Page 140 - Improving Machinery Reliability

112 Improving Machinery Reliability

dence of resonances with the excitation sources over a wide speed range; therefore,
stress calculations must be made to evaluate the adequacy of the system response.

Reciprocating Machinery. For reciprocating units such as compressors, pumps, or
engines, the harmonic excitation torques must be calculated and applied at the appro-
priate shaft location to calculate the stresses.25
Allowable Torsional Stresses. The calculated torsional stresses must be compared
to applicable criteria. The allowable values given by Military Standard 164 are
appropriate for most rotating equipment. The allowable zero-peak endurance limit is
equal to the ultimate tensile strength divided by 25. The MIL Spec uses this as a
global derating factor rather than calculating on the basis of individual factors
accounting for keyways, surface roughness effects, and the like. When comparing
calculated stresses to this value, the appropriate stress concentration factor and a
safety factor must be used. Generally, a safety factor of 2 is used for fatigue analysis.
When these factors are used, it can be shown that fairly low levels of torsional stress
can cause failures, especially when it is observed that the standard keyway (USA
Standard ANSI B17.1) has a stress concentration factor of 3.
We should not lose sight of the fact that process machinery is expected to live
much longer than military hardware, and that our process machinery manufacturer
has, perhaps:
1. no S-N curves
2. no intention of applying individual derating factors for either known stress rais-
ers or unknown superimposed stresses
3. no interest in determining coupling and misalignment-induced stress adders, etc.,
It would thus be reasonable to use a global derating value of 75, and, indeed,
world-class turbomachinery manufacturers such as Elliott, Dresser-Rand, Mannes-
mann-Demag, Sulzer, Mitsubishi, and surely many others, have both the experience
and analytical capability to virtually guarantee unlimited life of turbomachinery
shafts operating at relatively much higher mean torsional stresses. A typical example
would be steam turbine shafts with tensile strengths of 105,000 psi (ult.) and steady-
state torsional stresses of 10,900 psi, where this latter stress simply uses the standard
calculation formula

Nevertheless, 2,,,,/75 is not at all unreasonable for machines built by the “other”
manufacturers. A midwestern U.S. plant uses rotary blowers direct-coupled to 200
hp, 1800 rpm motors. The blowers came with 2% inch diameter shafts that had an
ultimate strength in tension of 80,000 psi. Although nominal stresses are thus only
2281 psi, the plant experienced many shaft failures with derating values as high as
80,000/2281 = 35. A typical torsional stress allowable thus becomes the ultimate ten-
sile strength divided by 75.