Synchronized manual transmissions are widely used globally in both commercial vehicles and passenger cars, although they are less common in North America. These transmissions contain a complex array of components and materials that deliver longer service and better performance when the correct, dedicated lubricant is used.
Manual transmissions come in two main types: synchronized and unsynchronized. Unsynchronized transmissions require manual synchronizing, which depends on the skill of the driver at each shift event to synchronize gear speeds, particularly on the downshift. Unsynchronized transmissions are usually only found in motorsport applications or heavy-duty commercial vehicles. North American heavy trucks are typically equipped with unsynchronized manual transmissions, whereas European truck manufacturers tend to favor synchronized manual transmissions.
Classic synchro unit with brass synchro rings.
A synchronizer does exactly as the name suggests. It equalizes its speed with that of the next gear to be engaged, allowing a smooth, crunch-free selection. Modern synchronized manual transmissions are of the “constant mesh” variety. This means that idling (free spinning) gears on a main shaft are in constant mesh with a corresponding set of gears, machined as one single component and forming a second “lay shaft.”
The most common synchronizer design is the “cone clutch” or “blocker ring” type. Typically, gears are arranged on the main shaft in pairs; for example, first and second gears are adjacent, as are third and fourth. In between each pair is a synchronizer unit fixed to the shaft. The two key components in the synchronizer unit are the sleeve and the “blocker,” or “synchronizer,” ring. Gears are selected by the sleeve, which can be moved in either direction by the gearshift mechanism. When the driver selects first gear, the sleeve will move to the first gear and lock onto its gear engagement teeth (also known as “dogs”). The gear is then effectively locked to the main shaft and drive is taken up. When the driver de-clutches and selects second gear, the sleeve moves the other way, de-selecting first gear and selecting second in the same way.
Before the sleeve can lock on to each gear, however, the speed of both sleeve and gear must be synchronized. This is accomplished by a blocker (synchronizer) ring, one of which sits between the synchronizer and each gear. 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, as the shift event is taking place. As the surfaces of this “cone clutch” grip, the rotational speed of the gear becomes synchronized with that of the synchronizer sleeve and gear selection can be completed.
These blocker rings were traditionally made of brass; the internal conical surface was faced with fine grooves in order to provide better grip on the surface of the gear cone. In an older transmission, synchronization begins to fail (leading to crunching gears) when the internal surface of these blocker rings becomes significantly worn and their ability to grip the gear is reduced.
Earlier or more basic synchronized manual transmissions are equipped with one blocker, or “synchro,” ring per gear. However, the latest generation transmissions now feature double or triple cone synchronizers on the lower gears to facilitate smoother shifting and shorten the synchronization phase. Materials technology has improved, too. Brass is being replaced by molybdenum-based materials in commercial vehicles, sinter compositions, phenolics in Japan, and carbon materials. Each is chosen for its wear and friction performance.
Commercial vehicle and passenger car synchronizers follow similar principles, but the choice of materials reflects the much higher torque commercial vehicle transmissions must transmit. A typical heavy duty synchronization ring can be made from steel coated with molybdenum or carbon, with torque capacities as high as 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:
3. Neutral détente
7. Blocking release
8. Engagement tooth contact
9. Full engagement
Lubricating synchronizers is a complex proposition. Clearly, there is a need to prevent wear, but the synchronizer blocker rings still need to generate sufficient friction to perform the synchronization. That same lubricant also has to protect bearings and seals and resist degradation in the face of increasingly extended drain periods. It must also survive higher temperatures caused by reduced airflow due to improved vehicle aerodynamics and the increased energy density typical of modern, high performance powertrains.
Considering the long and hard life of synchronizers and their mechanical complexity, it becomes easier to understand the importance of using the correct fluid. Maintenance mistakes that shorten the life of a manual transmission include filling with engine oil or even automatic transmission fluid (ATF).
Dedicated manual transmission fluids (MTF) offer far better protection against wear and pitting. They combine high temperature resistance with high levels of gear and bearing protection, and they are individually designed to adapt to the behaviors of various synchronizor materials. Additive and viscosity modifier technology can be tailored during the design process to meet individual OEM specifications, so as to provide a fluid that functions as an integral component of the transmission.
The trend is toward lower viscosity MTFs that reduce churning losses and improve fuel efficiency, without compromising protection. This is achieved through the use of robust additive and sophisticated viscosity modifier technologies. In North America, the trend is toward SAE 75W-80 and 75W-90 viscosity grades. In emerging markets like China and India, the trend favors SAE 80W-90.
Using dedicated fluids has a major impact on the cost of equipment ownership, reducing service costs and fuel consumption, and delivering improved reliability. There’s also an environmental benefit, thanks to extended drain intervals. And, from the drivability point of view, shift quality is also improved. Using a dedicated MTF to protect manual transmissions does not represent a significant additional cost compared to using an inappropriate fluid, but it does have major benefits for both owners and drivers.
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