Quieting down Otter's engine.

[barts]

Otter has an antique engine made by Bowditch Mfg. Company sometime in the 1890s... it's 2.5" x 2.5" bore and stroke, and the piston valve has no problems with the 150 psi steam the boiler happily produces. However, the connecting rod bearings are not so happy, and every time I've whacked the throttle wide open the knock has gotten a little louder despite ample lubrication. Since the engine is quite compact, the big end bearing is 1" diameter x 1" length bronze or brass; the cross head is 7/16" diameter. Both ends of the connecting rod have discernible play, so there's the knock(s).

So I ran some numbers for the big end at 180 psi:

* The surface feet/minute at 400 rpm is 3.14/12 * 400 = ~ 105 ft/minute.
* The pressure/unit area is (2.5/2)^2*3.14 * 180 = ~ 900 psi.

PV ~= 95000

Hmmm.

Now Bunting Bearings provides data in a handy chart here: http://www.buntingbearings.com/data.html

Now I look for an alloy that will meet P, V and P*V constraints....

I can see why the original cast bronze bearing didn't do the job - the P*V rating is prob. exceeded by 30% or so at full rock and roll.

It looks like aluminum bronze will work - alloy 954 is available from Mcmaster-Carr, and more than 25% margin on the PV rating, with 6x on the pressure rating and 2.5 x or so on the velocity.

Ok, that handles the big end, but the crosshead bearing also needs help. Rocking bearings are problematic since they don't develop proper hydrodynamic support; they're always scraping metal to metal. I think I'll fit a needle bearing here in the connecting rod. This means I'll drop down to a 3/8" wrist pin, but a hardened and ground pin (needed for the needle bearing) will easily handle the loads. If I have to I'll make a new rod to fit the needle bearing captive; the big end bearing just bolts on and that's a handy feature.

There's a nice winter project.... we're planning on another big year of steaming, so having a nice quiet engine that will take full throttle will be a big plus.

- Bart

[DetroiTug]

I'm not familiar with your engine, but If possible, I would opt for ball bearings on the big end and needle bearings on the wrist pin. If a new rod must be made it will be better than trying to fit a much larger bearing than either end of the rod was designed for. My engine has ball and needle on the rods, super quiet and no tending. If they fail, simply replace them.

I had a loose wrist pin in mine, because the rod was bored oversize by the original attemptee. I fixed it last spring, the engine is dead quiet now. Changed the whole boat; that clacking at high speed was annoying.

-Ron

[Lopez Mike]

Wonder what the heat buildup would be with a Delrin bearing. Fitted zero clearance or even maybe tighter. Is the crank journal smooth?

Has it knocked from the beginning and just gotten louder recently? With automotive plain bearings my experience has been that they run for years totally quiet but once they star to rap, things get worse in a hurry.

[barts]

The crank is one piece and not overhung, so there is no way to use ball bearings at the big end. The crank journal is pretty smooth. I've tightened up the clearance once, but that only lasted a year or so. Delrin won't take the guff here. The limiting PV for the best Delrin-AF seems to be less than 10000.

- Bart

[Lopez Mike]

My Evinrude 15 outboard has a one piece crank and a needle bearing big end connecting rod bearing BUT I'm sure that the crankpin surface is hardened. You could case harden your crank pins surface but then I think you might be inviting a crack.

I'll bet your big end bearing was getting loose for years and just got loose enough for you to hear in recent times. Have you considered just taking out a shim or two? Or if there are no shims, shaving the cap a few thou and adding some?

I'm planning on tearing down my Strath Warrego this Winter for paint and minor changes and one of the things I will be doing is documenting the various clearances.

The valve chambers at each end of the piston valve seem to accumulate crud and corrosion. I wish I knew of some anticorrosive coating I could apply to the cast iron that would stand the steam and heat. Also, the piston valve is hollow to let the exhaust steam travel from the upper end through to the lower end and thus out the exhaust connection. Mung collects there too. I wonder how long automotive exhaust paint would survive in there? They bake on some sort of glossy finish on expensive wood stoves. Time for a web search.

[barts]

My Evinrude 15 outboard has a one piece crank and a needle bearing big end connecting rod bearing BUT I'm sure that the crankpin surface is hardened. You could case harden your crank pins surface but then I think you might be inviting a crack.

The crank is original, and prob. cast or ductile iron. I'll leave that alone. If I ever have to replace it, I'll need to turn one from solid - there's not much meat there since they kept the engine length so short.

I'll bet your big end bearing was getting loose for years and just got loose enough for you to hear in recent times. Have you considered just taking out a shim or two? Or if there are no shims, shaving the cap a few thou and adding some?

Actually, there are no shims, and I tightened the bearing up up by machining some off the mating surfaces and boring a round hole a couple of years ago. I could certainly do this again, but I'd like to get this right. I'd like to be able to run the engine at full speed w/o worrying.

I'm planning on tearing down my Strath Warrego this Winter for paint and minor changes and one of the things I will be doing is documenting the various clearances.

The valve chambers at each end of the piston valve seem to accumulate crud and corrosion. I wish I knew of some anticorrosive coating I could apply to the cast iron that would stand the steam and heat. Also, the piston valve is hollow to let the exhaust steam travel from the upper end through to the lower end and thus out the exhaust connection. Mung collects there too. I wonder how long automotive exhaust paint would survive in there? They bake on some sort of glossy finish on expensive wood stoves. Time for a web search.

Power coat.... you can get formulations that work to 1200 F.

- Bart

[Lopez Mike]

The powder coating sounds interesting. It might be a bit of a trick to keep it off of the surfaces on the cylinder and the valve bore. Might help ever so slightly with heat loss issues.

Mike

[Cyruscosmo]

here is a link for ya Mike http://www.caswellplating.com/powder-co ... tions.html and grizzly sells a cheep powder gun.

Cheers

[Lopez Mike]

Cool. So if this stuff will take that much heat, hot do I have to bake it after spraying? I have visions of melting something on my engine!

Mike

[Cyruscosmo]

Each color comes with the temperature and duration of bake. The high temp silver from that link says 450 degrees for 20 minutes. I have not used any of the High Temp stuff yet but the other stuff I have used cured at 350 degrees and pretty much takes a hammer to remove. Would the steam engine even see temperatures over 350?

Oh and remember to put tape where you don't want the coating to stick. And use an oven that your wife does not. The smell of that stuff curing does not go good with roast!

Cheers
Scott