Millwright Certification Precision Shaft Alignment Techniques 2 — Questions and Answers

Question 1: What is soft foot, and why must it be corrected before performing shaft alignment?

It is a type of foundation crack
It occurs when one or more machine feet do not sit flat on the base, causing the frame to distort when bolts are tightened (Correct answer)
It is wear on the shaft coupling
It is a vibration frequency

Correct answer: It occurs when one or more machine feet do not sit flat on the base, causing the frame to distort when bolts are tightened

Soft foot occurs when a machine foot does not make full contact with the baseplate, causing the machine frame to flex when the hold-down bolt is tightened, which distorts bearing bores and creates unpredictable alignment changes.

Soft foot is analogous to a wobbly table. When a mounting bolt is tightened on a foot that does not sit flat, it pulls the machine frame down, distorting the bearing housings and shaft position. Types include: parallel soft foot (gap under the foot, corrected with shims), angular soft foot (foot rocks on a point, corrected with tapered or stepped shims), reactive soft foot (caused by pipe strain or coupling forcing the machine), and induced soft foot (one foot pulling another through frame stiffness). Check by loosening each bolt one at a time and measuring movement with a dial indicator. Acceptable soft foot is typically less than 0.05 mm.

Question 2: In reverse dial indicator alignment, what does each indicator measure?

Shaft speed and torque
Each indicator measures the radial position of the opposite shaft, giving both offset and angularity data (Correct answer)
Shaft diameter and bearing clearance
Temperature and vibration levels

Correct answer: Each indicator measures the radial position of the opposite shaft, giving both offset and angularity data

In reverse indicator alignment, two dial indicators are mounted on opposite shafts pointing at each other. Each reads the runout of the opposite shaft, and the combined data from both reveals both offset and angular misalignment.

Reverse dial indicator alignment uses two indicators mounted on brackets attached to each shaft, with each indicator reading the opposite shaft's rim or coupling surface. As both shafts are rotated together through 0, 90, 180, and 270 degree positions, each indicator provides four readings. The mathematical combination of both indicators' data yields both parallel offset and angular misalignment in both vertical and horizontal planes. The key requirement is that both shafts must rotate together. Bracket sag must be measured and compensated in the vertical calculations.

Question 3: What is thermal growth, and how is it accounted for during cold alignment?

Expansion of the coupling during operation
Predictable increase in machine height as components reach operating temperature, compensated by offsetting cold alignment targets (Correct answer)
Growth of the foundation concrete
Expansion of alignment shims

Correct answer: Predictable increase in machine height as components reach operating temperature, compensated by offsetting cold alignment targets

Thermal growth is the vertical and horizontal expansion of machine casings and bearing pedestals as they heat to operating temperature. Cold alignment targets are deliberately offset to ensure correct alignment at running temperature.

As machines reach operating temperature, their casings, bearing housings, and supports expand. A pump handling 200 degree C fluid may grow 0.5 mm vertically at the bearing centerline compared to ambient temperature. If aligned to zero offset when cold, the pump would be significantly misaligned when hot. Thermal growth targets are calculated based on: material coefficient of thermal expansion, temperature difference from ambient to operating, and the distance from the base to the shaft centerline. Growth (mm) = coefficient times delta-T times height.

Question 4: Why is it important to use proper shim practices during alignment?

Shims are decorative
Incorrect shimming introduces errors: corroded shims, too many shims, or wrong thickness shims prevent accurate and stable alignment (Correct answer)
Shims reduce vibration automatically
Shims are only used for appearance

Correct answer: Incorrect shimming introduces errors: corroded shims, too many shims, or wrong thickness shims prevent accurate and stable alignment

Poor shimming practices (using too many shim layers, corroded shims, bent shims, or inadequate shim area) create soft foot conditions, compress over time, and prevent the machine from maintaining its aligned position.

Shim best practices include: limit shim stack to 3-5 shims maximum per foot (more layers increase compressibility and soft foot risk); use pre-cut stainless steel shims (resist corrosion unlike mild steel); ensure shims fully cover the foot area; check shims are flat and not bent, kinked, or burred; remove old paint and rust from feet and base before shimming; and use the minimum number of shims possible (replace multiple thin shims with fewer thick ones when possible). Corroded or compressed shims are a leading cause of alignment drift.

Question 5: What alignment readings at the 12, 3, 6, and 9 o'clock positions indicate pure parallel (offset) misalignment with no angularity?

All readings are zero
The 12 and 6 readings are equal and opposite, and the 3 and 9 readings are equal and opposite (Correct answer)
The 12 reading is maximum and all others are zero
All four readings are identical

Correct answer: The 12 and 6 readings are equal and opposite, and the 3 and 9 readings are equal and opposite

Pure parallel offset produces equal and opposite readings at the 12/6 and 3/9 positions because the indicator traces a circle that is offset from center but not tilted, creating symmetric deflections.

In rim (radial) readings, the dial indicator traces the runout of one shaft relative to the other. Pure parallel offset means the shafts are parallel but displaced. This produces a sinusoidal pattern: if offset is purely vertical, the 12 o'clock reading is maximum positive and 6 o'clock is maximum negative, with 3 and 9 readings being zero. The offset amount is (TIR)/2 = (12 reading - 6 reading)/2. Most real-world conditions are a combination of both offset and angularity.

Question 6: What is the primary benefit of performing precision alignment on rotating machinery?

It makes the machine look better
It dramatically extends bearing, seal, and coupling life while reducing vibration and energy consumption (Correct answer)
It increases the machine's rated speed
It eliminates the need for lubrication

Correct answer: It dramatically extends bearing, seal, and coupling life while reducing vibration and energy consumption

Precision alignment reduces cyclic forces on bearings and seals, minimizes coupling wear, decreases vibration, and can reduce energy consumption by 2-17%, resulting in significantly longer component life and reduced downtime.

Studies consistently show that precision alignment provides substantial returns: bearing life can increase 2-8 times, seal life increases similarly, coupling life extends 5-10 times, vibration decreases significantly, and energy consumption drops 2-17%. The cost of precision alignment is typically recovered within months through reduced spare parts, less downtime, and lower energy bills. Modern standards suggest alignment within 0.05 mm offset and 0.05 mm/100 mm angularity for most industrial machinery.

Millwright Certification Precision Shaft Alignment Techniques 2

What is soft foot, and why must it be corrected before performing shaft alignment?