433A Alignment and Levelling 4 — Questions and Answers
Question 1: What instrument is commonly used to check the flatness of a large baseplate or sole plate?
- Dial indicator on a magnetic base
- Precision machinist level and straightedge (Correct answer)
- Laser shaft alignment system
- Ultrasonic thickness gauge
Correct answer: Precision machinist level and straightedge
A precision machinist level placed on a straightedge spanning the baseplate surface is the standard method for checking flatness and level of large baseplates and sole plates during installation.
Question 2: What is pipe strain and how does it affect alignment?
- Vibration caused by fluid flowing through pipes
- External forces from connected piping that distort the machine casing and shift alignment (Correct answer)
- Corrosion of piping near the machine
- The weight of piping on the baseplate
Correct answer: External forces from connected piping that distort the machine casing and shift alignment
Pipe strain occurs when connected piping exerts forces (from thermal expansion, improper supports, or poor fit-up) on the machine casing, distorting it and shifting the shaft alignment. It must be eliminated before alignment.
Question 3: When performing laser shaft alignment, why is it important to eliminate bracket sag?
- Bracket sag damages the laser sensors
- Bracket sag introduces a systematic error that makes offset readings inaccurate (Correct answer)
- Bracket sag prevents the shaft from rotating
- Bracket sag is only a concern with dial indicators
Correct answer: Bracket sag introduces a systematic error that makes offset readings inaccurate
Bracket sag causes the indicator or laser detector to droop under gravity when rotated to the bottom position, introducing a systematic error in the offset (rim) readings. Modern laser systems often have automatic sag compensation.
Question 4: What is the acceptable alignment tolerance typically expressed in?
- Degrees of angle only
- Mils (thousandths of an inch) for offset and mils per inch for angularity (Correct answer)
- Percentage of shaft diameter
- RPM deviation
Correct answer: Mils (thousandths of an inch) for offset and mils per inch for angularity
Alignment tolerances are expressed as offset in mils (thousandths of an inch or hundredths of a millimeter) and angularity in mils per inch (or mm/100mm) of coupling span. Tolerances become tighter at higher operating speeds.
Question 5: Why is it standard practice to move only the movable machine during alignment corrections?
- The stationary machine is always heavier
- The stationary machine is the reference and is already aligned to process piping and other equipment (Correct answer)
- Moving both machines is faster
- The stationary machine has no shims
Correct answer: The stationary machine is the reference and is already aligned to process piping and other equipment
One machine (typically the driver) is designated as the 'movable' machine while the other (typically the driven equipment) is the 'stationary' reference because it is already aligned to process piping, ductwork, or other fixed connections.
Question 6: What does a positive value at 12 o'clock and negative value at 6 o'clock on a rim indicator reading indicate?
- The movable machine shaft is higher than the stationary machine shaft (Correct answer)
- The movable machine shaft is lower than the stationary machine shaft
- There is angular misalignment only
- The indicator is malfunctioning
Correct answer: The movable machine shaft is higher than the stationary machine shaft
With the indicator mounted on the stationary shaft reading the movable shaft rim, a positive reading at 12 o'clock (top) indicates the movable machine shaft centerline is higher than the stationary shaft centerline.
433A Alignment and Levelling 4
What instrument is commonly used to check the flatness of a large baseplate or sole plate?