Bearing internal clearance

Bearing internal clearance is defined as the total distance either inner or outer ring can be moved when the other ring is fixed.

If movement is in the radial direction, it is called radial internal clearance; if in the axial direction, axial internal clearance.

Bearing performance depends greatly upon internal clearance during operation (also referred to as operating clearance); inappropriate clearance results in short rolling fatigue life and generation of heat, noise or vibration.

In almost all applications, the initial clearance in a bearing is greater than its operating clearance. The difference is mainly caused by two effects:

• Bearings are typically mounted with an interference fit on the shaft or in the housing. The expansion of the inner ring or the compression of the outer ring reduces the internal clearance.
• Bearings generate heat in operation. Differential thermal expansion of the bearing and mating components influences the internal clearance.

Sufficient internal clearance in a bearing during operation is important. Preload (clearance below zero) is possible for certain bearing types.

Radial internal clearance

The radial internal clearance is determined on the dismounted bearing. The radial internal clearance applies to bearings with an inner ring and is determined on the unmounted bearing. It is defined as the amount by which the inner ring can be moved in a radial direction from one extreme position to the other in relation to the outer ring

he radial internal clearance groups are defined in ISO 5753-1 and usually uses designation suffixes to indicate when the bearing internal clearance differs from Normal. ISO 5753-1 designates the groups by the word "Group" and a number

Operating clearance

The operating clearance is determined on a bearing still warm from operation. The operating clearance is determined on a mounted bearing still warm from operation. It is defined as the amount by which the shaft can be moved in a radial direction from one extreme position to the other.

The operating clearance is derived from the radial internal clearance and the change in the radial internal clearance as a result of interference fit and thermal influences in the mounted condition.

A normal operating clearance is usually achieved with internal bearing clearance CN

The operating clearance value is dependent on the operating and installation conditions of the bearing. A larger operating clearance is, for example, necessary if heat is transferred via the shaft, the shaft undergoes deflection or if misalignment occurs. An operating clearance smaller than CN should only be used in special cases, for example in high precision bearing arrangements. Normal operating clearance is achieved with an internal clearance of CN or, for larger bearings, more usually C3 if the recommended shaft and housing tolerances are maintained.

How to determine operating clearance

Operating clearance: 
S = S₀ - S_p - S_t

For the value of radial internal clearance refer to Bearing internal clearance tables.

Reduction in radial internal clearance due to fit

The radial internal clearance is reduced due to the fit as a result of expansion of the inner ring and contraction of the outer ring

Reduction in radial internal clearance: 
S_p = Δd + ΔD

Expansion of the inner ring: 
Δd ≈ 0.9 * U * d / F ≈ 0.8 * U

For very thin-walled housings and light metal housings, the reduction in the radial internal clearance must be determined by mounting trials.

Contraction of the outer ring: 
ΔD ≈ 0.8 * U * E / D ≈ 0.7 * U

Reduction in radial internal clearance due to temperature

The radial internal clearance can alter considerably if there is a substantial temperature differential between the inner and outer ring

Reduction in radial internal clearance due to temperature: 
ΔsT = α * d_M * 1000 * (ϑIR - ϑAR)

A larger radial internal clearance should be used for shafts running at high speeds, since adequate thermal compensation between the bearing, shaft and housing does not occur in this situation. ΔsT can, in this case, be significantly higher in this case than for continuous operation.

Axial internal clearance

The axial internal clearance sa is defined as the amount by which one bearing ring can be moved relative to the other, without load, along the bearing axis.

internal-clearance.png 
sa = axial internal clearance 
sr = radial internal clearance

Relationship between radial and axial internal clearance

With various bearing types, the radial internal clearance s_r and the axial internal clearance s_a are dependent on each other. Guide values for the correlation between radial and axial internal clearance are shown for some bearing types.

Y0 = axial load factor in accordance with product table.

Axial internal clearance calculation example

Example: deep groove ball bearing 6008-C3

For deep groove ball bearings, the calculation of the axial internal clearance is shown in the following example:

Axial internal clearance 
sa = 13 · 10 μm = 130 μm

internal-clearance-ratio.png 
Approximate calculation of the ratio of radial to axial internal clearance for deep groove ball bearings 
sa = axial internal clearance 
sr = radial internal clearance 
d = bearing bore diameter 
1, Bearing series