Synchromesh Gearbox : Definition, Parts, Working, Pros & Cons, Applications [PDF]

Ever since the inception of the constant mesh transmission system, inventors have been experimenting with various modifications to overcome its inherent limitations. The primary issue with the constant-mesh gearbox was its tendency to deliver a rough and noisy transmission.

One major drawback caused due to the use of dog clutches in the constant mesh system, which directly meshed with rotating gears, resulting in a harsh gear-shifting experience. This design also exhibited signs of wear and tear, leading to increased maintenance requirements.

In 1919, engineer Earl Avery Thompson introduced the synchromesh gearbox as a solution to these issues. Specializing in mechanical and electrical engineering, Thompson’s synchromesh transmission design first found application in Cadillac and LaSalle cars in 1928.

What is a Synchromesh Gearbox?

Thompson sold his synchromesh transmission patents to General Motors in 1924 and 1930 for a reported fee of approximately $1 million. Despite this innovation, non-synchronous transmission systems remained prevalent in most cars until the 1950s.

Porsche took a significant step in 1947 by adopting the split ring synchromesh system, which later became the dominant design. The 1952 Porsche 356 became the first car to feature the synchromesh system on all forward gears.

What is a Synchromesh Gearbox?

The synchromesh gearbox is a type of manual transmission system where gear changes occur between gears rotating at the same speed. In this system, gears on the layshaft remain fixed, while gears on the main shaft can either roll freely when disengaged or lock onto the layshaft when engaged.

Construction or Parts of Synchromesh Gearbox

Construction or Parts of Synchromesh Gearbox

1. Clutch shaft:

  • The input shaft carrying engine power output to the gearbox. Also known as the input shaft, it houses the input gear.

2. Layshaft:

  • An intermediate shaft with mounted gears transmitting motion from the clutch shaft to the output shaft. Also referred to as a countershaft.

3. Main shaft:

  • The shaft transmitting torque/power generated in the gearbox to the vehicle’s differential. Also known as the output shaft.

4. Needle bearing:

  • Mounted on the main shaft, locking the gear onto its teeth and facilitating synchronized rotation.

5. Synchronizer:

  • Comprising a synchronizer hub, sleeve, and blocker ring, it synchronizes the speed of engaged gears, ensuring smooth transitions.

6. Helical gears:

  • Including input, counter, and output gears mounted on respective shafts, these gears have helical cut teeth for quieter meshing.

7. Spur gears:

  • Dog’s teeth used to lock onto the synchronizer blocker ring, allowing synchronized rotation.

8. Idler gear:

  • Reverses the output shaft direction when engaged between the counter and output gears.

9. Shift fork:

  • Connected to synchronizers, it engages or disengages pairs of gears by moving synchronizers back and forth.

10. Shift rod:

  • Transfers force from the gear shifter to gears via the shift forks.

Synchromesh Gearbox Working Principle

The synchromesh gearbox operates on the principle of synchronizing gear and synchronizer fractions before engagement, ensuring matched speeds during engagement.

Step by Step Working of Synchromesh Gearbox:

#1 Gear Engagement:

  • When the driver needs to engage a gear, the clutch is disengaged, stopping the rotating parts.
  • The gear shifter is moved to the desired gear, initiating the synchronizer movement.
  • The blocker ring locks onto the gear’s teeth, syncing speeds.
  • Upon re-engaging the clutch, the synchronizer sleeve moves the gear, interlocking it with the output shaft.

#2 Shifting Gears:

  • For subsequent gears, the process involves disengaging the clutch, shifting gears using the shifter, and repeating the synchronizer engagement.

#3 Reverse Gear:

  • To engage reverse, the vehicle halts, and the clutch is disengaged.
  • The shifter is moved to reverse, introducing the idler gear between the first gear and counter gear.
  • Upon clutch re-engagement, the output shaft’s direction is reversed.

Advantages of Synchromesh Gearbox:

  • No declutching required: Unlike constant mesh gearboxes, synchromesh gearboxes do not necessitate declutching.
  • Reduced noise: Helical gears result in quieter operation compared to other gear types.
  • Smooth transition: Synchronizer devices facilitate smooth gear transitions.
  • No torque loss: Synchronizers ensure efficient power transmission without torque loss.
  • Less vibration: Synchromesh gearboxes experience lower levels of vibration.
  • Overcoming constant mesh issues: Clattering and grinding problems associated with constant mesh gearboxes are mitigated.
Pros and Cons of Synchromesh Gearbox

Disadvantages of Synchromesh Gearbox:

  • Costly: Manufacturing precision parts makes synchromesh gearboxes expensive.
  • Complex and bulky: The system’s intricate design and larger size pose challenges for servicing.
  • Space requirements: Greater space is needed for installation.
  • Potential damage: Improper gear engagement can lead to damage.
  • Load limitations: High loads may cause teeth to break, limiting its capacity.
  • Mixed gear systems: Some vehicles employ a synchromesh system only for higher gears to reduce costs.

Synchromesh Gearbox Application:

Most modern cars, and even racing cars with modifications, utilize synchromesh gearboxes for their ability to efficiently shift gears at high torque and speed.

Video on synchromesh gearbox


That was all about synchromesh gearbox. If you still have any questions, you can write us in the comments.

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