Automatic Transmission Basics

Automatic transmissions were developed as a way to allow gear shifting without the operators input. Automatic transmissions use two planetary gear sets to achieve the desired gear ratio. This kind of gear-set is called a “double-planetary” or “Simpson” gear set. A planetary gear set consists of a sun gear, planetary pinion with carrier and a ring gear or annulus. In a Simpson gear-set the two gear-sets share a sun gear. Planetary gear sets offer several gear ratio options in a small space. By driving one member of the gear-set and holding another, we get a gear ratio output on the last member.

Automatic transmission gear set

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Planetary Gear Set Side View

Automatic Transmission gear set side view

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The Gears

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First Gear

First gear is the hardest gear to learn, if you can understand first gear, the other gears should sound simple. First gear is complicated because it involves both planetary gear sets.

First Gear

In first gear the rear clutch drives the front annulus. The front carrier is considered held because is it splined to the output shaft which is spinning but much slower than the annulus/input shaft. The sun gear acts as an output for the front gear-set and as an input for the rear gear-set. The rear carrier is held by the one way clutch. The one-way clutch allows the rear carrier to turn with crankshaft rotation but not counter-crankshaft rotation. Since in first gear the sun gear is trying to rotate the rear carrier counter-crankshaft rotation, the one-way clutch holds the carrier. When the driver selects manual 1st or low the low and reverse band holds the rear carrier. Of coarse the rear annulus is splined to the output shaft so it is 1st gears output. A typical output for 1st gear would be a 2.6:1 gear ratio.

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Second Gear

Second Gear

In second gear the rear clutch drives the front annulus, the sun shell is held by the kickdown band which makes the front carrier the output. A typical output for 2nd gear would be a 1.4:1 gear ratio. The rear annulus is splined to the output shaft so it is spinning and driving the rear carrier. The rear carrier and annulus simply freewheel and don’t transfer engine torque.

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Third Gear

Third Gear

In third gear the rear clutch drives the front annulus and the front clutch drives the sun shell. The front carrier is the output. Nothing is really held by in third but since the sun and annulus are spinning at the same speed, the planetary pinions do not rotate and can be considered held. The result is a 1:1 gear ratio through the transmission. There is no change in torque or RPM. The rear carrier is freewheeling but since the sun gear is spinning and the rear annulus is splined to the output shaft which is also spinning, it is doing the same thing as the front carrier.

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Overdrive

With this gear-set a 4th gear is possible but there is no way to drive the rear carrier.

Overdrive

In overdrive the sun shell is held and the rear carrier is driven causing the carrier to rotate around the sun, pushing the rear annulus faster than input speed. A typical output for an overdrive or 4th gear would be a 0.7:1 gear ratio.

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Reverse

Reverse

In reverse, the sun gear is the input, the carrier is held by the low and reverse band causing the planetary pinions to spin but not rotate around the sun. The result is the rear annulus spinning in the opposite direction as the crankshaft.

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Getting More Gears

To get more gears out of a transmission, add more gear sets. The more planetary gear sets you have, (along with more driving/holding devices) the more gear ratios are possible. Some manufactures will use a ravigneaux (explained later) gear set to get the number of gears or the gear ratios they want. Manufacturers have used many combinations of gear-sets to get the ratios and the number of gears they want.

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Park

Park is a fairly simple system. None of the clutches are driven, none of the bands are holding. On the transmission output shaft there is a straight spur gear that has a parking pawl that prevents the shaft from turning. When the operator selects park, the parking pawl engages and slips between two of the teeth on the parking pawl gear. These are not as strong as you would think, it is always a good idea to also set the parking brake. This will keep the parking brake working and make sure the vehicle doesn’t move.

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Neutral

Neutral is the easiest range, in neutral nothing is driven, nothing is held, and the parking pawl is disengaged. So the engine is in no way connected to the wheels. The engine can rev up all it wants and the car won’t move and the car can roll away unless the brakes are applied.

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Driving and Holding Devices

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Multi-Disk Wet Clutches

Automatic Transmission wet clutchMult-disk wet clutches can be used as a driving device or a holding device. A basic 3-speed automatic has 2 of these. A front clutch that drives the sun shell/gear and a rear clutch that drives the front annulus. They consist of a drum, apply piston, return spring(s), steel drive plates, friction disks, pressure plate and C-clip. They are called a “wet clutch” because they need automatic transmission fluid to work properly. The drum can be steel or aluminium and houses all the clutches components. It may also provide an outer surface for a band to grab on to. The steel drive plates are splined to the drum and turn whenever the drum does. They sit between the friction disks. The friction disks spline to the member of the planetary gear set that the clutch controls. They have a friction material that will grab on to the steel drive plates when the friction disks and drive plates are pressed together by the apply piston. When the apply piston engages, the friction disks and the steel drive plates are forced together causing the drum and steel drive plates to turn or hold the friction disks and planetary gear member. The return spring(s) push against the pressure plate so when fluid pressure to the apply piston is released, the clutch disengages. The pressure plate is a thick steel plate that sits at the open end of the drum and keeps the friction disks and drive plates from being pushed out the opening. They are held in place by the C-clip.

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One-Way/Overrunning Clutch

Automatic Transmission one way clutchA one way clutch allows the outer race to turn in one direction only. A basic 3-speed automatic has one of these controlling the rear carrier. In this example, when the outer race is turned clockwise, the rollers are pushed up into the pockets and the outer race can turn freely. However, if the outer race is turned counter-clockwise the rollers are pushed down into the ramps causing them to wedge between the outer race and the inner race, stopping the outer race from turning. A typical one way clutch used in an automatic transmission would have many more rollers, as well as small springs to help push th rollers into the ramps.

 

 

 

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Brake Bands

Automatic Transmission brake bandBrake bands stop a planetary member from rotating by wrapping around a drum or shell. A basic 3-speed automatic has two of these. A kickdown band that holds the sun shell/gear, and a low and reverse band that holds the rear carrier. They have friction material along the inside to grab onto a drum or shell. When a brake band is to be applied, a servo receives fluid pressure and moves its piston to compress the band around its drum or shell. Bands have two advantages over wet multi-disk clutches as a holding device. As a band applies, it is drawn in by the rotating drum or shell, meaning that a band will “self-energise” or partially engage itself. The other advantage is that bands need less space and cost less than wet multi-disk clutches.

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Controlling Fluid Flow, Pressure and Temperature

Controlling where fluid goes, and at what pressure, is what decides what is driven or held. Fluid is picked up in the pan and pulled through the filter by the pump. Fluid is then pushed out of the pump to the valve body, which regulates, and distributes fluid pressure to the appropriate clutches, bands, gears and torque converter. If fluid is run hot all the time it begins to break down, even a few degrees too hot over the lifetime of the fluid can cause the fluid to break down early.

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The Transmission Pump

Automatic Transmission pumpThe transmission pump is what makes the fluid pressure and flow for the system. The pump is usually driven directly by the torque converter shell so it spins with the engine so the pressure needs to be regulated/limited. This is done by the pressure regulator and pressure relief valve. These valves are usually located in the valve body. The pressure regulator controls system line pressure based on system demands. The pressure relief valve is a safety in case pressures get too high.

 

 

 

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Transmission Oil Cooler

Automatic Transmission coolerMost transmissions do not have their own cooling system, they rely on ATF alone to cool and lubricate the system, so the oil must be cooled somehow. If the ATF is run too hot all the time, it will start to break down much faster. There are two main types of transmission oil coolers. The most common type uses airflow just like a radiator. Transmission cooler lines are run to the front of the car, just in front of the radiator where the cooler sits. The cooler will look just like a mini-radiator. The other kind of cooler uses the engines cooling system, much like an engine oil cooler (pictured). They can be a separate piece or they can be part of the radiator.
If while servicing a transmission, excessive metal fragments are found, it is a good idea to flush out or replace the transmission oil cooler.

 

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Lube Circuit

The lube circuit directs ATF to anywhere that needs lubrication, such as members of the planetary gear set. The circuit is usually fed by the transmission oil cooler.

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Transferring Rotational Torque From the Engine to the Automatic Transmission

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Torque Converter and Flexplate

Between the engine and transmission, there needs a place to allow some slip when the engine is idling and as we get the vehicle moving. A torque converter is a sealed unit which allows this to happen.

Automatic Transmission torque converter

Automatic Transmission TCThe crankshaft is bolted to the flexplate, which is a thin piece of metal that is also bolted to the torque converters shell. The flexplate can have the outer teeth which are turned by the starter to get the engine running or they can be on the torque converter. A flexplate is much lighter than a manual transmissions flywheel, on an automatic the torque converter weighs enough to maintain engine momentum between powerstrokes. The flexplate is bolted to the torque converter shell, which is one piece with the pump or impeller. The pump has fins that pull in transmission fluid in the middle of the torque converter and fling it to the outside and into the turbine. The turbine has fins that catch the fast moving fluid which rotates the turbine before returning the fluid to the middle to start the cycle again. This process actually creates torque. The turbine can never quite catch up to the speed of the pump so the torque converter actually works like a gear ratio, creating torque and losing RPM during acceleration. The turbine is splined to the transmission input shaft, so when the pump gets the turbine moving, the transmission gets rotational torque and if a gear is selected, the vehicle starts to move. When the engine is idling the pump can’t move enough fluid to push the turbine, especially when the operator has his/her foot on the brakes, or with the transmission in park. The stator comes into play as the vehicle accelerates. After the fluid comes back out of the turbine to get pulled in by the pump, it is flowing the wrong way for the pump to pick it back up again. Older torque converters, called fluid couplers, did not have a stator and were very sluggish while accelerating. The stator sits on a solid shaft coming out of the transmission that does not rotate, with a one-way clutch that allows the stator to freewheel once the pump and turbine approach the same speed. During acceleration the stator is locked on its shaft. The stator redirects the fluid coming out of the turbine from the wrong way to the right way for pickup. This makes a world of difference for acceleration, but once the turbine has caught up to the pumps speed, the stator is not needed and actually hurts fuel economy. The stator then freewheels and gets out of the way of fluid flow.

The TC makes 3 connections to the transmission. The transmission input shaft is driven by the TCs turbine, the stator rests on the stator support shaft and the transmission pump is driven by the TC shell.

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Lock-up Torque Converter

The turbine never completely catches up to the speed of the pump, (usually 90% max) this creates torque for acceleration but just wastes power while cruising. Lock-up torque converters have a lock-up clutch which locks the shell to the turbine to make the turbine spin at the same speed as the pump/shell. This improves highway fuel economy.

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Torque Converter Stall Speed

Stall speed is the maximum RPM the engine can turn the shell/pump in the torque converter, while the transmission is in gear, without turning the rest of the driveline. This is only really important on high performance or heavy duty applications because it indicates the maximum speed the pump can turn without turning the turbine.

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Transmission Management

TCM

Automatic Transmission control moduleThe transmission control module is an electronic device (a computer) that decides what gear to shift into and when. It takes input signals from gear monitoring sensors, the crank sensor as well as many other management sensors to decide what gear to drive in. It then commands the solenoids in the valve body, on or off, to control the spool valves and engage the appropriate gear. The TCM can be located external to the transmission or inside the transmission, possibly even one piece with the valve body to save the cost of wiring. This design causes the TCM to run very hot, something electronics do not like.

 

 

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Gear Monitoring Sensors

Automatic Transmission sensorThe gear monitoring sensors are usually PM generator style sensors which detect the speed of a planetary gear member. These are useful for the TCM to determine if a clutch or band is slipping, and if the correct gear ratio has been engaged.

 

 

 

 

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Valve Body

Automatic Transmission valve bodyThe valve body is what directs fluid to where it needs to go to drive in the desired gear, as well as regulate system pressures. Fluid is directed to the correct clutches and brake band servos, as well as the torque converter and lube circuit. They use a series of spool valves and check balls to do this. Older systems used a throttle valve and a governor valve which fought against each other to either force an upshift or a downshift. Modern systems have solenoids that are electronically controlled by the TCM (transmission control module) to move the spool valves.

 

 

 

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Other Transmission Designs

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Ravigneaux Gear-Set

Automatic Transmission ravigneaux gear setA ravigneaux gear set allows 4 gear ratios in a very small amount of space, They have 2 sun gears, one bigger and one smaller. They have two sets of planetary pinions, one set meshes with the front and rear annulus, the front sun, and the other set of planetary pinions. The other set of pinions mesh with the rear sun and the other set of planetary pinions.

 

 

 

 

 

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Non-planetary Transmissions

Some manufacturers have used a design similar to a standard transmission, that is able to shift on its own. Instead of synchronizers, wet multi-disk clutches are used to lock constantly meshed gears to a shaft to select a gear.

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CVT

CVT stands for Continuously Variable Transmission. These transmissions do not shift gears at all. They use two pulleys that change their size according to system demands connected by a metal belt. These transmissions can hold and engine in its powerband while changing the gear ratio to allow the vehicle to accelerate while the engine remains at its peak RPM to produce power. They can also allow the engine to operate at very low RPM on the highway to save fuel.

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DSG

DSG stands for Direct Shift Gearbox, these transmissions are essentially two transmissions in one. A 6-speed DSG, has a duel clutch located inside the transmission that can send power to either of two shafts. One shaft is in charge of gears 1,3 and 5, while the other is in charge of 2,4 and 6. As the vehicle stars off in 1st gear, the transmission has already engaged 2nd gear, all the transmission has to do is switch the powerflow to to the other shaft with the duel clutch to drive in 2nd gear. This is how these transmissions shift so quickly. While in 2nd gear the transmission has already selected 3rd and so on.

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Transbrake

A transbrake is a performance valve body modification used mostly on drag cars. It allows the transmission so sit in first and reverse at the same time, locking the planetary gear-set and slipping the torque converter, while the car is on the line waiting for the light to turn green. This allows the operator to rev the engine up to the torque converters stall speed. When the light turns green the operator releases the transbrake causing a sudden shock of torque through the driveline. Idealy you get the car to launch near the engines peak torque range without overly spinning the tires.

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