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Micro Stirling Mechanical Cryo-cooler teardown by Fraser

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Fraser:
Continued.....

Fraser:
So what was wrong with the SC3000 Micro Stirling Cryo-cooler ?

Tests showed that the brushless motor controller/driver has failed. This means that the field coil is not receiving the correct drive but the whole integrated coil and controller PCB can be replaced without breaching the coolers Helium gas seals. Sadly someone decided that opening the coolers sealed casing and releasing the Ultra High Purity (UHP) Helium was justified. The moment the seals were breached, the cooler effectively became scrap. The failed controller and drive PCB is potted into the top of the motors field coil housing and so would not be a simple repair. The QWIP detector end of the assembly also has a very unhealthy rattle so something has broken loose inside. This Integrated QWIP detector and Cryo-cooler assembly is beyond economic repair. Hence the teardown.

In the provided images the reader will see that the motor shaft incorporates a couple of offset sections that drive the Compressor Piston and Displacer connecting rods. It looks a little like piston connecting rods on the crank shaft of an Internal Combustion Engine. The offset sections of the motors drive shaft provide the correct phase relationship between the compressor Piston and Displacer movements for the Stirling Cooling effect. Ultra High Purity Helium is used as the gas within the coolers sealed casing as it it greatly improves performance and will not freeze at 77K. Any air contamination of the Helium Gas fill can lead to 'piston lock' as a result of freezing.

The cryo-cooler that I have disassembled for this thread showed signs of high run time hours so it was likely nearing the end of its operational life when it eventually suffered an electronics failure. There is solid material contamination within the motor magnet area and this may be the result of the high run time hours or the previous disassembly. If it is due to run-time hours, something was wearing and releasing black material into the ultra clean environs of the Compressor Piston and Displacer :(  I have another Micro Stirling Cryo-Cooler that sounds like its has bolts rattling around inside it when running. I suspect a bearing or connecting rod pin failure. Such a failure would likely distribute solid material around the ultra clean areas of the cooler and lead to its self destruction over time. Machines suffer from friction that casuses wear so it is not surprise to see a Rotary Stirling Cooler suffering as a result of its lack of liquid lubrication.

I trust that this teardown has been of interest to those who know of the technology but have not seen the internal workings of a unit before.

coppercone2:
what are the highest tolerances required for the parts in the cryo cooler?
and a university particle physics department with a accelerator might be a source of the UHP helium gas, I have seen big walls of filters / objects that had something about high helium purity on it (but the requirement for the feed gas was already some purer form of helium).

Looked like you need to send it through like 10 different canisters to get it cleaner.

I would not be surprised if they had additional surface finish steps in something like that, I doubt its just ground. Like burnishing or electro polishing.

Fraser:
I regret that I have no inside knowledge on these coolers and their tolerances. The UHP Helium was a challenge for me to obtain when I went searching for it from BOC. They would not even entertain supplying it to me and the cost was high for small amounts anyway. The Helium pressure inside the cooler is around 40 Atmospheres (~588 psi) and you need specialist equipment to purge the air out of the cooler, fill it with UHP Helium and then insert the Helium seal and grub screw, all without disconnecting from the coolers single fill port. I soon gave up on the idea of refilling my old Micro Cryo Coolers with UHP Helium. 

Spirit532:

--- Quote from: coppercone2 on October 02, 2023, 09:45:39 pm ---what are the highest tolerances required for the parts in the cryo cooler?

--- End quote ---
H6/h7 or thereabouts for the motion parts, minus the crankshaft. Gas dynamic bearings(running in helium, obviously) in operation, until it crashes and starts rubbing(like in this teardown).


--- Quote from: coppercone2 on October 02, 2023, 09:45:39 pm ---I doubt its just ground. Like burnishing or electro polishing.

--- End quote ---
Fine ground or lapped(honed), depends on which part.


--- Quote from: coppercone2 on October 02, 2023, 09:45:39 pm ---Looked like you need to send it through like 10 different canisters to get it cleaner.

--- End quote ---
You just buy UHP 6.0 helium from a supplier. Cleaning it is quite difficult, way cheaper to just buy. UHP isn't too expensive.


--- Quote from: Fraser on October 02, 2023, 09:55:07 pm ---I soon gave up on the idea of refilling my old Micro Cryo Coolers with UHP Helium. 

--- End quote ---
Should've sent them to me :)

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