Ever wondered just what is inside a modern automatic transmission? This article describes and informs on clutch packs, one-way clutch systems, torque converters and more.
The modern automatic transmission consists of many components and systems that are designed to work together in a symphony of clever mechanical, hydraulic and electrical technology that has evolved over the years into what many mechanically inclined individuals consider to be an art form. We try to use simple, generic explanations where possible to describe these systems but, due to the complexity of some of these components, you may have to use some mental gymnastics to visualize their operation.
Automatic transmissions contain many gears in various combinations. In a manual transmission, gears slide along shafts as you move the shift lever from one position to another, engaging various sized gears as required in order to provide the correct gear ratio. In an automatic transmission, however, the gears are never physically moved and are always engaged to the same gears. This is accomplished through the use of planetary gear sets.
The basic planetary gear set consists of a sun gear, a ring gear and two or more planet gears, all remaining in constant mesh. The planet gears are connected to each other through a common carrier which allows the gears to spin on shafts called “pinions” which are attached to the carrier.
One example of a way that this system can be used is by connecting the ring gear to the input shaft coming from the engine, connecting the planet carrier to the output shaft, and locking the sun gear so that it can’t move. In this scenario, when we turn the ring gear, the planets will “walk” along the sun gear (which is held stationary) causing the planet carrier to turn the output shaft in the same direction as the input shaft but at a slower speed causing gear reduction (similar to a car in first gear).
If we unlock the sun gear and lock any two elements together, this will cause all three elements to turn at the same speed so that the output shaft will turn at the same rate of speed as the input shaft. This is like a car that is in third or high gear. Another way that we can use a Planetary gear set is by locking the planet carrier from moving, then applying power to the ring gear which will cause the sun gear to turn in the opposite direction giving us reverse gear.
The illustration on the right shows how the simple system described above would look in an actual transmission. The input shaft is connected to the ring gear (dark grey), The Output shaft is connected to the planet carrier (light grey) which is also connected to a “Multi-disk” clutch pack. The sun gear is connected to a drum (orange) which is also connected to the other half of the clutch pack. Surrounding the outside of the drum is a band (blue) that can be tightened around the drum when required to prevent the drum with the attached sun gear from turning.
The clutch pack is used, in this instance, to lock the planet carrier with the sun gear, forcing both to turn at the same speed. If both the clutch pack and the band were released, the system would be in neutral. Turning the input shaft would turn the planet gears against the sun gear, but since nothing is holding the sun gear, it will just spin free and have no effect on the output shaft. To place the unit in first gear, the band is applied to hold the sun gear from moving. To shift from first to high gear, the band is released and the clutch is applied causing the output shaft to turn at the same speed as the input shaft.
Many more combinations are possible using two or more planetary sets connected in various ways to provide the different forward speeds and reverse that are found in modern automatic transmissions.
Some of the clever gear arrangements found in four and now, five, six and even seven-speed automatics are complex enough to make a technically astute lay person’s head spin trying to understand the flow of power through the transmission as it shifts from first gear through top gear while the vehicle accelerates to highway speed. On newer vehicles, the vehicle’s computer monitors and controls these shifts so that they are almost imperceptible.
A clutch pack consists of alternating disks that fit inside a clutch drum. Half of the disks are steel and have splines that fit into groves on the inside of the drum. The other half have a friction material bonded to their surface and have splines on the inside edge that fit groves on the outer surface of the adjoining hub. There is a piston inside the drum that is activated by oil pressure at the appropriate time to squeeze the clutch pack together so that the two components become locked and turn as one.
A one-way clutch (also known as a “sprag” clutch) is a device that will allow a component such as ring gear to turn freely in one direction but not in the other. This effect is just like that of a bicycle, where the pedals will turn the wheel when pedaling forward, but will spin free when pedaling backward.
A common place where a one-way clutch is used is in first gear when the shifter is in the drive position. When you begin to accelerate from a stop, the transmission starts out in first gear. But have you ever noticed what happens if you release the gas while it is still in first gear? The vehicle continues to coast as if you were in neutral. Now, shift into Low gear instead of Drive. When you let go of the gas in this case, you will feel the engine slow you down just like a standard shift car. The reason for this is that in Drive, a one-way clutch is used whereas in Low, a clutch pack or a band is used.
A band is a steel strap with friction material bonded to the inside surface. One end of the band is anchored against the transmission case while the other end is connected to a servo. At the appropriate time hydraulic oil is sent to the servo under pressure to tighten the band around the drum to stop the drum from turning.
On automatic transmissions, the torque converter takes the place of the clutch found on standard shift vehicles. It is there to allow the engine to continue running when the vehicle comes to a stop. The principle behind a torque converter is like taking a fan that is plugged into the wall and blowing air into another fan which is unplugged. If you grab the blade on the unplugged fan, you are able to hold it from turning but as soon as you let go, it will begin to speed up until it comes close to the speed of the powered fan. The difference with a torque converter is that instead of using air, it uses oil or transmission fluid, to be more precise.
A torque converter is a large doughnut-shaped fluid coupling (10″ to 15″ in diameter) that is mounted between the engine and the transmission. It consists of three internal elements that work together to transmit power to the transmission. The three elements of the torque converter are the Pump, the Turbine, and the Stator. The pump is mounted directly to the converter housing which in turn is bolted directly to the engine’s crankshaft and turns at engine speed. The turbine is inside the housing and is connected directly to the input shaft of the transmission providing power to move the vehicle. The stator is mounted to a one-way clutch so that it can spin freely in one direction but not in the other. Each of the three elements have fins mounted in them to precisely direct the flow of oil through the converter.
With the engine running, transmission fluid is pulled into the pump section and is pushed outward by centrifugal force until it reaches the turbine section which starts it turning. The fluid continues in a circular motion back towards the center of the turbine where it enters the stator. If the turbine is moving considerably slower than the pump, the fluid will make contact with the front of the stator fins which push the stator into the one way clutch and prevent it from turning. With the stator stopped, the fluid is directed by the stator fins to re-enter the pump at a “helping” angle providing a torque increase. As the speed of the turbine catches up with the pump, the fluid starts hitting the stator blades on the back-side causing the stator to turn in the same direction as the pump and turbine. As the speed increases, all three elements begin to turn at approximately the same speed.
Since the ’80s, in order to improve fuel economy, torque converters have been equipped with a lockup clutch (not shown) which locks the turbine to the pump as the vehicle speed reaches approximately 45 – 50 MPH. This lockup is controlled by computer and usually won’t engage unless the transmission is in 3rd or 4th gear.
The Hydraulic system is a complex maze of passages and tubes that sends transmission fluid under pressure to all parts of the transmission and torque converter. The diagram at left is a simple one from a 3-speed automatic from the ’60s. The newer systems are much more complex and are combined with computerized electrical components. Transmission fluid serves a number of purposes including: shift control, general lubrication and transmission cooling. Unlike the engine, which uses oil primarily for lubrication, every aspect of a transmission’s functions are dependent on a constant supply of fluid under pressure. This is not unlike the human circulatory system (the fluid is even red) where even a few minutes of operation when there is a lack of pressure can be harmful or even fatal to the life of the transmission. In order to keep the transmission at normal operating temperature, a portion of the fluid is sent through one of two steel tubes to a special chamber that is submerged in anti-freeze in the radiator. Fluid passing through this chamber is cooled and then returned to the transmission through the other steel tube. A typical transmission has an average of ten quarts of fluid between the transmission, torque converter, and cooler tank. In fact, most of the components of a transmission are constantly lubricated in fluid including the clutch packs and bands. The friction surfaces on these parts are designed to operate properly only when they are coated in oil.
The transmission oil pump (not to be confused with the pump element inside the torque converter) is responsible for producing all the oil pressure that is required in the transmission. The oil pump is mounted to the front of the transmission case and is directly connected to the hub of the torque converter housing. Since the torque converter housing is directly connected to the engine crankshaft, the pump will produce pressure whenever the engine is running as long as there is a sufficient amount of transmission fluid available. The oil enters the pump through a filter that is located at the bottom of the transmission oil pan and travels up a pickup tube directly to the oil pump. The oil is then sent under pressure to the pressure regulator, the valve body, and the rest of the components as required.
The valve body is the control center of the automatic transmission.
The valve body contains a maze of channels and passages that direct hydraulic fluid to the numerous valves which then activate the appropriate clutch pack or band servo to smoothly shift to the appropriate gear for each driving situation. Each of the many valves in the valve body has a specific purpose and is named for that function. For example the 2-3 shift valve activates the 2nd gear to 3rd gear up-shift or the 3-2 shift timing valve which determines when a downshift should occur.
The most important valve, and the one that you have direct control over is the manual valve. The manual valve is directly connected to the gear shift handle and covers and uncovers various passages depending on what position the gear shift is placed in. When you place the gear shift in Drive, for instance, the manual valve directs fluid to the clutch pack(s) that activates 1st gear. It also sets up to monitor vehicle speed and throttle position so that it can determine the optimal time and the force for the 1 – 2 shift. On computer controlled transmissions, you will also have electrical solenoids that are mounted in the valve body to direct fluid to the appropriate clutch packs or bands under computer control to more precisely control shift points.
The computer uses sensors on the engine and transmission to detect such things as throttle position, vehicle speed, engine speed, engine load, stop light switch position, etc. to control exact shift points as well as how soft or firm the shift should be. Some computerized transmissions even learn your driving style and constantly adapt to it so that every shift is timed precisely when you would need it.
Because of computer controls, sports models are coming out with the ability to take manual control of the transmission as though it were a stick shift, allowing the driver to select gears manually. This is accomplished on some cars by passing the shift lever through a special gate, then tapping it in one direction or the other in order to up-shift or down-shift at will. The computer monitors this activity to make sure that the driver does not select a gear that could over speed the engine and damage it.
Another advantage to these “smart” transmissions is that they have a self diagnostic mode which can detect a problem early on and warn you with an indicator light on the dash. A technician can then plug test equipment in and retrieve a list of trouble codes that will help pinpoint where the problem is.
These three components are important in the non-computerized transmissions. They provide the inputs that tell the transmission when to shift.
The Governor is connected to the output shaft and regulates hydraulic pressure based on vehicle speed. It accomplishes this using centrifugal force to spin a pair of hinged weights against pull-back springs. As the weights pull further out against the springs, more oil pressure is allowed past the governor to act on the shift valves that are in the valve body which then signal the appropriate shifts.
Of course, vehicle speed is not the only thing that controls when a transmission should shift, the load that the engine is under is also important. The more load you place on the engine, the longer the transmission will hold a gear before shifting to the next one.
There are two types of devices that serve the purpose of monitoring the engine load: the Throttle Cable and the Vacuum Modulator. A transmission will use one or the other but generally not both of these devices. Each works in a different way to monitor engine load.
The Throttle Cable simply monitors the position of the gas pedal through a cable that runs from the gas pedal to the throttle valve in the valve body.
The Vacuum Modulator monitors engine vacuum by a rubber vacuum hose which is connected to the engine. Engine vacuum reacts very accurately to engine load with high vacuum produced when the engine is under light load and diminishing down to zero vacuum when the engine is under a heavy load. The modulator is attached to the outside of the transmission case and has a shaft which passes through the case and attaches to the throttle valve in the valve body. When an engine is under a light load or no load, high vacuum acts on the modulator which moves the throttle valve in one direction to allow the transmission to shift early and soft. As the engine load increases, vacuum is diminished which moves the valve in the other direction causing the transmission to shift later and more firmly.
An automatic transmission has many seals and gaskets to control the flow of hydraulic fluid and to keep it from leaking out. There are two main external seals: the front seal and the rear seal. The front seal seals the point where the torque converter mounts to the transmission case. This seal allows fluid to freely move from the converter to the transmission but keeps the fluid from leaking out. The rear seal keeps fluid from leaking past the output shaft.
A seal is usually made of neoprene (similar to the neoprene in a windshield wiper blade) and is used to keep oil from leaking past a moving part such as a spinning shaft. In some cases, the neoprene compound is assisted by a spring that holds the neoprene in close contact with the spinning shaft.
A gasket is a type of seal used to seal two stationary parts that are fastened together. Some common gasket materials are: paper, cork, rubber, silicone and soft metal.
Aside from the main seals, there are also a number of other seals and gaskets that vary from transmission to transmission. A common example is the rubber O-ring that seals the shaft for the shift control lever. This is the shaft that you move when you manipulate the gear shifter. Another example that is common to most transmissions is the oil pan gasket. In fact, seals are required anywhere that a device needs to pass through the transmission case with each one being a potential source for leaks.
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A vehicle’s transmission system plays a critical role in delivering power from the engine to the wheels, enabling the vehicle to move and change speeds smoothly. In this article, we will provide an overview of the various transmission systems, their components, and their functions.
Understanding Manual Transmissions
Manual transmissions, also known as stick shifts, provide a more engaged driving experience as the driver is responsible for manually changing gears. In this article, we will delve into the inner workings of a manual transmission, exploring its key components and their functions.
Clutch: The clutch is the primary component that allows the driver to disengage and engage the engine from the transmission while shifting gears. The clutch assembly consists of several parts, such as the pressure plate, clutch disc, and flywheel. When the clutch pedal is pressed, the clutch disc disengages from the flywheel, allowing the driver to change gears. Releasing the pedal reconnects the clutch disc to the flywheel, transferring power back to the transmission.
Gear Shift Lever: The gear shift lever is the driver’s interface for selecting the desired gear ratio. By moving the lever, the driver engages various gear sets within the gearbox to change the transmission’s output speed.
Gearbox: The gearbox houses different sets of gears that provide various gear ratios. These ratios determine how many times the input shaft (connected to the engine) must rotate for the output shaft (connected to the wheels) to complete one rotation. Lower gears provide higher torque for acceleration, while higher gears offer lower torque but better fuel efficiency at cruising speeds.
Synchronizers: Synchronizers are essential components that help match the speeds of the input and output shafts during gear shifts. This ensures smooth gear engagement and prevents grinding or clashing of gears.
Linkage: The linkage system connects the gear shift lever to the transmission, transmitting the driver’s input to the gearbox.
Reverse Gear: The reverse gear is a unique gear set that changes the rotation direction of the output shaft, allowing the vehicle to move backward.
In a manual transmission, the driver must carefully balance clutch engagement and throttle input to ensure smooth gear shifts and avoid stalling the engine. Mastery of this skill set is often seen as a rite of passage for many driving enthusiasts.
Exploring Automatic Transmissions
Automatic transmissions offer a more convenient driving experience by automatically shifting gears based on the vehicle’s speed and engine load.
Torque Converter: The torque converter replaces the clutch in automatic transmissions, acting as a fluid coupling between the engine and transmission. It transfers power from the engine to the transmission using hydraulic fluid, allowing for smooth gear shifts and eliminating the need for a clutch pedal.
Planetary Gear Sets: Automatic transmissions use a series of planetary gear sets to achieve different gear ratios. These gear sets consist of a central sun gear, planet gears, and an outer ring gear, all working together to vary the transmission output speed.
Hydraulic System: The hydraulic system uses pressurized fluid to control the operation of clutches and bands, which in turn engage and disengage planetary gear sets. It also provides lubrication and cooling for the transmission.
Valve Body: The valve body is the control center of the hydraulic system, directing fluid flow to engage the appropriate gear sets based on driving conditions and electronic signals from the vehicle’s sensors.
Electronic Controls: Modern automatic transmissions use electronic controls and sensors to optimize gear shifts for improved performance, fuel efficiency, and driving comfort.
The Clutch: Function and Components
The clutch is a vital component in manual transmissions, responsible for connecting and disconnecting the engine from the transmission during gear shifts. It enables smooth gear changes and prevents damage to the gearbox. The clutch assembly consists of the pressure plate, clutch disc, and flywheel. When the driver presses the clutch pedal, the pressure plate disengages the clutch disc from the flywheel, allowing for gear shifts. As the pedal is released, the clutch disc re-engages with the flywheel, transferring engine power back to the transmission. Understanding the clutch’s operation and components is crucial for mastering manual transmissions and ensuring their longevity.
The Gearbox: Gear Ratios and Synchronizers
The gearbox is the heart of a transmission system, housing various gear sets that provide different gear ratios. These ratios determine the balance between torque and speed, allowing the vehicle to accelerate, maintain cruising speeds, and climb inclines efficiently. In manual transmissions, synchronizers are essential components that help match the speeds of the input and output shafts during gear shifts. This ensures smooth gear engagement and prevents grinding or clashing of gears. A well-maintained gearbox contributes significantly to a vehicle’s performance, fuel efficiency, and overall driving experience.
The Torque Converter: Role and Operation
The torque converter is a key component in automatic transmissions, serving as a fluid coupling between the engine and transmission. It replaces the clutch found in manual systems, allowing for seamless gear changes without driver intervention. The torque converter consists of three main elements: the impeller, turbine, and stator. As the engine spins the impeller, it transfers hydraulic fluid to the turbine, causing it to rotate and transmit power to the transmission. The stator redirects fluid flow between the impeller and turbine, maximizing torque transfer efficiency. A thorough understanding of the torque converter’s function is essential for comprehending automatic transmission operation and maintenance.
Transmission Fluids: Types and Maintenance
Transmission fluids lubricate, cool, and protect transmission components, ensuring smooth operation and longevity. Different transmissions require specific fluid types, such as automatic transmission fluid (ATF) or manual transmission fluid (MTF). Regular fluid checks and changes, as per the manufacturer’s guidelines, are crucial to prevent transmission issues and prolong its life.
Drivetrain Components: Driveshaft, Axle, and Differential
The drivetrain transfers power from the transmission to the wheels, enabling the vehicle to move. Key components include the driveshaft, which transmits torque from the transmission to the differential, the axle, which connects the wheels to the differential, and the differential itself, which distributes torque between the wheels, allowing them to rotate at different speeds during turns. A thorough understanding of these components is essential for maintaining and troubleshooting your vehicle’s transmission system.
Electronic Transmission Controls and Sensors
Modern transmission systems incorporate electronic controls and sensors to optimize gear shifts, enhance performance, and improve fuel efficiency. These components work in tandem with the vehicle’s Engine Control Unit (ECU) to monitor driving conditions and make real-time adjustments. Key sensors include speed sensors, throttle position sensors, and transmission fluid temperature sensors. By understanding the role of these electronic controls and sensors, drivers and technicians can better diagnose and address transmission-related issues.
Common Transmission Problems and Solutions
There can be numerous transmission problems that can affect the performance and functionality of a vehicle. Some common transmission problems include:
Slipping gears
Grinding or clashing noises during gear shifts
Difficulty shifting gears
Leaking transmission fluid
Overheating transmission
Delayed engagement or slow response in gear changes
Transmission warning light on the dashboard
Loss of power or poor acceleration
Erratic or harsh shifts
Complete transmission failure
Regular maintenance, such as checking transmission fluid levels and conditions, can help prevent many of these issues. However, some problems may arise from wear and tear or manufacturing defects, requiring professional diagnosis and repair.
Transmission Maintenance and Troubleshooting
Proper maintenance and troubleshooting are essential for transmission longevity. The best way to maintain a performance transmission involves the following:
Regular fluid checks and changes
Inspecting and adjusting linkage
Identifying common issues, such as slipping, grinding, or leaks
Consulting a professional for diagnosis and repair
Following manufacturer-recommended maintenance schedules
Upgrading and Modifying Transmission Systems
Are you looking to upgrade your vehicle’s performance and unlock its full potential? Modifying your transmission system can make a significant difference in your driving experience.
Imagine tackling challenging terrain or effortlessly accelerating on the open road, all thanks to a customized transmission tailored to your specific needs. Performance transmissions offer enhanced durability, quicker gear shifts, and improved torque transfer, giving you the edge you desire on the road or track.
Gearstar is a leading provider of performance transmissions, specializing in building custom solutions for a wide range of vehicles. Our expert technicians will carefully assess your needs and craft a transmission that delivers the power and performance you’ve been dreaming of.
Don’t wait to experience the thrilling transformation a performance transmission can bring to your vehicle. Contact Gearstar today to discuss your options and let our team of experts guide you through the process of upgrading your transmission system. Together, we’ll elevate your driving experience to new heights.
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