Biomedical Engineering Reference
In-Depth Information
contact to the less acceptable ''boundary'' condition, where increased contact
usually leads to higher friction and wear. This condition is sometimes referred to
as
mixed lubrication
.
Boundary lubrication is often described as a condition of lubrication in which
the friction and wear between two surfaces in relative motion are determined by
the surface properties of the solids and the chemical nature of the lubricant rather
than its viscosity. According to another common definition, boundary lubrication
occurs when the surface of the bearing solid is separated by a film whose thickness
is on the molecular level.
This concept is illustrated by the Stribeck curve derived from bearing friction
experiments. When the coefficients of friction for bearing, reported by Stribeck,
were plotted against a dimensionless grouping, now known as the
Gumbel number
,
of the form:
viscosity
speed/load (5.1)
it took the form shown in Figure 5.2. This relationship is widely used to indicate
the mode of lubrication in various bearing forms, since it exhibits little variation
for different configurations.
Boundary lubrication prevails in the left-hand side of the graph, where the
coefficient of friction is essentially independent of both speed and load. Friction
thus obeys the laws of dry friction, with coefficients of friction being
×
0.1.
In themixed lubrication region, where the total friction arises froma combination
of viscous shearing of the lubricant and direct asperity contact, the coefficient of
friction falls as the speed of sliding is increased or the load is decreased. In
this region, the coefficient of friction varies rapidly as the operating parameters
≈
Decreasing lubricant film thickness
Transition from
hydrodynamic and EHD
to boundary lubrication
10
−
1
10
−
2
Boundary
(molecular
dimensions)
Elastohydrodynamic
(EHD)
Hydrodynamic
Boundary
lubrication
10
−
3
Mixed lubrication
Fluid - film lubrication
Viscosity
×
speed
load
Figure 5.2
Stribeck curve and lubrication models.
