Page 357 - Analysis and Design of Machine Elements

Sliding Bearings
coalesce at and near the surface. Materials at the surface then easily spall out, degrad- 335
ing the element surface by either small deep pits or large shallow pits, releasing
work-hardened particulate contaminates [7]. Fatigue wear is a prevalent form of failure
in such machine elements as rolling contact bearings, gears and so on.
Fretting wear occurs at oscillatory sliding contact surfaces under large loads. It causes
damage at the asperities of contact surfaces, producing increased surface roughness and
micropits. Typical sites of fretting damage include interference fits, bolts, keyed and
splined joints.
Corrosive wear is undesired oxidation or rusting of metal surface caused by the com-
bined mechanical, chemical or electrochemical interaction with the environment. A typ-
ical example is the deterioration of lubricated surfaces in sliding bearings.
Wear process includes three stages. The first stage is break-in wear or a running-in
process, a beneficial process that allows wear to occur so that mating surfaces can adjust
to each other to provide smooth running. After running-in, the tips of asperities in con-
tact become flattened. During the second stage, wear increases with the operating time.
The time span at this stage decides product life. The last stage is destructive and is char-
acterized by a rapidly increased wear rate that leads to failure.
The severity of wear can be alleviated by reducing contact forces, relative speeds and
mating surface properties. And the most important is to maintain continuous lubrica-
tion between rubbing surfaces to prolong element life and postpone the advent of the
destructive wear stage.

12.2.3 Lubrication
The objective of lubrication is to reduce friction, wear and friction generated heat by
introducing a lubricant, usually petroleum-based mineral oil, between contact surfaces.
Lubricant films are usually assumed to be Newtonian fluids, following Newton’s law of
viscous flow.

12.2.3.1 Newton’s Law of Viscous Flow
In Figure 12.4, plate A moves at a velocity v on a film of a lubricant with thickness h.As
force F is applied on the plate, shear stress will generate within the lubricant. Assume

Figure 12.4 Laminar flow between parallel plates.