Summer STEM Challenge: A Vacuum Version of The Armstrong Crane

Helen of Troy’s beauty led to the Trojan wars in ancient Greece myth. Hers was the ‘face that launched a thousand ships’ that sailed to Troy. But the Armstrong Crane was no myth: it was this crane that allowed William Armstrong to build a company that would become a Colossus of the Victorian age. It supplied engines, bridges, guns and thousands of ships to the entire world.

What was this crane that launched a thousand ships? It used pulleys and strong cables to haul up a load, pulled by a piston propelled by high-pressure water. At its simplest, an Armstrong crane was a short quayside tower, with the hydraulically powered hook on its jib lifting heavy loads into ships. Here’s how to make a version of that crane.

Using air pressure from a vacuum cleaner rather than water pressure makes the project easier. A piston is pushed by atmospheric pressure on one side and lower pressure on the other. The tower and jib are pieces of wood. The cylinder is a length of large diameter (110mm) plastic pipe, with a soda bottle as the piston. Antique Armstrong cranes often have a horizontal cylinder at the bottom of the tower, but if you use a vertical cylinder then this can combine the tower and cylinder functions.

You can simply fix a string to a piston, taking it around the pulleys to carry it from cylinder to jib and down to the hook. However, the piston can carry a pulley that pulls on a loop of string, so if one end of the string is fixed, the other end will move twice as far as the pulley. The original Armstrong Cranes often used more pulleys to get an even bigger ‘gearing up’ effect. 

Everything built? Testing time! Apply vacuum and the piston will suck down, pulling the hook up. Remove the vacuum and the hook will return to the ground, but only if it’s heavy enough - a little more than 2x the piston weight - so make sure your hook is heavy.

In theory, with -150mBar vacuum on the area of the 110mm, then you have a force = pressure x area (divided by 2 because of the pulley gearing up) of 70N or 7kg. How much can your crane really lift?

If the vacuum cleaner has speed control, you can control the pull on the load electrically or you can wire the cleaner to a standalone TRIAC controller. More interesting, perhaps, is to make your own control valve. A simple valve can be made by bending a polypropylene (PP) pipe. As the bend exceeds a certain angle, it forms a ‘crimp’ across the pipe, stopping airflow. This kind of valve design works best if the bend may not straighten.

The top of the crane tower can be a plastic pipe coupler (minus its rubber seal to allow rotation) with the jib attached. Optionally, you can wrap strings around the coupler, using it as a ‘capstan’ to rotate the jib. The two pulleys on the top of the pipe mean you can pull down on either side to rotate the jib.

Finally, what about using a vacuum ‘sucker’ to pick up the loads rather than a hook? It’ll need a small subsidiary hose with its own flow valve.