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/...-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