurement of axial clearance.
• Spacer Adjustment: Uses precision-sized spacers for consistent preload in mass production.

Applied radially, mainly for cylindrical roller bearings through interference fits (inner ring/shaft or outer ring/housing). Interference amount requires application-specific calculation.

Applied axially, primarily for angular contact ball bearings, tapered roller bearings, and thrust bearings. Achieved through opposed mounting configurations:

• Back-to-Back (DB): Higher rigidity and moment capacity but temperature-sensitive, suitable for short spans like machine tool spindles.
• Face-to-Face (DF): Less temperature-sensitive but lower rigidity, ideal for long spans like automotive wheel hubs.

Deep groove ball bearings can accept axial preload when selected with larger radial clearance (C3/C4).

Preload force determination is critical - insufficient force yields no benefit while excessive force causes overload. Calculation must consider:

• Bearing type/size
• Load magnitude/direction
• Rotational speed
• Lubrication method
• Operating temperature
• Shaft/housing materials and structure

For new designs, calculation followed by experimental verification is recommended due to practical uncertainties.

Based on preload-elastic deformation relationship, measuring shaft displacement to determine preload. Uses shims, spacers, or intermediate rings adjusted per:

• Shaft/housing shoulder distances
• Total bearing width
• Required axial displacement
• Thermal expansion compensation
• Manufacturing tolerances
• Preload loss compensation

Measures bearing friction torque to determine preload, suitable for mass production due to speed and automation potential.

Direct measurement/application of preload force, though indirect methods are generally preferred for simplicity.

Performed separately for each bearing arrangement using nuts/shim/spacers with measurement verification for precise nominal preload achievement.

Components manufactured to standard tolerances are randomly assembled, assuming statistical improbability of extreme tolerance stacking. Requires tighter tolerances for accurate preload.

• Machine Tool Spindles: Enhances stiffness and reduces vibration for improved machining accuracy.
• Automotive Differentials: Increases rigidity and lowers noise for better handling and comfort.
• Wind Turbines: Boasts load capacity and service life for greater reliability.
• Robotic Joints: Improves stiffness and reduces clearance for superior motion precision.

Beyond optimal preload values, stiffness improvement plateaus while friction/heat rise dramatically shortens bearing life. Calculation complexity warrants modern engineering tools or professional consultation. During adjustment, minimize variation by properly seating rolling elements.

Specialized single-row or paired bearings facilitate simple, reliable adjustment or deliver predetermined preload post-installation, including:

• CL7C specification tapered roller bearings for automotive differentials
• Universal matched single-row angular contact ball bearings
• Paired single-row tapered roller bearings
• Paired single-row deep groove ball bearings