1. The three types of misalignment
Parallel misalignment, also called offset, occurs when the two shafts (pump and motor) are parallel in space — keep the same direction — but are displaced relative to each other laterally or vertically. Visually, it is like two parallel straight lines at different heights or at different lateral spacings. This type of misalignment generates cyclic radial load on bearings at every shaft rotation, manifesting mainly as vibration at the 1× rotation frequency (1×N).
Angular misalignment occurs when the two shafts are not parallel — they form an angle between them, converging or diverging at the extremities. Even if the coupling centers are aligned at a point, the shafts diverge along the set length, and the effect on bearings is a cyclic axial load that excites vibration modes at the 2× rotation frequency (2×N). In severe cases, it generates audible metallic noise and burnout of the elastic coupling in a few hours.
Axial misalignment, also called incorrect gap, is the distance between coupling faces outside manufacturer recommendation. Each coupling model (Falk, Renold, Lovejoy, John Crane Thomas) has a specific nominal gap value to respect — typically between 3 and 10 mm depending on size. Gap too small generates constant axial load on the pump thrust bearing; gap too large leaves the coupling under-engaged, with torque transmission loss and premature wear of elastic elements.
In practice, any motor-pump set presents combination of the three types simultaneously, and all three must be corrected in the same procedure. Correct order is: first axial gap (defines motor longitudinal position), then angular (corrects tilt) and finally parallel (corrects offset). Moving only one without redoing the others is the source of almost all poorly done alignment failures.
2. FB Bombas practical tolerances
Alignment tolerance depends on coupling type and operating speed. Rigid couplings (key-flange, flange-flange) do not absorb misalignment and require very tight tolerance: 0.02 to 0.05 mm TIR for sets up to 1,800 rpm, and 0.01 to 0.03 mm above that. Flexible couplings (elastomeric elements like Falk Wrap-Flex, Lovejoy Jaw, or claw couplings like Renold) absorb some fraction of residual misalignment and allow 0.08 to 0.15 mm TIR tolerance.
FB Bombas adopts as practical reference for FBCN, FBOT, FBE and FBEI sets: 0.05 mm TIR for rigid coupling and 0.10 mm TIR for flexible coupling, in both planes (parallel and angular), measured with pump and motor at ambient temperature.
In pumps operating with hot fluid (FBOT at 250-350 °C), thermal growth of the set displaces the pump shaft upward by values known to the manufacturer — typically 0.1 to 0.3 mm depending on temperature — and cold alignment must compensate this growth by positioning the motor slightly below target, so that the set is aligned in operation.
| Coupling type | Parallel (mm) | Angular (mm/100mm) | Speed (rpm) |
|---|---|---|---|
| Rigid | ≤ 0.05 | ≤ 0.03 | ≤ 1,800 |
| Flexible | ≤ 0.10 | ≤ 0.06 | ≤ 1,800 |
| Flexible high speed | ≤ 0.05 | ≤ 0.03 | > 1,800 |
3. Method 1 — Dial indicator (rim-and-face)
Dial indicator method is the classic, accurate and relatively cheap method — requires two or three dial indicators with magnetic base, adjustable supports and operator training. The rim-and-face configuration uses two indicators: one points radially to the opposite-side coupling hub (measures parallel misalignment), another points axially to the coupling face (measures angular misalignment).
Both shafts are rotated together at 0°, 90°, 180° and 270°, and readings at the four positions allow calculating vertical and horizontal corrections to apply to the motor feet.
Practical method accuracy is excellent (TIR of 0.01 to 0.02 mm) and it remains as the reference standard even after laser popularization. Limitations are time required (30 to 60 minutes per alignment by experienced operator) and sensitivity to support radial play — always verify before starting with "bar sag check": mount supports with indicators pointing down at distance equivalent to actual alignment, rotate 180° and measure how much the support bar deflects from its own weight.
This value is discounted from all subsequent readings.
4. Method 2 — Laser alignment
Laser alignment uses two optical sensors mounted on coupling hubs: the emitter projects a beam against the detector on the opposite side, and the software calculates in real time the parallel, angular and gap misalignment values in both planes. Reference manufacturers are SKF, Pruftechnik (Rotalign), Easy-Laser and Fluke. The big differentiator over dial indicator is speed: a complete alignment is done in 10 to 20 minutes by an operator with basic training.
FB Bombas recommends laser method for routine preventive maintenance in plants with multiple pump-motor sets — initial equipment investment (USD 6,000 to 20,000 depending on model) is offset in a few months by reduced machine downtime. For commissioning critical pumps (FBOT in powerhouse, FBEI in thermal fluid system, FBCN in fire under NFPA-20), laser method is also the standard choice for traceability — all reports are electronically recorded with date/time and measured values, facilitating audit and future technical support.
5. Method 3 — Straightedge + feeler gauge (emergency field)
The method of ground steel straightedge resting on coupling hubs, combined with feeler gauge to check the face, is the emergency field method. Rest the straightedge on the two hubs along the axis: if there is parallel misalignment, a gap appears under the straightedge on one side, measurable with feeler gauge.
Angular misalignment is verified with feeler gauge inserted at four points of the coupling face (12 o'clock, 3, 6, 9) — variation greater than tolerance indicates angular misalignment in that direction.
Accuracy is limited — typical steel straightedge has own deviation of 0.1 to 0.3 mm over 300 mm lengths, and feeler gauge depends on operator tactile sensitivity. The method is acceptable only as initial approximation to bring the set "into the field" of correct alignment, before final verification with dial indicator or laser.
In emergency maintenance without instruments available, this method is better than nothing — but the pump should not return to continuous operation without later alignment by more precise method.
6. Step-by-step alignment procedure
FB Bombas standard procedure for aligning any pump-motor set is: (1) shut down and lock electrically and mechanically (lockout/tagout); (2) remove elastic coupling elements, keeping only the hubs; (3) verify base flatness (frame or skid) with precision spirit level — the base is the absolute reference, any deviation propagates to the whole set; (4) verify and correct pump leveling and fixation (the pump is the reference, almost never moved); (5) using pump as reference, move only the motor; (6) start with axial gap: adjust motor longitudinal position to gap value specified by coupling manufacturer; (7) correct angular misalignment by adjusting shims under motor feet; (8) correct parallel misalignment by moving motor laterally; (9) retighten all bolts with correct torque; (10) reinstall elastic coupling elements; (11) measure again after final tightening — tightening may introduce residual deviation, must be confirmed; (12) record final values on standard datasheet.
7. Validation by ISO 10816 vibration
After alignment, the pump is started and vibration is measured on bearings (radial horizontal, radial vertical and axial) with portable vibration analyzer.
ISO 10816 establishes four severity zones according to machine category; for 15 to 300 kW industrial pumps on rigid foundation, limits are: zone A (newly commissioned and in optimal condition, vrms < 2.3 mm/s), zone B (acceptable long-term operation, 2.3 to 4.5 mm/s), zone C (restricted operation with monitoring program, 4.5 to 7.1 mm/s), zone D (unacceptable vibration, > 7.1 mm/s).
Well-aligned pump-motor set should operate in zone A right after commissioning. Persistent vibration in zone B/C/D after well-done alignment indicates other causes: residual impeller imbalance (recalibrate per ISO 21940 G2.5, FB Bombas standard), bearing play, foundation resonance, or partial cavitation. Vibration measurement at specific 1×N frequency (rotation) indicates imbalance or parallel misalignment; at 2×N indicates angular misalignment; at high frequencies indicates bearing problem. A modern analyzer provides the complete spectrum and allows precise root-cause diagnosis.
