Thermal-growth compensation and shim correction on a 2970 rpm pump drive
A 55 kW, 2-pole motor (2970 rpm) drives a centrifugal pump through a flexible coupling. The pump is hot at operating temperature; the motor is cold. A laser alignment check gives a parallel offset of +0.18 mm (motor shaft centreline above the pump centreline at the coupling) and an angular error of 0.10 mm per 100 mm, with the motor shaft rising towards the rear feet. The motor feet are 250 mm (front) and 650 mm (rear) from the coupling centre. The pump casing grows upward by 0.15 mm at the coupling when at operating temperature, and the motor is expected to grow 0.02 mm. The motor front feet have 0.70 mm of shims, the rear feet have 1.20 mm. The technician proposes to remove 0.18 mm from all feet and “call it done”.
1. Calculate the required cold parallel offset and the shim changes at the front and rear motor feet. State any assumption about thermal growth and sign convention.
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Take positive as the motor centreline above the pump centreline. At the coupling, the measured offset is +0.18 mm. When hot, the pump grows +0.15 mm and the motor +0.02 mm, so the motor must be set lower cold by 0.15 − 0.02 = 0.13 mm. The target cold offset at the coupling is therefore −0.13 mm, and the required change at the coupling is −0.13 − 0.18 = −0.31 mm.
The measured angular error is +0.10 mm per 100 mm, with the motor rising towards the rear. At the front feet, 250 mm from the coupling, the angular contribution is 0.10 × 250/100 = +0.25 mm. At the rear feet, 650 mm away, it is 0.10 × 650/100 = +0.65 mm. Assuming the target angular error is zero and thermal growth does not change the slope:
Front shim change = −0.31 − 0.25 = −0.56 mm (remove 0.56 mm). Rear shim change = −0.31 − 0.65 = −0.96 mm (remove 0.96 mm).
This leaves 0.14 mm at the front and 0.24 mm at the rear, which is feasible with the existing shim packs.
2. The technician proposes removing 0.18 mm from all feet. Explain why this is wrong and describe the correct alignment sequence.
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Removing 0.18 mm from every foot only tries to cancel the parallel offset at the coupling. It leaves the 0.10 mm/100 angular error untouched and ignores the 0.13 mm thermal growth requirement. At 2970 rpm, that residual angular error will load the coupling and bearings and will show up as vibration. The equal-shim approach also assumes the feet are coplanar and the readings are free of soft foot, which has not been checked.
The correct sequence is: isolate and lock off; check and correct soft foot with a dial indicator or the laser; clean the feet and shim stacks; take a cold alignment reading; calculate thermal compensation and angular correction; adjust the shims; torque the hold-down bolts in the correct sequence; recheck the alignment; run the unit to operating temperature and take hot readings. If the hot readings differ from the prediction, adjust again. Do not use the coupling to absorb misalignment.
3. What acceptance criteria would you set for the cold and hot alignment, and what would you check before signing off?
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For a 2970 rpm machine with a flexible coupling, a reasonable target is a cold parallel offset within 0.05 mm and an angular error within 0.03 mm per 100 mm, with the thermal growth compensated. The hot running check should confirm that the offset remains within about 0.05 mm and the angular error within about 0.03 mm/100 mm; the manufacturer’s limits take precedence if they are tighter.
Before sign-off, verify soft foot is no more than about 0.02 mm at each foot, the shim stacks are clean and have no more than three shims per foot, and all bolts are torqued. Check the coupling gap and element condition, run the machine, and record vibration and bearing temperature at steady state. If the alignment cannot be achieved because of insufficient shims or distorted feet, machine the feet or correct the base rather than accepting a compromise.