AUTOMOTIWE FINAL DRIVE
FINAL DRIVE
A final drive is that part of a power transmission system between the drive shaft and the differential. Its function is to change the direction of the power transmitted  by  the  drive shaft through 90 degrees to the driving  axles.  At the same time. it provides  a  fixed reduction between the speed of the drive shaft and the axle driving the wheels.
The  reduction or gear ratio of the final drive is determined by dividing the number of teeth on the ring gear by the  number of teeth on  the  pinion  gear.  In passenger  vehicles, this speed  reduction  varies  from about 3:1 to 5:1. In trucks it varies from about 5:1 to 11:1. To calculate rear axle ratio, count the number of teeth on each gear. Then divide the number of pinion teeth into the number of ring gear teeth. For example, if the pinion gear has 10 teeth and the ring gear has 30 (30 divided  by  10),  the  rear  axle  ratio  would  be  3:1. Manufacturers install a rear axle ratio that provides a compromise between performance and economy.  The average passenger car ratio is 3.50:1.
The higher axle ratio, 4.11:1 for instance, would increase acceleration  and pulling power  but  would decrease fuel economy. The engine would have to run at a higher rpm to maintain an equal cruising speed. 
The  lower  axle  ratio.  3:1,  would reduce acceleration and pulling power but would increase fuel mileage. The engine  would  run  at  a  lower  rpm  while maintaining the same speed. 
The major components of the final driveinclude the pinion  gear, connected  to  the drive shaft, and a bevel gear or ring gear that is bolted or riveted to the differential  carrier.  To maintain  accurate and  proper alignment and tooth contact,  the  ring gear and differential  assembly are mounted in bearings. The bevel drive pinion is supported by two tapered roller bearings, mounted in the differential carrier.  This pinion  shaft  is  straddle mounted.  meaning  that  a bearing is located on each side of the pinion shaft teeth. Oil  seals  prevent  the  loss  of  lubricant  from  the  housing where the pinion shaft and axle shafts protrude. As a mechanic, you will encounter the final drive gears in the spiral bevel and hypoid design.
Spiral Bevel Gear
Spiral bevel gears have curved gear teeth with the pinion and ring gear on the same center line. This type of  final  drive  is  used  extensively  in  truck  and occasionally in older automobiles. This design allows for constant contact between the ring gear and pinion. It also necessitates the use of heavy grade lubricants.
Hypoid Gear
The hypoid gear final drive is an improvement or variation of the spiral bevel design and is commonly used in light and medium trucks and all domestic rear- wheel drive automobiles. Hypoid gears have replaced spiral bevel gears because they lower the hump in the  floor  of  the  vehicle  and  improve  gear-meshing  action. As you can see in figure 5-13, the pinion meshes with the ring gear below the center line and is at a slight angle (less than 90 degrees).
Figure 5-13.—Types of final drives.
This angle and the use of heavier  (larger)  teeth  permit  an  increased  amount  of power to be transmitted while the size of the ring gear and housing remain constant. The tooth design is similar to the spiral bevel but includes some of the characteristics of the worm gear. This permits the reduced drive angle. The hypoid gear teeth have a more pronounc
ed curve and steeper angle, resulting in larger tooth areas and more teeth to be in contact at the same time. With more than one gear tooth in  contact,  a  hypoid design increases  gear  life  and reduces  gear  noise.  The  wiping  action of the teeth causes  heavy  tooth  pressure  that  requires  the  use  of heavy grade lubricants.
Double-Reduction  Final  Drive
In the final drives shown in figure 5-13, there is a single fixed gear reduction. This is the only gear reduction in most automobiles and light- and some medium-duty  trucks  between  the  drive  shaft  and  the wheels.
Double-reduction final drives are used for heavy- duty trucks. With this arrangement (fig. 5-14) it is not necessary to have a large ring gear to get the necessary gear  reduction.  The  first  gear  reduction  is  obtained through a pinion and ring gear as the single fixed gear reduction final drive. Referring to figure 5-14, notice that  the  secondary  pinion  is  mounted  on  the  primary ring gear shaft. The second gear reduction is the result of the secondary pinion which is rigidly attached to the primary ring gear, driving a large helical design翻译
gear which is attached to the  differential case. Double-reduction final drives may be found on military design vehicles, such as the 5-ton truck. Many commercially designed vehicles of this size use a single- or double-reduction final drive with provisions for  two  speeds  to  be incorporated

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