Torque Tube and Hotchkiss Drive
The propeller shaft transmits torque from the transmission to the differential and finally, it is transmitted to the rear wheels causing them to rotate. This torque not only rotates the wheels in one direction but it also attempts to rotate the differential housing in the opposite direction. This can be understood as follows:
As the propeller shaft turns the pinion, it forces the ring gear and wheels to rotate. It is the side thrust of the pinion teeth against the ring gear teeth that makes the ring gear to rotate. This side thrust also causes the pinion shaft to push against the shaft bearing. The thrust against the shaft bearing is in a direction opposite to the thrust of the pinion teeth against the ring gear teeth. Because the pinion bearings are held in differential housing, the housing tries to rotate in a direction opposite to the ring gear and wheel rotation. This action is known as rear end torque. To prevent excessive movement of the differential housing from this action, several methods of bracing the housing are used. The two most common types
of bracings used in modern automobiles are as follows:
1. Hotchkiss drive
2. Torque tube drive.
The figure shows Hotchkiss drive which consists of the propeller shaft, two universal joints, and a slip joint. The propeller shaft is not covered in a tube as in the torque tube drive system.
The spring or suspension is fixed 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 as 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 pinion shaft also changes its position. If there is only 1 universal joint at the front part 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 in its angularity as the axles move up and down with springs. Hotchkiss drive is used nowadays in most of the cars.
TORQUE TUBE DRIVE
In the torque tube drive system, the propeller shaft is enclosed or covered in a hollow tube. The torque tube is rigidly bolted to the differential housing at one side 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 for this type of drive. It is usually mounted in between the transmission and the propeller shaft. On many cars, a pair of truss rods are attached between the rear axle housing and the transmission end of the torque tube. The torque tube and the truss rods brace with the differential housing to prevent excessive movement of the differential housing. In other words, the rear-end torque is absorbed by this system.
It is to be noted that in the Hotchkiss drive, the springs taking the weight of the body, also take the torque reaction, driving thrust and the side thrust. In torque tube drive, the spring takes only the side thrust besides supporting the weight of the body.
|torque tube drive.
For better understanding watch the video below :