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ELECTRICAL EQUIPMENT 



use an exceptionally short shaft, it is sometimes possible to omit 

 the bearing immediately below the revolving field, but, in gen- 

 eral, it seems preferable to have a bearing at this point. The 

 thrust bearing must sustain not only the weight of the revolving 

 element but also the unbalanced water-thrust, and the top 

 bracket must, therefore, be of adequate strength and is usually 

 heavily reinforced, as shown in Fig. 205. 



There are two general classes of thrust bearings; those which 

 depend upon a film of oil between two plates, and those which have 

 hardened rollers between two hardened surfaces. The first class 

 may be subdivided into (a) those which are supplied with oil 

 under pressure and (6) those which revolve in a bath of oil under 

 atmospheric pressure. There are also combinations of the two 

 classes. In either case the bottom plate is stationary and some- 

 times mounted on a spherical self-aligning washer, while the top 

 one rotates with the shaft. 



Oil-pressure Bearings. In this type of bearing, oil under high 

 pressure is pumped into an annular chamber between a revolving 

 and a stationary disc (see Fig. 196), and the pressure required to 



FIG. 196. Assembly of an Oil-pressure Bearing. 



separate the plates is, of course, dependent on the superincumbent 

 weight and the area of the bearing plates. This type of bearing 

 is not extensively used with water-wheel-driven units, as when 

 taken in connection with the necessary pumps and auxiliaries it is 



