Page 247 - Improving Machinery Reliability

218 Improving Machitiety Reliability

steam are customary methods. The temperature required to achieve a given amount
of expansion can be calculated from:
e
AT=- (3-12)
ad
where e = diametral expansion (interference) in inches; a = coefficient of linear
expansion, inch per inch per OF; and d = initial diameter of the bore (before expan-
sion) in inches. Figure 3-86 represents this relationship graphically, with a assumed
to be 6.3 x low6 inch per inch. It should be noted that the AT values of Figure 3-91
must be added to the ambient temperature of the shaft when fitting a hub. Also, ease
of assembly may make it desirable to thermally expand the hub beyond the theoreti-
cally required expansion value (solid line) to an “easy assembly” value (dotted line).
Our earlier example might illustrate this point.
Here, a four-inch diameter AIS1 shaft transmits a maximum permissible torque of
125,664 Ib-in. This torsional holding requirement is to be achieved with a coupling
having a Ld2 = 56. According to Figure 3-89, the required press-fit pressure is
12,000 psi. Assuming further that the coupling d/D ratio is 0.68, Figure 3-90 shows a

Figure 3-91. Temperature change above ambient required to increase hub bore.