Rombic Drive - geometry design and gear selection

[mungewell]

Hi all,
I have know about the magic of Stirling engines for a while, but am only now getting deep into them and looking to design/build my own. This is for fun, and for mind exercising....

I have noted that crank driven engines have sinusoidal motion of the pistons, however that is not (necessarily) the case with a Rombic drive.

As the 'arms' and 'bar' lengths change relative to the gear size you can get some 'adjustments' to the motion, but I have not seen this discussed anywhere. Is this of use in Stirling Engines? In particular I am imagining a system where a displacer can be used to couple/isolate the hot piston from the hot end, raising it allowing for good heat transfer as the hot piston raises and then lowering the displacer during the cooling phase.

I attempted to do some math in the attached XLS, and if correct I could end up with:

rombic_drive.png (29.24 KiB) Viewed 1597 times

Additional gears are used to make the advancing of the Displacer and Cold Piston easier. The 'rod' for the displacer runs through the center of the 'rod' for the hot piston and through a hole in the piston. The Cold Piston faces the other direction, and is linked with a 'regenerator' tube.


Also, does anyone have advice for gear selection? The plastic ones from McMaster are a lot(!!) cheaper than metal, but are they up to the job?
https://www.mcmaster.com/6325K77/
https://www.mcmaster.com/57655K58/

Cheers,
Simon

[tom-rock]

Hi Simon,

since years I am building rhombic Stirling engines and indeed its an ingenious invention.
The rhombic geometry allows many possibilities of volume variations especially the general form (different lenghts of con rods etc.).
Look after the original analysis "The Philips Stirling Thermal Engine" of its inventor Roelf Jan Meijer.
If you search a bit you will find it, its free in the net and provides a deep insight the rhombic drive.

I used plastic (high quality plastics like POM and Torlon) gears in my engines for a long time.

Look for example in one of my videos: https://youtu.be/8JY65VfvKzQ

In my newest video I show some of the worn gears and present a timing belt drive.

Plastic gears are very quiet and good if the performance is not too high.
At a power of 300 watts they only last a few hours before they are worn.
Brass and especially Steel gears are a bit noisy but durable.

Many greetings

Ralf

[staska]

Hello.

You could use double sided tooth belt too. Gears have to be super precise and backslash free for rhombic engine.

I did tried try them - not for my mechanic skill.