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ON-LINE DIAGNOSIS OF MARINE DIESEL CRANKSHAFT CRACK

BASED ON MAGNETIC MEMORY

LI HAN-LIN, LIN JIN-BIAO, CAI ZHEN-XIONG, LU YONG

(Marine Engineering Institute, Jimei University, Xiamen, Fujian 361021) Abstract: This paper summarized recent diagnosis methods of marine diesel crankshaft crack and introduced the fundamental principle, development and application of metal magnetic memory testing (MMMT) diagnosis technolog y. C ompared with other methods, a scheme developed from MMMT combined with torsional sig nal to make on-line diagnosis of crankshaft crack was proposed, and the on-line diagnosis experimental platform of diesel was designed. Finally, comments were also made on the on-line MMMT method.

Keywords: marine diesel; crankshaft; crack; metal mag netic memory

1Introduction

Most of the crankshaft cracks on ocean marine diesel are deeply latent and difficult to detect artificially. In general, while the crack develops to a certain extent it will rupture and cause destructive breakage1. If inchoate stress concentration and tiny crack of crankshaft are detected on-line by Non-Destructive Testing (NDT) methods, the diesel can be stopped before the full development of the crack and thus rupture can be avoided. A new NDT method, MMMT developed rapidly recent years with advantages in such areas as cheap and simple sensors, measuring without impacting diesel operation and long-term on-line detection. Therefore, there will be a great foreground to use MMMT in crankshaft crack detection.

2Dynamic Diagnosis of Crankshaft Crack

2.1 Static Diagnosis of Crankshaft Crack

Crankshaft crack is one of the fatal faults in diesels. There are some static NDT methods to diagnose crankshaft crack2such as Ultrasonic Testing, Radiographic Testing, Magnetic Particle Testing, Penetrate Testing, Eddy Current Testing, Optical Holography,and Microwave Testing. These methods have taken an important part in crankshaft quality control and accident prevention. However, on the other hand there are several disadvantages of these methods. Firstly, all of them must find cracks already formed and can not detect unexpected ones. Secondly, they are unable to make on-line detection and fault diagnosis. Thirdly, most of them need surface pretreatment of the facilities before inspection. Finally, the accuracy of detection is affected by crankshaft configuration and personal skill.

2.2 Dynamic Diagnosis of Crankshaft Crack

At present there are some methods to diagnose shaft cracks on-line: rotor engine vibration displacement method, train wheel shaft temperature testing, acoustic emission and tortional vibration signal testing. However, vibration displacement method is unfit for diesel crankshaft crack diagnosis due to the strong vibration from reciprocating movement which makes it difficult to survey displacement and acceleration. Although there is local high temperature on the cracks, it is hard to inspect the change of the crankshaft temperature due to the high temperature of the lube oil in the diesels. Acoustic emission method is still in developing as great noise from diesel operation makes it difficult to measure high frequency acoustic wave emitted by crack. Therefore at present it is feasible to diagnose crankshaft crack on-line by using torsional vibration signal.

2.3 Crankshaft Crack Diagnosis Using Torsional Vibration Signal

Literature 3 brought forward a method using torsional vibration signal detected real-time from diesel to diagnose crankshaft fault. It studied crack fault diagnosis though measuring the characteristics of the torsional vibration of the shafting flywheel system, and presented that model frequency and damp were sensitive parameters of cracks. The torsional vibration measuring system of the diesel had several advantages such as: easy measuring, no direct contact, cheap sensor and longevity. It was a long-term reliable alarm method for diesel shafting crack. Research group in Marine Engine Institute of Jimei University has done the research of internal-combustion engine score fault with intelligent torsional vibration diagnosis system4, and it was a successful case of on-line fault diagnosis based on torsional vibration signal.

3Metal Magnetic Memory Testing Technique

In the year 1997, Russian professor A. A. Dubov brought forward correlative theory of metal magnetic memory on the World Conference on NDT, developed MMM instruments and formed MMM NDT technique5.

3.1 Theory of Metal Magnetic Memory Testing

Under the condition of the existence of the outside magnetic field, usually the magnetic field of the earth, there will appear a phenomenon in the stress concentration zone of ferromagnetic components with load that permeability of material besides the zone reduces and surface leak magnetic field increases, which is called magnetostriction. It will makes the surface magnetic field of the ferromagnetic accessory increase, and the magnetic field can remember the location of disfigurement and stress concentration zone, thus it is named metal magnetic memory.