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Shafts and Shaft Components 361
Necessary strength to resist loading stresses affects the choice of materials and
their treatments. Many shafts are made from low carbon, cold-drawn or hot-rolled steel,
such as ANSI 1020-1050 steels.
Significant strengthening from heat treatment and high alloy content are often not
warranted. Fatigue failure is reduced moderately by increase in strength, and then only
to a certain level before adverse effects in endurance limit and notch sensitivity begin
to counteract the benefits of higher strength. A good practice is to start with an inex-
pensive, low or medium carbon steel for the first time through the design calculations.
If strength considerations turn out to dominate over deflection, then a higher strength
material should be tried, allowing the shaft sizes to be reduced until excess deflection
becomes an issue. The cost of the material and its processing must be weighed against
the need for smaller shaft diameters. When warranted, typical alloy steels for heat treat-
ment include ANSI 1340-50, 3140-50, 4140, 4340, 5140, and 8650.
Shafts usually don’t need to be surface hardened unless they serve as the actual
journal of a bearing surface. Typical material choices for surface hardening include
carburizing grades of ANSI 1020, 4320, 4820, and 8620.
Cold drawn steel is usually used for diameters under about 3 inches. The nom-
inal diameter of the bar can be left unmachined in areas that do not require fitting of
components. Hot rolled steel should be machined all over. For large shafts requiring
much material removal, the residual stresses may tend to cause warping. If con-
centricity is important, it may be necessary to rough machine, then heat treat to
remove residual stresses and increase the strength, then finish machine to the final
dimensions.
In approaching material selection, the amount to be produced is a salient factor. For
low production, turning is the usual primary shaping process. An economic viewpoint
may require removing the least material. High production may permit a volume-
conservative shaping method (hot or cold forming, casting), and minimum material in
the shaft can become a design goal. Cast iron may be specified if the production quan-
tity is high, and the gears are to be integrally cast with the shaft.
Properties of the shaft locally depend on its history—cold work, cold forming,
rolling of fillet features, heat treatment, including quenching medium, agitation, and
tempering regimen. 1
Stainless steel may be appropriate for some environments.

7–3 Shaft Layout
The general layout of a shaft to accommodate shaft elements, e.g., gears, bearings, and
pulleys, must be specified early in the design process in order to perform a free body
force analysis and to obtain shear-moment diagrams. The geometry of a shaft is gener-
ally that of a stepped cylinder. The use of shaft shoulders is an excellent means of
axially locating the shaft elements and to carry any thrust loads. Figure 7–1 shows an
example of a stepped shaft supporting the gear of a worm-gear speed reducer. Each
shoulder in the shaft serves a specific purpose, which you should attempt to determine
by observation.

1 See Joseph E. Shigley, Charles R. Mischke, and Thomas H. Brown, Jr. (eds-in-chief), Standard Handbook
of Machine Design, 3rd ed., McGraw-Hill, New York, 2004. For cold-worked property prediction see
Chap. 29, and for heat-treated property prediction see Chaps. 29 and 33.