Maintenance Flashcards

A maintenance technician notices that a pump's mechanical seal is leaking intermittently only during startup. After startup, the leak stops. What is the MOST likely root cause?

Answer: Thermal expansion of the shaft causing temporary face separation during cold startup

Mechanical seals on cold equipment experience differential thermal expansion between the shaft and housing during startup. This briefly separates the seal faces until the components reach operating temperature and stabilize dimensionally. Once thermal equilibrium is reached, the leak stops. This is a classic cold-start seal behavior and does not necessarily indicate seal failure.

When performing a vibration analysis on a rotating machine, a dominant frequency peak appears at exactly 2× the running speed (2X). Which fault condition does this MOST specifically indicate?

Answer: Shaft misalignment or bent shaft

A 2X (twice running speed) vibration signature is the classic indicator of shaft misalignment or a bent shaft. Misalignment produces a force cycle twice per revolution because each rotation creates two deflection peaks. Unbalance produces a dominant 1X peak, bearing defects produce sub-synchronous or non-integer frequencies, and looseness typically shows multiple harmonics.

A technician is using an ohmmeter to check a three-phase motor winding for insulation breakdown to ground. All three phase-to-ground readings are above 1 MΩ, but the motor still trips on a ground fault during operation. What is the MOST probable explanation?

Answer: The insulation breakdown only occurs at operating voltage and temperature, which a low-voltage ohmmeter cannot detect

A standard ohmmeter uses very low test voltage (typically 1.5–9V), which is insufficient to stress motor insulation the way operating voltage (460V or higher) does. Insulation that appears healthy at low voltage can break down under full operating voltage and elevated temperature. A megohmmeter (megger) test at rated voltage — and ideally at operating temperature — is required to properly evaluate insulation integrity and would reveal this type of thermally-activated breakdown.

A hydraulic system maintains correct pressure at idle but pressure drops significantly under load. The relief valve setting and pump output have been verified as correct. Which component is the MOST likely cause?

Answer: A worn pump with excessive internal bypass (volumetric efficiency loss)

When a hydraulic pump wears internally, its volumetric efficiency drops — meaning it bypasses fluid internally from high-pressure to low-pressure ports. At idle (low flow demand), the pump can maintain pressure because little flow is needed. Under load, when the actuator demands high flow, the worn pump cannot supply sufficient volume, causing pressure to collapse. This is a key distinction from relief valve or line restriction problems, which would affect pressure regardless of load.

During a preventive maintenance inspection, a technician finds a V-belt drive where all belts appear properly tensioned and aligned, yet the sheave grooves show a polished, glazed surface. What does this condition indicate, and what is the correct corrective action?

Answer: The belts have been slipping chronically due to under-tensioning or overloading, glazing both belts and sheaves; replace belts AND refinish or replace sheaves

Glazed sheave grooves are the result of chronic belt slippage — the belt slides against the groove rather than gripping it, generating heat that polishes (glazes) both the belt sidewalls and the sheave groove surfaces. Installing new belts onto glazed grooves will cause the new belts to slip immediately on the smooth surface, failing prematurely. The corrective action is to replace the belts AND restore the sheave groove surface (by refinishing or sheave replacement) to re-establish proper friction grip.

A technician is troubleshooting a 480V three-phase motor that runs correctly in one direction but overheats and trips when direction is reversed using a reversing contactor. The overload relay is correctly sized. What is the MOST likely cause?

Answer: The motor fan is unidirectional and provides no cooling airflow when the motor runs in reverse

Many TEFC (Totally Enclosed Fan Cooled) motors use an external fan that is designed to move air in only one direction regardless of shaft rotation, but some smaller or older motors use a simple unidirectional fan blade. When such a motor runs in reverse, the fan moves air backward (or not at all), eliminating cooling airflow. The motor generates the same heat as in forward but cannot dissipate it, causing rapid overheating. This is a known design consideration when reversing-duty operation is required.