Bearing Clearance: Definition and Grouping Explained

Clearance is a critical concept in bearings, central to the bearing selection and maintenance process. Therefore, it is essential to elaborate the concept of bearing clearance, standard limits, selection principles and corresponding calculations.

Understanding the Concept of Bearing Clearance

Bearing clearance has a clear definition. For bearings, it generally refers to the relative distance of movement between one fixed ring and another. It can be understood as the gap between the inner and outer rings of the bearing.

In the case of cylindrical roller bearings, due to the existence of bearing clearance, the inner and outer rings of the bearing may reach a certain distance of relative movement in the radial direction.

This distance is the radial clearance of the cylindrical roller bearing. For NU and N series cylindrical roller bearings, due to their structure, these two types of bearings can achieve axial displacement.

There is no limit to the axial movement between the inner and outer rings of the bearing structure itself, so these bearings do not have the concept of axial clearance.

For cylindrical roller bearings with flanges such as the NJ series, the inner and outer rings of the bearing can move axially relative to each other. This movement is restricted by the internal structure of the bearing, but this value is generally the axial movement that can be achieved internally by the bearing, which is not included in clearance.

For deep groove ball bearings, the bearing race is an arc groove, so the axial movement of the bearing is restricted by the groove structure. The axial relative movement distance of the inner ring with respect to the outer ring is the axial clearance of the bearing; The radial relative motion distance between the bearing's inner and outer rings is the bearing's radial clearance.

In the general bearing catalogues, under standard conditions, the clearance of the following bearings refers to the radial clearance:

• Deep groove ball bearings
• Cylindrical roller bearings
• Spherical roller bearings
• Needle Bearings
• Self-aligning ball bearings

Under standard conditions, the clearance of the following bearings refers to the axial clearance:

• Angular Contact Ball Bearings
• Tapered roller bearings

Bearing clearance grouping and limit values

There are national standards for bearing clearances. Newly manufactured bearings follow these standards. The Chinese national standard GB/T4604 provides the standard limit values ​​for radial clearance in bearings.

According to these standards, bearing clearance in China is roughly divided into five groups: Group 2 (C2), Standard Group (C0), Group 3 (C3), Group 4 (C4) and Group 5 (C5).

The Standard Group, also represented by some brands such as CN, is the standard clearance. Group 2 has less slack than the Standard Group, Group 3 has more slack than the Standard Group, and Group 4 has more slack than Group 3.

In the application of motors and reducers in general, the most used clearances are the Standard Clearance and Group C3. More details about the principles and methods of specific selection will be described later.

In addition to the standard clearance grouping, in the field of engine applications there is also something called CM clearance. CM stands for Engine Release. In general, CM clearance is within the Standard clearance range (CN), but its average value is lower than that of CN. Represents a narrower segment within the Standard Clearance (CN), located at the lower end.

It is not difficult to understand that CM clearance is a subset of NC clearance, but its range is narrower and its average value is smaller than that of CN.

Therefore, it can be considered a type of backlash where the control accuracy is improved and the overall backlash is reduced. The subsequent sections on bearing clearance and performance will explain how this choice of clearance benefits the noise performance of the bearing, which is why it is chosen for engine application.

The clearances of commonly used deep groove ball bearings are shown in the following table:

The clearance for commonly used cylindrical roller bearings is shown in the table below: