Synchronisation

During a gear change, the synchronizer brings the gear and sliding sleeve to the same rotational speed before their teeth engage. In a manual main gearbox, this prevents gear grinding and reduces the load on the engagement teeth. A synchronizer assembly may consist of a cone, one or more synchronizer rings, a sliding sleeve, a hub and retaining components. Depending on the transmission design, the gearbox range section may also be synchronized. The assembly design and friction material determine how quickly rotational speeds are matched under load.

In a synchronized gearbox, selecting a gear causes the sliding sleeve to press the synchronizer ring against the cone of the corresponding gear. Friction matches their rotational speeds. Only then can the sliding sleeve move over the engagement teeth without grinding and engage the gear.

In an unsynchronized main gearbox with engine-controlled speed matching, the gears in the main gearbox do not have synchronizer rings. During a gear change, drive torque is interrupted and the main gearbox is shifted into neutral. The engine control system then increases or decreases engine speed to match the rotational speeds of the components being connected. The dog teeth of the selected gear then engage. In this type of dog-clutch gearbox, speed matching in the main gearbox is therefore not achieved through friction between a synchronizer ring and a cone.

MAN TGL and TGM vehicles with a manual ZF Ecolite or Ecomid gearbox use different synchronizer components from a MAN TGA, TGS or TGX with a ZF Ecosplit. Designs and gear ratios also differ between Ecosplit III, New Ecosplit III and Ecosplit IV. For Mercedes Atego, Axor, Actros and Arocs, as well as older SK vehicles, the correct application depends on the transmission fitted. These include Mercedes gearbox types G85-6, G131-9, G211-12, G240-16 and G281-12. The same approach applies to buses and vans: first check whether the gear or shift section in question is actually synchronized.

Alongside these manual gearboxes, automated types are also used, such as ZF AS Tronic, ZF TraXon and Mercedes PowerShift. When identifying these gearboxes, it is important to distinguish between a synchronized shift section and an unsynchronized main gearbox with engine-controlled speed matching.

Synchronizer rings are available with metal friction surfaces or specific friction linings, among other designs; single-cone and multi-cone designs are not necessarily interchangeable. Always identify parts using the vehicle’s chassis number, then verify the transmission identification and part numbers. Visual comparison or selection by model name alone is never sufficient: small differences in cone angle, fit, tooth profile or axial installation dimension can impair shifting.

Grinding during one specific gear change, increased shift effort or a delay before the gear engages may indicate a worn synchronizer. If a gear jumps out under load, the engagement teeth, sliding sleeve, shift fork and shaft end play also need checking. If several gears are difficult to engage, first check whether the clutch disengages fully and inspect the shift mechanism. This helps prevent the same problem from persisting after the gearbox has been repaired.

During a test drive, note whether symptoms occur on upshifts or downshifts, when cold or warm, and at different engine speeds. Then check the transmission oil level and the specified oil grade; incorrect oil can affect the friction characteristics of the synchronizer rings. Inspect the drained oil and magnet for metal particles. After disassembly, measure the axial end play of the shafts and shift sections with a dial indicator. Use a feeler gauge to measure the remaining clearance between the synchronizer ring and gear cone, and compare it with the specification for the transmission concerned. Also inspect cone surfaces for scoring or discoloration and measure wear on the shift forks and their guides in accordance with the manufacturer’s instructions.

Do not replace only a worn ring if its associated cone, hub or sliding sleeve is also damaged. Clean the oil passages, check the roller bearings and set the specified clearances using suitable shims. Install friction components clean and use only the specified transmission oil. After assembly, check shifting operation and carry out a test drive under the conditions in which the problem occurred. If several shift sections are extensively damaged, overhauling or replacing the complete gearbox may be more practical; technical support with identification can help prevent unnecessary downtime.