piston material Al or wood??

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piston material Al or wood??

Postby vaibhavkbarnwal » Thu May 21, 2009 11:19 pm

I am working on an alpha type Stirling engine for which I am using
aluminium cylinders and teflon pistons. While testing I realized that
teflon expands more than aluminium and thus it gets stuck when heated
, restricting its motion. Now I am thinking of replacing the teflon
piston with aluminium pistons(it will expand as much as the aluminium
cylinder does....i suppose) and use silicon sealant instead of using
O-ring to seal it.

Another option I am looking at is to use wooden piston as heating does
not have any effect on wood while the cylinder will expand... but not
sure it may give enough space to leak as the temperature would be
around 150 degree C.
Any help/suggestion will be appreciated.

Thank You

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Response to piston material Al or wood??

Postby bptdude___2569 » Thu May 21, 2009 11:36 pm


You ever heard of a material called Delrin, also known as Polypenco in the U.K. ?

It should be OK at 150 degree C, is machined like the soft metals, like the aluminum, will not expand or contract, and should be strong enough.

one idea for you, anyways.
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Response to piston material Al or wood??

Postby bptdude___2569 » Thu May 21, 2009 11:44 pm

Sorry, I messed up. I confused F and C degrees. Delrin will melt at 150 deg C.

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Response to piston material Al or wood??

Postby vaibhavkbarnwal » Fri May 22, 2009 4:40 am

It may not be used because of the reason you mentioned but even if it had a higher melting point I would still had the same problem which I had earlier.
Polymers in general have higher coefficient of thermal expansion than metals so while heating the piston gets stuck in the cylinder or is too tight to do reciprocating motion (I realised this very late [:(])

Materials apart from Al and wood can also be suggested. I am going to try with aluminium pistons .Hopefully it should do fine. Will update of it [:)]

Thank You
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piston material Al or wood??

Postby stan.hornbaker » Fri May 22, 2009 6:49 am

Wood would be an unusual material for a piston unless it were hard and dense. Aluminum running in brass, glass, steel cylinders with all running surfaces well polished and the piston sliding freely. Graphite in glass is commonly used with excellent results. A thin walled cast iron piston (min. wt.) in a C.I. cylinder should be OK also.
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Response to piston material Al or wood??

Postby joefish711 » Tue Jun 30, 2009 9:56 am

Graphite in glass or pyrex always works well. On the other hand, I would be interested to see how lignum vitae (http://en.wikipedia.org/wiki/Lignum_vitae) would hold up as a power piston. If you're determined to use wood, make sure it's sealed properly, otherwise it will slowly absorb water from the air, causing it to expand (poor piston).
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Response to piston material Al or wood??

Postby stan.hornbaker » Tue Jun 30, 2009 1:11 pm

A graphite piston in a glass cylinder is highly desirable. A graphite displacer adds to much weight/inertia to the moving parts. A hollow air tight displacer has minimum inertia. Brazed brass or stainless steel construction.


FYI a graphite piston/glass cylinder can usually be obtained from for the asking.
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