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barts
05-11-2008, 04:09 PM
Hi folks -

I'm refurbishing a 110 year old steam engine (2.5"x2.5") to use in our 19' steam launch; the existing engine I built from a refrigeration compressor is worn out after 18 years of service.

Like many small engines, the eccentric straps are bronze and run on cast-iron eccentrics; they're retained w/ a larger diameter section in the middle of each eccentric. The provision for oiling these straps is a small hole in the top of each one; needless to say, even though this engine has a piston valve and thus sees much lower valve actuation forces than slide valve engines, the straps have worn significantly. They're reasonable complex bronze castings, so I think I need to build up the worn bearing faces and then machine them true again. There's probably .015" or .020" clearance now; I think .003" would be a lot quieter.

Can one apply babbitt as one would use solder? I'd make a fixture to hold each eccentric in the four jaw, build up the surface and then machine the
diameter and clean out the mating groove. Babbitt is appealing as it's low temperature and doesn't affect the base metal.

The piston valve would also seem to be a good candidate for a similar treatment; it is bronze, has no rings and thus would need frequent
attention to stay steam-tight.

Comments? Other ideas?

The company that made this engine went out of business in the panic of
1893, so spare parts are unlikely to be available :-).

- Bart

sconisbee
05-11-2008, 04:37 PM
over bore then press in and silver solder/Tig in a bronze bushing perhaps? granted im no steam engineer so thats a guess

Oldbrock
05-11-2008, 05:03 PM
Without seeing the eccentric and strap the ideas here may or may not work. First, are the eccentrics round and parallel? If not they have to be turned to correct it. How thick are the straps? If you don't want to make them thinner you should machine babbitt thickness into the eccentric you should leave about 1/8" . You have to acid clean the straps and tin them as you would anything you are going to solder. Babbitt should be poured so you have to make an internal dam so you don't have to fill the whole thing. Make a circle,wood would do, hardwood if you have it, about 1/8" smaller than the eccentric and locate it and the strap concentric. Make sure they can't move then heat the strap to the melting point of babbitt, don't know it off the top of my head, and melt the babbitt and pour in into the annular space and let cool. Now you may set it up and bore it to fit the eccentric. I'm sure others will add to this or contradict me but I have done a few babbitt bearings in my time. Good luck, Peter

wierdscience
05-11-2008, 05:14 PM
Is the strap a split bore like a connecting rod?If it is you might consider shaving the clearance off the cap side of the split ,re-boring it round and letting it run like that.

Other alterantive would be bronze welding to build up the strap,if you don't have a torch or haven't done it before they do now have silicon bronze welding wire for the mig.

barts
05-11-2008, 05:44 PM
Yes, these are split straps.... the cast iron eccentrics are in good shape; they're about .003 different in size, but both are round. I had not considered turning down the eccentrics to add babbitt thickness; that's an interesting idea. The straps are quite thin and I'm loathe to remove much material. I had considered bronze welding to build up the straps, but that can be tricky....

I'll try and post a picture of the straps mounted on the eccentrics.

- Bart

J. R. Williams
05-11-2008, 06:29 PM
Barts
What is the diameter of the eccentrics? The application of Babbit or Aluminum bronze will require a light cut on the surface to get a good clean surface. Is the diameter such that a standard automotive or truck crankshaft bearing shell be cut down in width and tacked welded or soldered in place?

JRW

macona
05-11-2008, 07:06 PM
Once you know your eccentric is true Moglice would work great to build up the worn surfaces. I have used the stuff twice now and its great.

Rich Carlstedt
05-11-2008, 07:34 PM
Just file the bottom strap, thats what they did in the old days.
The strap does not need to be round ...sorry guys..they didn't worry about it
The clearance at the joints(sides) , means nothing, only the clearance at the top and bottom, which you correctly put at .003
This determines valve timing, while side play has no effect on valve timing or operation.

Rich

Mcgyver
05-11-2008, 08:27 PM
Just file the bottom strap, thats what they did in the old days.

that and install some sort drip oiler.

Oldbrock
05-12-2008, 01:24 AM
I was thinking of a drip oiler too. Could it be mounted remotely and flexible tubes leading to the straps? just another thought. Peter

Ian B
05-12-2008, 07:26 AM
Bart,

Can you machine up new eccentrics, or skim the existing ones and Loctite a ring onto them?

Ian

Davek0974
05-12-2008, 08:12 AM
Do it the right, old-school way :)

File or machine the caps a bit, bolt it all together and get it running;)

Adjust the fit by feel.

I am currently building a scale steam engine with three straps and have had plenty of discussions with people who are used to these problems. Its just wear and tear.

Good idea with the remote oilers, small flexi pipe going to a small tank with on/off taps should do well. My straps only have oil holes too, seems quite common.


Dave

Mcgyver
05-12-2008, 09:20 AM
Perhaps there's a flex hose that would work for this, prototypical practice was a drip oiler above and then a elongated cup to catch the oil and carry it to the connecting rod big end bearing...that's how they solved the problem of bearing motion, drip feed not.

however my memory sort of failed me...i thought there was a similar arrangement for the eccentrics...except thinking about it more there's too much movement in the reversing gear. Here's a pic i took showing the cup receiving oil for the connecting rod bearing and and also one at the top of the drag link....but unfortunately no ingenious device on the eccentric, just a plain old reservoir. I guess the Oiler's job was still safe.

http://i20.photobucket.com/albums/b201/michael0100/tripleexpansion.jpg

the problem with this set up is not way to keep crude out, and not way to turn the oil off. I wonder if took a large machine oil cup, you know the ones with the sprint loaded lids, removed some of the sides and epoxied in a glass liner if that might not work and look authentic. Or a small brass cylinder with cut outs and glass liner and a screw on top.

In a perfect world, there'd be no wear - mating parts would not touch gliding effortlessly on a a hydrostatic layer of the correct lubricant. Deviation from this ideal comes from not enough of the right lubricant being there at all times and/or contamination of the lubricant with grit/crude.

a simple cup with lid is better than nothing, better still if the sides were cut out so you could see the oil level with a glass sleeve installed....this one is smallish and is easily done like this or with a angled spigot, ie it could come right out of the eccentric and angle slightly to be parallel with the rod

http://i20.photobucket.com/albums/b201/michael0100/oilcup-1.jpg

here's a shot of a home made drip oiler, complete with adjustable needle valve and shut off....a lot of work but something like that could adapted and they're available commercially if you don't want to make them

http://i20.photobucket.com/albums/b201/michael0100/oilervertical.jpg