Gasket

Seals in the front axle prevent lubricants from escaping from wheel hubs, steering knuckle housings and, where applicable, driven front axle components, while keeping out water, dust and abrasive particles. These include hub oil seals, cassette seals, O-rings, flat gaskets and sealing rings for kingpins or CV joints. A defective seal often appears to be a minor issue, but in practice it quickly leads to grease loss, wheel bearing damage, corrosion on running surfaces and premature damage to hub or steering knuckle components. For MAN and Mercedes trucks, buses and vans, every seal must always be identified using the chassis number. Visual comparison or selection based on the model name alone is never sufficient, as the axle version, hub diameter, ABS configuration, bearing design and year of manufacture are decisive.

The most common front axle seals are radial shaft seals on the inner side of the wheel hub. These oil seals seal against a machined sealing surface on the steering knuckle or hub axle and retain the specified wheel bearing grease inside the hub. Certain versions use cassette seals: composite seals with integrated protective lips and their own running ring. These offer better resistance to dirt, splash water and eccentric movement, but require an exact fit for the shaft diameter and bore diameter.

O-rings and profile seals in steering knuckle housings also require attention. They prevent water and dirt from reaching the sliding surfaces, kingpins and bronze kingpin bushes. Driven front axles may have additional seals around axle shafts, CV joints or reduction housings. The choice depends, among other things, on the axle manufacturer, nominal dimensions, sealing width, direction of rotation and lubricant. NBR rubber is suitable for many mineral greases and oils, while FKM seals withstand higher temperatures and more aggressive lubricants better. Therefore, always identify parts using the chassis number.

A greasy inner rim surface, grease traces on brake discs or drum brake components, contaminated ABS sensors and recurring wheel bearing damage often indicate a leaking hub seal. On vehicles with disc brakes, grease on the brake disc can cause uneven braking torque and render brake pads unusable. In drum brakes, lubricant penetrates the friction lining, making replacement of the brake shoes generally necessary.

First clean the entire hub area and check after a test drive where the leak reappears. Then measure the radial and axial play of the wheel hub using a dial gauge. Excessive bearing play can cause the sealing lip to oscillate, resulting in leakage even with a new oil seal. Also inspect the sealing surface for grooves, corrosion pits, blue discolouration caused by overheating and wear marks. A new seal on a damaged running surface is not a durable repair; use a repair sleeve if available or replace the relevant hub or steering knuckle component. Also check the condition and correct fill level of the wheel bearing grease. Dark, burnt or metal-contaminated grease indicates roller bearing damage and requires further investigation.

Preferably replace a leaking hub oil seal together with the relevant wheel bearings if moisture, dirt or heat discolouration has been found. Remove the old seal without damaging the bore or sealing surface. Thoroughly clean all contact surfaces and inspect the bearing seats for fit and fretting. Install the new oil seal squarely using a suitable installation sleeve that applies pressure only to the metal outer casing; pressure on the sealing lip deforms the lip geometry and shortens its service life.

Lightly lubricate the sealing lip with the specified wheel bearing grease before installation, unless the manufacturer explicitly prescribes dry installation. Then adjust the wheel bearings according to the specified tightening procedure and check free rotation, axial play and the absence of rubbing after installation. Correct installation prevents repeated leaks, brake contamination and unplanned downtime. Thanks to technical support and a wide availability of axle parts, Braem can help identify the correct seal for the specific front axle version.