Track for Cam Follower

[dozer]

We've got an extremely heavy object riding on several cam followers. The cam followers ride on an 2" thick plate embedded in concrete. Here's the problem. Who ever "designed" (I use the term loosely) the embed plate didn't think about hardness. They just used A36 steel.

You guessed it, the embed plates have long since been installed, so I'm planning on bolting a hardened plate on top of the embed plate. Maybe Ryerson's Wearform 400 which has a hardness of 360 BHN.

Is there a way to calculate how thick this extra plate needs to be or can I just go with a 1/4" and call it good? The plate would be completely supported by the embed plate which is in a 3' thick reinforced concrete floor. The cam followers are 5" O.D. and have a load of about 29,000 lbs each. They move very slowly (9 in/min) and not very far or very often.

 

[GregLocock]

I'd have thought this was an ideal application of Hertzian contact stress theory. Just eyeballing it (depending on the width of the rail) I think you are in a world of hurt, but if you can replace the hardened strips when they fail then that may be a sensible approach. The railway people have this stuff down pat, they use a much smaller wheel loading and much bigger wheels.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[solider]

My only worry would be whether the original plate wears uniformaly - considering both along and across the embedded plate.
If the wear is not uniform, then the new hardened strip will not be uniformly supported - which may cause more issues for you.

But I agree with GregLocock, Hertzian contact stress should be considered. You will find the peak stress to be below the hardened plate surface when subject to a loaded, rolling cam follower. So make sure the depth of hardness exceeds the depth of the peak stress.

 

[diamondjim]

Just out of curiosity.
Have you done a hardness check
on the A36 steel? I wonder if
it has been work hardened by
your heavy loads.
How wide are the wheels?
1/4 thick seems like an adequate
thickness. Have the cam follower
wheels embedded into the soft
steel? Are you certain that the
cams share the loads equally?

 

[Pressed]

What is the rail failure, deformation or premature wear? If the hard plate addition doesn't work out, consider these.

If you have abrasive wear, compare the hardness of the follower to the rail. With expensive cams I always made sure the followers were 2-4 points softer on the Rockwell C scale as they were easier/cheaper to replace. This is not always true for every case and depends on the ratio of cam to follower OD and desired outcome (ie: harder followers don't always wear out softer cams). If you do reduce the followers hardness to be sacrificial to the rail you might also be testing if those followers can handle their load. Abrasive wear can also be attributed to particles/debris at the point of contact.

To prevent adhesive wear, one good combination of materials for cam and cam follower is soft steel and cast iron; which has been used for railroads. According to the articles (linked) the rail is 300 Bhn and the wheel is 363-401 Bhn.

If the rail material is deformed, brinelled or spalled, it might be better to replace the rail. Especially if it is below the point of follower contact; being embedded you might not see this, but any sign/indication would be bad.

On the practical side, is the rail flat and the roller cylindrical with the maximum amount of contact utilized?

Here are a few links of related information.

[URL unfurl="true"]http://www.rbcbearings.com/camfollowers/capacity.htm[/URL]
[URL unfurl="true"]http://mdmetric.com/prod/iko/ikonucf.pdf[/URL]
[URL unfurl="true"]http://www.mines.edu/fs_home/jsteele/411/Lectures/L07Wear/L15SurfaceFailure.pdf[/URL]
[URL unfurl="true"]http://www.highbeam.com/library/docfree.asp?DOCID=1G1:121498377&ctrlInfo=Round20%3AMode20e%3ADocG%3AResult&ao=[/URL]
[URL unfurl="true"]http://www.findarticles.com/p/articles/mi_m0BFW/is_10_98/ai_93699210[/URL]
[URL unfurl="true"]http://eng.monash.edu.au/railway/pdf/RCF.pdf[/URL]
[URL unfurl="true"]http://www.ktu.lt/lt/mokslas/zurnalai/mechanika/mech_51/BAZARAS151.pdf[/URL]

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[dozer]

Upon rereading my post I did speak in present tense. My bad, the heavy piece of equipment has not been installed yet. I was reviewing our supplier's drawings and calcs when I noticed he hadn't addressed the issue of rail hardness. I use the term "rail" loosely. It's just a flat plate embedded in the concrete.

What I ended up doing to determine the thickness of the wear plate is made it thick enough so that by the time the load spread out onto the embed plate is was less than 0.66Fy. This is a bastardization of section K1.3 of the AISC code that I think is conservative.

No, I didn't assume the load is carried uniformily by all of the rollers. Don't worry good engineering judgement was used to determine the loads on the rollers. Where good engineering judgement fell flat was this whole hardness issue. I could start a whole new thread on how often I come across subs, suppliers, vendors, and consultants who don't seem to know there you-know-what from a hole in the ground.

 

[unclesyd]

Look at your thickness on the wear plate because if the stresses are very high you will start ironing or wrinkling your plate. All rolling contact wear plates I've seen used all have been thicker than the 1/4" mentioned unless they were in chute or bins.