2 - Cam Background

Cams come in many shapes and varieties, but only aspects relevant to this model will be discussed here. The cam used in the Ready to Fly model (Figure 2) is a radial cam, meaning that the flat-faced follower in the model moves in a direction radial to the cam’s axis of rotation. The type of joint closure used in this model is force closure. Force closure means that an external force must be applied to the joint in order to maintain contact between the cam and follower. In this case, that external force is simply gravity. It is important to note that the force that maintains contact between the cam and follower must be applied in the direction that closes the joint and cannot become negative. Otherwise, contact is lost. In other words, if you turn the model upside-down, the follower will obviously fall away from the cam.

Figure 2. Cam used in Ready to Fly model.

I did not design the cam, but I still should mention the Fundamental Law of Cam Design since it was good to have in mind later on when I was plotting the motion profile of the follower. The Fundamental Law of Cam Design is stated in the textbook by Norton (2012) as follows: “The cam function must be continuous through the first and second derivatives of displacement across the entire interval (360 degrees).” Therefore, the motion profile plots I create from my measurements should be continuous.

The equations that I used to assess the cam in the Ready to Fly model mainly come from Figure 3, taken from the Design of Machinery textbook (Norton, 2012). The two vector loops I used, which will be discussed further in the Follower Motion and Fin Motion sections, are:

Where x is the horizontal distance between the axis of rotation and point of contact, R_b is the radius of the cam’s base circle, s is the displacement of the follower, and r and q define the Cartesian coordinates of the contact point with respect to the rotating axis coordinate system embedded in the cam (Norton, 2012). Eccentricity, or the distance between the follower’s axis of motion and the center of the cam, does not factor into these equations, but it does play a factor in sizing a cam when a roller follower is used. Also, pressure angle, the angle between the follower’s direction of motion and the axis of transmission, is zero for all positions of the cam and follower. Before I could put these equations to use, I had to build the model.

Figure 3. Vector loops used for a radial cam and flat-faced follower.