The fit between the bearing and bearing seat hole
When selecting a bearing fit, the various factors discussed in this section should be considered, as well as the general selection methods provided.
Rotation condition
The rotation condition refers to the movement condition of the bearing ring relative to the load direction. There are basically three different conditions: “rotating load”, “stationary load” and “non-directional load”.
The applicable condition of “rotating load” is that the bearing ring is rotating and the load is stationary, or the bearing ring is stationary and the load is rotating, then all points of the raceway bear the load during one revolution. A heavy load that does not rotate but swings, such as a load acting on a connecting rod bearing, is usually considered a rotating load.
The bearing ring bearing the rotating load, if installed with a clearance fit, will rotate (creeping or drift) on the bearing support surface, demonstrating it, and causing wear (friction corrosion) on the contact surface. To prevent this, an interference fit must be used. The amount of interference fit required depends on the operating conditions (see points 2 and 4 below).
Condition Demonstration 1
Inner ring rotation
Outer ring stationary
Constant load direction
Rotating load on inner ring
Static load on outer ring
An interference fit is required between the inner ring and the shaft because the load direction relative to the inner ring changes.
A clearance fit can be used between the outer ring and the bearing seat because the load direction relative to the outer ring is constant.
Example: belt driven shaft
Condition Demonstration 2
The inner ring is stationary
Outer ring rotation
Constant load direction
Inner ring has static load
Rotating load on outer ring
An interference fit is required between the outer ring and the bearing seat because the load direction of the outer ring changes.
A clearance fit can be used between the inner ring and the shaft because the load direction relative to the inner ring is constant.
For example: rollers on conveyor belts, car wheel bearings
The applicable condition of “static load” is when the bearing ring is static and the load is also static, or the bearing ring and the load rotate at the same speed, and the load always points to the same position on the raceway. Under these conditions, the bearing ring usually does not rotate on its bearing surface. Therefore, the bearing ring does not necessarily have an interference fit, unless it is necessary to do so for other reasons.
Condition Demonstration 3
Inner ring rotation
Outer ring stationary
The load rotates with the inner ring
Inner ring has static load
Rotating load on outer ring
An interference fit is required between the outer ring and the bearing seat because the load direction of the outer ring changes.
A clearance fit can be used between the inner ring and the shaft because the load direction relative to the inner ring is constant.
For example: vibration applications, vibrating screens or motors
Condition Demonstration 4
The inner ring is stationary
Outer ring rotation
The load rotates with the outer ring
Rotating load on inner ring
Static load on outer ring
An interference fit is required between the inner ring and the shaft because the load direction relative to the inner ring changes.
A clearance fit can be used between the outer ring and the bearing seat because the load direction relative to the outer ring is constant.
For example: rotary rock crusher, (carousel drive)
“Non-directional load” means the external load, shock load, vibration, and unbalanced load that fluctuate in high-speed machines. This will cause a change in the direction of the load, and it is not easy to accurately describe. When the load direction is uncertain, especially when there is a heavy load, it is recommended that the inner and outer rings have an interference fit. For the inner ring, the recommended fit for the rotating load is usually used. However, when the outer ring must move freely axially in the bearing housing and the load is not heavy, a slightly looser fit than the recommended fit for the rotating load can be used.
Load size
The interference fit of the bearing inner ring on its supporting surface will loosen as the load increases and the inner ring expands. Under the influence of the rotating load, the ferrule will start to creep. Therefore, the amount of interference fit must be related to the size of the load; the heavier the load, especially in the case of impact load, the greater the amount of interference fit required (Figure 15). Shock load and vibration also need to be considered.
The load size is defined as
P≤0.05C-light load
0.05C＜P≤0.1C-normal load
0.1C＜P≤0.15C-heavy load
P＞0.15C-very heavy load
The internal clearance of the bearing
The interference fit of the bearing on the shaft or in the bearing seat will cause elastic deformation (expansion or compression) of the bearing ring, resulting in a reduction in the internal clearance of the bearing. However, a certain minimum clearance must be maintained, see the section “Internal clearance of the bearing”. The original clearance and allowable reduction depend on the type and size of the bearing. Interference fit may result in a large reduction in clearance, so bearings with a larger original clearance than the normal value must be used to avoid bearing preload (Figure 16).
Temperature conditions
In many applications, the operating temperature of the outer ring is lower than that of the inner ring. This may result in a reduction in internal clearance.
During operation and service, the temperature reached by the bearing ring is usually higher than the temperature of the parts assembled with it. This may result in a loose fit of the inner ring on its bearing surface, and expansion of the outer ring may hinder the required axial displacement of the outer ring in its bearing seat. Quick start or seal friction may also cause the inner ring to fail to fit.
Therefore, the temperature difference and the direction of heat flow within the bearing arrangement must be carefully considered.
Operation accuracy requirements
In order to reduce elastic deformation and vibration, for bearings that require high operating accuracy, clearance fit should not generally be used. The bearing support surface in the shaft and the bearing housing should use a narrower dimensional tolerance, which is at least equivalent to tolerance class 5 for the shaft and at least equivalent to tolerance class 6 for the bearing seat. The cylindricity tolerance should also be smaller.
Design and material of shaft and bearing seat
The fit of the bearing ring on its supporting surface shall not cause uneven deformation (out-of-roundness) of the bearing ring. The reason for this may be the discontinuity of the bearing surface. Therefore, split bearing housings are generally not suitable for applications where the outer ring has a tighter interference fit, and the selected tolerance fit should not be tighter than the fit obtained by the tolerance group H (or at most K group). In order to ensure adequate support for the bearing rings installed in thin-walled bearing housings, light alloy bearing housings or on hollow shafts, the selected interference fit should be better than those normally recommended for thick steel plate or cast iron housings or solid shafts. The fit fit is tighter, see the section “The fit of the hollow shaft”. In addition, the shaft of some materials can adopt a looser interference fit.
Easy to install and remove
Bearings with a clearance fit are usually easier to install or disassemble than bearings with an interference fit. When the operating conditions require interference fit and convenient installation and disassembly are required, separable bearings or bearings with tapered bores can be used. Bearings with tapered bores can be directly installed on the bearing surface of the tapered shaft, or can be fitted on the optical shaft or stepped cylindrical shaft by means of an adapter sleeve or a withdrawal sleeve .
Displacement of non-fixed end bearing
If a bearing that cannot withstand axial displacement is used as a non-fixed end bearing, one of the bearing rings must always be able to move freely in the axial direction during operation. This can be achieved as long as a clearance fit is adopted for the bearing ring carrying a static load. When the outer ring bears a static load and axial displacement is allowed in the bearing seat hole, a hardened intermediate sleeve or sleeve is often installed on the outer ring, for example, in the case of a light alloy bearing seat. In this way, it can avoid the phenomenon of deformation of the bearing surface in the bearing seat due to the low material hardness; otherwise, over time, this will cause the axial displacement to be restricted, or even the displacement is blocked.
If cylindrical roller bearings, needle roller bearings or CARB toroidal roller bearings without ribs on the bearing ring are used, since the axial displacement occurs inside the bearing, the inner and outer rings of the bearing can be installed with an interference fit.
Bearing for your choosing
