Learn Automotive Transmissions and Transaxles Explained – Transmission vs Transaxle, Gear Types (Spur, Helical, Idler), Synchronizers, Gearshift Mechanisms, Power Flow (1st–5th, Reverse), Final Drive Ratios and Electrical Systems (VSS, Reverse Lockout, Shift Blocking)

Understanding Automotive Transmissions and Transaxles

Introduction:

The transmission or transaxle is a vital link in the powertrain of any modern vehicle. Its purpose is to use gears of various sizes to give the engine a mechanical advantage over the driving wheels. During normal operation, power from the engine transfers through the engaged clutch to the input shaft of the transmission or transaxle. Gears inside the housing alter torque and speed before passing power to other drivetrain components. Without the mechanical advantage that gearing provides, an engine generates only limited torque at low speeds, making it impossible to move a vehicle from a standing start. The crankshaft always rotates in the same direction, if the engine transmitted power directly to the drive axles, the wheels could only be driven in one direction. The transmission or transaxle provides gearing to reverse direction for backward movement. A neutral position stops power from reaching the drive wheels.

Transmission Versus Transaxle:

Vehicles are propelled by rear wheels, front wheels or both. The drive system determines whether a conventional transmission or a transaxle is used. Rear wheel drive (RWD) vehicles normally use a transmission, with gearing housed in a cast metal case attached to the rear of the engine. A drive shaft links the transmission output shaft to the differential and drive axles in a separate rear housing. The differential splits power and redirects it to the two rear axles. For many years, RWD was the conventional method. Front wheel drive (FWD) vehicles require a different design. The transaxle combines transmission gearing, differential, and drive axle connections into a single case located in front. Advantages include good traction on slippery roads because drivetrain weight is directly over the driving axles. It is also more compact and lighter than a RWD transmission. Transverse engine and transaxle configurations allow lower hood lines, improving aerodynamics.

Four‑wheel‑drive vehicles typically use a transmission and a transfer case mounted on the side or back of the transmission. A chain or gear drive inside the transfer case receives power and splits it to two separate drive shafts one to the front differential and one to the rear. Most manual transmissions and transaxles are constant‑mesh, fully synchronized units. Constant mesh means that whether or not a gear is locked to the output shaft, it remains in mesh with its counter gear. All gears rotate except reverse as long as the clutch is engaged. Fully synchronized means the unit uses rings and clutches to bring rotating shafts and gears to the same speed before shifts occur promoting smooth shifting. In a four speed unit, all forward gears are synchronized, reverse may or may not be synchronized.

Transmission Designs:

All automotive transmissions/transaxles have multiple forward speed gears, neutral, and one reverse. Five speed units are now common. Some early five‑speeds were four speeds with an add on overdrive gear. Overdrive reduces engine speed at a given vehicle speed, increasing top speed, improving fuel economy and lowering engine noise. Most late model five speeds incorporate fifth gear in the main assembly, as do six speed units. Fifth and sixth gears typically provide two overdrive gears, allowing manufacturers to use lower final drive gears for acceleration while reducing engine speed during highway operation.

Self Shifting Manual Transmissions:

Self shifting manual transmissions are available on some passenger cars and used in Formula One. They work like typical manuals, but electronic or hydraulic actuators shift gears and operate the clutch. The driver shifts using buttons or paddles on the steering wheel; some units also operate in fully automatic mode. These are not automatic transmissions with manual controls. They do not use planetary gear sets or torque converters. They are manual transmissions with a computer controlled actuator connected to shift forks and a clutch actuator. The computer shifts automatically at the correct time and sequence, activating the clutch precisely when the driver selects automatic mode. There is no gearshift linkage or cable, a sensor at the shifter sends a signal to the computer, which commands actuators to engage/disengage clutch and gears with very fast response times. Engine torque is controlled during shifts via throttle or ignition injection for smoothness.

Gears:

Gears in a manual transmission or transaxle transmit rotating motion, normally mounted on shafts and transferring motion between parallel shafts. Shafts and gears interact in three ways: the shaft drives the gear, the gear drives the shaft, or the gear freewheels on the shaft as an idler. Gear sets multiply torque and decrease speed, increase speed and decrease torque or transfer torque with no speed change.

Gear pitch: 

Refers to the number of teeth per unit of pitch diameter. It is found by dividing the number of teeth by the pitch diameter. For example, 36 teeth on a 6 inch pitch diameter gives a pitch of 6. Gears must have the same pitch to mesh: a five pitch gear only meshes with another five pitch and so on.

Spur gears: 

The simplest design, with teeth cut straight across parallel to the shaft. Only one tooth is in full contact at a time. Their straight teeth minimize the chance of popping out of gear, making them ideal for reverse. Their drawback is the clicking noise that becomes a whine at higher speeds.

Helical gears: 

Teeth are cut at an angle, allowing two or more teeth to mesh simultaneously, distributing load and making the gear very strong. They run quieter due to a wiping action during engagement. A disadvantage is axial thrust that moves the gear on the shaft, which must be absorbed by thrust washers and other components. Helical gears are right or left handed. On parallel shafts, one must be right handed and the other left handed. Spur and helical gears with teeth on the outside diameter are external gears; two external gears rotate in opposite directions. If an external gear meshes with an internal gear (teeth on the inside), both rotate in the same direction.

Idler gears: 

Placed between drive and driven gears to transfer motion without changing direction of rotation, because all three have external teeth. Idler gears are used in reverse gear trains to reverse output shaft rotation. In forward gears, input and output shafts turn the same direction; in reverse, they turn opposite, allowing the vehicle to go backward.

Transmission/Transaxle Design:

Internal components consist of parallel shafts with meshing gear sets of different ratios. Moving the shift lever selects ratios for different torque and speed outputs. Gears are mounted internally splined or keyed to a shaft, or manufactured as integral/clustered parts. Gears that need to freewheel are mounted on bushings or bearings. The case housing includes the main body, cover plates, extension housings, and bearing retainers, bolted together with gaskets for leak proof seals. The case is filled with transmission fluid for lubrication and cooling.

Transmission:

Most have three shafts: mainshaft (output shaft), input shaft and countershaft. Speed gears ride on the mainshaft. The input shaft is inline with the mainshaft but not directly connected, it rotates with the clutch and drives the countergear assembly. The countershaft (cluster gear) is several gears machined from one piece, mounted on roller bearings on a fixed countershaft that does not turn. Thrust washers control end play. Main gears (speed gears) on the output shaft transfer rotation from countergears to the output shaft, they freewheel until locked by their synchronizer. Power flows from the input shaft to the clutch gear, which meshes with the large countergear. The cluster rotates, turning the speed gears. No power output occurs until a speed gear is locked to the mainshaft by a shift fork moving its synchronizer. Power then travels through the selected gear to the mainshaft and driveline.

Transaxle:

Transaxles use similar principles but include differential gearing and drive axle connections. They typically have two shafts: input (driving) and output (driven). The input shaft is above and parallel to the output shaft. In transversely mounted transaxles the input shaft is self centering and does not need a pilot bearing. The output shaft's speed gears freewheel unless locked by synchronizers, they are in constant mesh with input shaft drive gears. Naming varies by manufacturer for clarity, this text uses input gear cluster for the input shaft and its drive gears, and pinion shaft for the output shaft. A pinion gear machined on the end of the pinion shaft is in constant mesh with the differential ring gear in the housing. When the pinion rotates, torque flows through the ring gear, differential, and drive shafts to the wheels. Some transaxles have a third shaft to offset power flow, it is added only when an extremely compact unit is required.

Synchronizers:

The synchronizer has multiple jobs. Its main task is to bring components rotating at different speeds to one synchronized speed, ensuring the pinion shaft and speed gear rotate together. It then locks them together, resulting in a clash free shift. In some transmissions, the synchronizer sleeve's spur teeth can act as a reverse gear. In modern units, all forward gears are synchronized. One synchronizer is placed between first and second gears, another between third and fourth, and a fifth gear also has one. Reverse normally lacks a synchronizer because it is selected at a stop.

The most common is the block or cone synchronizer. The sleeve surrounds the assembly and meshes with the clutch hub's external splines; the hub is splined to the pinion shaft and held by a snapring. The sleeve has an internal groove and an external groove for the shift fork. Three slots in the hub hold inserts (keys) that slide freely; insert springs keep the ridge of each insert in contact with the sleeve's internal groove. The sleeve is precisely machined to slide smoothly; alignment marks may ensure proper indexing. Brass, bronze, or powdered iron blocking rings are positioned at front and rear; some use additional friction material. Each blocking ring has three notches matching the insert keys and a set of beveled clutching teeth (dog teeth) for alignment. The inside of the blocking ring is a cone with grooves. The cone makes up half of the cone clutch; the other half is on the speed gear shoulder, which also has clutching teeth.

Operation:

In neutral or reverse, synchronizers are in neutral position and not rotating with the pinion shaft. Speed gears freewheel. To shift into first, the clutch is disengaged and the shifter moves the synchronizer sleeve toward first gear. Inserts move with the sleeve, forcing the blocking ring's cone against the gear shoulder. The grooves cut through lubricant, creating metal to metal friction, generating heat bronze/brass minimize wear on the steel shoulder. This friction synchronizes speeds, the sleeve then slides over the blocking ring's clutching teeth and then over the gear shoulder's teeth, completing engagement. Power flows from first gear to the sleeve, hub, mainshaft and driveline. To shift to second, the clutch disengages and the fork pulls the sleeve from first gear. Moving toward second, inserts force the forward blocking ring against the second gear shoulder, friction synchronizes and the sleeve locks them. Power then flows from second gear through the sleeve and hub to the pinion shaft.

Many manufacturers use multiple cone synchronizers (single, double or triple cone). For example, first and second gears may have triple cone, third and fourth double cone, fifth and sixth single cone. Double cone units have friction material on both sides of rings, reducing shift effort and increasing durability, triple cone adds a third surface. Multiple cone synchronizers allow high capacity in a smaller package, reducing transmission size and weight compared to a larger single cone.

Gearshift Mechanisms:

A typical five‑speed linkage has three shift rails and forks. Each rail/fork controls a synchronizer, and each synchronizer can engage two speed gears. Shift rails transfer motion from the gearshift lever to the forks. Forks are semicircular castings connected to rails with split pins and rest in the synchronizer sleeve groove.

Linkages:

Two basic designs: internal and external. Internal (direct) linkages are at the side or top of the transmission; the shifter is in direct contact with internal mechanisms. Shift rails have interlock and detent notches. Interlock notches prevent selection of more than one gear; when one rail moves, interlock pins hold others in neutral. Detent notches and spring‑loaded pins/balls give feedback when the collar is adequately moved. As the rail moves, a detent ball drops into the selected gear's notch, and an interlock pin moves into the other rails. External linkages operate similarly but use rods or cables outside the transmission to move levers that shift internal rails.

Transmission Power Flow: 

Input shaft rotates at engine speed clutch gear meshes with countergear, which rotates. Countergears mesh with speed gears, but none are locked to the mainshaft, so they spin freely and no torque is applied. All gear changes pass through neutral when changing gears, one gear disengages to neutral before the next engages.

First gear: 

Power flows through input shaft/clutch gear to countergear. First gear on the cluster drives first speed gear on the mainshaft. The first/second synchronizer moves to lock first gear to the mainshaft. Power drives the output shaft and driveline.

Second gear: 

The shift fork disengages first and moves to lock second speed gear. Power still goes through input shaft and countergear, but now the second countergear transfers power to the second speed gear locked on the mainshaft. Second gear is slightly smaller than first to balance speed and acceleration.

Third gear: 

The first/second synchronizer returns to neutral; the third/fourth synchronizer locks third speed gear. Power flows through third gear of the countergear to third speed gear, synchronizer, mainshaft, and driveline. Third gear is smaller than second, giving further torque decrease and speed increase.

Fourth gear: 

The third/fourth synchronizer locks the input shaft's clutch gear directly to the mainshaft, giving a 1:1 ratio. There is no torque multiplication; this ratio yields maximum speed output, used for cruising fuel economy. Downshifting to lower gears provides torque for passing or climbing.

Fifth gear: 

The fifth gear synchronizer engages fifth gear to the mainshaft. A large countershaft gear drives a smaller mainshaft gear, creating overdrive and reducing engine speed at higher vehicle speeds.

Reverse: 

A reverse idler gear is introduced between the countershaft reverse gear and the reverse speed gear (the external tooth sleeve of the first/second synchronizer). Both synchronizers are in neutral. The linkage moves the idler gear into mesh with the first/second synchronizer sleeve. Power goes from input shaft/clutch gear to countershaft, then to the idler (changing direction), then to the synchronizer sleeve (changing direction again), and finally to the mainshaft and driveline. Some Ford transmissions use helical reverse gears in constant mesh with first gear.

Transaxle Power Flows:

The following descriptions assume you are standing by the right front fender looking into the engine compartment. The example is a five‑speed, two‑shaft unit.

Neutral: 

No power to the differential. Synchronizer collars are centered; meshed drive gears are not locked to the output shaft, so gears spin freely and the output shaft does not rotate.

Forward gears: 

When first gear is selected, the first/second synchronizer engages first gear. Because the hub is splined to the output shaft, the input shaft's first gear drives the output shaft's mating first gear. The output shaft rotates at first gear ratio and drives the differential ring gear at that same ratio. For other forward gears, the appropriate shift fork engages the selected synchronizer, the input shaft's gear drives its mating gear on the output shaft, rotating at that ratio and driving the ring gear accordingly.

Reverse: 

On transaxles with a sliding reverse gear, the fork forces the reverse gear into mesh with both input and output shafts. This third gear reverses the normal rotation of first gear, allowing the vehicle to go backward.

Differential action: 

The final drive ring gear is driven by the output shaft and transfers power to the differential case, which holds the ring gear and pinion gear. Side gears connect to drive axles. In a RWD differential, power flow changes 90° between the drive pinion and ring gear. Most FWD cars do not need that change because the transverse engine already rotates in the correct direction; the differential only provides torque multiplication and divides torque between axle shafts for different speeds. Some transaxles do require the 90° change, used in rear‑engine RWD or longitudinally positioned FWD/AWD applications.

Final Drive Gears and Overall Ratios:

All vehicles use a differential for additional gear reduction beyond the transmission or transaxle gearing this is the final drive gear. In a transmission‑equipped vehicle, it is in the rear axle housing; in a transaxle, it is housed in the transaxle case.

Electrical Systems:

Although most manual transmissions are not electrically operated, some accessories are controlled or linked to them, and sensors provide information to the vehicle's computer. A few transmissions have electronically controlled or limited shifting.

Reverse lamp switch: 

Most manual transmissions have a separate switch on the transmission or on the shift linkage. If mounted in the transmission, the shift fork closes the circuit when reverse is engaged; if on the linkage, the linkage closes it directly.

Vehicle speed sensor (VSS): 

Most late model transmissions/transaxles have a VSS that sends an electrical signal to the powertrain control module (PCM) representing output shaft speed. The PCM calculates vehicle speed for cruise control, fuel/spark management, instrumentation (speedometer, odometer, upshift lamp), etc.

Reverse lockout system: 

Some vehicles electrically prevent shifting into reverse when moving forward. Typically, above 20 km/h, the PCM energizes a solenoid that pushes a cam and lock pin down, blocking reverse. At lower speeds or when stopped, the solenoid is de‑energized and a return spring holds the lock pin away, allowing free movement into reverse.

Shift blocking: 

Some six speed transmissions have shift blocking that prevents shifting from first to second or third when engine coolant is below a specified temperature, vehicle speed is between 20‑29 km/h and throttle is less than 35% open. This helps improve fuel economy. These transmissions also have reverse lockout. The PCM controls a skip shift solenoid that blocks the shift pattern from first to second/third; the driver may move the lever as if shifting to second but fourth gear is actually selected.