Motor–Pump Alignment After a Bearing Replacement
After a bearing replacement, a 15 kW, 2-pole (2970 rpm) centrifugal pump and its motor were re-coupled with an elastomeric coupling. Vibration at the motor drive-end bearing is 8.2 mm/s RMS overall, with a strong 2× running-speed component (99 Hz) and axial vibration of 4.1 mm/s. Dial indicator readings with the motor as the movable machine: parallel offset 0.12 mm (motor high) and 0.09 mm (motor to the right); face readings at a 100 mm diameter show 0.06 mm more gap at the top than at the bottom. A soft-foot check shows a 0.15 mm gap under the rear-left foot when that bolt is released. Motor feet span 320 mm. The pump casing runs at 78 °C while the motor frame is at 41 °C. The vendor alignment target is 0.05 mm parallel and 0.03 mm per 100 mm angular. Suction piping was re-bolted and pulls approximately 0.2 mm when the flange bolts are loosened. Production wants the unit back on line before the pump seals leak again.
1. Interpret the vibration and indicator data. State the dominant fault, the other faults that must be excluded before you align, and why the 2× component alone is not conclusive.
Read the worked answer
- 8.2 mm/s is above 7.1 mm/s, which puts this machine in ISO 10816-3 zone D (unacceptable) for a 15 kW class machine.
- A dominant 2× with high axial vibration (4.1 mm/s) is the classic signature of misalignment: parallel offset produces 2× radial, angular misalignment produces strong axial 1× and 2×.
- But 2× is not proof. Soft foot, a bent shaft, coupling wear, and mechanical looseness all produce 2× or its harmonics. A 0.15 mm soft foot is already confirmed by the check, and a soft foot distorts the frame and invalidates any alignment reading taken with the feet as found. Fix it first.
- Coupling: inspect the elastomeric element for heat, cracking, or wear, and check the axial gap against the vendor's dimension; a worn or wrong-gap coupling mimics misalignment axially.
- Pipe strain: a 0.2 mm flange pull is four times the 0.05 mm target; if the pump casing moves when the flange is loosened, the piping is springing the pump and any alignment will be lost.
- Looseness: check foot bolts, baseplate cracks and grout. A phase check across the coupling that shows 2× radial and axial 180° out of phase supports misalignment.
Order the work: pipe strain, soft foot, then alignment.
2. Give the correction sequence and the shim/move calculation for the vertical and horizontal errors, using the foot span. State the intermediate target you would work to.
Read the worked answer
- Pipe strain. Put a dial indicator on the pump coupling while loosening the suction flange. If the casing moves more than 0.05 mm, re-support or re-fit the piping before aligning; otherwise the pipe pulls the pump back after you finish.
- Soft foot. Shim the rear-left foot to below 0.03 mm, torque diagonally in stages, and re-check all four feet, since correcting one foot often changes another.
- Angular (vertical). 0.06 mm over 100 mm = 0.0006 mm/mm = 0.6 mm/m. Over the 320 mm foot span that is 0.6 × 0.32 = 0.19 mm of correction at the rear feet, added or removed in the direction that closes the top gap. The rear pair moves roughly 0.19 mm relative to the front pair.
- Parallel (vertical). Motor 0.12 mm high, so remove 0.12 mm at the front feet; at the rear feet apply 0.12 mm plus or minus the 0.19 mm angular component. In practice: correct the angular first, re-read, then remove the parallel shim.
- Horizontal. Move the motor 0.09 mm to correct the parallel offset, plus the horizontal angular component from side face readings. Use jacking bolts and check both planes after each move, because a horizontal move can disturb the vertical reading.
- Target. Aim finer than the vendor: 0.03 mm parallel and 0.02 mm per 100 mm angular, because the 37 °C metal temperature difference will move the machine once it runs.
- Thermal growth. The pump at 78 °C grows more than the motor at 41 °C, so the motor must be set low by the calculated or vendor-specified cold offset, not aligned cold to zero.
- Re-torque the coupling and feet, then re-measure before start-up.
3. What must you verify before signing the job off, and how would you confirm the alignment is still correct when the unit is hot?
Read the worked answer
- Final dial indicator readings recorded in both planes, against a tighter target than the vendor's, with date, ambient temperature, and the technician's name.
- Coupling gap and element condition checked against the vendor drawing; guard refitted.
- All four feet torqued to spec and soft foot re-checked after torquing.
- Pipe flanges checked for strain with the machine cold; supports fitted if there was movement.
- Start the unit, let it reach steady temperature (about 30–45 minutes at 78 °C casing temperature), and repeat the dial indicator readings. Compare against the calculated thermal growth.
- Take vibration in horizontal, vertical, and axial at motor drive end, motor non-drive end, and pump bearings. Expect below 2.8 mm/s, well inside zone A/B, not merely below the 7.1 mm/s limit.
- Check bearing temperatures, seal leakage, coupling temperature, and motor current against the nameplate value.
- If the hot readings drift outside target, shut down, cool, and correct the cold offset rather than accepting a hot misalignment.
- Record the results on the PM sheet and schedule a re-check after two weeks, since a new bearing and fresh grout settle.
Do not sign off on the basis of the vibration reading alone: a machine can run smooth at the bearing while the seal is being loaded by pipe strain or thermal growth.