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In hydrodynamic lubrication, sometimes
referred to as fluid film lubrication, the wearing surfaces
are completely separated by a film of oil. This type of
lubricating action is similar to a speedboat operating on
water. When the boat is not moving, it rests on the
supporting water surface. As the boat begins to move, it
meets a certain amount of resistance or opposing force due
to viscosity of the water. This causes the leading edge of
the boat to lift slightly and allows a small amount of water
to come between it and supporting water surface. As the boat’s
velocity increases, the wedge-shaped water film increases in
thickness until a constant velocity is attained. When the
velocity is constant, water entering under the leading edge
equals the amount passing outward from the trailing edge.
For the boat to remain above the supporting surface there
must be an upward pressure that equals the load.
The same principle can be applied to a
sliding surface. Fluid film lubrication reduces friction
between moving surfaces by substituting fluid friction for
mechanical friction. To visualize the shearing effect taking
place in the fluid film, imagine the film is composed of
many layers similar to a deck of cards. The fluid layer in
contact with the moving surface clings to that surface and
both move at the same velocity. Similarly, the fluid layer
in contact with the other surface is stationary. The layers
in between move at velocities directly proportional to their
distance from the moving surface. For example, at a distance
of ½ h from Surface 1, the velocity would be ½ V. The
force F required to move Surface 1 across Surface 2 is
simply the force required to overcome the friction 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 principle of hydrodynamic lubrication
can also be applied to a more practical example related to
thrust bearings used in the hydropower industry. Thrust
bearing assembly is also known as tilting pad bearings.
These bearings are designed to allow the pads to lift and
tilt properly and provide sufficient area to lift the load
of the generator. As the thrust runner moves over the thrust
shoe, fluid adhering to the runner is drawn between the
runner and the shoe causing the shoe to pivot, and forming a
wedge of oil. As the speed of the runner increases, the
pressure of the oil wedge increases and the runner is lifted
as full fluid film lubrication takes place. In applications
where the loads are very high, some thrust bearings have
high pressure-pumps to provide the initial oil film. Once
the unit reaches 100 percent speed, the pump is switched
off.
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