Recent automotive engines for high performance vehicles have been designed for higher speeds and outputs. Not only the combustion load but also the inertia force applied on the connecting rod has been increasing. Automotive engines have also become compact and lighter in weight for needs of lower fuel consumption. For these reasons, the rigidity of the connecting rod has been reduced in comparison with the increasing inertia force.
As a result, fretting damage may occur between two surfaces of the connecting rod big end bore and the bearing outer surface, causing breakage of the connecting rod itself.
Countermeasures for fretting such as a tighter bearing fit ( interference ) and higher rigidity of the connecting rod big end are generally adopted. But the details for these countermeasures can not be easily predicted at the design stage. Rather they are obtained only by durability tests on the actual engines. Then, authors tried to reproduce fretting phenomenon, using an actual connecting rod on the test rig, and experimented various effective factors of the fretting. The prediction of the fretting at the design stage is investigated, based on the structual analysis of the connecting rod big end using the FEM.
Consequently, it has been confirmed that the fretting is related to the magnitude of relative slip amplitude between the connecting rod big end bore and the bearing outer surface due to distortion of the connecting rod, and that the analysis of FEM is effective to predict the fretting.
FRETTING PHENOMENON HAS BEEN RECENTLY often observed on the outer surface of the bearing for high performance and compact engines. This is because the load on the connecting rod bearing has been increasing, and also the rigidity of the connecting rod big end has been decreasing. Fretting may occur due to microvibrations between the big end bore of connecting rod and the outer surface of the bearing.
Fig. 1 shows typical fretting that occurred on the connecting rod bearing of an actual engine. Fretting is observed similarly at both surfaces of the connecting rod big end bore and the bearing outer surface. In serious cases cracking can occur.
Also in some cases the cracking may result in breakage of the connecting rod. This is supported by the fact that beach marks caused by fatigue are observed with the starting point of the fretting when the broken-off section of the connecting rod is examined.
Fig.1Apperance of Typical Fretting Damage on Connecting Rod Bearing
Fig. 2 shows the results of EPMA at a fretted area. The fretting is considered as adhesive wear due to micromovements between two surfaces.
Factors causing fretting on the connecting rod bearing vary widely, and have complex interrelationships. Fig. 3 shows the cause and effect diagram of fretting occurring between the connecting rod big end bore and the bearing outer surface. Causes can be roughly divided into several groups; operating conditions, dimensions, materials and geometrical imperfection of the connecting rod and the bearing. When changing these factors, a lot of restrictions are present, so it makes difficult for improvement. Generally, most factors relating to operating conditions greatly affect engine performance, thereby having little freedom of change. Since improvement in machining accuracy and changing material and surface treatment usually lead to increased costs for the connecting rod and the bearing, it is desirable to follow conventional procedures and existing materials. However, improvement in the dimension and/or shape of the connecting rod and the bearing can be done relatively easy and at a comparatively low cost in the engine development stage, and is one of general countermeasures for fretting.
Fig.2Results of EPMA at Fretted Area
Fig 3Cause and Effect Diagram of Fretting Damage Occurring between Bearing and Connecting Rod
In actual experiences, increasing the bearing fit and/or the rigidity of the connecting rod has been often used into practice. However, the actual dimensions for these countermeasures can not be easily predicted, and in fact these have been determined experimentally by durability tests on actual engines.
In this paper, authors discuss the dimensions and shapes of the connecting rod big end and the bearing, and have investigated theoretical analysis to prevent the fretting.
Mizuno, Y., Sugita, M., Okamoto, Y., Ono, A. et al., "Fretting Phenomenon on Outer Surface of Connecting Rod Bearings for Automotive Engines," SAE Technical Paper 931022, 1993, https://doi.org/10.4271/931022.
