Spur Gears:

These gears contain straight teeth along their alignment parallel to the axes. These gears are illustrated in mesh in Figures (a) and (b). The contact among the two meshing teeth is along with a line whose length is equivalent to whole length of teeth. It might be viewed that in external meshing, the two shafts rotate opposite to each of other while in internal meshing the shafts rotate in the similar sense.

(a) External Meshing                        (b) Internal Meshing

If the gears mesh externally and diameter of the one of gear becomes infinite, the arrangement becomes 'Spur Rack and Pinion'. This is illustrated in given figure. It changes rotary motion into translatory motion, or vice-versa.

   Spur Rack and Pinion