How Planetary Gears Work行星齿轮是怎么工作的

Three gears nested inside each other can slow things down, speed them up, run them backwards, or blend two motors into one. It is why a Toyota Prius has no gearbox. Drag any model to look around it.

三个套在一起的齿轮，可以减速、增速、反转，还能把两台电机的转速“调和”在一起。丰田普锐斯没有传统变速箱，靠的就是它。每个模型都可以拖动，换个角度看。

Two gears先看两个齿轮

When two gears mesh, their teeth pass the contact point at the same rate. A gear with twice as many teeth therefore turns half as fast. And because the teeth push each other sideways, the two gears always turn in opposite directions.

两个齿轮啮合时，两边的齿以同样的节奏经过接触点。所以齿数多一倍的齿轮，转速就慢一半。又因为齿是互相侧推的，两个齿轮永远朝相反方向转。

drag to rotate拖动旋转
The orange gear has 18 teeth and turns at a steady speed. Change the number of teeth on the blue gear and watch its speed change.
橙色齿轮 18 个齿，匀速转动。改变蓝色齿轮的齿数，看它的转速怎么变。

Gears inside a gear齿轮里的齿轮

A planetary gear set puts a sun gear in the middle, several planet gears around it, and a ring gear with teeth on the inside that wraps around them all. The planets are held on a frame called the carrier, which can turn too.

行星齿轮组的中间是太阳轮，周围一圈是几个行星轮，最外面是一个齿长在内侧的齿圈，把它们整个包住。行星轮装在一个叫行星架的框架上，这个框架本身也能转。

drag to rotate拖动旋转
Pull the set apart to see how the four parts stack. Here the ring is held still and the sun drives, so the planets roll around inside the ring and drag the carrier along.
把它拆开，看四个部件是怎么叠在一起的。这里齿圈固定、太阳轮驱动，于是行星轮贴着齿圈内侧滚动，带着行星架一起转。

Every tooth in this model is placed by the same rule as a real gear set, so what you see is what would happen: the sun has 18 teeth, each planet 15, and the ring 48. Those numbers are not arbitrary. The ring has to equal the sun plus two planets (18 + 2×15 = 48), or the planets would not fit.

这个模型里每一个齿的位置都按真实齿轮组的规则摆放，所以你看到的运动就是真实会发生的运动：太阳轮 18 齿，每个行星轮 15 齿，齿圈 48 齿。这些数字不是随便取的：齿圈齿数必须等于太阳轮加上两个行星轮（18 + 2×15 = 48），否则行星轮塞不进去。

Hold one, drive another固定一个，驱动另一个

Here is where it gets clever. The set has three parts that can connect to the outside world: sun, carrier and ring. Hold any one of them still, turn another, and the third becomes the output. Each choice gives a different ratio.

巧妙的地方来了。这套齿轮有三个部件可以和外界相连：太阳轮、行星架、齿圈。固定其中任意一个，转动另一个，第三个就是输出。每一种选法都得到不同的传动比。

drag to rotate拖动旋转
Hold固定
Drive驱动
The held part turns grey. Try all six combinations: you'll find two ways to slow down, two ways to speed up, and two ways to run backwards.
被固定的部件会变成灰色。把六种组合都试一遍：你会找到两种减速、两种增速、两种反转。

An automatic transmission is essentially a few of these sets in a row, with clutches and brakes that grab different parts. Shifting gear means changing which part is held.

汽车的自动变速箱，本质上就是几组这样的行星齿轮串在一起，再用离合器和制动器去“抓住”不同的部件。所谓换挡，就是换一个被固定的部件。

One straight line一条直线管住全部

You don't have to hold anything. All three parts can turn at once, and their speeds always obey one rule: the carrier's speed is a weighted average of the sun's and the ring's, with the tooth counts as weights. Drawn as three vertical bars spaced by those tooth counts, the three speeds always sit on one straight line.

其实不必固定任何部件。三个部件可以同时转，而它们的转速永远服从一条规则：行星架的转速，是太阳轮和齿圈转速的加权平均，权重就是它们的齿数。把三个转速画成三根竖杆，杆的间距按齿数排，三个点永远落在同一条直线上。

drag to rotate拖动旋转
Drag any of the three dots up or down to set a speed (above the middle line is forward, below is backward). The line pivots, and the gears follow.
上下拖动三个圆点中的任意一个来设定转速（中线以上是正转，以下是反转）。直线会绕着转动，上面的齿轮跟着变。

This picture is called a lever diagram, and engineers use it to design transmissions. It also explains the trick in the next section.

这张图叫杠杆图，工程师设计变速箱时就用它。下一节的“把戏”，也要靠它来看懂。

The Prius trick普锐斯的把戏

A petrol engine is most efficient in a narrow range of speeds, but the wheels need every speed from standstill to motorway. A normal car bridges the gap with a gearbox. The Prius uses a single planetary set instead: the engine turns the carrier, the wheels are connected to the ring, and a motor-generator sits on the sun.

汽油发动机只在很窄的转速区间里最省油，可车轮需要从静止到高速的所有转速。普通汽车用变速箱来弥合这个矛盾。普锐斯只用了一组行星齿轮：发动机带动行星架，车轮连在齿圈上，一台电动发电机装在太阳轮上。

drag to rotate拖动旋转
Set the car speed and the engine speed independently. The generator on the sun takes up the difference, whatever it is. Try "Electric only" and look at the generator.
车速和发动机转速可以各自独立设定，两者之间的差额，全部由太阳轮上的发电机吸收。点“纯电行驶”，看看发电机在干什么。

That is the whole idea. Because the sun can absorb any speed difference, the engine can stay near its most efficient speed while the car speeds up or slows down, and the generator's electricity helps drive the wheels through a second motor. There are no gears to shift. With the engine off, the generator simply spins backwards. Past a certain car speed it would spin too fast, which is one reason the engine has to join in.

这就是全部的秘密。太阳轮能吸收任何转速差，所以车子加速减速时，发动机可以一直待在最省油的转速附近；发电机发出的电，再通过另一台电机帮着驱动车轮。整个过程没有挡位可换。发动机熄火时，发电机只是在反转；车速超过某个值，它会转得太快，这也是发动机必须介入的原因之一。

Everywhere once you look一旦留意，到处都是

Planetary gears pack a big reduction into a small, balanced package, with the load shared across several planets. You'll find them in bicycle hub gears, electric screwdrivers, wind turbines, robot joints and the wheel hubs of excavators, all built on the same straight line you dragged above.

行星齿轮能把很大的减速比塞进一个小巧、对称的结构里，负载还分摊在好几个行星轮上。自行车内变速花鼓、电动螺丝刀、风力发电机、机器人关节、挖掘机的轮毂里都有它，全都遵循你刚才拖动的那条直线。