CVA Shaft Alignment & Machinery Installation 2 — Questions and Answers
Question 1: What is 'bracket sag' and how does it affect dial indicator alignment readings?
- The thermal deflection of the coupling bracket under heat load
- The downward deflection of the dial indicator bracket due to gravity, which introduces a false reading that must be measured and subtracted (Correct answer)
- The looseness in the dial indicator stem caused by vibration
- The distortion of the indicator mounting bracket due to magnetic fields
Correct answer: The downward deflection of the dial indicator bracket due to gravity, which introduces a false reading that must be measured and subtracted
Bracket sag is the gravitational deflection of the dial indicator arm when it extends outward; this sag adds an error to bottom readings and must be measured independently and subtracted from alignment calculations.
Question 2: In laser shaft alignment, what does the detector (PSD) measure?
- The rotational speed of the shaft
- The position of the laser beam striking its surface, which is used to calculate shaft offset and angle (Correct answer)
- The temperature of the shaft surface
- The vibration amplitude at the coupling location
Correct answer: The position of the laser beam striking its surface, which is used to calculate shaft offset and angle
The position-sensitive detector (PSD) measures the exact X-Y coordinates where the laser beam strikes its surface; changes in beam position as the shafts are rotated are used to calculate misalignment values.
Question 3: What type of misalignment is indicated when vibration phase measurements show a 180° phase difference across the coupling in the radial direction?
- Soft foot
- Parallel (offset) misalignment (Correct answer)
- Resonance
- Looseness
Correct answer: Parallel (offset) misalignment
A 180° radial phase difference across the coupling is a classic indicator of parallel misalignment, as the two shaft centerlines are offset and move in opposition to each other.
Question 4: Which coupling type provides the greatest tolerance for angular misalignment while transmitting torque?
- Rigid flange coupling
- Gear coupling (Correct answer)
- Disc pack (diaphragm) coupling
- Jaw (spider) coupling
Correct answer: Gear coupling
Gear couplings use meshing gear teeth that can accommodate significant angular misalignment (typically up to 1.5° per gear mesh) while transmitting high torque loads.
Question 5: What is 'cold alignment target' or 'alignment offset' in precision machinery installation?
- The maximum permissible misalignment at operating speed
- An intentional misalignment introduced during cold installation to compensate for predicted thermal growth at operating temperature (Correct answer)
- The alignment tolerance for low-speed machinery below 1,800 RPM
- The zero reference position set before taking alignment measurements
Correct answer: An intentional misalignment introduced during cold installation to compensate for predicted thermal growth at operating temperature
A cold alignment target (thermal offset) intentionally misaligns the machine when cold so that, after thermal expansion at operating temperature, the shafts reach the desired aligned position.
Question 6: What is the recommended hold-down bolt tightening sequence to minimize soft foot when installing machinery?
- Tighten all bolts simultaneously using an air wrench
- Tighten diagonally opposite bolts in a star pattern, gradually increasing torque in multiple passes (Correct answer)
- Tighten from front to back, one row at a time
- Tighten the highest bolt first, then work downward
Correct answer: Tighten diagonally opposite bolts in a star pattern, gradually increasing torque in multiple passes
Tightening diagonally opposite bolts in a cross/star pattern and incrementally increasing torque prevents the machine frame from warping and ensures even clamping force across all feet.
Question 7: For a machine running at 3,600 RPM (60 Hz), what is the industry-standard acceptable residual misalignment for precision alignment?
- ≤ 0.010 inch (0.254 mm) offset and ≤ 1.0 mrad angle
- ≤ 0.002 inch (0.05 mm) offset and ≤ 0.5 mrad angle (Correct answer)
- ≤ 0.005 inch (0.127 mm) offset and ≤ 2.0 mrad angle
- ≤ 0.020 inch (0.508 mm) offset and ≤ 3.0 mrad angle
Correct answer: ≤ 0.002 inch (0.05 mm) offset and ≤ 0.5 mrad angle
ISO 10816 and ANSI/ASA S2.75 precision alignment standards for high-speed machinery (3,600 RPM) require residual offset ≤ 0.002 inch and angular misalignment ≤ 0.5 mrad to avoid excessive coupling and bearing loads.
CVA Shaft Alignment & Machinery Installation 2
What is 'bracket sag' and how does it affect dial indicator alignment readings?