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 ...
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