In this paper, a nonlinear dynamic model of the single-stage reciprocating compressor system with a rub-impact fault caused by subsidence is developed, considering the piston rod flexibility. Meanwhile, different rub-impact scenarios of the crosshead including no corner, a single corner, two opposite corners, and two adjacent corners of the crosshead impacting on the slide, are considered in this model. Numerical simulation results show that the subsidence, time-varying load, and piston rod flexibility have significant effects on the vibration response of the reciprocating compressor system with rub-impact, where the larger the subsidence and time-varying load, the more intense the vibration response. Moreover, compared to the rigid piston rod, the flexible piston rod causes the system to generate stronger shock. In the rub-impact, the crosshead appeared in three different motion modes: noncontact; permanent contact; and collision – and most of the collisions occur in the form of two adjacent corners impacting on the slide. In addition, strange attractors are observed in the phase space trajectories, and the existence of chaotic behavior is verified using the Poincaré section method and maximum Lyapunov exponent method.

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Multi-scale entropy (MSE) is a widely recognized feature extraction approach to mechanical fault diagnosis, for it can effectively estimate the complexity of nonlinear time series. For MSE algorithm, due to the sensitivity of entropy estimation on series length, the scale factors are often required to be limited to a small range. Nevertheless, in the existing MSE methods, the scale factors can only be set to positive integers with a fixed minimum step size, which may result in insufficient analysis precision and cannot provide high-quality feature vectors with sufficient eigenvalues for intelligent diagnosis in the limited scale range. In view of the above defects, this paper subdivides the scale factors and proposes dense multi-scale entropy. In the suggested method, the number of data points in the raw sequence is expanded on the premise of guaranteeing the characteristics of the original series. Based on this, the timescale of the original series is refined and more intensive scale factors with higher precision can be provided. The superiority of the method developed in this work is verified by using CWRU bearing and reciprocating compressor gas valve fault data sets, and the results indicate that the method of this paper can provide more precise analysis scale and feature vectors with higher quality for intelligent diagnosis.

The clearance faults on joint of moving mechanism are most common in a reciprocating compressor. In order to investigate the relationship between the clearance faults and the dynamic behavior of the moving mechanism, a dynamic model with clearances of a reciprocating compressor is built via software ADAMS. We take into the clearance size and clearance number into consideration and set clearance fault on the joint between the crank and connecting rod, the joint between connecting rod and crosshead and both joints of connecting rod, exploring the effect of these factor on the dynamic response. Then we make a non-linear analysis to estimate the chaos behavior. In the end, we conclude that the clearance size and the number of clearance both strongly influence the dynamic behaviors of the moving mechanism, so do the position where clearance fault happens. As the clearance size increases or the number of clearance fault increase, the acceleration will oscillate more violent and have higher amplitude, furthermore, it is more possible to show chaotic behaviors.

This paper is targeted on the vertical vibration of the crosshead in a reciprocating compressor, taking into consideration the crosshead subsidence. The traditional model of the compressor is usually a crank-slider mechanism system without considering the abrasion loss for any parts, thus neglecting the influence of the piston rod flexibility. A rigid-flexible model of slider-crank is described theoretically, in which the crank, connecting rod, and crosshead are treated as rigid bodies, while the piston rod connected to the crosshead is considered as a flexible body. Lagrange equation was adopted to establish the kinetic equation of the system, considering the mass of the crank, the linkage, and the crosshead. After modeling the mechanism, the simulation shows that the dynamic response of the crosshead will be greatly influenced if crosshead subsidence is considered. Additionally, the influence of the crosshead subsidence is also investigated, and some new phenomena arise especially the vertical vibration of crosshead.