Shafts are used in a variety of ways in all types of mechanical equipment. A shaft, usually a
slender member of round cross section, rotates and transmits power or motion. However, a
shaft can have a noncircular cross section and need not be rotating. An axle, a nonrotating
member that carries no torque, is used to support rotating members. A spindle designates a
short shaft. A flexible shaft transmits motion between two points (e.g., motor and machine),
where the rotational axes are at an angle with respect to one another. The customary shaft
types are straight shafts of constant or stepped cross section and crankshafts (Figure 9.1). The
former two carry rotating members such as gears, pulleys, grooved pulleys (sheaves), or other
wheels. The latter are used to convert reciprocating motion into rotary motion or vice versa.
Most shafts are under fluctuating loads of combined bending and torsion with various
degrees of stress concentration. Many shafts are not subjected to shock or impact loading;
however, some applications arise where such load takes place (Section 9.5). Thus, the associated
considerations of static strength, fatigue strength, and reliability play a significant
role in shaft design. A shaft designed from the preceding viewpoint satisfies strength requirements.
Of equal importance in design is the consideration of shaft deflection and
rigidity requirements. Excessive lateral shaft deflection can cause bearing wear or failure
and objectionable noise. The operating speed of a shaft should not be close to a critical
speed (Section 9.7), or large vibrations are likely to develop.
In addition to the shaft itself, the design usually must include calculation of the necessary
keys and couplings. Keys, pins, snap rings, and clamp collars are used on shafts to
secure rotating elements. The use of shaft shoulder is an excellent means of axially positioning
the shaft elements.
Figure 9.2 shows a stepped shaft supporting a gear, a crowned
pulley, and a sheave. The mounting parts, discussed in Section 9.8, as well as shaft shoulders,
are a source of stress raisers, and they must be properly selected and located to minimize
the resulting stress concentrations. Press and shrink fits (Section 9.6) are also used
for mounting. Shafts are carried in bearings, in a simply supported form, cantilevered or
overhang, depending on the machine configuration. Couplings connect a shaft to a shaft of power source or load. Parameters that must be considered in the selection of a coupling
to connect two shafts include the angle between the shafts, transmitted power, vibrations,
and shock loads. The websites www.pddnet.com, www.powertransmission.com, and
www.grainger.com present general information on shaft couplings.
