Synchromesh Gearbox – Construction, Working, Advantages

Synchromesh gearbox transmission system is an extension of a constant mesh type transmission system. In 1928, Engineers from General Motors developed a Synchromesh transmission system in order to overcome the problems of the constant mesh transmission system.

Synchromesh Gearbox |
Example of Synchromesh Gearbox

In the Synchromesh gearbox system, the dog clutch from the constant mesh gearbox is replaced with the synchromesh device. This synchromesh device makes the gearbox more compact, smooth, and noise-free operation of gears.

Usually, it comes with 5 speed 1 reverse manual transmission configuration. The best example of it is Maruti Suzuki Swift model.

Why there is a need for Synchromesh Gearbox?

Someone has said that necessity is the mother of all inventions. The synchromesh gearbox was developed in order to overcome problems of sliding mesh and constant mesh gearbox.
Let us discuss, why there was a need for the development of Synchromesh gearbox.
  1. Gear shifting response in constant mesh gearbox was not quick because the dog clutch used in constant mesh gearbox needed to mesh with pair of gears which is not a quick and fast responsive task.
  2. In the constant-mesh gearbox, there was a lack of a mechanism that can bring all the rotating shafts i.e. clutch shaft, layshaft, main shaft at the same rotating speed which was responsible for the harsh shifting of gears.
  3. This caused more wear and tear of the dog clutch resulting in more maintenance of the system.
  4. The constant-mesh gearbox was not that compact and it has to be less noisy.

These problems lead the engineers from General Motors to develop a special type of gearbox popularly known as Synchromesh gearbox which will overcome all the above problems.

Synchromesh Gearbox Construction and Working

The synchromesh gearbox is an extension of the constant mesh type gearbox.

Synchromesh type of gearbox is used over the constant type of gearbox because constant mesh type suffers from the problem of clashing of dog clutch due to the large difference in speeds of engaging gears.

Synchromesh gears ensure initial frictional contact amongst mating parts of gears to equalize the speed of gears pair and then push for smooth engagement of dog clutch with gear on the main shaft.

synchromesh gearbox construction diagram
synchromesh gearbox construction diagram

Construction of Synchromesh Gearbox

The synchromesh gearbox contains the following components:
  • Clutch shaft
  • Layshaft
  • Main shaft
  • Reverse idler shaft
  • Bearing
  • Transmission gears
  • Synchromesh device/synchronizer
Synchromesh Gearbox 3D view |
3D view of Synchromesh Gearbox

The Clutch shaft is the input shaft of the gearbox as it is connected to the output shaft of the engine.
Layshaft is the intermediated shaft on which gears are mounted which transmits power from the clutch shaft to the output shaft.

The main shaft is the splined output shaft. Synchronizers and gears are mounted on the main shaft.

Synchronizers are special shifting devices that have conical grooves cut over their surface which provides frictional contact to the gears that are meshed to equalize the speed of the clutch shaft, layshaft, and main shaft. This provides smooth shifting of gears.

Clutch Shaft

  • It is the input shaft to the gearbox.
  • Its outer end is connected to the clutch disc.
  • It has a gear machined at its inner end that meshes with a respective gear on the layshaft.

Lay Shaft

  • The layshaft is freely suspended in bearing mounted on the transmission case.
  • It has gears rigidly mounted/machined on it.

Main Shaft

  • It is the output shaft of the gearbox.
  • It has splines cut across its length to accommodate axial movement of gears on it.
  • its outer end is connected to the propeller of the shaft through a universal joint.


  • Generally, taper roller bearings are used.
  • These bearings are required to take radial and thrust loads during gear engagement.

Reverse idler shaft

  • It is a short shaft that supports the reverse idler gears.

Transmission gears

  • In synchromesh gearboxes, generally helical gears are used.
  • These gears can be grouped into
    Gears on the clutch shaft – rigidly attached/machined
    Gears on layshaft – rigidly attached/machined
    Gears on main shaft – free to rotate on main shaft
    Gears on reverse idler shaft – rigidly attached/machined

Synchromesh Devices

  • It consists of sliding dog members

Selector mechanism

  • The selector mechanism employed is similar to that used on the sliding mesh gearbox.

Transmission case

  • It provides support for bearings and shafts.
  • It also provides an enclosure for lubricating oil.
  • Generally, it is made up of aluminum to reduce weight.
  • It has a vent on its top surface to ensure atmospheric pressure inside the gearbox.

Working of Synchromesh Gearbox

Synchromesh Gearbox Layout |
The figure shows the construction and working of the synchromesh gearbox. The gears B, C, D, E are on the main shaft and are always in mesh with the corresponding gears on the layshaft. Hence, all the gears on the main shaft and layshaft start rotating as shaft A starts rotating.
F1 and F2 are the members free to slide on the main shaft. G1 and G2 are the ring-shaped members having internal teeth fit onto the external teeth of members F1 and F2. K1 and K2 are the dog teeth of B and D respectively. K1 and K2 also fit onto teeth of G1 and G2 respectively.
T1 and T2 are the balls that are supported by the springs. These tend to prevent the sliding of the members G1, G2 on F1, F2. However, when the force is applied through G1, G2 fork S1, S2 exceeds a certain value. Members G1, G2 slides over F1, F2.
For direct gear, the member G1 and thus member F1 is slid towards the left till there comes M1 and M2. Rub and friction make their speed equal. Further, pushing member G1 to the left causes it to override the ball and get engaged with the dog K1. Now, the drive to the main shaft is direct from B through F1 and splines. We have to give sufficient time for synchronization of speeds, otherwise, a clash may result.
For the second gear, the members F1 and G1 are slid to the right so that finally the internal teeth on G1 are engaged with L1. Then the drive to the main shaft will be from B through U1, U2, C, F1, and the splines.
For the first gear, the members G2 and F2 are moved towards the right. Then the drive will be from B through U1, U3, D, F2, and the splines to the main shaft.
For the reverse gear, the members G2 and F2 are slid towards the right. Then the drive will be from B through U1, U4, U5, D, F2, and the splines to the main shaft.

Advantages of Synchromesh Gearbox

  • Synchromesh gearbox creates negligible noise during engagement and disengagement of gear.
  • Avoids double-declutching.
  • Quick shifting of gears.
  • Compact in size.
  • Used in most modern cars.
  • No wear and tear of gears.
  • Low vibration.

Disadvantages of Synchromesh Gearbox

  • Expensive due to more number of gears and high manufacturing costs.
  • Not capable enough to handle high loads.
  • Damage possibility due to improper handling is high.

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