LTD Stirling Engine
I made an attempt at an LTD Stirling Engine several months ago and failed quite miserably. I know these things are temperamental but after my first attempt I did have quite a few ideas for improvements…nothing new, things that people have already been doing but stuff that I ignored! So over the last few months despite having 101 other projects on the go I started collecting materials. I decided on a glass cylinder with a graphite piston for the power piston and also point bearings as shown in Jan Ridders latest LTD simple stirling plans.
Here is the lump of graphite I`ve managed to get hold of… it is a fair lump!
And it seems to cut OK. I hacked a long block off the top and turned it down holding the vac under the tool to pull all the graphite dust away. This was just a test but I used the same process to make the displacer piston bushings.
The power piston will have a glass cylinder. I needed to know that I can cut test tubes before I got into this and this setup worked a treat. Turning the glass slowly in the lathe I eased the dremel cutter slowly into the tube. I believe you can cut it half way and snap it the rest of the way but I actually held a cloth under it and cut right through.
Here is a collection of most of the materials. The only thing missing is the 2.5mm silver steel which I forgot to include in the picture.
The container was purchased at Asda for £1 and is about 101mm OD. I rang a plastic company and for a 100mm OD clear acrylic piece of 20mm length they wanted £12 plus £10 postage!!!! I think this will do the trick nicely and it cut far more easily than the tapered Tesco spaghetti tube I used last time – the 20mm cut off length can also be seen above! You can also see the glass cylinder.
So I`ve now made some progress with the displacer cylinder, top and bottom have been cut and the top displacer rod hole bushed with graphite. The displacer cylinder will be permanently sealed once completed unlike some designs (and my last attempt) which used screws to clamp the cylinder in place. I think I had a problem with air leakage last time so this time I`ll stick to Jans idea of a permanent seal. I`m actually going to use sealant to permanently bond the bottom section of the aluminum to the plastic displacer cylinder. I`ve cut the top aluminum so that there is a rim of the same ID of the cylinder. This allows me to lock the cylinder onto the rim and I`ll use a small bead of silicon just on the outside join to seal the two as it is held tight without any bonding. If anything does go wrong it`ll be easier to cut this small seal if I need to disassemble the displacer again.
So that is where I`m at right now. My aim is to actually get this finished hopefully during the next couple of weeks. There isn`t an excessive amount of work in it now.
Still pushing on with this, I have now cut a new cylinder to size and with a better finish from a 16mm OD test-tube with 14mm ID. This is the graphite piston to a perfect polished fit. This is the first time working with graphite and I`m very impressed with the fit!
Wasn`t quite finished in this picture but you can see the mirror finish starting.
Since I don`t have a boring head for the mill and didn`t want to offset the plate in the mill, I luckly had this step drill that gave me just the 16mm by 3mm shoulder I needed just to sit the glass piston cylinder inside.
I then turned up two supports…
A little update here as I feel I am coming to the final straight!
Here is a picture of the parts so far but not including the glass cylinder and graphite piston – they are safe away from potential smashes! I`ve now cut the main parts for the crank shaft assembly – the displacer and piston rods, various size parts to the crank axis and of course the four webs.
I`ve also turned the tiny graphite bushes that sit between the webs and allow smooth movement of the rods.
I have now made a small error as at the same time I`ve not soldered the other axis to give the 90deg shift. I DID thread them onto the axis to keep them aligned whilest silver soldering these bits and my intention was to solder the axis for the 90 deg shift in my next “shop time”. Of course when I solder the next bits, it`ll loosen the current soldered parts!! I am strongly considering gluing the other parts that make up the crank assembly. Especially with it being a stirling engine, there are few excessive forces or temperatures to worry about.
I`m off work for 6 weeks at present and so I`m getting more done than I ever have! – I`m having a great time between the workshop and looking after my little one.
I was lying in bed last night thinking about gluing the rest of the crank shaft and then I thought again, should I really not be following the plans! I`d also ignorantly cut the crank shaft into all the right sized little bits to soft solder into position. It was only when I chose to read the text that comes with the plans does it explain to insert that as one long piece, soft solder and cut the small sections out. Of course, despite having to remake the crank axis, this method allows perfect alignment and the only thing I had to do was set the cranks 90deg apart. I thought about a fancy jig and then decided that I could do this using a couple of pin vices.
My soft soldering leaves a lot to be desired. My flame seems too fierce and I end up with a huge blob of solder around the part. However, some time, effort and a lot of sanding got rid of the most of it.
Here is everything assembled so far. Perhaps I`ll have some time for the displacer tonight!
After going on about the displacer and how pleased I was with the graphite, I`m now not sure! It seems to move freely and when I blow it, the pressure builds up and the piston shoots out. However, the fall through test doesn`t work very well, it still falls through… humm would you go with this piston or try and change it? The piston will be something that could be changed once all is assembled. I`m tempted to run with it….
I`m pretty sure that this is all the parts are now complete and I`ll spend tonight polishing them up and then finally sealing the displacer if I have time.
Here is the displacer being cut from foam board. It actually cut very nicely on the lathe with a sharp tool, much better and more accurate than using a craft knife.
This is the holder for the displacer being formed
I next needed to make a similar holder for the flywheel. The plans for the flywheel to be glued to the holder but I wanted to be able to change it from something more aesthetically pleasing, I`m not keen on two CDs stuck together..but again I want to stick to the plans until it runs!
To make the flywheel changeable, I wanted to drill 4 holes so that I could attach it with small screws. So onto the milling machine and mounting my new Boxford spindle nose attachment so I can use the Boxford chuck. Only ever cutting gears, believe it or not, I`ve never using it this way around so have only just run into this potential disaster!! It doesn`t fit!!!!! I need a solution for future!
Luckily with about 2mm clearance I could just centre spot the hole locations but it certainly wasn`t ideal.
Two CDs were glued together and attached to the holder
And now all the pieces in the box ready for polishing and assembly!
OK, I couldn`t resist a quick mock up..notice this was a temporary single disk on the flywheel, it seems to run very very smooth but then the power piston wasn`t in. It did give me an idea of what it would look like.
Hopefully further updates shortly! I will need to leave it at least 18 hours for the sealant to dry and of course that is if I get onto it tonight.
Thanks for the comments and interest so far.
A quick update… last night I sealed the cylinder to the top plate:
and then I sealed the side of the displacer onto the top plate:
This evening I managed to cut the counter weight but I have to admit I struggled with this. At one point the flywheel was clearly falling to the bottom weighted position. The instructions, as I read them, was to keep trimming material from the weight until the flywheel landed at random positions. After quite a bit of material coming off the weight, I did get to a position where it became a little more random but it still favoured one side – I am concerned the power piston brass centre is too heavy as it is this falling to the lowest position which is causing the flywheel to end up at one side. That said, it isn`t as bad as it was so I`m hoping it is balanced correctly. If it doesn`t work it`ll definitely be my first “check”.
All the bits together and a picture of my trying to balance the system without the bottom plate.
And now the bottom plate sealed in position:
I guess the proof will be in the pudding when I try it out tomorrow when the sealant has gone off!
There is a time when sometimes you are almost defeated and despite your best efforts a fresh set of eyes are required. In 2010, I must have a spent a large number of hours troubleshooting to get the LTD running and it has been sitting on my shelf ever since. When I read a message on HMEM that a fellow engineer and someone who I had communicated with via the internet before, had made an LTD in one night from start to finish, I got in contact. Within a few nights I had a message back with videos of it running.
As presumed at the time, the test tube glass power piston was not true enough and this has been lapped. The pillars holding the main crank shaft have also been aligned.
It came back to me in pieces and it took me two nights to reassemble it so it would work. I spent hours on it again and it just wouldn`t go! Anyway, when it finally set off I took these pictures.
So before I show, a huge huge thanks to Pete… I owe you one for sure as this thing has been driving me bonkers for years!
It runs every time now it is together and setup but I`m going to make so finer adjustments so it will run better.