Transmission Or Transaxle – Lesson 3 | Manual Transmission and Drivetrain
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Manual Transmission and Drivetrain : Lesson 3 – Transmission Or Transaxl
At a glance
Manual transmissions and transaxles
Typical manual transmission
The manual transmission is a vital link in the drivetrain of many modern vehicles. The manual transmission uses different size gears to give the engine a mechanical advantage over the driving wheels. Without this mechanical advantage, an engine can generate only limited torque at low speeds. Without enough torque, moving a vehicle from a standing start would be impossible.
During normal operating conditions, power from the engine is transferred through the engaged clutch to the transmission input shaft. The input shaft transfers this power to gears in the transmission, which alter its torque and speed and then send it to the rest of the drivetrain.
Manual transmissions are used on rear-wheel and four-wheel drive vehicles. On front-wheel drive vehicles, manual transaxles are used. Manual transaxles basically perform the same function as manual transmissions. However, transaxles also have one additional function: they contain the differential which provides final gear reduction and differential action for the wheels. The differential unit is mounted inside the transaxle housing.
The differential receives torque directly from the output gear of the transmission components inside the transaxle. It then uses a gear ratio to multiply this torque and transmits it to the wheels via half shafts.
Purpose of gears
The purpose of the gears inside a transmission or transaxle is to transmit rotating motion. Gears are normally mounted on a shaft and they transmit rotating
motion from one shaft to another. Gears and shafts act upon each other in one of three ways:
! The shaft can drive the gear.
! The gear can drive the shaft.
! The gear can be free to turn on the shaft.
Sets of gears can be used to multiply torque and decrease speed, decrease torque and increase speed, transfer torque and leave the speed the same, or change the direction of torque.
Gear rotation
Gear rotation inside a manual transmission or transaxle must be understood when performing diagnosis and repair.
A basic rule that applies to gears is that two external gears in mesh rotate in opposite directions.
This means that an engine that is driving a gear clockwise will cause any gear in mesh with this gear to rotate counterclockwise. To get that driven gear to Another basic gear rule is that when the third gear is added, the output from the gearset is in the turn the wheels in a clockwise direction, a third gear must be added.
Gears in mesh 1:1 ratio
- Clockwise rotating gear
- Counterclockwise rotating gear
Two gears connected by an idler gear
- Input gear rotating clockwise
- Idler gear rotating counterclockwise
- Output gear rotating clockwise
Gear design
There are many types of gears, and each has its own operating characteristics. Common gears that are found in manual transmissions and transaxles include:
! Spur gears
! Helical gears
! Spur bevel gears
Spur gears
The spur gear is the simplest gear design used in manual transmission/transaxles.
! Its main advantage is that its teeth are cut straight so it can slide in and out of contact with other gears.
! Its main disadvantage is that it is noisy during operation. Spur gears whine at high speed.
! If a spur gear is found in a manual transmission/ transaxle it is usually only used for reverse gear.
Helical gears are the most common types of gears used in manual transmission and transaxles. These gears are cut at an angle to the gear’s axis of rotation. This allows two or more teeth to be in full contact at all times during operation.
! The main advantage of helical gears is that they operate much more quietly and are much stronger than spur gears.
! The main disadvantage of helical gears is that they cannot be slid into and out of contact with their adjoining gears. They must maintain contact at all times. Helical gears are sometimes referred to as constant-mesh gears.
! Helical gears are used for all forward speed gears, and in some cases for reverse as well. Basic helical gears
Spur bevel gears
Spur bevel gears allow a gear to rotate on an axis that is 90 degrees offset from the gear which it contacts.
! Spur bevel gears are only used as pinion gears and side gears in the differential assembly of a manual transaxle.
Gear ratios
The ancient Greek engineer Archimedes once said, “Give me a lever long enough, and a place to put it, and I can lift the world.” This statement refers to the ability of a lever to multiply force. Transmission gears are basically a set of levers arranged in a circle. Transmission gears multiply the force by the differences in size and number of teeth in the gear. A gear ratio is a term that describes the differences in the number of teeth on the gears in mesh.
For example:
! Two gears are both the same size and have the same number of teeth.
! Each time the driving gear makes a complete rotation, so does the driven gear.
! Both gears are turning at the same speed, and because they are the same size and have the same number of teeth, they are turning with the same amount of torque.
! The only difference between the gears is that they are rotating in opposite directions.
! Gears the same size with the same number of teeth are considered a one-to-one gear ratio because the driving gear is rotating one time for each rotation of the driven gear.
! Gear ratios are usually written with a colon between them, so one to one would be written 1:1.
Reduction gear ratio
When the driving gear is smaller than the driving gear, the driving gear is turning slower than the driving gear, therefore the gears are in reduction. This is a reduction in the driven gear’s speed, which multiplies torque.
! The smaller gear has 12 teeth and is driving the larger gear that has 24 teeth.
! The 12-tooth driving gear is turning with 10 pounds of torque. But the 12-tooth gear rotates
! twice for every rotation of the 24-tooth-driven gear.
This causes the driven gear to have twice as much torque in every rotation. The driven gear now turns with 20 pounds of torque.
Gears in reduction
1 Driving gear
2 Driven gear
! This is a reduction gear ratio of 2:1.
Gear ratios (continued)
An example of the gear ratios in a manual transmission would be: ! Reverse = 3.40:1
! 1st gear = 3.97:1
! 2nd gear = 2.34:1
! 3rd gear = 1.46:1
! 4th gear = 1:1
! 5th gear = .79:1
As you can see, Reverse and 1st through 3rd gears are reduction gears. 4th gear is 1:1 meaning that the driving and driven gears have the same number of teeth and are rotating at the same speed. This is called direct drive.
If an engine that produces 407 Nm (300 lb-ft) of torque is connected to a drivetrain that has a 10:1 gear ratio, the result is that 4,070 Nm (3,000 lb-ft) of torque is applied at the wheels, which is the amount of power needed to move the 1,360 kg (3,000 lb) vehicle.
However, there is a drawback to reduction gear ratios. The driving gear must turn many more times than the driven gear. So an engine that is operating at 6,000 revolutions per minute (rpm) will only turn a drivetrain with a 10:1 gear ratio at 600 rpm.
Because of centrifugal force, once a vehicle begins moving, it does not require as much power to maintain its speed as it did to get it moving. Because of this force gear ratios can be changed to allow increased rotation speed.
Overdrive gear ratio
Overdrive gear ratio
Any time the driving gear is rotating slower than the driven gear it is called an overdrive gear ratio.
Overdrive ratios allow the drivetrain to actually turn faster than the engine because at high speed very little torque is needed to keep the vehicle moving. Since overdrive ratios allow the engine to operate at lower rpm they provide better fuel economy.
To determine the total gear ratio of the entire drivetrain, all that has to be done is to multiply the ratio of the specific transmission gear by the ratio of the differential. For example, assume you have a differential with a 3.78:1 ratio. To determine the actual gear ratio that is being used in any specific gear, just multiply that gear ratio by 3.78. If the transmission 1st gear has a 3.97:1 ratio, multiply it by the differential ratio of 3.78:1 and you find that the total gear reduction from the engine to the wheels is 15.01:1. So the torque of the engine is multiplied 15.01 times by the drivetrain.
Gears in overdrive
- Driving gear
- Driven gear
Powerflow
Basic manual transmission operation
To understand how modern transmissions work, we should first look at the operation of a basic 3-speed transmission. In this section, we will build up a simple set of gears to see how a basic 3-speed transmission works.
Powerflow
The path that power follows from the input shaft to the output shaft in a manual transmission is called power flow. Understanding power flow is essential for the diagnosis of manual transmission concerns.
Although the power flow through some transmissions may be slightly different because of differences in all manual transmissions power flow is very similar.
On the typical manual transmission, the input shaft is powered through the clutch and drives the countershaft (layshaft). The countershaft (layshaft) then transfers the power to the gear engaged to the output shaft by the synchronizer.
Gear reduction
To get a manual transmission into 1st gear takes the use of four gears and three shafts.
! A small gear on the input shaft from the engine drives a larger gear fastened to the transmission countershaft.
! Another smaller gear fastened to the countershaft drives a large gear on the third shaft, which is the output shaft.
Looking at the size of the gears you can see that there is gear reduction between the input shaft gear and the countershaft input gear. Additionally, there is more gear reduction between the countershaft 1st gear and the output shaft 1st-speed gear.
Note that the input shaft and the output shaft are turning in the same direction because the countershaft acts as an idler gear between them.
Basic transmission in first gear
- Input shaft
- 1st speed gear
- Output shaft
- Countershaft
Powerflow
Powerflow (continued)
Direct drive
3rd gear in our basic transmission is direct drive. In direct drive no gear reduction takes place.
! The input shaft is mechanically connected directly to the output shaft.
! Each rotation of the input shaft results in a rotation of the output shaft, giving it a 1:1 ratio.
Basic transmission in third gear (direct drive)
- Input shaft
- Output shaft
Powerflow
Reverse
To achieve reverse in a manual transmission requires the use of additional gear and shaft. This gear is commonly known as the reverse idler gear. In some transmissions, the reverse idler gear actually slides in and out of contact with its adjoining gears. In other transmissions, it is a helical gear that is constantly in the mesh.
! In reverse, power still enters the transmission through the input shaft and is transferred to the countershaft input gear.
! However, the countershaft reverse gear and the output shaft reverse gear are not in direct contact.
! In order for the countershaft reverse gear to transmit rotation to the output shaft reverse gear, the reverse idler gear meshes with the teeth on both gears.
! The normal rotation of the output shaft is reversed, allowing it to rotate in reverse.
- Note that the countershaft reverse gear is smaller than the reverse speed gear on the output shaft, providing a reduction gear ratio to multiply power in reverse. This gear reduction is needed since reverse can only be engaged from a standing stop.
Basic transmission in reverse
- input shaft
- Reverse speed gear
- Output shaft
- Reverse idler gear
- Countershaft
Manual transmission components
Although the operation of a manual transmission is very straightforward, many different components are needed to make its operation practical.
Synchronizers and speed gears
In a transmission, gears are named for the speed they are used with. For example, the gear that is used for 1st gear is called the 1st speed gear. All forward gears in modern transmissions are helical gears. Helical gears are quiet and have additional strength. However, because the teeth of helical gears are angled, they cannot be slid into and out of engagement with each other. For this reason, the speed gears are not directly splined to the shaft upon which they ride.
The inner diameter of the speed gears is smooth, allowing them to rotate freely on the shaft. When the gear needs to be connected to the shaft, the synchronizer sleeve moves over and engages the clutching teeth on the side of the gear.
! The synchronizer sleeve is locked to the speed gear.
! The synchronizer sleeve inner diameter has internal teeth that slide along the external teeth of the synchronizer hub outer diameter.
! The synchronizer hub is splined to the shaft’s inner diameter.
The speed gear is connected to the output shaft through the synchronizer, allowing the torque of the gear to be transmitted.
In most instances, each synchronizer works with two-speed gears because its sleeve can slide both forward and rearward.
For this reason, synchronizers will be named for the gears they control. For example, the 1-2 synchronizer works on both 1st and 2nd-speed gears.
Basic synchronizer and speed gear
- Synchronizer hub
- Synchronizer sleeve
- Blocking ring
- Clutching teeth
- Speed gear
Synchronizing gear and shaft speed
Another function of the synchronizer is to make the speed of the speed gears match that of their shaft before the gear is locked to the shaft. The rotating speed of the gear is different than the speed of the shaft. If the speed of the gear and shaft isn’t the same before the synchronizer sleeve engages the gear’s clutching teeth, both the sleeve and the clutching teeth could be damaged.
When a gear is selected, the shift fork forces the synchronizer sleeve toward the speed gear.
! A blocking ring, which has a cone-shaped inner surface, is pushed into contact with the cone-shaped shoulder of the speed gear.
! As the synchronizer sleeve continues to move, it compresses the inserts against the retaining springs.
Synchronizer operation
- Blocking ring
- Driven gear
- Synchronizer sleeve
! As it moves further, the sleeve splines mate with teeth on the blocking ring.
! Friction between the blocking ring and gear shoulder causes the gear, which is rotating freely on the shaft, to speed up or slow down to the same speed as the synchronizer.
Manual transmission components (continued)
Synchronizing gear and shaft speed (continued)
The blocking ring prevents the sleeve’s splines from engaging the gear’s clutching teeth until they are all rotating at the same speed.
! When the blocking ring (which is connected to the synchronizer) and the speed gear teeth are lined up, the synchronizer sleeve can slide over the gear’s clutching teeth, locking the gear to the shaft.
! As this happens, the compressed inserts move into a notch on the inner diameter of the sleeve, helping to hold the sleeve in place.
Synchronizer operation
- Point where sleeve and gear mesh
- Driven gear
- Blocking ring
- Synchronizer sleeve
Shift mechanisms
The transmission is shifted by means of shift mechanisms. Common components of the shift mechanisms include:
! Shift forks
! Shift rails
! Interlock plates
! Detents
Typical shift mechanism – exploded view
- Selector arm
- Interlock plate
- Shift rail
- 1-2 shift fork
- Inserts
- Selector arm plates
- 3-4 Shift fork
- Inserts
- Shift cover
Shift forks and shift rails
The transmission is shifted by means of shift forks that fit into a groove cut into the center of the synchronizer sleeve. The forks ride on shift rails that are moved by the driver using the gearshift. When the driver moves the gearshift, the selector shaft will move, causing the shift fork to move the synchronizer sleeve and engage the speed gear.
The shift forks usually have inserts on their tips that fit snugly into the synchronizer sleeve and prevent shift fork wear.
Shift fork and shift rail
- Shift rail
- Shift fork
- Shift fork inserts
Interlocks and detents
To prevent transmission damage, the shift mechanism uses interlocks. These devices can be connected to the selector shafts or the shift cover. Interlocks are designed to prevent the transmission from being shifted into more than one gear at a time.
Interlock plate
- Interlock plate
- Shift cover
Detents
Detents are used to hold the shift forks in position once a gear has been selected. Detents are usually a ball and spring design and can either fit into notches on the selector shafts or in a lever called the offset lever. Once the shift fork is moved, the spring of the detent forces the ball into a notch on the selector lever or shift rail, locking the shift fork in position.
Typical detents
- Detent spring
- Detent ball
- Detent plate
- Offset lever
Countershaft
The countershaft is made up of a series of gears that can be machined from one piece of hardened steel or may have a series of individual gears that are splined to a single shaft. Some countershafts do have synchronizers and smooth inner diameter gears as well as splined gears
Typical countershaft
Bearings
Typical transmission bearings
- Gear bearing
- Gear bearing
- Output shaft support bearing
All transmission shafts and many of the gears ride on bearings. These can be caged needle bearings, ball bearings, or tapered roller bearings. These bearings are designed to allow free rotation while providing the support necessary for the component. Many of the bearings inside a transmission require special service tools for removal and installation.
Housing
The shafts and gears of the transmission are contained in housing. The parts of the housing include the transmission case, the extension housing, and the top cover. The parts are bolted together with gaskets and seals, providing a leak-proof seal at all joints. The housing is filled with transmission fluid to provide constant lubrication and cooling for the spinning gears and shafts.
Typical transmission housing assembly
- Cover gasket
- Cover plate
- Transmission case
- Front bearing retainer
- Gasket
- Fill plug
- Gasket
- Extension housing
- Extension housing seal
Pump
Some new manual transmissions use a pump to improve cooling and lubrication of the internal components. Most pumps used in the manual transmissions are geroter-type pumps and are driven by the countershaft.
Oil cooler
Another new system found on some vehicles is the oil cooler. This oil cooler is part of the radiator and uses the engine coolant to also cool the transmission oil. The transmission pump sends the hot oil to the cooler through oil tubes. In the radiator, it is cooled using the engine coolant. The oil then returns to the transmission.
Typical oil cooler
- Oil cooler inlet
- Oil cooler outlet
- Oil tubes
Operation
Manual transaxle operation
The manual transaxle can basically be divided into two sections: the transmission section and the differential section. The transmission section uses the same types of components as a manual transmission. Shift forks, synchronizers, and gears are basically the same design. But there is one major difference between a manual transaxle and a manual transmission: there is no countershaft.
The countershaft is not needed because rotation from the input shaft and the main shaft (sometimes called the output shaft) is transferred to the differential, which then sends the torque to the wheels in the same direction of rotation as the input shaft (except when the vehicle is in reverse).
Manual transaxle
- Clutch assembly
- Differential assembly
- Output shaft
- Input shaft
Manual transaxle components
Although the operation of a manual transaxle is similar to the transmission, many different components are needed to make it operate.
Differential
The differential of the transaxle transfers power to the wheels, provides a final gear ratio, and enables the wheels to rotate at different speeds during turns. This is done using four small gears mounted inside a carrier, which in turn is driven by the output shaft output gear through a ring gear.
! Inside the carrier is two side gears. These gears are splined to the half shafts.
! Between the side gears, and providing connections between them are two pinion gears, which ride on a shaft that is supported by the carrier.
! The side gears are only connected to the carrier through the pinion gears. Therefore it is the pinion gears that actually drive them.
Basic differential components
- Output pinion
- Side gears
- Pinion gears
- Carrier
- Pinion shaft
- Ring gear
- Half shafts
Notes
Input and output shaft assemblies
Typical manual transaxle input and output shaft
- Reverse gear idler
- Reverse gear
- Fifth gear
- Fourth speed gear
- 3/4 synchronizer
- Third speed gear
- Second gear
- First gear
- Input shaft
- Output shaft
- Output pinion gear
- First speed gear
- 1/2 synchronizer
- Second speed gear
- Third gear
- Fourth gear
- Gear wheel (fifth speed gear)
- Fifth/reverse gear synchronizer
- Reverse speed gear
Input Shaft
The input shaft of the transaxle transfers crankshaft rotation to the output shaft assembly. Along the input shaft length are the drive gears for all the different gear ratios. Some of these gears are cut directly from the input shaft. Other gears with synchronizers just ride on the input shaft the same way the speed gears of a manual transmission ride on the output shaft.
On the input shaft, 1st, 2nd, and reverse gears are cut into the shaft itself. 3rd and 4th gears ride on the shaft and, during operation, must be locked to it through a synchronizer. 5th gear is a separate gear, but it is splined directly to the shaft.
Output shaft
The output shaft of the transaxle transfers power to the differential at the selected gear ratio. In the previous illustration of the output shaft, the 1st, 2nd, 5th, and reverse driven gears ride on the shaft and are engaged through the first/second or fifth/reverse synchronizer respectively. 3rd and 4th driven gears are cut into the output shaft. Additionally, the differential output gear is a separate gear but it is splined directly to the shaft.
Reverse idler gear
Constant-mesh reverse idler
- Reverse gear idler shaft mount
- Thrust washer
- Reverse idler gear
There are two basic types of reverse idler gears used in transaxles: constant mesh and spur-cut. The constant mesh is basically the same as those in manual transmissions, while the spur-cut is a sliding type that is engaged by a shift fork.
- Needle roller bearing
- Thrust washer
- Reverse idler shaft
Constant-mesh
The constant mesh is a synchronized, helical-type reverse idler gear.
! It is in constant mesh with both the reverse driving gear on the input shaft and the driven gear on the output shaft.
! Reverse is engaged when the fifth/reverse synchronizer engages the reverse-driven gear with the output shaft.
Spur-cut
moved by a shift fork along its own shaft.
! When engaged, the idler gear meshes between the spur-cut reverse drive gear on the input shaft and the spur-cut reverse driven gear on the outside diameter of the first/second synchronizer sleeve.
! This action reverses the rotation of the output shaft and differential and drives the vehicle in reverse.
Reverse idler gear
- Reverse idler gear
- Reverse idler gear shaft
Reverse synchronizer
Reverse engagement is synchronized by the fifth/ reverse synchronizer.
! When reverse is selected by the driver, the fifth gear synchronizer hub is pressed against the reverse blocking ring, which is pressed against the reverse blocking ring retainer.
! This retainer is connected to the input shaft.
! As the blocking ring cone surface engages the cone surface of the retainer, it stops rotation of the input shaft and allows the smooth meshing of the reverse idler gear and the reverse speed gear.
Typical reverse synchronizer
- Input shaft
- Blocking ring retainer
- Shift fork, fifth/reverse
- Fifth-speed cluster gear
- Fifth/reverse synchronizer
- Reverse blocking ring
Shift linkages
Because of the location of the transaxle, a shift linkage must be used between the transaxle and the shift lever inside the vehicle. There are two basic types of linkages.
Rod and clevis linkage
Rod and clevis linkage uses a rod that connects to the shift rails inside the transaxle. This rod and clevis allow movement of the shift rails, which in turn moves the shift forks inside the transaxle. This type of linkage also uses a stabilizer bar between the gearshift assembly and transaxle. The rod connects to the shift lever through support bushings.
Rod and clevis linkage and transaxle
- Stabilizer bar
- Shifter
- Rod support bushing
- Gearshift rod and clevis
Cable linkage
Some transaxles are equipped with a cable gearshift linkage. Because they are jointly connected to the floor assembly, the selector and shift cables may only be replaced as a pair. The cables on this type of linkage are connected to a selector mechanism on the transaxle. These cables work in combination with each other to allow the driver to select gears.
Cables and shifter
- Gearshift lever
- Shift cable
- Selector cable
- Gear selector lever on the transaxle
- Gearshift lever on the transaxle
- Cable guide
Shift linkages (continued)
Cable linkage selector mechanism
Cable linkage systems require the use of a selector mechanism. The selector and shift cables connect to this mechanism and movements of the mechanism’s selector levers determine the transaxle gear.
Selector mechanism
- Reversing lamp switch
- Selector finger
- Selector finger bracket
- Selector shaft
- Selector gate
- Selector mechanism housing
- Selector lever
- Gearshift lever
- Selector mechanism cover
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