3.6.5
1. Determine the temperature differential AT; °F (± from room temperature).
where the differential expansion is the total diameter change required. It includes the inference fit plus the slip clearance.
Some common coefficients of thermal expansion are listed in Table 3.2.
2. Calculate the desired expansion and shrinkage to find temperature change required for 1020 CRS, where the shaft OD is 01.2500 and the rotor ID is 01.2480.
These diameters give a 0.002-in interference fit. The minimum desired slip fit clearance is 0.003 in, and the differential expansion is 0.005 in.
The temperature change AT required on these parts to give 0.005-in expansion is calculated as follows.
A 615.38°F change in temperature is required. Therefore, the total temperature would be 615.38°F plus ambient (72°F in this case). One could heat the rotor to 687.38°F (364.10°C).The shaft temperature could be reduced to shrink the shaft in order to reduce the heat needed for expansion of the rotor.
For instance, cool the shaft to -75°F (-59°C), and heat the rotor to 540.38°F (282.43°C) to get the required deferential expansion.
3. Another method is to use the maximum change in temperature to determine differential expansion. The total possible change in temperature using dry ice at -100°F (-73°C) to cool the shaft and an oven at 700°F (371°C) to heat the rotor is 800°F (444°C).
Differential expansion = AT (coefficient of the thermal expansion) (shaft OD) = (800°F) [0.0000065 in/(in A°F)] 1.250 in = 0.0065 in
0.0065 in – 0.002 in interference = 0.0045 in clearance at these temperatures.
For practical purposes, one may use a dry ice temperature of -75°F and an oven temperature of 650°F. This allows for the extremely fast warming of the shaft and cooling of the rotor while assembling.
A minimum of 0.003 in clearance was calculated for all fits. The usual finished interference between the parts ranges from 0.0005 to 0.003 in.
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