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Clicking Over-Center Switches Mechanisms Lego Technics Mindstorms

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Uploaded on Jan 23, 2011

A personal memo showing how to make a few mechanisms in Lego using only elastic bands.

The 'battery holder' is a 4-lever linkage like a "Mole-grip" - I try to show why its called 'overcentre' by waving my finger over two of the linkages as they line up: that is centre. Before centre, I can push the jaws open, at centre the jaws are locked; over centre, the jaws push to a locked position. A lever un-jams the mechanism.

The push-switch doesn't work well because it uses two elastic bands which are supposed to be 'tuned' to overcome the friction (and I couldn't be bothered). The white elastic band is pulling apart the far ends of the grey linkages it's attached to (like circlip-pliers). Turning compression (elastic band) into expansion is a good trick. The band plus its two grey things are behaving like a torque-spring. The centre-point (dwell) happens when the torque-spring ends are closest to each other. The purple elastic band also has an over-centre moment as it crosses the torque-spring's attachment point. When I take off the purple elastic band, you get to see the bistable nature better with a nice tactile clicking rod.

The first lever switch is the simplest (kinda normal) over-centre elastic band. It's awkward for lego - kinda sloppy, but a stronger elastic band just pulls things apart. I wanted to use it for the useless machine we made but the elastic band covered too much space, so I ended up 'inventing' (probably re-inventing) a new way, which is the lever that slides over a sprung angled linkage. Again, it has a good tactile feel which is easy to adjust by changing the shape of the angled linkage or the tightness of the elastic band.

The last switch is made using a T bar and is more complex. It uses two rubber linkages to balance a lever that is shoving the T each way. A problem needs solving with this approach: the lever will push right through so it needs limiting. Another problem is that the ends of all the bars are rounded which means the when the T bar tries to push the lever away, it won't necessarily slide across the back of the T, it will stay on the tip. The solution to both these problems is the loose U-shaped bits: they block the lever and stop it pushing all the way past the T, and especially stop the rounded ends meeting each other. Then when the T pushes back, the loose U slides out of the way and persuades the lever to go back across the back of the T. You probably have to build one to get all that! It's the stiffest switch with a good tactile feel.

The parts are all from the Lego NTX 2.0 Mindstorms set.

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