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The ring and pinion gears are a matched set. They are lapped (meshed and spun together with an abrasive compound on the teeth) at the factory. Then one tooth on  each  gear  is  marked  to  show  the  correct  teeth engagement.  Lapping  produces  quieter  operation  and assures longer gear life. Spider Gears The spider gears are a set of small bevel gears that include two axle gears (differential side gears) and hvo pinion gears (differential idler gears). The spider gears mount   inside   the   differential   case.   A   pinion   shaft passes through the two pinion gears and case. The two side gears are splined to the inner ends of the axles. 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  drive  include 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. as shown in figure 5-13. 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). This angle and the use of Figure 5-13.—Types of final drives. 5-12

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