Page 243 - Improving Machinery Reliability

214 Improving Machinery Reliability

conventional heat-shrink methods, while hubs with larger bore diameters are proba-
bly best suited for hydraulic dilation fit-up.

Torsional Holding Requirement Must Be Defined

There is no uniformly accepted design practice governing either the fit-up integri-
ty or torsional holding requirement of coupling hubs on equipment shafts. One
school of thought opts for interference dimensions which will ensure slip-free trans-
mission of rated turbocompressor torque. Others believe that the interference-fit
dimensions and, in some cases, the entire coupling design should allow safe trans-
mission of the maximum allowable torque value for a given shaft material and nomi-
nal shaft diameter. The two approaches are illustrated in Figure 3-88.
Graphically representing the mean torsional stress T,,, as a function of torque T,

Figure 3-88 also shows typically accepted maximum allowable torque values for
various shaft diameters made of the more common turbomachinery shaft steels, AISI
1040, 4140, and 4340. Assuming a turbocompressor absorbing 7,000 hp at 4,500
rpm (155 hp/100 rpm) were to incorporate a four-inch diameter, AISI 4140 shaft, its
rated torque would be

Figure 3-88. Shaft torque versus stress for typical shaft sizes.