Power Transfer
While the functions of all power trains are basically the same, a variety of methods has been devised to achieve those functions. The principal methods that are used for transferring power in machinery can be classified into the following types:
- Gear
- Chain
- Friction
- Fluid
Gear
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| Illustration 1 | g01084125 |
By definition, a gear is a toothed wheel or cylinder that is used to transmit rotary or reciprocating motion from one part of a machine to another. Gears are the most common elements that are used in modern power trains. The gears represent one of the most efficient and cost-effective means of transferring engine power to the drive wheels of a machine. By varying the size and the number of gears, it is also possible to modify the power that is produced by an engine, in order to suit the work that is being performed.
The benefits of gear drives are no slippage and the gear drives can handle very high loads. However the gear drives are heavier than other types of drives and the distance between the input and output shafts is fixed by the diameter of the gears. Gears can be found in the following components: transmissions, differentials, axles and final drives
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| Illustration 2 | g01084126 |
The axle that is pictured in Illustration 2 is an example of a gear drive. In this particular application, the gears are able to handle very high torque loads at the final drive.
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| Illustration 3 | g01084127 |
A chain drive is a variation of a gear drive that is also used to transmit power from one rotating shaft to another. The gears, usually called sprockets, are not in mesh but instead are connected by a linked chain. The links of the chain mesh with the teeth of the sprockets so that the driven sprocket maintains a constant speed ratio with the drive sprocket. Track drives operate under the same principles as a chain drive.
Like gears, chain drives virtually eliminate slippage. Sprockets that are connected to the same side of the chain rotate in the same direction. Sprockets that are connected on different sides of the chain move in opposite direction. To avoid excessive wear, sprockets for roller chain drives should have 10 or more teeth. If a chain has an even number of spaces between links than the sprockets should have an odd number of teeth.
Roller Chains
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| Illustration 4 | g01084128 |
Roller chains are most commonly seen on heavy machinery. Roller chains provide an efficient means of carrying heavy loads at low speeds between shafts that are far apart. The roller chain is made up of alternate roller links and pin links. Roller links have two roller link side plates, two bushings, and two rollers. Pin links consist of two pin link plates and two pins. The side plates of the roller chain determine the pitch of the chain.
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| Illustration 5 | g01084130 |
Like gears, chain sprockets are often mounted on shafts with splines and keys. The slack side of a chain should be on the bottom whenever possible. On longer chain drives, an idler wheel or sprocket is often used on the slack side in order to maintain proper tension between the driving sprocket and the driven sprocket. Chains do stretch in use so chain tension must be adjusted (Illustration 5). This may be accomplished by moving one of the main sprockets or adjusting the idler sprocket, if equipped.
The benefits of chain drives are listed below:
- Little or no slippage
- Relatively inexpensive.
- Can maintain fixed ratio between rotating shafts.
- Resist heat, dirt, and bad weather.
- More powerful than belt drives.
Chain sprockets and shafts must be very carefully aligned in order to ensure full service life and correct tracking. Chain drives must be lubricated regularly in order to reduce wear, to protect against corrosion, and to prevent the link pins or roller bushings from galling or seizing.
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| Illustration 6 | g01084132 |
Large machines use different types of chain drives. The track-type tractor that is shown in Illustration 6 uses a version of a chain called a track to propel the machine. The track is driven by a sprocket.
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| Illustration 7 | g01084133 |
Smaller machines, such as the Skid Steer Loader that is shown in Illustration 7, use a chain to transfer power to the final drive and to the drive wheels. The chain is driven by a hydraulic motor through a sprocket (Illustration 5).
Friction
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| Illustration 8 | g01084134 |
Friction occurs when the surfaces of two objects rub together. This friction can be used to transmit motion and power from one object to another object. The amount of friction depends on the surface materials, the force with which the objects touch, and the temperature of the surfaces. Unlike gears and chains, friction drives allow some slippage to occur between components. This slippage is useful when a more gradual transfer of power is suitable.
One of the most common uses of friction is in a wheel. The friction between a driven wheel and the ground moves the wheel and the machine that is attached to it. The machine moves in the same direction the wheel is turning (Illustration 8).
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| Illustration 9 | g01084136 |
By using this same friction, power can be transmitted by bringing a driven wheel into contact with the surface circumference of a second wheel (Illustration 9). The second wheel will rotate in the opposite direction. Wheels that are used to transmit power in this manner are sometimes referred to as friction gears even though the wheels have no teeth.
The speed and torque of a friction wheel drive depends on the size of each wheel. A small wheel that drives a large wheel results in less speed and more torque. A large wheel that drives a small wheel results in less torque and more speed.
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| Illustration 10 | g01084137 |
Another common friction drive is the disc or the clutch. Clutches are used to cause two components to rotate together. When the clutch is engaged, the discs and plates are held together by springs or by hydraulic pressure. Friction causes the discs and plates to rotate together.
In a flywheel clutch, two disks are mounted on a shaft. One disc is connected to the engine. One disk is connected to the power train usually at the transmission. When the discs are not touching, the disc that is connected to the engine runs freely (Illustration 10, top diagram), while the disc that is connected to the power train is unaffected. When the discs are brought together, engine rotation is transferred by friction to the power train disc, which then turns in the same direction (Illustration 10, bottom diagram). The speed and torque of each friction disc is the same.
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| Illustration 11 | g01084138 |
Clutches are used in planetary transmissions to change the speed ratio between the input shaft and the output shaft. Clutches are also used in torque converters with lockup clutches in order to provide a direct connection between the input shaft and the output shaft.
The clutch discs and plates that are shown in Illustration 11 use friction to engage the clutch pack which transfers power to a transmission.
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| Illustration 12 | g01084139 |
Belts are a common means of transferring power from one wheel to another wheel. In a belt drive, (Illustration 12) the wheels are referred to as pulleys. Unlike wheels, that are driven by direct friction contact, pulleys rotate in the same direction. Belts also provide a more efficient power transfer than friction wheels, because the belt contacts more of the pulley surface.
The speed and torque of belt drives depends on the size of each pulley. A small pulley that drives a large pulley results in less speed and more torque. A large pulley that drives a small pulley results in less torque and more speed.
The benefits of a friction drive includes the ability to intentionally build slippage into the machine and a wide range of different materials can be used. The contact area should be a minimum of 180° on the driver. Friction drives are expensive and excessive slippage can cause accelerated wear and premature failure.
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| Illustration 13 | g01084140 |
The drive belts on the AP-1055B Asphalt Paver in Illustration 13 use friction to transfer power from the final drive to the ground.
Fluid Drives
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| Illustration 14 | g01084141 |
Fluid drives are another method of transferring power from the engine to the ground. Fluid drives have been active since the earliest developments in machinery. One of the most basic forms of fluid drive is the water wheel (Illustration 14). Many of the mills and factories of Colonial America were powered very efficiently by water wheels. Fluid drives are now used in some of the most sophisticated modern machinery, such as hydrostatic drives.
In fluid drives, fluid transfers power from the engine to the transmission or to the hydraulic drive motors.
The following drives are the two types of fluid drives:
- Fluid coupling
- Hydrostatic drive system
The fluid coupling or impeller/turbine provides a hydraulic connection between the engine and transmission. The fluid coupling performs the same task as a mechanical clutch. The fluid coupling uses hydraulic oil flow rather than friction discs to transfer power. The basic hydrostatic drive system consists of a hydraulic pump, lines, and a motor(s).
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| Illustration 15 | g01084143 |
In a fluid coupling or impeller turbine drive (Illustration 15), the impeller and turbine are placed close together in an enclosed housing that is filled with oil. The impeller is the driving member while the turbine is the driven member. Engine power spins the impeller. The impeller acts as a pump to move fluid toward the turbine. The swirling fluid pushes the blades on the turbine making the turbine turn. The turbine is connected to the power output.
The principles at work in transferring torque through fluids are called hydrodynamics. Hydrodynamics is the dynamics of incompressible fluids in motion.
Fluids in a hydrodynamic drive allow torque to be transferred with less impact than in a mechanical gear or a chain drive. The more gradual transfer of power places less strain on the drive line. This allows for a longer machine life.
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| Illustration 16 | g01084145 |
Fluid couplings such as torque converters operate on hydrodynamics. Torque converters can be found in most machines (Illustration 16) with power shift transmissions.
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| Illustration 17 | g01084146 |
In a hydrostatic drive system (Illustration 17), lines join a pump and a motor in a closed hydraulic loop. The pump is the central part of a hydrostatic drive. The pump changes mechanical energy into fluid energy. Lines carry the high-pressure fluid from the pump to the motor and low-pressure fluid from the motor back to the pump.
The motor converts the fluid energy to mechanical work. The motor is connected to the part of the equipment that performs the mechanical work of propelling the machine. Depending on the type of machine, the propelling component may be the final drives at the wheels, the differential, or the transmission.
Hydrostatic drives offer an infinite range of speeds and provide a relatively simple means of getting power to the ground.
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| Illustration 18 | g01084147 |
Illustration 18 shows a hydraulic pump, that provides oil flow to a hydraulic motor in a hydrostatic drive system. The pump is driven by the engine.
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| Illustration 19 | g01084148 |
Illustration 19 shows a hydraulic motor, which receives oil from the pump. The motor converts hydraulic energy into mechanical energy in order to propel the machine.
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| Illustration 20 | g01084150 |
The 902 Compact Wheel Loader that is shown in Illustration 20 is an example of a machine with a hydrostatic drive system.
The benefits of a fluid drive system include fewer parts, less wear, and infinite speed ranges.
Fluid drive systems are susceptible to leakage, contamination, and temperature related problems.