Synchronized manual transmissions are commonly employed in both commercial and passenger vehicles around the world, however, they are less prevalent in North America. When the proper, specific lubricant is used, these transmissions have a complex array of components and materials that provide longer service and higher performance.
There are two types of manual transmissions: synchronized and unsynchronized. Unsynchronized transmissions necessitate manual synchronization, which is dependent on the driver's ability to synchronize gear speeds at each shift event, notably on the downshift. Only motorsport applications and heavy-duty commercial cars use unsynchronized gearboxes. Unsynchronized manual transmissions are common in North American heavy vehicles, although synchronized manual transmissions are preferred by European truck manufacturers.
A synchronizer does exactly what it says on the tin. It adjusts its speed to match the pace of the next gear to be selected, resulting in a smooth, crunch-free selection. Constant mesh synchronised manual transmissions are the most common type of modern synchronised manual transmission. This means that idle (free spinning) gears on a main shaft are in constant mesh with a corresponding set of gears that are machined as a single component and form a second "lay shaft."
The “cone clutch” or “blocker ring” style of synchronizer is the most prevalent. Gears are usually located in pairs on the main shaft; for example, the first and second gears are next to each other, as are the third and fourth gears. A synchronizer device is attached to the shaft between each pair. The sleeve and the "blocker," or "synchronizer," ring are the two most important parts of the synchronizer unit. The sleeve, which may be moved in either direction by the gearshift mechanism, selects the gears.
The sleeve will shift to first gear and lock onto its gear engagement teeth (commonly known as "dogs") when the driver picks first gear. Drive is taken up once the gear is effectively locked to the main shaft. When the driver declutches and picks second gear, the sleeve travels in the opposite direction, de-picking first and selecting second in the same manner.
The speed of both the sleeve and the gear must be coordinated before the sleeve can lock on to each gear. A blocker (synchronizer) ring, one of which sits between the synchronizer and each gear, accomplishes this. As the shift event occurs, the inner face of the ring is conical, and this locates over a cone on the face of the hardened steel gear with a gripping action. The rotating speed of the gear is synchronised with that of the synchronizer sleeve as the surfaces of this "cone clutch" grasp, allowing gear selection to be accomplished.
These blocker rings were usually constructed of brass, with small grooves on the internal conical surface to provide stronger grip on the gear cone's surface. When the inside surface of these blocker rings becomes severely worn and their ability to grasp the gear is weakened in an older transmission, synchronisation begins to fail (leading to crunching gears).
One blocker, or "synchro," ring is installed in each gear in earlier or more rudimentary synchronised manual transmissions. However, newer transmissions have double or triple cone synchronizers on the lower gears to allow for smoother shifting and a faster synchronisation phase. Materials science has progressed as well. In commercial cars, sinter compositions, phenolics in Japan, and carbon compounds, brass is being replaced with molybdenum-based compounds. Each one was chosen based on its wear and friction properties.
The fundamentals of synchronizers in commercial vehicles and passenger cars are identical, but the materials used reflect the significantly higher torque that commercial vehicle transmissions must convey. A typical heavy duty synchronisation ring is comprised of steel coated with molybdenum or carbon, and can handle torques of up to 18,000 Nm (13,276 lb ft).
Although the process of synchronization might seem simple, in engineering terms it is defined by nine different stages. These are:
1. Disengagement
2. Neutral
3. Neutral détente
4. Pre-synchronization
5. Synchronizing
6. Synchronization
7. Blocking release
8. Engagement tooth contact
9. Full engagement
Synchronizer lubrication is a difficult task. Clearly, wear must be avoided, but the synchronizer blocker rings must still provide enough friction to achieve the synchronisation. The same lubricant must also protect bearings and seals, as well as withstand degradation during progressively long drain periods. It must also withstand higher temperatures due to lower airflow generated by enhanced vehicle aerodynamics and the increased energy density seen in current, high-performance powertrains.
When you consider the lengthy and difficult life of synchronizers, as well as their mechanical complexity, it's easy to see how important it is to use the right fluid. Filling a manual transmission with engine oil or even automatic transmission fluid is a common maintenance blunder that shortens its life (ATF).
Dedicated manual transmission fluids (MTF) provide significantly superior wear and pitting protection. They are specially engineered to adapt to the characteristics of various synchronizer materials, and they combine excellent temperature resistance with high levels of gear and bearing protection. In order to produce a fluid that operates as an essential component of the transmission, additive and viscosity modifier technology can be customized during the design phase to satisfy individual OEM criteria.
Lower viscosity MTFs are becoming more popular because they reduce churning losses and improve fuel efficiency without sacrificing protection. This is accomplished by the application of advanced additive and viscosity modifier technologies. The tendency in North America is toward SAE 75W-80 and 75W-90 viscosity grades. The trend in rising countries like China and India is towards SAE 80W-90.
Using dedicated fluids lowers the cost of equipment ownership by lowering servicing expenses and fuel usage while increasing reliability. There's also a benefit to the environment, thanks to the longer drain intervals. Shif quality has also been improved from the standpoint of drivability. Using a specific MTF to safeguard manual transmissions is not significantly more expensive than using an inappropriate fluid, but it has considerable advantages for both owners and drivers.
