The Gearbox

8/12/2019 The Gearbox

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The Gearbox (Transmission)

Types of gearing:

Various types of gearing are used on a motor vehicle. The gearboxes employ oneor more of the following:

1- Spur, teeth parallel to axis, used on sliding mesh.

2- Helical, teeth inclined to axis to form helix. 3- Double helical, two sets of opposing helical teeth.

4- Epicyclic or planetary, spur or helical gears rotating about centers which are

not stationary.

Gear ratio (single gear train):

The gear ratio, or velocity ratio, between a pair of gear

wheels is in inverse ratio to the number of teeth on each.Thus:

NB/NA = DA/DB= nA/nB

NB = NA (nA/nB)

Where:

NA= rev per min of gear A, nA = number of teeth on A

NB = rev per min of gear B, nB = number of teeth on B

DA = Diameter of gear ADB = Diameter of gear B

Power, Speed and Torque:

The power transmitted by a shaft is directly proportionalto the speed of revolution and the torque acting on it

Power [kW] = 2 N T / (60 x 1000) [N.m/s]

Then

TA NA = TB NB



For a given power, therefore, the torque is inversely proportional to the speed of

revolution and if the re min is reduced the torque will be increased in the sameratio (assuming 100% gear efficiency).

TB/T

A = n

B/n

A

Where:

TA = torque transmitted by A

TB = torque transmitted by B

Velocity or gear ratio (ig) = number of teeth on driven gear/number of teeth

on driver gear.

TB = TA (nB/nA) = TA/ ig

Compound gear train: If the number of teeth on each wheel is known, the relationship between the speedof wheels A and D can be determined as follows

For wheels A and B: NB/NA = nA/nB, i.e. NB=

NA (nA/nB)

Wheel B and C are fixed on the same shaft, so

NC=NB

For wheels C and D: ND/NC = nC/nD, i.e. ND =

NC (nC/nD)

Substituting NC = NB = NA (nA/nB) from above,

we get

ND = NA (nA/nB) (nC/nD)

Or ND/NA =

By inspection of the layout of the figure, it will be observed that wheels A and C

are driver gears while B and D are driven gears. Hence, from the above equation

Velocity or gear ratio (ig) = product of teeth on driven gears/ product of teeth

on driver gears

ND = NA (nA/nB) (nC/nD) = NA (nA nC / nB nD) = NA/ig

Example:



A double reduction set of gearing is as shown in the above figure. Wheel A is the

driver gear, wheels B and C fixed to the same shaft and wheel D is the final gear inthe train. The number of teeth on each wheel is A=20, B=50, C=40, D=30 teeth.

a- Determine the velocity ratio of the gearing system.

b- Calculate the speed of rotation of wheel D when wheel A rotates at 1800

rev/min.

c- Calculate the torque of wheel D when the torque of A is 100 N.m and the

effi ciency of the gear tr ain is 90%.

a- Velocity ratio = product of teeth of driven gears/ product of teeth on driver

gears

i.e. velocity ratio (i g ) = (n D n B / nC n A ) = (30 x 50) / (20 x 40) = 1.875

b- N D = N A / i g = 1800 / 1.875 = 960 rev/min

c- T D = T A i g g = 100 x 1.875 x 0.9 = 168.75 N.m

Types of Drives and gearboxes

There are many types of the car drives, usually classified accordance with number

of driving axles (4x2, 4x4, 4WD, AWD) and each type has a different gearing

arrangement. Also, gearbox (transmission) has different types (sliding-mesh,constant-mesh, synchro-mesh) some of them are old-fashion and had been

replaced, and some are in use in modern cars.

SLIDING-MESH GEARBOX:

The sliding gearbox was popular on cars up to about 1930, but it is rarely used.The basic layout of a 4-speed and reverse gearbox is shown in the figure. The

various spur-type gears are mounted on three shafts.

o Primary shaft (alternative names – clutch or first motion shaft)

o Layshaft (countershaft)

o Mainshaft (third motion shaft).



Primary shaft

This shaft transmits the drive from the clutch to the gearbox. At the end, the shaftis supported by a spigot bearing positioned close to the splines on to which the

clutch driven plate is connected. The main load on this shaft is taken by a bearing;normally a sealed radial ball type, positioned close to an input gear called a

constant mesh pinion. The gear is so named because it is always in mesh with alarger gear, a c constant mesh wheel, that I part of the layshaft gear cluster. Note

that a small driving gear is called a pinion and a large gear a wheel .

Lay shaft

This shaft, which is normally fixed to the gearbox casing, supports the various-

sized driving pinions of the layshaft gear cluster.

Main shaft

This splined output shaft carries spur gearwheels that slide along the shaft to

engage with the appropriate lay shaft gears. At the ‘front’ end, the main shaft is

supported by a spigot bearing situated in the centre of the constant mesh pinion. A

heavy duty radial ball bearing is fitted at the other end to take the force of the gearsas the attempt to move apart.

Gear positions

Neutral

All main shaft gearwheels are positioned so that they do not touch the layshaftgears. A drive is taken to the layshaft, but the mainshaft will not be turned in

neutral position.

First gear

The firs-speed gearwheel A on the mainshaft is lid backwards to engage with

pinion B on the layshaft; all other gears are positioned in neutral. In this gear, the

reduction in speed that occurs as the drive passes through the constant-mesh gears,E and F, is reduced further by the firs-speed gears, A and B.

The gear ratio (also called the movement ratio or velocity ratio) is given by

Ratio = (Driven/driver) x (driven/driver)

Ig1 = (F/E) x (A/B)

Noutput 1 = Ninput / ig1

Toutput 1 = Tinput x ig1 x g1



Power take-off arrangement

In addition to the mechanism use for

driving a vehicle along a road, a powersupply is often required for operating

external items of auxiliary equipment.

A light truck having a tipping

mechanism is one example, but the mostvaried application of power take-off units

is associated with specialized off-roadvehicles.

The figure shows a typical power take-off arrangement that is driven from the gearbox layshaft.

Disadvantages of the sliding mesh

Although the mechanical efficiency of the sliding mesh gearbox was high, it

suffered from two great disadvantages: 1- Gear noise due to the type of gear.

2- The difficulty of obtaining a smooth, quit and quick change of gear without the

great skill and judgment.

CONSTANT-MESH GEARBOX



The main feature is the use of the stronger helical of double helical gears which

lead to quieter operation. In this design, the mainshaft pinions revolves freely on

bushes or needle-roller bearings and are all in constant engagement with thecorresponding layshaft wheels. The gear operation is obtained by locking the

respective gear to the main shaft by means of a dog clutch. The layout of the box isshown in the figure.

With this arrangement the quieter-running helical gears can be employed, and

during gear changing the noise and wear are reduced by the simultaneousengagement of all the dogs instead of only a pair of gear teeth as on the sliding-

mesh gearbox.

With single helical pinions (double helical is economically impractical), the

driving loads on the teeth cause an axial thrust which must be resisted by thrustwashers, or shoulders, on the mainshaft.

CONSTANT-LOAD SYNCHRO-MESH



The figure shows unite main details of. Fundamentally the box is laid out in same

manner as a constant-mesh, with the exception that a cone clutch is fitted betweenthe dog and gear members. The initial movement of the selector a sleeve carries the

hub towards the gear and allows the cones adjusts the speed of the gearwheel tosuit the hub and mainshaft. Extra pressure on the lever will allow the sleeve to

override the spring-loaded balls, and positively engage with the dogs on the gear.

BAULK RING SYNCHRO-MESH

This system is designed to overcome the main disadvantage of the earlier design-

noise or crashing of the gears due to a quick change, by adding baulking ring to dothe job as shown in the figure.



ADDISIONAL GEAR RATIOS

Commercial vehicles having a relatively low power/weight ratio, and operatingunder unladen to fully loaded conditions, require additional gears for efficientoperation.

ALTERNATIVE RATIO GEARBOX:

A- One arrangement is to provide two pairs of alternative-ratio constant mesh

gears between the clutch shaft and layshaft. This doubles the number of indirect

gear ratios available.B- Another system is to use an auxiliary gearbox behind the main gearbox with a

choice of direct drive or a reduction to split the ratios in the main gearbox. Thisenables all the available gears to be used in sequence. The auxiliary gearbox may

be a layshaft type with constant-mesh gears, or epicyclic, and the gear change may

be power-operated electrically or by compressed air.

OVERDRIVE GEAR:

Sometimes, and particularly, for cars where economy with a lowered cursing

engine speed is desired, the epicyclic unit may provide an overdrive of

approximately 0.75:1. More recent practice is to incorporate fifth speed an indirectratio of some 0.75:1 to 0.85:1. A typical arrangement is an extra pinion on thelayshaft in constant mesh with a mineshaft pinion turning on needle-roller

bearings. This is engaged by a synchromesh unit splined to the mainshaft andoperated from the reverse selector.



THE ALL-INDERCT GEARBOX (TRANSAXLE):

The layshaft two-stage gearbox is used in both longitudinal- and transverse-engined front-wheel-drive case. However, many of the former employ a single-

stage, all-indirect gearbox. There is no direct drive and consequently no particular

advantage in 1:1 gearbox ratio.

TWO-SPEED TRNASFER GEARBOX:



A range of vehicles uses optional four-wheel drive- with additional ‘emergency’

low ratios- to provide a cross-country facility. This is usually accomplished by atwo-speed transfer gearbox. With layshaft and two pairs of constant-mesh helical

gears, attached to the end of the main gearbox are driven via short coupling shaftfrom the gearbox mainshaft.

Four- and All-Wheel Drive:

Four-wheel-drive (4WD) and all-wheel-drive (AWD) systems can dramatically

increase vehicle’s traction and handling ability in rain, snow, and off -road driving.The improved traction of 4WD and AWD systems allows the use of tires narrower



than those used on similar 2WD vehicles. These narrow tires are less expensive.

They also tend to cut through snow and water rather than hydroplane over it. Both4WD and AWD systems add initial cost and weight.

4WD versus AWD:

4WD systems are those having a separate transfer case. They also give the driverthe choice of operating in either 2WD or 4WD through the use of a shift lever or

shift button.

AWD systems do not have a separate transfer case. They use a front-wheel-drive

transaxle equipped with a viscous clutch, center differential, or transfer clutch. All-

Wheel-drive system does not give the driver the option of selecting 2WD or 4WDmodes. The system operates in continuous 4WD. All-wheel-drive vehicle are

usually passenger cars that are not designed for off-road operation. They are

designed to increase vehicle performance in poor traction situations, such as icysnowy roads, and in emergencies.

The Gearbox

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