ASVAB (Armed Services Vocational Aptitude Battery)Mechanical Comprehension (MC)Medium
A large gear with 60 teeth is meshed with a smaller gear that has 20 teeth. If the large gear rotates at 10 RPM, what is the rotational speed of the smaller gear?
- A3.33 RPM
- B30 RPM
- C10 RPM
- D20 RPM
Show answer & explanationAnswer & explanation
Correct answer: B. 30 RPM
In a gear train, the ratio of the number of teeth is inversely proportional to the ratio of their rotational speeds. So, (Teeth_large / Teeth_small) = (RPM_small / RPM_large). (60 / 20) = (RPM_small / 10 RPM). 3 = RPM_small / 10. RPM_small = 3 × 10 = 30 RPM.
Why the other options are wrong
- A. Incorrect. This would be if the small gear was driving the large gear, or a division error (10/3).
- C. Incorrect. This would mean no change in speed, which only happens if gears have the same number of teeth.
- D. Incorrect. This might be a direct subtraction or some other miscalculation.
Gear Ratio
The ratio of the number of teeth on the driven gear to the number of teeth on the driving gear, which determines the change in speed and torque.
- Gear Ratio = (Number of teeth on driven gear) / (Number of teeth on driving gear)
- Speed Ratio = (RPM of driving gear) / (RPM of driven gear)
- Gear ratio is inversely proportional to speed ratio.
Memory trick: Teeth Tell The Turning Times!