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 Post subject: Main Rod Bearing Repair
PostPosted: Sun Dec 28, 2014 9:29 pm 

Joined: Thu Apr 14, 2005 9:34 pm
Posts: 2327
Location: Copenhagen, Denmark
We have a main rod of type (2) from Figure 37 of Bruce (1952), "The Steam Locomotive in America". It is a bolted strap big end with full face wedge. The engine is a fast express engine that is operated at high speed. The bearing recently exhibited knock, and the bearing was over-adjusted, leading to heat and what appears to be bearing failure. The brass shows a hairline crack in the babbit, and some evidence of bearing delamination.

The current plan is to bore out the bearing and apply new babbit. Comments?

Main question - what is necessary to correctly locate the rotating center of the bearing and make a true cut? An opinion expressed here is that we need to take the whole rod and mount that in a mill/boring machine in order to correctly locate the bearing center and axis alignment. The suggestion is that the alignment of the bearing brass to the rod is unknown. However, we really do not have a machine large enough to hold the rod.

Comments? What would we do otherwise to locate the brass center and true up by machining the brass out of the rod? Do we need to machine this at all? Can we melt out and re-babbit the bearing as is?

Steven Harrod
Danmarks Tekniske Universitet
Institut for Systemer, Produktion, og Ledelse

 Post subject: Re: Main Rod Bearing Repair
PostPosted: Sun Dec 28, 2014 10:58 pm 

Joined: Sun Aug 22, 2004 7:19 am
Posts: 5699
Location: southeastern USA
The distance between centers of the bores in the rod brasses should be equal to the distance between the centers of the crankpin of the wheel to which the rod connects and the wrist pin in the crosshead when both are at the same extreme dead center (front or back), and the half of clearance in the cylinder divided equally by measurement at that end (hope this makes sense). You don't have to deal with compensating for angularity in the rod as you would if you went at top or bottom quarter.

The most practical way I have found to do this is by removing and rebabbitting the bearing halves separately - you can use the same mandrel for both - then clamping the halves together to bore them out with the split at the center between the halves. There's a margin to which you need to oversize the bore to account for keeping them running cool and lubricated with adequate lube clearance. There's probably some standard for the locomotives in your country or well known shop floor rule of thumb your machinists will know about. If you make them that correct diameter in the mill and set them up face to face, your engine crews can't force them into too tight a fit out on the road and burn them up again. If you trust your crews, you can leave a bit of adjustability in them....your call.

Anyhow, once you have the split brass bored and ready to install, clamp it up in place with the other end also clamped in place, with thin hard sheet metal strips running top to bottom across the brasses from split to split - these will give you reference points to measure between the centers of the bores. By either skimming off or shimming behind the fixed half of the brasses, you can decrease or increase the distance to achieve a good fit. So, you finagle the correct length in the rod, not in the mill, when you have everything fixed and can measure accurately how much adjustment you need in real space, rather than by trying to work it all out on paper and hoping.

The hardest part is finding the dead center of the crankpin and getting a good real (not theoretically correct) distance measured between the two centers with both set in the right helps if the crankpin and wrist pin are still mostly circular.

Good luck with your project - wish I could help with the air pump question but WABCO and NYAB pumps I've laid hands on don't have those connections, and nothing but lube lines run beneath the jackets.


Santayana: "He who does not remember the past is condemned to repeat it."
Corollary: "He who does is doomed to watch those who don't repeat it anyway."

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