2. FUNDAMENTALS OF LUBRICATION
The basic purpose of a lubricant is to reduce
friction and wear between two surfaces moving
relative to one another. In most cases a lubri-
cant also dissipates heat, prevents rust or
corrosion, acts as a seal to outside contami-
nants, and flushes contaminants away from
bearing surfaces. In order for the lubricant to
accomplish these functions, a lubricant film
must be maintained between the moving sur-
faces. This condition is known as fluid film
lubrication.
friction and wear between two surfaces moving
relative to one another. In most cases a lubri-
cant also dissipates heat, prevents rust or
corrosion, acts as a seal to outside contami-
nants, and flushes contaminants away from
bearing surfaces. In order for the lubricant to
accomplish these functions, a lubricant film
must be maintained between the moving sur-
faces. This condition is known as fluid film
lubrication.
2.1 Fluid Film Lubrication
Fluid film lubrication reduces friction between
moving surfaces by substituting fluid friction for
mechanical friction. Fluid film lubrication is il-
lustrated in
moving surfaces by substituting fluid friction for
mechanical friction. Fluid film lubrication is il-
lustrated in
Surface 2, separated by a film of fluid. The oil
film can be looked at as being made up of many
layers. The layer in contact with the moving
Surface 1, clings to that surface and moves at
the same velocity. Similarly, the layer in contact
with Surface 2 is stationary. The layers in be-
tween move at velocities directly proportional to
their distance from the moving surface. For
example, at a distance of 1/2 h from Surface 1,
the velocity would be 1/2 V. The force F, re-
quired to move Surface 1 across Surface 2 is
simply the force required to overcome the fric-
tion between the layers of fluid. This internal
friction, or resistance to flow, is defined as the
viscosity of the fluid. Viscosity will be discussed
in more detail later.
film can be looked at as being made up of many
layers. The layer in contact with the moving
Surface 1, clings to that surface and moves at
the same velocity. Similarly, the layer in contact
with Surface 2 is stationary. The layers in be-
tween move at velocities directly proportional to
their distance from the moving surface. For
example, at a distance of 1/2 h from Surface 1,
the velocity would be 1/2 V. The force F, re-
quired to move Surface 1 across Surface 2 is
simply the force required to overcome the fric-
tion between the layers of fluid. This internal
friction, or resistance to flow, is defined as the
viscosity of the fluid. Viscosity will be discussed
in more detail later.
The guide bearings of a vertical hydroelectric
generator, if properly aligned, have little or no
loading and will tend to operate in the center of
the bearing because of the viscosity of the oil. In
highly loaded bearings, like thrust bearings and
horizontal journal bearings, the fluid's viscosity
alone is not sufficient to maintain a film between
the moving surfaces. In these bearings higher
generator, if properly aligned, have little or no
loading and will tend to operate in the center of
the bearing because of the viscosity of the oil. In
highly loaded bearings, like thrust bearings and
horizontal journal bearings, the fluid's viscosity
alone is not sufficient to maintain a film between
the moving surfaces. In these bearings higher
fluid pressures are required to support the load.
If this pressure is supplied by an outside source,
it is called hydrostatic lubrication. If the pressure
is generated internally, that is within the bearing
by dynamic action, it is referred to as
hydrodynamic lubrication. In hydrodynamic
lubrication, a fluid wedge is formed by the
relative surface motion of the journals or the
thrust runners over their respective bearing
surfaces. The formation of this wedge is similar
to the wedge that forms under a speeding boat,
pushing the bow out of the water, or under water
skis, allowing the skier to skim across the water.
If this pressure is supplied by an outside source,
it is called hydrostatic lubrication. If the pressure
is generated internally, that is within the bearing
by dynamic action, it is referred to as
hydrodynamic lubrication. In hydrodynamic
lubrication, a fluid wedge is formed by the
relative surface motion of the journals or the
thrust runners over their respective bearing
surfaces. The formation of this wedge is similar
to the wedge that forms under a speeding boat,
pushing the bow out of the water, or under water
skis, allowing the skier to skim across the water.
illustrates the wedge action in a pivot-
ing shoe thrust bearing. As the thrust runner
moves over the thrust shoe, fluid adhering to the
runner is drawn in between the runner and the
shoe, causing the shoe to pivot, and forming a
wedge of oil. As the speed of the runner In-
creases, the pressure of this wedge increases
and the runner will be lifted vertically, and full
fluid film lubrication takes place.
moves over the thrust shoe, fluid adhering to the
runner is drawn in between the runner and the
shoe, causing the shoe to pivot, and forming a
wedge of oil. As the speed of the runner In-
creases, the pressure of this wedge increases
and the runner will be lifted vertically, and full
fluid film lubrication takes place.
a horizontal journal bearing. In Drawing "A", the
journal is at rest and the weight of the journal
has squeezed out the oil film at "E" so that the
journal rests on the bearing surface. As rotation
starts, as shown in Drawing "B", the journal has
a tendency to roll up the side of the bearing. At
the same time fluid adhering to the journal is
drawn into the contact area at "F." As the speed
increases the oil wedge is formed at "G." The
pressure of the oil wedge increases until the
journal is lifted off the bearing at "H" as shown in
Drawing "C". Drawing "D" shows the condition at
full speed. The journal is not only lifted
vertically, but is also pushed to the left by the
pressure of the oil wedge so that the resultant
force from the fluid pressure acts along the line
"PO." The minimum fluid film thickness at full
speed will occur at "J" and not at the bottom of
the bearing.
journal is at rest and the weight of the journal
has squeezed out the oil film at "E" so that the
journal rests on the bearing surface. As rotation
starts, as shown in Drawing "B", the journal has
a tendency to roll up the side of the bearing. At
the same time fluid adhering to the journal is
drawn into the contact area at "F." As the speed
increases the oil wedge is formed at "G." The
pressure of the oil wedge increases until the
journal is lifted off the bearing at "H" as shown in
Drawing "C". Drawing "D" shows the condition at
full speed. The journal is not only lifted
vertically, but is also pushed to the left by the
pressure of the oil wedge so that the resultant
force from the fluid pressure acts along the line
"PO." The minimum fluid film thickness at full
speed will occur at "J" and not at the bottom of
the bearing.
(FIST 2-4 11/90)
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