Mechanical Engineering - Machine Design - Discussion

This is wrong because addendum circle is always greater than dedendum circle.

The first answer (A) is incorrect it should be reversed to be "The dedendum is less than addendum".

Why dedendum is larger than addendum?

Meshing of mating teeth should occur on the involute portion of teeth. The teeth have involute profile from the base circles to addendum circles. If there is inaccuracy in manufacturing of gears, then the tip of tooth of one gear (always lies on its addendum circle) will dig the bottom portion of mating gear that results in interference. To avoid this bottom land of gears are lowered such that it lies on the periphery of the dedendum circle. The dedendum circle is made smaller than the base circle and also lies below the pitch circle at distance, 'b'. Addendum circle lies at distance 'a', beyond the pitch circle, such that b > a.

Guys the question said addendum not circle!

Very well said @Chandan.

Addendum is the distance from pitch circle to the teeth head, so it must be less than dedendum.

Please, give an explanation about option B.

Please, is it must for the pitch circle diameter to be greater than the base circle diameter.

Pitch circle diameter is the product of module and not no of teeth.

Module m: Pitch diameter divided by number of teeth: m= D/T.
Circular pitch p: The width of a tooth and a space, measured on the pitch circle. Cp= π*D/T.
Diametral pitch P: The number of teeth of a gear per inch of its pitch diameter. Dp = T/D.