Mechanisms: Gears

Even before the Industrial Revolution, gears of one kind or another have been put to work both for and against us. From ancient water wheels and windmills that ground grain and pounded flax, to the drive trains that power machines of war from siege engines to main battle tanks, gears have been essential parts of almost every mechanical device ever built. The next installment of our series on Mechanisms will take a brief look at gears and their applications.

As is often the case, evolution is the best inventor, and a geared mechanism linking the rear legs of juvenile planthopper insects predates the human invention of gears by a couple of billion years. Human use of gears dates back at least to third-century BC China, and the technology spread rapidly and widely. Within a few hundred years, precisely machined metal gears had enabled complex geared devices like the Antikythera mechanism to be built in Greece.

At its simplest, a gear is nothing more than a wheel with some sort of teeth cut into its circumference. The teeth are sized and shaped to mesh with teeth on other mechanical elements to transmit torque. Multiple gears connected in series are called a gear train, and if the diameters of gears in the gear train are different, the torque transmitted will be proportional to the difference. So, if the driving gear has a diameter of 1 cm and the driven gear is 10 cm across, the gear train will increase the torque 10-fold while reducing the rotational speed by a factor of 10.

To counter this, teeth can be cut at an angle to the axis of rotation. Skewing the teeth like this around the circumference of the gear results in a helical pattern, hence the name helical gear. Not only are helical gears quieter, they can also be crossed to transmit power at a right angle. The tradeoff is that because of the skewed teeth, helical gears impart thrust along their axes. The thrust can be dealt with using thrust bearings, like tapered roller bearings, or by using two helical gears with opposing teeth directions on the same shaft to cancel out the axial thrust. This results in the beautiful herringbone gear seen in many high-power applications like wind turbines.

For the longest time, producing metal gears was a complex process involving multiple machining steps to produce teeth with the desired geometry. Teeth can be cut by any number of machining operations, like broaching, milling, shaping, or grinding.

But gear cutting is time-consuming and expensive, so most gears these days are produced by some kind of molding operation. Plastic gears of the kind we hate to see when we look inside a power tool built to a price point are easily produced by injection molding, and despite their bad reputation, they can result in perfectly serviceable if not particularly long-lived gear trains. But metal gears can also be molded, with powdered metal gears now making a huge share of the market.

Powdered metal gears are produced by filling a mold with very fine metal alloy powder mixed with binders and lubricants. The powder in the mold is compressed by a hydraulic ram with a tool matching the shape of the mold, and the tremendous pressure fuses the metal particles together into a solid strong enough to be handled. The green parts are then heated to permanently fuse the particles into the final metal part which in many cases is ready to use with no further machining.

While powder metallurgy is out of reach for most home shops, DIY gears are very much doable by anyone with access to some basic machine tools. We’ll never get enough of watching [Chris] machine the gears and pinions of the Clickspring clock, and while those gears are highly specialized for the world of metrology, many of the same principles apply to gears for other applications. 3D-printing is making custom gear trains possible too, and the results can be surprisingly robust under the right conditions. And don't forget CNC routers, which are turning out gears large and small in all sorts of materials.

It's hard to even scratch the surface of what goes into the engineering behind gears — tooth geometry, pressure angles, lines of contact — nor can we cover the really interesting gears, like harmonic drives and epicyclic gears. But this is a start at least, and a taste of what you’re in for when you start adding gears to your builds. Open the floodgates of awesome gear projects in the comments!