Motion System Design

In the loop: From apples to banana peels

An apple falling from a tree led to the discovery of gravity. It happened around 1665, in the English countryside where Isaac Newton had gone to avoid the plague. The details are well documented. Not so in the case of the event that turned us on to the concept of friction.

My guess is that it was an accident — a slip, a wipeout, a case of the butterfingers; perhaps the very fall that brought our ancestors out of the trees. Whatever it was, it marked the beginning of a battle between mankind and one of the most dominant forces of nature that has raged ever since.

Friction, though not itself a force, is most readily observed in the force it produces. It’s always there, pushing back, resisting the relative motion (and the tendency for such motion) between any two bodies of mass in contact.

To anyone attempting to walk, catch a ball, or swing a bat, friction is an ally; but to every rotating shaft and linear slide, it’s an unforgiving enemy. If left unchallenged, it would halt every generating station, manufacturing plant, locomotive, and automobile on the planet. Indeed, the universe itself would run down if it contended with nothing other than this invisible power.

Over the years, the struggle against friction has given birth to many marvelous inventions. The wheel, bearings, rail travel, the use of lubricants, and magnetic levitation are some of the more notable examples. If not for the discovery of friction, we also wouldn’t know such modern conveniences as WD-40, Teflon, 3-IN-ONE oil, talc, cue chalk, stickum, pine tar, baseball cleats, and those banana colored signs that say “Caution Wet Floor.” Life just wouldn’t be the same if that fateful mishap hadn’t occurred.

But there’s another side to the story. In ancient times, when early civilizations were beginning to think expansionistically, friction (more or less) kept everyone in their place. By impeding mass movement, nature itself imposed practical limits on every effort to project power, reshape the environment, and subdue the Earth. In a sense, man had to overcome friction before he could conquer everything and everyone around him, and transportation technology became the weapon of choice.

Long before anyone heard of a Kenworth or Peterbilt, Egyptians were transporting huge stone slabs and other heavy loads (like battering rams) on the backs of wooden sleds. On one 4,000- year-old carving that depicts workers lugging a statue this way, you can clearly see someone pouring a liquid, presumably water, onto the path of the sliding rails; the first record of using a lubricant to reduce work.

Eventually, sleds dragged across the ground gave way to platforms rolled over logs; simple physics — rolling friction is less than sliding friction — but a not-sosimple implementation. An ancient lithograph from around 700 BC shows Assyrian slaves manually placing each log along the path of travel to form an endless series of rollers. Perhaps it was one of those log carriers who came up with the idea of wheeled transport.

Thanks to the wheel and some ingenious thinking, friction may be on the run today, but it is by no means a defeated foe. It takes energy to overcome energy, and therein lies the rub. Yes we can drive 70 mph down the interstate, but a good 20% of the engine’s power is spent overcoming its own friction as well as that of other components; over the course of a 100-mile trip nearly one gallon of gasoline must be burned to loosen friction’s grip.

Friction, as you know, also poses a barrier to productivity. Every time we try to raise manufacturing throughput we run into our old nemesis. If it’s not positioning accuracy and the resulting tolerance limits, it’s excessive wear and heat. If not that, it’s the cost of energy. One way or another, friction lets us know that it’s still a force to be reckoned with.

A bearing manufacturer once calculated the amount of energy we could save if only we could reduce the coefficient of friction in rolling bearings by 10%. It would be enough to provide half the power to four large countries — Australia, Brazil, Spain, and Sweden — or the equivalent of the combined output of 25 large nuclear reactors. In light of recent energy woes and how our ancestors must have stumbled onto the path of discovery, we ought to be tripping over ourselves to solve this slippery problem.

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