My First Rhombic Drive Engine

[Vjdeep]

Hello Everyone!
I have built a rhombic drive Stirling engine for a small science fair at my college. But the catch is that it isn't running at all. I've added a few photos of my engine below. Any help regarding it would be of great help. thanks in advance!
all the parts are made of aluminium.
The heating cylinder, flywheels and the holding brackets have been made of mild steel.
https://youtu.be/Gjt2Wd6YfdY

[Ian S C]

Hi Vjdeep, looks good, you have put in a lot of work on your motor.
What is the power piston, and cylinder liner made of?

The motor seems to have quite a bit of friction when it is turned in the video. The bearing surfaces in the linkage are aluminium on steel pins, this tends toward high friction, best to use ball races. What are the main bearings? There will be some added friction in aluminium gears over steel or cast iron, plastic could be a good way to go. I have a unfinished Rhombic on my work bench that has a pair of Nylon gears.

Ian S C

[Trevor]

Hi Vjdeep
Nice looking motor and well done. It appears that your piston and liner are both alloy - If this is so you will have an enormous amount of friction as alloy to alloy will gall and cause trouble to you. If you can change the liner to steel or cast iron. Ian knows a lot about this problems as he helped me on occasions with the same problem of using alloys.
Do love your work.
Trev

[Ian S C]

Vjdeep, If you can get old car shock absorbers(the telescopic ones) and pull them to bits (it can be messy), the tube inside is highly polished, and makes a good cylinder liner, they range from about 25 mm to over 40 mm. You also get a bar of best quality steel with chrome plated out side. I have not found a use for the vented piston with it's plastic (usually)piston ring. I don't know about metals in South India, but I often use the long cylindrical weights that were used here and in UK to balance windows so they could slide up and down easily, these window weights are made of poor quality cast iron, but sometimes you get lucky.

Below is a stove top fan, the displacer tube is the chrome plated mild steel tube from a vacuum cleaner, the cylinder is a bit of inner tube from a shock absorber with a thread cut on one end to screw into the cylinder head, the 30 mm piston is made from a piece of a window weight. The bearings are skate board size, 8 mm shaft size. From what I remember the stroke of the power piston is 20 mm.

Don't know if the craze took off in India, but it did here, Fidgits, a little spinning toy with up to 4 ball race bearings (poor quality), here the toys sold for a fraction of the price of the cheapest of single bearings bought from engineering suppliers.

Ian S C

[Vjdeep]

Hi Vjdeep
Nice looking motor and well done. It appears that your piston and liner are both alloy - If this is so you will have an enormous amount of friction as alloy to alloy will gall and cause trouble to you. If you can change the liner to steel or cast iron. Ian knows a lot about this problems as he helped me on occasions with the same problem of using alloys.
Do love your work.
Trev

hello Trevor,
thanks for the compliments. Yeah, both the piston and the linear are made of the same aluminium alloy, i wanted to reduce wait as much as possible so chose aluminium. and, i had an aluminium bar laying around of the required dia. so used it, realized my mistake late, I've actually changed the piston and its bore now to aluminium with rings to seal pressure and a bore which i got from my local hardware shop.

[Vjdeep]

Hi Vjdeep, looks good, you have put in a lot of work on your motor.
What is the power piston, and cylinder liner made of?

The motor seems to have quite a bit of friction when it is turned in the video. The bearing surfaces in the linkage are aluminium on steel pins, this tends toward high friction, best to use ball races. What are the main bearings? There will be some added friction in aluminium gears over steel or cast iron, plastic could be a good way to go. I have a unfinished Rhombic on my work bench that has a pair of Nylon gears.

Ian S C

Thanks for the reply Ian
the piston and its linear are made of Aluminium alloy, i've come to realize the error late. i have actually started on changing the bore/linear to that i purchased at my local hardware. Plastic gears of that dimension are hard to find, tried to 3D print it but was waay over my budget. I am planning to cut holes into my gears and reduce its weight. Didn't realize that the links would have that much friction, must look into it.

[Ian S C]

Friction is probably the most important thing to get rid of. Some people even take the shields off the ball races, and wash out the grease, replacing it with a very small amount of very light grade oil. It might be worth using a lubricant on the gears that is Teflon/PTFE based, use absolute minimum.

Ian S C

[Ian S C]

The nylon gears that I used in my motor came from a cake mixer. make sure that the gears do not mesh too tightly.
Ian S C

[Cosmoslab]

Hi Ian Mark here. I have thought for a long time looking in James Rizzo's book that a half rhombic drive might be possible. I have started building such. Now the problem with removing the gears and lower links is the downward thrust causes the piston to jamb against the cylinder wall which is an obvious failure. However after a lot of thought and sketches I might have the solution. I have made a guide block for the 6mm displacer rod placed between the piston and the rear link and displacer stop.
This has proved difficult to make as precision is paramount. I'd be interested to read your thoughts.
This is not entirely new there is a post on you tube: "Half rhombic drive".