Request for help finding a cam curve that will have lower maximum contact stress than a Parabolic curve (and link to CAD & JPEG drawings)

Hello everyone,
I would like to ask if anyone could please help me with a design I am struggling with due to space constraints. I have a Rapidshare link
listed below where you can download CAD and JPEG images of the cam and follower, along with output from my cam design software, for clarification of the problem.
This is related to my post regarding finding a cam material that can withstand high contact stress. I would like to try to attack this from another angle and try to find a cam curve that will have a lower maximum contact stress than the Parabolic curve I am now using.
The cam is a very small radial disk cam with an oscillating roller follower that has high contact stress between the cam and roller. Everything on the design is "locked in" i.e., I cannot make the cam or roller larger (except for cam thickness & roller length), I cannot increase cam versus follower displacement, or decrease follower versus cam displacement. This is a very slow moving cam oscillated manually by hand, so I don't have to worry about the dynamics of the curve, vibrations, etc..
I am presently using a Parabolic curve because it has the largest minimum radius of curvature and lowest contact stress of all the standards curves my cam design software can produce, i.e., Harmonic, Modified Sine, Modified Trapezoid, Cycloidal, and the "standard" Polynomials (3-4-5), (3-4-5-6), (4-5-6-7), etc..
At the link below you can download DWG and duplicate DXF drawings, along with duplicate JPEG images of the cam, from Rapidshare. The CAD files have layers you can turn on that show more curve detial than the JPEG's. I also included output text files from my cam design software and a text file with all dimensions, angular displacements of the cam and follower, and roller forces and spring rates ...
http://rapidshare.de/files/16364719/Rib_Cam_Eng-Tips.zip.html
This is a dual roller conjugate rib or blade cam that uses two cam follower rollers. The outer roller pushes into the outer profile (the profile furthest away from the cam rotation axis) and inner roller pushes into the inner cam profile (the profile closest to the cam rotation axis). The two rollers have different springs and spring rates.
The arrangement is fully explained in the Read-Me Microsoft Word file included with the drawings.
The inner profile is the profle with the highest contact stress due to its smaller radius of curvature. Even with the existing Parabolic curve, the outer profile of the cam has much more reasonable maximum contact stress at about 181,000.00 PSI using a 3/8" thick cam, which seems acceptable to me.
I would like to keep the maximum cam thickness at 3/8" as it may be difficult to increase thickness beyond that.
It seems it might not take much of a change to reduce the contact stress on the inner curve, perhaps at the expense of a larger pressure angle or some other trade off. The inner and outer profiles have to be the same curve type, so whatever changes I make to the inner profile, I will have to make to the outer profile.
Perhaps a cubic curve, elliptical curve, or a special Polynomial curve is the solution. Dynamics and vibrations are not an issue since the cam is so slow moving. I would have liked to explore the Stoddart, Duddley, Berzake, Thoren, Cycloid first half, and Harmonic first half curves, but my software won't produce those curves.
If there is a curve that produces a lower maximum contact stress and I could get a CAD file of the curves I could superimpose over the Parabolic curves for comparison in AutoCAD, that would be a very big help. Cam design software output for the curves would also be a very big help. If I know the cam angular displacement versus follower angular displacement for each degree or preferably each 0.25 degree of cam rotation, I can put that in my spreadsheet and double check the maximum contact stress & other things.
I wish my cam design software were not so limited, because I could then just experiment and zero in on the best compromise.
I would really appreciate any feedback or help anyone can offer.
Thank you.
Sincerely, John
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<snipped> I don't know if I am reading you correctly. If you are saying you have a parabolic curve on a cam, Are you meaning that the roller will have a higher radius distance and then fall in towards the center in this curve? If so.. Why does it need to be parabolic curve at all? Why not closer to a straight line between the two points with the center of the line being the same "radius" as the center of the parabolic curve would have? Or a lesser angled parabolic curve?
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Hi Spaceman,
Thanks for your reply. Are you the same Spaceman from Allexperts ? If so, we have talked before.
I think you have the basic idea, it's just a standard cam using a Parabolic curve. You can download the JPEG's if you want to see it.
I might be able to use a "modified constant velocity curve", which is a straight line with a radius added where the straight line blends with the cam dwells. This may be the best solution to the problem, I am not sure.
I'm not sure a lesser angled Parabolic would work, if the radius of curvature gets larger at each curve end, this will likely increase the angle of the Parabolic curve.
Thanks again, John
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That is not me, but i have read some of his answers in the UFO section. I am not that far out there. I think the truth is right around us and not only "out there". :)

If I get a few minutes extra soon I will register to download that file and look more closely.

What is the Software name you are using anyway?

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Hi Spaceman,

John2005: I'm using software from the book "Cams for industry" by John Reeve. It's an old DOS program. It's a good program and a good book, but it's geared for higher speed cams where dynamic performance and vibrations are a concern. It cannot make a modifed constant velocity curve, I should be able to make it in AuotCAD without too much trouble, but not as fast as with software. The software does not calculate contact stress, I had to make a spreadsheet for that, using the formulas from the cam design book by Clyde Moon you can get free here...
http://www.camcoindex.com/svcman/moonbook.pdf
I have also checked out about every cam design software demo out there, i.e., Dynacam, Camtrax, Cam-Designer, Antalytix Cams, etc.. I even found a freeware version of cam design software http://www.camme.it / but it is not in English. There does not seem to be a whole lot of software available to design cams, and all of the demos I tried were either very limited, high priced, or did not do what I really wanted or needed (demo or full version).

You don't have to register with rapidshare to download the files, just scroll down, click the "free" button, enter the code they give you, and download.
Thanks again, John
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Well, I seem to be at a loss on this one. Can you tell me what the arms that connect to the rollers are doing and why they have to be done at the same time? (unless it is a secret invention) :)
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Hi Spaceman,
Thanks for writing back. Without allot of drawings and perhaps somewhat of a lengthy description (even more so than my original post) it may be kind of hard to describe what the cam does, but you can basically just think of the rollers being attached to a spring force, the follower is very small. The weight of the follower is only about 2 pounds and has no real effect as far as forces are concerned. All of the roller force comes from the springs, but I cannot really reduce the spring force.
Thanks again, John
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