Deflection is not affected by strength,
but rather by stiffness as represented by the modulus of elasticity,
which is essentially constant for all steels. For that reason,
rigidity cannot be controlled by material decisions, but only by
geometric decisions.
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
inexpensive, 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 treatment
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 nominal 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 concentricity 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 quantity 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. Stainless steel may be appropriate for some environments.
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