Cold Alignment of a 3,600 rpm Motor to a Hot-Growing Pump
A 3,600 rpm, 200 hp motor drives a centrifugal pump through a 6-inch-diameter flexible coupling. Laser alignment, with the motor as the movable machine and a 10-inch sensor separation, shows the motor shaft centerline 0.010 in low at the coupling center, and the motor shaft sloping downward toward its rear at 0.0012 in per inch. The motor front feet are 10 in from the coupling center and the rear feet are 30 in. The pump vendor's data says the pump centerline rises 0.006 in from cold to hot. The coupling manufacturer's tolerance at this speed is 0.002 in offset and 0.0005 in/in angularity. Shim stock on hand: 0.001, 0.002, 0.005, 0.010, 0.020, and 0.050 in stainless.
1. Calculate the cold shim packs for the front and rear feet, and state whether the as-found condition is within tolerance.
Read the worked answer
Step 1 - Is the as-found condition acceptable?
- Offset: 0.010 in found vs 0.002 in allowed = 5x the limit.
- Angularity: 0.0012 in/in found vs 0.0005 in/in allowed = 2.4x the limit.
- No. Both are out, in the vertical plane. The horizontal plane must be corrected as well and checked separately.
Step 2 - Shim needed to fix offset and slope (before thermal).
Shim at distance d from the coupling center = offset + (slope x d)
- Front feet (d = 10 in): 0.010 + (0.0012 x 10) = 0.010 + 0.012 = 0.022 in
- Rear feet (d = 30 in): 0.010 + (0.0012 x 30) = 0.010 + 0.036 = 0.046 in
Step 3 - Thermal growth correction.
If the pump rises 0.006 in hot, the motor must be set 0.006 in high cold so the two centerlines meet at operating temperature. Add 0.006 in uniformly (assuming parallel growth at the coupling plane):
- Front: 0.022 + 0.006 = 0.028 in
- Rear: 0.046 + 0.006 = 0.052 in
Step 4 - Build the stacks with the fewest shims.
- Front: 0.020 + 0.005 + 0.002 + 0.001 = 0.028 in
- Rear: 0.050 + 0.002 = 0.052 in
Note the rear stack is the clean one. If the front stack must be reduced to three shims, order 0.023 or 0.028 in shim stock rather than stacking four.
2. Before you trust these numbers and start shimming, what conditions and checks must be satisfied?
Read the worked answer
The arithmetic is only as good as the setup. Verify in this order:
- Soft foot first. Check each foot one at a time with a dial indicator, target 0.002 in or less. Correcting soft foot after shimming means doing the alignment twice, and soft foot can also mimic an angular misalignment in the readings.
- Pipe strain. Loosen or disconnect the suction and discharge flanges and watch the alignment readings. If they move, the piping is springing the pump and must be re-fitted, not shimmed around.
- Reading validity. Check shaft and coupling runout, bracket sag, sensor mounting, and that the laser has a clear line of sight and is within its stated accuracy. Confirm the units and the sign convention before doing any math.
- Thermal data. The 0.006 in must come from vendor data or a hot-to-cold measurement, and you need to know whether both machines grow and in which direction. Horizontal growth matters too.
- Physical limits. Check that the motor feet are not already at the top of the jacking screws, that the baseplate is grouted and sound, and that the motor can be moved horizontally with the jacking bolts.
- Document and repeat. Record the as-found and as-left readings, torque the hold-downs to spec in a cross pattern, and re-check after tightening because tightening can move the machine.
3. After the work, the running machine shows 0.003 in of vertical offset at operating temperature and vibration is 0.15 in/s overall with 2x running speed dominant. What do you do next?
Read the worked answer
Do not simply re-shim on suspicion. Separate the possibilities with data.
What 2x running speed usually means:
- Angular or parallel misalignment that is still present.
- Looseness or a soft foot that opens up when the machine is hot.
- A coupling that is worn, binding, or has the wrong gap or spacer length.
- Occasionally a structural or foot resonance, which shows up at 2x but with a phase or impact signature that changes with speed.
Checks:
- Compare hot vs cold. You aligned cold with a 0.006 in thermal target. Re-read the machine hot. If it is 0.003 in off hot, either the thermal estimate was wrong or something moved after tightening. The correct next action is to measure at temperature, not to guess.
- Re-check soft foot hot, and re-check hold-down torque. A foot that is tight cold can lift when the baseplate grows.
- Look at the full spectrum and phase. 2x with strong axial 2x and 180-degree phase across the coupling points at misalignment. Look for harmonics of 2x and a 1x component that suggests looseness.
- Inspect the coupling for wear, fretting, or an incorrect gap.
- Compare with baseline. If the machine ran at 0.05 in/s after a previous alignment, the 0.15 in/s is a real change regardless of the number.
Decision: document the hot readings, correct the known error against the measured hot offset rather than the cold estimate, and re-run. If 2x persists after a verified hot alignment with no soft foot and a good coupling, look at the piping and foundation.