Millwright Certification Mechanical Systems 1 — Questions and Answers
Question 1: When aligning a shaft using a dial indicator, what does a total indicator reading (TIR) of zero at all measurement points confirm?
- The shaft is rotating at the correct speed
- The shaft centerlines are perfectly collinear (Correct answer)
- The bearing preload is within specification
- The coupling is properly lubricated
Correct answer: The shaft centerlines are perfectly collinear
A TIR of zero at all points around the rotation confirms that the two shaft centerlines are perfectly collinear — no angular or parallel offset exists. Any runout indicates misalignment that must be corrected before operation.
Question 2: A millwright notices that a ball bearing is running hot and emitting a high-pitched whine. Which condition is MOST likely the cause?
- Insufficient radial clearance due to over-tightening the locknut (Correct answer)
- Normal break-in during the first 100 hours of operation
- Excessive axial clearance in the bearing housing
- Mismatched grease viscosity during last lubrication cycle
Correct answer: Insufficient radial clearance due to over-tightening the locknut
Over-tightening a locknut preloads the bearing beyond design limits, eliminating internal clearance. The resulting metal-to-metal contact generates heat and the high-pitched whine characteristic of a preloaded or seized bearing.
Question 3: What is the function of a keyway and key in a shaft-to-hub connection?
- To prevent axial movement of the hub along the shaft
- To transmit torque between the shaft and the hub (Correct answer)
- To act as a sacrificial element that fails before the shaft under overload
- To maintain the concentric alignment between shaft and bore
Correct answer: To transmit torque between the shaft and the hub
A key fits into matching keyways machined in both the shaft and hub, creating a positive mechanical drive that transmits torque. While keys can act as shear elements under extreme overload, their primary designed function is torque transmission.
Question 4: When selecting the correct bearing for a location that experiences combined radial and thrust (axial) loads, which bearing type is MOST appropriate?
- Deep groove ball bearing
- Cylindrical roller bearing
- Angular contact ball bearing (Correct answer)
- Needle roller bearing
Correct answer: Angular contact ball bearing
Angular contact ball bearings are designed with a contact angle that allows them to carry significant axial (thrust) loads in addition to radial loads. Cylindrical roller bearings carry radial loads well but have very limited axial capacity; needle bearings are radial-load-only components.
Question 5: In a worm gear drive, the gear ratio is determined by dividing the number of teeth on the worm wheel by which value?
- The pitch diameter of the worm
- The number of starts (threads) on the worm (Correct answer)
- The helix angle of the worm
- The module of the worm wheel
Correct answer: The number of starts (threads) on the worm
Gear ratio in a worm drive equals the number of worm wheel teeth divided by the number of starts on the worm. A single-start worm advances the wheel one tooth per revolution; a two-start worm advances two teeth, halving the ratio and doubling output speed.
Question 6: What is the purpose of establishing a 'soft foot' check before performing a precision shaft alignment on a coupled machine set?
- To verify that the coupling guard is removable without tools
- To confirm the foundation bolts are torqued to specification
- To identify a condition where one or more machine feet do not make even contact with the baseplate (Correct answer)
- To measure the total weight distribution across all mounting pads
Correct answer: To identify a condition where one or more machine feet do not make even contact with the baseplate
Soft foot occurs when one or more machine feet do not make uniform contact with the baseplate, causing the frame to distort when bolts are tightened. This distortion shifts bearing centerlines and makes achieving stable precision alignment impossible until soft foot is corrected with shims.
Millwright Certification Mechanical Systems 1
When aligning a shaft using a dial indicator, what does a total indicator reading (TIR) of zero at all measurement points confirm?