Rigid couplings require precise alignment and are most commonly used in vertical units where the entire
weight is supported by thrust bearings in the motor or generator. Flanged and threaded couplings are the
most widely used rigid couplings. Flanged couplings are used on large vertical units and consist of
precisely machined flanges on each shaft, connected by a series of coupling bolts around the perimeter of
the flanges. Threaded couplings, used to connect the line shafts of vertical turbine pumps, are cylindrically
shaped with internal threads matching the external threads on the line shafts. The shafts to be coupled are
simply screwed tightly into either end of the coupling.
Flexible couplings are designed to accommodate slight misalignment between shafts and to some extent,
dampen vibration. The amount of misalignment allowable is completely dependent of the design of the
particular coupling. Since there are a number of flexible coupling designs, tolerances for misalignment
should be obtained from the coupling manufacturer. The flexibility of the
couplings can be provided
through clearances between mating parts, as in gear and chain couplings, or through the use of a flexible
material in the coupling, as in flexible disk and compression couplings. Horizontal pumps usually employ
some sort of flexible coupling to connect the pump to its driver.
If properly aligned, most couplings should require very little maintenance outside of periodic inspection,
and in some cases, lubrication. Over time, the alignment between the pump and its driver can deteriorate,
increasing stress on the coupling which can lead to a shorter life.
2.8 Shaft Alignment
Misalignment is a common and sometimes serious problem. Poor alignment can cause premature wear or
failure of bearings, overheating of shaft couplings, and in extreme cases, cracked or broken shafts. The
procedure for alignment depends on the type of equipment and its design.
Large vertical units, suspended from a thrust bearing in the motor or generator, require making the shaft
plumb and the guide bearings concentric. The procedure for aligning these units is discussed in detail in
Volume 2-1, Alignment of Vertical Shaft Hydro Units
The lineshafts of vertical turbine pumps are held in alignment by lineshaft bearings in the pipe column.
The proper alignment of the lineshaft depends on the proper assembly of the pipe column and the bearing
retainers. Depending on the design, the pump motor to lineshaft coupling may be aligned by the face and
rim method or the reverse indicator method described below. Refer to the pump manufacturer's instructions
for specific directions for assembly and alignment.
Horizontal Pump Alignment.
Horizontal pumps are usually coupled to the pump driver with a flexible
coupling. The amount of misalignment a flexible coupling can tolerate is dependent on the coupling's
design and characteristics, such as speed and torque, of the machines being coupled. The coupling's
manufacturer should provide installation instructions indicating the allowable tolerances for a particular
design. A horizontal pump can usually be aligned acceptably by either the face and rim method or reverse
indicator method. In most cases, the pump driver is aligned to the pump, as the pump is usually connected
to rigid piping and is more difficult of move.