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    ROLLING BEARING FAILURE

    6.0 Loss of internal clearance

    Click on photograph to enlarge 
    Figure 6a
    Loss of internal clearance failure
    Figure 6b
    Loss of internal clearance failure
    Figure 6c
    Damage to motor and pump following loss of clearance failure in pump drive-end bearing
    Main CharacteristicsThe optimum condition for a rolling bearing is when the internal radial clearance is zero. It is possible to run bearings with pre-load so that there is no residual internal clearance and rolling occurs by elastic deformation at the contacts; this is, however, a somewhat sensitive condition with higher friction losses and the risk of differential thermal expansion between the races loading the contacts beyond the elastic limit and preventing rolling. Pre-loaded bearings are normally only used where very accurate guidance of the shaft is required (e.g.in machine tools) and more normal practice is to run with a small residual internal clearance to ensure that rolling is maintained at the contacts.
    Once the clearance has been lost and the elastic limit exceeded, sliding occurs at the contacts. There is a large increase in friction and the process escalates, giving a condition of thermal runaway. Either the machine stalls if there is not sufficient power to overcome the friction, or the bearing heats up so much that it deforms plastically (figures 6a, b). With a bearing mounted on a rotating shaft, the inner race reaches the highest temperature and a characteristic of this type of failure is that the severity of damage is greatest at the inner race.
    CauseThe following mechanisms can lead to loss of clearance failure.
    1. Transient temperature differential during start-up or acceleration
      The shaft generally heats up faster than the bearing housing so that the temperature differential during the start-up can be much greater than the steady state value. This is particularly severe with hollow shafts or very thick housings.
    2. Use of bearing with incorrect internal clearance class for the application
      Where in normal circumstances there is a difference in temperature between shaft and housing (e.g. in centrifugal pumps handling hot liquids, hot gas fans, electric motors) a higher than normal clearance class of bearing should be used. This is particularly the case with chemical pumps using austenitic steel shafts that have a higher coefficient of thermal expansion than the ferritic steel of the bearing. In such cases a high clearance bearing should be used (e.g. C3) and, with hot fluids, C4. Particular care has to be taken during maintenance that the correct clearance class of replacement bearing is fitted.
    3. Excessive interference fit(s) of races
      About 80% of the interference fit is transmitted through the race. This is allowed for in the fits recommended by the bearing manufacturers.
    4. Insufficient fit of inner race on shaft
      Rotation of the bearing relative to the shaft leads to increased friction and heating of the inner race and hence loss of clearance.
    5. Complete loss of lubrication
      Loss of lubrication leads to increased friction and again to increased temperature differential.
    6. Inadequate lubricant viscosity to allow generation of elastohydrodynamic lubricant film
      This can occur with oil-lubricated double-row bearings and bearings mounted in pairs where there is insufficient circulation of the oil to the outer row or race to prevent it rising to a temperature where its viscosity becomes too low. In such cases the loss of clearance occurs in the outer bearing or row. A positive supply of lubricant to the outer bearing or race is required when dmn>3500 mm.rev/s (dm = mean bearing diameter).
    7. Gross overload resulting in excessive frictional heating
      A typical example of this is where two locating-type bearings are mounted on the same shaft with one mounted 'free' in its housing to allow for differential axial thermal expansion between shaft and housing. If the 'free' bearing fails to slide during a thermal change, e.g. on starting or stopping, a gross overload is created when the bearings have to resist the expansion (contraction) of the shaft. In practice this effect only becomes significant for bearings with outer diameter >100mm.
    8. Use of cooling jacket round bearing
      Where, because of the environment, cooling is required to control the bearing temperature, this should not be in the form of a complete water jacket round the bearing, which only serves to increase the temperature gradient between inner and outer races. Where cooling is necessary it should be applied to the oil, not to the bearing. Where bearing housings are fitted with cooling jackets, these should be filled with oil or glycol to allow convection cooling. The jacket should be fitted with a stand pipe to allow for thermal expansion of the liquid.
    Note 
    Possible Confusion with
    Other Types of Damage    		 
    CommentThis is a serious type of failure that can lead to catastrophic damage if sufficient power is available to continue driving the machine (figure 6c). Moreover, the failure occurs so suddenly that it is not possible to provide a monitor that will give adequate warning to allow protective action to be taken.