Constant mesh gear box | Components Construction and working

constant mesh gear box is upgrade from sliding mesh gear box. Is was developed considering all the limitations of sliding mesh type gear box. It is called so because in constant mesh gear box all gears which are solely responsible for reducing or increasing RPM are in constant / continuous engagement.

Components of Constant mesh gear box :

In constant mesh gear box we have three types of shafts i.e. input shaft or clutch shaft, lay shaft or fixed gear shaft and main shaft or gear shaft. In this coupler rings and splined gears are addition to sliding mesh type gear box.

Design of gears on main shaft and lay shaft is different. Gears on layshaft has internal spline for mounting on shaft but their position is kept fixed with help of spacers.

Gears on main shaft has smooth internal cylindrical design to accommodate bearing. These gears have external splines on one side for engaging with Internal spline of coupler ring. These splines are rounded at ends for smooth engagement with coupler ring.

Between these gears coupler ring having end rounded internal spline and groove on outer surface for fork placement are mounted over spline gears. These spline gears has external spline for coupler movement and internal spline for mounting on gear shaft. These splined gears are directly fit on main shaft.

Input shaft has splines at three locations for mounting of clutch plate , reverse gear and main drive gear.

Construction of Constant mesh gear box:

Input shaft has 2 gears on it ,main drive gear fixed at one position with spline while reverse gear able to slide back and forth over spline. This reverse gear has slot for placement of fork which gives it movement. One end of this shaft is mounting of clutch shaft.

fixed gear shaft has set of gears having different size and number of teeth according to gear ratios fixed at one positions on splines with help of spacers.

Main shaft has set of gears mounted on bearings having constant engagement with gears of fixed gear shaft. These gears can freely rotate over bearings. In between these gears spline gears are placed.

These gear are splined with main shaft and so can rotate with shaft. Coupler rings is moved with help of forks on this splined gear. As said above main shaft gears has set of external splines on both ends.

These splines are of same size as that of splined gears. so whenever coupler is moved with help of fork it slides over spline of gear. Coupler ring position is such that is occupies half main gear and half splined gear. this way power transmission is completed.

constant mesh gear box
Constant mesh gear box

Working of constant mesh gear box:

To understand working of constant mesh gear box first have a look at above image, one by one we will engage each gear. This layout is for 5 speed gear box i.e. 4 forward and reverse.

Power is coming from engine to input shaft and transmitting to fixed gear shaft. Now all gears of fixed gear shaft are rotating because all them are splined to shaft and further they are rotating all gears of main shaft. Main shaft is not in rotation because gears of this shaft are on bearing. Now let us engage gears one by one.

  • 1st gear: Move bigger coupler ring towards right and engage with 1st gear. Meaning of engage here is such that coupler ring is half on gear and half on splined shaft. This way power transmits from rotating gears to splined gear and than to main shaft because splined gear and main shaft are splined to each other.
  • 2nd gear: Move bigger coupler ring towards left and engage with 2nd gear. Now power is transmitting through 2nd gear.

One fork is placed at groove of coupler ring which performs both these movements.

  • 3rd gear: Move small coupler ring towards right and engage with 3rd gear. Power starts transmitting from rotating gear to splined shaft and than to main shaft. Out put of gear box is main shaft itself.
  • 4th gear: Move same coupler ring towards left and engage with 4th gear. Now is transmitting from 4th gear to main shaft.

2nd fork is placed at groove of small coupler ring to perform 3rd and 4th gear engaging movement.

  • Reverse gear: Now this is going to be bit confusing as in above layout reverse gears look far away from each other. This layout was made just for understanding mechanism easily. On left side of this layout actual arrangement of all three shafts is given. In this position input shaft can make direct contact with both main shaft and fixed gear shaft i.e. with fixed gear shaft through main drive gear and with main shaft through reverse gear.

Now to engage reverse gear, move reverse gear of input shaft towards right and engage with reverse gear of main shaft. This way power gets transmitted to main shaft as these both gears are splined to their shafts but difference is input shaft gear can slide while main shaft gear position is fixed.

3rd fork is placed at groove of this gear for its sliding movement. so we have total of three forks and whenever anyone of them is at engaged position other two are at their neutral positions.

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