Creating A Cam Profile

What actually is the cam profile? In the beginning, the 3-arc type cam profile was a lobe shape that had some dimensions on it like a blueprint. You could then physically machine the cam lobe shape from the dimensions. Anyone can create a cam profile this way. The only thing you know for sure about the finished cam profile is the maximum lift. None of the other data will be known until  the cam profile is actually made and then measured. Changing the dimensions of the arcs will change the measured data. The 3 arcs are the (1) base circle diameter, (2) the nose(tip) radius, and the (3) flank(side) radius. The distance between the base circle and the nose will obviously determined the maximum lift. The lift of the tappet at various places is measured out to four decimal places and the duration and velocity data is then calculated. That will usually provide enough data to know if the profile will work for the application. As the need for more control over the cam profile increased the number of arcs also increased. I am not sure but it looks like six different arcs were eventually used. That allowed some type of ramps to be integrated into the profile. An overall slow process but it worked. Remember a low rpm engine with light valve spring pressure is very forgiving to the cam profile design. This was all done before computers and involved mathematical equations were used to design the cam profile.

With computers a cam profile lift table can be created out to at least six decimal places. This lift table IS THE CAM PROFILE and can tell you almost everything you need to know. You just have to know how to read it. Below is a sample lift table on the opening side of a cam profile. There will also be one for the closing side. In this example they are the same (symmetrical cam profile).

  0    0.4200000
  1    0.4198850
  2    0.4195400
  3    0.4189650
  4    0.4181600
  5    0.4171250
  6    0.4158601
  7    0.4143653
  8    0.4126406
  9    0.4106862
10    0.4085022
11    0.4060888
12    0.4034464
13    0.4005754
14    0.3974762
15    0.3941494
16    0.3905959
17    0.3868165
18    0.3828125
19    0.3785850
20    0.3741358
21    0.3694665
22    0.3645793
23    0.3594766
24    0.3541612
25    0.3486360
26    0.3429046
27    0.3369709
28    0.3308390
29    0.3245139
30    0.3180007
31    0.3113052
32    0.3044337
33    0.2973929
34    0.2901904
35    0.2828341
36    0.2753327
37    0.2676954
38    0.2599321
39    0.2520534
40    0.2440706
41    0.2359955
42    0.2278409
43    0.2196199
44    0.2113466
45    0.2030356
46    0.1947023
47    0.1863625
48    0.1780327
49    0.1697300
50    0.1614721
51    0.1532769
52    0.1451628
53    0.1371486
54    0.1292531
55    0.1214954
56    0.1138944
57    0.1064687
58    0.0992368
59    0.0922166
60    0.0854250
61    0.0788782
62    0.0725911
63    0.0665770
64    0.0608476
65    0.0554124
66    0.0502788
67    0.0454511
68    0.0409312
69    0.0367175
70    0.0328051
71    0.0291854
72    0.0258463
73    0.0227722
74    0.0199441
75    0.0173400
76    0.0149365
77    0.0127169
78    0.0106757
79    0.0088146
80    0.0071388
81    0.0056538
82    0.0043632
83    0.0032667
84    0.0023599
85    0.0016333
86    0.0010727
87    0.0006597
88    0.0003725
89    0.0001874
90    0.0000799
91    0.0000262
92    0.0000054
93    0.0000004
94    0.0000000

The table lists the tappet lift at each degree of camshaft rotation. The first column is the degree of rotation and the second column is the tappet lift. Zero degree is the maximum lift of the cam profile. Do you know the duration at 0.050 for this profile? If you don't, you need to start at the beginning and carefully read all of my entries. The change in lift from one degree to the next is velocity. The change in velocity is acceleration. The change in acceleration is jerk. All these are important parameters to look at when analyzing a cam profile. A lift table with a resolution of at least six decimal places is necessary to adequately analyze these parameters. The lift table can be inserted into a spreadsheet program and these numbers can be calculated. A graph(curve) can then be created from these numbers. Lift, velocity, acceleration, and jerk curves can tell much about the cam profile design. If you have a computer camshaft lobe profiler or looked at a profile report, this is the kind of data you will see. Many engine builders use this data to help decide on which camshaft to use. The ramp designs and valve lash settings can also be analyzed. I also like to know the radius of curvature data and pressure angle data. That data is not always part of the lift table but should be available in the profile report.

As I said, the lift table is the cam profile. The lift table is the piece of data that you want to have. It will be able to tell you important information about the cam profile along with the ability to actually manufacture it. How the lift table is created is really not important. There are different ways the lift table can be created. The 3-arc cam profile talked about earlier could be computer profiled and the lift table created. Different cam profile design programs can create the lift table. The type of program used does not matter. On valve trains with a variable rocker ratio the valve lift data is designed first and then converted to the lobe lift table. The original cam profile design or an actual lobe should have a lift table that can be analyzed to determine if the cam profile design is a good one or a not so good one.

If you have a lift table or .s96 file that you would like analyzed, email it to me and I will give you my honest opinion on the cam profile design.

The answer to the duration at 0.050 question is 264.23 degrees at the crankshaft.

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