For the convenience of adjustment and disassembly, a large number of detachable connections such as threaded connections, keys and tight fits are used on the diesel engine. These parts mainly rely on the friction between the fittings to maintain the relative position. If there is no loosening device or the anti-loose device fails, it will easily loosen under the action of long-term repeated load (especially alternating load). Therefore, for connecting parts that are subjected to alternating loads during work and connectors that do not have automatic locking devices, the tightness of the connections should be checked frequently, and if it is found, it must be tightened in time.

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Common forms of damage to diesel engine fasteners are as follows.

1. The delayed fracture material is subjected to a stress lower than the static load fracture strength, but due to stress corrosion, fatigue, creep, etc., the fracture occurs after a period of time. It mainly occurs on externally threaded fasteners such as bolts and studs. This is a sudden increase in the number of months or years after the external force does not exceed the static or dynamic load allowed by the design. Sexual breakage. According to existing research, there are many reasons for delayed fracture, such as the environment in which the vehicle is used, the materials used in the parts and the manufacturing process, and the sensitivity of the shape and strength of the parts to delayed damage. It can now be prevented from design and manufacturing, such as using materials that are less sensitive to delayed damage, avoiding sensitive areas of strength, reducing or avoiding processes that may allow hydrogen to penetrate parts (such as electroplating, excessive pickling, etc.) ) or drive off hydrogen by post-processing.

2. The phenomenon that a fatigue fractured part breaks under a long-term work under an alternating load is called fatigue fracture. Due to the unique notch-sensitive structure of the threaded member, fatigue fracture is the most visible form of fracture of the threaded fastener under dynamic load conditions. It seems that by designing the dynamic fatigue strength of the part, there is enough room for it to be safe. However, the actual situation is not completely the same. The reason is that the fatigue life of the parts often has a certain limit, and the factors affecting the degree of dispersion are complicated. If the fatigue life is lower than the life of the whole vehicle, it will obviously break at some point in the use of the whole vehicle. Fatigue fractures are also an unexpected factor in sudden failures that may occur during vehicle use.

3. The reliability of the non-standard fastening screw fastening depends on the joint design, fasteners and fastening aspects. The fasteners are often replaced and disassembled during maintenance. Maintenance points across the country and with different levels of disparity are bound to have irregular tightening operations. Inferior fastening is the same as inferior fasteners.

4. Inferior fasteners If you use inferior parts, you are likely to malfunction or even cause an accident during driving. The fasteners for vehicles contain many varieties that are used in different parts and have great differences in importance. The evaluation of their importance and the different control are important to improve the safety of the vehicle. The proliferation of inferior fasteners used to repair parts inevitably leads to failures and even fatal failures in areas where the design requirements are inherently problem-free. This is another factor that hides unexpected faults.

Condition monitoring and fault diagnosis of diesel engines is very important, and the technology is more and more advanced. In the failure of the diesel engine, the connecting rod bolt is loose, the gap between the main bearing of the connecting rod bearing and the piston cylinder sleeve is abnormal, and the fault accounts for a certain proportion. If the gap between the gas force and the reciprocating inertial force is too large or too small, the engine's dynamic performance will change, which will change the vibration amplitude and properties of the diesel engine. For example, when the piston skirt gap in the diesel engine cylinder changes, under the action of the same side thrust, the speed and energy of the piston striking the cylinder liner, the number of impacts and the position change accordingly, thereby causing a change in the vibration characteristics of the cylinder liner. This change in vibration characteristics can be accepted by a sensor mounted in the appropriate position of the engine and transmitted to a vibrometer or spectrum analyzer for processing, and a diagnosis of the engine state can be made by comparison with a standard spectrum. If the connecting rod bolt is loose or the large bearing bush wears, the gap between the connecting rod bearing bush and the crank pin will be abnormal. The variation of the bearing bushing clearance, the vibration response power spectrum can be measured on the crankcase. As the gap increases, the power spectrum shows an increasing trend. This makes it possible to identify the impact characteristics of loosening of the connecting rod bolt or failure of the bearing bush. It is not difficult to find the characteristic frequency when the engine connecting rod bolt is loose from the power spectrum.

In the maintenance and repair of the diesel engine, the newly replaced bolts and the connecting rod bolts in use should be carefully inspected before installation and use. The inspection method: use a magnifying glass or a naked eye to check for defects, especially pay attention to check the head of the bolt, Thread and thread root; check the pitch and bending with the ruler and ruler; install the nut to see if it is too loose to check the thread wear, especially check whether the contact surface between the nut and the bolt head and the bearing is uniform Contact or fit is good; the bolts are hung with wire ropes, hammered with a hand hammer, and their sounds are compared with new bolts to determine the presence or absence of defects and fatigue.

If the bolt loosens in the hole where the upper and lower parts of the bearing are tightly fitted, the bearing cover is displaced by the centrifugal force, and the bolt is subjected to the shearing force. If the white alloy is exposed at the joint of the upper and lower bearings, it will be squeezed out when the bolt is installed, or the bolt may be bitten due to burrs on the edge of the hole. Check the matching clearance between the connecting rod bolt and the bolt hole, and must meet the design requirements; check whether the positioning pin is loose or worn; when the tightening nut is inserted into the split pin, check whether the split pin is in contact with the surface of the nut; measure the total length of the connecting rod bolt and Check the amount of elongation.

The life of the medium-duplex four-stroke engine connecting rod bolt is about 15,000 to 20,000 hours. If this time is exceeded, it should be replaced even if the appearance is not abnormal.

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