Millwright Certification Precision Shaft Alignment Techniques 1 — Questions and Answers
Question 1: What type of misalignment exists when two shaft centerlines are parallel to each other but do not share the same centerline?
- Angular misalignment
- Parallel (offset) misalignment (Correct answer)
- Torsional misalignment
- Axial misalignment
Correct answer: Parallel (offset) misalignment
Parallel (offset) misalignment means the two shaft centerlines run in the same direction but are displaced from each other radially. Angular misalignment, by contrast, occurs when the centerlines meet at a non-zero angle. Both types often exist simultaneously and must be corrected separately.
Question 2: What is bracket sag (indicator sag), and how does it affect alignment readings?
- It stiffens the indicator bracket, causing falsely low side readings
- It causes the indicator to droop under gravity, making the bottom reading appear more negative than the true shaft position — requiring a sag correction to be subtracted (Correct answer)
- It only affects laser alignment targets, not dial indicators
- It is a permanent deformation of the shaft caused by heavy couplings
Correct answer: It causes the indicator to droop under gravity, making the bottom reading appear more negative than the true shaft position — requiring a sag correction to be subtracted
When a dial indicator and its bracket are cantilevered off a shaft, gravity causes the assembly to sag downward. This makes the indicator read a more negative (lower) value at the 6 o'clock position than the actual shaft relationship warrants. Sag must be measured and mathematically corrected before calculating moves.
Question 3: What is a 'cold alignment target' (also called a cold offset or cold correction), and why is it used?
- A target used only when aligning equipment in freezing temperatures
- An intentional misalignment introduced during cold alignment so the machine will be in true alignment at normal operating temperature (Correct answer)
- A shim thickness specification from the equipment manufacturer
- A dial indicator zeroing procedure performed before the machine is started
Correct answer: An intentional misalignment introduced during cold alignment so the machine will be in true alignment at normal operating temperature
Many machines grow vertically and horizontally as they reach operating temperature. A cold alignment target intentionally offsets the shaft positions in the cold state by a calculated amount equal to predicted thermal growth, so the machine achieves true alignment only after warming up to operating conditions.
Question 4: Why do higher-speed rotating machines require tighter shaft alignment tolerances than lower-speed machines?
- Because lubricants are less viscous at high speeds, increasing friction
- Because misalignment-induced vibration forces and bearing loads increase with rotational speed, accelerating wear and reducing service life (Correct answer)
- Because high-speed shafts expand more axially than low-speed shafts
- Because couplings become more rigid at elevated RPM
Correct answer: Because misalignment-induced vibration forces and bearing loads increase with rotational speed, accelerating wear and reducing service life
Dynamic forces generated by misalignment grow with rotational speed. Even a small offset or angular error produces significantly higher cyclic stresses on bearings and couplings at 3600 RPM than at 900 RPM. This is why alignment tolerance charts reference tighter allowable limits as speed increases.
Question 5: When taking dial indicator readings during shaft alignment, why should both the driver and driven shafts be rotated together as a single unit?
- To check for bearing preload
- To eliminate errors caused by shaft runout and coupling eccentricity, ensuring readings reflect only the positional relationship between the two shaft centerlines (Correct answer)
- To measure the axial float of each bearing
- To confirm that the coupling is properly lubricated before startup
Correct answer: To eliminate errors caused by shaft runout and coupling eccentricity, ensuring readings reflect only the positional relationship between the two shaft centerlines
If only one shaft rotates while the indicator rides on the other, eccentricity in the coupling hub or runout in the shaft will contaminate the readings. Rotating both shafts as a unit means the indicator sees only the change in relative position between the two centerlines, which is the true alignment condition.
Question 6: What effect does pipe strain (also called pipe stress) have on precision shaft alignment?
- It has no effect once the coupling is installed
- It can pull or push the machine casing after alignment is complete, distorting the shaft positions and negating the alignment work (Correct answer)
- It only affects the driven machine, never the driver
- It increases thermal growth predictably and can be compensated by standard cold targets
Correct answer: It can pull or push the machine casing after alignment is complete, distorting the shaft positions and negating the alignment work
Pipe strain occurs when connected piping exerts forces or moments on a pump, compressor, or other machine casing. These external loads can shift or twist the casing after the alignment is verified, invalidating the work. Piping must be checked for strain — typically by loosening flanges and observing indicator movement — before finalizing alignment.
Millwright Certification Precision Shaft Alignment Techniques 1
What type of misalignment exists when two shaft centerlines are parallel to each other but do not share the same centerline?