Mechanical Maintenance Flashcards

A centrifugal pump operating at design flow shows normal vibration, but at reduced flow (30% of BEP) exhibits severe cavitation and shaft deflection. The MOST likely root cause is:

Answer: Recirculation vortices forming at the impeller inlet due to off-BEP operation

At flows well below the Best Efficiency Point (BEP), internal recirculation develops at both the impeller inlet and outlet. These recirculation vortices cause localized pressure drops below vapor pressure, producing cavitation damage and erratic radial forces that deflect the shaft — a phenomenon distinct from classic NPSH-deficit cavitation that occurs at high flow. The other options would typically manifest at all flow rates, not specifically at low-flow off-BEP conditions.

When measuring backlash in a worn spur gear set using a dial indicator held tangentially at the pitch circle, the reading is 0.018". The manufacturer specifies maximum allowable backlash of 0.012". Before condemning the gear set, the technician should FIRST:

Answer: Verify center-to-center distance of the gear shafts has not increased due to bearing wear

Excessive backlash can result from worn gear teeth OR from increased center distance caused by worn, loose, or failed shaft bearings. If bearing housings or bores are worn, the shafts spread apart, directly increasing backlash without any tooth wear. Condemning gears without verifying center distance risks replacing serviceable gears while leaving the actual cause (bearing failure) intact. The tight-point check (D) applies to runout/eccentricity, not excess backlash diagnosis.

A hydraulic cylinder controlling a press platen drifts downward 0.25" per hour under load with the directional control valve centered and blocked. The system uses a closed-center valve. Which component failure MOST specifically explains this symptom?

Answer: A leaking counterbalance valve permitting controlled load-induced flow

A counterbalance valve (load-holding valve) is installed to prevent exactly this condition — it holds a load against gravity by requiring pilot pressure to open. A faulty counterbalance valve with a worn poppet seat can crack open under sustained load pressure, allowing slow controlled drift even with the DCV blocked. Worn piston seals (A) would cause cylinder drift but fluid would transfer cap-to-rod internally, not to tank — the cylinder would collapse faster and system pressure would not be maintained. A relief valve (C) set too low would dump flow to tank during operation, not during hold. Contamination-related viscosity reduction (D) affects all leakage paths globally, not specifically the load-holding circuit.

During vibration analysis on a belt-driven fan, a technician identifies a prominent frequency peak at exactly 2× the belt's calculated fundamental frequency. This is MOST indicative of:

Answer: A single localized fault (splice or flat spot) on the belt

A single discrete defect on a belt — such as a worn splice joint, a flat spot from storage, or a hardened section — generates a vibration pulse each time that defect contacts the sheave. On most drives, each belt revolution produces two such contacts (once at the drive sheave, once at the driven sheave), resulting in a strong 2× belt frequency peak. If there were two defects 180° apart, this would be even more pronounced at 2×. Eccentric sheaves (B) produce peaks at shaft rotational frequency, not belt frequency. Resonant whip (C) appears as broadband energy, not a discrete harmonic. Mismatched belt tension (D) would show up as amplitude modulation at the difference frequency, not a clean 2× peak.

A precision machine tool spindle uses angular contact ball bearings mounted in a back-to-back (DB) configuration. After a thermal event, the spindle runs with increased noise and reduced radial stiffness, but axial preload readings are within spec. The MOST probable cause is:

Answer: The outer ring of one bearing has spun in its housing bore, relieving its interference fit

In a DB (back-to-back) angular contact configuration, the outer rings rely on interference fit in the housing to transmit radial loads. A thermal event — such as momentary overload, inadequate cooling, or grease breakdown — can heat the outer ring enough to thermally expand it beyond its interference fit, allowing it to spin or 'creep' in the bore. This damages the bore surface and eliminates the interference, resulting in reduced radial stiffness (the ring can now move slightly) and noise (fretting debris, eccentricity). Critically, preload in a DB pair is controlled by inner ring spacers or face-to-face contact, so it can remain nominally correct even after outer ring spin. Option B is wrong because increased contact angle increases, not decreases, axial and radial stiffness.

A maintenance technician is performing laser alignment on a coupled pump-motor set. After correcting both angular and offset misalignment in the horizontal plane, the vertical readings show the pump is 12 mils low at the coupling and 8 mils high at the far feet relative to the motor shaft centerline. This condition is called:

Answer: Short-sighted (concave) vertical angularity with offset in the same direction

When the coupling end reads low (–12 mils) and the far end reads high (+8 mils), the machine's shaft centerline crosses through the reference line — the near end is below and the far end is above. This is 'short-sighted' or concave angularity where offset and angularity partially cancel each other. The correction requires shimming the far (outboard) feet down or the near (inboard) feet up, with the shim amounts calculated using the feet-to-feet and feet-to-coupling distances. Soft foot (A) would produce inconsistent, non-repeating readings as bolts are tightened/loosened. Bracket sag (B) is a horizontal plane concern and would affect all readings uniformly. A bent shaft (D) would produce a once-per-revolution runout signal, not a static offset pattern visible in laser alignment readings.