A key having a square cross-section of side d/4 and length l is used to transmit torque T from the shaft of diameter d to the hub of a pulley. Assuming the length of the key to be equal to the thickness of the pulley, the average shear stress developed in the key is given by

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ISRO VSSC Technical Assistant Mechanical 14 July 2021 Official Paper
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  1. \(\rm\frac{16 T}{l d}\)
  2. \(\rm\frac{16 T}{l d^2}\)
  3. \(\rm\frac{8 T}{l d^2}\)
  4. \(\rm\frac{16 T}{l d^3}\)

Answer (Detailed Solution Below)

Option 3 : \(\rm\frac{8 T}{l d^2}\)
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Detailed Solution

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Explanation:

The Force of the shaft circumference is given by

\(\rm{P=\frac{P}{d/2}} =\frac{2P}{d}\)


Shearing area = width × length

\(\rm{\frac{d}{4}\times l=\frac{ld}{4}}\)

\(\rm{Average ~shear~ stress = \frac{P} {Shearing ~area}} \)

\(\rm{Average ~shear~ stress =\frac{\frac{2T}{d}}{\frac{ld}{4}}}=\frac{8T}{ld^2}\)

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