What is the Final Drive?
The Final Drive is the last gearing used while transmitting the engine power to the wheels. It has two purposes. Firstly, it turns the power flow at the right angle from the propeller shaft to the rear axle. Secondly, it also provides a mechanical advantage (leverage) from the propeller shaft to the rear axle. The Final drive contains a pair of spiral-bevel gears comprising a pinion that connects to the propeller shaft and a ring gear that connects to a flange on the differential case.
The pinion gets the drive from the engine while the crown-wheel is attached to the wheels and rotates them. Usually, there are 3 to 4 times more teeth on the ring gear than the pinion. Thus, it provides the final speed reduction between 3:1 to 4:1. The drive pinion is an integral part of the pinion shaft. Its spline-end attaches to the end of the propeller shaft or the rear universal joint.
What is the Final Drive Ratio?
The final-drive ratio is an extent by which the rotating speed of the driveshaft (output) from the gearbox finally reduces before it reaches the driven wheels. If you alter this ratio, then it also alters the effective ratio of every gear, thereby affecting the vehicle performance and economy. Hence, the manufacturers term it as the final drive ratio. Basically, it is the final ratio of the gear-train.
Basically, it is the ratio of the angular velocity of the gear that gives the input to the output ratio. You can easily calculate the final drive ratio directly from the number of teeth on the crown-wheel and pinion. The manufacturers define the final drive ratio and mechanical ratio in such a way that it results in a number which forms an ideal association.
How does the Final Drive work in a vehicle?
Let's refer to the diagram above. For e.g. there are 8 teeth on the pinion and 30 teeth on the crown-wheel as shown in the diagram. To rotate the crown-wheel (& thereby the wheels) by one rotation, the pinion needs to rotate 3.75 times more. Let's assume that the vehicle is traveling in the 4th gear whose gear-ratio is 1:1 which means the gearbox input is equal to the output.
In the 4th gear, to rotate the wheels by one rotation, the engine needs to rotate the pinion 3.75 times more. This means the engine itself needs to rotate 3.75 rotations in order to rotate the wheels by one rotation. Because
1 wheel rotation = 3.75 pinion rotations x 1 (1:1 ratio of the 4th gear in this case)
Therefore, 1 wheel rotation = 3.75 engine rotations (in 4th gear)
Thus, in this case, the engine turns 3.75 times to turn the wheels by one rotation. The engine speed always reduces by the final ratio to rotate the wheels by one turn. Similarly, in the 1st gear whose ratio is 3.78, the engine needs to rotate 3.78 x 3.75 times more to turn the wheels by one rotation. Hence, this ratio is known as the final drive ratio.
1 wheel rotation = 3.75 pinion rotations x 3.78 (3.78:1 ratio of the 1st gear in this case)
Therefore, 1 wheel rotation = 14.175 engine rotations (in 1st gear)
That's why the vehicles can climb the slopes easily in the first gear than other gears.
Need of the Final Drive in an automobile:
Generally, Automobile drive train has two or more zones where the manufacturers use the gearing. One is in the gearbox while the other is in the axle. Thus, it helps to deliver a great combination of the vehicle speed and torque at the wheels. Besides, the modern-day transmissions also employ the built-in differential gearing system. The manufacturers name such units as ‘Transaxle’. The transaxle contains numerous different sets of gears which serve the purpose. It also contains the Final-Drive that provides further speed reduction at the wheels.
What is a 'Transaxle'?
A transaxle is a combination of the transmission and final drive (axle) merged into a single unit. Or else, these could be two separate units that connect thru’ a driveshaft. The gear ratios in the transmission and the final drive are of vital importance. It is because they change the characteristics of the vehicle's performance.
Furthermore, the differential contains a mechanism which splits the torque equally between the two wheels. It also allows them to rotate at different speeds while moving along a curved path. In most modern cars, the manufacturers employ the transaxle unit to reduce the weight or to accommodate the Final-Drive in the narrower spaces. Dana, Eaton, and Divgi-Warner are some of the leading suppliers of the final drive to the automotive manufacturers.
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