According to the mechanical principle, the reciprocating inertia force is generated by the piston group and the small head of the connecting rod, and the rotary inertia force is generated by the large part of the connecting rod and the crank (commonly known as the crank). Among them, the connecting rod does a more complex movement when bearing the impact load, and its small head both does the reciprocating movement up and down with the piston, and does the rotating movement. In addition, the rotary force and inertia force generated by the piston in the four working strokes of the intake, compression, expansion and exhaust are extremely uneven, so as long as the natural vibration frequency of the crank connecting rod mechanism is close to or overlaps the frequency of the speed and load, resonance phenomenon will occur, which will cause the flutter and vibration of the speed of a certain area of the engine. This flutter and vibration gradually increase with the increase of the mileage of the motorcycle. If the motorcycle driver does not pay attention to avoid this resonance area (that is, the resonance generated at a certain speed, because of different models, the area of resonance speed is different), the crankshaft connecting rod and piston and other moving friction parts will be gradually damaged as the resonance phenomenon continues. At the same time, there are parts manufacturing errors on the crankshaft connecting rod mechanism of the influence.

(1) Combined crankshaft

In order to reduce its own vibration in the crankshaft mechanism is set up a balance device (some engines may also be equipped with a balance shaft), when the engine is running, it can balance a part of the inertia force. However, the mechanical manufacture of parts has accumulated errors (especially the combined crankshaft). Take a single cylinder engine for example, the crankshaft assembly is composed of two crank shafts and a crank pin, a connecting rod, and the size of the center distance between the crank pin holes on the crank shaft is the most critical technical parameter. Under normal circumstances, the more regular manufacturers, in the manufacture of crank shaft and paired press assembly, are based on the eccentricity tolerance of 0.003mm as a group, so assembled crankshaft, basically to ensure that the crankshaft component of the spindle journal radial runout value between 0mm and 0.03mm. Even if there is a manufacturing error, it can be reasonably corrected after assembly to ensure that the radial circular runout value of the crankshaft spindle journal is within the specified value range.

As we all know, some irregular enterprises, the production batch is small, the manufacturing of the crank shaft pin hole center distance difference is large (if the fake parts are more serious), not in accordance with the provisions of 0.003mm group for grouping assembly, so that the crankshaft after pressing assembly, the spindle neck radial runout value seriously exceeded the standard. Even if the correction method is adopted, the radial circular runout value of the out-of-tolerance crankshaft spindle journal cannot be controlled within the specified range. As a result, the crankshaft assembly not only vibrates greatly during the engine operation, causing partial wear when the piston runs up and down, but also the radial circular runout value of the two shaft ends is too large, causing serious wear of the bearing inner and outer rings supported, exceeding the clearance, and causing abnormal operation sounds. Resulting in crankshaft failure (FIG. 7-9). Part of the factor is caused by the non-standard operation of removing or replacing the crankshaft connecting rod assembly during the maintenance process.

In addition, if the parallelism of the connecting rod size head exceeds the specified 100:0.08, the piston installed in the small head hole of the connecting rod is skewed in the cylinder, resulting in partial wear of the piston and the ring, and when the fault is serious, the oil will be channeled to pull the cylinder.

(2) Integral crankshaft

The large displacement multi-cylinder engine adopts the form of integral crankshaft structure, and the main bearing installed on the crankcase and the connecting rod bearing in the big head are generally divided into two halves of the bearing bush. The bearing bush is divided into two pieces: one piece is installed in the crankcase bearing seat hole or connecting rod big head, and the other piece is installed in the bearing cover or connecting rod cover. When the engine is working, the lubricating oil pumped out by the oil pump is filtered by the crude oil filter and the fine filter, and the oil film is formed between the crankshaft spindle neck, the connecting rod shaft neck and the bearing bush with a certain oil pressure. At the same time, the lubricating oil is lubricated by the through oil pump to lubricate the motor friction pair parts. Due to the role of the oil film, the shaft journal and the bearing bush are not in direct contact, and they can rotate smoothly. In fact, the lubricating oil film plays a floating bearing role between the bearing shell and the journal. However, if the tightening torque is too large when tightening the connecting rod big head bolt, the oil film will disappear here, direct contact will occur between the metals, and the bearing will be burned in a short time. In addition, the scars, burrs, scum, etc. on the surface of the bearing, as well as the matching gaps that ensure the formation of the oil film are too large or too small, will destroy the oil film or make the oil film disappear until the bearing is burned. Therefore, the large displacement motorcycle instruction manual usually recommends the use of more superior quality of the full synthetic oil (API-SJ, SK... Above). The base oil base of the product has excellent flow characteristics and is suitable for use in extremely variable temperature conditions or in more demanding environments. In order to save oil costs, some users use ordinary four-stroke lubricating oil without authorization, resulting in frequent bearing shell burning and journal strain failure. In addition, the oil filter is not cleaned and maintained on time or replaced, which makes the oil dirty and seriously affects the normal lubrication between the crankshaft journal and the bearing shell.

(Text from the network, infringement please contact delete)