Summer STEM Challenge: Vacuum Pile-Driver

STEM Challenge #51: The pile-driver run

Pile-driving is like hammering nails into wood. Except the nails are 30cm or even metres in diameter and 10 or 50m long, and the wood is the Earth! Once in the ground, piles are joined together and form the foundations of buildings from skyscrapers to wind turbines. There are other ways, but a lot of piles are put in the ground by a falling weight. The weight – the hammer – is simply hauled up and then let drop. Here is how to make a pile-driver run off a vacuum cleaner that can hammer garden stakes and posts into the ground.

A mains vacuum cleaner can put out about 200mbar pressure – 20 per cent of atmospheric pressure – and a flow rate of tens of litres/second. With this kind of power, up to ~10kg force on an 8cm pipe, a wooden hammer-piston of a few kilograms can easily be sucked up a vertical pipe. A big chunk of wood, circular or with circular rings which fit the pipe to within a millimetre or so, makes a good hammer-piston. Try making a couple of hammer-pistons of different sizes, and work with the lighter one to start with.

Once it’s lifted up to the top of the Vacuum Pile-Driver pipe, you need to turn the vacuum off and let the hammer-piston drop freely. Just turning the motor off would take several seconds, which is far too slow. You need some kind of valve to let the air in, like the string-operated flap in the pic, to let air in almost instantly. It can be taped and glued to the top of the pipe.

Once you’ve put it together, you’ll need something to test it on. If the workpiece, whatever you are hammering, won’t allow the air in front of the hammer-piston to escape fast, you need to put a few large holes at the bottom of the pipe, although if you are using it to hammer a smaller diameter thing like a wooden stake, the holes are not needed. A grab-handle on the pipe will make it easier to handle, and a team of two is helpful to operate the Vacuum Pile Driver, especially if you have made a big one. 

Once everything is in place with the hammer-piston sitting on the workpiece and the valve closed, ensure that everybody’s fingers are out of the way, switch on the vacuum and, with luck, the piston will slide up to the top. Open the valve and whoosh! goes the piston downwards and thumps into the workpiece.

How big an impact can you get? Well, energy is (mass x g x height). So if you’re dropping a 2kg piston-hammer 1m you’ll get 2x10x1=20J. Not that much, you might think, but if that hammer blow is moving the target for, say, 2 milliseconds, then the instantaneous power is 20/(2e-3) = 10kW. That’s a lot of power! And if the hammer stops in 4 milliseconds from 4ms-1, then that’s a deceleration of (v/t) = 1,000g. That’s a serious lot of gs!

What about automating the action of the Vacuum Pile Driver somehow? A projecting rod on the top of the hammer-piston could push the flap valve open, allowing the hammer-piston to descend automatically once it has risen to the top. And what about getting the hammer-piston to re-ascend automatically, allowing a complete repeating action? Could a string from the hammer-piston to the valve do this somehow? Or maybe something that delays the closing of the valve?

If you liked this, you will find lots more fun science stuff in Neil Downie’s books, like ‘The Ultimate Book of Saturday Science’ from Princeton University, and for lots of other things (and a free copy of the ‘Exploding Disk Cannons’ book), visit www.saturdayscience.org. In line with this experiment, Neil’s current work includes developing a new ventilator system to support people with breathing difficulties – get more information on this great project here: Exovent.org. 

There is a back catalogue of STEM-related challenges from the past year to choose from if you are looking for more options. The IET also has a host of resources that adults can use to engage children with the world of STEM.