Seal water is provided on most larger seals to help cool and keep the seals clean. The seal water must be
clear clean water. Some type of filtration should be installed if there is any silt or sand in the seal water
supply as any contaminants can quickly damage the seals.
The purpose of the bearings is to locate and support the shafts of a pump or turbine. The bearings can
provide radial support (line or guide bearings), axial support (thrust bearings), or both. The most common
types of bearings are fluid film and antifriction bearings.
Fluid Film Bearings.
Fluid film bearings derive their load carrying capacity through the formation of an
"oil wedge" as the shaft or thrust runner rotates. The formation of this "oil wedge" is similar to the fluid
wedge that forms under a speeding boat, raising its bow out of the water. The force of the wedge in a
bearing must be sufficient to balance the load to the bearing surfaces.
Fluid film, or plain bearings, are normally used on turbines and large pumps and can be in the form of
sleeve bearings, either solid or split, tilting pads, or pivoted thrust shoes. These bearings usually consist of
a cast iron or steel bearing shell with a tin or lead based babbitt lining. Bronze bushings are used for line
shaft bearings in vertical wet-pit pumps and on some horizontal pumps.
The thrust and upper guide bearings of large vertical generators are insulated from the frame to prevent
circulating current from passing through the bearing. The bearing can be quickly damaged or destroyed if
not adequately insulated. Test terminals are usually provided to check the insulation. Refer to FIST,
Volume 3-11, Miscellaneous Power O&M
for more information on bearing insulation testing.
The antifriction bearing, through the use of rolling elements, utilizes the low
coefficient of rolling friction as opposed to that of sliding friction of the fluid film bearing in supporting a
load. The most common type of antifriction bearings are "ball" and "roller" bearings, referring to the shape
of the bearing's rolling elements. These bearings are also classified as "radial," "radial-thrust," or "thrust"
bearings according to the type of load they are meant to support.
An antifriction bearing is a delicate, precision-made piece of equipment, and a great deal of care should be
taken during installation. The bearing manufacturer will usually provide instructions and precautions for
the installation of a particular bearing, and these instructions should be followed closely. Cleanliness is
probably the most important thing to take into consideration in handling antifriction bearings. Any dust or
dirt can act as an abrasive and quickly wear the bearing's rolling elements; therefore, it is important to work
with clean tools and clean hands and to clean the bearing housings, covers, and shaft prior to installation.
The new bearing should not be cleaned or wiped prior to installation unless it is recommended by the
manufacturer. Bearings should be pressed onto shafts using adapters that apply even pressure to the inner
race only. Never hammer a bearing onto a shaft.
2.7 Shaft Couplings
Couplings are used to connect the shaft of a driver, such as a turbine or a motor, to the shaft of a driven
machine, such as a pump or generator. There are basically two types of couplings: rigid and flexible.