Technical Aptitude & Knowledge Flashcards

A compound gear train consists of a 10-tooth driver gear meshed with a 40-tooth gear on a common shaft with a 15-tooth pinion, which drives a 60-tooth output gear. If the input shaft rotates at 1,200 RPM, what is the output shaft speed?

Answer: 75 RPM

In a compound gear train, the overall gear ratio multiplies across each stage. Stage 1: 10/40 = 0.25. Stage 2: 15/60 = 0.25. Overall ratio: 0.25 × 0.25 = 0.0625. Output speed = 1,200 × 0.0625 = 75 RPM.

An electrician measures 480V across a three-phase motor's terminal block but only two of the three phase legs read the correct line voltage. The third leg reads 277V to ground instead of the expected value. What is the MOST likely cause?

Answer: An open neutral on a grounded wye system

In a grounded wye (star) system, an open neutral causes unbalanced loads to share the neutral return path through ground, resulting in one or more phase legs reading line-to-neutral voltage (277V on a 480V system) rather than line-to-line voltage. A blown fuse would read full voltage across the open point, not 277V.

A hydraulic cylinder with a 4-inch bore and a 2-inch diameter rod must extend and retract at the same linear speed. The pump delivers 10 GPM on the extend stroke. Approximately how many GPM must the pump deliver on the retract stroke to maintain equal speed?

Answer: 7.5 GPM

Piston area = π(2)² = 12.57 in². Rod-side (annular) area = π(2)² − π(1)² = 12.57 − 3.14 = 9.42 in². The ratio of rod-side to full-bore area = 9.42/12.57 ≈ 0.75. To achieve the same speed, the retract flow = 10 × 0.75 = 7.5 GPM, since less fluid volume is needed to fill the smaller annular cavity.

When performing a megohmmeter (megger) test on a motor winding, a technician records 50 MΩ at the start of a 10-minute test and 480 MΩ at the end. The Polarization Index (PI) is 9.6. How should this result be interpreted?

Answer: Winding insulation is in excellent condition

The Polarization Index is calculated as the 10-minute resistance reading divided by the 1-minute reading (480/50 = 9.6). IEEE standards classify a PI above 4.0 as 'excellent' for rotating machinery. A PI below 1.0 indicates moisture or carbon tracking. A high PI means the insulation is dry and polarizing properly over time — a sign of healthy insulation.

A technician is troubleshooting a pneumatic control circuit where a double-acting cylinder fails to retract after fully extending. The solenoid valve shifts correctly when manually actuated, and air pressure is confirmed at the valve inlet. What is the MOST likely cause of the fault?

Answer: Flow control valve installed backwards on the retract line

If the solenoid valve shifts correctly under manual actuation and supply pressure is confirmed, the valve itself is not the fault. A flow control valve (meter-out type) installed backwards acts as a check valve, blocking exhaust air from leaving the cylinder's rod side and preventing retraction. This is a common installation error that produces exactly this symptom — extend works, retract is blocked.

A technician uses an oscilloscope to probe a 60 Hz AC circuit and observes a sine wave with a peak value of 169.7V. A True RMS multimeter placed across the same terminals reads 120V. A standard averaging multimeter calibrated for pure sine waves reads 118.8V on the same terminals. Which of the following conclusions is CORRECT?

Answer: The circuit contains harmonic distortion causing the averaging meter to under-read

For a pure sine wave, peak = RMS × √2, so 120V RMS × 1.414 = 169.7V peak — consistent with the oscilloscope. However, averaging meters apply a fixed form factor (1.1107) calibrated for pure sine waves. If harmonics are present, the waveform's average-to-RMS ratio changes, causing the averaging meter to read incorrectly while the True RMS meter accurately captures the actual power-equivalent voltage. The 1.2V discrepancy (118.8 vs 120) indicates harmonic content.