Power Train
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| Illustration 1 | g01084038 |
A power train is a group of components that work together to transfer power from the source where power is produced to a point where power is used to perform work. This definition might be compared to a freight train. A freight train is a group of components, a locomotive and cars, that transfers freight from where the freight is produced to where the freight is needed.
The term power train is not new. Power train has been used since the earliest times in order to describe the components that transfer power from one place to another place. For example, in early water-powered mills (Illustration 1) used in earlier times, the term power train was used to describe the machinery that carried power from the water wheel to perform work such as milling flour, weaving cloth, or sawing lumber.
In a typical modern industrial machine, the power train transfers power from the rotating flywheel of an engine to the road wheels or tracks that do the work of propelling the machine. The power train does more than just transfer power. If an engine were coupled directly to the drive wheels of a vehicle, the vehicle would run constantly at engine speed.
The power train provides a means of disconnecting and controlling the use of engine power. The the basic functions of the power train are listed below:
- Connect and disconnect power from the engine to the drive wheel(s).
- Modify speed and torque.
- Provide a means for reverse direction.
- Equalize power distribution to the drive wheels enables the vehicle to turn.
Power
Power is a term that is used to describe the relationship between work and time. Power is defined as the rate of performing work or transferring energy. In other words, power measures how quickly work is done. Power is equal to the work done divided by the amount of time it takes to do the work or P=W/t.
Work
Work is equal to the force that is applied to move an object multiplied by the distance the object travels. Force is a measure of the pushing power that is exerted by one object against another object.
According to physicist Isaac Newton's laws of motion, work equals force multiplied by the distance an object is moved. (W = F × d)
Substituting the definition of work into the definition of power shows that power is equal to the force applied to move an object multiplied by the distance that the object travels, divided by time. (P = F × d/t)
Torque
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| Illustration 2 | g01084039 |
Torque is a twisting effort applied to an object that tends to make the object turn about the axis of rotation. The amount of torque is equal to the magnitude of the applied force, multiplied by the distance between the object's axis of rotation and the point where the force is applied. Just as a force that is applied to an object tends to change the linear rate of motion of the object, a torque that is applied to an object tends to change the object's rate of rotational motion.
The amount of torque that is available from a source of power is proportional to the distance from the center at which torque is applied. In Illustration 2 the lever has more torque as the fulcrum gets closer to the object of power application (right diagram). But the lever must also be rotated farther to get this torque.
Mechanical Power Train
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| Illustration 3 | g01084045 |
The power trains that are used in most of today's construction machinery can be classified into three basic types:
- Mechanical
- Hydrostatic
- Electric
In a mechanical power train, power from the engine is transferred through a coupling (clutch or torque converter) to the transmission. From the transmission power is transferred to the differential, final drives, and to the wheels or tracks.
The major components of a typical mechanical power train are listed below:
Engine - Provides the power to operate the vehicle and the coupling device.
Coupling - Connects the engine power to the rest of the power train. Flywheel clutch couplings can disconnect the engine power from the rest of the power train. This allows the engine to run while the machine is not moving. Torque converters and torque dividers always provide a fluid coupling in order to connect the engine to the remainder of the power train. The connection can be direct if the machine is equipped with a lockup clutch.
Transmission - Controls output speed, direction, and torque of the power that is delivered to the remainder of the power train.
Differential - Transmits power to the final drive and wheels, while allowing each wheel to rotate at a different speed.
Final drive - Connects power to the wheels or tracks.
Ground engagement - Propels the machine through wheels or tracks.
The machines that are shown in Illustration 4 and Illustration 5 are equipped with mechanical power trains.
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| Illustration 4 | g01084046 |
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| Illustration 5 | g01084048 |
Hydrostatic Drive
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| Illustration 6 | g01084049 |
In hydrostatic drives fluid is used to transmit engine power to the machine's final drive. Power from the engine is transferred to a hydraulic pump. The hydraulic pump provides oil flow to a drive motor. The drive motor transfers power to the transmission or directly to the final drive.
The major components of a typical hydrostatic power train are listed below:
Engine - Provides the power to operate the vehicle and the hydraulic pump(s).
Pump(s) - Produce fluid flow to power the drive motor(s).
Motor(s) - Provide power to the transmission or final drive.
Transmission (if equipped) - Controls the output speed, the direction and the torque of the power delivered to the remainder of the power train.
Differential (if equipped) - Transmits power to the final drive and wheels while allowing each wheel to rotate at a different speed.
Final drive - Connects power to the wheels or tracks.
Ground engagement - Propels the machine through wheels or tracks.
The machines that are shown in Illustration 7 and Illustration 8 are equipped with hydrostatic drive power trains.
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| Illustration 7 | g01084051 |
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| Illustration 8 | g01084052 |
DC Electric Drive
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| Illustration 9 | g01084055 |
In a DC electric drive, electricity is used to transmit engine power to the machine final drive. Power from the engine is transferred to an AC generator. The electricity from the AC generator is used to power the motors at the final drive.
Engine - Provides the power to operate the vehicle.
AC Generator - Converts the mechanical power from the engine into electricity.
Rectifier - Converts the Alternating Current AC into Direct Current DC.
Field Exciter - Controls speed of motors.
DC Motors - Provide power to the final drive.
Final Drive - Connects power to the wheels.
Ground Engagement - Propels the machine through the wheels.
AC Electric Drive
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| Illustration 10 | g01084056 |
An AC electric drive operates the same as a DC electric drive except that a DC to Variable AC Inverter controls the motor speed and the electric motors are AC motors, which provide power to the final drive.
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| Illustration 11 | g01084058 |
Electric drives are used on some competitive mining trucks. Most competitive mining trucks have a DC electric drive, but recent larger mining trucks have an AC electric drive.
Mining trucks with mechanical drives generally have a higher power train efficiency and higher operating speed on an uphill grade. Competitive mining trucks also rely on dynamic braking instead of using oil cooled disc brakes.