Biomedical Engineering Reference
In-Depth Information
Fixed
3
2
2
1
1
1
ab
a
abc
Fixed
Sliding
Rolling
Gliding
Figure 5.1
Kinematic conditions of contact for the various
test configurations: sliding, rolling, and gliding. Adapted
from McGloughlin and Kavanagh [7].
friction between two bearing surfaces, the sliding is sometimes classified into
further two groups, sliding and gliding [7, 8]. The kinematic conditions are
represented graphically as shown in Figure 5.1. Sliding is a condition where the
contact point on the lower bearing surface remains stationary relative to the contact
point on the upper bearing surface which moves. Rolling is a condition where the
relative contact point velocities of both the lower and the upper bearing surfaces are
equal. Finally gliding is a condition where the contact point on the upper surface
is stationary and the contact point on the lower surface moves.
The primary function of a lubricant is to reduce friction or wear (or both) between
moving bearing surfaces in contact with each other. There is a variety of lubricants
including the synovial fluid in human joints. The following types of lubrication are
considered in order of decreasing film thickness [9]:
•
Hydrodynamic lubrication
•
Elastohydrodynamic lubrication
•
Mixed lubrication
•
Boundary lubrication.
In hydrodynamic lubrication, the load is supported by the pressure developed
due to the relative motion and geometry of the bearing surfaces. In hydrodynamic
or fluid-film lubrication, there is no contact between the bearing surfaces. Friction
arises purely from shearing of the viscous lubricant.
In contact situations involving high loads such as gears, ball bearings, and other
high-contact-stress geometries, there are two additional requirements. The first is
that the surface deforms elastically leading to a localized change in the geometry
which favors lubrication. The second is that the lubricant becomes more viscous
under the high pressure that exists in the contact zone. Here, the lubricant pressures
existing in the contact zone approximate those of dry contact Herzian stress. This
is the definition of elastohydrodynamic lubrication, sometimes abbreviated as EHL
or EHD.
Although the most important function of a lubricant is to prevent contact, a
better fundamental understanding about the transition from hydrodynamic and
elastohydrodynamic lubrication to boundary lubrication is required. This is the
region where lubrication goes from the desirable hydrodynamic condition of no
