crankpin size of material selection

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Dale Grice
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crankpin size of material selection

Postby Dale Grice » Mon Jan 09, 2017 3:02 pm

What do you use for size of stock for crankpins? Given that a pin is .750 +0 -.001 on print. It has been suggested to me that the mill finish on 1018 crs is good enough for use in a bronze, oilite, or fiberglide bearing. In my mind a little polish with 400 grit emery would yield a better bearing surface.

My 3/4 crs stock is .7493. When I polish it, it gets down to 0.748. And I still see some very small pits.

RBC fiberglide bearing cjs1216 has a minimum diameter of 0.7480. I haven't looked up a bronze or oilite bearing minimum shaft diameter. Joe Nelson's book states .002 clearance on rod pins/bearings.

To get rid of my minor pits, I would have to get too far undersize.

Do most of you use the same size stock needed as in my example above? Or do you get oversize stock, and turn it down? Or I thought maybe getting ground stock out of Mcmaster. But they do not carry ground 1018 crs.

The collet chuck I am using is running 0.0002 out. So I figure if I use 3/4 stock, the 2 tenths out of round would not be an issue.

Just looking for insight from those of you that have been there and done it.
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blff cty lcmtv wrks
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Re: crankpin size of material selection

Postby blff cty lcmtv wrks » Mon Jan 09, 2017 4:32 pm

imho, i would use a piece of 1 inch stock, turn it down close to the finish diameter. polish, sand, etc to get it really smooth. then fit the bushing to the crank pin. take care on the other end where you press the pin into the wheel. get the hole on the wheel sized first, and make the pin to fit. do realize if you press said bushing into the rod end, you just might squash the bushing and make the hole a bit under size. thank god for reamers. a good reamer is like a paintbrush, it will cover up a lot of sins. i really don't think it will hurt if you have a bit of wiggle room between the crankpin and the bushing.

back to the crank pin material. when you machine for the rod end bushing, leave about a 16th of an inch for a shoulder. this will let the rod stand off a bit from the wheel. i hope i made myself clear.

have you considered the black moly rods for bushing mateial. works real good.

bigc
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Trainman4602
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Re: crankpin size of material selection

Postby Trainman4602 » Mon Jan 09, 2017 8:44 pm

Hi Dale

You should go one step further and harden the pins. Believe me the pins will ware egg shaped. I have had this problem on several locomotives it is impossible to fix with out pressing of the driver from the axle and pressing out the old pin and replace it. I have tried to file them more round and make a special bushing to fix the problem. NO GOOD!

Do yourself a favor and either case harden the CRS or use some other material that can be hardened.

Even a further step would be to grind them.
ALLWAYS OPERATING MY TRAIN IN A SAFE MANNER USING AUTOMATIC AIR BRAKES

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BigDumbDinosaur
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Re: crankpin size of material selection

Postby BigDumbDinosaur » Tue Jan 10, 2017 2:58 am

Dale Grice wrote:What do you use for size of stock for crankpins?

I'd use something better than C1018 cold-rolled. Aside from surface finish qualities—the lack of them, in the case of ordinary cold-rolled, the mills do not always guarantee how round the material will be. You might discover that your "round" piece of C1018 is anything but round.

The other issue that was brought up by the OP was the cyclic loading the crankpins will see as a consequence of normal running. Unless your crankpins are made from a relatively tough grade of steel they will eventually become slightly elliptical due to a combination of wear and physical distortion—and will ruin the rod bearings in the process. C1018 is really too soft for this sort of application, but often gets used because it's cheap material, readily available and not difficult to machine.

Unless you want to go through the process of heat treating your crankpins, I'd suggest using materials that are reasonably tough but not too difficult to machine. My two choices would be C1144 or ASTM A311, aka StressProof®, which is a more refined form of C1144. You will need to do some polishing on these materials (more on this below) to get the desired finish, unless you order TGP stock (turned, ground and polished). TGP needs no finishing, as long as it isn't rusty (don't buy rusty steel, no matter how cheap).

Heat treated pins will be much tougher and probably will outlast the rest of the locomotive. :) The material choices in this case would be C1144, ASTM A311, 4130 or 4140, which are listed in order of machining difficulty and ultimate strength and toughness. You would turn the parts to within 0.002" of desired diameter, heat and quench, and then draw the hardness back to around R30C or thereabouts. You'd then finish your pins to size, using various grades of abrasive cloth (not emery!) and cutting oil. Your final step will be to polish the wear surface with crocus cloth and cutting oil while the lathe is spinning fast (1200-1500 RPM), which with some patience will achieve around a 20 micro-inch finish, close to that achieved on an automobile crankshaft's bearing surfaces.

Alternatively, if you have access to suitable equipment you can grind the heat treated pins to size and then give them a light "dusting" with crocus and oil to get the abrasive marks out that will be produced by the grinder wheel.

Any of the steels I mentioned are best machined with a carbide under oil flow to get the best possible finish. A high speed steel cutter will work with C1144 and ASTM A311, but probably won't survive cutting 4130 and 4140. These last two are chromium-molybdenum alloys with considerable toughness, so plan accordingly if you decide to use them. Cutting any of these steels dry will likely produce a poor finish and make cause chatter problems in lighter lathes.
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Marty_Knox
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Re: crankpin size of material selection

Postby Marty_Knox » Tue Jan 10, 2017 10:35 am

I use ground and polished 1144 Stressproof the size the drawing shows for the running surface. If a shoulder is called for I just use a bronze thrust washer. I bore the hole in the wheel for a press fit. For a specific example, the Allen Mogul main crankpin is shown as made from 1" diameter stock, turned to 7/8" where it is pressed into the wheel, and 3/4" where it runs in the bearing. I make it from 3/4" Stressproof. A lot less work, and I have a polished running surface.

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BigDumbDinosaur
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Re: crankpin size of material selection

Postby BigDumbDinosaur » Wed Jan 11, 2017 4:35 pm

Marty_Knox wrote:I use ground and polished 1144 Stressproof

Pedantic note: C1144 is not StressProof® (note the spelling), the latter which is a trademark for a steel that was developed in the 1930s from AISI 1144. In other words, StressProof is a derivative of C1144, but C1144 itself is not StressProof.

Generally speaking, StressProof will achieve better metallurgical properties following heat treat than C1144, especially in ultimate strength and uniformity of hardness. However, for hobby use, StressProof can be used as is without heat treating.

TGP will be within 0.001 inches of the nominal size, but will not exceed it. I used 1 inch diameter TGP StressProof to make the axles for both my locomotive and my control car. When checked with a vernier micrometer, the stock diameter was 0.9997 inches. Both locomotive and control car have disc brakes, with the caliper brackets riding on the axles to maintain radial position relative to the rotors. The brackets have a 1 inch diameter bronze bushing that was reamed to 1.001 inches after press-fitting, providing the necessary clearance. An occasional shot of chassis grease keeps everything running smoothly.
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mattmason
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Re: crankpin size of material selection

Postby mattmason » Wed Jan 11, 2017 5:37 pm

I use oil-hardening drill rod. Right on size and pretty hard. I do not harden them, but have not had much in the way of wear after 6 years.
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BClemens
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Re: crankpin size of material selection

Postby BClemens » Wed Jan 11, 2017 6:33 pm

I use the inner rings for roller/needle bearings of appropriate size (usually metric) and look specifically for ones grooved and drilled for lubrication. These are pressed or loctite-ed to the crank pin and used as the bearing race. If used for the inner race, a masonry bit will drill a hole (grind debur) in these for location aligned with a grease/oil hole in ---crank pin/axle or whatever. Positive bearing surface with a commercially hardened bearing race surface - roller/needle/or plain.

BC

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Greg_Lewis
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Re: crankpin size of material selection

Postby Greg_Lewis » Wed Jan 11, 2017 11:18 pm

mattmason wrote:I use oil-hardening drill rod. Right on size and pretty hard. I do not harden them, but have not had much in the way of wear after 6 years.

Be careful here. I discovered, much to my dismay, that drill rod can be tri-lobed and by several thou. This will not show up with a casual micrometer check, since the three lobe nature of the error cancels out. You have to set the rod on good v-blocks and rotate it while checking with a quality dti on a stand. It took $30 of my money and a half a day of frustrating results to learn this.
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BigDumbDinosaur
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Re: crankpin size of material selection

Postby BigDumbDinosaur » Thu Jan 12, 2017 1:06 am

mattmason wrote:I use oil-hardening drill rod. Right on size and pretty hard. I do not harden them, but have not had much in the way of wear after 6 years.

Would not be my choice for axles or crankpins. There's the tri-lobing problem previously mentioned by Greg, plus drill rod is on the brittle side. It's not a structural material.
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daves1459
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Re: crankpin size of material selection

Postby daves1459 » Fri Jan 13, 2017 12:36 am

Consider using "Stress Proof" steel. Stress Proof is a trade name for a pre-heat treated steel and is readily available from most of the internet metal suppliers like Online Metals and Speedy Metals. Stress Proof is hardened to the Rockwell "C" 26-27 range which makes it tough in bending and hard enough for good wear against bronze bearing materials. After hardening Stress Proof is post treated to improve stability and machining. It can easily be machined with high speed tool bits, has good chip forming characteristics, and gives a good surface finish.

Another pre-heat treated steel is "Fatigue Proof". Fatigue Proof is a little harder then Stress Proof at Rockwell "C" 30, but, has near the same machining characteristics. Unfortunately it is more difficult to find for the home machinist in small quantities.

For reference for the yellow cast bearing bronzes like SAE 660 steel hardened to Rockwell "C" 30 will have about the same wear rate as the bronze. For the softer orange bronzes such as Naval Bronze, Tobin Bronze, phosphor bronze, and etc. they wear equal to softer steels like Stress Proof. Whatever steel you use make sure the journal area against the bronze is well polished.

Lastly, tool steels such as oil hardening (O-1) and water hardening (W-1) while wear well have the problems listed above also have low fatique strength and tend to crack in bending unless the internal corners have a radius and relatively large diameters are used. Avoid air hardening steels altogether unless you have access to commercial heat treating facilities. If you must have a harder shaft look into 41L40. It is easily obtainable as Stress Proof and machines easily because of the lead in it. I.E.; the "L" in the designation. It will harden to the Rockwell "C" 50 range using a torch, oil quench, and 350 degree F temper and not be brittle. Again, for reference, shafts that run in needle and roller bearings are hardened to Rockwell "C" 58 to 62.

Dave S.

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BigDumbDinosaur
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Re: crankpin size of material selection

Postby BigDumbDinosaur » Fri Jan 13, 2017 4:42 pm

daves1459 wrote:Consider using "Stress Proof" steel. Stress Proof is a trade name for a pre-heat treated steel and is readily available from most of the internet metal suppliers like Online Metals and Speedy Metals. Stress Proof is hardened to the Rockwell "C" 26-27 range which makes it tough in bending and hard enough for good wear against bronze bearing materials. After hardening Stress Proof is post treated to improve stability and machining. It can easily be machined with high speed tool bits, has good chip forming characteristics, and gives a good surface finish.

Another pre-heat treated steel is "Fatigue Proof". Fatigue Proof is a little harder then Stress Proof at Rockwell "C" 30, but, has near the same machining characteristics. Unfortunately it is more difficult to find for the home machinist in small quantities.

For reference for the yellow cast bearing bronzes like SAE 660 steel hardened to Rockwell "C" 30 will have about the same wear rate as the bronze. For the softer orange bronzes such as Naval Bronze, Tobin Bronze, phosphor bronze, and etc. they wear equal to softer steels like Stress Proof. Whatever steel you use make sure the journal area against the bronze is well polished.

Lastly, tool steels such as oil hardening (O-1) and water hardening (W-1) while wear well have the problems listed above also have low fatique strength and tend to crack in bending unless the internal corners have a radius and relatively large diameters are used. Avoid air hardening steels altogether unless you have access to commercial heat treating facilities. If you must have a harder shaft look into 41L40. It is easily obtainable as Stress Proof and machines easily because of the lead in it. I.E.; the "L" in the designation. It will harden to the Rockwell "C" 50 range using a torch, oil quench, and 350 degree F temper and not be brittle. Again, for reference, shafts that run in needle and roller bearings are hardened to Rockwell "C" 58 to 62.

Dave S.

I guess you didn't read any of the other posts before yours. :D
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