Hub reduction

A hub reduction unit on a driven front axle reduces the rotational speed between the drive shaft and the wheel hub, while increasing the available drive torque at the wheel. This system is used on heavy MAN and Mercedes trucks with driven front axles, particularly in construction transport, off-road operations, heavy distribution transport and vehicles with permanent or selectable four-wheel drive. Depending on the axle version, this may include MAN vehicles of the F2000, M2000, TGA, TGS, TGX and TG3 types or Mercedes vehicles of the SK, Axor, Actros and Arocs types. Always select a hub reduction unit, planetary gears, sun gears, ring gears, hub housings and seals based on the chassis number. Visual comparison or selection based on the model name alone is never sufficient.

The reduction unit generally consists of a planetary gear set in or at the wheel hub. The drive shaft from the front axle differential transmits torque via a steering joint and a half shaft to the sun gear. This sun gear drives several planetary gears, which run in a planet carrier and mesh with the internally toothed ring gear. The planet carrier or hub transfers the reduced torque to the wheel. By locating the reduction in the hub, the load on the differential, half shafts and propshaft is reduced at high wheel torques.

Compatibility is determined not only by the vehicle type, but also by the front axle code, reduction ratio, brake version, wheel hub version, half-shaft splines and the design of the steering knuckle and steering joint assembly. Left- and right-hand components are not always interchangeable either. The gear module, number of teeth, tooth flank hardening quality and fit of the planet shafts in the planet carrier must match exactly. An incorrect reduction ratio causes stress in the driveline, especially when four-wheel drive is engaged, and may lead to overheating or gear tooth breakage.

A faulty hub reduction unit can often be identified by a rattling, knocking or grinding noise near one front wheel, especially under traction or when changing between accelerating and engine braking. Oil leakage at the hub seal, metal particles in the gear oil, excessive hub temperature and noticeable play at the wheel are important warning signs. In severe cases, stripped gear teeth, seized planetary gears or a cracked planet carrier may occur.

First check the oil level and condition of the specified axle oil. If in doubt, have the oil analysed: ferromagnetic particles often indicate wear in gears, planet shafts or roller bearings; bronze-coloured particles may originate from plain bearings or bearing cages. Then compare the temperature of both hub reduction units after an identical drive using an infrared thermometer. A clearly higher temperature on one side indicates internal friction, insufficient lubrication or an excessively tight bearing fit.

With the vehicle safely jacked up, check the radial and axial wheel play using a dial indicator. Distinguish between play in wheel bearings, kingpins and the internal reduction gears. Have a second mechanic slightly rotate the driveline and measure the angle of rotation before the wheel starts to move. Excessive free play may indicate worn splines, worn planet shaft pins or damaged half-shaft spline connections. Also check the steering joint for play; a defective joint can transfer shock loads directly into the hub reduction unit.

In cases of limited damage, hub reduction units can be overhauled with new roller bearings, seals, planet shaft pins and matching gear components. Always replace gears as a set when the tooth flanks are worn, discoloured or chipped. Do not combine used planetary gears with a new sun gear or ring gear: differences in tooth contact and wear pattern significantly reduce service life. Check the hub housing for cracks, oval bearing seats and damaged sealing surfaces.

Use only the specified transmission oil and filling quantity. An excessively high level causes heat build-up and leakage, while an excessively low level breaks down the lubricating film on gear teeth and bearings. Adjust bearing axial play and preload according to the axle specification, using a dial indicator and the specified tightening torques. For a fast and technically correct repair, parts identification based on the chassis number and front axle number is essential; this prevents incorrect reduction ratios and incompatible hub components.