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davidh
03-29-2008, 04:27 PM
many small engines use aluminum for connecting rod bearings, actually they just machine the rod and make no inserts.

i have a new 5 h.p. air compressor that has a totally knocked out rod bearing and the crank is a bit chewed up and required re-sizing. the manufacturer does not sell parts, just compressors (made off shore). and its splash oiled. . .

i brought the crank and rod to my engine rebuilder and he tried to find a undersized insert to match the reground crank to no avail.

i told him i could machine my own bearing if it could be made of aluminum and have considered pouring a tube big enuf to hold and machine in my lathe then split and tangs pressed , using melted down auto pistons that i have from engines of the 60's and 70's.

i suppose a guy could use brass but i have the aluminum. . . .


the compressor turns about 900 rpm, thats why i considered and aluminum machined set of inserts.

help anyone ?

davidh

einstein
03-29-2008, 04:47 PM
Keep after your pouring temperature, if that one goes gassy it won't machine well and might cut the shaft from the oxide buildup in the pores.

Also, I don't know how well aluminum would hold up there, though it sounds like it would run ok if the original was bare aluminum.

I'm surprised that they are not using SAE-841 bearing bronze for that actually. It would probably explain why it's eaten it up so badly, if the machine was operated with low oil for any length of time that would do it.

Though SAE 841 doesn't cast properly due to the fact that it is designed to be porus and through powder metallurgy actually carries a fair amount of oil already a part of it. It would probably outlast the remaining life of that machine though if you made the inserts out of it.

I think you can get tubes of it, and it machines very easily.

rocco
03-29-2008, 05:03 PM
Have you considered babbitt? It's an old fashion bearing material but it should work well for this application. I'm not sure if babbitt would stick to aluminium, it might be necessary to copper plate the bearing surface of the rod to prep it for the babbitt.

davidh
03-29-2008, 06:32 PM
i believe the original inserts were babbit, typical automotive type, but i had not considered making those. . . never seen it done. the engine rebuilder tried to get something to use that was sized right but could not . i;ve seen a lot of articles on re-babbiting but never read them. i must not have thought i could or would ever need to. . .

the 841 brass sounds like the thing to use. i would not have had a clue what to use. is this similar to oilite type material ?

i know about the dang alum temps getting too gassey, been there. it gets really wierd when its poured too hot. . .

im getting the crank and rod back this week so i will know what size it was ground down to.

i know it was run without oiil as the sump pan on the engine was installed incorrectly and it literally ran out of oil and quit. this rod is the only one that was injured, the high pressure one is perfect, no marks or discolor at all.

i just hated to scrap the thing, it sells for 7 or 8 hundred us bucks. . .

ten thousand thanks for the information.
davidh

einstein
03-29-2008, 06:48 PM
the 841 brass sounds like the thing to use. i would not have had a clue what to use. is this similar to oilite type material ?


Based on a google search, SAE 841 bronze is oilite.
Here's a typical product listing, but I noted the term oilite in the google search that included this. (http://www.atlasbronze.com/sintered_bronze_product_sheet.html)
Here's one that lists the same material as an oilite bearing material. (http://www.smallparts.com/products/descriptions/tbb-sbb-fbb.cfm)

I've made bearing inserts out of it before, it's nice stuff though it can be a touch pricy.

Also, I don't know if babbit actually has to stick to the host material. On my 1915 nelson bros engine, the babbit inserts can be pried out of the cast iron rod and main bearings using a screwdriver once the caps and shaft are removed from them, though they were likely cast in-place it doesn't look like an overly complicated method.

Mine rely on grease cups though to lubricate them.

davidh
03-30-2008, 01:52 PM
wow ! they are proud of their product. ($$$$) still, i will probably go for it. it is the primary (lo-pressure) piston in a two stage pump and should work fine. i am going to do some reading up on babbit. hell,i need another interest to add to my resume`.
davidh (the old guy)

Bobnova
03-30-2008, 05:29 PM
All the old IC engines used to have bearings cast into 'em as einstein said, the process was cast the bearing, bolt it up, take it apart, scrape the high spots off, bolt it together, take it apart, scrape the high spots off, bolt it together, and so on.
Then once it was "good enough" they would tow it up to ~30mph and dump the clutch, as the fit was too tight for the starter to turn the engine.
Once it was being dragged along the friction between bearing and crank would "machine" it all flat-ish.


Things have come a long way since then.

HAVEHEATWILLCAST1
03-30-2008, 10:46 PM
What size is this rod bearing anyhow?
How do they know what size to grind the crank too :?:

Could you not get a copper tube or something (diff bearing) and have them grind the crank to fit it's thickness?

Have them off-set the grind and make a stroker out of it. :D

einstein
03-31-2008, 03:41 PM
All the old IC engines used to have bearings cast into 'em as einstein said, the process was cast the bearing, bolt it up, take it apart, scrape the high spots off, bolt it together, take it apart, scrape the high spots off, bolt it together, and so on.
Then once it was "good enough" they would tow it up to ~30mph and dump the clutch, as the fit was too tight for the starter to turn the engine.
Once it was being dragged along the friction between bearing and crank would "machine" it all flat-ish.


Things have come a long way since then.

Starter?
30 MPH?
Where could you be in such a devil of a hurry?

http://i31.photobucket.com/albums/c376/sbssean1987/Mecha/th_94d34d73.jpg (http://s31.photobucket.com/albums/c376/sbssean1987/Mecha/?action=view&current=94d34d73.flv)

There's the engine I was talking about. You can see the main bearing caps between the flywheels. I didn't do the babbit on it myself, it was already redone when I got the engine. But it looks like the babbit was cast in and scraped, and set with a few shims in the mains so it could turn without binding too badly.

Likely it was started with the mains slightly loose and then as they ran-in they were tightened down until the shims had the spacing correct. It'll free-wheel for almost 30 seconds between hits if I take all the load off of it.

Btw, it is supposed to sound like that. The whole "variable displacement" thing being promoted lately in cars and trucks is not a new idea, it actually dates back to engines like this one that only fired when they needed more power.

Bobnova
03-31-2008, 04:34 PM
Wow, now thats old school!

rocco
03-31-2008, 09:56 PM
I took apart a 48 Chevy engine a few months ago, it had cast in place babbitt rod bearings.

davidh
04-03-2008, 04:07 AM
the old chev is of the times of plastigage. i remember it well. i think i still have some somewhere.

those old hit and miss engines are cool as can be. i can't tell by the picture but does this one have a pot full of water around the head for cooling ? i've always wanted one but apperantly not bad enuf causes i still don;t.

the engine guy is going to grind just enuf off the crank to clean it up. . . the 841 brass sounds like the thing to use. i'll keep ya all up on it. . .

HAVEHEATWILLCAST1
04-03-2008, 04:44 AM
plastigage :shock:
I'm haveing a thread spitting brainism. :P

http://www.backyardmetalcasting.com/forums/viewtopic.php?t=2271&start=0&postdays=0&postorder=asc&highlight=

einstein
04-03-2008, 05:22 AM
those old hit and miss engines are cool as can be. i can't tell by the picture but does this one have a pot full of water around the head for cooling ? i've always wanted one but apperantly not bad enuf causes i still don;t.

That's a hopper cooled engine alright. At one point in the video it splashes a bit, there's water to within 2" of the top of the cooling hopper. Though I think one of the flywheels is slightly off balance, at it's governed speed it vibrates a little bit which leads to the splashing. The fact that it is mounted on an old riding mower so it can move itself about the yard if I line up the other belt doesn't seem to help any, the tires like to let it bounce and splash all over itself.

This one doesn't have the water in contact with the head though, the water only cools the cylinder walls. Head itself is air cooled, but has no fins to assist this.

I've never had a problem with it, it doesn't fire often enough even at the highest load I can get it to overheat or anything bad. I have had the water boiling a few times during the summer though, running it up the hill to my workshop to power a grindstone and then back down to where it is kept will usually make it steam a bit.

lodcomm
04-03-2008, 05:44 AM
For an excellent tutorial on planning, pouring and shimming babbit
get gingery's book on it:

http://www.lindsaybks.com/dgjp/djgbk/babbitt/index.html

Mcmaster/carr sells babbit ingots.

cheers

reubenT
04-04-2008, 02:55 AM
many small engines use aluminum for connecting rod bearings, actually they just machine the rod and make no inserts.


that's the way this onan was and I'm just reproducing it, I'd recommend using a product called MBL in all bearing and gear lubricant, (except where friction cluches are used, it'll cause them to slip) Used in gas engines it will keep the engine going if it were run out of oil.
(I carelessly drove my truck to town and back once when the oil had run low and it was rattling from lack of oil to the lifters, didn't hurt it a bit)
I put it in an engine that had quite a rattle to it and it quieted it down and ran fine for a long time. (that one is still a good engine but it sit's in a defunct homemade log skidder waiting for another job.)
http://www.eimi.com/mbl.html
I love that stuff, it really cuts down on friction.

sandcrab
04-20-2008, 04:25 PM
The inserts for the 1/3 scale Chevy were rolled from Sterling Silver sheet from a jewelry supply house and then reamed and hand scraped to rod
or main journal size after tightening the caps. Precision inserts have several thousandths crush when the caps are set. The crush is really
what holds the insert in place, not the small indents.

On a compressor the pressure are no where near as high as they are in an internal combustion engine so I would think that Aluminum sheet
could be rolled to fit the rod and then machined and hand scraped to give the clearance for the splash oiling. Just make sure when the cap is
bolted on that it presses the aluminum shell into the rod. You can get plastigage at the auto parts supply and use it to size your new insert.

Rather than pouring babbit bearings some of the miniature engine builders machine the inserts from an ingot and then remelt the turnings
in a lead ladle and are ready to do it again. It works best with two pieces mated in a four jaw so that when they are parted off they are
each a true semi circular shell half.

Just my $.02 :)

Bobnova
04-20-2008, 05:05 PM
If you're feeling really advanced (at least, advanced enough to mimic 50's nascar tech) you can scrape out a thin wedge (couple thousanths) where the two halves of the bearing shell meet. With bearings that don't have some material removed there those two edges poke inward slightly when the cap is torqued down, pinching the journal. If you take a bit of metal off it gets crushed to a circular shape.

Pretty minor and probably not worth it on a air compressor or hobby motor, but on the nascar motors of the day it was worth a decent chunk of horsepower and durability.
Now if you look, every single bearing shell you find will have a little bit of metal removed from that area, it's one of the very few things the auto makers picked up without waiting 30 years.