The segmented shoe type bearings are adjustable to allow adjusting the bearing clearance and the
position of the center of the bearing. The sleeve type journal hearing may be doweled in place, or
the bearing shell may be a tight fit in the upper or lower bridge. Both the sleeve type and the
segmented shoe bearings used on generators are partially submerged in an oil bath and lubricate
through the rotation of the shaft.
3. OBJECTIVES OF VERTICAL SHAFT ALIGNMENT
In a perfectly aligned vertical shaft hydrounit, all the rotating components would be perfectly
plumb and perfectly centered in the stationary components at any rotational position. The thrust
bearing shoes would be level, with each shoe equally loaded and the thrust runner would be
perfectly perpendicular to the shaft. As the shaft turns, perfectly centered in the guide bearings,
the only loading on the guide bearings would be from mechanical and electrical imbalance. As
alignment deviates, loading on the guide bearings will increase and so will vibration levels. Any
increase in vibration from misalignment will decrease the factor of safety for operation in severe
circumstances, such as rough zone operation. If a unit has a moderate vibration problem caused
by misalignment, the driving forces that occur with draft tube surging or mechanical imbalance
may be enough to cause damage to the unit.
Since a perfect alignment isn't possible, we need guidelines or tolerances to let us know when we
are "close enough." Table 1 lists tolerances for use in aligning a vertical shaft hydrounit. These
are general tolerances, and some judgement must be used in specific cases. In most cases, a unit
can easily be aligned within these tolerances, but in some special circumstances, it may not be
possible without major modifications. When a major modification is required, such as moving
the generator stator, the possible consequences of not doing it should be compared to the benefits
before making a decision.
To meet the tolerances of table 1, concentricity, circularity, straightness, perpendicularity, and
plumb must be addressed. The following are definitions of these characteristics as they apply to
vertical shaft alignment.
By definition, concentric refers to anything sharing a common center. In the alignment of a
vertical shaft unit, the stationary components are considered concentric when a single straight
line can be drawn connecting the centers of all of the components. This straight line will be
plumb or within the allowable tolerances for plumb.
The concentricity of the stationary components can be checked by measuring clearances, or if the
unit is completely disassembled, such as during an overhaul, a single tight wire can be used as a
plumb reference. Clearance measurements, i.e., bearing, turbine seal ring, and generator air gap,
can be used to locate their centerlines with reference to the shaft. If the unit is disassembled, the
upper and lower bridges and the head cover can be installed temporarily and a single tight wire
hung through the unit. The concentricity is determined by measuring from the stationary