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A gearbox, also known as a gear case or gear head, is a gear or a hydraulic system responsible for transmitting mechanical power from a prime mover (an engine or electric motor) into some form of useful output.
A gearbox is a set of gears for transmitting power from one rotating shaft to another. They are used in a wide range of industrial, automotive and home machinery application.
Gear heads are available in different sizes, capacities and speed ratios. Their main function is to convert the input provided by an electric motor into an output of lower RPM and higher torque.
Functions of a Gearbox
• A gearbox is precisely bored to control gear and shaft alignment.
• It is used as a housing/container for gear oil.
• It is a metal casing for protecting gears and lubricant from water, dust and other contaminants.
Gearbox Specifications
There are a number of performance specifications which must be considered while choosing a gearbox for different industrial applications. Some of the important specifications are :
• Gear ratio : The ratio may be specified as x : 1, where x is an integer.
• Output torque
• Maximum input power
• Maximum input speed
• Gearing arrangement
• Reducer output
• Shaft Alignment
The Gearbox (Transmission)
Types of gearing:
Various types of gearing are used on a motor vehicle. The gearboxes employ one or 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 A
DB = Diameter of gear B
Power, Speed and Torque:
The power transmitted by a shaft is directly proportional to 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 same ratio (assuming 100% gear efficiency).
TB/TA = nB/nA
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 speed of 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
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).