Pressure angle, also known as the angle of obliquity,[1] is in general the angle at a pitch point between the line of pressure which is normal to the tooth surface, and the plane tangent to the pitch surface. The angle formed by the common tangent to pitch circle and common tangent to Dedenda Circle throughout the pitch point is known as Pressure Angle. The pressure angle gives the direction of the normal to the tooth profile. The pressure angle is equal to the profile angle at the standard pitch circle and can be termed the “standard” pressure angle at that point. Standard values include 14.5, 20 and 25 degrees. Earlier gears with pressure angle 14.5 were more commonly used because for a given pressure angle, cosine would be larger for smaller angle, thus resulting in more power transmission and less pressure on the bearing. But for a given material, smaller pressure angles correlate with weaker teeth. To run gears together properly one must match pressure angles.[2]
Just as there are three types of profile angles, there are also three types of corresponding pressure angles: the transverse pressure angle, the normal pressure angle, and the axial pressure angle. [3]