# In an epicyclic gear train, the sun gear on the input shaft is 20 teeth external gear. The planet gear is a 40 teeth external gear and the ring gear is a 100 teeth internal gear. The ring gear is fixed and the sun gear is rotating at 60 rpm counter clockwise. The arm attached to output shaft will rotate at ________

In an epicyclic gear train, the sun gear on the input shaft is 20 teeth external gear. The planet gear is a 40 teeth external gear and the ring gear is a 100 teeth internal gear. The ring gear is fixed and the sun gear is rotating at 60 rpm counter clockwise. The arm attached to output shaft will rotate at ________

(a) 5 rpm

(b) 10 rpm

(c) 20 rpm

(d) 30 rpm

The question was asked in a job interview.

This intriguing question comes from Mechanical Power Transmission in chapter Mechanical Power Transmission of Farm Machinery

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Correct answer is (b) 10 rpm

Easy explanation: $\frac{Ns-Na}{Nr-Na}=\frac{-Tr}{Ts}$

$\frac{60-Na}{0-Na}=\frac{-100}{20}$

6Na = 60

Na = 10 rpm.

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