Stirling engines Ball bearings

From what Ive read, shiny wire does a better job of being a bearing on a tin can Stirling engine, but I want to see just how much extra friction a proper ball bearing race or two would add to the mix.

The friction levels are very low on these little engines.

They have to be or they just dont work.

But I want to try to make a more robust version, and perhaps even make one that is capable of doing some work.

Perhaps.

But probably not.

Any attempt to make a bigger tin can Stirling engine would also involve a lot more weight. And more weight means more wear and friction. Ball bearings will be the solution, as long as there is enough power left over to overcome the extra friction that bearings have.

That sounds a little contradictory, but bearings are really good at dealing with extra load on the rotating surfaces, but they involve a little extra friction than say... hovering in space.

So, with this in mind, I looked into small bearings.

They cost a lot.

A 12mm (internal) ball bearing race is a very standard thing that industry makes. They cost around $2 each retail, and are a weighty, very strong thing you might find in a motor bike, or in the centre of a bicycle wheel. They carry a lot of weight, and last for ever. But they have way too much friction.

The little bearings I want that are only around 1.5mm in diameter (internal) all seem to cost around $20 each. They are nothing like the robust 12mm ball bearing races I looked at.

Tiny.

Fragile. (actually not really, but compared to the 12mm version...)

Fragile.

And expensive.

But my fishing real has a dozen of the things. They cant really cost that much. It must just be that there is no real retail trade in them. I need three or four, so I thought of buying a $20 K-mart fishing reel, and pulling it to bits, and thats probably what Ill end up doing. But in the mean time, I really want to know if a bearing will be too costly in terms of friction.

I found a little computer cooling fan in my electronics junk drawer. I figured that should have at least one bearing.

These little fans dont put a lot of stress on their little bearings, but they last for ever and spin really fast, with little friction.

Last for ever, fast.

Perfect.

I started by removing all the bits that didnt look like a bearing.

How hard could it be.

Very, it turns out.

Thats the little bearing inside the small brass tube.




I spent a lot of time and energy trying to get it out.

I started by trying to knock out the pin by gently placing a centre punch (made of thick wire) on the centre axle, and smacking it with a hammer as hard as I could.

That didnt work so well.

Actually that didnt work at all.

I finally got it to give some ground by taking a hacksaw to it, and cutting through all the hard plastic surrounds that held the coils to the little motor.

This left me with a much more manageable bit of kit, that even looked like it might finally surrender its bearings.

In fact, this would be perfect if I could just get the pin out of the centre, so I could put the Stirling engines wire crank through the centre.

Centre.

Difficult

I put one end in over the opening of a little socket so the punch could get through and hit it hard.

Nothing happened, so I turned it over and hit it again.

Thats my trusty hammer at the top of the frame.

Camera shy hammer.

This time it worked.

Theres a little flange on one end of the pin that was making it impossible to tap out.

An amazingly strong little flange.

I hit it hard.

A lot.



Now that the shaft was out, I needed to knock out the bearings. I tried inverting the socket so its outside fit inside the brass sleeve, and rested against the bearing. I hit it again.

And again.

And some more.

In the end I just kept cutting.

The hacksaw was the only thing making any progress.

Who would think there would be a time where a hammer failed me.

Theres no real danger of damaging the little bearings here, as the brass sleeve is soft and bearings are made of insanely tough... stuff.

The bearings came out with ease, once the brass was cut through.

It turns out that the brass sleeve is really a brass sleeve with a divider in the middle.

No amount of hammering was ever going to get the bearings out.





Oddly, after all that hammering, the bearings still roll reasonably.

Reasonably.

The reason the little fan was part of my electronics junk drawer in the first place, was because it no longer turned. I think it was the cooling fan that I replaced on my rectifier, so it was never going to work perfectly.

The bearings spun freely enough after rotating them for a while with my drill. but there was a little bump in each revolution.

This pic is of the new ball bearing raced tin can Stirling engine running on my stove-top at around 200rpm.






200rpm is around the same speed on the same heat setting as it ran before, but it no longer runs from the heat of a single candle at only 36rpm, so the bearings have increased friction a little.

The brass sleeve was replaced with a cable tie for each bearing, and it turns out that cable ties fit nicely into my adjustable screw in bearing mounts.

Having the adjustable wire crankshaft I made turned out to be worthwhile, as I would never have been able to make just this one change. The smallest change in the crank shaft, in either the displacer crank, or the power piston crank makes a lot of difference as to how well my little Stirling engine runs, so it would be very hard to tell if it didnt work because of the bearings, or because of the different crank I would have been forced to make.

So.

A success as far as this little engine up goes. The bearings are small, but should be more reliable than the a plain wire on wire bearing. And it seems the friction loss is only around a quarter of a candle.

An interesting addition to this learning thing.





120 Things in 20 years measures the amount of friction in a tin can Stirling engine in "candles". I think I just invented a new metric.

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Fire Quicker more durable ethanol stove

A while back I made a pretty convincing ethanol camp stove out of a coke can.

It worked well and looked like this when it was running.

Ive used it a bit since then and its definitely a practical addition to any backpack.

The problem is its starting to show signs of aging. The can expands and contracts, and there is now a crease where extra gas flows giving an uneven flame.




In my model (also not my design) I used an inner sleeve with holes at the top to let the gas out. The result is that the trapped ethanol (spirit/alcohol/methanol) between the outside wall, and the inner wall boils the spirit, and creates a gas jet that looks a lot like a normal BBQ burner.

One of the problems was trying to get the top of a coke can to fit into the bottom of a coke can. Its tricky because (of course) they are the same size. It can be done, but involves gently stretching the outside one and it takes a lot of messing about.

Now Im being pretty fussy here. The little burner works really well, but it did tend to leak fuel around creases that formed in the join between the top and the bottom sections.

Someone on a Stirling engine forum pointed me towards a different method of making one. This method involves no holes, but it just encourages those creases that mine developed by itself, and I think it might be a winner.

Theirs also used a drink can, but I think it might make a more robust version if I used a tinned food can.

So, to start with, I peeled a normal, every day food tin.

The first step was to trim it to size.

I needed the top to fit into the bottom, so the first thing to do was punch a hole in it to enable a cutting tool to get in there.

I used that mystery tool that pocket knives have.

Probably a leather working tool or something.

Who knows, just punch a hole in the can.

Next jam in some tin snips or something to make a nice cut to separate top from bottom.

My tin snips are stupid, so I ended up using scissors.

This isnt the best way to use scissors, so dont do this at home unless you actually own the scissors.





One very good method of cutting a tin can, is to just tear it by grabbing a ragged tail of tin with a pair of pliers and twist. If theres a groove to follow, it actually makes a pretty straight cut.

As I said, I ended up using scissors to trip everything to it was nice and neat.

I made the inside sleeve (the one on the left), slightly taller than the other so the pot could sit on the rim and let the gas escape from the gap between the outside sleeve.

I bent a series of grooves in the base of the inner sleeve so that they would allow the flow of gas, and also allow the inside sleeve to fit inside the outside sleeve.

This was simply a case of grasping with pliers, and twisting them to the left, and slightly down toward the centre.

Next I inverted the inside sleeve and carefully positioned the outside sleeve over it so that I might press the two together by stamping down with my foot.

Which failed completely.

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Stirling engine Slow motion power piston video

Opps.

For some reason the video wasnt displayed on my last post.

Here is the video for those that missed it...

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Stirling engines Slow motion balloon power piston

I made a few adjustments to my little tin can Stirling engine. It now spins twice as fast, at a rate of around 210rpm. Not that that means much because the more flame you put under it the faster it goes. But its around twice as fast as it was originally. Quite a success as far as bettering a terribly inefficient Stirling engine goes.

If you ask me.

The main adjustment I made was to replace the two sections of shaft that make contact with the supports. The original shafts and bearing surfaces were made of galvanised fencing wire.

I replaced them with a thinner grade of stainless wire. Actually its welding wire, and is the same stuff I use whenever I mention stainless wire in the construction of my fishing lures or anything else on the blog.

Pictured here in this uncomfortably framed, but interestingly red image, is the new wire, the old wire, and a match.

The thin stainless wire makes a huge difference. It even runs without the 8g counter weight now.

The counter-weight is there to mirror the weight of the displacer, so without it, the power piston has to lift all that weight on its own.



With the counter-weight and a minimum sized flame, it can now tick along as slowly as only 32 rpm.

Stately.

To run as slowly as 32 rpm, I found it also needed a small drop of very light lubricant (fishing reel/sewing machine oil). But its important to note that the shaft for the displacer - the one that goes through the small hole in the can, should not be oiled. The oil burns, and leaves a sticky residue which will stop the engine.  As seen by the improvement by the slight reduction in friction, the smallest extra friction will kill these little engines. Use graphite, or just leave it with nothing.

If you did lubricate the displacer shaft, its also possible that oil or Vaseline could get into the displacer container, and being flammable, might eventually find its way to igniting if everything was just right.

Everything is very rarely just right, and a Stirling engine is a very safe thing to make and use because there are no pressurised containers. The making involves some sharp bits of tin can, and should probably not be built by kids, but as a finished item, its as safe as any small candle is, so probably qualifies as relatively child friendly.

Lets say... As child friendly as a birthday cake.

Anyway, it looks like this in slow motion (sorry for the poor picture quality)...

[edit from the future - Opps, for some reason the video wasnt dropped into place.] Here it is...




Even more stately.

Its currently clunking away on my desk, running at around 60rpm on these two little flames, and has been doing so for an hour. One flame is about the size a birthday candle, and the other is around half the size of a birthday candle.

My point is it isnt using much heat compared to the last version.





I find its sounds...

oddly soothing.



K-chunk K-chunk



120 Things in 20 years thinks that if ever I disappear, it might be because Im off on a Stirling engined bike trip around Australia... in slow motion.

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