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How Automatic Transmissions Workby Karim NiceIntroduction to How Automatic Transmissions WorkIf you have ever driven a car with an automatic transmission, then you know that there are two big differences between an automatic transmission and a manual transmission: There is no clutch pedal in an automatic transmission car. There is no gear shift in an automatic transmission car. Once you put the transmission into drive, everything else is automatic. Both the automatic transmission (plus its torque converter) and a manual transmission (with its clutch) accomplish exactly the same thing, but they do it in totally different ways. It turns out that the way an automatic transmission does it is absolutely amazing!Automatic Transmission Image GalleryIn this article, well work our way through an automatic transmission. Well start with the key to the whole system: planetary gearsets. Then well see how the transmission is put together, learn how the controls work and discuss some of the intricacies involved in controlling a transmission. Just like that of a manual transmission, the automatic transmissions primary job is to allow the engine to operate in its narrow range of speeds while providing a wide range of output speeds.Photo courtesy DaimlerChryslerMercedes-Benz CLK, automatic transmission, cut-away modelWithout a transmission, cars would be limited to one gear ratio, and that ratio would have to be selected to allow the car to travel at the desired top speed. If you wanted a top speed of 80 mph, then the gear ratio would be similar to third gear in most manual transmission cars. Youve probably never tried driving a manual transmission car using only third gear. If you did, youd quickly find out that you had almost no acceleration when starting out, and at high speeds, the engine would be screaming along near the red-line. A car like this would wear out very quickly and would be nearly undriveable.So the transmission uses gears to make more effective use of the engines torque, and to keep the engine operating at an appropriate speed. The key difference between a manual and an automatic transmission is that the manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios, while in an automatic transmission, the same set of gears produces all of the different gear ratios. The planetary gearset is the device that makes this possible in an automatic transmission. Lets take a look at how the planetary gearset works. Planetary Gearsets & Gear RatiosWhen you take apart and look inside an automatic transmission, you find a huge An ingenious planetary gearset A set of bands to lock parts of a gearset A set of three wet-plate clutches to lock other parts of the gearset An incredibly odd hydraulic system that controls the clutches and bands A large gear pump to move transmission fluid around The center of attention is the planetary gearset. About the size of a cantaloupe, this one part creates all of the different gear ratios that the transmission can produce. Everything else in the transmission is there to help the planetary gearset do its thing. This amazing piece of gearing has appeared on HowStuffWorks before. You may recognize it from the electric screwdriver article. An automatic transmission contains two complete planetary gearsets folded together into one component. See How Gear Ratios Work for an introduction to planetary gearsets. From left to right: the ring gear, planet carrier, and two sun gearsAny planetary gearset has three main components: The sun gear The planet gears and the planet gears carrier The ring gear Each of these three components can be the input, the output or can be held stationary. Choosing which piece plays which role determines the gear ratio for the gearset. Lets take a look at a single planetary gearset. One of the planetary gearsets from our transmission has a ring gear with 72 teeth and a sun gear with 30 teeth. We can get lots of different gear ratios out of this gearset.InputOutputStationaryCalculationGear RatioASun (S)Planet Carrier (C)Ring (R)1 + R/S3.4:1BPlanet Carrier (C)Ring (R)Sun (S)1 / (1 + S/R)0.71:1CSun (S)Ring (R)Planet Carrier (C)-R/S-2.4:1Also, locking any two of the three components together will lock up the whole device at a 1:1 gear reduction. Notice that the first gear ratio listed above is a reduction - the output speed is slower than the input speed. The second is an overdrive - the output speed is faster than the input speed. The last is a reduction again, but the output direction is reversed. There are several other ratios that can be gotten out of this planetary gear set, but these are the ones that are relevant to our automatic transmission. You can try these out in the animation below:So this one set of gears can produce all of these different gear ratios without having to engage or disengage any other gears. With two of these gearsets in a row, we can get the four forward gears and one reverse gear our transmission needs. Well put the two sets of gears together in the next section. Compound Planetary GearsetThis automatic transmission uses a set of gears, called a compound planetary gearset, that looks like a single planetary gearset but actually behaves like two planetary gearsets combined. It has one ring gear that is always the output of the transmission, but it has two sun gears and two sets of planets. Lets look at some of the parts: How the gears in the transmission are put togetherLeft to right: the ring gear, planet carrier, and two sun gearsThe figure below shows the planets in the planet carrier. Notice how the planet on the right sits lower than the planet on the left. The planet on the right does not engage the ring gear - it engages the other planet. Only the planet on the left engages the ring gear. Planet carrier: Note the two sets of planets.Next you can see the inside of the planet carrier. The shorter gears are engaged only by the smaller sun gear. The longer planets are engaged by the bigger sun gear and by the smaller planets. Inside the planet carrier: Note the two sets of planets.Automatic Transmission GearsFirst GearIn first gear, the smaller sun gear is driven clockwise by the turbine in the torque converter. The planet carrier tries to spin counterclockwise, but is held still by the one-way clutch (which only allows rotation in the clockwise direction) and the ring gear turns the output. The small gear has 30 teeth and the ring gear has 72, so the gear ratio is: Ratio = -R/S = - 72/30 = -2.4:1So the rotation is negative 2.4:1, which means that the output direction would be opposite the input direction. But the output direction is really the same as the input direction - this is where the trick with the two sets of planets comes in. The first set of planets engages the second set, and the second set turns the ring gear; this combination reverses the direction. You can see that this would also cause the bigger sun gear to spin; but because that clutch is released, the bigger sun gear is free to spin in the opposite direction of the turbine (counterclockwise). Second Gear This transmission does something really neat in order to get the ratio needed for second gear. It acts like two planetary gearsets connected to each other with a common planet carrier. The first stage of the planet carrier actually uses the larger sun gear as the ring gear. So the first stage consists of the sun (the smaller sun gear), the planet carrier, and the ring (the larger sun gear). The input is the small sun gear; the ring gear (large sun gear) is held stationary by the band, and the output is the planet carrier. For this stage, with the sun as input, planet carrier as output, and the ring gear fixed, the formula is: 1 + R/S = 1 + 36/30 = 2.2:1The planet carrier turns 2.2 times for each rotation of the small sun gear. At the second stage, the planet carrier acts as the input for the second planetary gear set, the larger sun gear (which is held stationary) acts as the sun, and the ring gear acts as the output, so the gear ratio is: 1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1To get the overall reduction for second gear, we multiply the first stage by the second, 2.2 x 0.67, to get a 1.47:1 reduction. This may sound wacky, but it works. Third GearMost automatic transmissions have a 1:1 ratio in third gear. Youll remember from the previous section that all we have to do to get a 1:1 output is lock together any two of the three parts of the planetary gear. With the arrangement in this gearset it is even easier - all we have to do is engage the clutches that lock each of the sun gears to the turbine.If both sun gears turn in the same direction, the planet gears lockup because they can only spin in opposite directions. This locks the ring gear to the planets and causes everything to spin as a unit, producing a 1:1 ratio. OverdriveBy definition, an overdrive has a faster output speed than input speed. Its a speed increase - the opposite of a reduction. In this transmission, engaging the overdrive accomplishes two things at once. If you read How Torque Converters Work, you learned about lockup torque converters. In order to improve efficiency, some cars have a mechanism that locks up the torque converter so that the output of the engine goes straight to the transmission. In this transmission, when overdrive is engaged, a shaft that is attached to the housing of the torque converter (which is bolted to the flywheel of the engine) is connected by clutch to the planet carrier. The small sun gear freewheels, and the larger sun gear is held by the overdrive band. Nothing is connected to the turbine; the only input comes from the converter housing. Lets go back to our chart again, this time with the planet carrier for input, the sun gear fixed and the ring gear for output. Ratio = 1 / (1 + S/R) = 1 / ( 1 + 36/72) = 0.67:1So the output spins once for every two-thirds of a rotation of the engine. If the engine is turning at 2000 rotations per minute (RPM), the output speed is 3000 RPM. This allows cars to drive at freeway speed while the engine speed stays nice and slow. ReverseReverse is very similar to first gear, except that instead of the small sun gear being driven by the torque converter turbine, the bigger sun gear is driven, and the small one freewheels in the opposite direction. The planet carrier is held by the reverse band to the housing. So, according to our equations from the last page, we have: Ratio = -R/S = 72/36 = 2.0:1So the ratio in reverse is a little less than first gear in this transmission. Gear RatiosThis transmission has four forward gears and one reverse gear. Lets summarize the gear ratios, inputs and outputs:GearInputOutputFixedGear Ratio1st30-tooth sun72-tooth ringPlanet carrier2.4:12nd30-tooth sunPlanet carrier36-tooth ring2.2:1Planet carrier72-tooth ring36-tooth sun0.67:1Total 2nd1.47:13rd30- and 36-tooth suns72-tooth ring1.0:1ODPlanet carrier72-tooth ring36-tooth sun0.67:1Reverse36-tooth sun72-tooth ringPlanet carrier-2.0:1After reading these sections, you are probably wondering how the different inputs get connected and disconnected. This is done by a series of clutches and bands inside the transmission. In the next section, well see how these work. Clutches and Bands in an Automatic TransmissionIn the last section, we discussed how each of the gear ratios is created by the transmission. For instance, when we discussed overdrive, we said: In this transmission, when overdrive is engaged, a shaft that is attached to the housing of the torque converter (which is bolted to the flywheel of the engine) is connected by clutch to the planet carrier. The small sun gear freewheels, and the larger sun gear is held by the overdrive band. Nothing is connected to the turbine; the only input comes from the converter housing. To get the transmission into overdrive, lots of things have to be connected and disconnected by clutches and bands. The planet carrier gets connected to the torque converter housing by a clutch. The small sun gets disconnected from the turbine by a clutch so that it can freewheel. The big sun gear is held to the housing by a band so that it could not rotate. Each gear shift triggers a series of events like these, with different clutches and bands engaging and disengaging. Lets take a look at a band. BandsIn this transmission there are two bands. The bands in a transmission are, literally, steel bands that wrap around sections of the gear train and connect to the housing. They are actuated by hydraulic cylinders inside the case of the transmission.One of the bandsIn the figure above, you can see one of the bands in the housing of the transmission. The gear train is removed. The metal rod is connected to the piston, which actuates the band. The pistons that actuate the bands are visible here.Above you can see the two pistons that actuate the bands. Hydraulic pressure, routed into the cylinder by a set of valves, causes the pistons to push on the bands, locking that part of the gear train to the housing. The clutches in the transmission are a little more complex. In this transmission there are four clutches. Each clutch is actuated by pressurized hydraulic fluid that enters a piston inside the clutch. Springs make sure that the clutch releases when the pressure is reduced. Below you can see the piston and the clutch drum. Notice the rubber seal on the piston - this is one of the components that is replaced when your transmission gets rebuilt. One of the clutches in a transmissionThe next figure shows the alternating layers of clutch friction material and steel plates. The friction material is splined on the inside, where it locks to one of the gears. The steel plate is splined on the outside, where it locks to the clutch housing. These clutch plates are also replaced when the transmission is rebuilt. The clutch platesThe pressure for the clutches is fed through passageways in the shafts. The hydraulic system controls which clutches and bands are energized at any given moment. FROM: /automatic-transmi自动变速箱的工作原理耐斯卡瑞姆介绍自动变速箱的工作原理如果你有一个自动变速器的汽车，那么你肯定会知道自动变速器和手动变速器之间有2个很大的区别： 一个自动变速器没有离合器踏板。 自动变速器中没有齿轮换挡。一旦你把变速器放在汽车上传递动力，其他一切都是自动的。自动变速器（加上它的液力变矩器）和手动变速器（其离合器）完成同样的事情，但它们以完全不同的方式来完成它。它证明了自动传输的方式是绝对惊人的！在这篇文章中，我们将通过自动变速器来工作。我们从整个系统的关键来看-行星齿轮组。然后我们将看看自动变速器是如何组装的，学习如何控制工作和讨论一些参与控制传输的复杂性。就像一个手动变速器，自动变速器的主要任务是让发动机在其窄的速度范围内工作，同时提供了较大变速范围的输出速度。Mercedes-奔驰 CLK, 自动变速器,剖面立体图没有传动装置，汽车将被限制在一个传动比，并且该比例将被选择允许汽车行驶在所需的最高速度。如果你想要一个80英里的最高时速，那么在大多数的手动变速器中，齿轮的比例将与第三个档位相似。 你可能从来没有尝试过使用第三档驾驶一辆手动变速的汽车。如果你做了，你会很快发现，你几乎没有加速启动时，而是直接以很高的速度启动，引擎是工作在警戒线附近的。一辆这样的车会磨损得很快，几乎是无法驾驶。 因此，变速器利用齿轮使发动机的扭矩更有效地利用，并以适当的速度保持发动机运转。手动变速器和自动变速器之间的主要区别是，手动变速器锁止或打开不同的齿轮，使输出轴实现各种齿轮比，而自动变速器中，齿轮一样产生所有不同的传动比。行星齿轮组是装置在自动变速器上。让我们看一看行星齿轮系是如何工作的。行星齿轮系和齿轮传动比 当你拆开看自动变速器里面，可以发现，在一个相对狭小的空间里布满了很多零件。在这些零件中有： 一个精巧的行星齿轮系 一套锁止齿轮的钢带 三个锁住另外一些齿轮的湿式片式离合器 控制离合器和钢带的复杂液压系统 为变速器提供液压的齿轮泵 关注的中心是行星齿轮系。变速器所产生的所有不同的传动比决定于齿轮系尺寸的大小。变速器里其他的零部件是协助行星轮系产生不同传动比的。这件装置已出现在材料的工作方式。你可以从电动丝刀的文章中找到。自动变速器包含两个完整的行星齿轮组合为一个组件。其介绍可参见为齿轮系工作方式。从左到右：内齿圈，行星架，和两个太阳轮 任何行星齿轮组有三种主要的组件： 太阳齿轮 行星齿轮和行星齿轮架 齿圈这三个组成中的任何一组成都可以作为输入或输出，或者被固定。选择哪一条中所起作用决定的齿轮组的速度比。让我们来看看在一个行星齿轮组。 一个从我们的行星齿轮装置，内齿圈齿数为72，太阳轮齿数为30。我们可以得到许多不同的传动比的齿轮组。输入输出固定计算齿轮速比A太阳轮(S)行星架(C)内齿圈(R)1 + R/S3.4:1b行星架(C)内齿圈(R)太阳轮(S)1/(1 + S/R)0.71:1C太阳轮(S)内齿圈(R)行星架(C)- R/S-2.4:1 同时，锁定任何三个组件连接在一起，将锁定整个装置在1:1时速比。注意上面列出的第一个速比为减速-输出速度比输入速度慢。二是超速的输出速度比输入速度快。最后一种方案也是减速，但输出速度的方向相反。这个行星齿轮系还可以得到其他很多的速比，而刚才的这些速比仅和我们的自动变速器有关。因此这一套行星齿轮系可以产生所有需要的不同的齿轮传动比，却不需要接合或不接合其他齿轮。把两套行星齿轮系置于一行，我们可以得到四个前进档和一个后退档。下一部分内容我们将这两个齿轮系组装在一起。复合行星齿轮系 该自动变速器采用了一套齿轮，称为复合行星齿轮组，看起来像一个单行星齿轮装置，但实际上就像两个行星齿轮组的组合。其中包含一个总是作为输出的内齿圈、两个太阳和两套行星装置。 让我们看看部分的零件：变速器中的齿轮的组装从左到右：内齿圈，行星架，和两个太阳轮 下图显示了行星架是行星轮的载体。可以看到在右边的行星比左边的行星低。右边的行星没有与内齿圈啮合-它与其他行星接合。只有左边的行星上的内齿圈啮合。行星架：两个行星轮 在下个图中，您可看到在行星架的内部，短行星轮与小太阳轮啮合，长行星轮同时与大太阳轮和小太阳轮啮合。 行星架内部：两个行星轮自动变速档位第一档 在第一个档位，较小的太阳齿轮是由在液力变矩器的涡轮机驱动的顺时针方向。该行星架试图逆时针旋转，但仍由单向离合器（这只允许在顺时针方向旋转）和环形齿轮轮流输出。小齿轮有30齿，齿圈有72齿，速比为：Ratio = -R/S = - 72/30 = -2.4:1 所以旋转速率为负2.4:1，这意味着输出方向与输入方向相反。但输出方向确实是相同的输入方向，这是在那里的两套行星的伎俩。第一套与第二套啮合，而第二套输出至内齿圈，这样的组合把方向转回原先输入的方向。你可以看到，这也会导致更大的太阳齿轮自转，但由于离合器被释放，更大的太阳齿轮是自由旋转的相反方向的涡轮机（逆时针）自转。 第二档变速器做一些单独的事情来获得二档所需的传动比。它像相互连接的两套行星轮和共同的行星架相连接。行星架的第一阶段实际上是使用较大的太阳齿轮用作内齿圈。因此，第一阶段由太阳（较小的太阳齿轮），行星架和“内齿圈”（大太阳轮）。输入是小太阳齿轮，内齿圈（大太阳齿轮）是由带固定的，而输出是行星的载体。对于这个阶段，随着太阳作为输入，行星架作为输出，和内齿圈固定，公式是：1 + R/S = 1 + 36/30 = 2.2:1 该行星架为每一个小太阳齿轮旋转2.2次。第二阶段，