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A Study on Automobile Differential System

A Study on  Automobile Differential System

INTRODUCTION

       First of all description of differential, differential is an arrangement of gears in an epicyclical train permitting the rotation of two shaft at different speeds, used on the rear axle of automotive vehicles to allow different rates of wheel rotation on curve. Differential is an integral part of all four wheelers. The differential technology was invented centuries ago and is considered to be one of the most ingenious inventions human thinking has ever produced. In this seminar we will understand why a differential is needed in an automobile and its working in a logical manner.

            In automobiles and other wheeled vehicles, the differential allows the outer drive wheel to rotate faster than the inner drive wheel during a turn. This is necessary when the vehicle turns, making the wheel that is traveling around the outside of the turning curve roll farther and faster than the other. The average of the rotational speed of the two driving wheel equals the input rotational speed of the drive shaft. An increase in the speed of one wheel is balanced by a decrease in the speed of the other.

PARTS OF DIFFERENTIAL SYSTEM

Power from the engine is transferred to the ring gear through a pinion gear. The ring gear is connected to a spider gear. The spider gear lies at the heart of the differential and special mention should be made about its rotation. Spider gear is free to make 2 kinds of rotations, one along with the ring gear (rotation) and second on its own axis(spin).

       The spider gear is meshed with 2 side gears. we can note that both the spider and side gears are bevel gears. Power flow from the drive shaft to the drive wheels follows the following pattern. From the drive shaft power is transferred to the pinion gear first, since the pinion and ring gear are meshed, power flows to the ring gear. As the spider gear is connected with the ring gear, power flows to it. Finally from the spider gear, power gets transferred to both the side gears.

WORKING OF DIFFERENTIAL



           When a car turns a corner, one wheel is on the "inside" of a turning arc, and the other wheel is on the "outside." Consequently, the outside wheel has to turn faster than the inside one in order to cover the greater distance in the same amount of time. Thus, because the two wheels are not driven with the same speed, a differential is necessary. A car differential is placed halfway between the driving wheels, on the front, rear, or both axes. In rear-wheel drive cars, the differential converts rotational motion of the transmission shaft which lies parallel to the car’s motion to rotational motion of the half-shafts (on the ends of which are the wheels), which lie perpendicular to the car’s motion.

DIFFERENT CASE STUDIES

Differential when car travels in a straight line (Wheels at same speed)

)
        In this case the spider gear rotates along with the ring gear but does not rotate on its own axis. So the spider gear will push and make the side gears turn, and both will turn at same speed. In short, When the vehicle moves straight, spider-side gear assembly will move as a single solid unit.
         When the car is traveling straight, both wheels travel at the same speed. Thus, the free-wheeling planet pinions do not spin at all. Instead, as the transmission shaft turns the crown wheel, the rotary motion is translated directly to the half-shafts, and both wheels spin with the angular velocity of the crown wheel (they have the same speed).

Vehicle takes a right turn.



         When properly meshed, the side gear has to have the same peripheral velocity as that of the spider gear. Technically speaking, both the gears should have the same pitch line velocity. When the spider gear is spinning as well as rotating, peripheral velocity at the left side of spider gear is the sum of the spinning and rotational velocity. But at the right side, it is the difference of the two, since the spin velocity is in the opposite direction at this side. This fact is clearly depicted in above Fig. This means the left side gear will have higher speed compared to the right side gear. This is the way the differential manages to turn left and right wheels at different speeds.

Vehicle takes a left turn.


           In this case the left wheel get more resistance, because it is in inner side. So now the spider wheel will rotate about its own axis. And the result of this combined rotation left wheel speed will reduce and right wheel speed will increase.


OTHER FUNCTIONS OF DIFFERENTIAL


Apart from allowing the wheels to rotate at different rpm differential has 2 more functions. First is speed reduction at the pinion-ring gear assembly. This is achieved by using a ring gear which is having almost 4 to 5 times number of teeth as that of the pinion gear. Such huge gear ratio will bring down the speed of the ring gear in the same ratio. Since the power flow at the pinion and ring gear are the same, such a speed reduction will result in a high torque multiplication.
One specialty of the ring gear, they are hypoid gears. Hypoid gears have more contact area compared to the other gear pairs and will make sure that the gear operation is smooth. The other function of the differential is to turn the power flow direction by 90 degree.

DRAWBACK OF A STANDARD DIFFERENTIAL


The differential we have gone through so far is known as open or standard differential. It is capable of turning the wheels at different rpm, but it has got one major drawback. The problem is an open differential always tries to balance the torque. That's a hard statement to get a grasp on, but it means that if the spider gears are pushing on both drive gears and one of them offers lots of resistance (tire sitting on pavement) and the other side offers no resistance (up in the air, or sitting on a patch of ice), then it will find a happy balance where both sides are receiving almost no torque at all. All the rotational energy is guided to the side with the least resistance. In the end, that side spins very fast and the pressure on each drive gear is the same.. Almost no torque is needed to spin one wheel, and since the open differential always sends the same amount of torque to both output shafts, almost no torque is going to the other side as well.
To overcome this problem, Limited Slip Differentials are introduced.

LIMITED SLIP DIFFERENTIAL


Limited slip differentials (LSD) are used in automobile to overcome the traction difference problem of drive wheels. Consider a situation where a vehicle fitted with a standard differential moves straight, and one drive wheel is on a surface with good traction and the other wheel is on a slippery track.

In a standard differential the left and right axle rotations are completely independent. Since one wheel is on a slippery track, the standard differential will make that wheel spin in excessive speed, while the good traction wheel will remain almost dead. This means high power supply to the slippery wheel and low power flow to the good traction wheel. So the vehicle won’t be able to move.

          The main advantage of a limited-slip differential is demonstrated by considering the case of a standard (or "open") differential in off-roading or snow situations where one wheel begins to slip or lose contact with the ground. In such a case with a standard differential, the slipping or non-contacting wheel will receive the majority of the power, while the contacting wheel will remain stationary with the ground. The torque transmitted will be equal at both wheels, and therefore, will not exceed the threshold of torque needed to move the wheel with traction. In this situation, a limited-slip differential prevents excessive power from being allocated to one wheel, and thereby keeping both wheels in powered rotation.


CONCLUSIONS


The differential is found on all modern cars and trucks, and also in many all-wheel-drive vehicles. In automobile and other wheeled vehicles, the differential allows each of the driving wheels to rotate at different speeds, while supplying equal torque to each of them. In case of open differential traction problems are high, and more engine power loss will occur. So in order to avoid this traction problem a new type of differential is proposed.
       Limited slip differential prevents excessive power from being allocated to one wheel, and thereby keeping both wheels in powered rotation. The Torsen differential exhibits a torque biasing characteristic which matches available engine power to changing traction conditions. Torsen differentials are used in off-road and high performance all-wheel drive vehicles.

REFERENCES

  1. A study on vehicle differential system: Chandrakant singh, Lalit kumar: - International Journal of scientific research and management (IJSRM) 
  2. The development of a differential for the improvement of traction control ; The institution of mechanical engineers


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