Page 13 – The Spool Piston

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”Getting a Stirling to run, is an exercise in friction reduction”,
said Malcolm on completion of the assembly of his first Stirling engine - a kit.
Yes, when the power source is limited, it is essential to minimise the friction.

Andy Ross initiated the idea of the spool piston.
It reduces the friction between the piston and its cylinder.

For the beta engine, it has long been the practice to drive the displacer by a con rod that
passes through the centre of the piston. A good sliding fit that has to be gas tight.
The consequence is that usually, the fixing for the piston con rod is off centre.

This generates slight side forces on the piston. Also, it can be seen that, with every stroke
of the piston, it does a slight dance. It twists one way, then on the return, twists back again.

Andy’s piston has its con rod attached in the centre. Friction is reduced and power increased.

The piston is made up of two discs joined with a tube. The top disc is the gas seal.
The piston con rod can now attach to the lower disc in the centre.

The displacer con rod passes through the centre of the upper piston disc as usual.

Between the two discs – the connecting spool tube is cut away on one side.

A peg is fixed to the end of the displacer con rod at right angles and protrudes out of the tube.

This peg can then connect, via a ball joint, to a second, lower, displacer con rod.
This lower con rod passes through a slot in the lower spool piston disc.

It’s hard to describe. Have a look at the photo.

The result, is that of the two items, piston and displacer, the piston has the better linkage.
The piston is the one that actually has the force of the engine on it.