Page 146 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

growth or shrinkage is understood, the few extra minutes re-
quired will assure the best possible final alignment.
Metals, like most materials, expand when heated. The
amount they expand is expressed as the coefficient of thermal
expansion. This coefficient is expressed as the change in length per
degree of temperature rise. This is expressed mathematically as:

c = dL/dT (8.1)

By rearranging Equation (8.1) the amount of thermal growth
for a given length (L) can be expressed as:

dL = c × dT × L (8.2)

Equation (8.2) states the change in length is equal to the dif-
ference in temperature times the coefficient of thermal expansion
times the original length.
In general, most steels or cast iron frames have a coefficient
of thermal expansion of about .0063 mils per inch per degree Fahr-
enheit, with aluminum and bronze being about twice that amount.
Table 8-3 lists the coefficient of expansion in mils per inch per
degree Fahrenheit, for some common metals for use between 32°F
and 212°F.

Table 8-3. Coefficients of Expansion
————————————————————————————————
COEFFICIENT OF
MATERIAL EXPANSION
————————————————————————————————
Soft Forged Iron .0063
Cast Iron .0059
Soft Rolled Steel .0063
Hardened Steels .0056
Nickel Steel .0073
Aluminum .0094
Bronze .0100
————————————————————————————————