Pump Alignment: Flexible Coupling and Shaft Alignment

Why Alignment Matters

A pump and motor connected by a flexible coupling must share a common rotational centerline as closely as possible. Even a flexible coupling tolerates misalignment only within narrow limits; it does not eliminate the effects of poor alignment. Misalignment causes excessive vibration, accelerated bearing wear, seal leakage, coupling failure, and shortened equipment life. Achieving correct alignment at installation — and re-checking it after any maintenance — is one of the highest-value skills in industrial maintenance.

Types of Misalignment

Parallel misalignment (also called offset misalignment) exists when the two shaft centerlines are parallel but not collinear — they are offset side to side or up and down. It is measured as the distance between the two centerlines at the coupling face.

Angular misalignment exists when the two shaft centerlines meet at an angle rather than running parallel. Think of a shallow V-shape between the two shafts.

Most real installations have a combination of both. The alignment procedure corrects each in the vertical plane first, then in the horizontal plane.

Soft-Foot: Correct It First

Before measuring anything at the coupling, check for soft-foot — a condition in which one or more motor feet do not make full, solid contact with the base plate. Soft-foot causes the shaft centerline to shift unpredictably when you tighten or loosen hold-down bolts, making precise alignment impossible.

To detect soft-foot: snug all four hold-down bolts, then loosen one bolt at a time while watching a dial indicator placed on the motor housing or shaft. A reading change greater than 0.002 in when a bolt is loosened indicates soft-foot at that foot. Correct soft-foot by inserting the correct thickness of shim stock under the low foot before proceeding. Never proceed to precision alignment with uncorrected soft-foot.

Rough Alignment: Straightedge and Feeler Gauge

Rough alignment brings the shafts close enough that the precision dial indicator method can be applied safely and effectively. The target for rough alignment is typically within 0.010 in parallel offset and angular gap visible to the eye.

1. Place a precision straightedge across the tops and sides of both coupling hubs. The straightedge should contact both hubs evenly with no visible gap on either side.

2. Slide a feeler gauge into any gap between the straightedge and either hub. A gap greater than 0.010 in requires correction.

3. Adjust motor position by adding or removing shim stock under the motor feet for vertical correction, or by tapping the motor laterally with a soft mallet for horizontal correction.

4. Re-check with the straightedge and feeler gauge after each adjustment.

5. Confirm rough alignment is within 0.010 in before mounting dial indicators.

Precision Alignment: Rim-and-Face Method

The rim-and-face method uses two dial indicators simultaneously — or one repositioned — to measure both parallel offset (rim reading) and angular misalignment (face reading).

Setup:

• Lock out and tag out the equipment per your facility's LOTO procedure before mounting any indicators or reaching near the coupling. Lockout/tagout (LOTO) is required by OSHA 29 CFR 1910.147 and must be completed before work begins.
• Mount one dial indicator so its plunger contacts the rim (outside diameter) of the stationary coupling hub. This indicator measures parallel offset.
• Mount a second indicator so its plunger contacts the face (flat end surface) of the stationary coupling hub. This indicator measures angular misalignment.
• Zero both indicators at the 12 o'clock (top) position.

Taking readings:

Rotate the movable shaft (motor side) through four clock positions: 12 o'clock (top), 3 o'clock (right side), 6 o'clock (bottom), and 9 o'clock (left side). Record all indicator readings on the alignment data form at each position. Keep the stationary shaft fixed; rotate only the shaft carrying the indicator fixtures so both indicators sweep simultaneously.

Interpreting readings:

• The rim indicator shows total indicator runout (TIR) for parallel offset. Half the TIR at 6 o'clock (bottom reading with the top zeroed) equals the actual parallel offset.
• The face indicator shows angular misalignment. Half the TIR at 6 o'clock equals the angular gap across the coupling diameter.

Correcting misalignment:

Add or remove shim stock under the rear motor feet to correct angular misalignment; add or remove equal shims under all four feet to correct parallel vertical offset. After each correction, re-rotate and re-read. Repeat until both readings are within the tolerance on the manufacturer's coupling specification sheet — commonly ±0.002–0.005 in depending on coupling type and operating speed.

PPE and Safety Rules for This Lab

• Safety glasses are required from entry to exit.
• Apply LOTO before touching the coupling or mounting any indicator. Verify zero energy before any hand contact near the shaft.
• Never reach across a rotating shaft. Rotate the shaft by hand only after confirming LOTO is in place.
• Keep shim stock edges away from fingers; edges are sharp — use gloves when handling raw shim stock.
• Report any cracked or bent coupling hubs to the instructor immediately; do not operate.

Completing the Lab

Record every reading, every shim change, and the final result on your alignment data form. The instructor will verify your final alignment with a reference dial indicator setup. Both parallel and angular misalignment must be within the manufacturer's stated tolerance or you will repeat the exercise. Accurate record-keeping is as important as the alignment itself — in industry, alignment records are kept with the equipment maintenance history.