shaft-alignment

name	shaft-alignment
description	Rotating machinery shaft alignment — types of misalignment (parallel/angular/combined), rim-and-face method, reverse indicator method, laser alignment (Optalign, Rotalign), thermal growth correction, soft foot detection and correction, alignment tolerances by speed (API 686), four-bolt correction, alignment report, and coupling selection for misalignment tolerance.
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Rotating Machinery Shaft Alignment — Complete Skill

Types of Misalignment

Definitions

Parallel (offset) misalignment: Shaft centerlines are parallel but offset; measured in mm at the coupling faces Characterized by: two bends in shaft system; loads on bearings in shear

Angular misalignment: Shaft centerlines intersect at coupling but at an angle; characterized by one bend Measured in mm/m or mrad

Combined misalignment: Most common real-world condition — both offset and angular simultaneously Both must be corrected simultaneously

Axial (end-float): Excessive axial gap or preload on coupling; not a shaft alignment issue per se — coupling selection

Measurement Methods

Rim-and-Face Method

Setup: Dial indicators on one shaft; rotate shaft with indicators bearing on opposite coupling Rim indicator: measures radial offset (parallel misalignment) at coupling face Face indicator: measures angular misalignment across coupling face

Reading convention: Record at 0°, 90°, 180°, 270° (top, right, bottom, left) Rim total indicator reading (TIR): offset = TIR/2 (eccentricity) Face TIR: angular = TIR / measurement diameter

Limitations: Sag correction required (indicator bracket sag under gravity) Coupling must be rotatable together (both shafts must turn) — difficult for some equipment Not suitable for large gap couplings

Sag measurement: Measure bracket sag with indicators at 12 o'clock position, then free-hang; record sag value (mils) Subtract sag from bottom reading: corrected bottom = raw bottom - sag

Reverse Indicator (Reverse Dial) Method

Setup: Two indicators on different shafts, each reading on the opposite shaft coupling OD Indicator A: reads on B side; Indicator B: reads on A side Rotate shafts together through 360°; record readings at 4 positions

Calculation: Define: A = rim readings on Machine A; B = rim readings on Machine B Offset at Machine B: O_B = (A_bottom - A_top) / 2 Offset at Machine A: O_A = (B_bottom - B_top) / 2

Advantages over rim-face: No face indicator needed (avoids face runout/coupling wobble errors) More accurate for long spans between shaft ends Standard method for industrial rotating machinery per API 686

Example: Machine train: electric motor (fixed) + pump (to be moved) A indicators (on motor, reading pump coupling): top = 0, right = -2, bottom = +4, left = +2 (mils TIR) O_pump_vertical = (4-0)/2 = +2 mil (pump shaft high relative to motor at that plane)

Laser Alignment System

Types: Optalign PLUS (single-laser, rotating reflector); Rotalign Ultra (dual-laser heads) Each head has laser source and detector; heads communicate wirelessly Result: offset and angle in horizontal and vertical planes; correction values (shim and move amounts)

Procedure (laser):

1. Mount brackets and heads on each shaft (coupling halves or shaft adapters)
2. Enter shaft dimensions (span between measurement planes; foot positions from coupling)
3. Rotate shafts to 3–5 positions (12, 3, 6, 9 o'clock)
4. Laser system solves for 3D misalignment
5. Software calculates: Front foot shim Δ_FF, Rear foot shim Δ_RF, lateral move at front, lateral move at rear

Accuracy: ±0.001 mm (1 μm) for high-end systems; 10–100× more accurate than dial indicators Advantage: no bracket sag; automatic thermal offset compensation if known

Alignment Tolerances

API 686 Tolerances (Machinery Alignment Standard)

Offset tolerances vs. coupling type and speed:

Operating Speed	Offset (parallel) [mm]	Angularity [mm/m]
< 1,000 RPM	≤ 0.250	≤ 1.00
1,000–3,600 RPM	≤ 0.050	≤ 0.50
> 3,600 RPM	≤ 0.025	≤ 0.25
> 7,200 RPM	≤ 0.013	≤ 0.125

Note: these are ambient/cold alignment targets; thermal growth must be subtracted from targets

Coupling manufacturer limits: Gear coupling: offset ≤ 0.2° angular; offset capability 1–3 mm (with penalty on life) Flexible disc (Metastream): offset ≤ 0.3 mm per unit; angle ≤ 0.5°/unit Rubber jaw/spider: more tolerant but creates vibration if misaligned > 0.5 mm

Thermal Growth Correction

Purpose

Problem: alignment is set cold; machine operates at elevated temperature → thermal growth changes misalignment Must pre-align to offset position (called "cold alignment target") so that at operating temperature alignment is correct

Thermal growth estimation: ΔH = α × L × ΔT [m; α = thermal expansion coefficient; L = vertical height of shaft centerline above baseplate; ΔT = operating minus ambient] For carbon steel: α = 11.7×10⁻⁶/°C; for pump centerline at 300 mm above base, ΔT = 80°C: ΔH = 11.7×10⁻⁶ × 0.300 × 80 = 0.000281 m = 0.28 mm

Growth data sources: Engineering calculation (approximate); OEM data (more reliable); OPRA (on-site measurement using proximity probes before/after heat-up)

Application: If pump grows upward 0.2 mm more than motor at operating temperature: Cold alignment target: pump shaft 0.2 mm LOW relative to motor (so at temperature they align)

Soft Foot Detection and Correction

Soft Foot Types

Parallel soft foot: one or more feet not in full contact with baseplate (gap exists cold) Angular soft foot: foot plane not parallel to baseplate (one corner raised) Spring soft foot: foot rocks on curved baseplate or debris under foot Induced soft foot: from pipe strain forcing machine out of position

Detection procedure:

1. Tighten all foundation bolts; measure dial indicator on machine casing near each foot
2. Loosen one bolt; record indicator reading; re-tighten; move to next bolt
3. Soft foot: indicator changes > 0.05 mm (2 mils) when bolt loosened
4. Repeat after shimming soft foot; repeat until all feet < 0.05 mm change

Correction: Parallel: add shim equal to gap measured Angular: taper shims (machined) or step shims Pipe strain: correct piping first; then re-check soft foot Maximum allowable soft foot before alignment: < 0.05 mm (0.002") — API 686

Four-Bolt Correction Calculation

Shim Change Calculation

Vertical correction: Shim change at front feet = misalignment_at_front_bearing_plane [mm] Shim change at rear feet = misalignment_at_rear_bearing_plane [mm] (Calculated by similar triangles using foot span and coupling-to-foot distances)

Example calculation: Coupling offset (vertical): +0.10 mm (pump high at coupling); angular: +0.20 mm/m (gap at top) Span from coupling to front feet: L_FF = 200 mm; span to rear feet: L_RF = 600 mm Front shim: δ_FF = offset - angular × L_FF = 0.10 - 0.20×0.200 = +0.06 mm (add 0.06 mm shim) Rear shim: δ_RF = offset - angular × L_RF = 0.10 - 0.20×0.600 = -0.02 mm (remove 0.02 mm shim)

Lateral moves: Same calculation in horizontal direction; use jack bolts to move machine laterally

Shim materials and sizes: Stainless steel shim stock: 0.025 mm to 5.0 mm Minimum shim: 0.025 mm; stack maximum: 4 shims (more creates soft foot due to stacking) For > 3 mm adjustment: use machine shim (solid machined, no stacking)

Alignment Report Content

Required documentation (API 686 Section 2.7):

1. Machine train identification; date; technician; instrument
2. As-found alignment readings (dial or laser; 4 positions)
3. Soft foot readings (as-found and corrected)
4. Thermal growth corrections applied (source; values)
5. Shim changes made (location; amount; from/to shim stack)
6. Lateral moves made
7. Final (as-left) alignment readings
8. Comparison with tolerance
9. Sign-off

Standards and References

Standard	Scope
API 686	Machinery installation and installation design for petroleum, chemical
ISO 10816	Mechanical vibration — evaluation of shaft alignment
ANSI/ASA S2.75	Shaft alignment methodology for rotating machinery
Piotrowski "Shaft Alignment Handbook"	Comprehensive reference
Murray "Alignment of Rotating Machinery"	Practical guide

Output

Provide: machine train description (equipment type; operating speed [RPM]; coupling type; base type: grouted/baseplate), as-found misalignment (measurement method: rim-face/reverse indicator/laser; vertical offset [mm]; horizontal offset [mm]; vertical angularity [mm/m]; horizontal angularity [mm/m]), soft foot (each foot reading change [mm]; any foot > 0.05 mm: corrective action), thermal growth (operating T_motor [°C]; T_pump [°C]; calculated ΔH_motor, ΔH_pump [mm]; cold target offsets), shim corrections (front feet left/right Δ_shim [mm]; rear feet left/right Δ_shim [mm]; lateral moves [mm]; direction), as-left alignment (final readings; offset [mm]; angle [mm/m]), tolerance check (vs. API 686 at operating RPM; pass/fail per plane), and applicable standard (API 686; ISO 10816; alignment method reference).