Construction and working of Synchromesh gearbox

Construction and Working of Synchromesh Gearbox

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 diagram

Construction of Synchromesh Gearbox

The arrangement and functions of various components of a typical synchromesh gearbox are mentioned below :
  1. Clutch shaft
  2. Layshaft
  3. Main shaft
  4. Bearings
  5. Reverse idler shaft
  6. Transmission gears
  7. Synchromesh Device
  8. Selector mechanism
  9. Transmission case

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 the 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 the main shaft – free to rotate on the 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.

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