Page 354 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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or large diameter shafts. Split bearings (Figure 12.1b), on the other hand, facilitate such
assembly and allow adjustment of bearing clearance. They are especially suitable for
supporting heavy elements like crankshafts.
Sliding bearings include radial bearings and thrust bearings according to loading
directions. Radial bearings, more commonly called journal bearings or sleeve bearings,
are cylindrical and carry radial loads perpendicular to the shaft axis. Thrust bearings
are generally flat and carry axial loads parallel to the shaft axis. Although the design and
analysis of thrust bearings is also important, a detailed study of thrust bearings is not
covered here due to limited space.
Lubrication is an important factor in classifying sliding bearings. Accordingly, we
have boundary and hydrodynamically lubricated bearings. In a boundary-lubricated
bearing, the journal and bearing have intermittent contact, that is, only a partial
lubricant film exits between contact surfaces. Boundary-lubricated bearings are used
successfully in light service, low speed or unimportant applications, such as electric
fans, office machinery and home appliances. In a hydrodynamically lubricated bearing,
continuously supplied lubricant is fed by the rotating journal into a wedged-shaped
region, lifting the rotating journal off the stationary bearing. As the journal and bearing
are completely separated, there is no metal-to-metal contact, resulting comparably low
friction. Hydrodynamically lubricated bearings are usually for high rotating speeds
with impact and momentary overloads.
Finally, according to the load carrying mechanism, we have hydrodynamic and
hydrostatic bearings. Unlike hydrodynamically lubricated bearings, lubrication in a
hydrostatic bearing is obtained by continuous supply of externally pressurized lubri-
cant to the bearing interface at a pressure high enough to separate the surfaces with
a relatively thick film of lubricant. The hydrostatic lubrication can achieve extremely
low friction at all times, including at startup and low-speed operation. However, it
requires an expensive and complicated external fluid pressurization system. Hydrostatic
bearings are commonly used in machine tools, huge telescopes and antenna systems,
where slow, smooth and accurate operation is required.

12.2 Working Condition Analysis

The analysis and design of sliding bearings involves substantial knowledge of tribology,
which studies friction, wear and lubrication. Just like strength theory, tribology plays a
significant role in machine element life. For a machine element to function successfully,
design must not only satisfy strength and rigidity requirements, but also tribological
demands. This section will introduce some basic tribological principles that will be used
in sliding bearing design.

12.2.1 Friction
Friction is regarded as a force resisting relative movement between surfaces in con-
tact. Although friction generates frictional heat and causes power losses, not all fric-
tion is undesirable. For instance, belt drives rely on friction to transmit torque and
motion.