The spur gear is the last gear we will look at and the
most important as far as we are concerned. We will be looking at the gear terms
and how to draw the gear teeth using Unwins construction. Firstly, we will
discuss the spur gear itself.
A spur gear is one of the most important ways of
transmitting a positive motion between two shafts lying parallel to each other.
A gear of this class may be likened to a cylindrical blank which has a series of
equally spaced grooves around its perimeter so that the projections on one blank
may mesh in the grooves of the second. As the design should be such that the
teeth in the respective gears are always in mesh the revolutions made by each is
definite, regular and in the inverse ratio to the numbers of teeth in the
respective gears. This ability of a pair of well made spur gears to give a
smooth, regular, and positive drive is of the greatest importance in many
engineering designs. An example of two spur gears in mesh are shown below.