Paper solution summer -2012 be VIII sem subject: automobile engg



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Construction:

Figure shows the construction of differential.It consist of bevel pinion which gets power from transmission and the bevel pinion is further mesh with crown wheel ,which turns with bevel pinion.The cage is attached with crown wheel which consist of sun and planet gears and the half shaft s are supported in it.



Working:

When power transmits from Gear box to bevel pinion the pinion turns which in turn turns the crown wheel which is in mesh with it. As the cage is attached with crown wheel it turns with crown wheel and as both the half shaft is supported in cage it also start rorating with cage.

But when the vehicle takes a turn the inner wheel due to resistance rotates at slower speed and outer wheel rates at faster speed. If suppose the wheels are rotating with ‘N’ Number of rpm and the rpm of inner wheel due to resistance reduces to ‘n’ number of rpm then the rpm of inner wheel will be ‘N-n’ and the rpm of outer wheel will be ‘N+n’ and thus outer wheel rotates at higher speed. Thus differential provides relative motions between the wheels and avoid skidding of vehicle while taking turn.

i) Rear axle:



Rear axle in most of the cases is live axle and is used to transmit the power from differential to driving wheels.The rear axle is not a single piece but it is in two halves and is known as half shaft. There are various types of forces act on rear axle like:

  1. Total rear weight of the vehicle,causing both bending and shearing action.

  2. Side thrust on the wheels while cornering which imposes bending load and an end thrust.

  3. Driving Torque while driving.

  4. Breaking torque while breaking .

Depending upon the method of supporting the rear axle and mounting the rear wheels ,the axle are of following three types:

  1. Semi-flaoting rear axle.

  2. Three floating rear axle.

  3. Fully floating rear axle.

ii)Torque tube drive:



Fig shows Hotchkiss drive which consists of propeller shaft, two universal joints slip joint The propeller shaft is not enclosed in a tube as in the torque tube drive. The spring is rigidly in the middle to the rear axle. The front end of the spring is rigidly fixed on the frame, while the rear end is supported on a shackle. The rear-end torque is absorbed by the.rear end springs. When the car is moving forward, the rear-end torque causes the front halves of the springs to be compressed the rear halves of the springs are expanded. Two universal joints, one at each end of the propeller shaft, are required in Hotchkiss drive. The reason for this is obvious. When the springs deflect, the propeller shaft also changes its position. If there is only one universal joint at the front end of the propeller shaft it will bend under this condition. Therefore, another universal joint at the rear end of the propeller shaft is used. The slip joint compensates for differences in length of the propeller shaft caused by changes its angularity as the axles move up and down with the springs. Hotchkiss drive is used now-a-days in most of the cars



iii) TORQUE TUBE DRIVE :




In torque tube drive, the propeller shaft is enclosed in a hollow tube. The tube is rigidly bolted to the differential housing at one end and is fastened at the other end to the transmission through a somewhat flexible joint The tube incorporates bearings which support the propeller shaft. Only one universal joint is necessary on this type of drive. It is usually placed between tbe transmission and tbe propeller shaft. No sliding joint is needed in tbe propeller shaft On many cars, a pair of truss rods are attached between tbe rear axle housing and the transmission end of die torque tube. The torque tnbe and the truss roads brace tbe differential housing to prevent excessive differential housing movement In other words, the rear-end torque is absorbed by these members.

It is to be noted that in Hotchkiss drive, tbe springs taking die weight of the body, also take the torque reaction, driving thrust and the side thrust In torque tube drive, the springs take only the side thrust besides supporting weight of tbe body.

iv)Forced cooling system.







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