Brake drum

Brake drums, together with brake shoes, brake linings, brake camshafts and slack adjusters, form the friction component of drum brakes. They are mainly found on axles of vehicles operating under heavy, variable or contaminated conditions, such as MAN TGL, TGM, TGS and TGX, Mercedes Atego, Axor, Arocs and Econic, as well as buses including MAN Lion’s City, Mercedes Citaro and various Neoplan models. Therefore, always identify brake drums using the chassis number; visual comparison or selection based solely on the model name is never sufficient.

A brake drum must match the wheel hub, brake anchor plate, brake shoes and wheel fastening used exactly. Critical parameters include the nominal internal diameter, maximum wear diameter, drum width, hub centring, pitch circle diameter, number of wheel bolt holes and the position of any ABS- or EBS-related components. Whether it is an integral drum-hub assembly or a separate brake drum also determines the installation procedure.

For MAN D08, D20, D26 and D38 drivetrains and Mercedes OM 906, OM 936, OM 457, OM 470 and OM 471 applications, the same vehicle type may have several axle loads and brake sizes. A 17.5-inch, 19.5-inch or 22.5-inch wheel therefore does not in itself indicate the correct drum diameter. Cast-iron brake drums must also be matched to the friction grade of the brake lining: overly aggressive friction material can cause thermal stress, heat checking and accelerated wear, while excessively soft linings can lead to fading under high axle loads.

The most common fault patterns are oval wear, taper, heat cracks, blue discolouration, deep grooves and excessive internal diameter. Oval brake drums cause a pulsating brake feel, vibrations in the chassis or steering system, and uneven brake shoe engagement. A shiny or locally burnt running surface often indicates a sticking brake camshaft, defective slack adjuster, insufficient brake shoe return or a brake cylinder that does not fully release.

After removal, measure the internal diameter at several points using a brake drum gauge or internal micrometer: at a minimum, at different heights and at 90° to each other. Compare the largest measured value with the maximum wear diameter specified by the manufacturer. At the same time, check runout, mounting face flatness and centring on the wheel hub using a dial gauge. Also inspect the brake shoes for unevenly worn brake lining, loose rivets, glazing, and oil or grease contamination. Always replace the brake drum in case of heat cracks, dimensional limits being exceeded or clear ovality; machining is only acceptable if the remaining wall thickness and maximum diameter remain within specifications.

Replace brake drums per axle, together with suitable brake shoes or brake linings when the contact pattern is no longer uniform. Combining a new drum with old, glazed friction linings results in a longer bedding-in period and may cause local overheating. Thoroughly clean the mounting faces of the wheel hub and drum: corrosion, dirt or burrs cause runout and can load wheel bolts unevenly. Use a calibrated torque wrench for the specified tightening torques and follow the correct crosswise tightening sequence.

A recurring practical issue is a new brake drum that overheats again. In that case, do not only inspect the drum, but check the complete mechanical actuation system: free movement of the brake camshaft in the bushes, condition of the S-cams, return springs, automatic slack adjusters and brake cylinder operation. On pneumatic systems, also check for residual pressure or leaking control valves. With technical support and extensive stock, Braem can help keep downtime during brake overhauls to a minimum.