Page 362 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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steady-state position, offset from the loading direction, with an eccentricity e,between
the centre of bearing and the centre of journal, as shown in Figure 12.7c.
To sum up, a journal bearing experiences boundary, mixed-film and hydrodynamic
lubrication from startup to normal operation. At a normal operating speed, the rotating
journal draws a sufficient amount of lubricant of certain viscosity into a wedge-shaped
zone between the bearing at a velocity high enough to create a pressure large enough to
lift the journal off the bearing. Thus, the external load applied to the journal bearing is
supported by a continuous film of oil lubricant.

12.2.5 Potential Failure Modes
Typical failure modes of a sliding bearing include abrasive wear, adhesive wear, scuffing,
galling and seizure, fatigue and corrosion.
Foreign particles from operating environment, oxidized wear particles or grit may
cause scraping or scratching on the mating element surfaces. In particular, during
startup, shutdown or when a journal contacts with a bearing, abrasive wear will
accelerate, causing an impaired profile and increased clearance, leading to malfunction
of the bearing. Abrasive wear can be reduced by using filters in lubrication systems.
When a sliding bearing carries a heavy load or the supplied lubricant is insufficient, the
lubricant film may be so thin that metal-to-metal contact occurs at the bearing interface.
Combined with continuous relative motion of the journal and bearing, adhesive wear
may occur, leading to the damage of bearing surfaces. In serious cases, adhesive wear
results in scoring, scuffing, galling or even seizure.
When a sliding bearing is subject to a cyclic load or a cyclic small amplitude sliding,
cracks may generate and fatigue or fretting wear may occur. Finally, acid forma-
tion during oxidation of a lubricant may induce unacceptable corrosion on bearing
surfaces.

12.3 Load Carrying Capacities

12.3.1 Boundary-Lubricated Bearings
The design of boundary-lubricated bearing is largely an empirical process based on doc-
umented user experiences. The primary design variables of unit bearing load p, sliding
velocity v and their product pv should be within allowable values.
Unit bearing load p is the ratio of the applied bearing load F to the projected area of a
bearing, that is, the product of journal diameter d and bearing width B,thatis,
F
p = ≤ [p] (12.3)
dB
Because excessive velocity may speed up the wear process, the linear velocity of jour-
nalmustbewithinallowablevaluesand is expressedas
dn
v = ≤ [v] (12.4)
60 × 1000
In addition to the individual consideration of load capacity, p, and sliding velocity,
v,the pv factor is an important variable for the evaluation of temperature rise and