Fig 4Line of action14 and 22 teeth, 20°P: pitch point
Involute teeth always touch somewhere on one straight line: the line that is tangent to both base circles. It crosses the line of centres at the pitch point P, where the two pitch circles touch. The angle it makes with the common tangent is the pressure angle φ. Since all contact happens on this line, the tooth force always points along it too. That is why every gear force breaks down into a tangential part (does the work) and a radial part (pushes the shafts apart), in the ratio tan φ.