Sliding Mesh Gearbox

Sliding mesh gearbox is the simplest type of gearbox. It looks similar to a constant mesh gear box, except that the main shaft gears are not always in contact with the counter shaft gears.

The individual gear ratio is obtained by sliding the selected gear wheel axially. The gear wheels are splined in the main shaft and can be slid to obtain different gear ratios.

One major problem with sliding mesh is the absence of synchronizer units as found in the constant mesh. While changing the gear ratio, the speeds have to be matched before engagement of the gears. Due to the unavailability of synchronizer teeth, the gears might collide with each other and generate a lot of noise. This can even damage the gears.

The gears are provided with spur straight teeth, in order to avoid any side thrust while engagement. This type of gearbox is used in very few vehicles where compact assembly is required. The shaft and gears are made of low alloy nickel-chromium-molybdenum steel.  

Design:

The input shaft from the engine drives the counter shaft. Both input and counter shaft gears are in constant mesh with each other. All the other gears on the counter shaft are rigidly fixed and rotate with the shaft. The main shaft or the output shaft is held in the same axis as that of the input shaft.

The main shaft gears are not in constant mesh with the counter shaft gears. The gear wheels on the main shaft can be slid axially to achieve different gear ratios. The different gear ratios are obtained as following:

Neutral Gear:

In this case, none of the main shaft gears are meshed with the counter shaft gears. Hence, the drive from the input shaft is not transferred to the main shaft. The wheels remain stationary.

1^st Gear:

In 1^st gear, the gears D and F are meshed to form a lower gear ratio. Since the gear F is bigger (30 teeth) and has more number of teeth than the gear D (10 teeth), the main shaft rotates at a lower speed. The torque is highest and speed is lowest at 1^st gear. This is to facilitate easy movement of the car from standstill.

If the counter shaft is rotating at 1000 rpm (N1), then the main shaft speed (N2) can be calculated as

N1/N2 = T_F/T_D

N2 = N1 X (T_D / T_F)

N2 = 1000 X (10/30)

N2 = 333.33 rpm

Where,  N1 = speed of counter shaft

              N2= speed of main shaft

              T_D = number of teeth in gear D

              T_F = number of teeth in gear F

2^nd Gear:

In 2^nd gear, the gear E is slid and meshed with gear C. The number of teeth in gear E (T_E) is reduced, or example T_E = 22. The number of teeth in gear C is increased (T_C= 15). This increases the speed of the car and the torque is reduced.

If the counter shaft speed (N1 = 1000 rpm), then the main shaft speed (N2) is

N2 = N1 X (T_c / T_e)

N2 = 1000 X (15/22)

N2 = 681.81 rpm

3^rd gear or Top gear:

In 3^rd gear, the main shaft is slid axially to mesh with the input shaft. In this case, the drive from the input shaft is directly transferred to the main shaft. The input shaft has a gear with internal teeth that mesh with the main shaft gear with external teeth. The vehicle can achieve top speed in top gear and the torque is lowest at this point.

Reverse Gear:

In reverse gear mechanism, an idle gear ‘I’ is used in between the gears G and F. The gears G and F are not in direct contact. The idle gear I is driven by G and the gear I drives the gear F. In this way, the direction of rotation of the main shaft is reversed and hence the vehicle moves backward.