Does a graphite piston wear down?

[rlukens]

General question. Mods, feel free to move this.
I'm close to building my first flame eater. Many different designs, but I favor the graphite piston if for no other reason than it has less mass to change directions.
I've purchase a chunk of graphite. Does it not wear quickly and change the tolerances to required for a vacuum engine?
TIA
Russ

 

[el gringo]

https://www.youtube.com/watch?v=GMqkD0Bua28


Graphite pistons seem to work with aluminum cyl. The fit is ~ -.0003, as close as I can measure with 'minus pins' and micrometer.

Ray M

I have run these ~ 1 hour with no noticeable reduction in performance, I see no graphite residue in the cylinders.

 

[Blogwitch]

Hi Russ,

There are a couple of limitations and good thoughts on the use of graphite.

The first one is that it is easy to machine, although rather dirty to do so, and to reach those fine tolerances use newspaper wrapped around a backing board to wear the surface down to what is required. It is the same material used in pencils, and you will notice that they do wear down rather quickly.
This brings up the next part. The bore needs to have a super smooth surface otherwise the piston will wear down very quickly, the rough bore would act like a file, so you will soon be making a spare
The advantage is that you do not have to lubricate the piston/bore as graphite is a very good dry lubricant.

Personally, I would always go for cast iron cylinder and piston, self lubricating and seems to last forever.

John

 

[rlukens]

John,
I tend to agree with your thinking, anything less than a micro finished bore would surely eat the piston. However many others appear to have used graphite succesfully. I'm wondering if the tenths tolerances are quickly gone.
I've got a couple of old CI window weights, but now that I've bought the graphite I'll so with it.
I'm headed out West for a couple of weeks. Maybe I'll change my mind.

And yes, that stuff has got to be messy.

Russ

 

[el gringo]

I agree with all of your comment. All my IC engines use cast iron pistons. I use graphite on these vacuum engines because of light weight. Yep, need a mirror smooth cyl bore. As for long lasting I have a feeling these little guy will run as long as I want with the original piston

 

[el gringo]

Window weight cast iron is a sure way to mess up cutting tools and cylinder bores. Ask me how I know...:wall:

 

[Cogsy]

The Jan Ridders engine in my avatar has a graphite piston running in an aluminium bore. I had to make several pistons til I got the correct fit (they almost seem to 'compress' and get a glazed look to them) but once I got it running I have never needed to change it. Of course this engine probably only has around an hour or two of running on it total, and is currently gummed up with dust and crud from sitting on the shelf, so I don't know how much work you'd get from one. They are quick and easy to make though, and as el gringo says, the weight difference in a low power engine is a bonus.

 

[rlukens]

Window weight cast iron is a sure way to mess up cutting tools and cylinder bores. Ask me how I know...:wall:

Mr. Gringo,
I've already beat the hell out of several brazed carbide tools. Lots of pot holes and who knows what in that old stuff. I'll take your advice and buy a chunk of decent CI. Or... go with graphite on the vacuum engine.
Did I read that you used graphite with an aluminum cylinder? At that low operating temperature I guess expansion isn't a problem?

 

[el gringo]

Mr. Gringo,
I've already beat the hell out of several brazed carbide tools. Lots of pot holes and who knows what in that old stuff. I'll take your advice and buy a chunk of decent CI. Or... go with graphite on the vacuum engine.
Did I read that you used graphite with an aluminum cylinder? At that low operating temperature I guess expansion isn't a problem?

yes, I seem to remember aluminum grows ~ .001 for each 1" diameter @ 500f.. don't think these little guys get to 300f so the expansion would still be in tolerance. [and look at all the wear to be saved on the graphite piston]
Ray M

 

[Blogwitch]

Just a bit of advice if you are going to try to get cast iron from a sash weight.

If you come from the US or Canada, don't even try. They seemed to have used any old dross to cast them with, maybe end of day just to get rid of anything left in the furnace.

If you come from the UK, go ahead, I have yet to have one that didn't turn down beautifully and being around 100 years old or older, the cast iron is very fine grained, super smooth and finishes are like chrome.
The loop end was always cast in the downwards position to make sure the loop didn't break because of inclusions. I always cut 3" off the opposite end of the loop, just to get rid of any inclusions, then everything after that should be perfect..After the outer scale is removed you should end up with 13" long x 1 3/8" diameter.

When I get my lathe remotored I will do a vid showing how easy it is.

BTW, I have never had to buy any cast iron in all the years I have been into ME, except when it was included with a kit of some sort, all because of sash weights. I still have about 30 to 40 sitting outside ready to give up it's secret. In fact the cylinder blocks for the 'paddleducks' engine was the size it was because that was the size I could get from a lump of sash weight diameter..

John

 

[werowance]

Window weight cast iron is a sure way to mess up cutting tools and cylinder bores. Ask me how I know...:wall:

amen to that. and will destroy a bandsaw blade in seconds. I had to try it my self as well....they are hard as a rock. even left one weight in my outdoor wood boiler which I keep fired up year round for hot water in summer heat in winter for 2 years. I figured that would soften it up some. nope still hard as a rock.
durabar is great stuff though

 

[neil_1821]

I successfully used a graphite piston with a Stirling engine, the cylinder was a glass test tube so was perfectly smooth. Tolerances on the test tube aren’t great so took a few attempts to get a good fit without air leaking past and there is small graphite reside in the tube but that does aid lubrication. I turned to graphite after many failed attempts with a polished brass piston in a lapped cylinder as well as using peak in a glass cylinder. It’s ideal for low powered engines

 

[mcostello]

I have some graphite cylenders if some body wants to do it the other way round.

 

[el gringo]

Wonder how a graphite piston would work with a graphite cylinder??? Any shedding of material would simply adhere to the other surface??? (you heard it here first)
Ray M

 

[Cogsy]

Wonder how a graphite piston would work with a graphite cylinder??? Any shedding of material would simply adhere to the other surface??? (you heard it here first)
Ray M

But isn't that basically the definition of galling? The material shed from one surface would build up a high spot on the other and cause more wear, and repeat... I'd be interested in seeing the experiment though.

 

[el gringo]

Me too:thumbup: Hope someone is setting it up now, maybe the modeler who has the graphite cylinders?
pss...I'm not sure of how much galling is involved with equal materials of such low frictional values at very low or zero delta temperature. [ie: TFE verses TFE]:wall:

Ray M