Pressure plate

The clutch pressure plate, also known as the pressure assembly, clamps the clutch disc against the flywheel with a precisely defined axial force. This reliably transmits engine torque to the gearbox input shaft. In MAN and Mercedes trucks, buses and vans, this component is subjected to high torque loads, frequent disengagements and significant thermal cycles. A pressure assembly with insufficient clamping force causes clutch slip; a pressure plate that does not disengage fully, on the other hand, causes difficult gear shifting and accelerated wear of the friction lining on the clutch disc. Always identify a clutch pressure plate using the vehicle chassis number. Visual comparison or selection based solely on model name, engine capacity or engine type is never sufficient.

Clutch pressure plates differ in, among other things, outside diameter, bolt pattern, installation height, release bearing contact diameter, type of release system and nominal clamping force. They generally consist of a steel or cast-steel clutch cover, a machined pressure ring, tangential leaf springs and a diaphragm spring. The pressure ring must remain flat and thermally stable; even slight distortion results in uneven pressure distribution across the clutch disc. Heavy-duty applications use pressure assemblies with higher clamping force, reinforced spring packs and heat-resistant pressure surfaces.

The correct version depends on the engine-gearbox combination, the flywheel and the permissible torque to be transmitted. This is relevant for, for example, MAN D08, D20, D26 and D38 engines and Mercedes OM 906, OM 457, OM 460, OM 471 and OM 473 engines. Different flywheels, clutch diameters and conventional or automated transmissions may also be fitted within a single engine family.

Typical fault symptoms include a slipping clutch under load, a burning smell, increasing engine speed without corresponding vehicle acceleration, judder when engaging and an abnormal engagement point. A weakened or overheated diaphragm spring reduces clamping force. Blue discolouration, hairline cracks or localised heat spots on the pressure ring indicate prolonged slipping or overloading. Worn contact surfaces on the diaphragm spring fingers may also occur when a release bearing runs out of alignment, is continuously preloaded or has insufficient free play.

First rule out operating system faults. Check the hydraulic release master cylinder, lines for leaks, air ingress and insufficient travel. On a pneumatic-hydraulic system, measure the operating travel according to the vehicle specification and check that the release bearing returns fully. During removal, the pressure plate, clutch disc, release bearing and flywheel are assessed as a single wear group. Measure the flatness of the pressure ring using a straightedge and feeler gauge; also check the height of the diaphragm spring fingers and the free movement of the tangential leaf springs. A pressure assembly with uneven fingers, cracks, heat-damaged material or a distorted pressure ring must not be reused.

Preferably replace a faulty clutch pressure plate together with the clutch disc and release bearing. Do not fit a new pressure assembly to a flywheel with grooves, heat cracks, runout or a friction surface outside tolerance. Also inspect the pilot bearing, gearbox input shaft splines, release fork, guide sleeve and crankshaft oil seal for wear or engine oil leaks. Oil or grease on the friction lining causes almost immediate slipping, even with a new pressure plate.

Degrease the flywheel and pressure surface only with a suitable residue-free cleaning agent. Centre the clutch disc using the correct alignment tool and tighten the pressure assembly mounting bolts crosswise and gradually to the specified tightening torque. Use new bolts where specified by the manufacturer. For fast identification and technical support with MAN and Mercedes applications, Braem can assist with selection based on the chassis number and the corresponding clutch components.