Showing posts with label Differential Assembly. Show all posts
Showing posts with label Differential Assembly. Show all posts

Construction & Working of Differential Assembly


Torque is supplied from the engine, via the transmission, to a drive shaft (British term: 'propeller shaft', commonly and informally abbreviated to 'prop-shaft'), which runs to the final drive unit that contains the differential. A spiral bevel pinion gear takes its drive from the end of the propeller shaft, and is encased within the housing of the final drive unit. This meshes with the large spiral bevel ring gear, known as the crown wheel. The crown wheel and pinion may mesh in hypoid orientation, not shown. The crown wheel gear is attached to the differential carrier or cage, which contains the 'sun' and 'planet' wheels or gears, which are a cluster of four opposed bevel gears in perpendicular plane, so each bevel gear meshes with two neighbours, and rotates counter to the third, that it faces and does not mesh with. The two sun wheel gears are aligned on the same axis as the crown wheel gear, and drive the axle half shafts connected to the vehicle's driven wheels. The other two planet gears are aligned on a perpendicular axis which changes orientation with the ring gear's rotation.


Input torque is applied to the ring gear (blue), which turns the entire carrier (blue). The carrier is connected to both the side gears (red and yellow) only through the planet gear (green) (visual appearances in the diagram notwithstanding). Torque is transmitted to the side gears through the planet gear. The planet gear revolves around the axis of the carrier, driving the side gears. If the resistance at both wheels is equal, the planet gear revolves without spinning about its own axis, and both wheels turn at the same rate.

If the left side gear (red) encounters resistance, the planet gear (green) spins as well as revolving, allowing the left side gear to slow down, with an equal speeding up of the right side gear (yellow).

Thus, for example, if the vehicle is making a turn to the right, the main crown wheel may make 10 full rotations. During that time, the left wheel will make more rotations because it has further to travel, and the right wheel will make fewer rotations as it has less distance to travel. The sun gears (which drive the axle half-shafts) will rotate in opposite directions relative to the ring gear by, say, 2 full turns each (4 full turns relative to each other), resulting in the left wheel making 12 rotations, and the right wheel making 8 rotations.
The rotation of the crown wheel gear is always the average of the rotations of the side sun gears. This is why, if the driven roadwheels are lifted clear of the ground with the engine off, and the drive shaft is held (say leaving the transmission 'in gear', preventing the ring gear from turning inside the differential), manually rotating one driven roadwheel causes the opposite roadwheel to rotate in the opposite direction by the same amount.



When the vehicle is traveling in a straight line, there will be no differential movement of the planetary system of gears other than the minute movements necessary to compensate for slight differences in wheel diameter, undulations in the road (which make for a longer or shorter wheel path), etc.


·         This is a depiction of an open differential, which is commonly found in most vehicles. They are quite trouble-free, but do have one disadvantage. On a dry road with good traction, the power is evenly applied to both wheels. When one of the tires hits ice or a slippery surface, it begins to spin and the majority of torque is directed to the spinning wheel, leaving very little for the wheel with the good traction. This is how vehicles can get stuck in snow or mud.
·         Another type of differential is the limited slip differential, which is an option on most new cars. It has a distinct advantage by having a set of clutches and springs within the differential. Their function is to apply pressure to the side gears should one of the tires begin to slip. By applying pressure to the opposite wheel from the one spinning, it allows for more torque to be applied to the wheel with traction. If is far superior to the open differential when it comes to traction in bad weather.





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Need of a Differential


Car wheels spin at different speeds, especially when turning. Each wheel travels a different distance through the turn, and that the inside wheels travel a shorter distance than the outside wheels. Since speed is equal to the distance traveled divided by the time it takes to go that distance, the wheels that travel a shorter distance travel at a lower speed. Also note that the front wheels travel a different distance than the rear wheels. 

For the non-driven wheels on your car -- the front wheels on a rear-wheel drive car, the back wheels on a front-wheel drive car -- this is not an issue. There is no connection between them, so they spin independently. But the driven wheels are linked together so that a single engine and transmission can turn both wheels. If your car did not have a differential, the wheels would have to be locked together, forced to spin at the same speed. This would make turning difficult and hard on your car: For the car to be able to turn, one tire would have to slip. With modern tires and concrete roads, a great deal of force is required to make a tire slip. That force would have to be transmitted through the axle from one wheel to another, putting a heavy strain on the axle components. 
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Advantages & Disadvantages of All or 4- Wheel Drive


The differential is found on all modern cars and trucks, and also in many all-wheel-drive (full-time four-wheel-drive) vehicles. These all-wheel-drive vehicles need a differential between each set of drive wheels, and they need one between the front and the back wheels as well, because the front wheels travel a different distance through a turn than the rear wheels.

Part-time four-wheel-drive systems don't have a differential between the front and rear wheels; instead, they are locked together so that the front and rear wheels have to turn at the same average speed. This is why these vehicles are hard to turn on concrete when the four-wheel-drive system is engaged.


The Advantages & Disadvantages of All Wheel Drive
All Wheel Drive (or AWD) is a system in which all four wheels of a car operate simultaneously to improve traction and handling. While it is possible for a car to have continuous AWD capabilities, it is far more common for one pair of wheels to engage only when sensors detect that the other pair has begun to slip. There are both advantages and disadvantages to AWD systems

1.    Traction-

In intermittent AWD systems, the rear wheels engage when sensors detect slippage from the front wheels. Under these circumstances, the vehicle effectively detects and compensates for dangerous driving conditions such as standing water, snow, ice or gravel that could otherwise compromise control of the vehicle. By engaging the second set of wheels, the vehicle experiences two additional points of contact on the surface of the road, allowing greater likelihood that its tires will grip the surface and allow the driver to retain control. The additional weight of AWD systems also encourages more grip on the road and the added points of contact distribute the vehicle's weight more evenly over points of propulsion.

2.    Fuel Efficiency-

The primary disadvantage of an AWD vehicle is its cost. The drive train and related equipment necessary to provide both continuous and intermittent AWD is complex and expensive, often requiring sensors and computers that are not necessary on two- or four-wheel-drive vehicles. This cost increases the initial market value of the vehicle and can also affect the cost of repairs. In addition to these costs, AWD systems require more fuel to power the additional wheels and are less fuel efficient than comparable two-wheel-drive vehicles.

3.    Braking Distance and Collision Avoidance-

While the weight of AWD vehicles improves their handling, it also increases the distance they require to stop. In a scenario where the vehicle must make a sudden stop and cannot swerve or turn, a collision becomes more likely than with a lighter car. Under similar circumstance, but ones in which an accident can be avoided by turning, AWD vehicles offer superior collision avoidance than similar vehicles with less effective handling and turning capabilities.


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Advantages & Disadvantages of Rear Wheel Drive


Advantages of Rear Wheel Drive-

  1. Better handling in dry conditions - accelerating force is applied to the rear wheels, on which the down force increases, due to load transfer in acceleration, making the rear tires better able to take simultaneous acceleration and curving than the front tires.
  2. More predictable steering in low traction conditions (ie: ice or gravel) because the steering wheels maintain traction and the ability to affect the motion of the vehicle even if the drive wheels are slipping.
  3. Less costly and easier maintenance - Rear wheel drive is mechanically simpler and typically does not involve packing as many parts into as small a space as does front wheel drive, thus requiring less disassembly or specialized tools in order to replace parts.
  4. No torque steer.
  5. Even weight distribution - The division of weight between the front and rear wheels has a significant impact on a car's handling, and it is much easier to get a 50/50 weight distribution in a rear wheel drive car than in a front wheel drive car, as more of the engine can lie between the front and rear wheels (in the case of a mid engine layout, the entire engine), and the transmission is moved much farther back.
  6. Steering radius - As no complicated drive shaft joints are required at the front wheels, it is possible to turn them further than would be possible using front wheel drive, resulting in a smaller steering radius.
  7. Towing - Rear wheel drive puts the wheels which are pulling the load closer to the point where a trailer articulates, helping steering, especially for large loads.
  8. Weight transfer during acceleration. (During heavy acceleration, the front end rises, and more weight is placed on the rear, or driving wheels).
  9. Drifting - Drifting is a controlled skid, where the rear wheels break free from the pavement as they spin, allowing the rear end of the car to move freely left and right. This is of course easier to do on slippery surfaces. Severe damage and wear to tires and mechanical components can result from drifting on dry asphalt. Drifting can be used to help in cornering quickly, or in turning the car around in a very small space. Many enthusiasts make a sport of drifting, and will drift just for the sake of drifting. Drifting requires a great deal of skill, and is not recommended for most drivers. It should be mentioned that front wheel drive and four wheel drive cars may also drift, but only with much more difficulty. When front wheel drive cars drift, the driver usually pulls on the emergency brake in order for the back wheels to stop and thus skid. This technique is also used for 'long' drifts, where the turn is accomplished by pulling the e-brake while turning the steering wheel to the direction the driver desires. With drifting, there is also the importance of 'counter-steering' - where while temporarily out of control, the driver regains it by turning the wheel in the opposite direction and thus preparing for the next turn or straight-away.

Diadvantages of Rear Wheel Drive-
  1. More difficult to master - While the handling characteristics of rear-wheel drive may be more fun for some drivers, for others having rear wheel drive is less intuitive. The unique driving dynamics of rear wheel drive typically do not create a problem when used on vehicles that also offer electronic stability control and traction control.
  2. Decreased interior space - This isn't an issue in a vehicle with a ladder frame like a pickup truck, where the space used by the drive line is unusable for passengers or cargo. But in a passenger car, rear wheel drive means: Less front leg room (the transmission tunnel takes up a lot of space between the driver and front passenger), less leg room for center rear passengers (due to the tunnel needed for the drive shaft), and sometimes less trunk space (since there is also more hardware that must be placed underneath the trunk).
  3. Increased weight - The drive shaft, which connects the engine at the front to the drive axle in the back, adds weight. There is extra sheet metal to form the transmission tunnel. A rear wheel drive car will weigh slightly more than a comparable front wheel drive car, but less than four wheel drive.
  4. Higher purchase price - Due to the added cost of materials, rear wheel drive is typically slightly more expensive to purchase than a comparable front wheel drive vehicle. This might also be explained by production volumes, however. Rear drive is typically the platform for luxury performance vehicles, which makes read drive appear to be more expensive. In reality, even luxury performance front drive vehicles are more expensive than average.
  5. More difficult handling on low grip surfaces (wet road, ice, snow, gravel...) as the car is pushed rather than pulled. In modern rear drive cars, this disadvantage is offset by electronic stability control and traction control.
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    Advantages & Disadvantages of Front Wheel Drive


    Advantages of Front Wheel Drive-

    1. Interior space: Since the powertrain is a single unit contained in the engine compartment of the vehicle, there is no need to devote interior space for a driveshaft tunnel or rear differential, increasing the volume available for passengers and cargo.
    2. Cost: Fewer components overall
    3. Weight: Fewer components mean lower weight
    4. Fuel economy: Lower weight means better gasoline mileage
    5. Improved drivetrain efficiency: the direct connection between engine and transaxle reduce the mass and mechanical inertia of the drivetrain compared to a rear-wheel drive vehicle with a similar engine and transmission, allowing greater fuel economy.
    6. Assembly efficiency: the powertrain can be often be assembled and installed as a unit, which allows more efficient production.
    7. Slippery-surface traction: placing the mass of the drivetrain over the driven wheels improves traction on wet, snowy, or icy surfaces. Although heavy cargo can be beneficial for traction on rear-wheel drive pickup trucks.
    8. Predictable handling characteristics: front-wheel drive cars, with a front weight bias, tend to understeer at the limit, which is commonly believed to be easier for average drivers to correct than terminal oversteer, and less prone to result in fishtailing or a spin.
    9. Better crosswind stability.
    10. Tactile feedback via the steering wheel informing driver if a wheel is slipping.
    11. Front wheel drive allows the use of left-foot braking as a driving technique.



    Diadvantages of Front Wheel Drive-

    1. The center of gravity of the vehicle is typically farther forward than a comparable rear-wheel drive layout. In front wheel drive cars, the front axle typically supports around 2/3rd of the weight of the car (quite far off the "ideal" 50/50 weight distribution). This is a contributing factor in the tendency of front wheel drive cars to understeer.
    2. Torque steer can be a problem on front wheel drive cars with higher torque engines ( > 210 N·m ) and transverse layout. This is the name given to the tendency for some front wheel drive cars to pull to the left or right under hard acceleration. It is a result of the offset between the point about which the wheel steers (which falls at a point which is aligned with the points at which the wheel is connected to the steering mechanisms) and the centroid of its contact patch. The tractive force acts through the centroid of the contact patch, and the offset of the steering point means that a turning moment about the axis of steering is generated. In an ideal situation, the left and right wheels would generate equal and opposite moments, cancelling each other out, however in reality this is less likely to happen. Torque steer is often incorrectly attributed to differing rates of twist along the lengths of unequal front drive shafts. However, Center-point steering geometry can be incorporated in the design to avoid torque steer. This is how the powerful Citroen SM front-wheel drive car avoided the problem.
    3. Lack of weight shifting will limit the acceleration of a front wheel drive vehicle. In a rear wheel drive car the weight shifts back during acceleration giving more traction to the driving wheels. This is the main reason why nearly all racing cars are rear wheel drive. However, since front wheel cars have the weight of the engine over the driving wheels the problem only applies in extreme conditions.
    4. In some towing situations front wheel drive cars can be at a traction disadvantage since there will be less weight on the driving wheels. Because of this, the weight that the vehicle is rated to safely tow is likely to be less than that of a rear wheel drive or four wheel drive vehicle of the same size and power.
    5. Due to geometry and packaging constraints, the CV joints (constant-velocity joints) attached to the wheel hub have a tendency to wear out much earlier than their rear wheel drive counterparts. The significantly shorter drive axles on a front wheel drive car causes the joint to flex through a much wider degree of motion, compounded by additional stress and angles of steering, while the CV joints of a rear wheel drive car regularly see angles and wear of less than half that of front wheel drive vehicles.
    6. The driveshafts may limit the amount by which the front wheels can turn, thus it may increase the turning circle of a front wheel drive car compared to a rear wheel drive one with the same wheelbase.
    7. In low traction conditions (ie: ice or gravel) the front (Drive) Wheels lose traction first making steering ineffective.



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    Differential Introduction


    A differential is a device, usually, but not necessarily, employing gears, capable of transmitting torque and rotation through three shafts, almost always used in one of two ways: in one way, it receives one input and provides two outputs—this is found in most automobiles—and in the other way, it combines two inputs to create an output that is the sum, difference, or average, of the inputs.
    In automobiles and other wheeled vehicles, the differential allows each of the driving roadwheels to rotate at different speeds.

    The differential has three jobs:
    1.      To aim the engine power at the wheels
    2.  To act as the final gear reduction in the vehicle, slowing the rotational speed of the                              transmission one final time before it hits the wheels
    3.    To transmit the power to the wheels while allowing them to rotate at different speeds (This is the one that earned the differential its name.) 

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