Stirling Engine : Laura

[Brian JohnParticipant @brianjohn93961]

I used a mini blow torch today to add some extra heat but that made no difference. I want to try a slightly larger diameter D. piston (12.6mm) but I could not make one up as I had some problems with the lathe today.

I still think the main problem is the bearings and nothing much will happen until the new ones arrive.

I have also remembered that I removed the Hylomar sealant from the cylinder holders as it was very messy. I could have air leakage there but others have used sealant and still had no luck with their engines. I will leave things as they are for the moment.

Edited By Brian John on 18/02/2016 06:23:22

[HopperParticipant @hopper]

If you are getting 24rpm out of it you are doing well!!. Keep running it so it wears down all the high spots and frees up.

I would wash the bearings out with petrol or white spirit or thinners or whatever you have about the place. And then don't oil them. For this type of non-load bearing use they dont really need oil. If you feel you absolutely must put something on them, try some silicone spray or some non-gumming clock oil or gun oil.

And +1 on Ian SC's suggestion to loosen off the screws on the bearing caps, and on the bearing pedestal feet, and let it all find its own position as it runs. If your crank is ever so slightly out of alignment, this will allow it to move that little bit to reduce the misalignment.

And the ice cube on the displacer fins trick is a good one.

40 degrees in the house? It's ok to turn on the air con you know. Even the locals are whinging about this latest heat wave. I have to bale out of my tin shed when it hits 35. Sitting inside in air-con splendor at 30 degrees now. Luxury that is!

[AjohnwParticipant @ajohnw51620]

The trick with Hylomar is very little well spread Brian. They don't like leaks but there is one version that uses them.

I still wonder if too much heat is part of the problem because if the displacer gets too hot along it's length I feel they will have problems working. Perhaps it would be worth trying a candle like this one, or a much thinner wick.

It might make an interesting project for Brian at some point. Mix of hand work and finding some way of holding parts on the lathe to enable the use of a slitting saw. It's by J Jonkerman. There is a bit of milling as well but I suspect that could be don on a face plate really or a vertical slide of course. Plans are on the web.

John

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[HowiParticipant @howi]

Brian you do NOT need a larger diameter diisplacement cylinder, there needs to be 2 or 3 mm space between it and the glass tube, air needs to freely move past the displacer, you might want to consider making the displacer smaller in length also. There needs to be a certain minimum volume of air in the system for efficiency. There needs to he sufficient air to expand and provide drive energy. You could make up a displacement cylinder from tightly wrapped wire wool without having to machine one. One of my engines uses this method for the displacer, like you I thought making it larger in diameter would make it work better, I can tell you that it does not.

[Ian S CParticipant @iansc]

With the wire wool displacer there should be minimal clearance, the idea is to use this as a moving regenerator, and the air should pass through the steel wool.

Ian S C

[AjohnwParticipant @ajohnw51620]

The design is probably basically correct so should work as is. It might be worth moving the displacer back away from the end of the tube if that is getting to hot.

They are curious things. The regenerator should be hotter at one end than the other, just like the displacer. The idea of the regenerator is to heat the cold air as it passes through it. I'm inclined to say really ????? but some sort of balance must be achievable.

This is a low temperature one and the gap around the displacer is very visible. It's made using stuff from Lidl.

https://www.youtube.com/watch?v=5_50RuMcc28

Annoying. I had been wondering if they could be made like this – no power piston as such, just a piece of light rubber. Somebody now has done it. NASA seem to use a similar set up with what are referred to as free pistons. No con rod.

John

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[Brian JohnParticipant @brianjohn93961]

I substituted a whole new work cylinder and work piston assembly on the engine today. This piston was a slightly looser fit and more free running while still giving a decent seal. I am getting ten revolutions with no heat applied. But it still will not run.

One thing I did not notice on the youtube video was how slow the engine runs. It obviously has to be very freewheeling as there is not much force produced ; it is very unlike a steam engine.

I am hoping the new bearings will make a difference when they arrive (maybe next week) but I doubt that they will improve on the current 10 revolutions. I will try them out when they arrive though as I am running out of ideas.

I had intended to make a new D. cylinder cover (part 24) in case I am getting leakage but I am having second thoughts about this. The D. piston rod is 2.93mm and the centre hole was reamed using a 3H7 reamer so this should be as good as it gets. Anyway, I do not have a working tailstock on my lathe at the moment so I cannot make a new part.

I am still not too sure about sealing the cylinder holders (frames) with Hylomar. They are such a tight fit that even a small amount will ooze everywhere and perhaps block that connecting hole between the two cylinders. You just have no way of knowing once everything is tightened up what has happened in there.

One other thing I can try is to make gaskets for all the cylinder covers. I have the correct gasket paper so this might be worth doing. Why do steam engines need gaskets but Stirling engines do not : I would have thought a good seal is even more important on a Stirling engine ?

 

Edited By Brian John on 20/02/2016 06:41:36

[pgk pgkParticipant @pgkpgk17461]

A steam engine will be running at higher pressures so more chance of blowing through the junctions. If your design is anyhting like mine then the flanges of the cylinders are quite wide BUT o the design I'm following the recommendation is to seal with silicone sealant.. just a tiy amount biased towards the outer side of the flange and when i have undone them again then no sign of it getting inside – not that that has helped get mine running…(yet??)

[KettrinboyParticipant @kettrinboy]

Hi Brian , you say the displacer rod is 2.93mm dia and a 3mm h7 reamer will cut around 3.02 mm if your lucky so that would give a near 0.1 mm clearance between rod and bush , ive found 0.02-0.03 mm is the optimum clearance to get a seal and allow free movement of the rod , and thats in a 30mm long bush , i think yours is around 10mm long is it so you may be getting too much leakage at that point a piece of 3mm silver steel rod should be 2.99-3.00mm so perhaps remaking the displacer rod from that would bring the clearance back to something that should be good enough to work, really hope you get this thing running sounds like your close but if certain things are not right these engines just wont go., i would try a paper gasket between the brass frames as one blob of sealant squidging out will block the passage and you guessed it the motor will not run. lets hope these new bearings are an improvement

regards Geoff

 

 

 

Edited By Kettrinboy on 20/02/2016 09:18:22

[Brian JohnParticipant @brianjohn93961]

I order 3mm stainless steel rod from China or HK and this is what I am sent : 2.93mm. I doubt that I will get any better result by ordering from another supplier. I have found that these things always seem to come out of one factory. It is meant for use in RC helicopters and cars.

From where can I purchase 3mm stainless steel or silver steel rod which is EXACTLY 3MM ?

[AjohnwParticipant @ajohnw51620]

No where is probably the answer to that Brian. Your best bet would be to buy 1/8" dia an very carefully polish it down. When it fits but is a bit stiff the best option would probably be to lap the two together with silvo/brasso etc but the diameter along the bar will have to be VERY consistent.

Then if you reamed in the lathe using a lubricant and at a slow speed immediately after drilling the hole things should work out. If I remember correctly from when Hopper was working on it your tailstock alignment is good. If not the reamer will produce a hole that is larger than it should be.

Only problem is that 1/8 dia might just turn out to be 50u bigger than 3mm. Stainless is usually a couple of thou down so that would leave it circa 100u over size.

Edit. There is an old trick for polishing bar I just remember and haven't actually tried. Books usually mention lead laps but it's possible to make them out of wood, pref a hard wood. 2 hinged pieces. Space the business end in this case say with 1 or 2 thickness's of photo copy paper. Drill a 1/8" hole centred on the gap, rub some lapping paste into it and then use it on the bar in the lathe moving it rapidly back an forth using finger pressure to to press it onto the work. At this sort of size there would be no need for the hinge really.

The same sort of thing can be done on holes using a dowel with a split in the end. There is a video  of this being done on rifle barrels by people dressed in ancient clothes. I reckon they have it wrong though, pushing the lap through rather than pulling but maybe the Brits made better rifle barrels. In this case, make the split etc and turn it to a suitable size then use.

John

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Edited By Ajohnw on 20/02/2016 11:05:39

[Roger Provins 2Participant @rogerprovins2]

eBay item number 131720676238 is in UK and said to be ground stock. I would send the seller a message seeing if he will select you a piece that's precisely 3.00mm Many eBay sellers are very helpful.

[Brian JohnParticipant @brianjohn93961]

The alignment on my tailstock WAS good…but not now. I am having problems with the tailstock. I have tried turning small steel parts in my lathe before this and it just does not work.

I think my best bet is to make part 24 again, drill out to 2.7mm…check the fit with the steel rod…drill out to 2.8mm, then check the fit…drill out to 2.9mm check the fit.

I also wonder about this heavy flywheel. I know this is the correct flywheel for the original design but I have a feeling that a lighter flywheel would be better for what I have built….it might be worth a go while I am waiting for the new bearings.

Roger : those steel rods are very accurate. Thank you, I will send them a message now.

 

Edited By Brian John on 20/02/2016 11:23:59

Edited By Brian John on 20/02/2016 11:26:27

Edited By Brian John on 20/02/2016 11:32:26

[Ian S CParticipant @iansc]

Brian, the size of the rod matters little, as long as it fits. One way to do it is to drill a whisker under size, now take a bit of steel the same size as the displacer rod, at one end cut a sloped flat about 20* (don't measure it, it doesn't matter), this will ream brass without hardening. After it has gone through, file across the flat, this should leave a very small burr on one side of the flat, put it through again.

How smooth is the bore of the power cylinder?

Ian S C

Edited By Ian S C on 20/02/2016 11:39:07

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Brian John on 20/02/2016 11:21:28:

I think my best bet is to make part 24 again, drill out to 2.7mm…check the fit with the steel rod…drill out to 2.8mm, then check the fit…drill out to 2.9mm check the fit.

.

Brian,

Have a look at the sketch [posted by Ian S C] of a ToolMaker's Reamer, on this thread.

You could make one very simply, from an offcut of your existing material.

The tiny burr that we discussed should then increase the bore to a nice sliding fit.

MichaelG.

.

Ian beat me to it !!

Edited By Michael Gilligan on 20/02/2016 11:44:32

[HopperParticipant @hopper]

Posted by Brian John on 20/02/2016 10:01:08:

From where can I purchase 3mm stainless steel or silver steel rod which is EXACTLY 3MM ?

Minitech in Brisbane. Talk to them on the phone and tell what you need it for. They have 3mm silver steel.

I would leave the stainless alone. It tends to pick up and gall so is probably not great for friction-free running.

[AjohnwParticipant @ajohnw51620]

My feeling is that a toolmakers reamer will produce a hole with more clearance than the same sized real reamer much like D bits do. I've only used a D bit on brass and at a much larger diameter.

Exact size silver steel – I did know of one source that was exact as the company that was selling it had it made in a few sizes specifically for one of their customers.

The marine shafting linked to is very close tolerance but what about the hole ? I still think you would be better off reducing 1/8 dia to fit maybe plus a bit of lapping. You wont need your tailstock for that.

I don't know if you have tried a smaller more localised flame but it is possible to put too much heat in these things. The temperature differential has to be maintained for them to work.

John

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[Brian JohnParticipant @brianjohn93961]

Things I have tried today :

1. machined the contact surfaces of the work piston where it meets the frames as I was not happy with the fit.

2. removed, cleaned and re-oiled the bearings. They did not seem as bad as I thought but it will be interesting to compare them with the new ones when they arrive. I no longer think the bearings are the problem.

3. tried a slightly smaller D. piston (12.4mm) but went back to the recommended 12.5mm.

4. reduced heat input by using a tealight candle.

All very frustrating…nothing has worked. I do not think friction is my problem. I am now getting about 14 revolutions (unheated) when turning by hand. What is everybody else getting ?

The only things left to try are the paper gasket between the cylinder holders (frames) and a slightly larger (3mm) D.piston rod.

 

Edited By Brian John on 21/02/2016 06:52:25

[pgk pgkParticipant @pgkpgk17461]

Lots of things to try still.. if one can cope with the frustrations of dissapointment..

Ceramic bearings, high speed bearings, bearings rated abec7, removing the seals and cleaning them off all lubricants that could cause drag..

Honing the cylinder and lightening the piston, different materials for piston/cylinder to see if one can reduce friction.

Similar for the d piston shaft.. find a way of honing the bore and adding perhaps a teflon sleeve to the rod..

paintng all joints with soapy water and some sort of low pressure test

Or just pop a small hole through the baseboard, add an airlne and blow it over the flywheel rim.. make a youtube vid without sound and pretend to gloat

[KettrinboyParticipant @kettrinboy]

Hi Brian

When the engine is assembled in a ready to run state as you turn the engine over you should feel some compression or bounciness this shows that the work cylinder piston is sealing and that there are no bad leaks elsewhere in the engine , with the displacer cylinder or work cylinder removed then spinning the engine over and getting 14 revs would be good as it suggests no problems with friction in the linkages, on my similar engine when it is all assembled it will spin only one revolution because of the compression that should be there , with the displacer cap off it will do about 4-5 revs with a good spin of the flywheels , I still think the displacer rod fit on your engine is too slack and could be the one thing stopping the engine running and getting this right will help this engine to run , on a short displacer bush of say10 mm length you need to aim for around 0.02mm of clearance between rod and bush to keep a good enough seal and free sliding of the rod., you mentioned your flywheel elsewhere , I think this engine running properly can spin this flywheel but it may take some time to get up to whatever its top speed turns out to be ,theres nothing to stop you experimenting with lighter aluminium flywheels if the engine seems to struggle with the current one but I,d try getting the engine running with the one you have first as it does look good doesnt it.

regards Geoff

[Brian JohnParticipant @brianjohn93961]

No, there is no springiness there at all. I know I have a good fit on the work piston/cylinder (the ''pop'' test). So we have a leak or leaks. Tomorrow, I will pull the whole thing apart and make paper gaskets for all cylinder covers (5X) and the cylinder holders (frames).

Galling : surely there is no danger of stainless steel rods galling at these low speeds ?

[Ian S CParticipant @iansc]

I was playing with the little Bohm motor I got at Jaycar, before tea this evening, it wouldn't go, took the piston out, wiped the piston and bore with a bit of paper towel, put it together and away it went. The two fly wheels on it are about 40 mm diameter brass, and I think the motor would be better if they were at least 50 mm. A well designed motor will actually run (very fast) without a flywheel at all, the only difficulty is turning the motor enough to start it. In this state the motor has virtually no torque.

Ian S C

[Neil WyattModerator @neilwyatt]

Looking at all the various comments, I would be surprised if the sole problem is the loose fit of the displacer rod.

What about putting a blob of oil where it enters the gland, if there are bubbles when the engine is turned over then no point in doing anything until a closer fit is achieved.

Neil

[Ian S CParticipant @iansc]

Here's the little Bohm, I just mentioned it because of how little it takes to make these motor not go.

Ian S C

[AjohnwParticipant @ajohnw51620]

Some one mentioned a different test for the power piston earlier Brian. Drops though and doesn't with a finger on the end of the cylinder. If it was pressed when a finger is on the end sealing of the chamber there should be some springiness / resistance. A bit like an air bearing – a tough thing to make at that level.

A pop test without knowing exactly what it is sounds like the piston is pulled out with the end cover on – that may well pop if it's pulled out fast enough.

Ian's reamer might give you the sort of hole tolerance that Geoff mentions which in practice is more or less the same as a decent reamer. One thing I find odd about reamers these days is that if I measure one rather than being exactly on size they tend to be mid way in the tolerance band. The advantage of Ian's reamer is that it could be tried on a piece of scrap and polished down a touch if needed – it would have to be by rather minute amounts. Bigger D bits tend to produce holes up to 1 1/2 thou over their size, more or less twice what a reamer tends to give.

John

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