Abstract
The chains for the 2nd order moment compensator in the marine diesel engines are designed usually to have extremly large safty margin. However, in a certain ship with the relatively large counter weight to reduce the hull vibration, it is possible to occur the non-linear vibration of the chain by the two node torsional vibration of the crank shaft.
For this problem, the non-linear vibration analysis of the chain was carried out with the measured spring characteristics by means of the chain tightener mechanism, which was represented by the form of Tq=k1⋅θ+k3⋅θ3.
In order to avoid such a non-linear vibration of the chain, it is important to increase the socalled ith order critical speed nc= (60/2π) √k1/I/i of the chain, which is related to the linear part of the chain spring characteristic k1, comparing with the rated engine speed nMCR. This can be obtained by increasing k1 or decreasing the moment of inertia I of the counter weight. Also, the increase of k1 is able to be realized by increase of the initial chain tension F0 or by stepping up the size of the chain and etc.
From this point of view, in this paper, a security evaluation method for the chain was considered in relation to the value of nc/nMCR, on the basis of the reaults of the non-linear vibration analysis of the chain.
