BEARINGS OF ALTERNATING CURRENT GENERATORS BASIC INFORMATION
Although, antifriction bearings are occasionally used on alternators of smaller ratings, the great majority are furnished with oil-lubricated babbitted bearings. For horizontal shafts at small ratings ring oiled bearings are used, but at higher ratings recirculation of externally cooled oil is used.
Two principal types of thrust bearings are used on vertical
alternators: the pivoted-shoe type and the spring type.
The adjustable pivoted-shoe type, introduced in the United
States by Albert Kingsbury, consists of a flat rotating collar or runner of
steel or fine-grained cast iron resting on a stationary member consisting of
several babbitted segmental shoes pivoted near their center on adjusting screws,
which, by changing the elevation of the shoes, can provide equal loading on
each.
The screws also permit small adjustments in rotor elevation
to correct generator and turbine clearances.
The bearings are immersed in oil. In operation, a thin, wedge-shaped
film of oil is formed between the runner and the shoe. The oil is continuously
circulated by the rotation of the runner and is cooled by either radiation or
water cooling, usually within the oil bath but occasionally by an external
system. Some of the larger bearings are cooled by means of water circulate
through tubes embedded below the babbitt surface.
The spring-type bearing is inherently self-equalizing; that
is, each shoe carries very nearly the same amount of load. A variation of the
pivoted-shoe bearing, in which the shoes are supported on a system of
interconnected levers, provides the same self-equalizing feature.
The spherical bearing is another variation of the
pivoted-shoe thrust bearing, in which the runner is part of a sphere and the
shoes of corresponding shape. This type of bearing restrains lateral movement
of the shaft, serving the dual function of thrust and guide bearing.
Horizontal-shaft alternators occasionally require thrust
bearing, as, for example, a singleimpeller reaction turbine having unbalanced
hydraulic thrust which must be restrained by the bearing. Thrust bearing
designs for this application are generally of the pivoted-shoe type, either
adjustable or equalizing.
Some thrust bearings, particularly of the adjustable
pivoted-shoe type, may be provided with load cells for measuring and equalizing
the thrust on the shoes. These may be of the hydraulic or straingage type, the
latter is more common in modern applications. In addition to providing a check
on the adjustment of the shoe loadings, these devices provide information about
the hydraulic thrust characteristics of the turbine.
Guide bearings for vertical alternators are oil-lubricated
babbitted rings. These are frequently segmented to facilitate assembly and may be
composed of individual shoes which are radially adjustable.
Guide bearings usually are partly immersed in an oil bath
with oil circulated by the pumping action of sloping grooves in the babbitt
surface. Occasionally, a separate lubrication system is provided which
introduces oil at the top clearance of the bearing, collects it at the bottom,
and recirculates it.
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