Screw Archimedes

Who needs a lever, man? I’m Marty McFly.

Let’s say, for the sake of argument, that you needed to move a heavy piece of equipment you’re helping to build. And you have the constraints of not wanting to tip it, and the course you need to navigate has some spots with less than 10 cm of overhead space. (This ignores one doorway that’s actually 10 cm too low — yes, we’ve done the equivalent of having built a boat in our basement. The solution there will either be slapping Daffy Duck with a frying pan in the shape of the equipment and having him run through the door, or a sawzall.) You need a smooth ride, because it’s an expensive, somewhat delicate trinket representing a several dollars and a few person-years of labor.

I tried designing a cart, but couldn’t meet all of the constraints — anything low enough would tend to bottom out on the incline (oh, that’s right, the path isn’t level the whole way. Is that a problem? I need this soon.) One day I was stressing and kvetching about it in front of the right person, who suggested air bearings/air casters. The heretofore unconnected link between the physics and the application clicked, and I knew that was the answer. Float the sucker on air. A small industrial blower and lots of small holes.

Here’s a demo of the system with a dummy load. There are also some lead blocks there, too. (cue rimshot). 160 kg for the optical table, lead and support structure, and another 110 kg or so for me.

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(if you want a soundtrack, turn on a vacuum cleaner. Any “Ishmael” wisecracks about my pasty-white legs will be subject to retaliatory editing)

As you might imagine, everyone involved has taken a test ride. For safety’s sake we don’t ride on the sled pads by themselves, true hoverboard-style — they’re only a few kg, which means that the unstable equilibrium is really unstable — a few degrees off kilter and a sled would tend to squirt out from underfoot at a few g’s of acceleration when trying to mount it. By adding the extra mass, that acceleration drops below 1 m/sec^2, which is manageable. You can see the slight drift as I step on the sled.

The lift is several cm, which is enough to get over door thresholds and deal with inclines. Polyethylene sheets can cover porous and rough areas that disrupt the airflow. If I can find a way to add a propulsion system I’m Luke Skywalker on his speeder, at least until I reach the limit of the extension cord.

Some interesting effects came to light with the practice run we did. Even though one knows the concept of centripetal acceleration and friction, it’s quite another thing to experience the differences between this mode of travel and a wheeled cart (which I did, having carted the lead bricks into the test area). You tend to drift while on the sled, and if your floor isn’t level, you find the gradient; this becomes very dramatic when the incline is several degrees. When you want to stop, you have to do so actively, since friction is basically absent, and this also shows up when taking a corner — the centripetal force wheels automatically generate is absent, so you have to exert it yourself.

Bonus: Hovercraft designs

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