Page 257 - Industrial Power Engineering and Applications Handbook

101236 Industrial Power Engineering and Applications Handbook
IMP2 and IMP3 Castrol where
MP greases 2 and 3 G, = quantity of regrease in grammes. This does not
Lanthax EPl and 2 for large motors } include the grease initially placed in the covers
and other spaces
Servogem-2 and 3 Indian Oil D = bearing outside diameter (mm)
Titex HT - for large motors Corporation B = bearing width (mm).
Lithon-2 Hindustan
Lithoplex-2 for large motors Petroleum Table 10.3 provides a ready reference for the quantity of
Balmer grease and its regreasing interval as prescribed by a leading
NLGI-2 and NLGI-3 Lawrie motor manufacturer.
These grades possess a high drop point, of the order of 10.4.4 General precautions
1 80"C, and a working temperature range from -30°C to
+130"C. When long idle storage is foreseen, arrangements may
be made to rotate the rotor periodically by hand by 90"
10.4.1 Cleaning bearings or so to shift the position of balls or rollers. This will
prevent the grease from leaving any marks on the races
For cleaning bearings and bearing housings, before and avoid any static indentation on the bearings.
applying the grease, only the following fluids should be After the recommended operating hours, as shown in
used: benzone, white petrol and benzole (white petrol is Table 10.3, the bearings should be taken out and washed
the most recommended). thoroughly as prescribed above and fresh grease applied.
The bearing covers and the bearing housing should also
10.4.2 Relubricating interval be washed.
The relubricating interval depends upon the running
conditions of the bearing, such as the bearing type, size, Note
1 For dust- and chemical-laden atmospheres as in cement plants,
speed, operating temperature and also the type of grease chemical industries and refineries and areas in alkaline and
used. The re-greasing time is thus related to the service saline environments reduce the regreasing intervals by roughly
conditions of the grease and is represented by 30%.
2 For larger frames contact the manufacturer.
3 The free space in the bearing should be completely filled while
(10.1) the free space in the bearing housing should be filled between
30-50%.
where 4 For more details refer to the product catalogues published by
RG = re-lubricating interval in hours of operation the leading bearing manufacturers.
K = factor as noted below
for ball bearings - 1 10.4.5 Prevention from shaft currents
for cylindrical roller bearings - 5
for spherical roller bearings or In an electric motor the stator windings are in a circular
taper roller bearings - 10 form. Each phase is wound identically and spaced equally
d = bearing bore diameter in (mm). to each other. Under ideal conditions the electric field
produced by these current-carrying conductors must be
The relubricating period obtained from this expression balanced and neutralized, as in a three-core cable, pro-
is valid for operations up to 70"C, when measured at the ducing no residual field. (For more details see Section
outer ring. This interval should be halved for every 15°C 28.8). But this is not always so. While some field in
rise in temperature above 70"C, which is unlikely to space may always be present in all ratings, it assumes
occur in an electric motor. For operating in areas that are cognisable strength in large LT and HT machines (2 2
hazardous or contaminated which may affect the life of MW), using circular laminations and all machines using
the grease, the regreasing interval may be reduced, segmented stator punchings.* Such space fields induce
depending upon the installation's environment. an e.m.f. between the two shaft ends. This is not a direct
A more generalized graph from a leading bearing result of proximity but may be due to asymmetry in the
manufacturer is reproduced in Figure 10.6, and covers magnetic circuit because of harmonics present in the
practically all sizes and types of bearings to determine system, a non-ideal flux distribution, due to slight
the relubricating period. disorientation or variation in the punching for the slot
When in operation, the bearings should be regreased positions, or an eccentric air gap between the stator and
through the grease nipples or grease injectors, if provided the rotor. The phenomenon of disorientation in the
on the motor, after the recommended number of running punchings for the slot positioning is independent of motor
hours, as prescribed in Table 10.3. If grease nipples are current and influences large LT and HT machines equally.
not provided, the motor may be temporarily stopped and In HT motors even electrostatic effects between the
regreased after removing the bearing end covers. windings of the stator and the rotor due to stator voltage
10.4.3 Quantity of regrease
*Segmented stator punchings are made in two halves and are used
G, = 0.0050 . B grammes (1 0.2) for very large machines (> 1.5 MW).