Bearing clearance, also known as bearing clearance, refers to the radial or axial movement of a bearing before it is installed on a shaft or in a bearing housing. A bearing ring is fixed and the bearing can move to the side that is not secured.
This movement is normally classified into radial clearance and axial clearance. The operating clearance of a bearing during use significantly influences its service life, temperature rise, noise and vibration characteristics.
Radial Clearance
When a bearing carrying radial load is not preloaded, its radial clearance G is defined as the average radial distance produced when the outer ring, without external load conditions, moves from the eccentric limit position on one side to the opposite extreme position .
Axial Clearance
For a bearing capable of supporting bidirectional axial load and not preloaded, its internal axial clearance G is defined as the average axial distance produced when a ring, without external load conditions, moves from the axial limit position on one side to the position opposite extreme.
Allowable tolerance for shaft neck rounding
| Newly machined shaft | Old unprocessed shaft | ||||
| Bearing diameter (mm) | High speed above 1000 rpm | Low speed below 1000 rpm | Bearing diameter (mm) | High speed above 1000 rpm | Low speed below 1000 rpm |
| 50~70 | 0.01 | 0.03 | 50~70 | 0.03 | 0.05 |
| 70~150 | 0.02 | 0.04 | 70~150 | 0.04 | 0.06 |
Maximum allowable wear value for bearings
| Bearing diameter (mm) | Radial Clearance (mm) | Axial Clearance (mm) |
| Less than 30 | 4D/1000 | 0.2 |
| 35 to 70 | 3.5D/1000 | 0.3 |
| 75 to 100 | 3D/1000 | 0.3 |
| Above 100 | Not exceeding 0.3 | 0.3 |
Note: D – Bearing internal diameter or shaft neck diameter
Standard for original radial clearance of new bearings
| Nominal bearing diameter (mm) | Single row spherical roller bearing (threads) | Single row short cylindrical roller bearing (threads) | Double row spherical roller bearing (threads) | Apply radial load (in MPa) during measurement. | The allowable wear value after use is (in threads). | ||||
| To exceed | For | Minimum | Maximum | Minimum | Maximum | Minimum | Maximum | ||
| 18 | 24 | 1.0 | 2.4 | 0.5 | 10 | ||||
| 24 | 30 | 1.0 | 2.4 | 0.5 | |||||
| 30 | 40 | 1.2 | 2.6 | 1.0 | 20 | ||||
| 40 | 50 | 1.2 | 2.9 | 2.0 | 5.5 | 1.0 | |||
| 50 | 65 | 1.3 | 3.3 | 2.5 | 6.5 | 1.0 | 20 | ||
| 65 | 80 | 1.4 | 3.4 | 3.0 | 7.0 | 5.0 | 8.0 | 1.0 | |
| 80 | 100 | 1.6 | 4.0 | 3.5 | 8.0 | 6.0 | 10.0 | 1.0 | |
| 100 | 120 | 2.0 | 4.6 | 4.0 | 9.0 | 1.5 | |||
| 120 | 140 | 2.3 | 5.3 | 4.5 | 10.0 | 1.5 | 30 | ||
The maximum movement of one ring of a fixed bearing and the other ring capable of moving in the radial or axial direction is the bearing clearance. In most cases, the greater the radial clearance of the bearing, the greater the axial clearance.
According to the condition of the bearing, the clearance can be divided into: original clearance, installation clearance and working clearance.
The installation clearance directly affects the normal operation of the bearings.
Too small a gap can lead to a greater increase in bearing temperature or even cause the bearing body to become stuck; If the gap is too large, it can cause significant vibration in the equipment and generate a lot of noise.
Original release:
It is the clearance when the bearing is free before installation, generally determined during processing and assembly.
Installation release:
Also known as mating clearance, it is the clearance when the bearing is assembled with the shaft or bearing seat but has not yet started to function. The installation clearance is generally smaller than the original clearance, mainly because the bearing's inner ring expands or the outer ring shrinks after installation.
Work release:
This is the clearance when the bearing is in operation. During operation, the bearing will reduce clearance due to temperature rise and thermal expansion of the inner ring, and increase clearance due to elastic deformation of the contact position of the rolling element and raceway under load.
Reference standards for mounting engine bearings
| Bearing type | Bearing inner diameter and shaft adjustment method with tolerance | |||||
| Nominal bearing inner diameter (mm) | Allowable bearing inner diameter tolerance (mm) | Permissible shaft tolerance (mm) | Adjustment method | Shaft neck interference value and bearing inner ring fit (difference between shaft diameter and actual bearing inner diameter) (millimeters) | ||
| To exceed | For | |||||
| Single row radial ball bearing | <18 | 0-1.00 | 0.2 | GB | +1~+2 | |
| 18 | 30 | 0-1.00 | 0.2 | GB | +1~+2 | |
| 30 | 50 | 0-1.20 | 1 | GB | +2~+3 | |
| 50 | 80 | 0-1.50 | 1.2 | GB | +2~+3 | |
| 80 | 120 | 0-2.00 | 1.3 | GB | +3~+5 | |
| 120 | 180 | 0-2.5 | +1.9（+2.8）+0.3（+1.2） | GB | +4~+7 | |
| Single row cylindrical roller bearing | 30 | 50 | 0-1.20 | 2.9 | GB | +1~+3 |
| 50 | 80 | 0-1.50 | 3.4 | GB | +2~+4 | |
| 80 | 120 | 0-2.00 | +2.8（+3.5）+1.2（+1.2） | GB | +4~+6 | |
| 120 | 180 | 0-2.5 | 9.2 | GB | +4~+7 | |
| Double row spherical roller bearing | GB | |||||
| Bearing outer diameter and housing cover adjustment method with tolerance | |||||
| Nominal bearing outer diameter | Permissible bearing outer diameter tolerance (mm) | Allowable housing cover tolerance (mm) | Adjustment method | Clearance between the bearing outer ring and the housing cover hole (mm) | |
| To exceed | For | ||||
| 18 | 30 | 0-0.90 | 0.9 | G-d | 0~3 |
| 30 | 50 | 0-1.10 | 1 | G-d | 0~3 |
| 50 | 80 | 0-1h30 | 1 | G-d | 0~3 |
| 80 | 120 | 0-1.50 | 1.1 | G-d | 0~3 |
| 120 | 160 | 0-2.50 | 1.3 | G-d | 0~3 |
| 180 | 260 | 0-3.50 | 1.2 | G-d | 0~3 |
| 260 | 315 | 0-3.50 | 1.7 | G-d | 0~3 |
| 80 | 120 | 0-1.5 | 1.1 | G-d | 0~3 |
| 120 | 180 | 0-2.5 | 1.3 | G-d | 0~3 |
| 180 | 260 | 0-3.5 | 1.4 | G-d | 0~3 |
| 260 | 315 | 0-3.5 | 1.7 | G-d | 0~3 |
| 120 | 180 | 0-2.5 | +2.7（+2.7）-1.4（0） | G-d | 0~3 |
Radial clearance selection:
The radial clearance of a bearing must be chosen based on specific conditions; smaller is not necessarily better. The radial clearance of bearings is divided into five groups. Group 0 is the standard base radial clearance group.
Group 0 bearings are commonly applied in general operating conditions, conventional temperatures and frequently used interference fits. Bearings with larger radial clearances are suitable for special operating conditions such as high temperatures, high speeds, low noise and low friction. Bearings with even larger radial clearances are suitable for precision spindle bearings and similar uses.
| Axial clearance of deep groove ball bearing | ||||||
| Ga=0.4w√ GrDw |
(C3) | |||||
| Nominal inner diameter（d） | 0.4 | Gr. | Dw | (Square root) | Gá | Range |
| ≤30 | 0.4 | 8 | 3.5 | 0.08 | 0.032 | 0.02-0.05 |
| >30~50 | 0.4 | 27 | 4 | 0.1 | 0.04 | 0.03-0.06 |
| >50~80 | 0.4 | 38 | 5 | 0.14 | 0.056 | 0.05-0.08 |
| >80~100 | 0.4 | 51 | 7 | 0.19 | 0.076 | 0.07-0.10 |
| >100~120 | 0.4 | 61 | 8.5 | 0.23 | 0.092 | 0.09-0.12 |
| >120~140 | 0.4 | 68.5 | 9 | 0.25 | 0.1 | 0.10-0.14 |
Axial clearance selection:
For deep groove ball bearings and tapered roller bearings, when they are mounted face-to-face or back-to-back, the internal clearance or preload typically requires the axial position of a bushing to be determined and the performance and operating requirements of the bearing configuration. bearing must be considered.
The axial clearance and radial clearance of these types of bearings generally only need to satisfy one of these values.
How to choose bearing clearance?
Selection of radial clearance for bearings is crucial as it is one of the critical factors that determine whether bearings can function properly.
Proper selection of radial clearance can ensure reasonable load distribution between the rolling elements of the bearing. It can limit the axial and radial displacement of the shaft (or housing), ensure the rotational accuracy of the shaft, and allow the bearing to operate under certain temperatures, reducing vibration and noise. This is advantageous for improving the service life of the bearings.
The difference between the theoretical clearance and the clearance generated by the interference fit of the housing or shaft with the bearing after the expansion or contraction collar is installed is called “installation clearance”.
When adding or subtracting the dimensional changes accumulated due to thermal variations inside the bearing, it is called “operational clearance”.
Operating clearance refers to the clearance that exists when the bearing is mounted on a machine and is loaded and rotated. The effective clearance plus the elastic deformation generated by bearing loads is known as “operational clearance”.
As shown in Figure 2, the bearing has the longest fatigue life when the operating clearance is marginally negative. However, as the negative clearance increases, the fatigue life of the bearing decreases noticeably.
Therefore, when selecting clearance for bearings, it is generally appropriate to have a slightly positive or zero value for operating clearance.
When selecting radial clearance for bearings, the following factors must be taken into consideration:
- The working conditions of the bearings, such as loads, temperature and speed.
- The requirements for bearing performance such as rotational accuracy, friction torque, vibration and noise.
- The interference fit between the bearing and the shaft or housing, which reduces the radial clearance of the bearing.
- The temperature difference between the bearing's inner and outer rings during operation reduces the radial clearance.
- The difference in the coefficient of thermal expansion between the shaft and housing can cause the bearing radial clearance to increase or decrease.
Based on experience, the ideal operating clearance for ball bearings is close to zero, while roller bearings must maintain a small operating clearance.
In components that require good support rigidity, bearings can allow a certain amount of preload.
Under normal working conditions, it is recommended to choose the basic component first to obtain adequate operating clearance for the bearing. If the basic component does not meet the requirements, an auxiliary component must be chosen.
The auxiliary component with large radial clearance is suitable for bearings with interference fit between the bearing and the shaft or housing. The auxiliary component with small radial clearance is suitable for applications that require high rotational accuracy, strict control of housing axial displacement, and reduced noise and vibration.
Furthermore, to improve bearing rigidity or to reduce noise, the operating clearance must be further reduced, while to compensate for the severe increase in bearing temperature, the operating clearance must be increased further. Specific analyzes must be carried out based on the conditions of use.
