Stirling Engine : Laura

[Ian S CParticipant @iansc]

Brian, I think you would be pushing your luck a bit there, drilling 15 mm OD tube to 13 mm. Ian S C

[Brian JohnParticipant @brianjohn93961]

Today I called the manufacturer of the thin walled tubing and they assured me that it is genuine stainless steel. I cannot get the thick walled stainless steel to run for more than 7 minutes. It is much better quality (marine grade) but it just will not work. The thin walled tubing will run for the full 20 minutes of the meths burner. So I have rebuilt the second engine with a thin walled hot cap today and it runs well. The brass cap on the end of the tube fits over the end rather then inside the tube as on the other engine. This gives a neater appearance and is easier to make.

Time to call it quits on this one now. But I am puzzled why it does not run well with the thick walled tubing…any opinions ?

NOTE : I know the flame on the meths burner is bigger than it should be. I will reduce it later.

https://www.youtube.com/watch?v=YRyWAWmO5vU&feature=youtu.be

Edited By Brian John on 21/06/2016 10:31:50

Edited By Brian John on 21/06/2016 10:33:26

[HopperParticipant @hopper]

Posted by Brian John on 21/06/2016 10:11:59:

…But I am puzzled why it does not run well with the thick walled tubing…any opinions ?

Simple, the thicker wall conducts more heat down the length of the displacer cylinder to the cool end, so the temperature differential between hot and cool ends is less. It is this differential that make a Stirling engine work. The hotter the hot end, the more the air expands when displaced into the hot end. The cooler the cool end, the more the air contracts when displaced to the cool end. Hence, higher temperature and therefore higher system pressure at one end of the cycle, and lower temperature and lower pressure at the other = more work done on the work piston.

[Brian JohnParticipant @brianjohn93961]

Thank you, that makes sense.

NOTE : I forgot to use the special stainless steel flux I purchased especially for these hot caps. I just used my usual EZI-WELD 602 flux which seems to have worked anyway. I only just realised my mistake now.

[Ian S CParticipant @iansc]

With the thick tube you can cure the problem with a bit of lathe work. Hold the hot cap in the lathe chuck by the hot end(closed end), and tur a bit from abut 1/3rd from the closed end for about a 1/3rd to about half the thickness, this thin bit works as a thermal dam, slowing the heat transfer, and is what I should have done on my larger BETA motor, I instead thinned the area at the end that is in direct contact with the flame, that area has now distorted, and at some speeds rubs on the displacer. Ian S C

[Brian JohnParticipant @brianjohn93961]

Thanks Ian, I have been thinking of thinning the whole length but, as you have pointed out, that may not be necessary. So you think only 1/3 of the length needs to be turned down….1/2 might be better ?

I have drilled a small shallow centre hole in the end cap before soldering everything together (I always do this now). So I can mount the collar in the chuck and use a dead centre to support the closed end while turning with a LH tool.

[Brian JohnParticipant @brianjohn93961]

I have reduced the diameter over about 1/2 of the length : from 15.7mm down to 15.2mm. It was no problem to do this as I put a small centre hole in the end of each hot cap I make before I solder it onto the tube. I was surprised at what a good finish I got on the stainless steel (0.1mm cuts). This was not even on auto feed. I wish I could get such a good finish on brass !

NOTE : I have not tried it out yet.

[Brian JohnParticipant @brianjohn93961]

Ian : This has been very successful. The engine ran for the full 20 minutes of the meths burner ; I am surprised at the difference. Tomorrow I will reduce the diameter along the whole length of the cylinder. I am not sure why I didn't think of this myself…it is much easier then using a boring bar to reduce the wall thickness from the inside !

[Ian S CParticipant @iansc]

No, leave it alone, the thick part acts as a heat store. Well you can do it, and see if there is a difference, but it looks ok the way it is. The motor should run as long as it has fuel, you could run it all day. Ian S C

[Brian JohnParticipant @brianjohn93961]

Well, I did it and now the engine does not work at all. My goodness…these things are fickle ! I will make up another hot cap but I will use the thin walled stainless steel tubing from now on as I know that works.

[Ian S CParticipant @iansc]

My goodness…these things are fickle! That's one reason mine don't get painted, once you start mucking about with them, you can have the problem of getting it going all over again. Ian S C

[Brian JohnParticipant @brianjohn93961]

I have found it is better to solder the brass collar first in case excess solder has to be removed. The end cap can then be soldered on with the hot cap in an inverted position. Also, I can machine off most of the brass end cap until it is almost parallel with the stainless steel tube and I can then place it all the way into the lathe chuck so that I can machine the underside of the brass collar again if need be. See the photo for the end results.

But I have another problem with the bearings. The silver steel rod I am using all seems to be a bit undersized at 5.93mm and this is a loose fit in the roller bearings. This is allowing the axle/crank to move sideways and the flywheel rubs on the brass frame causing the engine to stop. I have tried a number of things but nothing is working. I have used these roller bearings in the bearings supports before with no problem so I am not sure what is going wrong here. It is a standard item with the Beng's kits. Perhaps the previous axle was slightly larger in diameter. There is not much between too big and too small !

Any suggestions would be welcome ? I thought 6mm ground silver steel was supposed to actually be 6.0mm …mine is all 5.93mm ?

Edited By Brian John on 26/06/2016 15:07:07

Edited By Brian John on 26/06/2016 15:09:51

Edited By Brian John on 26/06/2016 15:11:39

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Brian John on 26/06/2016 15:06:19:

But I have another problem with the bearings. The silver steel rod I am using all seems to be a bit undersized at 5.93mm and this is a loose fit in the roller bearings. This is allowing the axle/crank to move sideways and the flywheel rubs on the brass frame causing the engine to stop. I have tried a number of things but nothing is working. I have used these roller bearings in the bearings supports before with no problem so I am not sure what is going wrong here. It is a standard item with the Beng's kits. Perhaps the previous axle was slightly larger in diameter. There is not much between too big and too small !

Any suggestions would be welcome ?

.

Loctite 'Bearing Fit'

MichaelG.

[Brian JohnParticipant @brianjohn93961]

I will use bearing lock as a last resort but I should not need it. My 6mm ground silver steel is all 5.94/5.95mm. Is this correct ?

I have got this engine running well now by making adjustments to the D.piston diameter ; further experiments with piston diameter should result in more improvements. It will now run for longer than 20 minutes but the crankshaft keeps slipping sideways in the bearings causing the flywheel and crank arms to rub on the bearing supports. I did not have this problem with the first engine I built but I do not know the diameter of the crankshaft in that one.

[roy entwistleParticipant @royentwistle24699]

Is there no way that you could introduce spacers or washers to stop the crankshaft moving sideways ?

Roy

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Brian John on 28/06/2016 13:19:12:

(a) I will use bearing lock as a last resort but I should not need it.

(b) My 6mm ground silver steel is all 5.94/5.95mm. Is this correct ?

.

(a) [quote from your earlier post] … Any suggestions would be welcome ? [/quote]

(b) that is probably comfortably within the manufacturing specification: As a matter of interest; have you checked it for roundness [an old and interesting story, which I shall leave you to research]. … If you want better, you will probably need to use either PGMS or something exotic and pricey.

MichaelG.

.

P.S. For reference; here is a supplier that actually quotes tolerances.

… and another, who takes PayPal

Edited By Michael Gilligan on 28/06/2016 13:36:15

Edited By Michael Gilligan on 28/06/2016 13:45:05

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Brian John on 28/06/2016 13:19:12:

My 6mm ground silver steel is all 5.94/5.95mm. Is this correct ?

.

Brian,

I'm not being funny, but; earlier you wrote that it was all 5.93 diameter.

How good are your measuring instruments ?

MichaelG.

.

P.S. … Who supplies your Silver Steel, and do they quote a specification ?

[SillyOldDufferModerator @sillyoldduffer]

Posted by Brian John on 28/06/2016 13:19:12:

I will use bearing lock as a last resort but I should not need it. My 6mm ground silver steel is all 5.94/5.95mm. Is this correct ?

…

I see Michael got in before me but I have a jotted note that claims silver steel is +0 / -0.015. I think that was from the first lot I ever bought, so it might vary.

You didn't say how you measured the diameter, so it might be your tool or technique that's out. Apologies if you're an expert or are naturally good at it but taking measurements can be harder than it seems. Here's 10 successive measurements of my 0.25" rod that demonstrate that I'm a clumsy oaf when it comes to using a calliper: 6.35, 6.35, 6.38, 6.39, 6.37, 6.33, 6.36, 6.34, 6.36, 6.32

I do better with a micrometer, but only if I'm careful to keep the anvils straight.

I measured some of my stock and got these results – note the errors!

Rod Micrometer Caliper

0.25" (6.35) 6.35 6.31 (Error 0.04)

5/16" ( 7.938) 7.94 7.93 (Error 0.01)

5mm 4.99 4.96 (Error 0.03)

You did me a big favour. I thought my stock of silver steel was all metric and it isn't! My 6mm rod is actually 0.25" and my 8mm rod is 5/16".

I've also had problems like yours after reusing a small bearing. On first insertion the axle rod was a tight fit. After a couple of re-fittings the rod and bearing became too loose. Most annoying: I replaced the bearing and now see in another of Michael's posts that there's a Loctite.

Keep up the good work. I've learned lots from your threads.

Cheers,

Dave

[Brian JohnParticipant @brianjohn93961]

Michael : I probably forget to zero my digital callipers before measuring ; either way it is NOT 6.0mm. Your idea of using bearing lock is a good idea but I want to see if I can avoid using it for now. I think there are things to be learnt here.

Minitech supply my silver steel and I don't think they give a specification but I will check with them tomorrow.

I might try fitting two sets of roller bearings in each bearing support. There is enough room to do that. I have no idea why that should be any better…just a hunch !

But in case nothing else works, I will buy some bearing lock tomorrow morning

NOTE : I have just tested a new 12.9mm piston and it goes like the clappers. ID of the cylinder is14.5mm. No more testing required on this point.

 

 

Edited By Brian John on 28/06/2016 14:58:47

Edited By Brian John on 28/06/2016 14:59:46

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Brian John on 28/06/2016 14:56:11:

Minitech supply my silver steel and I don't think they give a specification …

.

Here you go, Brian **LINK**

MichaelG.

[Brian JohnParticipant @brianjohn93961]

I tried the engine with four roller bearings (two per side) and it was no better. It seemed like a good idea last night but I don't know why I thought that !

I purchased some bearing retaining compound (Chemtools 8609 from Coles Air Tools) . It only took a very small amount. It says fixture in 30 minutes and full cure in 24 hours but it gripped in about 10 seconds. It was a good thing I had the correct position marked and that I did not have to adjust it afterwards ; that would not have been possible. I cleaned all the surfaces of oil using methylated spirits prior to using the bearing lock so I am confident that it should hold.

I think this has solved my problem but I will run it a few more times before I declare it a success.

Could I have used Loctite (222 or 263) thread locker for this ?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Brian John on 29/06/2016 08:07:29:

I purchased some bearing retaining compound (Chemtools 8609 from Coles Air Tools) .

… Could I have used Loctite (222 or 263) thread locker for this ?

.

Glad to know that you are making good progress

I don't know the Chemtools product, so can't really comment.

Of the Loctite products; I would use 'Bearing Fit' … because it's specifically engineered for the job.

222 or 263 would 'work' but would probably be much more difficult to dismantle if/when you need to.

MichaelG.

[Brian JohnParticipant @brianjohn93961]

Here is the specifications for the 8609 retaining compound. My bottle says fix time is 30 minutes but the website says 10 minutes…I would say about 30 seconds so be careful !

http://www.chemtools.com.au/product/adhesives-threadlockers/retaining-compounds/general-purpose-2/

 

Edited By Brian John on 29/06/2016 08:37:18

[Michael GilliganParticipant @michaelgilligan61133]

Thanks for the link, Brian

Just skimming through the Technical Data Sheet, 8609 seems more of a general purpose retaining compound [similar characteristics to Loctite 601 and 638], whereas 'Bearing Fit' is intended for relatively easy dismantling.

On the matter of 'fix time' … These products are anærobics and, to an extent, they will cure more rapidly in thin films than thick layers. Also [but not relevant to your shaft-to-bearing assembly] curing is accelerated in the presence of Copper.

MichaelG.

.

Edit: Here is the TDS for Loctite 641 Bearing Fit

Edited By Michael Gilligan on 29/06/2016 08:58:41

[Ian S CParticipant @iansc]

Are you using roller bearings, or ball bearings, you should be using the latter, roller bearings have a bit much friction, and if you use four instead of two, you double the friction. Ian S C

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