MANUAL TRANSMISSION

PURPOSE OF A MANUAL TRANSMISSION

A manual transmission is designed to change the

vehicle’s drive wheel speed and torque in relation to

engine

speed and torque

TRANSMISSION FEATURES

A manual transmission should:• be able to increase torque to the drive wheels  for

quick acceleration• supply different gear ratios to match load conditions• provide a reverse gear• provide an easy means of shifting gears• operate quietly with minimum power loss

GEAR FUNDAMENTALS

Gears are round wheels with teeth machined on

their perimeters

They transmit turning effort from one shaft to

another

When gears are different sizes, output speed and

torque change

GEAR DRIVE

Small gear driving a larger gear

Large gear driving a smaller gear

GEAR RATIO

The number of revolutions a drive gear must turn

before the driven gear completes one revolution

Calculated by dividing the number of teeth on the

driven gear by the number of teeth on the drive gear

Gear Ratio = 

GEAR RATIO

If the drive gear has 12 teeth and the driven gear

has 24 teeth, the gear ratio is two-to-one 

Gear Ratio = =

   = 2 or written 2:1

TRANSMISSION GEAR RATIOS

First gear 3:1

Second gear 2:1

Third (high) gear 1:1

Reverse gear 3:1

GEAR REDUCTION

Occurs when a small gear drives a larger gear

Increases turning force (torque)

Used in lower transmission gears

OVERDRIVE RATIO

Results when a larger gear drives a smaller gear

Output gear speed increases

Output torque is reduced

GEAR TYPES

Two gear types are commonly used in

automotive transmissions:

spur gears

helical gears

SPUR GEARS

Somewhat noisy

Teeth are cut parallel to the centerline of the

gear shaft

Used for sliding gears such as reverse gear

HELICAL GEARS

Teeth are machined at an angle to the centerline

Quieter and stronger than spur gears

Used for main drive gears which are in constant

mesh

GEAR BACKLASH

Distance between the meshing gear teeth

Allows lubricating oil to enter the high-friction

area between the gear teeth

Allows the gears to expand during operation

MANUAL TRANSMISSION LUBRICATION

Bearings, shafts, and gears are lubricated by oil

splash lubrication

As gears rotate, they sling oil around inside the

transmission

Typically, 80W or 90W gear oil is used

TRANSMISSION BEARINGS

Bearings reduce the friction between the surfaces

of rotating parts

Three basic types are used:• ball bearings• roller bearings• needle bearings

Used between shafts and housing, or between

gears and shafts

TRANSMISSION BEARINGS

Three types of antifriction bearings used• ball bearings• roller bearings• needle bearings

MANUAL TRANSMISSION CONSTRUCTION

TRANSMISSION CASE

Supports the bearings and shafts

Provides an enclosure for gear oil

Made of cast iron or aluminum

Drain plug and fill plug are provided

typically, the oil level should be level with the

bottom of the fill plug hole at operating temperature

EXTENSION HOUSING

Bolts to the rear of the transmission case

Encloses the output shaft

Holds the rear oil seal

FRONT BEARING HUB

Covers the front transmission bearing

Acts as a sleeve for the release bearing

MANUAL TRANSMISSION

TRANSMISSION SHAFTS

At least four shafts are commonly used:• input shaft• countershaft• reverse idler shaft• output shaft

INPUT SHAFT

Transfers rotation from the clutch disc to the

countershaft gears

Anytime the clutch disc turns, the input shaft

gear turns

COUNTERSHAFT

Holds the countershaft gears into mesh with the

input gear and other gears

Located slightly below and to one side of the

input shaft

Normally, it is locked in the case and does not

turn

REVERSE IDLER SHAFT

Supports the reverse idler gear,

Allows reverse idler gear to mesh with gears on

both the countershaft and output shaft

OUTPUT SHAFT

Holds the output gears and synchronizers

Connects to the drive shaft to turn the wheels

Gears are free to revolve on the shaft, but the

synchronizers are locked on the shaft by splines

TRANSMISSION GEARS

Input shaft gear turns countershaft gears.

Countershaft gears turn

output shaft gears

GEAR RANGES

Gear reduction

Direct drive

Output shaft rotation is reversed

INPUT GEAR

Machined part of the steel input shaft

COUNTERSHAFT GEAR

Several gears machined from a single piece of steel

REVERSE IDLER GEAR ASSEMBLY

OUTPUT SHAFT GEARS

SYNCHRONIZERS

Synchronizers have two functions:

o prevent the gears from clashing (grinding)

during engagement

o lock the output gear to the output shaft

SYNCHRONIZER THEORY

When the synchronizer is away from an output

gear, the gear freewheels (spins freely) on the output

shaft

When the synchronizer slides against the output

gear, it is locked to the synchronizer and to the

output shaft

Power flows through the output shaft to the

drive wheels

SYNCHRONIZER CONSTRUCTION

Hub is splined to the output shaft

SYNCHRONIZER OPERATION

When the driver shifts gears, the synchronizer

sleeve slides on its splined hub toward the output gear

Blocking ring cone rubs on the side of the drive

gear cone, causing friction between the two

Output gear, synchronizer, and the output shaft

begin to spin at the same speed

SYNCHRONIZER OPERATION

As soon as the speed is equalized, the sleeve can

slide over the blocking ring and spur gear teeth on

the output gear

This locks the output gear to the synchronizer

hub and to the shaft

Power flows through that gear to the drive wheels

FULLY SYNCHRONIZED TRANSMISSION

All the forward output gears use a synchronizer

Allows the driver to downshift into any lower

gear (except reverse) while the vehicle is moving

SHIFT FORKS

Transfer movement from the gear shift linkage to

the sleeves

SHIFT FORK ASSEMBLY

Movement of shift linkage moves the shift fork

EXTERNAL SHIFT ROD LINKAGE

INTERNAL SHIFT RAIL LINKAGE

INTERNAL SHIFT RAIL LINKAGE

COLUMN SHIFT MECHANISM

TRANSMISSION TYPES

There are several types:

three-speed

four-speed

five-speed

Some transmissions have overdrive in high gear

Extra gear ratios are needed for small, low-horsepower

engines

TRANSMISSION POWER FLOW

FIRST GEAR

Linkage rods move the shift forks so that first gear

synchronizer is engaged to the first output gear

Input shaft gear turns the countershaft gears

First gear is locked to the output shaft

Small gear on countershaft drives larger gear on the

output shaft

Gear ratio is about 3:1

SECOND GEAR

First gear synchronizer is slid away from first

gear

Second-third synchronizer is then engaged

Power flow is through second gear on the output

shaft

Gear ratio is about 2:1

THIRD GEAR

Synchronizer is slid over the small teeth on the

input shaft gear

Synchronizer locks the input shaft directly to the

output shaft

All the output shaft gears freewheel on the shaft

Power flow is straight through the transmission

Gear ratio is 1:1

REVERSE

Synchronizer is moved into the reverse gear on

the output shaft, locking the gear to the output shaft

Power flows through the countershaft, reverse

idler gear, reverse gear, and to the output shaft

NEUTRAL

All the synchronizer sleeves are located in the

center of their hubs

All the output shaft gears freewheel on the

output shaft

No power is transmitted to the output shaft

OVERDRIVE

In many transmissions, high gear is an overdrive

Gear ratio is less than 1:1

e.g. 0.87:1

Overdrive increases fuel economy

FIVE -SPEED, OVERDRIVE TRANSMISSION

OTHER TRANSMISSION DESIGNS

Many transmission design variations are used by

the numerous auto manufacturers

Whether rear-wheel-drive, front-wheel-drive, or

all-wheel-drive, all transmissions use similar

principles

FRON T-ENGIN E, FO UR- W HEEL D RIVE VEHICLE

Worm gear on the output shaft drives the

speedometer gear and cable

Gear on the output shaft turns a plastic gear on

the end of the speedometer cable

Cable runs through a housing up to the

speedometer head

SPEEDOMETER DRIVE

VEHICLE SPEED SENSOR

Most vehicles now use a sensor with an

electronic speedometer

A sensor may be mounted at the transmission

location where the speedometer cable would be

driven, or in the back of the speedometer head, with

a conventional speedometer cable

MANUAL TRANSMISSION SWITCHES

BACK-UP LIGHT SWITCH

Closed by the action of the reverse gear shift

linkage

When shifted into reverse, the linkage closes the

switch

Switch carries current to the back-up lamps