Gearbox shafts and gears

In the main gearbox, input shafts, layshaft assemblies and output shafts transmit engine torque through gear pairs with different ratios. The correct combination determines not only the selected gear, but also the direction of rotation, noise level and load on the roller bearings and synchroniser components. When carrying out a repair, shafts and gears must therefore be assessed as interacting components, not solely on the basis of visible damage.

The input shaft carries engine torque into the gearbox. The layshaft and the selected gear pairs then transfer torque to the mainshaft. In the range section, the planet carrier forms part of the planetary gear train. If one of these components is damaged, also check the associated gear teeth and bearings.

Specifications vary by vehicle, engine torque and gearbox variant. MAN TGL and TGM vehicles may have ZF Ecolite or Ecomid manual gearboxes, while TGA, TGS and TGX vehicles may, for example, have a ZF Ecosplit, AS-Tronic or TraXon. Gear ratios, tooth profiles and shaft lengths can differ even between versions with the same number of gears. The exact gearbox version is equally important for Mercedes Atego, Actros and Arocs. Buses and vans have different loads and designs: a gear set is not interchangeable simply because the vehicles share the same brand name.

Other gearboxes include the ZF 6S-850, 9S-1110, 16S-2220 and 12AS-2330 in MAN vehicles, and the Mercedes G 85-6, G 131-9, G 211-12 and G 281-12, alongside Telligent and PowerShift 3. The exact gearbox code must also be checked for these variants.

Always identify parts using the vehicle’s chassis number, then verify the gearbox code, version and relevant part numbers. A visual comparison or selection based on the model name alone is never sufficient. When checking an individual shaft or gear, verify the gear ratio, number of teeth, gear module, helix angle, keyway or spline profile, bearing seats and position of retaining rings, among other details. The material and heat treatment of the tooth surfaces must also suit the intended load.

A whining noise in one gear is more likely to indicate a specific gear pair; noise in several gears may be related to shaft support or roller bearings. Knocking during changes in load, jumping out of gear and metal particles in the transmission oil call for immediate inspection. Look for pitting on tooth flanks, broken teeth, scuffing caused by inadequate lubrication, worn shaft journals and damaged splines. Overloading, incorrect transmission oil, a low oil level or a faulty bearing may be the underlying cause. Replacing only the visibly damaged gear will not prevent the problem from recurring.

Start with a road test and record which gear, load and engine speed produce the symptom. Then check the oil level, oil condition, leaks and drain plug magnet for steel particles. Locate the noise using a suitable contact microphone or stethoscope on the gearbox housing. After disassembly, measure shaft end play and runout with a dial indicator. Measure gear backlash with a dial indicator, check shaft diameters with a micrometer and assess the contact pattern using marking compound. Compare all measurements with the tolerances for the specific gearbox code.

Replace damaged gear pairs in the specified combination and inspect the roller bearings, bearing seats, spacer rings, synchroniser components and lubrication passages at the same time. A new gear set fitted to a bent shaft or in a worn bearing seat will again cause noise and accelerated wear. Thoroughly clean the housing and oil passages, set the specified clearances and preload, and fill with transmission oil that meets the gearbox specification. After assembly, check gear selection, leaks and noise during a road test. Where extensive damage has resulted from driving without sufficient lubrication, assessing the complete gearbox is wiser than carrying out an isolated repair.