Mechanical Engineering - Machine Design - Discussion

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.

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.

Addendum > Dedendum.

Pitch circle = [(πD)/T] .
D= Pitch cycle dia, T= Number of teeth.

Dedendum dia is a base circle dia .
so, Base circle < Pitch circle is always.

That's why a correct answer is an option [C].

Addendum- it is the radial height of a tooth above the pitch circle.its standard value is one module.

Dedendum- it is the radial depth of a tooth below the pitch circle. It's standard value is 1.157m.

@All.

It is written pitch circle diameter =module * a number of teeth.

As we know that;
Module=pitch circle dia/no.of teeth.
So, option B is also correct.

Dedendum = 1.157*addendum.
Addendum = radial distance from pitch circle to top of tooth.
Dedendum = radial distance from pitch circle to base of the tooth.

Dedendum greater then addendum is the Right answer.

Dedendum= addendum + clearance so dedendum always greater then addendum.

Addendum is 1m.

Dedendum is addendum+clearance =1m + 0.157m=1.157m.
i.e 0.157 larger than addendum.

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

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