The right tolerance is the one from the set's manufacturer and the coupling's, and it tightens with speed. Without it, the manual for the SKF TKSA 20 laser alignment tool gives a guideline table: from 1,000 to 2,000 rpm, up to 0.10 mm offset and 0.08 mm per 100 mm angular deviation; from 3,000 to 4,000 rpm, 0.05 mm and 0.06 mm per 100 mm. The FBEI manual MAN001-10 quotes, from the coupling catalog, radial deviation below 0.05 mm and angular below 0.6°. Align all three quantities — axial distance, angle and offset — with a dial indicator or laser, and repeat the measurement after tightening the anchor bolts and connecting the piping: the pump leaves the factory pre-aligned, but transport and installation misalign the set.
At a glance
- Pump-motor alignment corrects three simultaneous misalignments — parallel, angular and axial — preserving bearings, mechanical seal and coupling.
- Tolerance tightens with speed: without a manufacturer value, the SKF TKSA 20 table accepts 0.10 mm offset and 0.08 mm per 100 mm from 1,000 to 2,000 rpm.
- On a dial indicator, the rim reading is twice the true offset, and the angle is the face-reading difference divided by the measuring diameter.
- In hot service, align cold while offsetting thermal growth α × L × ΔT: 250 mm of shaft height and a 50 K rise lift the shaft about 0.14 mm.
- Under ISO 10816-7, a general-duty pump up to 200 kW should stay at or below 3.2 mm/s when commissioned and 5.1 mm/s in unrestricted continuous operation.
1. The three types of misalignment
Parallel misalignment, or offset, happens when the pump and motor shafts point in the same direction but are displaced from each other vertically, horizontally or in both planes. It is expressed in millimeters, measured at the coupling plane. It imposes a cyclic radial load on the bearings at every shaft revolution.
With a dial indicator, the reading on the hub rim is twice the true offset, because the indicator travels both sides of the shaft: a total indicated variation of 0.20 mm corresponds to 0.10 mm of displacement.
Angular misalignment happens when the centerlines form an angle with each other. It is expressed in degrees or, more commonly in the field, in millimeters of gap per 100 mm of length — 0.6° is about 1.05 mm per 100 mm, because tan 0.6° ≈ 0.0105. On a face reading, divide the difference between opposite readings by the measuring diameter: a 0.06 mm difference on a 120 mm circle gives 0.05 mm per 100 mm.
The effect on the bearings is a cyclic axial load that wears the coupling and the seal.
Axial misalignment is the distance between the coupling faces, or between the shaft ends, outside the coupling manufacturer's value. Each model and size has its own distance — it is in the coupling catalog and on the set's drawing. Too small a distance pushes the pump shaft against the motor and overloads the thrust bearing; too large a distance reduces the engagement of the flexible element and speeds up wear.
On a motor with shaft end float, measure with the rotor in the running position indicated by the motor manufacturer.
In practice, every set has all three deviations at once, in the vertical and horizontal planes, and the corrections interact: shimming one motor foot changes angle and offset together. The usual sequence is to set the axial distance first, then correct the vertical plane with shims under the motor feet and, last, the horizontal plane, by moving the motor sideways. At the end, measure everything again with the bolts tight, because tightening also moves the motor.
2. Alignment tolerances by speed
Tolerance tightens with speed: the faster the shaft turns, the more load cycles per minute the deviation imposes on the bearings. The governing reference is the machine manufacturer's, which accounts for the whole set; the coupling manufacturer states what the coupling withstands, which is usually looser. Without a specific value, the manual for the SKF TKSA 20 alignment tool gives a table of acceptable maximum misalignment, which it presents as a rough guideline and which is reproduced below.
A flexible coupling accommodates small deviations in operation, but that does not justify poor alignment — the FBEI manual itself makes this point.
The FBEI manual MAN001-10 quotes, from the coupling catalog, radial deviation below 0.05 mm and angular below 0.6°. The radial value matches the table's 3,000 to 4,000 rpm band; the angular value, about 1.05 mm per 100 mm, is the coupling's limit and sits more than ten times above the machine guideline for 1,750 rpm (0.08 mm per 100 mm) — for bearing life, aim at the machine table.
In hot service, align cold with a target that offsets thermal growth: shaft height grows by roughly α × L × ΔT. For steel (α ≈ 11.5 × 10⁻⁶ /K), 250 mm from base to centerline and a 50 K rise of the support, the shaft rises about 0.14 mm. The actual value depends on the construction and on each side's temperature; confirm it on the drawing or with engineering before applying it.
| Speed | Maximum offset | Maximum angular | Inch equivalent |
|---|---|---|---|
| 0 to 1,000 rpm | 0.13 mm | 0.10 mm per 100 mm | 5.1 mils; 1.0 mil/in |
| 1,000 to 2,000 rpm | 0.10 mm | 0.08 mm per 100 mm | 3.9 mils; 0.8 mil/in |
| 2,000 to 3,000 rpm | 0.07 mm | 0.07 mm per 100 mm | 2.8 mils; 0.7 mil/in |
| 3,000 to 4,000 rpm | 0.05 mm | 0.06 mm per 100 mm | 2.0 mils; 0.6 mil/in |
| 4,000 to 6,000 rpm | 0.03 mm | 0.05 mm per 100 mm | 1.2 mils; 0.5 mil/in |
3. Method 1 — Dial indicator (rim-and-face)
The dial indicator method is the classic one: accurate and relatively inexpensive, it requires two or three dial indicators with magnetic bases, adjustable brackets and a trained technician. The rim-and-face setup uses two indicators: one points radially at the opposite coupling hub (measuring parallel misalignment) and the other points axially at the coupling face (measuring angular misalignment).
Both shafts are turned together to 0°, 90°, 180° and 270°, and the readings at the four positions are used to calculate the vertical and horizontal corrections to apply at the motor feet.
Before measuring, check the bracket sag: mount the bracket with the indicator at the same span as the real alignment, on a rigid pipe, zero with the indicator on top and rotate 180°. The bottom reading is the bracket's own deflection, which must be subtracted from the vertical readings.
Also confirm that the readings add up — the sum of the side readings should equal the sum of the top and bottom readings — and repeat the full turn when they do not. With the readings at the four positions and the distances from indicator to motor feet, you calculate the shim correction and the lateral move for each foot.
4. Method 2 — Laser alignment
Laser alignment uses measuring units fixed to each shaft or hub: the system reads the relative position of the shafts as they rotate and calculates, in the vertical and horizontal planes, the offset, the angle and the correction at each motor foot. The systems also help detect soft foot and let you enter the thermal growth compensation, so you can align cold while aiming at the hot condition.
The distance between coupling faces is, as a rule, still measured separately, with a caliper or feeler gauge.
Its advantage over the dial indicator is speed and records: the correction comes out calculated on the spot, repeating the measurement after each adjustment is easy, and the report is saved with date, values and the tolerance used. That matters in plants with many sets and on critical pumps, such as fire pumps, where commissioning traceability counts.
With any instrument, enter the distances between sensors, coupling and feet correctly, and define which machine stays fixed — normally the pump, tied to the piping — and which one moves.
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.
The FBEI manual accepts a straightedge and feeler gauge when a dial indicator or laser cannot be used, observing the gap recommended by the coupling manufacturer. The method's limit is resolution: the straightedge only shows a visible gap and the feeler gauge depends on the user's touch, so it serves to bring the set close to alignment, not to prove 0.05 mm.
Use it in an emergency and redo the alignment with a dial indicator or laser before leaving the pump in continuous operation.
6. Step-by-step alignment procedure
Work sequence: (1) switch off and lock out the drive and isolate pressure and fluid; (2) check the base — level, anchor bolts tight and grout cured; (3) check that the piping meets the nozzles in line, without forcing the pump; (4) check for soft foot by loosening one foot at a time with an indicator on it and shimming any foot that lifts; (5) keep the pump as the fixed machine and move the motor; (6) set the axial distance from the coupling catalog; (7) correct the vertical plane with shims under the motor feet; (8) correct the horizontal plane by moving the motor; (9) tighten the bolts in the specified sequence and torque; (10) measure again — tightening moves the motor; (11) turn the shaft by hand at least one revolution to check that it turns freely, then refit the flexible element and the coupling guard; (12) record the final values, the tolerance used and the thermal condition of the measurement.
7. Vibration after alignment: what ISO 10816-7 covers
After start-up, vibration shows whether the set was assembled well. ISO 10816-7 evaluates industrial rotodynamic pumps above 1 kW by r.m.s. velocity, in mm/s, measured on the bearing housings, and separates category I (high reliability, such as toxic liquids or critical service) from category II (general use), with different limits up to and above 200 kW.
According to the summary published by Europump, a newly commissioned category II pump up to 200 kW should stay at or below 3.2 mm/s (zone A); unrestricted continuous operation goes up to 5.1 mm/s (zone B), and above 8.5 mm/s vibration can damage the machine (zone D). For category I up to 200 kW, the limits drop to 2.5, 4.0 and 6.6 mm/s.
With a flexible coupling, the standard applies to the pump; the motor is assessed under ISO 20816-3, which replaced ISO 10816-3. Positive displacement pumps — such as the FBE and FBEI gear pumps — are outside its scope.
That is why the overall vibration reading confirms, but does not replace, the geometric measurement. Record vibration right after commissioning, in the radial horizontal, radial vertical and axial directions, and use that value as the baseline: an increase over time calls for investigation even before the zone changes.
When a spectrum is available, misalignment usually shows up as components at 1× and 2× running speed and high axial vibration; unbalance, looseness, base resonance and cavitation produce other signatures and have their own diagnoses. If vibration stays high with the alignment within tolerance, the cause is elsewhere.