Cam Profiles Simplified... part 1

A cam profile is nothing more than a mechanical device that uses a rotational motion in order to create a lifting motion in something. Similar to a lever, but instead uses a rotational motion to create lift, whereas a lever uses a linear motion.

Unrelated to cam profiles for camshafts, there are spiral cams that use a spinning motion to create lift. A screw type bottle jack would be one example of this type of cam.

A cam profile used for camshafts will exert the lifting motion onto a tappet. Lifter and follower are sometimes used to describe the same part. Picture the cam profile and the tappet in contact with each other on a vertical axis. Now, imagine rotating the cam profile and lifting the tappet. The cam profile lifts the tappet to maximum lift and then brings the tappet back down. A very simple process, right?

Simple, but when a cam profile is designed, certain parameters must be observed. If the profile is divided into 360 degrees, then we can observe what is happening at each degree of rotation. The first thing you will notice is how much the tappet moved (lifted) for each degree the profile was rotated. The lift per degree of rotation varies as the profile is rotated. It will begin with very little lift per degree and increase until the maximum lift per degree is reached. The tappet lift per degree of profile rotation is called the velocity and is expressed in inches per degree.

The velocity is important in the design of a flat tappet profile. The diameter of the flat tappet will determine the maximum velocity that can be designed into the profile. If the maximum velocity is exceeded, the contact point will run off the edge of the tappet and cause immediate damage. Roller tappets obviously do not have this problem.

Some other common parameters are acceleration and jerk. Acceleration is the rate of change in velocity and jerk is the rate of change in acceleration. Both are important in the design of cam profiles. Years of experience will determine these values.

Another important value for a flat tappet profile is what's called the nose radius. This determines the shape of the nose of the profile. Ideally, you want the profile to have a broad rounded nose. If the nose radius is too sharp then premature wear will take place. On a roller profile, there is what's called a negative radius. This is when the radius is inverted. On one or both sides of the lobe profile (the flank), the shape will go slightly inward and then back out again creating a negative radius in that area. If the negative radius is small enough, a special setup with a smaller wheel is necessary to grind the profile shape. This is labor intensive and will cost more. This type of profile is also harsh on the valve train requiring more maintenance. Again, years of experience will determine the nose radius and how much negative radius is acceptable.

To sum it up, the mechanical operation of a cam profile is fairly simple; to properly design a cam profile is a little more complicated.