Machinery's Handbook, 31st Edition
SHAFT ALIGNMENT 2725 In Fig. 8 observe the rim indicator settings at positions A, B, C, and D. Indicators at posi tions A and B are set correctly, indicators at C and D are not. A slight tilt in the mounting of the indicators will make too little difference to matter, but any tilt over a few degrees should be avoided. Just as important, the face indicator should be set perpendicular to the coupling face with the centerline of the indicator stem parallel to the axis of the fixed unit coupling shaft. Fig. 9a shows a setup with rim and face indicators set at odd angles that will cause incor rect readings. Note the difference between the arrangement of Fig. 9a, and the correct one shown in Fig. 9b.
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c Fig. 9b. Correct Positioning of Dial Indicators
c Fig. 9a. Incorrect Positioning of Dial Indicators
Preparations.— Before starting an alignment process several important factors need to be considered, among them the following. Is temperature rise in the coupled machinery a consideration? What is the indicator sag of the dial indicators in use? Is coupling runout a consideration? Are there improperly supported or “soft feet” in the movable machine? Are the motor feet, jack bolts, and shims clean? Can the movable unit be moved later- ally far enough to permit alignment, or is movement restricted by bolts (“bolt bound”) or nearby obstructions? Is the driving unit powered off and locked out? Thermal Effects: Metals increase in length as temperature increases, and decrease in length as temperature decreases. Rotating equipment at rest slowly attains the temperature of the surrounding environment, the ambient or room temperature. In service, machinery such as electric motors and fuel-burning engines generate heat and increase in temperature during operation; other machinery such as pumps may warm or cool during operation. If initial alignment adjustments are made when machines are at ambient or room tem perature, thermal movement will occur in various parts of the machines as they reach operating temperature. The various dimensions will not increase or decrease uniformly. In a motor-pump combination, for example, the motor shaft height above the base may increase while the pump shaft height may decrease. The amount of thermal expansion can be estimated by the formula Δ e = α H Δ T : where Δ e = change in height of shaft H = vertical height or distance between shaft centerline and shims α = expansion coefficient of material in μ in/in- ° F (for stainless steel 7.4, mild steel 6.3, cast iron 5.9, bronze 10, and aluminum 12.6). Note: 1 m in/in-°F = 1.8 m m/m-°C Δ T = the difference between operating temperature and ambient (initial measure ment) temperature Example 1(a): A pump is to be installed and aligned with a motor. The cast iron pump delivers fresh water at 34 ° F and is at present at 75 ° F. The pump shaft centerline is 12 inches above the baseline of the pump. The carbon steel motor is also at 75 ° F and is expected to rise to 140 ° F when in operation. The motor shaft centerline is 12 inches above the base of the motor. At operating temperature, compare the expected thermal growth of the pump and motor shaft centerlines. For the pump, the change in shaft height: . . e H T 0 0000059 12 34 75 0 002903 inch # # ∆ = α ∆ = − = − ^ h
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