Internal clearance term should be interpreted as displacement of one ring with regard to another.
There are two types of clearance, depending on a direction of displacement: radial clearance, axial clearance.

Radial internal clearance (bearing is only capable to carry radial load, and it is not pre- loaded) is a mean value of radial distances between first ring (that can be displaced from one most-eccentric position to another) with regard to the second ring, taking into consi- deration that no external force acts on a ring. Mean arithmetic value is calculated basing on different relative angular displacements of one ring with relation to another and rings with relation to rolling elements.

Axial internal clearance (bearing is capable to carry axial loads in both directions, and it is not preloaded) is a mean value of axial distances between first ring (that can be displaced from one most extreme axial position to another) with regard to the second ring, taking into consideration that no external force acts on a ring.
Mean arithmetic value is calculated basing on different relative angular displacements of one ring with relation to another and rings with relation to rolling elements.

Internal clearance in a mounted bearing is smaller then in a bearing before mounting because of the fact that bearing fitting on a shaft/into the housing causes some deformation of rings.
Depending on a fitting conditions inner ring is expanding and the outer ring is contracting. Because of deformations, in both cases internal clearance is getting smaller. Furthermore,there are some factors that have influence on clearance during bearing work (operational clearance), such as: thermal expansion of bearing elements and co-acting parts and loads that are occurring. Bearings are ma- nufactured with different clearances which allow to choose the one is which appropriate for a given working conditions.
The most commonly used bearings are those with standard design and clearance.
Values of normal clearance are determined in such a way that in normal working conditions with recommended fitting the bearing will have proper operational clearance. Abnormal working conditions associated with temperature or fitting characteristics require different clearance class which are marked with alphanumeric symbol from C1 to C5 or special clearance values.

Charts contain numerical values of radial clearance (before bearing mounting; they also omit load that comes from measurement equipment) for most common bearings designs. For double-row angular contact ball bearings the axial clearance is given because in this case this parameter is more important during bearing mounting design.
Typically, bearings with tapered bore have greater clearance than the bearings with cylindrical bore because usually they are mounted with greater interference fit.

Radial internal clearances

Axial internal clearances

Internal radial clearance measurement methods

Measurement of radial internal clearance under load (acc. to Fig.2)

Bearing prepared for radial clearance check is placed onto the measurement tool sleeve. Inner ring of a bearing is fixed by a clamp. Under the force P perpendicular to the bearing’s axis of revolution the outer ring moves from one extreme position to the other with regard to the fixed inner ring.
Radial clearance is shown as a difference of readings from gauge placed on the outer ring, where the reading is a value of displacement of inner ring with regard to outer ring.
Measurement is made 3 times at minimum with different angular positions of outer ring with regard to gauge. Consecutive measurements should be made with aprox. 60° angular displacement.

Measurement of radial internal clearance using plate (acc. to Fig.3)

Measurement of clearance using plate is performed in case of big size bearing or when other measurement devices are unavaiable. Inner ring is fixed to the plate during such measurement and the outer ring is moving from one extreme position to another due to crosswise acting load (manual or mechanical) in the direction perpendicular to bearing’s axis of revolution.
Measurement is performed 3 times, each one after revolving the inner ring for about 60°.

Internal axial clearance measurement method

The kinematic scheme below shows principles of measurement of clearance using special tooling.
Measurement of axial clearance in the range of outer diameters: D=40÷250mm and width B÷40mm is performed on special devices under measurement load Axial clearance of bearings with outer diameters D>250mm and widths B>40mm are measured using universal methods.

Calculations of elastic deformation caused by measurement load during clearance measurement

1. radial
   • - for deep groove ball bearings
     
     where:
     α – bearing’s angle of action
     Q – measurement load
     D_k – ball diameter
   • – for self-aligning ball bearings
     
     gdzie:
     α – bearing’s angle of action
     Q – measurement load
     D_k – ball diameter
2. axial
   • - for angular contact ball bearings
     
     where:
     α – bearing’s angle of action
     Q – measurement load
     z – number of rolling elements of first row
     D_k – ball diameter