Brake camshaft

A brake camshaft is the mechanical actuating component in pneumatically operated drum brakes. Via the brake adjuster or automatic slack adjuster, the stroke of the brake chamber is converted into a rotary movement. The S-shaped cams at the end then press the brake shoes in a controlled manner against the inner wall of the brake drum. Brake camshafts are mainly used on the axles of MAN F90, F2000, M2000, L2000, TGL, TGM, TGA, TGS and TGX trucks, MAN Intercity, Lion’s City and Lion’s Coach buses, as well as Mercedes SK, Atego, Axor, Actros, Arocs, Econic, Citaro, Intouro and Tourismo vehicles. Drum brakes also remain relevant for vehicles used in construction transport, off-road work and intensive urban transport due to their enclosed design and resistance to dirt contamination.

A brake camshaft must never be selected solely on the basis of the vehicle model. The correct part must always be determined using the vehicle chassis number; visual comparison or selection based only on the model name is never sufficient. Within a single vehicle series, variations may occur in axle manufacturer, drum brake diameter, brake shoe width, left- or right-hand version, splines, bearing bushes, seals and brake adjuster position.

Technically, brake camshafts are available with different shaft diameters, effective lengths, spline profiles and cam geometries. The S-cam must match the brake anchor plate and the specified brake shoes exactly. A different cam radius or incorrect shaft length changes the spread of the brake shoes, the required brake chamber stroke and the available braking force. Also check the fit of the bearing bushes in the brake anchor plate: excessive radial play causes the cam to engage at an angle, while an excessively tight fit can lead to seizure. Depending on the version, hardened steel brake camshafts are combined with plain bearings, bronze bearing bushes, plastic bushes, dust rings and sealing rings.

Wear usually occurs not on the cam itself, but at the bearing points, splines and contact surfaces between the S-cam and the brake shoe rollers. Typical fault symptoms include uneven brake lining wear, vehicle pull during braking, insufficient braking force on one wheel, excessive brake chamber stroke, hot brake drums or a brake that does not fully release after braking. Corrosion caused by water, road salt and dust, damaged seals or seized bearing bushes are common causes in buses and off-road vehicles.

Secure the vehicle, depressurise the relevant brake circuit according to the workshop procedure and remove the brake drum, brake shoes and brake adjuster. First check whether the automatic slack adjuster is functioning correctly; incorrect adjustment can cause the same symptoms as a worn brake camshaft. Then measure the radial play of the shaft at the bearing bushes using a dial indicator. Compare the measured value with the tolerance specified by the axle or brake manufacturer. Check the splines for rounding, torsional damage and fretting corrosion. Inspect the S-cams for flat wear, grooves, hairline cracks and blue discolouration caused by thermal overload. Turn the shaft by hand: movement must be smooth, without noticeable binding or jerking.

Replace a brake camshaft when the cams are worn, the shaft is bent, the spline connection has play or the bearing fit is outside tolerance. Preferably install new bearing bushes and seals as well and, if worn, new brake shoe rollers. Reusing damaged bushes will quickly result in play again and may cause uneven wear of the new brake linings. Thoroughly clean the bore in the brake anchor plate and press in bearing bushes straight, without damaging the sliding surface. Use only the specified heat- and water-resistant lubricant on the lubrication points; grease on brake linings, brake drums or friction surfaces is unacceptable.

After installation, the brake shoes, return springs, brake adjuster and brake chamber stroke must be checked as one system. Adjust the drum brake according to the vehicle specification and carry out a brake test on a suitable brake test bench. A correct brake camshaft not only restores even braking performance, but also limits overheating, drum damage and unplanned downtime. The availability of technical parts and support with chassis-number identification helps ensure repairs are carried out without incorrect fitment.