In both the pivoting shoe thrust bearing and
the horizontal journal bearing, the minimum
thickness of the fluid film Increases with an
Increase in fluid viscosity and surface
speed and decreases with an Increase in
load.
the horizontal journal bearing, the minimum
thickness of the fluid film Increases with an
Increase in fluid viscosity and surface
speed and decreases with an Increase in
load.
2.2 Elastohydrodynamic Lubrication
In instances of very high unit loads and
high speeds, as is experienced in
antifriction (ball and roller) bearings, cams,
and some gears, it Is difficult to establish or
maintain an oil film with adequate thickness
for normal hydrodynamic lubrication. In
these special cases, the lubricant is
compressed and extremely high pressures
are developed. The high pressures
increase the lubricant's viscosity and
elastically deform the metal surfaces. This
allows the load to be spread over a larger
area and Increases the load carrying
capacity. This is referred to as
elastohydrodynamic lubrication. In this
realm of lubrication an increase in load
deforms the metal surfaces rather than
affecting the oil film thickness, as the oil film
is actually more rigid than the metal.
high speeds, as is experienced in
antifriction (ball and roller) bearings, cams,
and some gears, it Is difficult to establish or
maintain an oil film with adequate thickness
for normal hydrodynamic lubrication. In
these special cases, the lubricant is
compressed and extremely high pressures
are developed. The high pressures
increase the lubricant's viscosity and
elastically deform the metal surfaces. This
allows the load to be spread over a larger
area and Increases the load carrying
capacity. This is referred to as
elastohydrodynamic lubrication. In this
realm of lubrication an increase in load
deforms the metal surfaces rather than
affecting the oil film thickness, as the oil film
is actually more rigid than the metal.
2.3 Boundary Lubrication
A well designed fluid film bearing will
operate with a full fluid film under most
circumstances, but under less than ideal
conditions, such as during start up and shut
down, the fluid film may become so thin
that contact may be made between the
rubbing surfaces. This condition is called
boundary lubrication.
operate with a full fluid film under most
circumstances, but under less than ideal
conditions, such as during start up and shut
down, the fluid film may become so thin
that contact may be made between the
rubbing surfaces. This condition is called
boundary lubrication.
Under boundary lubrication conditions, the
lubricant's viscosity alone cannot provide
sufficient lubrication. To compensate for
this, lubricant additives may be required, or,
in the case of hydroelectric generator thrust
bearings, an outside pressure source or
hydrostatic system may be used.
lubricant's viscosity alone cannot provide
sufficient lubrication. To compensate for
this, lubricant additives may be required, or,
in the case of hydroelectric generator thrust
bearings, an outside pressure source or
hydrostatic system may be used.
Some reciprocating equipment, such as
pistons in compressors or engines, and
slow moving equipment, such as turbine
wicket gates, rely on boundary lubrication
entirely. Gear teeth also depend on
boundary lubrication to a great extent. In
order for boundary lubrication to be
effective, that is to reduce friction and
provide damage control to the rubbing
surfaces, a very thin film of lubricant and/or
additive must be maintained. This is
accomplished through the use of various
extreme pressure additives, oiliness
additives, friction modifiers, and solid
lubricants, such as graphite or molybdenum
disulfide.
pistons in compressors or engines, and
slow moving equipment, such as turbine
wicket gates, rely on boundary lubrication
entirely. Gear teeth also depend on
boundary lubrication to a great extent. In
order for boundary lubrication to be
effective, that is to reduce friction and
provide damage control to the rubbing
surfaces, a very thin film of lubricant and/or
additive must be maintained. This is
accomplished through the use of various
extreme pressure additives, oiliness
additives, friction modifiers, and solid
lubricants, such as graphite or molybdenum
disulfide.
5 (FIST 2-4 11/90)