Gear boxes

GEAR BOXES
By: Ratnadeepsinh Jadeja

Introduction
Gear box is a speed and torque changing device between the engine and the
driving wheels.
It serves the following purposes in the transmission system of an automobile.
1. It exchanges engine power for greater torque and thus provides a
mechanical advantage to drive the vehicle under different conditions.
2. It exchanges forward motion or reverse motion.
3. It provides a neutral position to disallow power flow to the rest of power
train.
When driving power is required, It reduces the engine speed and transmits
stronger torque to the driving wheels and when high running speed is desired,
it transmits high speed low torque to the wheels.

Gear Ratios
• When one meshing gear rotates, the teeth of that gear cause the teeth
of the other gear to move so that the other gear also rotates.
• The relative speed of the two meshing gears (gear ratio) is determine by
the number of teeth of the two gears.
𝐺𝑒𝑎𝑟 𝑟𝑎𝑡𝑖𝑜 =
𝑁𝑜. 𝑜𝑓 𝑡𝑒𝑒𝑡ℎ 𝑜𝑛 𝑑𝑟𝑖𝑣𝑒𝑛 𝑔𝑒𝑎𝑟
𝑁𝑜. 𝑜𝑓 𝑡𝑒𝑒𝑡ℎ 𝑜𝑛 𝑑𝑟𝑖𝑣𝑖𝑛𝑔 𝑔𝑒𝑎𝑟
=
𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑑𝑟𝑖𝑣𝑖𝑛𝑔 𝑠ℎ𝑎𝑓𝑡
𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑑𝑟𝑖𝑣𝑒𝑛 𝑠ℎ𝑎𝑓𝑡
=
𝐸𝑛𝑔𝑖𝑛𝑒 𝑠𝑝𝑒𝑒𝑑
𝑃𝑟𝑜𝑝𝑒𝑙𝑙𝑒𝑟 𝑠ℎ𝑎𝑓𝑡 𝑠𝑝𝑒𝑒𝑑

Types of Gear Boxes
The following types of gear boxes are used in automobiles:
1. Selective type
I. Sliding mesh
II. Constant mesh
III. Synchromesh
1. Progressive type
2. Epicyclic or planetary type

Selective Type Gear Boxes
• It is that transmission in which any speed may be selected from the neutral position. In this
type of transmission, neutral position has to be obtained before selecting any forward or
reverse position.
• Advantages of selective type gear boxes:
1. Simple in construction.
2. Relatively free from troubles.
3. Light and small.
4. Low production cost.
• Disadvantages:
1. Gear ratio not being continuous but being in steps (3 to 5 steps), making it necessary to
shifts gears each time when vehicle running conditions change.
2. Noisy in operation.

Sliding mesh
gear box
(crash-type
gear box)

Progressive Type Gear Boxes
• These gears boxes are usually used in motor cycles. In this type of transmission, the
gears pass through the intervening speeds while shifting from one speed to
another. There is a neutral position between two positions as shown below in the
case of four speed gear box:
1
NEUTRAL
2
NEUTRAL
3
NEUTRAL
4
These gear boxes are a combination of sliding and constant
mesh gear boxes. The various gear speeds are obtained by
sliding the dog clutch or gear to the required position.

Epicyclic or
Planetary
Gear Box

Automatic Transmission
• Transmission in which various speeds are obtained automatically is known as
automatic transmission. The driver merely selects the general car condition such as
neutral, forward or reverse.
• Automatic transmission needs no gear change lever and clutch pedal since clutch
and transmission is a combined unit and works automatically. (In semi-automatic
transmission only the clutch is operated automatically)
• The transmission is operated hydraulically and is often known as hydraulic
transmission. This transmission employs fluid coupling , torque converter and
epicyclic gear arrangement. When all the different devices are combined into one
unit, they perform their duties collaborately.

Fluid coupling/Fluid flywheel
• Fluid coupling is a hydraulic unit that replaces a clutch in a semi or fully automatic
system, and transmits the engine torque to a transmission system. Since the coupling
is a major part of the engine flywheel assembly, it is also called a fluid flywheel or
fluid drive acting as an automatic clutch.

Fluid coupling/Fluid flywheel
• The efficiency of power transmission may be as high as 98 %
• The efficiency of the hydraulic coupling is defined as the ratio of power output to
power input
• The ratio Wt/Wp is known as speed ratio.
• A fluid coupling always slips by about 2 to 4 percent when transmitting full load. It
means that the turbine is always running slightly slower than the impeller and as such
complete disconnection of the drive is not possible. Thus the fluid coupling is not
suitable for ordinary gear box and is generally used with epicyclic gears to provide a
semi or fully automatic gear box.

Hydraulic torque converter
• Hydraulic torque converter is a device used for transmitting increased or decreased
power from one shaft to another. A variable torque is impressed on the driven
member without the use of a gear train or clutch.

Advantages of automatic transmission
• Following are the advantages of automatic transmission:
1. Smoother running under all conditions.
2. Noise free gear shifting.
3. No shock or jerky driving.
4. Simplified driving control.
5. Less physical and mental fatigue to the driver.
6. Improved acceleration in hill climbing.
7. Less wear and tear due to planetary gearing.
8. No clutch pedal and gear lever and hence simplification of driver’s compartment.
9. Reduced fuel consumption.
10. Longer life.

Overdrive
• Overdrive is a device interposed between the transmission (gear box) and propeller
shaft to permit the propeller shaft to turn faster (or overdrive) than the transmission
main shaft.
• The overdrive permits the engine to operate only about 70 % of the propeller shaft
speed, when the vehicle is operating in the high speed ranges.

Gear boxes

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Gear boxes