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Heavy Duty Equipment Technician Red Seal Exam Prep · Question

A technician is rebuilding a planetary gear set. The sun gear has 24 teeth, and each planet gear has 18 teeth. If the ring gear is held stationary and the sun gear is driven, what is the approximate speed reduction ratio if the planet carrier is the output?

For a planetary gear set where the ring gear is stationary, the sun gear is input, and the carrier is output, the gear ratio is 1 + (Ring Gear Teeth / Sun Gear

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Question: A technician is rebuilding a planetary gear set. The sun gear has 24 teeth, and each planet gear has 18 teeth. If the ring gear is held stationary and the sun gear is driven, what is the approximate speed reduction ratio if the planet carrier is the output?

Answer options:

  • 1.67:1
  • 2.33:1 ✅ 3.00:1
  • 4.20:1

Correct answer: 3.00:1

Explanation: For a planetary gear set where the ring gear is stationary, the sun gear is input, and the carrier is output, the gear ratio is 1 + (Ring Gear Teeth / Sun Gear Teeth). The number of teeth on the ring gear is Sun Teeth + (2 * Planet Teeth) = 24 + (2 * 18) = 24 + 36 = 60 teeth. So, the ratio is 1 + (60 / 24) = 1 + 2.5 = 3.00:1. (CSA Z62.1-19, Heavy Duty Equipment Mechanics)

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