Prototyping

The "Hulk" Spring

The “Hulk” Spring

After trying out the first set of shape memory springs, we figured that we have to put them to use. Half a year ago, we roughly assumed, that a closing force of 3 kg would be appropriate for our mechanism. The result however meant, that we got a relatively huge spring with about 1mm wire diameter and around 6mm spring diameter.
Now a 1 mm Nickel Titanium wire is VERY stiff, which means there was hardly a way for the company to make a loop on both ends for fixating the spring onto our mechanism. Our fault, we didn’t really need 3 kg force. Since we needed to make a prototype of our mechanism anyway, I decided to make a “beefed up” version of it, that can show off the basic function and put our big Hulk Hogan shape memory springs to good use.

 

 

For quick prototyping, I often use glass fiber sheets. They’re cheap and easy to handle (don’t forget to put a mask on though…).
A quickly made model on the computer looked something like this:

CAD Model

CAD Model

Let’s get started then:

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To explain what’s going on, have a look at the picture below. Essentially there is a middle slider element (red color)), which will slide left and right, depending on whether the mechanism is activated or not. From each side we have a spring pulling (kind of like a tug of war). The shiny silver spring is just your average mechanical spring, while the other bronze colored spring is our shape memory spring. Now, when the mechanism isn’t active (means the shape memory spring temperature is cool), the mechanical spring force is higher than it’s “opponent’s” force. However, if we activate the mechanism by heating the shape memory spring up, the memory spring “wins” the tug of war and the slider moves.

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Now let’s show you a video of it in action. To activate it, we send a current through the shape memory spring, which heats it up and makes it contract (with our hulk-force of 3 kg). You can see the slider moving (from black to red). Red essentially means that the mechanism is open. Afterwards, the spring cools down again and it’s 3 kg drag force goes down to a fraction of it.

 

Now that’s not all. We also got access to a Makerbot 3D printer, which is great to make actual sized parts, that we can then show to the machine shop, to check if something is doable in real life and not just in squeaky clean CAD software. Therefore, we printed a quick model of the latest mechanism draft.

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