In involute gears, pressure angle is

Concept:

Pressure angle 

• The angle between the pressure line and common tangent to the pitch circle is called Pressure angle or angle of obliquity.

Involute profile

• It is defined as the locus of the point on the line which rolls without slipping on the fixed circle.
• In involute gears, the pressure angle, from the start of the engagement of teeth to the end of the engagement, remains constant. It is necessary for smooth running and less wear of gears.
• But in cycloidal gears, the pressure angle is maximum at the beginning of engagement, reduces to zero at pitch point, starts increasing and again becomes maximum at the end of engagement. This results in less smooth running of gears.