Page 241 - Improving Machinery Reliability

212 Improving Machinery Reliability

which would have to be used for keyless shafts, and which is reproduced here for the
sake of c~rnpleteness.~~
Since the gear-coupling-equipped shaft is loaded in torsion and potentially bend-
ing, we employ the expression for combined torsion and bending and obtain the fac-
tor of safety (from Equation 3-6):

n= Thus,

1
n= = 1.90
/( ,.95..0..86n)I +3(2.9- 2,180 I 10,900).
52,500 80,000
52,500 80,000
Using the same mathematical expression to calculate the factor of safety for the shaft
equipped with the contoured diaphragm coupling:

The value

1.95 -
( 52,500 80,000 2
122
in the previous expression is so small that applying the equation for pure torsion
(Equation 3-5) would have given the same factor of safety, n = 2.25.

Shaft Still Adequate Under Uprate Horsepower Conditions

At this point, we may want to recall that the preceding analysis was made to deter-
mine whether the steam-turbine shaft would require replacement should the output
horsepower be increased beyond the manufacturer's maximum permissible rating of
17,600 HP. Rather than engaging in a debate as to safe maximum absolute values of
stress in turbine shafts, we merely investigated what factor of safety the shaft design
embodied while equipped with a suitable gear coupling transmitting 17,600 HP at
6,400 rpm:
(63,000) (17,600) = 173,250 lb -in.
Shaft torque T =
6,400
Steady torsional stress 7, = -
16T (16) (173,250) = 9,680 psi
=
7cd3 7c(4.5)3