Egg Cup Stirling
Apparently I cannot suppress my propensity to minimize
and simplifying things. After my first five coffee cup Stirlings I wanted
to make a model that should run on a egg-cup filled with hot water.
Forced by the small dimensions I had to implement some construction
Except for the proportional smaller dimensions for the cylinder with
the displacer and the cold cylinder with the working piston this model
above all differs with respect to the crank shaft and the bearing of
it. This crank shaft is a one piece steel wire with diameter 1.8mm.
I made a special bending jig to make the curved eccentrics with right
dimensions and under the angle of 90°.
Later I changed this crank shaft as you can see
on the video and on the extra picture on the right bottom of this page.
May be even less difficult to make and in fact somewhat better to keep
the driving rods in place. The present drawing plan is based on this
new version of the crank shaft.
I made point bearings as you mostly encounter in clockworks. For that
both ends of the crank shaft are grinded conical with a top angle of
60°. The crank shaft is fixed between two 3mm thread ends (screwed
in the pillar stands) which have conical cavities with top angle 90°.
The right adjustment for the bearing clearance and for the horizontal
position of the crank shaft is simple a matter of screwing this thread
ends somewhat in and out.
I made a glass displacer cylinder, but you can use transparant plastic
as well (acrylate PMMA). In the aluminium plates there are grooves in
what the cylinder is cemented air-tight with silicone kit so you don't
need the the six srews on the circumference. This decreases the heat
transfer from the bottom plate to the upper plate. Dismantling is somewhat
more difficult then, but in fact that is not necessary anymore if cementing
is done as last assembly treatment.
I made the displacer from 2mm thick plexi-glass. The advantage is that
the displacer axis can be screwed directly in the displacer so you can
skip the aluminium mandrel that you need with soft polystyrene.
All together this became a very simple little engine with only 16 parts,
all easy to make. May be
this is also the reason why it fascinates so much seeing this engine
run on a egg-cup with hot water.
Placed on a egg-cup with boiled water this little thing
is running smoothly and noiseless during 5 to 10 minutes with a speed
of about 200 rpm max. This speed is attained after 2 minutes and decreases
gradually due to the cooling down of the water in the cup. Of
course this relative short run-time is caused by the low heat capacity
of the small amount of water in the egg-cup. It is possible to run this
engine for a longer time or even continuously placing it on a substrate
that keeps warm, i.e. on a household teapot warmer. But don't exceed
°C to avoid damage to the plastic parts.