By: Michael Rauscher
The easiest lobe profile to design is a pushrod (translating) design which is fairly straightforward as the lobe description comes the tappet action. Now the OHC design is another step more complicated. Because the cam actuates a rocker arm the action is not translating (up & down), but oscillating, it pivots. This pivoting action cause a change in valve tip action an is different wheather the lobe is advancing or retreating in relation to the rocker arm as it rotates. So if were to use a symmetical (open and close profiles are the same) cam profile (shape) on an OHC design, we would have the open or close different from each other due to the rocker arm action.
The method to design an OHC cam is to design the valve action and not the lobe action as done in a translating design. After we design the valve action we use the valvetrain geometry to convert to the lobe shape establishing what the lobe would look like to obtain that valve action. The conversion process utlizes the Law of Sines as the basis to convert the valve action to the lobe action which forms the lift table and ultimately the finished lobe.
The most common method to design a lobe profile is to use polynomials in the basic format as:
With Y = lift and θ = angle of rotation or X
The exponents establish the derivative control such as Velocity, Acceleration and Jerk. The exponent values can be any positive integer value which changes the shape of the curve an infinite amount of ways. To the unfamiliar, lobe design is not asking the mathematics directly for a particular shape to obtain the duration @ .050" lift, but a sequence of manipulating the exponents to obtain the desired shape to effect the duration required at .050" lift. There are many other criteria involved that must also be controlled such as the radius of curvature either positive or negative, pressure angle, etc. So the desgin process can sometimes require substantial time and tedious study and a great deal of experience.
Base circle radius and follower dimensions play a very important role in the design process and the limitations the base circle radius and follower size may define.
Dynamics is another large contributor to the complications of the design process.