| As one of the key moving parts of diesel engine,the connecting rod is subjected to complex alternating load during work,which has a great influence on its structural strength and reliability.To some extent,the reliability of connecting rod directly affects the normal operation of the whole diesel engine.The marine low-speed diesel engine is different from other medium-speed and high-speed diesel engines.The structure of connecting rod is complex and its size is large.The stress state and size of connecting rod in working process are also different from those of other diesel engines.The fatigue strength of connecting rod is required to be higher.With the rapid development of finite element technology and computers,large-scale general finite element analysis software has been able to perform structural strength analysis of complex connecting rod.In order to avoid the disadvantage of single static strength analysis of connecting rod,it is necessary to continue static strength test,transient strength analysis,fatigue strength check and modal analysis of connecting rod,so as to ensure the design of connecting rod that meets the technical specifications and has reliable workability.In this paper,the connecting rod of a low-speed diesel engine is taken as the research object.Firstly,the static strength of the connecting rod under assembly and maximum combustion pressure is analyzed.It is found that the pre-tightening force of the bolt and interference force mainly affect the stress at the large and small ends of the connecting rod,and the combustion pressure in cylinder and recprocating inertia force of the components affect the strength of the connecting rod body.On the basis of strength calculation,the fatigue strength is checked,the working life and fatigue safety factor of the connecting rod are obtained,and the reliability of the connecting rod is evaluated.After that,the crank-connecting mechanism model is assembled,and the force is not directly simplified.The force is directly applied to the surface of the piston for strength calculation,and compared with the calculation result of the simplified load,the rationality of the cosine simplification in the range of 120° is verified.Then contact analysis of connecting rod bushing is carried out.The influence law of friction coefficient and interference on contact condition is obtained by orthogonal simulation test.The most suitable interference and friction coefficient are determined according to the influence law.This parameter will make the bushing fretting wear best.Secondly,the static strength test was carried out on the connecting rod with the reduction ratio of 1/4 using the electro-hydraulic servo universal testing machine,and compared with the static strength calculation result of the 1/4 connecting rod,which verified the reasonable of the simulation calculation of the static strength of the connecting rod.Then the modal analysis of the connecting rod is carried out as the starting point of the dynamic analysis of the connecting rod.The natural frequency and mode shape in the finite modal order of the connecting rod are obtained,and the vibration characteristics of the connecting rod are determined.Finally,the transient strength analysis of the crank-connecting rod mechanism is carried out,and the stress variation and distribution of the connecting rod before and after the top dead point are studied.It is found that there is a difference between the stress on the left and right sides of the connecting rod body before and after the top dead point,and the stress distribution varies with the crankshaft rotation angle.On the basis of strength calculation,the fatigue strength of the connecting rod is checked and compared with the results of static strength calculation and fatigue strength checking of the connecting rod mentioned above.Combining the results of static and transient strength calculation and fatigue strength check,the overall strength of the connecting rod can be better evaluated. |
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