Fatigue test method of automobile engine connecting rod

How to make good use of 160F diesel engine connecting rod bolts?

The breakage of connecting rod bolts in operation will cause serious ramming accidents. It not only causes damage to the cylinder head and cylinder liner, deforms and bends the connecting rod, but also causes major economic losses such as ramming the body and breaking the crankshaft. In order to avoid the ramming accident caused by broken connecting rod bolts, the following points should be noted during use and maintenance:

1. The connecting rod bolt is subjected to a large alternating impact load during engine operation and is an important part of the engine. It is made of high-quality alloy steel after quenching and tempering and precision machining. It has high strength and impact toughness and is not available. Ordinary ordinary Luo Gui or inferior parts are substituted. If the bolt material, heat treatment and processing accuracy do not meet the technical requirements, it will cause insufficient mechanical strength and deformation and fracture accidents.

2. Check carefully before assembly. When it is found that there are scratches, slide fasteners, cracks, dents, necking or cracks on the bolts (using oil immersion inspection or magnetic inspection), or the bolts and nuts are loosely fitted, or the bolts cannot be closely matched with the bolt holes, or use Observation by comparison method, the bolt length is 2% longer than the new standard bolt length, should be replaced.

3. Check the matching clearance between the connecting rod bearing and the connecting rod journal. If the clearance is too large, it is easy to cause the connecting rod bolt to break. At this time, the new connecting rod bearing should be replaced or the bearing should be enlarged, and it should be matched or scraped.

4. Check the connection between the connecting rod bolt or nut and the bearing surface of the connecting rod end cover. If there are burrs or unevenness, it should be repaired, otherwise the connecting rod bolt will be easily broken due to the additional torque.

5. When assembling, use a torque wrench to alternately connect the connecting rod bolts (generally 3-4 times) gradually and evenly to the specified button moment. If the tightening torque is insufficient, there will be a gap in the connecting surface of the connecting rod, and the bolt will be easily broken or sheared when it is impacted; if it is over-tightened, the bolt will be elongated and deformed, and the strength will be reduced. The torque requirements of connecting rod bolts of various types of engines are different. In the absence of technical information, the safety torque range can be estimated based on the bolt diameter, such as MU connecting rod bolts, the safe torque is 60-80Nm; M12 bolts , The torque is 80-100Nm; M14 bolts, the torque is 100-120Nmo.

6. If it is found that the torque of a bolt on the same connecting rod exceeds the specified value when bolting, the two connecting rod bolts on the connecting rod should be loosened and then tightened alternately and in turn, and the two bolts should be tightened uniformly. It is not allowed to loosen only the over-tightened bolt, otherwise the bolt will produce additional bending stress and be prone to fatigue fracture.

7. In the process of tightening the connecting rod bolt, you should try to turn the crankshaft to check whether it can rotate freely. If the crankshaft is difficult to rotate after the bolt is tightened, you should find the cause. Never loosen the connecting rod bolt to turn the crankshaft.

8. In order to prevent the connecting rod bolt or nut from loosening during the work, it should be locked with a brand new cotter pin, wire or locking plate; if the slot on the nut is aligned with the end of the hole on the bolt, in principle, only go inward Unscrew to align; used cotter pins, iron wires or locking tabs should not be used again.

9. During use, the tightness of the connecting rod bolts should be regularly checked. When it is loosened, the cause should be found. Check the bolts for damage, and then use a power wrench to tighten the bolts to the prescribed value in accordance with the prescribed torque.

10. When the connecting rod bolt is loosened or stretched during work, before the complete loosening and breakage, due to the looseness of the connecting rod bearing, a large knocking sound and vibration will be emitted. At this time, it should be stopped immediately to avoid greater occurrence. accident.

11. When a connecting rod bolt is slipped, cracked or broken, the two connecting rod bolts on the same connecting rod should be replaced in pairs, and they cannot be used together.

12. When the engine burns tiles many times or once, the connecting rod bolt material is subjected to high temperature annealing, the metallurgical structure changes, the mechanical strength decreases, and the bearing capacity decreases, and it should be replaced.

13. When the engine has a speeding, cylinder biting or rollover accident, the careful inspection of the connecting rod bolts must not be ignored. When the bolts are found to be deformed or cracked, they should be replaced in time.

14. In order to prevent fatigue fracture damage of connecting rod bolts, after the engine has worked for a total of 6000-7000 hours, even if the outer surface of the bolt is obviously damaged, it should be replaced.

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