8.3 Prealignment: Soft Foot, Runout, Pipe Strain, and Shims

Prealignment is not optional

Every hour spent taking alignment readings on a machine that has soft foot, pipe strain, or a bent shaft is wasted. The readings will not repeat, the corrections will not hold, and the machine will be back on the work list within months. The sequence is fixed:

1. Check and correct soft foot.
2. Check shaft and coupling runout.
3. Check and correct pipe strain.
4. Determine the thermal growth allowance and set the cold offset targets.
5. Only then take alignment readings — Chapter 9.

Soft foot

Soft foot means one or more machine feet do not make solid, planar contact with the base. Tightening the bolt then distorts the machine frame, moves the bearings, and changes the shaft position. A machine with soft foot gives different alignment numbers depending on which bolts are tight.

Four kinds:

Measuring it. With all bolts torqued, set a dial indicator on top of one foot, zero it, loosen that bolt only, and read the movement. Retighten and move to the next foot. Any foot that lifts more than about 0.002 inch requires correction. A four-foot machine is checked one foot at a time, all the way around, and rechecked after each correction because fixing one foot changes the others.

Shaft and coupling runout

Before alignment, confirm the two shafts are straight and the coupling hubs are true.

• Mount an indicator against the shaft, near the coupling, and rotate the shaft. This reads shaft bend and bearing condition.
• Mount an indicator against the coupling hub rim and face and rotate. This reads hub bore eccentricity and how squarely the hub sits.
• A common working limit is about 0.002 inch total indicator reading for both, though the manufacturer's tolerance governs.

Excessive runout must be resolved before alignment. Aligning to a bent shaft simply builds the bend into the alignment, and the machine vibrates at running speed no matter what the numbers said.

Pipe strain

Piping that does not fit is pulled into place with the flange bolts, and the force is then transmitted into the pump casing, distorting it and moving the shaft.

Check: with the machine bolted down and aligned, set dial indicators to read the shaft position vertically and horizontally. Loosen the suction and discharge flange bolts one flange at a time and watch the indicators. Movement beyond about 0.002 inch indicates unacceptable pipe strain.

Correct it at the pipe, not at the machine. Re-fit the spool, adjust the pipe supports, check that spring hangers are unpinned and set correctly, and confirm that expansion joints are installed with their control rods adjusted. Shimming the machine to chase pipe strain leaves the casing distorted; the wear rings and seals still see the load.

Thermal growth

Machines are aligned cold and run hot. The alignment must therefore be deliberately offset when cold so that it is correct at operating temperature.

$\Delta L = C \times L \times \Delta T$

where the growth is in inches, C is the coefficient of thermal expansion, L is the length from the foot or anchor point to the shaft centerline in inches, and the temperature change is in degrees Fahrenheit.

Worked example. A hot pump's shaft centerline sits 14 inches above its feet. The casing runs 180°F while the feet stay near 90°F, a change of 90°F. For carbon steel:

$\Delta L = 0.0000063 \times 14 \times 90 = 0.0079 \text{ in}$

The pump shaft will rise about 0.008 inch as it heats. The motor, which stays much cooler, rises far less. The machine is therefore aligned cold with the pump shaft set low relative to the motor by roughly that difference, so that the two come into alignment at temperature.

Where the manufacturer publishes cold alignment targets, use those rather than a calculation — they include effects the simple formula does not capture.

Shims