automatic transmission

The transmission is a group of parts that transmits power from a vehicle’s engine to the drive axle, which in turn makes the wheels go around. If you do not use a clutch to switch gears, your vehicle has an automatic transmission. If you use a clutch, your transmission is manual. Automatic transmissions contain an oil-like fluid to cool and lubricate the mechanism. The level and quality of this fluid should be checked regularly (see your vehicle owner’s manual) and replaced if needed. If your transmission performance seems good, the fluid is clear and red, and you have not had to add fluid, leave your transmission alone.

Just like that of a manual transmission, the automatic transmission’s primary job is to allow the engine to operate in its narrow range of speeds while providing a wide range of output speeds. Without 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.

You’ve probably never tried driving a manual transmission car using only third gear. If you did, you’d 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 engine’s 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. When you take apart and look inside an automatic transmission, you find a huge assortment of parts in a fairly small space. Among other things, you see:

* 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. An automatic transmission contains two complete planetary gearsets folded together into one component. Any planetary gearset has three main components:

* The sun gear
* The planet gears and the planet gears’ carrier
* The ring gear

automatic transmission

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. Let’s 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. Also, 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. 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.

manual transmission

Manual transmissions work by use of a clutch that connects the engine flywheel to the transmission input shaft – the input shaft of the transmission therefore turns at the same rpm as the engine. To understand the basic idea behind a standard transmission, the diagram below shows a very simple two-speed transmission in neutral:

The green shaft comes from the engine through the clutch. The green shaft and green gear are connected as a single unit. (The clutch is a device that lets you connect and disconnect the engine and the transmission. When you push in the clutch pedal, the engine and the transmission are disconnected so the engine can run even if the car is standing still. When you release the clutch pedal, the engine and the green shaft are directly connected to one another. The green shaft and gear turn at the same rpm as the engine.)

The red shaft and gears are called the layshaft. These are also connected as a single piece, so all of the gears on the layshaft and the layshaft itself spin as one unit. The green shaft and the red shaft are directly connected through their meshed gears so that if the green shaft is spinning, so is the red shaft. In this way, the layshaft receives its power directly from the engine whenever the clutch is engaged.

The yellow shaft is a splined shaft that connects directly to the drive shaft through the differential to the drive wheels of the car. If the wheels are spinning, the yellow shaft is spinning.

The blue gears ride on bearings, so they spin on the yellow shaft. If the engine is off but the car is coasting, the yellow shaft can turn inside the blue gears while the blue gears and the layshaft are motionless.

The purpose of the collar is to connect one of the two blue gears to the yellow drive shaft. The collar is connected, through the splines, directly to the yellow shaft and spins with the yellow shaft. However, the collar can slide left or right along the yellow shaft to engage either of the blue gears. Teeth on the collar, called dog teeth, fit into holes on the sides of the blue gears to engage them.

When the collar is between the two gears (as shown in the first figure), the transmission is in neutral. Both of the blue gears freewheel on the yellow shaft at the different rates controlled by their ratios to the layshaft.

From this simple overview, you can answer several questions:

1) When you make a mistake while shifting and hear a horrible grinding sound, you are not hearing the sound of gear teeth mis-meshing. As you can see in these diagrams, all gear teeth are all fully meshed at all times. The grinding is the sound of the dog teeth trying unsuccessfully to engage the holes in the side of a blue gear.

2) The transmission shown here does not have “synchros” so if you were using this transmission you would have to double-clutch* it. Manual transmissions in modern passenger cars use synchronizers to eliminate the need for double-clutching. A synchro’s purpose is to allow the collar and the gear to make frictional contact before the dog teeth make contact. This lets the collar and the gear synchronize their speeds before the teeth need to engage. The cone on one gear fits into the cone-shaped area in the collar, and friction between the cone and the collar synchronize the collar and the gear. The outer portion of the collar then slides so that the dog teeth can engage the gear.

3) You can also see how a small linear motion in the gear shift knob allows you to change gears. The gear shift knob moves a rod connected to the fork. The fork slides the collar on the yellow shaft to engage one of two gears.

*Double-clutching was common in older cars and is still common in some modern race cars. In double-clutching, you first push the clutch pedal in once to disengage the engine from the transmission. This takes the pressure off the dog teeth so you can move the collar into neutral. Then you release the clutch pedal and rev the engine to the “right speed.” The right speed is the rpm value at which the engine should be running in the next gear. The idea is to get the blue gear of the next gear and the collar rotating at the same speed so that the dog teeth can engage. Then you push the clutch pedal in again and lock the collar into the new gear. At every gear change you have to press and release the clutch twice, hence the name “double-clutching.”

clutch

Clutches are useful in devices with two rotating shafts. In these devices, one of the shafts is typically driven by a motor or pulley, and the other shaft is driving another device. In a drill, for instance, one shaft is driven by a motor and the other is driving a drill chuck. The clutch connects the two shafts so that they can either be locked together and spin at the same speed, or be decoupled and spin at different speeds.

In a car, you need a clutch because the engine spins all the time and the car wheels don’t. In order for a car to stop without killing the engine, the wheels need to be disconnected from the engine somehow. The clutch allows us to smoothly engage a spinning engine to a non-spinning transmission by controlling the slippage between them. To understand how a clutch works, it helps to know a little bit about friction.

The flywheel is connected to the engine, and the clutch plate is connected to the transmission. When your foot is off the pedal, the springs push the pressure plate against the clutch disc, which in turn presses against the flywheel. This locks the engine to the transmission input shaft, causing them to spin at the same speed. The amount of force the clutch can hold depends on the friction between the clutch plate and the flywheel, and how much force the spring puts on the pressure plate. When the clutch pedal is pressed, a cable or hydraulic piston pushes on the release fork, which presses the throw-out bearing against the middle of the diaphragm spring. As the middle of the diaphragm spring is pushed in, a series of pins near the outside of the spring causes the spring to pull the pressure plate away from the clutch disc. This releases the clutch from the spinning engine.

The most common problem with clutches is that the friction material on the disc wears out. The friction material on a clutch disc is very similar to the friction material on the pads of a disc brake, or the shoes of a drum brake — after a while, it wears away. When most or all of the friction material is gone, the clutch will start to slip, and eventually it won’t transmit any power from the engine to the wheels. Another problem sometimes associated with clutches is a worn throwout bearing. This problem is often characterized by a rumbling noise whenever the clutch engages.

clutch components

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transmission differential

The differential is a device that splits the engine torque two ways, allowing each output to spin at a different speed. The differential has three jobs:

1) To aim the engine power at the wheels
2) To act as the final gear reduction in the vehicle, slowing the rotational speed of the transmission one final time before it hits the wheels
3) To transmit the power to the wheels while allowing them to rotate at different speeds (This is the one that earned the differential its name.)

Car wheels spin at different speeds, especially when turning. Each wheel travels a different distance through the turn, and that the inside wheels travel a shorter distance than the outside wheels. When a car makes a turn, the wheels must spin at different speeds. For the non-driven wheels on your car — the front wheels on a rear-wheel drive car, the back wheels on a front-wheel drive car — this is not an issue. There is no connection between them, so they spin independently. But the driven wheels are linked together so that a single engine and transmission can turn both wheels. This requires the outside wheel to turn faster so the pinion gears do a balancing act and allow the inside wheel to slow down while the outside wheel speeds up. While turning a corner the outside wheel may do 110% of the vehicle speed while the inner wheel does 90%.

Power flow through the differential is as follows:

* the drive pinion rotates the ring gear
* the ring gear rotates the case
* the pinion shaft, as it rotates with the case, forces the pinion gears against the side gears which in turn rotate the axles

transmission differential