Statement (I): In short open-belt drives, an idler pulley is used in order to increase the angle of contact on the smaller pulley for higher power transmission.

Statement (II): The idler pulley facilitates changing the speed of the driven shaft, while the main or driving shaft runs at constant speed.

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ESE Mechanical 2014 Official Paper - 2

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Both Statement (I) and Statement (II) are individually true and Statement (II) is the correct explanation of Statement (1)
Both Statement (-) and Statement (II) are individually true tut Statement (II) is NOT the correct explanation of Statement (1)
Statement (I) is true but Statement (II) false
Statement (I) is false but Statement (II) is true he

Answer 1

Concept:

F5 S.C Madhu 19.06.20 D3

There are two advantages of using an idler pulley:

Initial tension in the belt can be increased from time to time by adjusting the position of adjustable weight or by increasing the adjustable weight value. Due to increase in initial tension, power transmitting capacity of belt drive gets increased.
Angle of contact on the smaller pulley increases which further increases the ratio between friction tension on both the sides because \(\frac{{{{\rm{T}}_1}}}{{{{\rm{T}}_2}}}\) = \({e^\theta }\). Hence, in effect power transmitting capacity of belt drive gets further increased.

It does not facilitate changing the speed of the driven shaft. Therefore, statement II is incorrect.