Page 242 - Improving Machinery Reliability

Machinery Reliability Audits and Reviews 213

Alternating torsional stress z, = (0.2) (9,680) = 1,940 psi

MF = (173,250) (0.3) = 52,000 lb-in.
MT = (173,250) (sin 0.057) = 173 lb-in.

Mtotal = -,/(25,025)2 + (52,000 + 173)’

Mtota! = 57,864 lb-in.
(57,864) (2.25) 64
0, = = 6,472 psi
(n:) (4.514

(173,250) (0.03)
0, = = 770 psi
(4.5) n: (2.25)2 0.94
1
n=
1,940 9,680
52,500 80,000 52,500 80,000
1.95 6,472 + 770)2 + 3 (2.9 ~
= 2.14
The shaft factor of safety, while certainly adequate for operation at 17,600 hp with
gear-type couplings, was thus shown to be only 2.14 in the original design case with
conventional gear couplings. Equipping the same shaft with contoured diaphragm
couplings made operation at 19,600 hp not only possible, but actually increased the
factor of safety to 2.25.46

Reliable Shaft-Hub Connections for Turbomachinery Couplings

Coupling hubs for turbocompressors must fit very tightly on compressor shafts.
Not only does the interference fit have to be high enough to prevent slipping at maxi-
mum applied torque, but potentially serious weakening of shafts due to fretting
action must also be avoided. The term “fretting” describes component damage that
occurs at the interface between contacting, highly loaded metal surfaces when sub-
jected to extremely small relative (or vibratory) motion. On the other hand, the cou-
pling hub must be designed for easy removal in case rapid access to compressor-
shaft seals should become necessary.
Satisfying both of these requirements does not generally present any serious prob-
lems for equipment shafts up to approximately three inches nominal diameter. It
does, however, require progressively more attention and design sophistication as
shaft sizes reach eight or more inches on cracked gas and propylene compressors in
modern ethylene plants.
Keyless coupling-hub engagement is a logical choice for many of these applica-
tions. Hubs up to about three inches nominal bore can be effectively mounted by