Custom Cam Design
By: Michael Rauscher
There is a common misunderstanding when it comes to cam design. Anyone can claim to design cams, and in a way they do, but in a very limited way. What they really do is design or spec where they want the valve events; usually only 2 events, duration at .050” and max lift. Most of the top engine shops establish where they want the valve to be in regards to crankshaft position.
In order to truly design a cam a lift table must be used to establish the valve events. This is not 2 events but all 360 events of the lobe.
I am one of the few that design OHC profiles and I do so by designing the valve lift curve and then working backwards through the geometry establishing what the cam lobe profile needs to look like to generate the valve lift profile.
The mathematics are quite involved, as commonly used in automotive designs, using 7th order polynomials, B Splines, Circular arcs, etc. in order to create the curves and control the desired derivatives of lift such as velocity, acceleration, jerk and snap which all effect the dynamics of the valve train.
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.
We have a published article on the development and manufacturing of a camshaft for further reading. Click Here to Read
Here is an example of cam design for a popular 4 cylinder 4V OHC. One is a lobe summary (450292C.txt), another is an S96 of the lobe design (450292CC.txt) which is fed into the CNC grinder program, and another is an S96 file (450292CV.txt) of the valve action which can be used for investigation or other studies. When examining any S96 file one must scroll down the zeros describing the base circle to see any lift values.
The S96 files are 396 rows of 8 decimal precision that describes the base circle and lift; 360 degrees and the remaining 36 rows redefines the base circle portion again. This format was designed by Harvey Crane (http://www.harveycrane.com) during the early years of cam design. His website is full of educational material and well worth a look.
All the design data in the linked text files have been generated by me and they describe the cam or valve motion, the summary almost has the valve train geometry and the opening and closing ramps, the exponents, constants, etc.
Now if anybody says the design cams ask them to provide a sample of design data. Most are unable to provide the raw data. Many of them are designing .050” timing and max lift which is a small part of the overall cam profile.
One note of interest, cam designers are usually not engine designers nor well versed in thermodynamics. Most of what they learn about can design comes from customer feedback and basic experience. Cam salesmen go off what worked well in specific applications and apply that to future requirements.
When we receive the finished cams we inspect them on our verification machine and compare them to the design file. Our requirements of lift error are less than .0005” and if it is exceeded we have a problem with the vendor and will not place them into the market until new ones are made that meet tolerances.