Wheel hub

A rear axle wheel hub forms the load-bearing connection between the axle shaft and the wheel and, on driven axles, also transmits torque. The hub not only supports the vehicle weight and varying load, but also absorbs radial forces, axial forces when cornering and braking forces. On MAN TGL, TGM, TGA, TGS, TGX and TG3, Mercedes Atego, Axor, Actros, Arocs and Econic, as well as on buses such as MAN Lion’s City, Mercedes Citaro and Neoplan coaches, the design differs according to axle type, braking system, hub reduction and wheel mounting. Different hub configurations are also used on vans such as the MAN TGE and Mercedes Sprinter. A wheel hub must therefore always be identified using the vehicle chassis number. Visual comparison or selection based solely on the model name is never sufficient.

Rear axle wheel hubs are available as conventional driven hubs, free-running hubs for non-driven axles and hubs fitted with a planetary hub reduction unit. On a conventional driven rear axle, the axle shaft transmits differential torque directly to the hub. In hubs for axles with planetary hub reduction, often used in heavy haulage, construction applications and certain bus axles, the axle shaft sun gear meshes with planetary gears and a ring gear mounted in a hub reduction unit. This reduction decreases the load on the differential and axle shafts, but places high demands on gear backlash, lubrication and sealing.

The hub generally runs on two tapered roller bearings or on a pre-set bearing package. These bearings operate in oil-lubricated conditions using transmission oil from the rear axle or a separate oil supply in the hub reduction unit. Correct bearing fit on the axle stub and hub bore, the specified preload and the position of spacer rings or adjusting nuts are crucial. Insufficient preload causes tilting play and uneven raceways; excessive preload increases bearing temperature and can lead to seizure of the rollers and races. Also check the pitch circle diameter, number of wheel bolts, centring of brake drums or brake discs and the connection for ABS sensor rings.

A humming or whining noise that increases with road speed, hot hub caps, metal particles in the rear axle oil and oil traces on the inside of the rim often indicate problems with the wheel bearings, hub seals or the hub itself. On hubs for axles with planetary reduction, rattling noises under driving or engine braking load may also indicate damaged planetary gears, a dented ring gear or excessive gear flank backlash.

Allow the wheel to rotate freely and first check axial and radial play using a dial indicator on the wheel rim or hub flange. Compare the measured value with the factory specification for the relevant axle. Then rotate the wheel by hand and check for resistance, rough running or dragging brake components. After a test drive, measure the temperature of both hub areas using an infrared thermometer; a clear temperature difference between left and right requires further investigation. Check the oil level and assess the transmission oil for a burnt smell, discolouration and ferromagnetic wear particles. Finally, inspect the seal running surfaces, wheel bolt holes, flange face and ABS tone ring for scoring, cracks or deformation.

Replace a wheel hub when the bearing seats are worn, the hub flange is cracked or the wheel bolt holes have become oval. In the event of bearing failure, both roller bearings, races, seals and – where specified – spacer rings must be renewed as a technically compatible package. Thoroughly clean the hub housing, remove old sealing residue and inspect the axle stub for discolouration, pitting corrosion and dimensional loss. Install bearings using suitable fitting tools on the correct ring; applying force through the rollers causes immediate damage.

After installation, fill with the specified transmission oil, adjust the bearing preload according to the factory specification and correctly secure adjusting nuts or lock rings. After a short test drive, check again for leaks, hub play and temperature. Correct identification by chassis number, combined with technical support and a wide availability of parts, reduces unnecessary downtime for trucks, buses and vans.