Cam Profile Design Limitations

I would recommend reading the previous entries before starting here. I have intentionally not explained in detail what some things are, since I have already done so in the previous entries.

Every design method has the same limitations to deal with when designing a cam profile. These limitations will make it difficult to prove which method is better.

A flat tappet profile has a velocity and a nose radius limit. The tappet diameter alone will determine the maximum velocity that can be designed into the profile. The nose radius is determined by a combination of the base circle diameter, lift, and nose acceleration.

A roller tappet profile has a pressure angle and a negative radius limit. The pressure angle is the angle between where the lobe and roller wheel make contact. It is mostly determined by a combination of the base circle diameter, roller wheel diameter, and the lift. The negative radius is the concave area on the opening and/or closing flank of the profile. It is determined by a combination of the base circle diameter, lift, acceleration and roller wheel diameter.

Now you can see the importance of a large base circle diameter and roller wheel diameter when designing a cam profile. That is the reason for the trend to the larger camshaft journals. Strength is also a factor.

All of these limitations can cause premature wear and mechanical damage to the valve train if they are exceeded far enough.

Fortunately, there are some design "tricks" that can help deal with these limitations. On some flat tappet profiles, the maximum velocity can be dwelled (DMV) for a certain number of degrees, to increase the lift and duration without causing a sharp nose radius. This will not work for all flat tappet profiles. There are some profiles that just cannot be made. For example, design me a cam profile for the following: Small Block Chevy 1.8685 journal diameter, 0.842 OEM flat tappet, 240-degrees at 0.050 tappet height, 0.400 lobe lift. Sounds simple enough, right? Please send me the basic lift table when you are finished. You will become my new best friend. Thank you in advance. A technique can be applied to roller tappet profiles that will dwell the radius of curvature for a certain number of degrees. This will keep the maximum acceleration from going too high and creating too small of a negative radius. An acceleration curve chart will immediately reveal if one of these techniques was used to design the cam profile.

(a little secret) As long as there are limitations, all cam designers will eventually end up with basically the same cam profile design, for a given application. Some designers will just spend more time on their designs to make them the best that is possible within these limitations.