Upon completion and review of this chapter, you should be able to:
In 1939, the big bit of automotive news was the Hydramatic Drive introduced by Oldsmobile. The Hydramatic was a combination of a liquid flywheel and a fully automatic transmission. “The ’40 Olds shifts without a clutch? What will they think of next?”
Throughout the years, the automatic transmission has evolved into a complex machine with electronic, hydraulic, and mechanical components. It is safe to say that this evolution will continue for years to come. The focus of this chapter is on the mechanical components of an automatic transmission.
This chapter also looks at the different designs of automatic transmissions and the major subassemblies that are part of a typical transmission. Before you can thoroughly understand the purpose of each and every part of a transmission, you must have an understanding of the roles and construction of the major subassemblies. Each of these subassemblies is covered in detail in later chapters.
Transmission designs vary from manufacturer to manufacturer and within each manufacturer. These variations may be simply different electronic controls for shifting or for the torque converter or the number of forward speeds a transmission has. Variations also result from the way a particular model of transmission is constructed and the vehicle it will be installed in. These topics are the focus of this chapter.
One of the primary variations in the design of an automatic transmission is based on the driveline of the vehicle the transmission was designed for. As you know, automobiles are propelled in one of three ways: by the rear wheels, by the front wheels, or by all four wheels. The type of driveline helps determine whether a transmission or a transaxle will be used.
Vehicles propelled by the rear wheels normally use a transmission. Transmission gearing is located within an aluminum casting called the transmission case assembly (Figure 5-1). The transmission case assembly is attached to the rear of the engine, which is normally located in the front of the vehicle. A driveshaft links the output shaft of the transmission with the differential and drive axles located in a separate housing at the rear of the vehicle. The differential splits the driveline power and redirects it to the two rear drive axles, which then pass it on to the wheels.
An RWD transmission with its identification numbers and production information highlighted.
Longitudinally mounted engines are sometimes said to have “north-south” orientation, whereas transverse engines are oriented “east-west.”
The front wheels propel front-wheel-drive (FWD) vehicles. For this reason, they must use a drive design different from that of a rear-wheel-drive (RWD) vehicle. FWD vehicles are typically equipped with a transaxle and have no need for a separate differential and drive axle housing. A transaxle is a compact unit that combines the transmission gearing, differential, and drive axle connections into an aluminum housing located in front of the vehicle. This design has two primary advantages: good traction on slippery roads due to the weight of the powertrain components being directly over the driving axles of the vehicle, and transverse engine and transaxle configurations that allow for lower hood lines, thereby improving the vehicle’s aerodynamics.
Not all FWD vehicles have a transversely mounted engine. Some have the engine mounted longitudinally and use a transaxle that looks like a conventional transmission design modified to drive the front wheels directly.
Four-wheel-drive vehicles typically use a transmission and transfer case. The transfer case mounts on the side or back of the transmission. A chain or gear drive inside the transfer case receives power from the transmission and transfers it to two separate driveshafts. One driveshaft connects to a differential on the front drive axle. The other driveshaft connects to a differential on the rear drive axle.
There are a few mid-engine and rear-engine cars out there. These can use either a transmission or a transaxle. To simplify things, you can look at the driveline and tell whether or not the vehicle has a transmission or a transaxle. If there is a separate drive axle unit with a differential, a transmission is used. If the drive axles extend from the transmission unit, it is a transaxle.
All automotive transmissions and transaxles are equipped with a varied number of forward speed gears, a neutral gear, and one reverse speed. Transmissions can be divided into groupings based on the number of forward speed gears they have. A growing number have six and a few have as many as nine. Of course, there are a wide variety of continuously variable transmissions (CVTs), as well.
Transmissions are designed and built for particular applications. Things that must be considered in the design of a transmission are the power output of the engine, the weight of the vehicle the transmission will be installed in, and the typical workload of the vehicle. All of these play a part in the size and strength of the parts and materials used in a transmission. Obviously, high-powered and hard-working vehicles need larger and stronger materials.
Overall transmission/transaxle size is also an important design consideration. This often dictates the placement of the various shafts inside the transmission and the size of the components. Transmission designs also vary according to the type of compound planetary gearsets used.
Transaxle design is most affected by the intended application. Since these units attach directly to the engine and that combination must fit between the two drive wheels, size is important. A variety of shaft arrangements can be found in today’s transaxles. Long shafts may be divided into two parts and the ends of each connected by gears or chains. Final drive units vary in design as well. Common final drive setups for FWD vehicles use helical gears, planetary gears, hypoid gears, and/or a chain drive.
It is important to know that no two transmission models are exactly alike, regardless of their outward appearance. Keep in mind that these variations are based on external and internal components.
Although there are many different designs, all transmissions and transaxles rely on gears, shafts, bearings, and apply devices to function. All automatic transmissions, except CVTs, have a torque converter to connect and disconnect the engine’s power to the transmission. All have an input shaft to transfer power from the torque converter to the internal gearsets and drive members. They also have gearsets to provide the different gear ratios, a reverse gear, and a neutral position. All have an output shaft to transfer power from the transmission to the final drive unit. They also have a hydraulic system that includes the pump, valve body, pistons, servos, brakes, and clutches.
In the past, manufacturers identified their transmission models with internal codes that represented the design of the transmission. Although technicians who worked on transmissions learned to know the differences between the models, there was little logic used to denote the features of a particular design.
Recently, manufacturers began using more definitive codes for model identification. Most use an alphanumeric code and the features of each transmission model are explained in the service information. Let’s take a look at the transmission model codes for some manufacturers.
Interpreting a General Motors transmission code will reveal the work capacity of the transmission, the number of forward gears, its directional placement in the vehicle, and if it has electronic controls. A 6L80 transmission is a six-speed, longitudinally mounted, light-to-medium duty, electronically controlled unit. The 6 in the model number designates the number of forward gears, the Lshows that it is a longitudinally mounted unit, and the 80 is the torque capacity rating.
Other commonly used GM transmissions are the 6T75 and the 6T30. Both of these are found in cars and light SUVs. Both of these models are transversely mounted six-speed units with electronic controls. The internal construction of these units is different, as noted by the different product series numbers. The 6T75 unit is designed for heavier duty than the 6T30. As you might have noted, the E designation is not always added; possibly since electronic automatics have dominated production in the past 20 years.
Chrysler uses a similar system (Figure 5-2). The first character denotes the number of forward gears. The second character is the duty rating. In contrast to GM’s use of a two-digit code for duty rating, Chrysler uses a single-digit code. Using a 62TE transmission as an example of Chrysler’s transmission code, the 6 indicates there are six forward speeds, the 2 is the duty rating, Tindicates the transmission is designed for FWD, and Eindicates that the unit is fully electronic.
Transmission model designation code interpretation for Chrysler vehicles.
Ford Motor Company uses similar logic for model designations; however, its duty rating is more specific. The 4F27E transmission is used in the Focus. The model code is broken into four character groups. The 4 indicates that the transmission is a four-speed unit. The F denotes that this is a transaxle for FWD vehicles. The 27 is the duty rating and represents the maximum input torque. In this case the 27 represents 270 lbs.-ft. (365 Nm). The Emeans the transaxle is fully electronically controlled.
The importance of using service information to decipher the codes is apparent when considering the model code for another commonly used Ford transmission. The 6R140 is a four-speed transmission for RWD pickups like the Ford Super Duty. This transmission is electronically controlled, but the model doesn’t contain an E.
Toyota has two primary classifications of transmissions/transaxles, the A–and U–series. The A-series are two- to eight-speed automatic transmissions for front-wheel-drive, all-wheel-drive, or rear-wheel-drive vehicles built by Aisin-Warner. The U-series are automatic transmissions for FWD applications built primarily by Toyota.
The designations of individual transmissions are defined by an alphanumeric system. This system is used for both automatic and manual transmissions. The first character is the series of transmission. The second numerical digit states the number of forward gears. The last numeric digit denotes the version of the base transmission. This number typically changes according to the application. These numbers may be followed by a letter which denotes special features of the transmission. Examples of these letters are as follows: L=Lockup torque converter , E= Electronic control ,F=Four-wheel-drive , and H=AWD with a transversely mounted engine.
To show the designations define the transmission, let’s look at the AA80E. This eight-speed automatic transmission is used in current Lexus models, including the LS460, GS460, and IS-F. This transmission is built by Aisin-Warner and is electronically controlled. The 0 after the 8 shows this is the first version of this transmission.
The weight of the transmission or transaxle is supported by the engine and its mounts and by a transmission mount. These mounts are not only critical for proper operation of the transmission, but also isolate transmission noise and vibrations from the passenger compartment. The mountings for the engine and transmission keep the powertrain in proper alignment with the rest of the drivetrain and help to maintain proper adjustment of the various linkages attached to the housing.
An engine oscillates (vibrates) as it runs. Since the transmission is directly connected to the engine, those vibrations carry through to the transmission. Faulty mounts will not only allow these vibrations and the resulting noise to transfer into the vehicle, but can also cause internal transmission problems. When an engine is mounted transversely, the inherent vibrations of the engine are easily transmitted to the vehicle’s suspension and wheels. Also, most FWD vehicles use compact in-line 4-cylinder or V6 engines that do not run as smoothly as larger engines. For these reasons, the manufacturers of FWD vehicles have developed many different mounting systems for their engines and transmissions.
The typical mount bolts to the transaxle and is connected to a plate or to the surface on the transaxle housing. A bolt passes through a rubber insulator and connects the mount to the transaxle (Figure 5-3). This type of mount is used on the side and toward the top of the engine/transaxle assembly. With this mount is a lower mount located between the subframe and the transaxle (Figure 5-4). These two mounts keep the assembly in place, but do little to control noise and vibration; therefore, additional mounts are used.
The through bolt in a transaxle mount. This bolts passes through a rubber isolator in the mount.
A lower engine mount for an FWD vehicle.
Another common way to suppress vibrations and noise is the use of an engine mount strut. This strut limits the rocking motion of the engine and connects the top of the engine to the frame of the vehicle (Figure 5-5). Again, the connecting points between the two parts of the mount are made through an insulator. Some vehicles use more than one of these struts or have an additional strut mounted to the side of the engine.
An engine mount strut.
Some models have a lower engine mount that connects the lower front of the engine to the frame of the vehicle (Figure 5-6) and an upper engine mount of the rear of the assembly (Figure 5-7).
A lower engine mount.
An upper engine mount.
A few models use an adjustable mount that responds to vibrations (Figure 5-8). This type of system relies on the action of a solenoid on a hydraulic mount. The solenoid responds to the commands from the PCM and decreases and increases the fluid pressure at the mount.
An electronically controlled engine mount.
Isolation of vibrations is not as much of a requirement for FWD vehicles with a longitudinally mounted powertrain. The engine and transmission in these models are mounted in much the same way as in a RWD vehicle (Figure 5-9). Although these mounts are less complex, they are still critical to the overall operation of the vehicle.
Mounts for a longitudinally mounted transaxle.
Chapter 5, Transmission Case Service
The basic shape and size of a transmission/transaxle reflects its design to some degree. Automatic transmission housings are typically aluminum castings. The torque converter and transmission housings are normally cast as a single unit; however, in some designs the torque converter housing is a separate casting. Transaxle housings are typically comprised of two or three separate castings bolted together; one of these castings is the torque converter housing (Figure 5-10). Although they serve many of the same purposes as a transmission, transaxle housings are considerably different in appearance from transmission housings.
A converter housing with associated components separated from the transaxle.
The converter housing is sometimes called the bellhousing.
The surfaces at which the sections of a transmission/transaxle mate are critical to the operation of the unit. Proper mounting surfaces are necessary to keep the various shafts aligned and to provide a good seal. It is important to remember that a poor seal will not only cause fluid leaks, but will also allow dirt to enter the unit. Dirt is a transmission’s most feared enemy.
Transmission housings are cast to secure and accommodate the following components:
Torque is multiplied and speed is decreased according to the gear ratio.
An example of the fluid passages cast into a transmission housing.
Various sensors and switches attached to a late-model transaxle housing.
This parking pawl assembly is just one of the many shafts and linkage parts that pass through or into a typical transmission housing.
The planet carrier is the bracket in a planetary gearset on which the planet pinion gears are mounted on pins and are free to rotate.
A transmission/transaxle case assembly is a precisely machined and designed unit. It has many more important roles than simply housing the internal components.
The planet gears can be referred to as the pinion gears.
Increasing the engine’s torque is generally known as operating in reduction because there is always a decrease in the speed of the output member, which is proportional to the increase in the output torque.
Nearly all automatic transmissions rely on planetary gearsets (Figure 5-14) to transfer power and multiply engine torque to the drive axle. A simple planetary gearset consists of three parts: a sun gear, a carrier with planetary pinions mounted to it, and an internally toothed ring gear or annulus. The sun gear is located in the center of the assembly and meshes with the teeth of the planetary pinion gears. Planetary pinion gears are small gears fitted into a framework called the planetary carrier. The planetary carrier can be made of cast iron, aluminum, or steel plate and is designed with a shaft for each of the planetary pinion gears (Figure 5-15).
A single planetary gearset.
Planetary gear configuration is similar to the solar system, with the sun gear surrounded by the planetary pinion gears. The ring gear surrounds the complete gearset.
Planetary pinion gears rotate on needle bearings positioned between the planetary carrier shaft and the planetary pinions. The carrier and pinions are considered one unit—the mid-size gear member (Figure 5-16).
The pinion gears and carrier assembly for a typical front planetary unit.
The planetary pinions are surrounded by the annulus or ring gear, which is the largest part of the simple gearset. The ring gear holds the entire gearset together and provides great strength to the unit.
Each member of a planetary gearset can spin (revolve) or be held at rest. Power transfer through a planetary gearset is only possible when one of the members is held at rest, or if two of the members are locked together.
Any one of the three members can be used as the driving or input member. At the same time, another member might be kept from rotating and thus becomes the held or stationary member. The third member then becomes the driven or output member. Depending on which member is the driver, which is held, and which is driven, either a torque increase or a speed increase is produced by the planetary gearset. Output direction can also be reversed through various combinations. Refer to Figure 5-17 for a summary of the possible outcomes of the different combinations available in a planetary gearset.
The basic laws of planetary gear operation.
|Sun Gear||Carrier||Ring Gear||Speed||Torque||Direction|
|1. Input||Output||Held||Maximum reduction||Increase||Same as input|
|2. Held||Output||Input||Maximum reduction||Increase||Same as input|
|3. Output||Input||Held||Maximum increase||Reduction||Same as input|
|4. Held||Input||Output||Maximum increase||Reduction||Same as input|
|5. Input||Held||Output||Reduction||Increase||Reverse of input|
|6. Output||Held||Input||Increase||Reduction||Reverse of input|
|7. When any two members are held together, speed and direction are the same as input. Direct 1:1 drive occurs.|
Compound gearsets combine simple planetary gearsets so load can be spread over a greater number of teeth for strength and also to obtain the largest number of gear ratios possible in a compact area. A limited number of gear ratios are available from a single planetary gearset. To increase the number of available gear ratios, gearsets can be added. The typical automatic transmission with four forward speeds has at least two planetary gearsets. Some transmissions are fitted with an additional single planetary gearset that is used to provide additional forward gear ratios.
The Simpson geartrain is an arrangement of two separate planetary gearsets with a common sun gear, two ring gears, and two planetary pinion carriers (Figure 5-18). One half of the Simpson gearset or one planetary unit is referred to as the front planetary and the other planetary unit is the rear planetary. The two planetary units do not need to be the same size or have the same number of teeth on their gears. The size and number of gear teeth determine the actual gear ratios obtained by the compound planetary gear assembly. The Simpson gearset was a very popular design with three- and four-speed transmissions.
A Simpson planetary gearset.
The different gear ratios and direction of rotation are the result of applying torque to one member of either planetary unit, holding at least one member of the gearset, and using another member as the output. For the most part, automobile manufacturer use the same parts of the planetary assemblies as input, output, and reaction members; therefore, they have similar power flows.
A Ravigneaux gearset has two independent sun gears, two sets of planet gears, and a common ring gear (Figure 5-19). This gearset provides forward gears with a reduction, direct drive, overdrive, and a reverse operating range. The Ravigneaux offers some advantages over a Simpson geartrain. It is very compact. It can carry large amounts of torque because of the great amount of tooth contact. It can also have three different output members. However, it has a disadvantage to students and technicians, it is more complex and therefore its actions are more difficult to understand.
A Ravigneaux gearset.
The Ravigneaux geartrain is designed to use two sun gears, one small and one large. They also have two sets of planetary pinion gears, three long pinions, and three short pinions. The planetary pinion gears rotate on their own shafts that are fastened to a common planetary carrier. A single ring gear surrounds the complete assembly.
The small sun gear is meshed with the short planetary pinion gears. These short pinions act as idler gears to drive the long planetary pinion gears. The long planetary pinion gears mesh with the large sun gear and the ring gear.
The Lepelletier design connects a simple planetary gearset to a Ravigneaux gearset. In this design, the input shaft is connected to the ring of the simple planetary gear and can simultaneously be connected to the carrier and large sun gear of the Ravigneaux gearset using separate clutches (Figure 5-20). The output shaft is connected to the ring of the Ravigneaux gear. The Lepelletier gearset has become very popular with many manufacturers because of its ability to provide more gear ranges than previous designs.
A Lepelletier gearset.
Rather than rely on a compound gearset, automatic transmissions use two or three simple planetary units in series (Figure 5-21). In this type of arrangement, gearset members are not shared. Instead the holding devices are used to lock different members of the planetary units together.
Two planetary units connected in tandem with the ring gear of one gearset connected to the planet carrier of the other.
The combination of the planetary units functions much like a compound unit. The tandem units do not share a common member, rather certain members are locked together or are integral with each other. The front planetary carrier is locked to the rear ring gear and the front ring gear is locked to the rear planetary carrier.
Honda non-planetary–based transaxles are rather unique in that they use constant-mesh helical and square-cut gears (Figure 5-22) in a manner similar to a manual transmission. These transaxles have a main shaft and countershaft on which the gears ride. To provide the forward gear ratios and a reverse gear, different pairs of gears are locked to the shafts by hydraulically controlled clutches. Reverse gear is obtained through the use of a shift fork that slides the reverse gear into position. The power flow through these transaxles is also similar to that of a manual transaxle.
A cutaway of a constant-mesh helical gear automatic transmission.
Certain parts of the planetary gearset must be held while others must be driven to provide the needed torque multiplication and direction for vehicle operation. “Planetary gear controls” is the general term used to describe transmission bands, servos, and clutches. In many modern transmissions clutches alone are used, without the use of bands and servos.
Although many of the latest models no longer use bands, many transmissions in use utilize bands. A band is used as a brake and is positioned around a rotating drum. The band brings a drum to a stop by wrapping itself around the drum and holding it. The band is applied hydraulically by a servo assembly. Connected to the drum is a member of the planetary geartrain. The band holds a member of the planetary gearset by preventing the drum from rotating and the attached planetary gear member becomes the reaction gear for the gearset. Bands have excellent braking characteristics and require a minimum amount of space within the transmission housing.
The servo assembly converts hydraulic pressure into a mechanical force that applies a band to hold a drum stationary. Simple and compound servos are used in modern transmissions.
In an automatic transmission, both sprag and roller overrunning (one-way) clutches are also used to brake members of the planetary gearset. These clutches operate mechanically. An overrunning clutch allows rotation in only one direction and does not require hydraulic activation.
In contrast to a band or one-way clutch, which only brake a planetary gear member, multiple-disc clutches are capable of both holding and driving gearset members.
A multiple-disc clutch has a series of friction discs that have internal teeth that are sized and shaped to mesh with splines on the clutch assembly hub. In turn, this hub is connected to a member of the planetary gearset that will receive the desired braking or transfer force when the clutch is applied or released.
Multiple-disc clutches are enclosed in a large drum-shaped housing that can be either a separate casting or part of the existing transmission housing. This drum housing also holds the other clutch components: cylinder, hub, piston, piston return springs, seals, pressure plate, friction plates, and snap rings.
Many modern transmissions like the 6L80, and several others, use a design called clutch-to-clutch. These transmissions do not use the traditional accumulators or servos to accomplish shift timing; instead the application and release of clutches is controlled by the shift and pressure control solenoids within the valve body. The electronic controls have the ability to release one clutch and apply another clutch, thereby shifting the transmission to another range.
Chapter 5, Reassembly and Testing
The housing contains the basic parts that transfer power and change gears. Fitted into the housing is a key component, the torque converter. A torque converter uses hydraulic fluid flow to connect and disconnect the power of the engine from the transmission. The converter is a doughnut-shaped device filled with ATF. Inside the converter are the parts that make it work.
Like most parts of an automatic transmission, this component has many different designs and shapes. A torque converter is bolted to the engine and not the transmission. However, the transmission is designed to support the unbolted end. This end rides on a bearing or bushing in the transmission.
The torque converter not only transfers engine power to the transmission, it also drives the transmission pump through its outer housing. The pump is a critical component because it supplies fluid flow for the entire transmission, including the torque converter.
The valve body contains the valves and orifices that control the operation of a transmission. The valve body is typically bolted to the bottom of the transmission housing and is covered by the oil pan. In response to the movement of the valves, fluid is routed to the different hydraulic apply devices. This action causes a change in gears.
Although the pump is the main source of fluid flow, there are many devices involved in the flow and pressurization of the fluid. All of these work together to enable the transmission to shift at the correct time and with the correct feel.
Transmissions have at least two shafts, an input shaft and an output shaft. The input shaft connects the output of the torque converter to the driving members inside the transmission. Each end of the input shaft is externally splined to fit into the internal splines of the torque converter’s turbine and the driving member in the transmission. Normally, the front clutch pack’s hub is the driving member. In many transmissions there is a tube, called the stator shaft, which surrounds the input shaft. The stator shaft is splined to the torque converter’s stator and is a stationary shaft.
The output shaft connects the driven members of the gearsets to the final drive gearset. The rotational torque and speed of this shaft varies with input speed and the operating gear. The output shaft may be splined to any member of each planetary gearset. For example, in a Simpson gearset, the carrier of the input gearset and the ring gear of the reaction set are splined to the output shaft.
Some transaxles have additional shafts. These shafts are actually a continuation of the input and output shafts. They are placed in parallel where the rotating torque can be easily transferred from one shaft to another. The shafts are divided to keep the transaxle unit compact.
When a component slides over or rotates around another part, the surfaces that contact each other are called bearing surfaces. A gear rotating on a fixed shaft can have more than one bearing surface; it is supported and held in place by the shaft in a radial direction. The gear tends to move along the shaft in an axial direction as it rotates, and is therefore held in place by some other components. The surfaces between the sides of the gear and the other parts are bearing surfaces.
A bearing is a device placed between two bearing surfaces to reduce friction and wear. Most bearings have surfaces that either slide or roll against each other. In automatic transmissions, sliding bearings are used where one or more of the following conditions prevail: low rotating speeds, very large bearing surfaces compared to the surfaces present, and low use. Roller bearings are used in circumstances including high-speed applications, high loads with relatively small bearing surfaces, and high use.
Transmissions use sliding bearings that are composed of a relatively soft bronze alloy. Many are made from steel with the bearing surface bonded or fused to the steel. Those that take radial loads are called bushings and those that take axial loads are called thrust washers (Figure 5-23). The bearing’s surface usually runs against a harder surface, such as steel, to produce minimum friction and heat wear characteristics.
Locations of the various bushings, bearings, and thrust washers in a typical transmission.
Bushings are cylindrically shaped and usually held in place by press fit. Since bushings are made of a soft metal, they act like a bearing and support many of the transmission’s rotating parts. They are also used to precisely guide the movement of various valves in the transmission’s valve body. Bushings can also be used to control fluid flow. Some restrict the flow from one part to another while others are made to direct fluid flow to a particular point or part in the transmission.
Often serving both as a bearing and a spacer, thrust washers are made in various thicknesses. They may have one or more tangs or slots on the inside or outside circumference that mate with the shaft bore to keep them from turning. Some thrust washers are made of nylon or Teflon, which are used when the load is low. Others are fitted with rollers to reduce friction and wear.
Thrust washers normally control free axial movement or endplay. Since some endplay is necessary in all transmissions because of heat expansion, proper endplay is often accomplished through selective thrust washers. These thrust washers are inserted between various parts of the transmission. Whenever endplay is set, it must be set to manufacturer’s specifications. Thrust washers work by filling the gap between two objects and become the primary wear item because they are made of softer materials than the parts they protect. Normally, thrust washers are made of copper-faced soft steel, bronze, nylon, or plastic.
Chapter 5, Extension Housing
Torrington bearings are thrust washers fitted with roller bearings. These thrust bearings are primarily used to limit endplay but also to reduce the friction between two rotating parts. Most often Torrington bearings are used in combination with flat thrust washers to control endplay of a shaft or the gap between a gear and its drum.
The bearing surface is greatly reduced through the use of roller bearings. The simplest roller bearing design leaves enough clearance between the bearing surfaces of two sliding or rotating parts to accept some rollers. Each roller’s two points of contact between the bearing surfaces are so small that friction is greatly reduced. The bearing surface is more like a line than an area.
If the roller length to diameter is about 5:1, or more, the roller is called a needle and such a bearing is called a needle bearing. Sometimes the needles are loose or they can be held in place by a steel cylinder or by rings at each end. Often the latter are drilled to accept pins at the ends of each needle that act as an axle. These small assemblies help prevent the agony of losing one or more loose needles and the delay caused by searching for them.
Many other roller bearings are designed as assemblies. The assemblies consist of an inner and outer race, the rollers, and a cage. Roller bearings are designed for radial loads. Tapered roller bearings are designed to accept both radial and axial loads and are rarely used in automatic transmissions. Ball bearings are constructed similarly to a roller bearing, except that the races are grooved to accept the balls. Ball bearings can withstand heavy radial loads, as well as light axial loads.
5-6c Snap Rings
Many different sizes and types of snap rings are used in today’s transmissions. External and internal snap rings are used as retaining devices throughout the transmission. Internal snap rings are used to hold servo assemblies and clutch assemblies together. In fact, snap rings are also available in several thicknesses and may be used to adjust the clearance in multiple-disc clutches. Some snap rings for clutch packs are waved to smooth clutch application. External snap rings are used to hold gear and clutch assemblies to their shafts.
Early automobiles were driven by belts and ropes around pulleys mounted on the driving wheels and engine shaft or transmission shaft. As there was always some slippage of the belts, one wheel could rotate faster than the other when turning a corner. When belts proved incapable of withstanding greater power and speed, automobile builders borrowed an idea from the bicycle and applied sprockets and chains to the driveline. Although this provided more strength, it also provided no slippage. Soon it became apparent that some sort of differential gearing to be developed to permit one wheel to turn faster than the other.
The last set of gears in the drivetrain is the final drive. In most RWD cars, the final drive is located in the rear axle housing. Most FWD vehicles have the final drive located within the transaxle. Some FWD cars with longitudinally mounted engines locate the differential and final drive in a separate case that bolts to the transmission.
A transaxle’s final drive gears provide a way to transmit the transmission’s output to the differential section of the transaxle. There are four common configurations used as the final drives on FWD vehicles: helical gear, planetary gear, hypoid gear, and chain drive. The helical, planetary, and chain final drive arrangements are found with transversely mounted engines. Hypoid final drive gear assemblies are normally found in vehicles with a longitudinally placed engine.
RWD final drives normally use a hypoid gearset that turns the power flow 90 degrees from the driveshaft to the drive axles. On FWD cars with a transversely mounted engine, the power flow axis is naturally parallel to that of the drive axles; therefore the power doesn’t need to be turned. Simple gear connections can be made to connect the output of the transmission to the final drive to the drive wheels.
A hypoid assembly in a transaxle is basically the same unit as would be used on RWD vehicles and is mounted directly to the transmission. The drive pinion gear is connected to the transmission’s output shaft and the ring gear is attached to the differential case. The pinion and ring gearset provides for a multiplication of torque.
The teeth of the ring gear usually meshes directly with a gear on the transmission’s output shaft. However on some transaxles, an intermediate shaft is used to connect the transmission’s output to the ring gear.
The differential case rotates with the final drive output gear and is supported by tapered roller bearings on each side. The differential case contains four bevel gears: two pinion gears and two side gears. The pinion gears are installed on a pinion shaft that is retained by a pin in the differential case. The pinion gears transmit power from the case to the side gears. Each side gear is splined to one of the halfshafts that transmit power to the front wheels.
Helical gears are gears with teeth that are cut at an angle or are spiral to the gear’s axis of rotation. Some transaxles route power from the transmission’s output shaft through two helical-cut gears to a transfer shaft (Figure 5-24). A helical-cut pinion gear attached to the opposite end of the transfer shaft drives the differential ring gear and carrier (Figure 5-25). The differential assembly then drives the axles and wheels.
Some transaxles use a transfer shaft and gear to move the output to the final drive unit.
A helical gear differential for a transaxle.
Helical final drive gearsets require that the centerline of the pinion gear is at the centerline of the ring gear. The pinion gear is cast as part of the main shaft and is supported by tapered-roller bearings. The pinion gear is meshed with the ring gear to provide the required torque multiplication. Because the ring is mounted on the differential case, the case rotates in response to the pinion gear.
Rather than use helical-cut gears in the final drive assembly, many transaxles use a simple planetary gear (Figure 5-26). The sun gear of this planetary unit is driven by the output shaft of the transaxle. The final drive sun gear meshes with the final drive planetary pinion gears, which rotate on their shafts in the planetary carrier. The planetary carrier is part of the differential case, which contains typical differential gearing: two pinion gears and two side gears.
A final drive unit that utilizes a planetary gearset.
The final drive pinion gears mesh with the ring gear, which has lugs around its outside diameter. These lugs fit into grooves machined inside the transaxle housing. The lugs and grooves hold the ring gear stationary. The final drive pinion gears walk around the inside of the stationary ring gear and drive the planetary carrier and differential case. This combination provides maximum torque multiplication from a simple planetary gearset.
When the ring gear is held and input is sent to the sun gear, forward gear reduction takes place. This gear reduction is the final drive gear ratio.
Chain-drive final drive assemblies use a multiple-link chain to connect a drive sprocket, connected to the transmission’s output shaft, to a driven sprocket (Figure 5-27), which is connected to the differential case. This design allows for remote positioning of the differential within the transaxle housing. Final drive gear ratios are determined by the size of the driven sprocket compared to the drive sprocket. The driven sprocket is attached to the differential case, which provides differential action for the drive wheels.
The chain setup for a final drive unit in a transaxle.
A gasket is typically used to seal the space between two parts that have irregular surfaces.
A hard gasketis one that will compress less than 20 percent when it is tightened in place.
Gaskets are used to seal two parts together or to provide a passage for fluid flow from one part of the transmission to another (Figure 5-29). Gaskets are easily divided into two separate groups, hard and soft, depending on their application. Hard gaskets are used whenever the surfaces to be sealed are smooth. This type of gasket is usually made of paper. A common application of a hard gasket is the gasket used to seal the valve body and oil pump against the transmission case. Hard gaskets are also often used to direct fluid flow or seal off some passages between the valve body and the separator plate.
A typical application of a gasket in an automatic transmission.
A soft gasket is one that will compress more than 20 percent when it is tightened in place.
A composition gasket is one that is made from two or more different materials.
RTV stands for room temperature vulcanizing, which means this sealant will begin to solidify at room temperature and form a seal prior to being heated by the operation of the transmission.
Gaskets that are used when the sealing surfaces are irregular or in places where the surface may distort when the component is tightened into place are called soft gaskets. A typical location of a soft gasket is the oil pan gasket, which seals the oil pan to the transmission case. Oil pan gaskets are typically a composition gasket made with rubber and cork. However, some transmissions use a RTV sealant instead of a gasket to seal the oil pan.
Chapter 5, Seals
A static sealprevents fluid from passing between two or more parts that are always in the same relationship with each other.
A dynamic seal is one that prevents leaks between two or more parts that do not have a fixed position and move in relation to each other.
Because valves and transmission shafts move within the transmission, it is essential that the fluid and pressure be contained within its bore. Any leakage would decrease the pressure and result in poor transmission operation. Seals are used to prevent leakage around valves, shafts, and other moving parts. Rubber, metal, or Teflon materials are used throughout a transmission to provide for static seals and dynamic seals. Both static and dynamic seals can provide for positive and nonpositive sealing. A definition of each of the different basic classifications of seals follows.
Three major types of rubber seals are used in automatic transmissions: the O-ring, the lip seal, and the lathe-cut seal or square-cut seal (Figure 5-30). Rubber seals are made from synthetic rubber rather than natural rubber.
Three types of seals shown in their typical position and mountings.
Synthetic rubber is made from neoprene, nitrile, silicone, fluoroelastomers, and polyacrylics.
O-rings are round seals with a circular cross section. Normally an O-ring is installed in a groove cut into the inside diameter of one of the parts to be sealed. When the other part is inserted into the bore and through the O-ring, the O-ring is compressed between the inner part and the groove. This pressure distorts the O-ring and forms a tight seal between the two parts (Figure 5-31).
A typical application of an O-ring seal.
O-rings can be used as dynamic seals but are most commonly used as static seals. An O-ring can be used as a dynamic seal when the parts have relatively low amounts of axial movement. If there is a considerable amount of axial movement, the O-ring will quickly be damaged as it rolls within its groove. O-rings are never used to seal a shaft or part that has rotational movement.
Lip seals are used to seal parts that have axial or rotational movement. They are round in order to fit around a shaft and into a bore. The primary sealing part is a flexible lip (Figure 5-32). The flexible lip is normally made of synthetic rubber and shaped so that it is flexed when it is installed to apply pressure at the sharp edge of the lip. Lip seals are used around input and output shafts to keep fluid in the housing and dirt out. Some seals are double-lipped.
Sealing action of a lip seal.
When the lip is around the outside diameter of the seal, it is used as a piston seal (Figure 5-33). Piston seals are designed to seal against high pressures and the seal is positioned so that the lip faces the source of the pressurized fluid. The lip is pressed firmly against the cylinder wall; as the fluid pushes against the lip a tight seal is formed. The lip then relaxes its seal when the pressure on it is reduced or exhausted.
A typical application of a lip seal.
The word toroidal infers that the spring is doughnut shaped.
A square-cut seal is often called a lathe-cut seal.
Lip seals are also commonly used as shaft seals. When used to seal a rotating shaft, the lip of the seal is around the inside diameter of the seal and the outer diameter is bonded to the inside of a metal housing. The outer metal housing is pressed into a bore. To help maintain good sealing pressure on the rotating shaft, a garter spring is fitted behind the lip. This toroidal spring pushes on the lip to provide for uniform contact on the shaft. Shaft seals are not designed to contain pressurized fluid; rather, they are designed to prevent fluid from leaking over the shaft and out of the housing. The tension of the spring and of the lip is designed to allow an oil film of about 0.0001 of an inch. This oil film serves as a lubricant for the lip. If the tolerances increase, fluid will be able to leak past the shaft, and if the tolerances are too small, excessive shaft and seal wear will result.
A square-cut seal is similar to an O-ring; however, a square-cut seal can withstand more axial movement than an O-ring can. Square-cut seals are also round seals but have a rectangular or square cross section. They are designed this way to prevent the seal from rolling in its groove when there are large amounts of axial movement. Added sealing comes from the distortion of the seal during axial movement. As the shaft inside the seal moves, the outer edge of the seal moves more than the inner edge, causing the diameter of the sealing edge to increase, which creates a tighter seal (Figure 5-34).
Sealing action of a rubber seal as a piston moves in its bore.
Metal seals are often called steel rings.
There are some parts of the transmission that do not require a positive seal and where some leakage is acceptable. These components are sealed with ring seals, which fit into a groove on a shaft (Figure 5-35). The outside diameter of the ring seals slide against the walls of the bore that the shaft is inserted into. Most ring seals in a transmission are placed near pressurized fluid outlets on rotating shafts to help retain pressure. Ring seals are made of cast iron, nylon, or Teflon.
Metal sealing rings are fit into grooves on a shaft.
Hook-end seals are also referred to as locking-end seals.
Three types of metal seals are used in automatic transmissions: butt-end seals, open-end seals, and hook-end seals. In appearance, butt-end and open-end seals are much the same; however, when an open-end seal is installed, there is a gap between the ends of the seal. When a butt-end seal is installed, the square-cut ends of the seal touch or butt against each other. Hook-end seals (Figure 5-36) have small hooks at their ends, which are locked together during installation to provide better sealing than the open-end or butt-end seals provide.
Hook-end sealing rings.
Some transmissions use Teflon seals instead of metal seals. Teflon provides for a softer sealing surface, which results in less wear on the surface that it rides on and therefore a longer-lasting seal. Teflon seals are similar in appearance to metal seals except for the hook-end type. The ends of locking-end Teflon seals are cut at an angle (Figure 5-37) and the locking hooks are somewhat staggered.
Scarf-cut seals; notice that the ends of the seal are cut at opposing angles.
Teflon locking-end seals are normally called scarf-cut rings.
Many late-model transmissions are equipped with solid one-piece Teflon seals. Although the one-piece seal requires some special tools for installation, it provides for a nearly positive seal. These Teflon rings seal much better than other metal sealing rings.
GM uses a different type of synthetic seal on some late-model transmissions. The material used in these seals is Vespel, which is a flexible but highly durable plastic-like material. Vespel seals are found on 4T60-E and 4T80-E transaxles.
The coding used to identify a GM transmission model typically includes what kind of information?
Transmission housings are cast to secure and accommodate many important components. List at least five of them.
What are the three common designs of compound planetary gearsets. How do they differ?
What are the input and output shafts connected to in a typical transmission?
Describe how the clutch-to-clutch transmission shifts without servos and accumulators.
Why are Torrington bearings used in automatic transmissions?
Why might a chain drive be used in a transaxle’s final drive unit?
An automatic transmission uses specially designed snap rings. What is so special about them?
What is the purpose of a servo?
When a transmission is described as having two planetary gearsets in tandem, what does that mean
Multiple-disc clutches are enclosed in a large drum-shaped housing that can be either a __________ or part of the __________ .
_____________normally control free axial movement or endplay.
In the operation of some automatic transmissions, both _______ and ____________ overrunning clutches are used to hold members of the planetary gearset.
The four common configurations used as the final drives on FWD vehicles are the __________gear, _________ gear, ___________ gear,_______ and .
Planetary _________ gears are small gears fitted into a framework called the planetary carrier.
Certain parts of the must be held while others must be driven to provide the needed torque multiplication and direction for vehicle operation.
The three major types of rubber seals used in automatic transmissions are the ___________ , the __________, ___________and the seal.
The three types of metal seals used in automatic transmissions are the __________, __________ ,_______ and seals.
Transmission gaskets are made of ______,________ ,_________ ,__________ ,______ or__________ .
A Ravigneaux gearset has two independent ___________ gears, two sets of gears, and a common gear.
Which of the following is used in all automatic transmissions?
All of the above
Overrunning clutches are capable of .
Braking a member of a planetary gearset
Driving a planetary gear member
Both A and B
Neither A nor B
While discussing current manufacturer transmission coding:
Technician A says Ford model numbers do not indicate the duty rating of the transmission. Technician B says Chrysler model numbers indicate the number of planetary units used in the transmission.
Who is correct?
Both A and B
Neither A nor B
Technician A says piston seals are designed to seal against high pressures. Technician B says the seal is positioned so that the lip faces the source of the pressurized fluid.
Who is correct?
Both A and B
Neither A nor B
Technician A says a multiple-disc clutch can be used to drive a planetary member.Technician B says a multiple-disc-clutch can be used to brake or hold a planetary member.
Who is correct?
Both A and B
Neither A nor B
Technician A says a nonpositive static seal will allow some fluid leakage between two parts that do not move in relationship to each other.
Technician B says a positive dynamic seal allows a controlled amount of leakage between two parts that move in relationship to each other.
Who is correct?
Both A and B
Neither A nor B
Which of the following statements is not true?
A torque converter uses hydraulic fluid flow to connect and disconnect the power of the engine to and from the transmission.
The transmission pump is driven by the input shaft.
The valve body contains the valves and orifices that control the operation of a transmission.
The transmission pump is the main source of fluid flow in a transmission.
Technician A says the output shaft connects the output of a driving member to the final drive unit. Technician B says the input shaft connects the impeller of the torque converter to the front driven member in the transmission. Who is correct?
Both A and B
Neither A nor B
Technician A says engine/transaxle mounts isolate the inherent vibrations of the assembly from the passenger compartment. Technician B says improper mounting of an engine and/or transmission can cause problems with the transmission. Who is correct?
Both A and B
Neither A nor B
Technician A says a Simpson gearset is two planetary gearsets that share a common sun gear .Technician B says a Ravigneaux gearset has two sun gears, two sets of planet gears, and a common ring gear.
Who is correct?
Both A and B
Neither A nor B