Page 236 - Industrial Power Engineering and Applications Handbook
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Transmission of  load and suitability of  bearings  8/215
       No rotating  mass  can  be balanced  up  to 0 micron, for   load farther away from the shaft collar, it is recommended that the
       obvious reasons. While rotating they are out of balance,   pulley be mounted on a separate jack shaft, supported on two pedestals
       which gives rise to rotating forces and adds to the radial   as illustrated in Figure 8.15. Provided that the motor shaft can be
       load on the bearing. This indirectly  affects the running   made longer, to support such a pulley and have sufficient strength,
       life  of  the  bearing,  in  addition  to  other  factors  noted   to take that load, one pedestal may also be adequate to support the
                                                      free end of the motor shaft as shown in Figure 8.16. The pulley is
       above. The selection of type and size of bearing, is thus   now  mounted between  the  shaft collar and  the pedestal. A jack
       governed by the speed of the machine, the type and weight   shaft or additional pedestal to support the motor shaft may also be
       of loads and the required life of the bearing which can be   necessary when the ratio of pulley diameters exceeds 6:l.
       expressed by                                      In  some cases, reinforcement  of  the  shaft by  increasing the
                                                      shaft diameter, employing a better grade of steel and using a superior
                                                      grade of bearings, may also meet the load requirement. The bearing
              106
       Ll0 = - (g)' hours                      (8.4)   bore, however, may pose a limitation in increasing the shaft diameter
            60. N,                                    beyond a certain point, say, beyond the diameter of the shaft collar.
                                                      If a larger shaft diameter is required either a larger frame size of the
       where                                          motor may be employed, which  may be uneconomical, or a jack
       L,, = Rated life of  bearings at 90% reliability, i.e. 90%   shaft or pedestal may be used as noted above. Replacing standard
            of bearings produced by a manufacturer will exceed   bearings with larger bore bearings to use a shaft of greater diameter
            this life                                 may not be possible in the same frame, due to pre-sized end shields
        C = basic dynamic load rating in kg or N (provided by   and bearing housings which, for motors up to 250 kW, are normally
            the  bearing  manufacturer).  It  is  the  load  which   cast and have fixed dimensionshoulding patterns.
            will give a life of  I  million revolutions
        P = equivalent dynamic bearing load in kg or N   Questionnaire to determine the suitability of the
        p  = exponent of the life equation, which depends upon   motor shaft and the bearing for the required
            the type of contact between the races and the rolling   belt drive
            elements. It is recommended as 3 for ball bearings
            and  10/3 for roller bearings             For critical loads and belt drives particularly, the user is
        N, = speed of the machine in r.p.m.           advised to seek the opinion of the motor manufacturer to
                                                      determine the mechanical suitability of the motor selected,
       With this equation the life of the bearing can be determined   its shaft and the DE bearing for the load to be transmitted,
       for different  load  conditions  and  is predetermined  for   according to the drive system being adopted. The following
       the type of  drive and  service requirements. To select  a   are some important parameters that may help to determine
       proper bearing, therefore, the type of application and the
       loading ratio (ClP) should be carefully selected to ensure
       the required minimum life. Bearing manufacturers' product   Pedestal bearing-,     -
       catalogues  provide  the  working  life  of  bearings  for
       different load factors and may be referred to for data on
       C, C,  and other parameters.
                                                                                      I  Abearing

       8.7  Suitability of rotors for pulley
            drives                                                                            se
                                                                                              me
       In belt drives particularly,  it is advisable that reference
       be  made  to  the  motor  manufacturer  to  determine  the
       suitability  of  the rotor  shaft  and  the  driving  end  (DE)   Figure 8.15  Arrangement of a jack  shaft
       bearing for transmission of the required  load. A typical
       format of a questionnaire is also given below for providing
       the manufacturer for load, belt and pulley data. The format   7
       is suitable for all drives that may be subjected to excessive                     Motor
                                                                                         Shaft collar
       forces on the shaft. These data will enable the manufacturer                       Pulley
       to  determine  the  following  parameters  to  check  the   I
       suitability of bearing and shaft strength and make suitable                       Long shaft
       changes if warranted:                                                        7
                                                                                         Pedestal
                                                                                         bearing
         Load acting on the motor bearings
         Bending moment at the motor shaft due to pulley and
         load
         Possible deflection of  the shaft                                               Base
                                                                                         frame
       Note  The shaft deflection should not be more than  1 1 % of the air
       gap between the stator and the rotor. For loads that exert more force
       and  torsional  stress  on  the  motor  shaft  and  bearings  than  is
       permissible, due to the larger width of pulleys which may shift the   Figure 8.16  Arrangement of  a long shaft
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