Sign-Off on a 3,600 rpm Pump/Motor Alignment
A mechanic has finished aligning a 100 hp (75 kW), 3,600 rpm motor to a centrifugal pump using a laser alignment kit and wants your signature so the unit can go back in service. His report, taken with the machine at ambient temperature, shows 0.010 in parallel offset (motor high) and 0.003 in per 10 in of angular misalignment at the coupling. The site tolerance for this machine is 0.002 in offset and 0.0005 in/in angular. While checking, loosening the right-rear motor foot bolt makes the indicator rise 0.006 in. The pump casing runs at 180 °F and the motor at 120 °F; the vendor lists thermal growth of +0.002 in for the pump and +0.004 in for the motor. Motor feet are 12 in (front) and 32 in (rear) from the coupling center.
1. The mechanic says the 0.006 in soft foot is "close enough" and that he will re-check it after the machine is bolted down. Do you accept the job? Explain what soft foot is doing to his readings and the order in which the work must be done.
Read the worked answer
Reject the sign-off. 0.006 in is roughly three times the 0.002 in alignment tolerance for this machine, so the alignment numbers cannot be trusted.
Why it matters:
- Soft foot means one foot does not sit flat. When you torque that bolt, you bend the motor frame or the baseplate, which tilts the shaft and changes both the offset and the angular reading the laser just measured.
- The reading is not stable - it moves depending on which bolts are tight - so any shim calculation made now is built on a moving target.
Correct order of work:
- Check all four feet one at a time (one bolt loose, the others torqued). A foot that rises more than about 0.002 in is soft.
- Find the cause: burr or paint on the foot, worn shim, bent foot, corroded baseplate, pipe strain on the pump, or a distorted frame. Fix the cause - do not just stack shims on top of a burr.
- Correct with the fewest shims possible, re-torque, and re-check all four feet until the soft foot is within tolerance.
- Only then perform rough alignment, then final alignment, and re-verify soft foot after final torque.
2. Using his readings and the thermal data, calculate the shim correction at the front and rear motor feet. State the direction of correction and show the arithmetic, including any assumptions.
Read the worked answer
Step 1 - angular correction at each foot plane (angular error × distance from coupling ÷ 10 in):
- Front foot: 0.003 × 12/10 = 0.0036 in
- Rear foot: 0.003 × 32/10 = 0.0096 in
Step 2 - add the parallel offset (motor is high, so shims come out):
- Front: 0.010 + 0.0036 = 0.0136 in
- Rear: 0.010 + 0.0096 = 0.0196 in
Step 3 - apply thermal growth. At operating temperature the motor grows 0.004 in and the pump 0.002 in, so the motor rises about 0.002 in relative to the pump. To arrive at line-hot, the motor must be set about 0.002 in low when cold, which means removing an extra 0.002 in of shim:
- Front ≈ 0.0136 + 0.002 = 0.016 in removed
- Rear ≈ 0.0196 + 0.002 = 0.022 in removed
Assumptions and cautions:
- Readings were taken cold, so the thermal correction applies. If they had been taken hot, it would not.
- These are starting values, not final values. Re-measure after shimming; never chase the numbers in one pass.
- Check that 0.016 in and 0.022 in of shim actually exist under those feet. If not, something else is wrong (raised base, bad grout, wrong spacer) - do not force it with a hammer or by leaving a foot floating.
- Soft foot must be corrected before any of this is applied.
3. The mechanic re-measures 0.001 in offset and 0.0004 in/in angular and wants to start the pump immediately. What must still be confirmed before sign-off, and what do you check after 30–60 minutes of running?
Read the worked answer
Before start-up:
- Soft foot re-verified after final torque, not just after shimming.
- Coupling condition: correct gap/end-float per the coupling manufacturer, no worn elastomer, guard in place.
- All foot bolts torqued to spec and lockouts/nuts installed; shims clean, flat, and no more than a few thin shims under one foot.
- No pipe strain on the pump flanges; check flange gaps before bolting up suction and discharge.
- Confirmation that the readings were taken with the machine at the same condition (cold) as the reference.
After 30–60 minutes at operating temperature:
- Re-shoot the alignment hot and compare against the thermal target - the goal is line-hot near zero, not cold near zero.
- Check bearing temperatures and vibration baseline (overall velocity and 1X/2X content). A 3,600 rpm machine should be smooth; a rise in 2X often points to residual angular error or soft foot.
- Look for foot bolt loosening, shim migration, or paint cracking at the baseplate - signs the frame is being stressed.
- Record the cold and hot readings so the next alignment starts from known thermal behavior.