Gear is also a gear wheel, used in a gearbox
This aims to describe the main types of gear and their arrangement as elements within a gearbox.
Gear wheels & configurations
A gearbox is made up of a series of gearwheels mounted on shafts. Over the years, engineers have thought up many different types of gear wheel and many different arrangements to give a huge range of reduction ratios and other properties. Here are some of the more common.
Spur gear
This is the most basic type of gear wheel. It is the kind of gear wheel you might see in a toy or in a simple drawing. Basically it is a circular wheel, with teeth cut into it. It is a flat shape, extended in the third dimension to make a gear-shaped prism. The teeth are a special shape based on a curve called an involute, which ensures that the gear ratio does not change as the gears engage, transmit their power and then disengage.
The basic spur gear is just fine for delivering small amounts of power, such as in a wristwatch, but as the amount of power increases, the intermeshing teeth start to chatter and vibrate because as two of the gear teeth meet, the full power of the drive is transmitted suddenly to the tooth, with no gradual lead-in. Spur gears are relatively cheap to make, so they tend to be used on low-power drives, or drives where cost is more important than noise.
Helical gear and herring bone gears
One step up from the spur gear. Imagine that 2-dimensional shape of the gear wheel. If a spur gear is made by projecting that shape vertically upward, the helical gear is made in exactly the same way, except that the gear turns a little as it projects upward. So the gear teeth on a spur gear are parallel with the gear axis, but the teeth on a helical gear are not. They make an angle with the gear axis, called the helix angle.
When two matching helical gears engage, the power is gradually transmitted from tooth to tooth, and so there is much less chatter as the gears co-rotate.
The drawback to the helical gear is that the angled teeth introduce a thrust component to the forces as they are transmitted from one gear to the next, so either the designers have to use a thrust bearing to react the load. Alternatively, they put in a second pair of gear wheels with the opposite helix angle. These two gears are often mounted together, and the teeth form a herring bone pattern, which gives rise to the name: herring bone gears.
In both spur and helical gears, the shafts carrying the gears are normally parallel, so that the drive does not usually turn a corner. Although in some arrangements of helical gears, the shafts are not parallel.
Bevel gears
The bevel gear tooth takes the same involute form as a spur gear, but these gears are made in the shape of a cone—or rather a frustum, rather than a two-and-a-half dimensional prism. The bevel gear is designed to turn the drive through an angle, usually 90o. As in almost all gearing systems, there is usually a speed reduction as well whenever a set of bevel gears is used. Bevel gears usually come in pairs, so with one smaller than the other, and having a smaller cone angle. The larger gear has a wider cone angle, and the angles and sizes are chosen to give the correct ratios and angles.
This diagram courtesy of WonkoTheSane:
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Hypoid gears
these are used most often in an automotive differential, the arrangement at the back of the car which allows the wheels to turn a corner while still provide the driving effort. As the car turns a corner, the inside wheel has to go a shorter distance than the outside wheel, and that means the two wheels cannot be on the same axle, or there will be a lot of sliding and wear. The differential is a special type of gearbox used to transmit drive to the rear wheels, while at the same time allowing them to rotate relative to each other. The reason for using a hypoid gear arrangement is that the drive shaft does not have to be in the same horizontal plane as the line joining the two rear wheels. This arrangement means the drive shaft does not have to protrude into the passenger compartment quite so far as it would if an ordinary gearbox were used.
Worm and wheel
Also called a worm gear, this is one of the more unusual types of gear arrangement in that almost all other gears will work with the input shaft driving the output, or the other way around. The worm and wheel does not do that. The output shaft cannot (normally) be used to drive the input shaft. Also, the worm/wheel offers a very high gear ratio, delivering a very large speed reduction and a large torque multiple in a simple and compact arrangement.
The worm/wheel drive is a very simple idea. The input shaft is attached to a helical screw, called the worm. This engages with the wheel, one tooth in each pitch of the thread. As the input shaft turns, it the worm acts like a screw, forcing the wheel to move forward by one tooth pitch per revolution of the input shaft. The wheel might have 100 or so teeth, giving a gear ratio of 100:1. However, the worm/wheel is quite inefficient because of high friction losses, so the drive does not increase the torque by the full 100 times, but it is, nevertheless, a simple, cheap way of getting a very high drive torque.
Arrangements and designs
The various types of gear can be arranged into different types of
gearbox, for example: