Bearing Internal Clearance Knowledge

What is bearing internal clearance?

Bearing internal clearance (radial internal clearance) is the total amount of space that allows one bearing ring to move about the other, either axially or radially.

Radial Clearance

More specifically, radial play, or clearance measured perpendicular to the bearing axis, equals average outer ring raceway diameter minus average inner ring raceway diameter minus (2 x ball diameter)

Radial Play Tables

Axial Clearance

Axial clearance pertains to the space measured along the bearing axis. The allocation of resources for axial play is approximately tenfold greater than that for radial play.

Different Clearance Classes

In nearly all applications, the initial clearance within a bearing surpasses its operational clearance. This distinction primarily stems from two factors:

• Bearings are typically affixed with an interference fit on the shaft or within the housing. The inner ring’s expansion or the outer ring’s compression diminishes the internal clearance.
• Operational heat generation by bearings contributes to differential thermal expansion in both the bearing and its mating components, influencing the internal clearance.

Maintaining adequate internal clearance during bearing operation is crucial. Some bearing types allow for preload (clearance below zero.)

To facilitate the selection of the optimal initial internal clearance for achieving the desired operational internal clearance, bearings are offered in various clearance classes. ISO has standardized five clearance classes for numerous bearing types.

Selection of Bearing Clearance

For best results, the right bearing clearance must be chosen. A bearing’s clearance, which is determined by a number of factors, including thermal expansion and an initial fit, influences how efficiently it operates.

Internal Clearance in Specific Applications

Relation between Radial Internal Clearance and Angular Clearance

During bearing operation, external factors like shaft deflection induce a moment load, creating angular clearance between the inner and outer rings. Exceeding allowable clearance may lead to abnormal stress, elevated temperatures, or flaking. Angular clearance, represented by θ1 and θ2, must be considered in device design and bearing installation. The formulas of θ1 and θ2 are defined by pitch circle diameter (dm), outer diameter (D), inner diameter (d), and the width (Dw) of the bearing.

θ1=2×tan-1 (2×(Δr×(ro+ri-Dw)) 0.5/dm)

θ2=2×tan-1((Δr×(ro+ri-Dw)) 0.5/dm)

θ1=Angular clearance around the center of the bearing

θ2=Angular clearance around the center of the bottom ball

dm=Pitch circle diameter (mm); generally, dm = (D+d)/2

Calculation of Clearance

Operating clearance (Δ):

This represents the elastic deformation-included space during bearing operation at a constant temperature.

Temperature-Induced Clearance Reduction (δt):

Due to temperature differences between the inner and outer rings, clearance decreased.

Clearance reduction from fitting (δf):

Interference fitting contracts the outer ring and expands the inner ring, decreasing internal clearance.

Clearance increase from load (δw):

Application of load induces internal clearance increase due to elastic deformation.

Meaning of Symbols

Factors Affecting Internal Clearance

Internal clearance in a bearing, the space between rolling elements and inner and outer rings, is influenced by fitting and temperature.

The fitting of the inner ring

A tight fit reduces clearance (increases preload), while a loose fit may increase it.

The fitting of the outer ring

Like the inner ring, fitting the outer ring affects clearance. Tight fit decreases, loose fit increases.

The temperature difference between inner ring and outer ring

Temperature variances can cause thermal expansion, impacting internal clearance. Higher temperature may reduce clearance.

Maintaining proper clearance is crucial for bearing performance. Manufacturers provide fitting and clearance guidelines, and regular monitoring is essential for efficient operation and extended service life.

Axial Load, Axial Displacement, and Change in Contact Angle

The application of axial load on a ball bearing results in elastic deformation, causing the contact angle to exceed its initial value and leading to axial displacement.

Axial load: Fa

Axial displacement: δa

Preload: a special type of negative clearance

Typically, bearings exhibit internal play during operation. However, introducing negative clearance, known as preloading, is beneficial arrangement rigidity.

Preloading is suited for bearings allowing play adjustment during installation, such as angular-contact ball bearings. This involves mounting two bearings in a face-to-face or back-to-back configuration to create a duplex set with a preload.

In various applications, preloading serves to:

Main shafts of Machine Tools and Precision Instruments

• Ensure precise radial and axial positioning
• Maintain shaft rotational accuracy
• Increase overall rigidity

Main Shafts if Machine Tools and Pinion Shafts in Car Boxes

• Enhance bearing rigidity
• Optimize gear meshing for improved performance

Small Electric Motors

• Mitigate noise from axial vibration and resonance

High-Speed or High-Acceleration Applications with Angular-Contact

• Prevent slippage between rolling elements and raceways caused by gyratory moments

Types of preload

Position Preload

A positional preload is established by securing two bearings in axial opposition and subjecting them to a preload. Once in a position, their arrangement remains constant during operation.

Practically, three approaches are typically employed to achieve positional preload.

• Installing a duplex bearing set with pre-adjusted stand-out dimensions and axial clearance.
• Employing a spacer or shim of the precise size to achieve the desired spacing and preload.
• Utilizing screws or nuts to establish axial preload, in this scenario, measuring the starting torque is crucial for validating the correct preload.

Achieving Preload and Enhancing Rigidity

When fixing the inner rings of bearing A and B axially, the space measuring 2 δa0 is eliminate. Each bearing is subjected to a preload of Fa0.

Bearing rigidity refers to the correlation between load and axial displacement in a duplex set under a specified axial load (Fa).

Consistent Pressure Preload

Utilizing a coil or leaf spring enables the establishment of a constant-pressure preload. This ensures that even if the relative position of the bearing changes during operation, the magnitude of the preload remains relatively stable.

Consistent Pressure Preload and Rigidity

The graph illustrates the rigidity of duplex bearings under a constant-pressure preload. The deflection curve of the spring is nearly parallel to the horizontal axis, indicating lower spring rigidity compared to the bearing. Consequently, the rigidity under a constant-pressure preload approximates that of a single bearing with a preload of Fa0.

Preload Table Guide

FAQ

What is the maximum theoretical bearing clearance?

A bearing’s maximum theoretical bearing clearance is the separation between its rotating and stationary components. It depends on the bearing when we talk about the maximum theoretical clearance.

Why is the correct C code of your bearings so important?

A correct C code is needed for bearings:

Bearings have to match in order to function as intended.

• Ensures resilience to damage, robustness, security, and durability.
• To avoid early failure and downtime, confirm that the bearing can sustain the necessary loads and conditions.