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the contacting point between two mating flanks moves from Rb2 to Rb1 as
shown to the right in Figure 3 (movement along the line of action).
Figure 2: Three general cases of motion transmission with a bar and a crank
The line of action in parallel axes cylindrical gearing is a straight line, connect-
ing the two base circles. If the surface normal vectors N1 and N2 are within the
line of action, then the vector product |Ni x Ri| remains constant during a com-
plete mesh cycle. The consequent application of this principle leads to the
construction of an involute, as shown to the right in Figure 3.
Figure 3: Line of action between base circles (left) and subsequent
involute development
The line that forms the tooth surface elements while traveling from position “a”
to “f” along the line of engagement (line of action) is always perpendicular to
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