LTD stirling - PTFE or brass power piston instead of graphite?

[mnbylcr2]

Hi Guys

I`m just trying to build my first LTD and most plans call for graphite power piston. Well I have a made a nice true, reamed and lapped bronze cylinder but I`m trying to find a suitable graphite substitute for the piston. I have a lump of PTFE, brass or bronze. I can also get hold of Delrin, I`d have to order it but at a reasonable cost.

I can only find one supplier of graphite in the UK but they are asking for £45 !!!! Well out of my price range.

Will PTFE work? - anyone used PTFE or perhaps for bushes as well?

Chris

 

[b.lindsey]

Haven't tried PTFE so can't say if it would work or not though I suspect it would. The only concern would be thermal expansion, but in such a small part that may be negligible. Before giving up on graphite you may check to see if there are any tool and die shops nearby that use ram type EDM and if so they may have some bits and pieces of graphite in various diameters for a reasonable cost.

 

[woodchip85]

I may be able to find you a piece of graphite depending on the size you need, let me know and ill get back to you.
(I work for a shop with EDMs so we have bits lying round/old electrodes)

 

[mnbylcr2]

pm sent - woodchip!!

 

[David Morrow]

For future reference, you may want to try the Graphite Store.

http://www.graphitestore.com/items_list.asp/action/prod/prd_id/25/cat_id/22

As an example a piece 1" x 12" is $18 US so you get enough for several projects.

Contact info :

Address: GraphiteStore.com, Inc.
1348 Busch Parkway
Buffalo Grove, IL 60089
US
Phone: 800-305-1664 (Toll Free US only)
847-279-1619
Fax: 847-279-1926
E-mail: [email protected]

 

[Krown Kustoms]

Be careful when machining graphite, it conducts electricity.
If the dust finds its way to your motor its finished.
I had a friend making a rocket engine nossel on an old south lathe and fried the motor due to graphite dust on the com.
-B-

 

[David Morrow]

Be careful when machining graphite, it conducts electricity.
If the dust finds its way to your motor its finished.
I had a friend making a rocket engine nossel on an old south lathe and fried the motor due to graphite dust on the com.
-B-

Very good tip. My first thought is to keep the vacuum on it while machining but it may just be a different motor that gets fried.

 

[Maryak]

Very good tip. My first thought is to keep the vacuum on it while machining but it may just be a different motor that gets fried.

Check and see if your motors are full enclosed, my lathe motor is so it's sealed from the outside air.

Best Regards
Bob

 

[RICHARDDV]

just a thought --try using an air venturi vacumn on your air hose and duct it outside thru a filter bag to get it out of the shop. I use a electric leaf blower thru a 3 " pvc y fitting to clear sawdust out of my shop planer. any high velocity air source could be used--richard

 

[mnbylcr2]

David - thanks for the suggestion regarding the graphite store. Being in the UK, I don`t know how much extra postage will be and also how to physically pay. The only way that is easy is paypal and most of these places don`t accept paypal.

mnbylcr2

 

[craynerd]

OUCH!! : :

Direct quote from an email response I have just had from them: for postage alone for 1x12 graphite rod:

""We are looking at $131.19 to ship one or several 1x12 rods to the U.K. via UPS.""

Hummm!

 

[deverett]

OUCH!! : :

Direct quote from an email response I have just had from them: for postage alone for 1x12 graphite rod:

""We are looking at $131.19 to ship one or several 1x12 rods to the U.K. via UPS.""

Hummm!

One thing not mentioned is that couriers charge in addition to their carriage fees, a 'clearance agency fee' if customs duty is payable. Guess how I found out?! Since that time, I have got my suppliers to use USPS Priority Mail for anything posted from the US to Europe. It works out considerably cheaper. Ask them for a quote by that route.

Dave
The Emerald Isle

 

[craynerd]

Thanks for the info Steamdave but I wouldn`t even consider it if it was half the cost, infact make that quarter! That price was postage, I`ve still got the cost of the graphite on-top. I`m still certain that somewhere in the UK will supply me with a smaller piece.

 

[chrismark]

Hello.
I have used aluminum for the piston.

Here's the video; [ame]http://img190.imageshack.us/i/s6301181.mp4/[/ame]

The machine has now run over 1000 hours.

Chrismark.

 

[black85vette]

I made one with a Delrin piston and a brass cylinder. Not the best but it did run.

I also checked into these folks:

http://www.airpot.com/html/make_your_own.html

You get a matched set with a piston and glass cylinder. I got a quote on the 160 unit and it was $27 usd for the pair. If I ever go back and make another one to run on the heat of my hand I will use this for the power piston.

 

[David Morrow]

Here are some interesting comments from Jerry Howell's web site :

"Over the years I have made pistons from many different materials and of the ones I have tried, graphite is far superior."

"Now lets consider graphite. It is extremely self lubricating due to its molecular structure. The molecules are arranged like a stack of cards and the bond is so weak that they readily slide over one another. Even so, graphite is quite hard and mechanically stable but it is easily machined. It can easily tolerate temperatures up to red hot, yet has a very low thermal expansion rate. No other material has all these properties! Are there any drawbacks to graphite? There is one, it is dirty to handle and makes a mess when machined. The dust and granules make a mess that even resist being wiped away dry. I keep my shop vacuum wand as close to the work as possible and try to prevent any dust from getting away. The dust won't hurt your machine tools though and if anything it may be a benefit. "

 

[Orrin]

From my extensive reading about home-built Stirling engines I've come across several pieces of advice regarding the use of Delrin or PTFE (Teflon) for pistons. They were all the same: Don't bother.

I'm not saying it won't work; but, why buck the odds? Many experimenters have good luck with graphite. Some experimenters have had poor luck with Delrin and Teflon; they are not dimensionally stable. The high coefficient of thermal expansion of Delrin and PTFE will cause binding as the engine warms up.

Graphite is not that hard to obtain. Machine shops that do EDM work should have plenty of scraps to give away.

Orrin

 

[David Morrow]

Chris, I'm doing a bit of catching up and order some bits of tooling and supplies over the next day or so. I have to order some graphite rod for a couple of engines I'm making. I also plan to get some other sizes just to keep on hand for future projects. What diameter and length do you need ? I think the order I'll be placing will be for 12 inch lengths. If I get something close to what you need, I'll cut and mail a piece to you if you can stand the delay.

 

[Ed T]

How about machinable ceramic?? I have machined some with fairly ordinary equipment and it was no big deal. It is brittle, but I suspect the graphite is too. Certainly doesn't have the lubricity of graphite, but it may be more available/cheaper. I suppose that one could insert some graphite pencil lead into holes around the outside of the piston to keep everything moving smoothly. Just a thought, don't know if it'll work.

 

[Stan]

I am no expert on the different kinds of graphite, but when EDM electrode material was highly recommended, I started watching ebay. Before long I found a seller with 'buy it now' prices on blocks of various sizes. I bought a block 4"x12"x2"+ for less than the usual suppliers want for a 12"x 3/4" rod.

It makes more mess because I just saw off a piece and chuck in a four jaw and whittle out what I need. It is nice material to work with and not too brittle. I have dropped thin wall pistons onto the floor and they didn't break (not a a recommended practice but necessary for serious research). Otherwise, once the pistons are machined to micron tolerance, I put them away and forget about them.

Seriously. when you have a finished cylinder bore, you can turn the graphite to within a thou or two and then use 600 grit cloth to bring it to a tight fit and then use the pack of the cloth strip to give a final polish.