Machine Design and Shop Practice (MDSP) Inhouse Practice Exam

The size of the gear expressed as the pitch circle diameter is set by the gear’s size parameters (the module or diameter pitch and the number of teeth), not by the pressure angle. The pressure angle, however, determines the shape of the tooth through the base circle, with the base diameter related to the pitch diameter by d_b = d cos(phi). Rewriting gives d = d_b / cos(phi). So, for a fixed pressure angle, the pitch diameter is proportional to the base diameter (with the constant factor 1/cos(phi)). That makes the statement about being proportional to the base diameter the most accurate way to describe its dependency among the given options. The other choices either ignore the size parameters or incorrectly tie the pitch diameter directly to the pressure angle.