Jackhammers (pneumatic drills)
by Chris Woodford. Last updated: May 17, 2014.
Twenty thousand years ago, if you'd needed to dig a hole in rough ground, chances are you would have found yourself swinging a sharpened deer antler over your head. Modern pickaxes are based on pretty much the same idea. The long wooden handle and metal blade act like levers to magnify the force you generate with your back muscles and arms. It's simple technology, but it's very effective.
Today, if you want to dig a hole in a hurry and there's a thick lump of concrete or asphalt in your way, you're most likely to use a jackhammer, also known as a pneumatic (air-powered) drill, rock drill, or pavement breaker. A strong and skilled road worker can swing a pickaxe 10 times a minute or more, but a jackhammer can pound the ground 150 times faster—that's 1500 times a minute! Pretty amazing, but how exactly does it work?
Photo: Left: A typical pneumatic jackhammer drill being used in a road repair project. Picture by John P. Curtis courtesy of US Navy. Right: Jackhammers aren't just used for construction: since they offer the fastest way of breaking through concrete and stone, they're often vital tools in emergency rescue work. Here, a worker from the US Naval Air Station Sigonella Fire and Rescue Team is using a pneumatic jack hammer to smash through concrete during a training exercise. Picture by Gary A. Prill courtesy of US Navy.
Pneumatics is the power of compressed air
You've probably never handled a jackhammer, but you use exactly the same technology every time you ride on a bicycle or travel by car. The rubber tires that carry you smoothly down the road are inflated with air, so the force of your weight pushing down is exactly balanced by the pressure of the air pushing you upward. Tires are an example of pneumatic technology, which means they use the force of air pressure. (You may have heard of a similar technology called hydraulics that uses the force of liquid pressure.)
You can't see air, but it's a surprising thing. It's a mixture of gases, mostly nitrogen and oxygen, with its molecules constantly zooming back and forth like angry bees. When air is trapped in a container, such as a bicycle tire, molecules of gas are repeatedly crashing into the rubber walls and bouncing back again. Each time one of these collisions happens, the molecules give a tiny push to the rubber. With millions of collisions happening all the time, the air exerts quite a pressure (defined as the force acting per unit of area) on the rubber—and that's what keeps the tire inflated. (The hotter the air is, the faster the gas molecules move, the more energetically they collide, and the more pressure they exert. That's why tires inflate more on hot days and after a long car journey.)
You might have seen pneumatics in action elsewhere. Blowpipes are another good example. When those angry savages from your comic books blow poisoned darts at their enemies, they're using air pressure to force a missile down a tube at high speed. In olden days, big department stores used pneumatic transport tubes to send money or messages rapidly from one floor to another.
Steam engines use pneumatics too; instead of air, they use high-temperature, high-pressure water vapor (steam) to push pistons back and forth and turn wheels at high speed. Vacuum cleaners, which use suction to remove dirt from soft furnishings, use the same principle in reverse—sucking air in rather than blowing it out.
Left: A construction worker using a pneumatic drill. Note the red compressed air hose coming out of the left-hand side of the drill, which is supplied by the large green portable air compressor (marked Sullair), hooked up to the pickup truck on the left of the photo. Picture by Renae Kleckner courtesy of US Air Force.
Inside a jackhammer
Back to jackhammers. The first time you saw someone digging a hole in the road with a tool like this, you probably thought the equipment was electric or powered by a diesel engine, right? In fact, the only energy involved in making a jackhammer pound up and down is supplied from an air hose. The hose, which has to be made of especially thick plastic, carries high-pressure air (typically 10 times higher pressure than the air around us) from a separate air-compressor unit powered by a diesel engine.
The air compressor is a bit like a giant bicycle pump that never stops blowing air. When the worker presses down on the handle, air pumps from the compressor into the jackhammer through a valve on one side. Inside the hammer, there's a circuit of air tubes, a heavy piledriver, and a drill bit at the bottom. First, the high-pressure air flows one way round the circuit, forcing the piledriver down so it pounds into the drill bit, smashing it into the ground. A valve inside the tube network then flips over, causing the air to circulate in the opposite direction. Now the piledriver moves back upward, so the drill bit relaxes from the ground. A short time later, the valve flips over again and the whole process repeats. The upshot is that the piledriver smashes down on the drill bit over 25 times each second, so the drill pounds up and down in the ground around 1500 times a minute.
Artwork: This little animation shows what happens inside a drill. Note how the blue valve at the top flips back and forth so the air changes direction. This makes the orange piledriver pound up and down, bashing the gray drill bit repeatedly into the ground. Note that this is a considerable simplification of what happens in a real drill, where the arrangement of valves, air passages, and so on is much more complex. You can get a sense of how much more complex real drills are from the illustration of Charles Brady King's original drill design, below.
Jackhammers, and the air compressors that power them, come in all different shapes and sizes. The drill bits on the end are interchangeable too. There are wide chisels, narrow chisels, and tools called moil points for fine work. A skilled drill operator can loosen chunks of road in just 10-20 seconds, making light work of what our ancestors—with their antler picks—would have found truly backbreaking work!