Understanding Cam and Follower Pairs

A cam and follower mechanism is a fundamental part of many machines, used to convert rotary motion into reciprocating or oscillating motion. It consists of a cam (a specially shaped rotating or oscillating member) and a follower (a member that is in contact with the cam and follows its movement).

What is a Kinematic Pair?

In mechanical engineering, a kinematic pair is a joint between two links that permits relative motion between them. Kinematic pairs are classified based on the type of contact between the elements and how the contact is maintained.

Types of Kinematic Pairs

Based on the type of contact, pairs are classified as:

• Lower Pair: When the contact between the elements is a surface contact (e.g., a shaft rotating in a bearing, a sliding block in a guide).
• Higher Pair: When the contact between the elements is a line or point contact (e.g., a ball bearing, a cam and follower, gear teeth meshing). A cam and follower system is generally considered a higher pair because the contact between the cam profile and the follower is typically a point or line contact.

Based on how the contact is maintained (closure), pairs are classified as:

• Self-Closed Pair (or Form Closed Pair): The contact between the two elements is maintained by the geometric shape or form of the mating parts themselves. Gravity might also play a role in maintaining contact in some cases.
• Forced Closed Pair (or Force Closed Pair): The contact between the two elements is maintained by the application of an external force, usually from a spring, gravity (when not inherent to the form), or hydraulic/pneumatic pressure.

Cam and Follower Without a Spring

A typical cam and follower system involves the follower riding on the cam profile. For the follower to continuously follow the cam's shape, the contact between them must be maintained throughout the motion cycle.

When a spring is used, it pushes the follower against the cam, ensuring constant contact regardless of the cam's profile or orientation. This is an example of a forced closed pair because the spring provides the external force to maintain contact.

However, if there is no spring, the contact must be maintained by other means. This can happen if:

• The cam profile is such that the follower is always positively constrained by the cam's shape (e.g., a grooved cam where the follower fits into a groove).
• Gravity acts on the follower in such a way that it always pulls the follower towards the cam.

In these cases, the closure is achieved through the geometry or the inherent forces (like gravity), not an external force like a spring actively pushing the follower onto the cam. This type of closure, where contact is maintained by the form of the mating parts or gravity without an explicit external forcing device like a spring, is called self-closure or form closure.

Therefore, a cam and follower system that operates without a spring relies on self-closure (either by gravity or cam profile geometry) to maintain contact. It forms a self-closed pair.