Bearings

Bearing systems in a driven rear axle comprise more than just wheel bearings. Depending on the axle design, these include tapered roller bearings and roller bearings in the wheel hubs, pinion bearings around the input shaft, differential bearings for the differential housing and, in the case of hub reduction, bearings for sun gears, planet carriers and planetary gear sets. These bearings keep rotating components in position both axially and radially, carry vehicle weight and driveline forces, and ensure correct gear mesh between the pinion and crown wheel. They are found in MAN TGL, TGM, TGA, TGS, TGX and TG3, as well as Mercedes Atego, Axor, Actros, Arocs, Econic and older SK or F2000 vehicles. Buses such as the MAN Lion’s City, Mercedes Citaro and Neoplan coaches may also use axle-specific bearings.

The correct bearing is determined by the axle code, gear ratio, hub version, brake configuration and maximum axle load. Driven axles with conventional wheel hubs generally use oil-lubricated tapered roller bearings. These combine high radial load capacity with the ability to absorb axial forces when cornering. Rear axles with planetary hub reduction have additional bearings in the reduction gear housing, where gear forces and thermal loads can increase considerably.

In the differential, preloaded tapered roller bearings ensure a stable position of the crown wheel. The pinion is usually supported by two bearings with a predefined preload, often adjusted using spacer rings, shims or a collapsible spacer. Incorrect bearing height, preload or backlash causes gear noise, increased contact pressure and accelerated damage to the bevel gear set. In motor vehicles with MAN D08, D20, D26 or D38 engines and Mercedes OM 936, OM 470, OM 471 or OM 473 engines, drivetrain loads depend heavily on engine torque, rear axle reduction and use in distribution, long-haul or construction transport.

Parts must always be identified using the vehicle chassis number. Visual comparison or selection based solely on the model name is never sufficient, as identical vehicle series may have different axle manufacturers, hub dimensions, bearing widths, oil seals and gear ratios.

A humming or whining noise that increases with vehicle speed often indicates worn wheel bearings or damaged pinion bearings. If the noise changes noticeably when accelerating and decelerating, the pinion bearings, backlash and condition of the crown wheel and pinion should be checked in particular. A dull knocking sound when cornering, uneven tyre wear or noticeable play at the wheel hub may indicate insufficiently preloaded hub bearings, worn bearing races or a damaged hub seat.

First check the oil level and condition of the rear axle oil. Metal particles, glittering wear debris or a burnt smell indicate damage to roller bearings or gears. Then measure the radial and axial play at the wheel hub using a dial gauge, while loading the hub according to specification. Check the temperature of both hub areas after a test drive using an infrared thermometer; a clear temperature difference between left and right indicates increased friction. If differential damage is suspected, check the backlash with a dial gauge on the crown wheel and assess the contact pattern using gear marking compound.

Always replace bearings together with their corresponding bearing races when pitting, discolouration caused by overheating, flaking or noticeable roughness is present. Reusing a worn bearing race with a new roller bearing significantly reduces service life. Thoroughly clean the bearing housing, check the fit for oval wear and measure the hub or housing seat. A fit that is too loose causes the outer ring to creep; a fit that is too tight can reduce internal bearing clearance to an unacceptable level.

Install new tapered roller bearings with the specified rear axle oil or assembly grease where required by the manufacturer, replace oil seals and adjust bearing preload precisely. For pinion and differential bearings, pinion depth, preload and crown wheel backlash must be set according to the axle specification. A complete bearing overhaul is technically justified when the axle housing, gears and hub seats are reusable. In the event of broken teeth, severely worn seats or blue discolouration caused by oil loss, overhauling the complete rear axle or replacing the relevant axle module is often the most reliable solution.