Lifespan of bearings
The lifespan of bearings is defined as: the number of revolutions – or the number of operating hours at a given constant speed – that the bearing rotates before the first signs of material fatigue appear on one of the rings or rolling elements.
Similar bearings operating under the same conditions achieve different service lives. In order to properly determine the service life, we assume that the bearings have been carefully mounted, properly lubricated and treated.
The lifespan of a bearing depends on the dynamic load factor, the loads and the speed. Other factors also play an important role, such as the choice of shaft and housing fit, the alignment of the various structural parts and the operating temperature.
The theory for the lifespan calculation – laid down in ISO standard 281 – is based on the relationship between load and the dynamic load factor that is corrected on the basis of a number of factors. The formula is based on the fact that 90% of the bearings achieve the calculated service life.
Axle and house fits
To achieve the optimum lifespan of a bearing, the selection of the correct bearing is of course a first requirement. However, this is not sufficient because the correct bearing will fail prematurely if the shaft and housing fit are not properly matched.
The ISO tolerances for shaft and housing along with tolerances for the bore and outer diameter of a bearing determine the fit. The ISO tolerances are established in a form of tolerance fields with respect to the zero line. The location of the tolerance field is indicated by a letter. Large letters are used for houses and small letters for axles.
The choice of a fit strongly depends on the operating conditions. The right choice is determined by the rotation ratio. A rotational ratio refers to the movement of the bearing ring – the inner or outer ring – in relation to the direction of the load. Rotating load occurs when the ring is stationary and the load rotates, or when the ring rotates and the load is stationary. A stationary load is the situation where both the ring and the load are stationary or both are rotating at the same speed. An indefinite load is also possible. This can be used if the direction of the load varies due to, for example, impacts or vibrations.
The material of the shaft or housing is also a factor to consider. For example, aluminum expands more when heated than does steel. In addition, other factors also play a role, such as a thin-walled housing, a hollow shaft or high vibrations.