FAA Aviation Mechanic General (AMG)Physics, Math and Fluid LinesMedium
A two-stage gear reduction drive consists of a 60-tooth drive gear meshing with a 20-tooth gear on the intermediate shaft, which also carries a 40-tooth gear meshing with a final 20-tooth output gear. If the input shaft rotates at 1,800 RPM, what is the output shaft speed?
- A100 RPM
- B300 RPM
- C600 RPM
- D900 RPM
Show answer & explanationAnswer & explanation
Correct answer: B. 300 RPM
Each stage ratio equals driven/drive teeth: stage 1 = 60/20 = 3, stage 2 = 40/20 = 2. Total reduction = 3 × 2 = 6. Output speed = 1,800 / 6 = 300 RPM.
Why the other options are wrong
- A. This would result from an incorrect total reduction ratio of 18.
- C. This uses only one gear stage's ratio instead of both combined.
- D. This uses an incorrect ratio of 2 instead of the combined ratio of 6.
Compound Gear Train Ratio
In a compound gear train, the overall speed ratio is the product of the individual gear stage ratios.
- Single stage ratio = driven teeth / drive teeth
- Compound ratio = product of all stage ratios
- Output speed = Input speed ÷ total ratio
Memory trick: 'Multiply each mesh to find the total squeeze' on shaft speed.