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How tight piston rings?

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Alan Worland09/07/2011 21:40:51
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I have built a ST No 4 engine which has been assembled as I have machined the various parts, and before I took it all apart for 'final finishing' I put an air line on the inlet, and with very low air pressure it could tick over quite slowly and sounded great!
My concern is that the double piston rings are going to be too tight - the bore has a good finish within and the ring ends have a gap (very small but not touching)
The rings just seem too 'springy'!
Am I worrying too much or is there anything I can do to take some of the spring out?
 
Alan
Jens Eirik Skogstad10/07/2011 03:53:42
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Springy??? If the engine has good compression and has good torque under starting since it is a steam engine, nothing wrong with the piston ring. It will take a time to get piston ring seated to cylinder under running-in period. Too tight ring gap can give more friction between pistonring and cylinder if the piston ring is expanded by heat of working temperature, also if you see the steam engine will loss revolution when the steam engine is in working temperature (let us say the "cold" steam engine is running fast and will reduce revolution when the steam engine is in working temperature caused by expanded piston rings due too little ring gap.).. Read this article about piston rings: http://www.btinternet.com/~sylvestris/rings/rings.htm
Alan Worland10/07/2011 21:33:10
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21 photos
Thanks! What an interesting article. Just had a measure of the force required to close the gap, and they (two rings) both measure about 2.5lbs, so they would appear to be, if anything, a bit on the low side. Although I still feel they are tight in the bore!
I shall assemble the engine and run it unloaded with low pressure air and plenty of oil and see how they perform.
Jens Eirik Skogstad11/07/2011 21:01:38
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400 forum posts
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The pressure will press the piston ring against cylinder wall under work and keep tight against leakage in cylinder. The machined surface is not perfect worked by tools, then the steam engine/internal combustion engines need running-in to make fit perfect between piston ring and cylinder. But the cylinder surface need a lot of "oil in pockets", also the cylinder need cross hatchet cylinder honing on the surface. After running-in is done, there is still visible cross hatched tool marks by honing tool. It means lubricating the piston rings are good and will last long time so long the oil is available by "oil in pockets" in cylinder surface..
 
I has a TVR1A twin steam engine with O-ring as piston ring, it took me about 8 hour running-in to make perfect fit between O-ring and cylinder. It need a lot of oil under running-in. Now the steam engine is running fine with oil in steam by displacement lubricator. Without oil, the O-ring has enormous friction against cylinder. Piston ring is better than O-ring and need less oil, the real big steam engine need up to 2-3 drops oil per minute with mechanical lubricator under normal use (source: Book for mechanics by Peder Lobben, printed 1935). You need the displacement lubricator with valve to control the oil consumpion and keep steam engine well lubricated without spend too much oil.
Alan Worland11/07/2011 23:05:47
247 forum posts
21 photos
I have honed the bore (although I cant really 'see' any cross hatching) I can see plenty of pors for the oil to be retained in.
Seems like it would be a better idea to run it in under steam then? or just give it loads of oil in the compressed air?
I have a displacement lubricator to be fitted in the supply line.
Jens Eirik Skogstad13/07/2011 06:49:55
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400 forum posts
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You can take running-in the steam engine with air, but you must stop steam engine often to lubricate the cylinder. The solution is the fog oil lubricator for air pressure tools.
 
Better to run in the steam engine with steam because you get all parts expanded by hot steam "to right size" + enough oil in steam.
 
Pors to retain the oil in cylinder wall is good since it is used cast iron as cylinder material. Other material such as bronze, brass, aluminium, best to have cross hatched honed surface.
Alan Worland13/07/2011 22:23:08
247 forum posts
21 photos
At one point I didn't think the rings were going to open up enough without breaking, but they went on! 0.0035 inch gap and the assembly seems quite 'smooth' in the bore.
I am continuing to assemble the engine and am looking forward to having it running!
I have a displacement lubricator for the engine, do you think this would be ok fitted into the steam chest or would it be better in the supply pipe, but close to the steam chest?
Jens Eirik Skogstad14/07/2011 06:09:57
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400 forum posts
22 photos
Better in the supply pipe, but close to the steam chest.

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