This Standard takes into account many modern theories and has
been substantiated, in part, by laboratory testing.
This Standard is not intended as a textbook. It is written for
use by those skilled in the art of shaft design and stress
calculations. This Standard provides a common basis for discussion
and understanding between persons involved in shaft design.
A design procedure is presented for computing the diameter of
either hollow or solid rotating steel shafts under combined cyclic
bending and steady torsional loading for unlimited life. The method
is based on an elliptical variation of fatigue strength with
increasing torque as exhibited by combined stress fatigue data.
The corrected reversed-bending fatigue limit of the shaft to be
designed is used in the recommended design formula. It is
calculated from the fatigue limit of the material from rotating
beam specimen data, such as that listed in Appendix B. Fatigue
modifying factors are cited to correct the polished, unnotched
specimen data for a number of service factors. These service
factors include surface condition, size, reliability, temperature,
duty cycle, fatigue stress, concentration, and miscellaneous
Although the present design procedure ensures that the shaft is
properly sized to provide adequate service life, it is not the only
shaft design consideration. A shaft must also be stiff enough to
limit deflections of key power transfer elements such as gears and
pulleys, and also have sufficient stiffness to minimize
misalignment through bearings and seals. Furthermore, the shaft
must have sufficient radial and torsional stiffness or lack of
stiffness to avoid unwanted vibrations at the operating speed,
These factors have not been included in this Standard,although
checking them is good design practice. Calculation procedures for
these factors may be found in most machine design textbooks. An
example of how to calculate shaft deflection and critical speed is
given in Appendix C.
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