TRANSMISSION SYSTEM - GEAR BOX

Necessity Of Gear Box In An Automobile  

• The  gear  box  was  necessary  in  the  transmission  system  to maintain engine  speed at the  most  economical value  under  all  conditions  of vehicle movement. 
• An ideal  gear box would provide an  infinite range of  gear ratios, so that the engine speed should be kept at or near that the maximum power was developed what ever the speed of the vehicle.
 Function Of A Gear Box 
• Torque  ratio  between  the  engine  and wheels to  be  varied  for rapid acceleration and for climbing gradients.
 • It provides means of reversal of vehicle motion.
 • Transmission  could  be  disconnected  from  engine  by  neutral position of gear box 

CLASSIFICATION 

•  AUTOMATED TRANSMISSION

• MANUAL TRANSMISSION 

Stages of automatic transmission  

·        Park(P) :- selecting the park mode will lock the transmission, thus restricting the vehicle from moving.

·        Reverse( R) :- selecting the reverse mode puts the car into reverse gear, allowing the vehicle to move backward.

·        Neutral (N) :- selecting neutral mode disconnects the transmission from the wheel.

·        Low (L) :- selecting the low mode will allow you to lower the speed to move on hilly and middy areas.

·        Drive (D) :- selecting drive mode allows the vehicle to move and accelerate through a range of gears

 Selective Type Gear Box  

    
  • It  is  the  transmission  in  which  any  speed  may  be  selected  from  the neutral  position
.     
  • In   this  type  of  transmission  neutral  position  has  to  be obtained  before  selecting  any        forward  or  reverse  gear.  Some  selective  type gear boxes are,

       1.  Constant mesh gear box with positive dog clutch.

       2.  Constant mesh gear box with synchromesh device.

       3.  Sliding mesh gear box 

 Sliding mesh gear box

It was the simplest and oldest type of gear box.
1.  The clutch gear was rigidly fixed to the clutch shaft.
2.  The  clutch  gear  always  remains  connected  to  the  drive  gear  of countershaft.
3.  The other lay shaft gears are also rigidly fixed with it.
4.  Two gears are mounted on the main shaft and could be sliding by shifter yoke when shifter was operated.
 5.  One  gear  was  second  & top speed  gear  and  the  other  was  the  first  and  reverse speed gears. All gears used are spur gears.
 6.  A reverse idler gear was mounted on another  shaft  and  always  remains connected to reverse gear of counter shaft
  
 FIRST GEAR 

• By operating  gearshift  lever, the larger gear  on main  shaft was made to slide  and mesh  with  first  gear  of  countershaft.  

• The  main  shaft  turns  in  the same direction as clutch shaft in the ratio of 3:1.

 SECOND GEAR

 • By  operating  gear  shaft  lever,  the  smaller  gear  on  the  main  shaft  was made to slide and mesh with second gear of counter shaft.
  • A gear reduction of approximately 2:1 was obtained.  

TOP GEAR

• By  operating gearshift  lever,  the combined  second speed gear  and top speed gear was forced axially against clutch shaft gear.  
• External teeth on clutch gear mesh with internal teeth on top gear and the gear ratio was 1:1. 

 REVERSE GEAR

• By  operating  gearshift lever,  the  larger  gear of  main shaft was  meshed with  reverse  idler  gear.
 • The  reverse  idler  gear  was  always  on  the  mesh  with counter  shaft  reverse  gear.  Interposing  the  idler  gear, between  reverse  and main shaft gear, the main shaft turns in a direction opposite to clutch shaft. 

NEUTRAL GEAR

• When  engine  was  running  and  the  clutch  was  engaged,  clutch  shaft  gear drives the drive gear of the lay shaft and thus lay shaft also rotates.
• But the main shaft remains stationary as no gears in main shaft are engaged with lay shaft gears. 

 CONSTANT MESH GEARBOX 

Explanation about the constant mesh gearbox

• In this type of gearbox, all the  gears of the main shaft are in constant mesh  with  corresponding  gears  of  the  countershaft.  

• The  gears  on  the  main shaft  which  are bushed  are  free  to  rotate.

• The dog clutches are  provided  on main shaft.

• The gears on the lay shaft are, however, fixed. • When  the  left  Dog  clutch  is  slide  to  the  left  by  means  of  the  selector mechanism, its  teeth  are engaged  with  those  on  the  clutch  gear  and  we  get the  direct  gear

• The  same  dog  clutch,  however,  when  slide  to  right  makes contact with the second gear and second gear is obtained.

• Similarly  movement  of  the  right  dog  clutch  to  the  left  results  in  low gear and towards right in  reverse gear. Usually the helical gears are used in constant mesh gearbox for smooth and noiseless operation 

SYNCHROMESH GEARBOX 

During engaged the synchromesh gearbox
Explanation about the synchromesh gearbox
 • This  type  of  gearbox  was  similar  to  the  constant  mesh  type  gearbox. 
• Instead of  using  dog  clutches  here  synchronizers are  used.
 • The modern cars use helical gears and synchromesh devices in gearboxes, that synchronize the rotation of gears that are about to be meshed  SYNCHRONIZERS
• This type of gearbox was similar to the constant mesh type in that all the gears  on  the main  shaft  are in  constant mesh  with the  corresponding  gears on the lay shaft.
 • The gears  on the lay shaft are fixed to it while those on the main  shaft  are free to rotate on the  same. 
• Its working was also  similar  to  the constant mesh type, but in the former there was one definite improvement over the  latter.
 • This  was  the  provision  of  synchromesh  device  which  avoids  the necessity of  double-declutching. 
• The  parts  that ultimately  are to be engaged are  first  brought  into  frictional  contact,  which  equalizes  their  speed,  after which these may be engaged smoothly.
• Figure shows the construction and working of a synchromesh gearbox. In most of the cars, however, the synchromesh devices are not fitted to all the gears as was shown in this figure.
 • They are fitted only on the high gears and on the  low  and  reverse  gears  ordinary  dog  clutches  are  only  provided.
 • This  was done to reduce the cost.