What I Learned Today

In reading “The How and Why of Mechanical Movements,” I’ve gone from levers to linkages to wheels to pulleys to screws, plus a really fascinating discussion of friction and its use in machines. One of the cleverest devices I’ve seen so far is the differential pulley or differential hoist, used in auto repair shops and factories to allow humans to lift very heavy loads (like car engines).

The differential pulley is composed of two pulleys with different diameters mounted on the same axle. The rope (or chain, with toothed pulleys, to prevent slippage) is looped in opposite directions over the two pulleys, with the weight to be lifted suspend from one downward loop. If you pull on the other (unloaded) loop hanging down, relative to the larger-diameter pulley, then for each meter of rope/chain it releases, the smaller pulley must take up fractionally more rope/chain, yielding a net lift for the suspended weight. The diameter difference means that you had to pull more rope/chain than the distance the weight was lifted, yielding a mechanical advantage (i.e., you don’t have to pull as hard, you just have to do it for longer). This is difficult to explain in words, and much easier to grasp from a diagram; click the image at right to enlarge it. (I personally found the differential windlass, which was a precursor to the differential hoist, a useful aid in understanding the latter.) At first it seems counter-intuitive that the weight should hang suspended without pulling the second loop tight as well, until you observe that the key is that the two pulleys share the same axle and are wound in different directions. Thus the weight pulls equally on both sides of the axle and it does not turn, until an additional force (your hand) upsets that balance. (All drawings are from A First Course in Physics, by Robert Andrews Millikan and Henry Gordon Gale (Ginn and Company, 1906), as cited in Donald Simanek’s lecture on simple machines.)

If it wasn’t obvious, I’m really enjoying this book (and learning a lot). The current chapter just covered automobile clutches and brakes and several different kinds of bearings. The next chapter moves on to gears and transmissions (actual automobile transmissions, manual and automatic). Given its richness and re-read value, I’m likely to want to keep this one much longer than the library permits. Amazon to the rescue!