Cam terminology — the pressure angle of a cam–follower system is the angle between the direction of follower motion and a normal drawn to which reference curve?

Introduction / Context: Pressure angle is a crucial parameter in cam design because it controls side thrust and potential jamming in the follower guide. Designers aim to limit it for smooth motion and lower contact stresses.

Given Data / Assumptions:

• Translating or oscillating follower with a defined motion law.
• Pitch curve is the trajectory of the follower center (roller center) relative to the cam.
• Normal is taken at the point of contact along this curve.

Concept / Approach: By definition, the pressure angle is the angle between the follower’s line of motion and the common normal to the pitch curve at the contact point. Using the pitch curve (not merely the base or pitch circle) accounts for the actual geometry that drives the follower.

Step-by-Step Solution:

Verification / Alternative check: For roller followers, the pitch curve is the cam profile offset by the roller radius; the definition still references the pitch curve normal.

Why Other Options Are Wrong:
pitch circle/base circle/prime circle — circles define basic sizes but not the instantaneous geometry of contact and force transmission.

Common Pitfalls: Confusing the base circle (used to generate profiles) with the functional pitch curve; ignoring that excessive pressure angle increases side load on guides.

Final Answer: pitch curve.