graphite yarn no thanks o-rings yes please

Have read that it is ill advised to use graphite rope packing for piston rings when used in a cast iron cylinder. Am wondering if this is true in all applications across the board or weather it's use is possible in a large , low pressure , low r.p.m. steam cylinder application where the cast iron cylinder could be highly plished?

Hi Richard,

Graphited packing worked well on small engines (bigger ones have almost always used metal rings) for nearly 200 years. then decent temperature-stable o-rings were invented. thou o-rings generally seem to work better, there is no reason why graphited yarn won't work but beware excessive piston clearances that could blow it out in no time.

Neil

The cylinder I am using to build up a grasshopper steam engine is 12" bore dia. Am wanting to get as good a seal as possible in order to maxamise condensing vacume. O rings seem prefferable but would they hold up in a cast iron application? Will be steaming with graphite introduced into the steam line.Also, I am thinking graphite packing would tend to burnish cylinder where as am thinking o rings might score cylinder if particles in steam are taken up.

Edited By richard orr on 18/09/2013 03:37:05

Hi Richard,

As long as you aren't using superheat (unlikely for a beam engine!) then o-rings ought to last. Yes graphited yarn is said to improve the finish, but I can't speak on o-rings causing scoring.

Neil

On a slightly different tack, but still about sealing cylinders:

Wasn't there an article in ME moons ago about using PTFE rings to virtually eliminate friction, but backed by 'O' rings to provide the needed pressure against the cylinder walls?

Didn't René Etter of RSME, experiment with PTFE and found that it creeped if not suitably contained.

If memory serves, he did overcome the tendency of that material to creep by containing it, only allowing a small amount to 'show', but I cannot remember if he was using it for rings or 'D' valves.

Geoff - Bad day today so no workshop.

Piston rings made from sheet PTFE would give good low friction, these type of rings are used in car shock absorbers, and I'v pulled a good few of those to bits, and they never seem to be worn out. When I rebuilt the Stuart S 9 earlier in the year I thought of using teflon, but decided that as I had some cast iron, I used that. Ian S C

The engine is indeed a 12" bore. double acting steam cylinder. ( Quite a story to my finding it ) It is going into a large sidewheel steamboat that I have been building over the past 14 years. The primary power will come from pressure rather then vacume. However, with a cylinder of that diameter, there is a ot of power to be gained from vacume also

I cut off the steam box and will use the D valve port face to make a type of poppet valve for the exaust that will also serve as a water slug relief port . Intake will be through bored out water drain valves. Intake pressure will be 30 p.s.i.g. expanded down to 10 p.s.i.g. R.p.m. will be from 80 to 120.

I think you could make a ring from PTFE(maybe bronze impregnated), a strip of sheetthe thickness of the required ring, and cut to the width, the joint would be best if it were stepped. For even lower friction, carbon impregnated PTFE is available. Ian S C