Tech.view: Air superiority

By N.V. | LOS ANGELES

BEFORE computers became ubiquitous, anyone studying aeronautics had to spend long, torturous hours grappling with equations in a textbook by Ludwig Prandtl, an engineering professor at Göttingen University during the 1930s, whose theoretical insights laid the foundations for Germany's aerial supremacy in the years leading up to the second world war.

Today, few bother with such mind-numbing formulae. A branch of engineering called “computational fluid dynamics” has rendered such theoretical approaches obsolete. Instead, computers perform millions of calculations using algorithms and numerical methods to simulate the way the airflow behaves as it passes over a solid object. Coming from a bygone age, your correspondent has a well-thumbed copy of the classic German textbook on his shelf. He also has, courtesy of Professor Prandtl, an abiding respect for the compressible nature of air.

As well as aeroplanes, streamlined trains and slippery racing cars, anything from wind tunnels to jackhammers is stuff to be savoured. A special place in the pantheon of compressible air is reserved for the modern dentist's drill. Those old enough to have suffered the drilling nightmares caused by that articulated contraption from dentistry's dark ages, with its painful whirling of wire-belt drives, will know precisely why. Even closer to his heart, though, are those pneumatic hand tools that make DIY a pleasure instead of a chore.

For much of his adult life, your correspondent has wanted an air compressor for use at home. Unfortunately, he could neither afford one, nor could he find a place to house such a large, noisy, smelly, diesel-powered machine. Over the past couple of years, though, he's noticed that air compressors have become ever cheaper and more powerful, thanks largely to the industrious Chinese. Better still, being electrically powered instead of relying on an internal-combustion engine, they can be used indoors.

You can now buy a perfectly adequate three-horsepower compressor that runs off a 120-volt household electricity supply for less than $180 (anything beefier needs a special 240-volt supply, like a washing machine). A half-inch impact wrench capable of loosening the rustiest of wheel nuts can be had for $80. A nailing gun for timber framing costs about the same; a paint gun around half as much. Air tools for cutting and nibbling thin sheets of metal are $20 or so.

After years of relying on electricity to power his tools, your correspondent has finally taken the plunge and equipped his workshop with compressed air. What impresses him most is not just how cheap air tools are, but the staggering amount of torque they deliver compared with electrical tools. The little impact wrench he now uses was a third the price of the huge electric hammer-drill he bought several years ago to bolt his car hoist to the garage floor to prevent it slithering around during an earthquake. The air drill weighs a little over half as much as the hefty electric one, and yet delivers more than 20 times the torque.

An air tool's other great advantage is that it has no heavy electric motor, and fewer moving parts, inside it to lug around. As a result, air tools spin faster, do not overheat, shrug off abuse and are more reliable, as well as being lighter and easier to manhandle. There is a lot to be said for having one powerful electric motor back at the compressor turning electrical energy into potential energy stored in compressed air in a 10 gallon tank, instead of having a wimpy electric motor in each of the various hand tools used for drilling, cutting, nailing, shaping, loosening, sanding, grinding, polishing and painting.

The air tool with the nearest thing to a motor inside it is an impact wrench. This has a tiny turbine with vanes like a paddlewheel in the handle. Pressing the trigger causes high-pressure air to flow into the tool, spinning the turbine and converting some of the compressor's stored energy into rotational energy before venting to the atmosphere.

In pricier models, a small planetary (epicyclic) gear-set steps the speed of the rapidly spinning turbine down to 3,000-4,000 revolutions per minute, while raising the torque even further. A ratchet arrangement forces a spring-loaded “hammer” to bang repeatedly into an “anvil” on the tool's driveshaft. Like a conventional hammer—which generates a powerful, but short, impulse from a fairly weak, but long, swing of the arm—an impulse wrench concentrates many thousands of rotational blows a minute into freeing a rusted lug nut.

Air does this better than electricity because it endows the tool with a far greater power-to-weight ratio. And weight is a disadvantage in hand tools, not just because it makes the job tiring. With common hammer drills, for instance, the combined mass of the ratchet mechanism, the drill bit itself and the chuck that locks it in place is often comparable to the mass of the rest of the tool. That makes transferring energy from the motor to the tip of the drill very inefficient. As a result, the drill then has difficulty penetrating materials like concrete.

The answer is to use a lighter design all round, and to separate the hammering action from the drilling motion—as is the practice in air wrenches. The two functions can be separated in electric drills, but it makes them considerably more expensive than they would be otherwise.

Meanwhile, the universal electric motors used in power tools may be compact, able to run at high speed and good at coping with varying loads, but they have a built-in limitation. As the load on an electric power tool increases, the torque rises to handle the work, while the motor slows down as a consequence—and eventually stalls when the load gets excessive. Do that too often and the heat generated will burn the insulation around the electrical windings and destroy the motor.

The tiny turbine in an air drill, by contrast, has nothing to burn out—and simply goes on churning out a large dollops of torque, almost irrespective of the load exerted on it. Such is the charm of compressed air. Professor Prandtl, by all accounts the most impractical of men, would have been delighted, though hardly surprised.