FAA Aviation Mechanic General (AMG)Physics, Math and Fluid LinesMedium
A drive gear with 40 teeth meshes with a driven gear having 10 teeth. If the drive gear rotates at 100 rpm, what is the rotational speed of the driven gear?
- A400 rpm
- B40 rpm
- C100 rpm
- D250 rpm
Show answer & explanationAnswer & explanation
Correct answer: A. 400 rpm
Gear speed is inversely proportional to tooth count: driven speed = drive rpm × (drive teeth/driven teeth) = 100 × (40/10) = 400 rpm.
Why the other options are wrong
- B. Incorrectly divides instead of multiplying by the gear ratio.
- C. Assumes no speed change, ignoring the gear ratio.
- D. Results from using an incorrect ratio calculation.
Gear Ratio and Speed
The speed of meshing gears is inversely proportional to their number of teeth; a smaller gear turns faster than a larger one.
- Driven rpm = Drive rpm × (Drive teeth/Driven teeth)
- More teeth = slower rotation, more torque
- Fewer teeth = faster rotation, less torque
- Used in gearboxes, accessory drives, and reduction gearing
Memory trick: Small gear spins fast, big gear spins slow — teeth trade speed for torque.