Request for material recommendation for cam and follower with high contact stress Question
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Posted by: john2003 ®

03/23/2006, 20:03:11

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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 http://www.camcoindex.com/svcman/moonbook.pdf.

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 ½” 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 www.matweb.com 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 www.Matweb.com…

UDDEHOLM VANADIS 6® Hot Work Tool Steel
Carpenter Speed Star® 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® 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.

Thank you for your help.

Sincerely,
John








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Re: Request for material recommendation for cam and follower with high contact stress
Re: Request for material recommendation for cam and follower with high contact stress -- john2003 Post Reply Top of thread Forum
Posted by: zekeman ®

03/24/2006, 10:17:36

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What are you trying to move? Maybe there is a noncam solution such as a linkage mechanism that will do it. Why don't you tell us or show us what motion/displacement/force specs you have? From what you describe, I doubt that you will find a material that will work using your cam unless you find another profile that wll reduce the stresses. I doubt very much that a parabolic profile answers that need. A blended polynomial cam would have to be considered and optimized to minimize contact stresses, since it appears that you are only interested in moving the follower between two discrete positions without regard to the motion.







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Re: Request for material recommendation for cam and follower with high contact stress
Re: Re: Request for material recommendation for cam and follower with high contact stress -- zekeman Post Reply Top of thread Forum
Posted by: john2003 ®

03/24/2006, 15:24:31

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Hi Zekeman,

Thanks for your reply.

I must use a cam for this device. Other mechanisms have been well researched, but only a cam will do.

The Parabolic curve gives the lowest contact stress of most of the standard curves, i.e., Modified Sine, Cycloidal, Harmonic, Modified Trapezoid, and most of the standard Polynomial curves.

I got a demo of the cam design program Dynacam, and it had a few curves that seemed to produce much lower stresses (179,000 to 181,000 PSI). The curves were Thoren, Stoddard, Duddley, Berzake-e, Berzak-d, and "Cycloid first half" & "harmonic first half". All other curves I have come across produce much higher stresses. I don't have experience with these curves.

Responsiveness between cam versus follower movement after leaving the dwells is important on the design. The demo program won't let me export anything, so I cannot really compare the responsiveness of the above mentioned curves with the Parbolic. At the very least I would like to superimpose the curves over the Parbabolic & standard curves in AutoCAD, just to get a feel for them.

If I had a displacement output file showing cam angular displacement versus follower angular displacement for each degree of cam rotation using the above curves, that would tell me what I need to know.

At a price of over 2,000 the Dynacam program is a little pricy for one job.

I wish I had some way to explore the Thoren, Stoddard, Duddley, and Berzake curves, before making a final decision on material. The cam design software I have, won't produce these types of curves. It could be that the Parbolic is already the best compromise, but it would be nice to be sure. A custom polynomial may also do the job.

Thanks again for your help,
John








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