When selecting a fit between inner bearing ring and shaft and between outer bearing ring and housing, the load situation should be considered.
Load conditions
There are three different load situations, characterized as "rotating load", "stationary load" and
"direction of load indeterminate".
"Rotating load" pertains if the bearing ring rotates and the load is stationary, or if the ring is stationary and the load rotates so that all points on the raceway are subjected to load in the course of one revolution.
A bearing ring subjected to a rotating load will tend to turn on its seating if mounted with a clearance fit, and may result in wearing (fretting corrosion) of the surfaces involved. To prevent this, an interference fit must be specified. The interference required is dictated by the operating conditions.
"Stationary load" pertains if the bearing ring is stationary and the load is also stationary, or if the ring and the load rotate at the same speed, so that the load is always directed towards the same position on the raceway.
"Direction of load indeterminate" represents variable external loads, shock loads, vibrations and unbalance loads in high-speed machines. These give rise to changes in the direction of load, which cannot be accurately described. When the direction of load is indeterminate and particularly where heavy loads are involved, it is desirable that both rings have an interference fit. For the inner ring the recommended fit for a rotating load is normally used. If the outer ring must be free to move axially in the housing, a somewhat looser fit than that recommended for a rotating load may be used.
Operating and load conditions for interference fit between inner ring and shaft (condition 1)
• Rotating inner ring
• Stationary outer ring
• Constant load direction and
• Rotating load on inner ring
• Stationary load on outer ring
Interference fit is required between inner ring and shaft as the load is varying in direction relative to the inner ring.
Clearance fit can be used between outer ring and housing as the load has a constant direction relative to the outer ring.
An example of this load situation is belt-driven shafts.
Operating and load conditions for interference fit between inner ring and shaft (condition 2)
• Stationary inner ring
• Rotating outer ring
• Load rotates with outer ring or
• Rotating load on inner ring
• Stationary load on outer ring
Interference fit is required between inner ring and shaft as the load is varying in direction relative to the inner ring.
Clearance fit can be used between outer ring and housing as the load has a constant direction relative to the outer ring.
An example of this load situation is merry-go-round drives.
Operating and load conditions for interference fit between outer ring and housing (condition 1)
• Stationary inner ring
• Rotating outer ring
• Constant load direction and
• Stationary load on inner ring
• Rotating load on outer ring
Interference fit is required between outer ring and housing as the load is varying in direction relative to the outer ring.
Clearance fit can be used between inner ring and shaft as the load has a constant direction relative to the inner ring.
An example of this load situation is conveyor idlers and car wheel hub bearings.
Operating and load conditions for interference fit between outer ring and housing (condition
2)
• Rotating inner ring
• Stationary outer ring
• Load rotates with inner ring
or
• Stationary load on inner ring
• Rotating load on outer ring
Interference fit is required between outer ring and housing as the load is varying in direction relative to the outer ring.
Clearance fit can be used between inner ring and shaft as the load has a constant direction relative to the inner ring. Examples of this load situation are vibratory applications and vibrating motors.
Magnitude of the load
The interference fit of a bearing inner ring on its seating will be loosened with increasing load as the ring deforms. Under the influence of rotating load the ring may begin to creep. The degree of interference must be related to the magnitude of the load; the heavier the load, the greater the interference fit required.
Bearing internal clearance
An interference fit of a bearing on a shaft or in a housing means that the ring is elastically deformed (expanded or compressed) and the bearing internal clearance is reduced. A certain minimum clearance should remain.
Temperature conditions
In many applications the outer ring has a lower temperature in operation than the inner ring.
This might lead to reduced internal clearance.
In service, bearing rings normally reach a temperature that is higher than the one of the components to which they are fitted. This can result in a reduction of the fit of the inner ring on its seating, while an outer ring expansion may prevent the desired axial displacement of the ring in its housing. Temperature differentials and the direction of heat flow in the bearing arrangement must therefore be carefully considered.
Running accuracy requirements
To reduce resilience and vibration, clearance fits should generally not be used for bearings where high demands are placed on running accuracy. Bearing seats on the shaft and in the housing should be made to tolerance grade 5 for the shaft and to tolerance grade 6 for the housing. Tight tolerances should also be applied to the cylindricity.
Displacement of the non-locating bearing
If non-separable bearings are used as non-locating bearings it is imperative that one of the bearing rings is free to move axially at all limes during operation. Adopting a clearance fit for the ring that carries a stationary load will provide this.
If cylindrical roller bearings have one ring without flanges, needle roller bearings or
CARB toroidal roller bearings are used, both bearing rings may be mounted with an interference fit because axial displacement will take place within the bearing.