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Author Topic: Bimetallic strips, purchase or make  (Read 9674 times)
Gozdom
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« on: February 26, 2010, 09:15:35 pm »

For a future lamp project, I'd need some bimetallic strips, much larger than those found in BM thermometers. Where could I get bimetal as a sheet, or what appliance could yield it?

Also, is it possible to make bimetal in a workshop? I understand soldering aluminum to brass is beyond me. What about riveting them together? Could it work? I reckon it should!

They are to be used in the lamp mechanics as kind of linear actuators.
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JingleJoe
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« Reply #1 on: February 28, 2010, 06:32:42 am »

I understand soldering aluminum to brass is beyond me.
No it isn't.
You just need heat and solder and clean surfaces. I have used a candle and a cigarette lighter to solder in the past, it worked fine. I currently use an alcohol burner alot for large things as larger things need more heat than my gnarly old soldering iron can deliver.

What do you want to do with this anyway? (linear actuator doesn't explain alot to me Huh) Can't you just use solid brass?
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elShoggotho
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« Reply #2 on: February 28, 2010, 06:37:13 am »

I understand soldering aluminum to brass is beyond me.
No it isn't.
You just need heat and solder and clean surfaces. I have used a candle and a cigarette lighter to solder in the past, it worked fine. I currently use an alcohol burner alot for large things as larger things need more heat than my gnarly old soldering iron can deliver.

What do you want to do with this anyway? (linear actuator doesn't explain alot to me Huh) Can't you just use solid brass?
Linear, as opposed to rotating. Something's gonna move back and forth, or up and down.
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JingleJoe
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« Reply #3 on: February 28, 2010, 06:52:20 am »

Ah I see ... That still doesn't explain exactly what function it will serve.

P.S. Where the fhtagn is the spell check!?
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Narsil
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« Reply #4 on: February 28, 2010, 02:48:26 pm »


You'll get much better results with steel and copper (this is the usual combination) as brass and aluminium have very similar coefficients of thermal expansion.

Rivets along the length of the strip should work if they're set properly.

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Gozdom
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« Reply #5 on: February 28, 2010, 03:44:45 pm »

This source: http://physics.csustan.edu/Ian/HowThingsWork/Topics/Temperature/Thermometers/BimetallicStripThermometer.htm
says aluminum expands most and iron the least, brass being between the two. Incorrect?

The lamp would look like a large egg (this is the actual light source) with a spider-like mechanical creature standing guard over it. The strips would be parts of the legs, expanding as they heat up, lifting the spider.
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Narsil
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« Reply #6 on: February 28, 2010, 04:40:01 pm »


Steel has a linear coefficient of thermal expansion of about 12, copper 16.5, brass 18.5 and aluminium 23.8.

Steel also has the advantage of having good spring properties. So steel and aluminium would, theoretically be the most sensitive but could give problems with galvanic corrosion so steel and brass or copper may be more practical.
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Gozdom
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« Reply #7 on: March 02, 2010, 03:20:19 pm »

OK, so no one has ever made a piece of bimetallic strip. Bad luck. I put together a strip of steel/brass with screws about an inch apart, and it doesn't move at all. I'll try soldering then. How come I can't buy bimetallic sheet like any other material? Duh.
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Mr. Boltneck
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« Reply #8 on: March 02, 2010, 04:26:54 pm »

I haven't been able to find the actuator sort on the usual sites (Smallparts.com, or Amazon Industrial), although Thomas Registry may be of interest: http://www.thomasnet.com/prodsearch.html?which=prod&what=bimetallic+actuator&cov=NA.
In addition, I seem to recall that some earlier versions of electrical car locks and other powered bits used a bimetallic actuator in many cases. You may be able to do a bit of research on makes and years, and then go grave-robbing at the wrecking yard. I think that the 1970's and early 1980's may be about right.
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Horse Brass
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« Reply #9 on: March 03, 2010, 01:33:34 am »

When I was a kid I made a small bimetallic strip. I used a 6 inch long piece of thin copper sheet (almost foil) which I found at school, and some aluminium foil from the kitchen folded over a couple of times. I put about half a dozen staples through it to hold it together. (It was very thin copper sheet.  Smiley ) When I poked it into the flame of a gas stove, it bent about 20-30 degrees if I remember rightly.
The piece I made would have been too weak for what you want, but if I were experimenting today, I'd start with the thinest materials I had and work up to thicker pieces to find a balance between strength and flexibility.

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Gozdom
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« Reply #10 on: March 03, 2010, 01:55:31 am »

Dammit I may have to dump the project or find an easier method like a DC motor or clockwork (or both). It just came to mind that it may be very hard to produce a bimetallic strip thick enough to lift something heavier than a plastic stick (like thermometers). Steampunk is my pastime but not metallurgical chemistry.

Unfortunately, a motor produces noise, which is undesirable for a desk ambience lamp.
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Gozdom
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« Reply #11 on: March 03, 2010, 02:09:40 am »

I'll post a drawing of the spider/lamp mechanism as I vaguely designed it, perhaps someone comes up with a better idea. Or makes it. The spider standing over the lamp should move up a bit as it heats up, and descend when switched off. Even better: it moves up where it cools down, then goes back slowly. It should be a slow, almost plantlike movement (otherwise it is disturbing).

Apart from bimetal, any substance could do it that expands a large amount when put near a lamp (maybe around 80-100 degree Celsius). Like steam engines. But then it needs a well sealed piston that can withstand overheating, is not tooo heavy and maintenance-free. A single  moving part could do the trick, in the spider body, mechanically connected to the legs.
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Narsil
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« Reply #12 on: March 03, 2010, 02:44:20 am »


I think you might struggle to make that work with thermal expansion from any solid, the volumetric expansion of steel is something in the order of 0.003% per degC, so with the temperature range you're talking about the actual magnitude of expansion is going to be small. It's not impossible by any means but it would need careful design  to create a linkage to generate the required degree of movement. Thermometers working on this principle use long coiled strips to generate a measurable deflection, but the force they produce is tiny.

The force generated by thermal expansion is the same as would be required to produce the same extension by a mechanical load.

Probably the best way to do it via thermal expansion is to use a substance which has a boiling point somewhere in that range and use a large diameter expansion cylinder but that's certainly getting into a fairly serious amount of technical complexity.

Another option is to keep the bi-metallic strip but use it as a sensor to trigger a separate mechanism providing the movement.

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Mr. Boltneck
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« Reply #13 on: March 03, 2010, 06:05:42 am »

How about shape-memory alloys, like Nitinol as actuators. Silent, and electrically operated or with external heat.
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Gozdom
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« Reply #14 on: March 03, 2010, 08:39:00 am »



Another option is to keep the bi-metallic strip but use it as a sensor to trigger a separate mechanism providing the movement.


Yes, this seems to be the only viable solution, bimetal serving as a switch for a motor.
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Captain Quinlin Hopkins
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« Reply #15 on: March 03, 2010, 12:43:01 pm »


Though interesting, it bends back to shape once heated, but after cooling you must manually bend it back to shape.  Now if you could somehow direct the heat from you lamp to various pieces of Nitinol, then you could, in fact, create motion as the opposite piece would bend forcing the original piece to straighten.  This would work well in a slow controlled manner, and I think you might have a winner with this. 


And since you are looking at creating a desktop illumination, you might look into this. 
A combination of moving and a light source could prove quite interesting. 

Kenetics can be quite fun if you take the time to work out the details sufficiently. 


If you feel it necessary to continue the bimetallic route, then understand what metals work with what, and how much heat is needed to move the piece. 
This site is a great reference for all things engineering. 
And if/when this site locks you out, just clear your cookies and restart your browser to start again.(I use ccleaner to clear all traces of browsing, which seems to work well)
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