Request for material recommendation for a cam and roller follower with high contact stress

Hello everyone,
I would like to ask if anyone could please help me with the following
situation.
I have 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, and I am using a Parabolic curve, which should give the
best minimum radius of curvature and lowest contact stress of just
about any curve that is located between two dwell points. 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..
The maximum contact stress between the cam and roller using a 3/8"
thick cam & 3/8" long roller is 331,228.24 PSI. I used the formulas
in the cam design manual by Clyde Moon to calculate the contact stress
along the curve, with the aid of a spreadsheet. I downloaded the design
manual from
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It's difficult to make the cam thicker than 3/8" due to various
design constraints, but there is a small chance I could go to a
thickness of 7/16" or possibly =BD" at the very extreme. This would
give a maximum contact stress of 306,657.76 PSI & 286,852.07 PSI
respectively.
The maximum contact stresses occur at the point of maximum angular cam
displacment, and 90% of the time the cam is not rotated that far. The
average maximum contact stress that the cam sees 90% of the time is
probably in the range of 220,000 to 252,000 PSI depending on cam
thickness. Still, it seems I should design for maximum stress along the
entire cam profile.
If the device fails there is a zero percent chance that anyone would
get hurt or injured. I don't think I have the luxury of working with
normal safety factors (if any), since the design is on the edge.
My main concern is that I need to avoid plastic deformation, and I need
to be reasonably sure that any elastic deformation of the cam or roller
will not cause the roller to roll rough or slide, i.e., if the pressure
causes a large enough flat spot on the roller, there would be sliding
or rough rolling. I am more concerned about these two factors than wear
or fatigue, since the cam rotates so slow and intermittently.
Can anyone please recommend a material and hardness combination for the
cam and follower that would withstand this type of contact stress? I
want to use something that is as cost effective as possible to machine,
heat treat, and work with. What metal properties do I need to be most
concerned with ? I would think compressive yield and shear strength
would be the two most important properties to consider, along with how
easy the material is to work with.
I found the following materials listed below on
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that
have compressive yield strengths of over 300,000 and 400,000 PSI,
depending on how hard they are (usually between 60 & 64 Rockwell C).
However, I am not sure how difficult they are to machine and work with
prior to heat treatment. The site gave no machining rating, but said
the ASTM 897 grade 5 machines well.
The cam is a very small "rib" cam that has two rollers. One roller
works on an inner profile and one roller works on the outer profile.
The stresses listed above are for the inner profile, since it has the
highest stresses. The cam rib gets thin right at the cam high point
(about a .120" wide rib over a short span) in case this could be a
problem during heat treatment.
Materials Found on
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UDDEHOLM VANADIS 6=AE Hot Work Tool Steel
Carpenter Speed Star=AE High Speed Steel (Red-Hard) (AISI M2)
Spray Formed Grade ROLTEC SF Cold Work Tool Steel
Spray Formed Grade WEARTEC SF Cold Work Tool Steel
ASTM 897 Grade 5 (230-185-00), Austempered Ductile Iron
UDDEHOLM ELMAX=AE Powder Metallurgy Stainless Mold Steel
Regarding the cam follower roller, I will be pressing the .1875" OD
roller onto a 2mm OD hardened steel dowel pin so the roller "rotates
with" the dowel/ shaft. Each end of the shaft is then supported by a
low friction self lubricating bushing. I think this arrangement will
allow the roller to roll well without sliding between the roller OD and
cam profile. I was going to use stock tool steel (i.e, A2, D2, 0-1, W-2
etc.) drill rod for the roller since it already comes in the OD I need
and is held to close tolerances. There will be no lubricant between the
cam profile and roller OD. I have also considered glass bead blasting
the cam profile to increase friction between the cam profile and roller
OD, to help insure that the roller always rolls well with no sliding
between the cam and roller OD.
My concern with the roller is finding stock round 3/16" OD bar that
can handle the high contact stress. It seems to me that it probably
needs to be hardened to handle this type of stress. However, when the
center of the 3/16" OD rod is drilled out so that it can be pressed
onto the 2mm OD dowel, it leaves a thin wall. I am concerned that the
roller will distort or crack during heat treatment. I need to make the
rollers as cost effectively as possible, and due to the way they are
assembled, I cannot make the roller and shaft as one piece.
The parts are so small I don't think material cost is a big issue, I
am worried that the high strength materials will be hard to work with.
I would appreciate any recommendations on the most cost effective
materials (easiest to work with) I could use for the cam and follower,
and the best heat treatment method for small parts that have thin
walls. =20
Thank you for your help.
Sincerely,
John
Reply to
John2005
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