Best Way to Make a Releasing Mechanism from a Bimetallic Strip?

How about soldering? I think you can get solder strips (like tinsel), and sandwich them between the two metals. You might even be able to heat the whole assembly quickly by applying a suitable voltage from end to end.

Hi,

I am also working on similar project. i.e.

" Solar Dish Stirling Engine System"

My project involves,

1) Heat Engine / Stirling Engine

2) Solar reflecting type concentrator

3) Tracker for solar concentrator

Please let me know your contact details.

my email ID is:

sweetechnologies at gmail dot com

Mobile: +91-9623557207

Madhav Chowdhary

Pune city / India

solarflower.org at gmail dot com

I'd like to see what you're working on.

Soldering would work well, but a constraint of the project is that everything be made as simply as possible with absolute minimum skills and tools.
So no soldering.

I will send mail to you

In the late sixties MIT researchers made just such a device to track their parabolic collector. A shade strip was designed to keep a coiled strip of bimetal always in its shade. The strip will be exposed to the extent that the sun rays reaching it generated enough torque. The problem was when the sun was hidden by clouds for extended periods. As far as I remember, it was manually brought back to rest position at day end. Since the application was to power a single domestic hot plate using molten Sodium Nitrate this was considered adequate. Unfortunately I could not trace details. The report was in a news letter of Volunteers in Technical Assistance (VITA), which seems to have metamorphosed now. Bioramani

Interesting.

The issue of periods without direct sun is pretty much why I didn't end up using that kind of approach for the tracking during the day, instead eventually developing a very simple heat engine (ethanol steam powered) which provides constant motion with the ability to catch up to the sun after less than 3.5 hoursof occlusion.

To have a bimetallic strip or similar achieve this would require alternate shading and illuminating of the strip, so that it could cool and reheat. This would be possible, but ultimately more hassle than my approach.

The strip switch I plan to use is simply for the reset of the system's orientation. It should still be able to achieve several hours of range (in case it's cloudy at dawn, etc) but only needs to trip once per day, rather than once per degree of sun movement.

My design takes all it's energy from the sun, there's no need for manual resetting.

It does not oscillate. There is no hunting or alternate heating /cooling. What happens is that as the shading begins the torque decreases. An equilibrium is reached with the coil partially in the sun at a torque to hold it oriented. As the angle increases towards evening more and more area of the coil remain exposed to generate the higher torque. The bimetal coil forms one end of the axle. The other end of the coil is firmly anchored. The far end of the axle runs in a journal bearing. The shade piece is mounted directly above the coil on a radial spoke attached to the axle after the bimetal torque coil.The shade piece also moves in an East-West arc. Since for small angle deviation cos(theta) is quite nearly unity, small angular deviations do not matter. Bioramani

Ah, I see. Good idea, tho it sounds tricky to get accurate enough to use with a parabolic dish..

But how is that interfered with by periods of cloud? Shouldn't it be able to catch up once the sun returns?

You are right. I am recalling from memory of what I used in the early seventies. All I remember now is that short periods did not matter. If the coil cooled a lot the response was quite slow. There was some issue connected with that. I had done some work then on solar stills and such, which is why this much at least stuck in the memory. The device consisted in a parabolic long trough with a linear focal axis and not a parabolic mirror with a single point focus. The collector was a double walled vacuum tube with sodium nitrate in the inner chamber. A small error in orientation shifts the axis only slightly. The diameter of the tube is large enough to compensate for this. A small iron container (Sodium nitrate has no corrosion on iron) insulated all round with a removable insulating cover over the exposed top surface collects the molten salt. Once the salt is all melted inside, the 'Hotplate' is disconnected and taken in with another plate attached to the collector. The latent heat released as the chamber is is used does the cooking. Since it is a change of phase, the system is essentially an isothermic one. Bioramani

A few tips on such arrangements. First, you already know that you want to select a pair of metals with large difference in expansion coefficient. You might also consider small difference in emf to minimize corrosion potential. You must take seriously the method you use to connect the two metals to each other. Most often they are welded. You need to connect them continuously to resist the shear stress between them that is the source of the bi-metal bending. Just fastening the ends together with rivets or bolts won't give you what you want. Also, the thicker the metals, the more force you will generate and the smaller the displacement. There are closed mathematical formulas to solve these problems.

> You might also consider small difference in emf to minimize corrosion potential.

This may be why steel is generally used with copper rather than aluminium?
I was thinking of placing a thin sheet of rubber between the metals to reduce the heat flow between them (as I'll be directly heating only the more expansive metal), would this also reduce corrosion, or is there some obvious reason why this shouldn't be done?

> Just fastening the ends together with rivets or bolts won't give you what you want.

I've seen quite a few references to riveted strips, it seems like it's viable. Clearly continuous attachment would give better results, but as I say I'm wanting the easiest means of construction (the device is meant for self sourcing and assembly in the developing world).

I found this for calculating bend radius, but not force:

Define L = strip length, dL = L1-L2 at that temp. difference, and T = thickness of one metal layer. When bent to the required radius r, the inner-outer layer arc length difference due to a radius difference of T equals dL.

Thus theta*((r+T/2)-(r-T/2)) = dL, where theta = L/r.
Then LT/r = dL ==> r = LT/dL.
So look up the thermal linear expansion coefficients of brass and steel, take the difference and multiply by delta temperature; the result is dL/L. Go from there.
I found coefficients for brass = 19E-6 /deg K and steel = 13E-6 /deg K.
Then dL/L = 6E-6*105 = 6.3E-4, and r = 0.0025/6.3E-4 = 3.97 m (answer).

I haven't completely understood your proposed mechanism, but I do understand you wish to withdraw a release pin. In this case, rather than using a bimetalic strip, use a bimetalic rod/tube. A steel rod inside a slightly shorter brass tube, with the two joined at one end only, will withdraw the steel rod when the system gets hot, and will apply orders of magnitude greater force than the bending of a bimetallic strip of similar mass. There would be some form of spring system required (probably on the plate having the pin socket hole) to allow the rod to snap back into place and prevent jamming when clouds pass over.

That's a very interesting idea, but I'm having difficulty finding info on it. Can you point me towards and diagrams etc?

Here's a diagram:

I've shown a cap at the left end, but the rod could be joined to the tube lots of ways (solder or braze would be excellent, but it could be a pin, threads, or just a crimp in the tube). The right side of the tube would be attached firmly to the moving part of your assembly, parallel to, but some distance away from (the farther the better, to reduce the force required), the axis around which it rotates. The left end needs a loose mount, so the tube can slide as it heats and cools.

A plastic tube around most of the length of the rod (heat shrink tubing?) would prevent electrolysis between the steel and brass, and pretty much guarantee that they won't be jammed by rust or crud. The latch plate could be springy itself (blue spring steel is relatively cheap; you might need a carbide drill to make the hole, but the hole would then last a long time), or it could be a thicker part with a hinge and separate spring. At least the tip of the steel rod where it latches should also be very hard to avoid wear. One possibility would be to use large piano wire for the rod; then the whole rod would be hard and very strong. If you leave a little extra sticking out, you could re-grind the tip square and readjust the latch plate when the tip wears. Of course the latch plate would be attached to the non-moving support for your collector, so the rod will snap into the hole when the assembly is rotated to the morning position.

If properly mounted, This tube could be quite small in diameter; I think a heavy-wall 1/8" OD tubing should be adequate. In fact if the assembly is almost balanced at latch position, then 3/32" tubing with 1/16" rod should work. In this case, there probably wouldn't be enough room for shrink tubing, so you would want to spray most of the rod with Krylon or something similar to prevent electrolysis.

The length of tube and rod required would depend on how sharp the edges are where the rod fits into the latch plate hole, how much play there is in your bearings, How flexible the assembly is, how precisely you can locate the latch plate, and of course how much temperature difference you get. Painting or otherwise coating the outside of the tube with a dark surface will increase the ∆T. This tubing comes standard in 3' lengths. For home-made units, you might need the full length to get reliable operation; If you can get fairly good precision, half that length, or maybe even one foot should do. You already have the formulas and coefficients to calculate the ∆L available, but there is no substitute for experimentation!

Laminating low modulus material between the two strips will reduce the bending by a considerable amount. The thicker the rubber and the lower the hardness (durometer) the more motion you will loose. While we are on the subject, more mass will lower the response time of the strip.

Intermittent joining with bolts or rivets will reduce the overall efficiency. I can't tell you how much, but I believe one could make an accurate prediction.

Your equation for motion is only good if the two members have the same elastic modulus, or if the individual thicknesses are adjusted to provide the same bending stiffness.

Man, this is why I love cr4, it's where all the actual experts hang out.

@Dkwarner

That looks a pretty comprehensive solution and is within the brief I'm working to. I'm a little concerned tho about the foot of length; due to my design as it currently is there's not really anywhere I could put that, especially as it would need to be tangential the main shaft.
I'll give it some thought.

@Welderman

Cheers, will take that into consideration.

Looks like it's time to do some sperimentin.

Any way you could post a sketch or photo of your assembly? How big is it? Then we could see how to do it.

I was assuming parallel to the main shaft, not tangential to it, but there are lots of possibilities.

The release pin needs to be as far as possible from the axis to reduce the force on it. So place the bimetallic tube along the shaft, with a lever out to the actual pin. Another possibility would be to have the bimetallic tube perpendicular to the shaft and perpendicular to the sun's rays (vertical in the morning) on the east side of the shaft. I can make some more sketches if necessary...

My mistake, the pin is parallel to the main shaft.

I'm using a bike wheel hub bearing as the contact between the shaft and the collector. This has the advantages of low friction, an easy bolt on point for the collector, and a bunch of pre-drilled holes around the circumference where the spokes were. However this does mean the pin will be sitting at a radius of about 30-40mm. (Inch and a half).

The collector is balanced tho, so the force on the pin should only be the return weight, which will be very little. But then there's wind...

A lever out isn't really an option as the angle between the shaft and collector is largely arbitrary, there's no set point at which it releases or returns to.

I'll take a crack at a pre-vis. Photos of the collector without the shaft are here:

http://solarflower.blogspot.com/2010/07/main-parabolic-trough-collector.html

The end boards are 100 x 48 cm.

That helps, but I need to know where the axis is located. I suspect the axis will be fairly close to the focus line for balance. If so, the bimetallic tube could perfectly fit parallel to the axis and just inside the apex of the parabola. At that point, it would be shaded from direct sunlight by the collector tube, but still get enough heat to work. If you have a square meter of collection area, then you have room for nearly a full meter of tubing! It looks like that would be around 20cm from the axis, which would probably work, although I'd prefer a bigger lever arm.

With that in mind, why not just mount the bimetallic tube along the east rim of the parabola? That would place the pin around 50cm from the axis, and near the bottom in the morning, where it would easily interact with the support (where the latch plate must be mounted).

I don't have time right now, but maybe later today or tomorrow I can make a sketch...

Dick

Ah. I get you now. The lever is to magnify the tube's displacement. That's a good idea. So is mounting it along the outside surface of the parabola.

The axis is the focus, in the top photo it's inhabited by a black copper tube. I checked the balance just now, its 50 grams heavier on one of the bottom corners (which is to say, adding 50 g to the opposite corner balances it) which is ok as it needs to be heavier on one side anyway to reset.

The bottom of either outer wooden beam would be the best place to mount the tube, yes. Sturdy and faces east at the end of the day. That'd give a displacement of 52 cm.

Maybe, maybe not! If you increase the displacement with a lever, then you decrease the force proportionately. Adding any kind of mechanism also introduces 'play', which then requires greater movement of the actuator. My other task is going slower than I thought, so I won't be able to make the sketches 'till tomorrow, but I think I've got it figured out.

Dick

How about this: take a long bike brake cable, strip off 30cm or so of the sheath, use the steel wire cable as the low expansion inner, with an aluminium tube as the high expansion outer. Run the rest of the cable to the pin directly on the hub.

There's no amplification of force, but it'd be very easy to construct. And the force required shouldn't be too much, as I say the collector is pretty well counterbalanced. Even if there's wind gusting when it's trying to release it'll just pop at any point the wind subsides for a second.

Any luck?

I'm thinking this approach is probably the best one, would love to see some numbers if you've got them.

Sorry, my work keeps getting in the way... I think I can work something up tomorrow, while I'm in the airport and on the plane to San Diego (That's where my real work is...).

Dick

Here's a start. This is how I'm picturing the bimetallic tube mounted on your collector:

Brown and blue represent the mounting structure, where the angle between the two legs is chosen so the latch plate (green) can mount directly on one leg. I've shown this so the right view is looking down into the axis of the collector in morning position, so the latch plate is mounted under an upper leg. It could also be mounted on a lower leg, with a mirror of these images. Since the lower leg is shorter, it would be stiffer. The bimetallic tube/rod (red) is mounted on the east (bottom in the morning) support bar, where its weight will automatically de-balance the unit. Of course the fixed mount would be as close as possible to the latch plate, and the other end of the tube/rod would be held loosely. The tube/rod would pass through a hole in the end plywood.

If this image isn't clear, I can send a PDF to your email. Hopefully I'll get you more later today.

Dick

The shape of the mount is determined by the tracking system; you can see the water wheel + shaft assembly in this video

http://solarflower.blogspot.com/2010/07/well-its-official.html

with the bike wheel bearing on the end being where the collector directly mounts, and the spoke holes ideally being where the release pin will insert.

When the pin releases the off-balance collector turns on the bearing shaft. (I'm probably not getting these terms exactly right). I therefore need a bunch of holes, as it's not set at which point on the shaft the pin will reconnect.

If that makes sense.

Did you have any numbers on the length of the tube, amount of offset, etc?

> Wouldn't you be happier with a latch that is primarily sensitive to solar radiation?

Whatever works best. What did you have in mind?

I'm writing this on a plane, so I can't refer to the expansion values you provided, or otherwise look them up, and once I get there, I'll be busy again, so you will have to put the numbers in yourself.

If I'm not mistaken, the formula for calculating the expansion is:∆L=L*∆T*(k2-k1); cm=cm*C°*(cm/cm*C°), where ∆L is the change in length, L is the full length, ∆T is the change in temperature, k2 is the coefficient of thermal expansion of the tube, and k1 is the coefficient of thermal expansion of the rod. Of course the coefficients must be in Celsius values.

In any case, since your ∆T is going to be relatively small, and your assembly is to be homemade, there is no way you will be able to hold the position of the moving collector at a constant tiny spacing from the mounting structure. Therefore, the latch plate must have sufficient spring to compensate for variation in this spacing, and the unlatching must be by the rod retracting inside the end of the tube. Here is a detail of the Latch Plate as I imagine it:

Now I'm off the plane, and have seen the video. So now you need flowing water as well as sunlight? It is not at all obvious how the water wheel will be controlled so the unit tracks the sun. The water wheel looks very crude to me, but I'm a machinist. It also strikes me as a major source of problems. Now I'm thinking 'welderman' is on to something.

Most of my training was in electronics; I would use a totally different approach. This unit is so light, that a very small electric gearmotor driving a worm gear similar to the one you have now could do the tracking, powered by a small solar cell. There are several possible arrangements that could automatically track the sun. I would think that one or more should be readily available on the internet, but I haven't looked. 20 years or so ago, my high school electronics kids made little robotic cars that would follow a flashlight wherever it went. Unfortunately, I'm going to be busy this week, so I doubt if I'll have a chance to elaborate 'till the following week.

There may be a problem. A 1 meter aluminium rod heated 100 ºC will expand 2.3 mm. Figuring that with .5 meters, 60 ºC and minus the expansion of the steel I get a figure of only 0.36 mm. This I think won't be enough to do very much of anything.

> there is no way you will be able to hold the position of the moving collector at a constant tiny spacing from the mounting structure.

?

The pin is to secure the collector to the main shaft, and release it from the shaft in the morning so that it can drop back to the starting position. Then reconnect it again.

> So now you need flowing water as well as sunlight?

I thought the term 'water wheel' might be misleading, but not sure what else to call it.

No, the wheel is for liquid ethanol, which is pumped onto it by the simple heat engine which drives the tracking system. I have a basic schematic here:

http://solarflower.blogspot.com/2010/06/heat-engine-which-drives-device-has.html

Now, being a fairly shrewd professional type you will have noticed that this setup is _incredibly weird_. But this is what it's evolved into over the last year and, despite it's strangeness actually seems so far to be working very well. At least, each separate component works well on its own, I'm just now getting up to the point of plugging it all together.

"The Solarflower is an open source solar energy collector which can be made very easily from common recycled and salvaged materials, using basic tools and skills.

It can be made almost anywhere, is portable, involves no inputs or emissions and has no real limit on how much power it can produce."

This is all very crude, yes. It's meant to be. Or at least, it's meant to be easily makeable by anyone who wants to, without requiring a high level of skill or investment.

What kind of problems do you foresee for the wheel? It doesn't seem to jam, and is very low friction. It's probably important to point out that it's not going to be running any kind of revs, just one half turn every six minutes or so.

> a very small electric gearmotor driving a worm gear

That would be a good way of doing it, yes. The brief for this project tho is, as I say, extreme ease of fabrication. It's mostly for use in the developing world, where materials, tools and skills are probably not quite up to the level of light sensors and motors. My heat engine consists a bit of copper tube, a glass jar, some thinnish pipe, two bike inner tube valves and ethanol. Constructing it requires a hammer and scissors.

> I doubt if I'll have a chance to elaborate 'till the following week.

No worries, thanks much for the input. Very informative.

Without having repeated the math, 0.36 mm sounds about right. That IS enough motion to catch and release, IF both the pin and the hole it fits into are hard and have sharp edges.

I failed to see the connection between your 'water wheel' and the ethanol engine. The first time I looked at it, I didn't view the video all the way through, and I thought the cups were cut up beer cans - it would take a lot of ethanol to make that rotate. If 2 ml of ethanol at a time can make it rotate, that's an incredibly low power. What mechanism do you propose to use to regulate the speed so it tracks the sun?

What you need is a 1 RPD (Revolution Per Day) motor. A large wind-up clock is 2 RPD; add one more 2:1 gear ratio and you have it. You might even be able to find a 24 hour clock to do it directly

> 0.36 mm sounds about right.

Hmm. Given the design brief, ie bits of scrap shackled together, I'm not confident I can work to those kinds of tolerances. If I had access to properly machined parts this whole thing would be easy, most of the challenge has been designing to the level of people without professional level skills using less than ideal materials and tools.

> I failed to see the connection between your 'water wheel' and the ethanol engine.

It's a little tricky to explain without visual aids but basically it works like this:

When the main collector is pointed directly at the sun nothing happens other than energy gets collected.

As the sun moves off by a degree the hotspot of a compound parabolic box collector shifts on to a copper boiler containing 2-3 ml of ethanol, which starts to boil.

The vapour leaves the boiler from a tube at one end and enters a chamber with about 100 ml liquid ethanol, forcing it out through another tube to the top of the 'water' wheel assembly, pouring into a cup and turning the wheel, which turns the shaft to which the main collector and box collector are attached. The eths then runs out the bottom of the wheel into a reservoir from which it can re-enter the boiler. The main collector now points back to the sun, the box collector's hotspot shifts off the boiler, which stops producing vapour.

OR

If the sun has been behind a cloud or something, and reappears more than one or two degrees later (up to 50º) the above still happens, except that about the time all 100 ml of liquid in the chamber has been forced out, the 2-3 ml in the boiler has boiled away. The vapour in the chamber starts to collapse, which sucks in liquid from the reservoir below the wheel, through the boiler, refilling it and the chamber.

The boiler is still receiving heat, since the sun is still off center, so starts to boil again. This keeps going until the sun is re-centered.

If that makes sense. I really need to do a little animation of all that working.

So, like I say, it's a weird setup. But it's insanely easy to construct, doesn't really cost anything in materials, and so far seems to work.

> I thought the cups were cut up beer cans

They are.

Well, actually they're whiskey and cola, which I has to buy at €2.20 a piece as I'm currently in Germany where it's practically impossible to find aluminium cans. Something about a recycling surcharge.

> it would take a lot of ethanol to make that rotate. If 2 ml of ethanol at a time can make it rotate, that's an incredibly low power.

Unloaded, ie just turning the wheel, gearing and shaft without the collector takes about 25 ml of ethanol. So not a lot. The collector is evenly balanced, minus the return weight, which will probably be about 50 grams. The worm gear takes things down by 120 times.

So the forces are very small. One half turn of the wheel rotates the shaft 1.5 degrees, or 6 minutes of sun movement.

> What you need is a 1 RPD (Revolution Per Day) motor.

I thought about clockwork mechanisms, and they're definitely a valid way of doing it, but there's a couple problems I have with them.

They require an external power source, and ok a battery or lifted weight /wound spring isn't a huge issue, but the more the device can just be set and forgotten, the better.

They don't track the sun directly, just where it should be. Most of the time that shouldn't be a problem, but means you have to keep an eye on it to make sure it hasn't gotten out of synch.

You either have to find a lot of big clocks capable of generating sufficient force to turn the collector, or make a separate mechanism which the clock then regulates.

The 2:1 gearing would have to be perfectly accurate or it'll creep.

But to be honest, if I was just making a couple of these things for myself this might well be the way I'd go. However for the general public in most parts of the world, I think (hopefully) my approach is overall more practical.

After thinking about your project, I wonder whether a bimetal strip is your best answer. These strips apply force and deflection based on their temperature. This means that your latch will be affected by ambient temperature fluctuations as well as solar radiation. Wouldn't you be happier with a latch that is primarily sensitive to solar radiation?

Any ideas? Anything that creates displacement from sunlight would be fine, long as it can be easily fabricated.

How about a spring loaded piston with an enclosed liquid (possibly in a bladder for sealing) that boils, similar to your heat engine? In fact I was thinking the other day about something along that line to be the heat engine itself. The boiling of the liquid would move a piston carrying a shade, so at extension the liquid would stop boiling.There would be some kind of mechanism that latched the shade in the shading position until most of the liquid had condensed. it would then snap back to its original position and start the next cycle. This would run a ratchet to advance the rotation, and of course a second shade mounted on the main unit would block the sun once it reached the correct position, to prevent going on past aiming at the sun.

A properly designed ratchet could automatically release once it reached the end-of day position, and the off-balance would return it to the morning position, ready for the next day.

This exact idea has been lurking in the background for almost the entire life of this project. Every couple of months I take it out and re-examine it again, see if I shouldn't actually be doing that instead.

And it seems fairly workable and reasonable on paper.

In the end tho, it's not that there's anything wrong with the idea, it's just that once you've actually built it, simple as it would be, I think it still wouldn't be as simple as what I'm going with.

Also, I don't like pistons. When I was trying to make the vapour on/off device I was playing round with them a bit, and the only thing which even comes close in friction versus sealing is a syringe, but ethanol vapour tends to seriously mess with the rubber seal, very quickly. Same with bladders.

And you'd lose a bit of time each cycle as the shade flips on and the boiler is still boiling, then time for it to cool, then to heat up again.

But, yes, if not for the current design this is almost certainly what I'd be going with.

Glass syringes don't have rubber seals. Nothing to swell or degrade. If you want lower friction, graphite pistons work nicely in glass or Pyrex syringes.

Made up a quick and dirty bimetallic strip; aluminium and steel, 160 mm long by about 10 mm wide by some fraction of a mm thick each. Held by four rivets.

With the heat of one candle (50 W?) it quite happily lifted 200 grams 8mm in about 30 seconds.

In the final application I'll be using a similar amount of concentrated solar as the heat source, but over the entire length of the strip, not one point as with the candle flame. Not sure what effect this will have, but seeing as I probably only need to move half the resistance half the distance, hopefully this'll work.