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Plain bearing material selection

Bill Simmons

Cast Iron
Joined
Mar 5, 2006
Location
Auburn CA
I am rebuilding a trans-axle for a 580k Case backhoe. Amount many issues the spider gears and their cross shaft pins are highly worn, about .040 combined wear. I bought aftermarket gears. Due to crazy prices on OEM pins and really poor quality in the aftermarket pins (I know, I bought some), I am considering making the new pins my self. They are about 2.5 inches long by 0.875 in diameter. The features required are some flats and one cross hole of about 0.2 in diameter.

I do not have a way to measure hardness of the original pins but they will not cut with HSS but will cut with Carbide tooling. I am just an amateur machinist and do not have much knowledge of metallurgy. I do not think that the pins would be highly stressed as about 5/8 of an inch is supported in bore on each end leaving about 1.25 inches between supports on a .875 inch pin. I believe I want a material with the max hardness I can machine to minimize wear. Please make some recommendations for a suitable material.

My primary tools are a Takisawa TSL 800 and a manual Gorton 2-32 mill.
 
Were the after market pins hardened? As in 'cannot be nicked with a good corner of a file'? If not, you might be better off looking for a heat treatment to improve them. Same goes for whatever you could make yourself. 4140 prehardened is not hard enough to call hardened.....more like strengthened.

I'd use 8620 and get it carburized for .030" case depth, then hardened to Rc60. This gives you a ductile center and a very hard case, which I feel will be more shock resistant than 4140 heat treated right through, at Rc60, if they can even get that out of it.
 
55 is about all you’ll get out of 4140 and it will be glass. I would temper it back to no more than 52RC and still not hold my breath.
 
The after market pins can be readily scratched with a file and slightly cut with the broadside of the file on the corner of the pin. The file just skates across the edge of the OEM pins but I can turn them with carbide using light cuts. Not sure how to translate that to some RCxx value. My center punches will barely mark both with no evident dimple to the naked eye.
 
(Good)Files are usually about 65RC and carbide is around 70 the last time I checked. It sounds like the aftermarket pins while not quite as hard as the OEM still have had a bit of effort put into them so quality issues aside I would shoot for no less hard than the aftermarket.

What was your plan exactly? To finish machine then harden or to rough them, send for hardening and finish machine? Because the possible warpage will affect material selection.
 
I do not have a plan yet. I was hoping that a material could be found in the machinable range (since I can machine the OEM pins)and thus not have to heat treat, but hoping and wishing may not get the job done. Currently the ends of the OEM pins are about .002 under size but either the pins or the holes are not round as it takes a little twisting and jiggling and a mild push to get them to "slide" in. The only critical dimension is the diameter. If it was prehardened to around 6xRC what would be the recommended type of drill bit for the .2 dia. cross hole.
 
It sounds like you could buy some of these:

McMaster-Carr

and a 1/4" four flute carbide endmill to cut away sections prior to drilling or for cutting the flats. Case hardened to ~.060 deep, but once you cut through it with the carbide endmill you won't have any issue drilling (at ~500rpm) with a good HSS drill.

Be patient with the milling, you want as stiff a setup as possible to support the cutter, at about 1K rpm. Don't "jump" into the part or during cutting, you want slow, uniform feeding.

You can use the 3" length version if you want a little more support in the outer bores (if they're long enough).
 
If I was trying to clean up a pre-existing ~.2 hole a carbide ball nose either plunging or interpolated. Making the hole from scratch a straight flute D-bit style drill but I’m not sure if your machine has the rigidity needed. A sharpened masonry drill bit can work in a pinch.

McMaster-Carr something like this.
 
The dowel pins look interesting but I am not sure of the hardness. They have undersized pins and oversize pins listed as RC50 and RC60 respectively but neither of these are available in a large enough diameter. The just "alloy steel" pins that you selected are not rated for hardness. Also their pin stock which is also RC60 is not large enough in diameter. All are described as being the strongest pins they have.
 
Well upon further research of the dowel pins I found the following. These pins are are speced to ASME B18.8.2. That standard says (I found it in MSC catalog) that core hardness shall be RC50 to RC58 and a case hardness of RC60 minimum. So hardness should be as you stated. I may order tomorrow which will get them here Monday. I am trying to think this through. Since the pins are a smidge over size and I need a couple of thousands under, I may order longer pins and so they can be chucked in the lathe and turned for to size for the desired length and reversed in the chuck and and then parted (probably abrasively). I will first try and see how well I can cut .001 deep with an old pin. I don't like trying to take so little.
 
If you can, I'd open up the holes a tiny bit to accept the stock dowel pin, as it's likely that the hole is already worn, and the pin has a good cylindrical geometry and smooth finish. It might even fit fine as-is, due to bore wear.

Let us know how things work out for you.
 
Why not use some Thompson 60 case harden shafting? McMaster-Carr has it. Buy a length of it, set up a abrasive cut off wheel on your chop saw and cut them to length, debur then and add the cross hole to them. Grind/mill a flat where needed. And you got it.
 
Thanks all for your suggestions. Milland said to let you know how things worked out and I often wonder about the rest of the story after reading suggestions. So here it is.

I now have the pins done and installed. I used the Thompson 60 rod, cutting to length using my abrasive saw. I was able to finish pins to length on the lathe with carbide tooling. The rod was about half a thou under nominal. The old pins were about .002 under nominal. Using a snap gauge I found the bores to be about .002 out of round. I cleaned the bores up with a .875 reamer. The new pins pushed in with light pressure. My biggest concern was drilling the .200 holes. I used a stub length solid carbide drill. It took a lot of pressure to get started but it made the three holes. I was really worried about breaking it on exit but it survived. The flats cut relatively easy using a .5 inch 5 flute carbide end mill.

Bill
 
Sometimes with very hard material it's better to just avoid the exit entirely. Drill halfway through from one side then halfway through from the other. Then your "exit" is in a softer area in the middle of the shaft.
 








 
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