In article ,
"Ed Huntress" wrote:

"Joseph Gwinn" wrote in message
...
In article ,
"Ed Huntress" wrote:

[snip]
Anyway, thanks for the tip. I would love to go for this in a big way, but
it isn't in the cards right now. I'm engaged in conversation with Dr.
Senft on
Stirling engine lubrication, and the outcome is going to eat me up for
months to come. d8-)

Is there a good solution? I recall that this was one of the big
issues.

You'll have to read my article, should I succeed in gathering enough info
to
write one. There are solutions that work; different solutions for
different
realms, from low-temperature-differential types to fractional-horsepower
mule motors, up to 100-hp-plus automotive engines. The solutions are all
different. Senft has put me on to a guy who apparently is one of the
world's
experts on automotive Stirlings, and who knows the big-time lubrication
solutions, both for kinematic engines and for free-piston types. I
haven't
talked to him yet. I'm looking forward to doing so.

Please keep us posted. It's even on topic, too.

I will if I get anything useful. There's a big book about the history of the
Philips Stirlings, which I've never seen. I'll have to ask one of the
experts if it contains any good info on lubrication.

The basic story is that free-piston Stirlings are using dynamic gas
bearings -- which is to say, the pistons are designed to center themselves
on a film of the working gas. Low-temperature-differential Stirlings
generally run dry, often with graphite pistons (another issue; it's not as
clear as it seems, because synthetic graphite is not lubricious). Other
bearings in those engines are dry-running ball bearings.

Small stationary engines sometimes are made with oiled bottom ends and, in
the case of beta Stirlings, with Rulon or other syntheic seals to keep the
oil out of the hot end. And sometimes they run dry, too. Here it's important
to avoid lateral loads on the pistons, by using rhombic drives, Scotch
yokes, or other mechanisms that result in straight-line forces applied to
the pistons.

I don't know about the big automotive types. There have been a number of
successful ones, from Ford, Saab, and others, so there must be a way. I'm
looking forward to learning more.

The basic issue, BTW, is that oil that gets into the heat exchangers ruins
their efficiency. And oil that gets into the hot end will carbonize. This is
one of the big engineering problems with Stirlings, as you've apparently
heard.

I had heard that seals were a big problem, and noticed that only NASA
seemed to be able to use Stirling cycle engines for anything, but didn't
know the details.


[snip]

Yes. But a torsion box (box-beam) made of concrete has lots of natural
damping. As you say, the filled-base commercial machines have been
attempts
to build in damping with low shipping weight (and lower costs) for the
machines.

It's a very tricky engineering problem, but it doesn't require a lot of
knowledge or heavy math. It just requires a good feel for materials and
structures, combined with a lot of patient thought and analysis.

As I said, Slocum runs hot and cold on this in the book. The savings
were not dramatic, even if he used aluminum weldments for the machine
frame. (Yes, it's a torsion box, bent into a C-form.) I think Slocum
did build such machines, so the tradeoff summary wasn't just a
theoretical musing.

Slocum patented (5,799,924 and 5,743,326) a vibration damper consisting
of a tube with a greased steel rod within, with the space between tube
and greased rod filled with poured-in epoxy resin. The grease acts as a
mold release, so the rod is floating, and later acts as the goop layer
to absorb vibration. I think that this was intended for boring bars, as
an alternative to solid carbide. And one could make the tube from solid
carbide as well.

I suppose one could embed a greased inner box frame inside the box beam
machine frame before filling with resin. From his patent drawings,
Slocum is thinking this way.


Joe Gwinn

If it gets too complicated, it can become a solution in search of a problem.
g When I was writing about machine tools and people were trying to sell me
on the Hexapod, it became one of those. Likewise, an
elliptical-piston-machining lathe I was once involved with.

My impression is that Slocum has come to this conclusion about concrete
cored machine tools as well. But when you mentioned trying to design
concrete machine tools, I wanted to be sure you knew of this literature.

And his damped boring bar is something a HSM could easily fabricate.

Joe Gwinn