Hub reduction

A hub reduction unit reduces the speed of the axle shaft immediately before the driven wheel, while increasing the available torque at the wheel hub. This system generally consists of a sun gear on the axle shaft, several planetary gears with needle bearings or plain bearings, a planet carrier and a fixed ring gear. These components are installed in a hub reduction housing mounted on the wheel hub. The reduction ratio relieves the differential, propeller shaft and axle shafts under high drive torques. This is why planetary hub reduction units are mainly found on heavily used MAN TGA, TGS, TGX and TG3 vehicles, Mercedes Actros, Arocs, Axor and Econic trucks, as well as certain MAN Lion’s City, Lion’s Coach, Neoplan and Mercedes Citaro buses.

The version depends on the axle series, permitted axle load, brake configuration, gear ratio, hub diameter and axle shaft connection. The gearing of the sun gears, the number of planetary gears, the size of the bearing seats and the position of oil channels also vary by version. Parts must therefore always be identified using the vehicle chassis number. Visual comparison or selection based solely on the model name is never sufficient, even when hub housings or planet carriers appear identical externally.

Typical faults include pitting on gear flanks, broken teeth, excessive radial play in planetary gears, damaged sun gear teeth and leakage through hub seals or flange seals. A rattling or knocking noise that is particularly audible during acceleration, engine braking or under maximum load often indicates gear backlash or bearing damage. A continuous humming noise, on the other hand, may originate from the wheel bearings, but must always be distinguished from damage in the planetary gear assembly.

Start the diagnosis by checking the oil level and condition of the specified transmission oil in the hub reduction unit. Dark oil, a burnt smell or visible metal particles indicate overheating and internal wear. If in doubt, carry out an oil analysis: ferromagnetic wear particles usually indicate gears or rolling bearings, while non-ferrous metals may point to bronze thrust washers or bearing cages. Then, with the axle raised, check the circumferential play at the wheel and measure radial and axial movement using a dial indicator. Compare the measurement with the factory tolerance for the relevant axle. Rotate the wheel slowly and use a stethoscope on the hub housing to listen for local knocking or grinding bearing noises.

During disassembly, sun gears, planet carriers, ring gears, axial thrust washers, needle bearings and mating surfaces must be systematically inspected. Never replace a damaged gear individually if the mating gear teeth show signs of pitting, blue discolouration or material displacement. Gears that have worn in together must be assessed as a technically matching set. Also inspect the hub housing for worn bearing seats, hairline cracks around the wheel bolt holes and deformation of the sealing surfaces.

Thoroughly clean all components and remove metal debris from the hub housing, oil channels and magnetic drain plug. Install new seals using the correct installation tool to prevent the sealing lip from folding over or the fit from being damaged. Adjust bearings and axial play only according to the specified tolerances, and tighten fastening bolts to the correct torque and required angle of rotation. After installation, fill only with the specified transmission oil to the correct level. A test drive under load, followed by checks for temperature differences, leaks and oil contamination, helps prevent repeated failures and costly downtime. Braem supports this with a wide availability of axle parts and technical identification based on the chassis number.