Connecting rod

Connecting rods and connecting rod bearings in air compressors convert the rotary motion of the compressor crankshaft into the linear stroke of the compressor piston. These internal compressor components are crucial to the output, operational reliability and oil-free operation of the pneumatic braking system. A defective connecting rod assembly can lead to insufficient air pressure build-up, increased engine oil consumption through the compressed air system or severe mechanical damage to the compressor housing. Always select connecting rods and connecting rod bearing shells based on the chassis number and, where available, the compressor part number. Visual comparison or selection based solely on the model name is never sufficient.

Depending on the engine family, year of manufacture and compressed air requirements, MAN and Mercedes trucks and buses use a range of single- and twin-cylinder compressors. In applications including MAN D08-, D20-, D26- and D38-engines or Mercedes OM 904-, OM 906-, OM 457-, OM 470-, OM 471- and OM 473-engines, not only the compressor capacity differs, but also the crankpin diameter, centre-to-centre distance, piston pin diameter and lubrication configuration. Compressors may also vary by version on MAN TGL, TGM, TGS and TGX or Mercedes Atego, Axor, Actros, Arocs, Econic and Citaro vehicles.

A compressor connecting rod generally consists of a small end for the piston pin, a rod body and a big end for the compressor crankshaft. Depending on the design, the small end runs on a pressed-in bronze plain bearing bush or a needle bearing, while the big end is fitted with two-piece connecting rod bearing shells, a roller bearing assembly or a split bearing cap. The bearings used must withstand alternating gas and inertia forces at high speeds without play, seizure or loss of the oil film. Correct fitment, bearing grade and any colour or size markings are therefore essential; a bearing with apparently identical dimensions may have a different tolerance class or lubrication groove.

Metallic knocking from the compressor, increasing noise at idle speed, low air output or abnormal oil contamination in air tanks are important warning signs. Excessive radial play on the crankpin causes knocking noises and may cause the bearing shells to spin in the connecting rod big end. Axial play of the piston pin is more likely to indicate a worn bearing bush, an enlarged piston pin bore or a damaged piston pin. Seized bearings often result in blue discolouration, scored running surfaces and aluminium or copper particles in the compressor oil or engine oil, depending on the lubrication circuit.

First check whether the problem is genuinely mechanical. Measure the build-up time and pressure increase according to the vehicle specification, check the compressor drive and rule out leaks, a faulty pressure regulator or a saturated air dryer. Then inspect the delivery line for excessive oil deposits and check the compressor oil supply line for blockages, kinks and leaks. After dismantling, measure the crankpin with a calibrated micrometer at several positions and different angles. Taper and ovality must remain within the compressor specification. Determine bearing oil clearance using Plastigage or by measuring the bore with an internal micrometer; at the same time, assess the flatness of the bearing cap and the condition of the fastening bolts.

In the event of bearing damage, do not replace only the connecting rod bearing shells. Always inspect the compressor crankshaft, piston, cylinder wall, piston pin and oil supply. A new bearing set on a scored or oval crankpin will fail quickly and may cause the compressor to seize. Use the specified tightening torque and the correct angle tightening procedure for connecting rod bolts; stretch bolts must not be reused. Fit bearing shells dry and free from grease in their seats, but apply clean, specified engine oil to the running surfaces before initial start-up.

After installation, the compressor must rotate freely without mechanical resistance. Then allow the system to build pressure in a controlled manner and check for abnormal noises, oil leaks and oil carry-over into the compressed air circuits. Timely overhaul using correctly identified parts reduces downtime and prevents oil or metal particles from contaminating brake valves, air tanks and EBS (Electronic Braking System) components.