Gears are used to transmit power between shafts rotating usually at different speeds. Gears do several important jobs, but most important, they provide a gear reduction in motorized equipment. This is key because, often, a small motor spinning very fast can provide enough power for a device, but not enough torque. With a gear reduction, the output speed can be reduced while the torque is increased.

Another thing gears do is adjust the direction of rotation. For instance, the differential of car has to turn power 90 degrees to apply it to the wheels.

The ratio is determiced by the distances from the center of the gear to the point of contact. For instance, in a device with two gears, if one gear is twice the size of the other, the ratio would be 2:1.

The problem with this type of gear is that the distance from the center of each gear to the point of contace changes as the gears rotate. This means that the gear ratio changes as the gear turns, meaning that the output speed also changes. If you used a gear like this in your car, it would be impossible to maintain a constant speed-you would be accelerating and decelerating constantly.

Many modern gears use a special tooth profile called an involute. This profile has the very important property of maintaining a constant speed ratio between the two gears.

Some of the many types of gears are illustrated below:

Spur Gears

A pair of spur gears for mounting on parallel shafts are the most common type of gears, which have straight teeth. Sometimes, many spur gears are used at once to create very large gear reductions.

The spur gear can be really loud. Each time a gear tooth engages a tooth on the other gear, the teeth collide, and this impact makes a noise. It also increases the stress on the gear teeth.

To reduce the noise and stress in the gears, most of the gears in your car are helical.

Helical Gears

Like spur gears helical gears connect parallel shafts, however the teeth are not parallel to the shaft axes but lie along helices about the axes.

When two teeth on a helical gear system engage, the contact starts at one end of the tooth and gradually spreads as the gears rotate, until the two teeth are in full engagement.

This gradual engagement makes helical gears operate much more smoothly and quietly than spur gears. For this reason, helical gears are used in almost all car transmissions.

Because of the angle of the teeth on helical gears, they create a thrust load on the gear when they mesh. Devices that use helical gears have bearings that can support this thrust load.

Bevel Geras

Bevel gears are useful when the direction of a shaft’s rotation needs to be changed. They are usually mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well.

The teeth on bevel gears can be straight, spiral or hypoid. Straight bevel gear teeth actually have the same problem as straight spur gear teeth-as each tooth engages, it impacts the corresponding tooth all at once.

Just like with spur gears, the solution to this problem is to curve the gear teeth. These spiral teeth engage just like helical teeth: the contact starts at one end of the gear and progressively spreads across the whole tooth.

Worm Gears

A worm and worm wheel gives a large speed ratio but with significant sliding. It is common for worm gears to have reductions of 20:1, and even up to 300:1 or greater.

Many worm gears have an interesting property that no other gear set has: the worm can easily turn the gear, but the gear cannot turn the worm. This is because the angle on the worm is so shallow that when the gear tries to spin it, the friction between the gear and the worm holds the worm in place.

A single pair may be inadequate for certain sources and loads, in which case more complex combinations such as the gearbox, known as gear trains,are necessary.

参考译文：

齿轮用于两个不同转速的轴之间的动力传递。齿轮有很多作用，但最重要的作用是动力设备的减速。这主要因为，通常，当小小的电机高速运转时，虽然能提供足够的功率，但不能提供足够的力矩，如果使用齿轮减速，就可以降低输出速度，同时提高力矩。

齿轮的另一个作用是改变旋转方向。例如，汽车的差速器要将动力转90°传给车轮。

传动比是由两轮的中心到啮合点的距离所决定的。例如，两个齿轮的传动机构中，如果一齿轮的尺寸是另一个的齿轮的两倍，那么传动比就是2：1 。

这种齿轮传动的缺陷在于，每个齿轮的中心到啮合点的距离在运转的过程中是变化的。也就是说，当齿轮转动时，齿轮的传动比是变化的，就意味着输出速度也会变化。假如你的车内使用这种齿轮，那么你就不能保持稳定的速度——你就一直在加速或减速。

很多现代齿轮使用一种被称为渐开线的齿廓。这种齿廓的齿轮具有特别重要的性质，就是保持两齿轮间稳定的传动比。

下面介绍其中的几种齿轮：

直齿轮

安装在两根平行轴上的直齿轮是最常见的齿轮，这种齿轮带直齿，有时一次用多级齿轮传动可以获得较大的降速比。

直齿轮运行时噪声较大。每次一齿轮和另一齿啮合时，齿间就发生冲击，从而产生噪音，同时也增加了吃面上的压应力。

为了降低噪音，减小应力，我们的轿车多使用斜齿轮。

斜齿轮

和直齿轮一样，斜齿轮也安装在两根平行的轴上，但是，斜齿轮上的齿与轴线并不是平行的，而是成一角度的。当两齿啮合时，齿面由一个头开始啮合，并在运转时逐步扩大接触面，直到全齿完全啮合。

逐步啮合的过程使斜齿轮在运转时，比直齿轮更平缓、噪音更低。正因为如此，几乎所有的轿车的传动系统都用斜齿轮。

因为斜齿轮的齿有两个角度，所以，其啮合时就会产生轴向推力，所以采用斜齿轮的机构均使用能承受轴向载荷的轴承。

锥齿轮

当两传动轴旋转时需要改变传动方向，采用锥齿轮是一种十分有用的方法。通常，锥齿轮的两轴呈90°，但也可以设计为其他角度。

锥齿轮的齿可以使直齿，螺旋形或者准双曲面。直齿锥齿轮和直齿轮同样存在同样的问题——齿的啮合过程中始终存在冲击。

就像直齿轮一样，这个问题的解决也是将轮齿变成曲线。螺旋锥齿轮的啮合和斜齿轮是一样的，齿面由一个断面进入啮合，逐步扩大接触面，知道全齿完全啮合。

涡轮蜗杆

涡轮和蜗杆可以获得较大的降速比，但是存在严重的相对滑动。通常的涡轮蜗杆的降速比可达到30：1，甚至达到300：1或更大。

很多涡轮蜗杆具有其他齿轮传动没有的一种有趣的特质，蜗杆能很容易的带动涡轮，但是涡轮却带不动蜗杆。这是因为涡轮蜗杆间的升程角很小，当涡轮要带动蜗杆时，涡轮蜗杆间的摩擦力就把蜗杆锁住了。

对于各种负载和动力，仅仅靠一对齿轮是不够的，如在变速箱中采用多种齿轮的复杂组合。