Mechanical Maintenance Flashcards

A technician measures 0.003" of radial play in a ball bearing that has a manufacturer-specified C3 internal clearance. Under normal operating temperature rise, what is the most likely outcome?

Answer: The bearing will operate correctly because C3 clearance is designed to accommodate thermal expansion

C3 internal clearance is a greater-than-normal clearance designation specifically chosen for applications where thermal expansion will reduce running clearance. The 0.003" radial play measured cold is intentional — as the bearing reaches operating temperature, the inner race expands and the clearance closes to the optimal running range. Replacing or preloading a C3 bearing would cause it to fail under heat.

When performing a laser shaft alignment on a coupled pump-motor set, the technician achieves perfect cold alignment. After 4 hours of operation at 180°F, severe vibration develops. The most probable cause is:

Answer: Failure to account for thermal growth offset during the cold alignment procedure

Hot equipment runs at a different shaft centerline height than when cold due to thermal growth of the machine feet and casing. Proper laser alignment procedure requires applying a thermal growth offset (also called 'cold offset' or 'alignment target') so that when the machine reaches operating temperature, the shafts are truly aligned. A perfect cold alignment without thermal offset correction will result in misalignment — and vibration — once the machine heats up.

A maintenance technician is analyzing a vibration spectrum and observes a dominant peak at exactly 2× running speed (2X) with no significant peak at 1X. The most likely root cause is:

Answer: Parallel (offset) misalignment between coupled shafts

Parallel (offset) misalignment characteristically produces a dominant vibration frequency at 2× running speed (2X RPM) with relatively low 1X amplitude. Angular misalignment typically shows a dominant 1X peak, often with a high axial component. Mass imbalance is predominantly a 1X phenomenon. Looseness typically produces sub-harmonics or a series of harmonics. The 2X dominant signature without 1X elevation strongly points to parallel offset misalignment.

A hydraulic cylinder is experiencing gradual drift (slow rod retraction under load) despite new seals and no visible external leakage. The technician has verified that the directional control valve is centered. The NEXT most logical diagnostic step is:

Answer: Perform a cylinder bypass test by pressurizing the rod-end port and measuring flow at the cap-end port with the load held

When external leakage is ruled out and valve function is confirmed, drift indicates internal cylinder bypass — the piston seal is allowing fluid to cross from the high-pressure side to the low-pressure side. The proper diagnostic is a cylinder bypass (piston seal integrity) test: pressurize one port, cap the other, and measure any flow bypassing past the piston. Increasing relief pressure would not fix internal leakage and could cause damage. Air bleed would not cause sustained load drift.

A V-belt drive system shows rapid, repeated belt failure. The belts are the correct cross-section and length, sheave alignment is verified, and tension is set per manufacturer spec. Inspection shows the failed belts have cracked, glazed sidewalls. The MOST likely cause is:

Answer: Sheave groove wear causing the belt to ride on the groove bottom rather than the sidewalls

Glazed, cracked sidewalls with otherwise correct belt selection and tension most indicate the belt is bottoming out in the sheave groove — a condition caused by worn sheave grooves. When grooves wear, the effective pitch diameter changes and the belt contacts the groove bottom instead of being wedged by the sidewalls. Metal-to-metal contact at the bottom generates heat, glazes the belt sidewalls, and dramatically reduces power transmission through slippage. A simple go/no-go gauge or sheave groove gauge confirms worn grooves.

During preventive maintenance on a 3-phase induction motor, a technician measures the following winding resistances with a calibrated ohmmeter: T1-T2 = 2.4 Ω, T2-T3 = 2.4 Ω, T1-T3 = 4.8 Ω. The insulation resistance to ground is 800 MΩ. What does this indicate?

Answer: The T1-T3 measurement reveals an open between terminals with T2 in series, indicating a fault in one phase coil

In a wye-connected motor, measuring T1-T3 places the T1 and T3 phase coils in series. If each phase coil has equal resistance R, then T1-T2 = 2R = 2.4 Ω, so R = 1.2 Ω per coil. T1-T3 should also equal 2R = 2.4 Ω. But the measured 4.8 Ω for T1-T3 equals double the expected value. This indicates T2's coil is open — current flows from T1 through the T1 coil, backward through the T3 coil to T3, measuring two coils in series (2.4 Ω), but the opened T2 coil forces current through the longer path. The high insulation resistance rules out a ground fault.