Synchromesh Gearbox – Construction, Working, Diagram

Discover the synchromesh gearbox, a vital component of modern vehicles. Learn about its construction, workings, and advantages for smooth shifting. See where it’s commonly used and explore its unique features. Check out our detailed diagram to visualize the gearbox’s structure.

What Is Synchromesh Gearbox?

A synchromesh gearbox is the latest iteration of the constant mesh model in manual transmissions. It facilitates gear changes between rotating gears that maintain the same speed. The gears in the layout shaft of a synchromesh gearbox can either rotate freely or be locked in place.

This model is an upgrade to the previous dog clutch design. The key component in a synchromesh gearbox that maintains stable transmission is the synchronizer. This is a clutch device that permits components to rotate at different speeds and uses friction cones to synchronize their motion.

The synchromesh gearbox consists of two interlocking components: the central cone and the bolt ring. The cone is the array part and the ring serves as the synchronizer. When the components spin at the appropriate speed, the bolt ring engages the gear, slowing or speeding it as it enters the cone through friction.

As a result, the synchronizer and gear attain a balanced speed and rotate at the same speed. The gears on the shaft are attached to these components, while the gears on the layout shaft are able to rotate freely.

 Learn MoreWhat Is Sliding Mesh Gearbox?

Synchromesh Gearbox Diagram

What Is A Synchromesh Gearbox?
Synchromesh Gearbox Diagram

Synchromesh Gearbox Construction

1. Main Shaft Gear

A diaphragm shaft serves as a link between the descending apparatus and gears to the output shaft. As depicted in the figure, gears B, C, D, and E are rotatable gears that mesh with the appropriate gears on the appropriate shaft, with the main shaft as the axis of rotation. The continuous rotation occurs in shaft A and the left shaft, and all changes are reflected in the main shaft.

2. Layout Gear

This refers to the central shaft that facilitates the transfer of rotational motion from the input shaft to the final output shaft, being equipped with appropriately-sized gears. The fixed gears on the countershaft (Lashel) are labeled U1, U2, U3, and U4, as shown in the figure.

3. Clutch Shaft

This shaft serves as the intermediary between the engine output and the transmission box, being inserted into the transmission mechanism.

4. Cone Synchromesh

This device features two buttons on its side, the first being a hollow cone and the second being a ring with teeth. The gear engages with the cone and teeth during the operation.

5. Synchronizer

Synchronizer boxes are equipped with special shifting mechanisms that incorporate conical grooves on their surface for friction contact with the gear to be engaged. This ensures that the main shaft, input shaft, and clutch shaft are rotating at equal speeds, facilitating smooth gear shifting.

6. Gear Lever

This is an operator-controlled shifting lever utilized to select the desired gear, such as gears 1, 2, 3, 4, 5, or reverse.

 Learn MoreWhat Is Constant Mesh Gearbox?

Synchromesh Gearbox Working

Construction Of Synchromesh Gearbox
Construction Of Synchromesh Gearbox – Image Source

The gears located on the clutch shaft, lay shaft, and main shaft rotate continuously in unison as long as the clutch shaft is powered by the engine. When the driver intends to shift into first gear, they must first disengage the clutch, which halts the rotation of these parts.

Subsequently, the driver operates the gear shifter to move into first gear, activating the shift rod. The shift rod then causes the fork to move the synchronizer collar toward the first gear. The blocker ring functions by coming into contact with the gear’s teeth or straight-cut teeth, thereby locking onto the gear and synchronizing its speed.

Upon the driver reengaging the clutch, the blocker ring prompts the rotation of the synchronizer sleeve in tandem, effectively aligning the speeds of the rotating components. Once the speeds are aligned, the synchronizer sleeve moves the gear into an interlocking position with the output shaft.

To engage the second gear, the driver disengages the clutch and utilizes the gear shifter to disengage the first gear and select the second gear position. This results in the fork moving the synchronizer from the first gear to the second gear. The blocker ring and sleeve are then pushed, bringing the speeds into synchronization and interlocking the second gear.

The fifth gear is engaged by the driver disengaging the clutch and shifting the gear shifter from the fourth to the fifth gear position. The fork then moves the synchronizer from the fourth to the fifth gear. The blocker ring and sleeve are subsequently pushed, synchronizing the speeds and interlocking the fifth gear.

Synchromesh Gearbox Principle

A common problem in gearboxes arises when the stationary gear is shifted while the other gear is rotating at a higher speed. To address this issue, the principle of “engaging gears only after equalizing their speeds” is employed. This means that the gears must come into contact with each other before engagement, allowing for their speeds to be equalized.

Synchromesh Gearbox Advantages

  1. Elimination of the need for de-clutching, as is the case with constant mesh gearboxes.
  2. Lower noise levels due to the utilization of helical gears with teeth cut at a specific angle.
  3. Smooth gear transitions are facilitated by the presence of a synchronizer device.
  4. No torque loss occurs during transmission due to the use of synchronizers, leading to improved power transmission.
  5. Reduced vibration of the gearbox.
  6. The synchromesh gearbox effectively eliminates the clattering and grinding problems commonly encountered with constant mesh gearboxes.

Synchromesh Gearbox Disadvantages

  1. High cost of manufacture due to the complex array of components and intricate design.
  2. Prone to failure when gear teeth are unable to engage as a result of unequal rotational speed, resulting in a loud, grinding noise.
  3. Vulnerability to damage from improper handling, owing to its delicate construction.
  4. Inadequate capacity to handle heavy loads, due to limitations in its design.

Synchromesh Gearbox Applications

1. Automobiles – Synchromesh gearboxes are extensively used in passenger cars, sports cars, and racing vehicles to ensure smooth and efficient gear shifting.

2. Heavy-duty vehicles – These gearboxes are also employed in heavy-duty vehicles like trucks, buses, and construction equipment to provide reliable and smooth gear shifting.

3. Motorcycles – Motorcycles also utilize synchromesh gearboxes to achieve seamless gear shifting while in motion.

4. Aerospace – In the aerospace industry, synchromesh gearboxes are employed in certain aircraft to ensure precise and efficient control over the engine and propellers.

5. Industrial machinery – These gearboxes are commonly used in a variety of industrial machinery, such as printing presses, wind turbines, and elevators, to provide smooth and precise control of the machinery’s movements.


The synchromesh gearbox used in which vehicle?

Synchromesh gearboxes are commonly used in various types of vehicles, including automobiles (passenger cars, sports cars, racing vehicles), heavy-duty vehicles (trucks, buses, construction equipment), motorcycles, and some aircraft. They are favored for their ability to provide smooth, efficient, and precise control of the machinery.

What is a synchromesh gearbox?

A synchromesh GEARBOX is a manual transmission system that employs synchronizing mechanisms to achieve smooth and effortless shifting of gears. The use of friction cones is central to the functioning of a synchromesh gearbox, as they allow for the synchronization of the rotational speed of the gears, resulting in more accurate and seamless gear shifting.

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