Cams, Followers and their Types

A cam is a rotating or sliding piece in a mechanical linkage that drives a mating component known as a follower. From a functional viewpoint, a cam-and-follower arrangement is similar to the linkages. The cam accepts an input (rotary or linear) motion and imparts a resultant (linear or rotary) motion to a follower.

Cam Nomenclature

Figure 1 CAM nomenclature

Cam profile: The Cam profile is the outer surface of the disc cam.

Base circle: The base circle is the smallest circle, drawn tangential to the cam profile.

Trace point: Tracepoint is a point on the follower; tracepoint motion describes the follower's movement.

Pitch curve: The pitch curve is the path the tracepoint generates as the follower is rotated about a stationary cam.

Prime Circle: The Prime circle is the smallest circle that can be drawn to be tangential to the pitch curve, with its center at the cam center.

Pressure angle: The pressure angle is the angle between the direction of the follower movement and the normal to the pitch curve.

Pitch point: Pitch point corresponds to the point of maximum pressure angle.

Pitch circle: A circle drawn from the cam center that passes through the pitch point is called a pitch circle.

Stroke: The most significant distance or angle through which the follower moves or rotates.

Types of Cams

Figure 2 Various custom cams

Cams can be classified into the following three types based on their shapes. They are:

1. Plate or disk cams: Plate or disk cams are the simplest and most common type of cam. A plate cam is illustrated in Figure 3 (a). This type of cam is formed on a disk or plate. The radial distance from the disk's center varies throughout the cam's circumference. Allowing a follower to ride on this outer edge gives the follower a radial motion.
2. Cylindrical or drum cam: Figure 3 (b) illustrates a cylindrical or drum cam. This type of cam is formed on a cylinder. A groove is cut into the cylinder, with a varying location along the axis of rotation. Attaching a follower that rides in the groove gives the follower motion along the axis of rotation.
3. Linear cam: A linear cam is illustrated in Figure 3 (c). This type of cam is formed on a translated block. A groove is cut into the block with a distance that varies from the translation plane. Attaching a follower that rides in the groove makes the follower motion perpendicular to the translation plane.

Figure 3 Cam types

Types of Followers

Followers are classified based on their motion, position and shape. The details of followers' classifications are shown in Figure 4 and discussed below

Figure 4 Follower types

1. Based on Follower Motion

Based on the follower motion, followers can be classified into the following two categories:

1. Translating followers are constrained to motion in a straight line, as shown in Figures 4 (a) and 4 (c).
2. Swinging arms or pivoted followers are constrained to rotational motion, as shown in Figures 4 (b) and 4 (d).

2. Based on the Follower Position

Based on the follower position, relative to the center of rotation of the cam, is typically influenced by any spacing requirements of the machine. The position of translating followers can be classified into the following two categories:

1. An in-line follower exhibits straight-line motion, with the line of translation extending through the center of rotation of the cam, as shown in Figure 4 (a).
2. An offset follower exhibits straight-line motion, with the line of motion offset from the center of rotation of the cam, as shown in Figure 4 (c).

In the case of pivoted followers, there is no need to distinguish between in-line and offset followers because they exhibit identical kinematics.

3. Based on Follower Shape

Finally, the follower shape can be classified into the following four categories:

1. A knife-edge follower is formed to a point and drags on the edge of the cam. The follower shown in Figure 4 (a) is a knife-edge follower. It is the simplest form, but the sharp edge produces high contact stresses and wears rapidly, so this type of follower is rarely used.
2. A roller follower consists of a follower that has a separate part, the roller that is pinned to the follower stem. The follower shown in Figure 4 (b) is a roller follower. As the cam rotates, the roller maintains contact with the cam and rolls on the cam surface. This is the most commonly used follower, as the friction and contact stresses are lower than those for the knife-edge follower. However, a roller follower can jam during steep cam displacements.
3. A flat-faced follower consists of a follower that is formed with a large, flat surface available to contact the cam. The follower shown in Figure 4 (c) is flat-faced. This type of follower can be used with a steep cam motion and does not jam. Consequently, this type of follower is used when quick motions are required. However, any follower deflection or misalignment causes high surface stresses. In addition, the frictional forces are greater than those of the roller follower because of the intense sliding contact between the cam and the follower.
4. A spherical-faced follower consists of a follower formed with a radius face that contacts the cam. The follower shown in Figure 4 (d) is a spherical-face follower. As with the flat-faced follower, the spherical- face can be used with a steep cam motion without jamming. The radius face compensates for deflection or misalignment. Yet, like the flat-faced follower, the frictional forces are more significant than the roller follower's.