In most automobile vehicles the final drive is embodied in the rear axle. But in various popular vehicles with front-wheel drive and a few special-purpose vehicles with four-wheel drive, it becomes necessary to consider final drives as units independent of their position. So, there are different types of Final Drive used in Automobiles.
In this article, I will discuss the different types of Final drive used in automobiles.
Different Types of Final Drive
The propeller shaft transmits torque from the transmission to the differential and 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-
Read more – What is the Universal Joint in Automobile?
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 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 bracing used in modern automobiles are as follows :
- Hotchkiss drive
- Torque tube drive
Each of these drives is discussed below-
#1. Hotchkiss Drive
This shows the Hotchkiss drive which consists of a propeller shaft, two universal joints, and a slip joint. The propeller shaft is not enclosed in a tube as in the torque tube drive.
The spring 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 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 the length of the propeller shaft caused by changes in its angularity as the axles move up and down with the springs. Hotchkiss drive is used nowadays in most cars.
#2. Torque Tube Drive
In a 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 that support the propeller shaft. Only one universal joint is necessary for this type of drive.
It is usually placed between the transmission and the propeller shaft. No sliding joint is needed in the propeller shaft. In 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 the 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, the springs take the weight of the body, and also take the toque reaction, driving thrust, and side side thrust. In torque tube drive, the springs take only the side thrust besides supporting the weight of the body.
Conclusion on Different Types of Final Drive
In an automobile vehicle, one form of a final drive commonly used in motorcycles is the chain drive. This is the simplest and most efficient system, but it does not find more general use, because of the difficulties of cleanliness and lubrication. In this article, I discussed the different types of final drives in automobiles.