Notes on evaluating brake disks - AH42.10-P-9406-07SLR

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MODEL 199 without CODE U28 (19-inch brake system)

If the brake disks on an axle have a different degree of wear, determine and eliminate the cause.

If all the brake disks on an axle show signs of increased wear, then the cause is attributable to a high vehicle load.

Always replace brake disks in pairs.

This ensures proper function of the brake system.

It is essential to check the insides and outsides of the brake disks for signs of wear.

The following signs of wear can occur:

Friction track chipped
"Erosion" on edge of brake disk
Crack formation across the friction track

Fig 1: Identifying Brake Disk Edge Length
Courtesy of MERCEDES-BENZ USA

Sign of wear: Friction track chipped

The friction track is chipped to a minor extent due to thermal stresses on the wear-resistant friction layer (2). Up to an edge length (a) of 19 mm these chips on the friction track are inconsequential.

As of an edge length (a) of 20 mm the brake disk should be replaced.

Fig 2: Identifying Thickness Of Brake Disk Outer Edge

Courtesy of MERCEDES-BENZ USA

Sign of wear: "Erosion" on edge of brake disk

Due to oxidation processes the carbon fibers form ash and the basic material at the outer edge (1) of the brake disk gets "like charcoal".

The thickness of the eroded area on the outer edge (1) of the brake disk reduces more quickly than the wear-resistant friction layer (2).

The erosion on the outer edge (1) of the brake disk is partially invisible due to the lining material which is smeared on.

If the wear-resistant friction layer (2) breaks away from the outer edge (1) of the brake disk, see area (A), as well as in the event of sub-surface corrosion of the wear-resistant friction layer (2) due to oxidation of the basic material, the brake disk should be replaced.

The minimum permissible thickness of the brake disk is 34 mm at the outer edge (1), see dimension (b).

Fig 3: Locating Brake Disk With Thermal Stress Crack

Courtesy of MERCEDES-BENZ USA

Sign of wear: Crack formation at right angles to the friction path

The stress loading on a brake disk is greatest in the connecting area, thermal stress cracks (arrows) can occur at these points.

If a thermal stress crack (arrows) is found on the brake disk, it must always be replaced.