An esoteric nuts and bolts question for the old shop hands

[Anonymous]

So I found myself a volunteer position machining parts for a Steam Locomotive rebuilding crew. (Neat puzzle duplicating parts you've never seen before and have no prints for. Hence the handle<G>) The current project is an old ALCo tank locomotive.

There are pins that allow the drawbars on the lead and trailing trucks to pivot slightly. They're captive and sit vertically in the harness, which is bolted to the locomotive frame. Each pin has shoulders such that the middle is 3/4" larger in diameter than the ends. One end of each of those pins has a slight taper (say 0.050" in 1"). The other end has no taper. The pins are close to being identical in dimension and seem to be of a hardened material.
My question to the universe is:

What the devil is that taper for, and how tight do the clearances need to be for it to do whatever it is that it does? I ask because on one end of the locomotive, the holes that the pin fits into are so badly and unevenly worn that I'm putting a bushing in so the pin won't fall out as soon as we get her back in service. At the other end, one pin is a heavy press fit, but using the other gives about 1/32" of diametrical clearance. (hard to get a mike on it- the bolts on that end are frozen.)
The prints we have for the locomotive don't show the pins in any sort of detail.

Any ideas?
I can make a quick sketch to post somewhere if that helps.

[dldance]

crazytechie - first this sounds like a very interesting project even if it does have it's craziness. Second - I have NO machining experience either on steam engines or anything else. However, I did take a very detailed mechanical drawing class many years ago in engineering school that has taught me alot about how things fit together. From that perspective, that taper on the pin sounds like a Morris taper. That would allow one end of the pin a wedge/interference fit into a blind hole to prevent the pin from falling out. The other end of the pin would have lateral forces and need some play to allow the turck to track. A similar taper has been used to attach the chuck to a drill press. That is my guess.

Good Luck.

dd

[Anonymous]

Ok: sketch to follow; I'll put it up as a JPG. Look for it sometime tomorrow. Photos of the offending parts will have to wait- shop's a goodly drive away and traffic out here is horrible. . . and likely to remain so until the 26th. I'll try and get an ACAD drawing together, too.

The engine in question is ALCo C/N 67544, built 1927. 2-6-2T (28"/44"/28"-- that's a whole lotta Rosie!!<G>) 15x24, with superheat. I can *almost * read the order number, etc. on the tattered 5th-generation erection drawing I have here.

She's not that complex. . . just a little tired. Lots of lumber companies had their way with her, and 'bout ran the wheels off . . . .Lots of wear distributed through. Lucky for us she's never been seriously hurt, so the frame is pretty close to straight. Check:

http://www.ncry.org/roster/s_03rd/rd3.shtml

for the short story. We got her as "Kit, Locomotive, Steam, 1:1 scale. Batteries not included, full assembly required. Model may require additional parts and paint to match photo on box. Ages 16 and up." There's something funny/sad about a steam locomotive up on blocks. . .

I'd just hate to have her break or get on the ground with a load of people watching because I made a bad assumption about what something does and didn't leave enough wiggle room.

We tried the Morse taper theory. . . one pin didn't go all the way into the hole, and we had to bounce the whole assembly around pretty hard to get it togeather. The other pin is sufficiently loose in the same exact assembly (wear or ?) that it falls out unless the bottom of the harness is present.
Which tells me we're not understanding something about the assembly.

Our sister locomotive (Quincy #2, also ALCo 2-6-2T) is so worn out that we frequently look at #3 to see how to fix #2. . .

[Anonymous]

ok, so here's a crude sketch of what I'm talking about.



please forgive the quality-- I was sitting in my truck waiting for traffic to thin out when I did that sketch.

I figure that the only danger I'm in is making things too tight-- the locomotive ran with the wear as found. (just not very well) It just bugs me not knowing why there's a taper on one end and not on the other!


a slightly bigger version of the pic can be found at:

http://www.pleasantonmodelrr.org/trains/trains_sketch001.jpg

or ask specific questions and I'll try to elucidate with the camera next week.

[tdmidget]

Are you sure that the taper was originally machined on this part and not the result of wear or cold flow? Heat treatment in those days was frequently done by the color of the metal and is not very consistant. Therefore one end may well be softer than the other. If both ends were taped I would theorize that it was to compensate for wear by removing shims but if it is ovious that it was machinied with only one taper that would apparently not be the case.
Have you tried contacting Doyle McCormick or others with restoration experience? Would love to know what you finally determine to be the original concept and dimensions.

[Anonymous]

tdmidget-

I hadn't thought about the 'quality-of-heat-treatment' aspect before, but what you say does make some sense. If the pins were quick-and-dirty (forge) case-hardened or poorly quenched at some point, that could certainly make for a differential in end-for-end hardness and subsequent wear. (I keep forgetting how far materials science has come!)

There is some evidence of cold-flow on one of the pins. . . . not so much on the other. Shame i don't have a hardness tester ready to hand. . . a series along the length of the pin would be enlightening and might answer the question of "intentional vs wear" quite nicely.

i'm still trying to wrap my head around the near symmetry of the tapers given the disparity in wear in the rear vs the front.

I have no idea if the assembly originally had shims; I'm guessing not, based on the wear patterns on the rest of the parts.

Where can I get contact information for Mr. McCormick?

[tdmidget]

I don't know how to contact doyle McCormick. I'm surprised that you have not been inumdated with post from he ( who has been known to post on this forum) and others invovled in restoration. I suggest using Google and trying outfits like steamtown and other museums and operating groups. Sorry I can't be of more help.
Scott

[jchnhtfd]

crazy -- this is an area where I am NOT, repeat NOT, a qualified expert. Not even an unqualified expert... that being said, however, I kinda like the Morse taper concept (fat lot of steam engines I've worked on -- like zero, zilch!). Why? In aircraft work, which I do know something about, one of the objectives is to make sure that all of the rotation is in one place and on one bearing surface, and that nothing else moves or even frets. From your sketch, I just wonder if it is possible that the taper was -- as dd says -- meant to be an interference fit, to prevent the pin from rotating in the harness and frame and force all the movement to be between the pin and the drawbar. Then, somewhere along the line, the thing got loose and started wearing where it hadn't ought to?

[tdmidget]

Possible jchnhtfd but I dont think so. There would be many many shocks in this application strong enough to break a taper fit. I think if prevention of rotation was the object a pin through one end might have been more durable. however I don't think this would be a major consideration- look at freight car center pins.

[Anonymous]

Well, the die is cast.

I finally decided that the correct way to figure out problems with a 1920's locomotive is to attack the problem like a 1920's mechanic. So i got out the trianglar file and took a series of swipes at the pin, all up and down its length. . . . Standard blacksmith operating procedure in the early 20th for testing if and how much something has been hardened. (depth of filecut after x swipes is proportional to the hardness at that point. . . . no mark = way hard. try a file on the back of a pocket knife sometime to see what i mean.)

The pin appears to have been *mildly* case-hardened, but the hardening on the ends was worn almost completely away-- hence the taper. On that basis, I went after the pin with carbide tooling and cut the ends back to being straight. Then I took a gander at what Machinery's Handbook had to say about low-speed shafitng and made some bushings based on those guidelines. . . I may provide some grease fittings and make it part of the hostling routine to put a grease gun on them before each run day-- one more small chore for the guy <me, probably> that climbs under every weekend. Ah, the glory. <G> These engines live a pretty pampered life under our care, so I don't think I'm going to try re-hardening the pin. If it wears, maybe i'll try making another from some modern material and harden that.

I'll post back late in the year after we've run her a bit and tell how everything turns out. Thanks for your comments, everyone!

[modorney]

The bottom cylinder is, indeed a pure cylinder. It should be a tight (an interference fit, almost). When you put the pin in the hole, it should go all the way in.

The central cylinder (3 inches long) should bear the load of the drawbar, and gravity will make the bar drop down, so you want a full bearing surface.

The top cylinder is tapered, because you want the holding bar to keep the pin rigid. So, the holding bar may flex a bit, when you tighten it. And, since this arrangement probably saw a fair amount of disassembly, and reassembly, often in the field, the taper provided an easy way for everything to stay rigid.

If everything got loose, it would eventually rattle around, leaving a quarter inch of slop, and the movement would tear the pin mounting apart.

A more sophisticated assembly, like shrink fit, is better, but impractical in the field, especially in a cold climate. However, around here (Pleasanton) a shrink fit would be OK.

[tdmidget]

Hmmm makes some sense. BUT if the object here is to compensate for wear and maintain a basically zero clearance around the pin, why not taper both ends and as you said put compression on it by bowing the top bar? Wear on either end would be taken up by the tapers until the shoulder met the bars.