Why are two meshing gears made of different face widths?

[k.udhay]

Hi,

I observe in an automobile transmission that two meshing gears of same material are never of same face widths. Either the driver of driven is a couple or more millimeters wider than the other. Most of the time (not always) the pinion is wider than the gear.

Though I can assume that the pinion is wider for reinforcement purpose, why is it not always the case? Thanks.

 

[Simon Bridge]

The face contact ratio must be bigger than one:
iirc: the main reason is just to make sure that one gear is always inside the width of the other ... so there's no overhang on each side to catch on something. Margin for error type of thing.

Checking:

The face widths of pinions are usually wider than face widths of mating gears to insure full face contact
without critical adjustment.
-- http://www.wmberg.com/catalog/pdf/b00k2-16.pdf​