How Do Synchromesh Transmissions Work?

Since its inception, the automobile has been subject to constant and relentless refinement to the point where the average car now possesses some extremely clever and complex engineering.

One of the more impressive components in any automobile is its transmission or ‘gearbox’, and whilst most cars do not benefit from the technology, the synchromesh gearbox is what the single-clutch manual transmission aspires to be. At least for the time being.

Synchromesh - MAT Foundry

SLIDING MESH

The problem every transmission is looking to overcome is how to connect two moving parts rotating at different speeds without damaging them. Without the assistance of a modern transmission, the driver would have to attempt to manually match the engine speed of the vehicle (RPM) with the transmission speed (wheel speed) after disengaging the previous gear and before engaging the next one – slowing down gear changes, reducing vehicle speed, and wasting fuel.

This type of transmission is called a ‘sliding mesh’, as you have to slide the gears in and out of contact with each other, with the gear stick directly moving the gears and controlling the contact.

Because of the margin of error inherent when attempting to match engine and wheel speeds, the sliding mesh transmission is susceptible to damage caused by the gears grinding against each other when the speeds aren’t compatible.

As transmissions are precisely engineered with extremely small tolerances, the small fragments of metal that would chip off gears could cause significant damage resulting in costly repairs. At least with it being the most basic of transmissions, it is also the most robust, and could take more punishment than other types of transmission.

CONSTANT MESH

The sliding mesh system was subsequently improved upon to create the now ubiquitous ‘constant mesh’ transmission - which, as you can guess, devised a method to change between gears without interrupting the connection. It is the standard system for most cars.

The constant mesh transmission moved the problem of connecting two moving parts away from where the gears contacted each other to where the gears contacted the driveshaft which powered the wheels. Each gear was only loosely connected with the drive shaft, allowing the gear to rotate at a differing speed to the shaft and making gear changes easier.

This was accomplished with a device called a dog clutch. Several of these were positioned on the drive shaft between the gears, and were fixed to drive shaft. When ‘shifting gears’, it was these clutches instead of the gears that were moved by the stick – pushing them into contact with the gears. The loosely mounted gears would already be moving at some speed through their contact with the driveshaft, and the dog clutch moving at full speed would take them the rest of the way as they meshed together resulting in a smoother transition.

SYNCHROMESH

Synchromesh transmissions are a further refined version of the constant mesh system, although less common. How it improves the system is by adding another stage to the process of connecting the gears to the drive shaft through the dog clutch.

It splits the dog clutch in two – a gear fixed to the drive shaft called the synchronizer hub, and a collar around the outside of it that could slide back and forth called the shift sleeve.

A new component was added to the gears themselves – the synchroniser cone - and a further moving part called the synchronizer ring was introduced that surrounded the cone.

This is where it gets a bit complicated.

The collars, or shift sleeves, are now the components controlled by the gear stick - and they can slide halfway in either direction onto the synchroniser rings. This forces the rings against the synchroniser cones attached to the gears, and through the increased friction caused by the widening cone it can either speed up or slow down the gear to match the speed of the shift sleeve and synchronizer hub.

Once the speeds are closely enough matched, the sleeve can continue to slide over the blocker ring and directly mesh with both the cone and the gear, connecting everything together and transferring power to the drive shaft.

Unbelievably all of this happens in the fraction of second it takes you to shift gear, providing you with an even smoother gear change.

So the next time you seamlessly slide through the ratios, take a moment to appreciate all the thought and work that went into creating the complex arrangement of cogs helping you on your way.