Fault analysis of crankshaft fracture of second-hand semi-trailer engine
A second-hand semi-trailer engine crankshaft with good quality should have a long service life under normal use. However, if there are defects in the manufacturing process or improper use and maintenance, it will often lead to the fracture of the crankshaft.
(1) The most common fracture form of crankshaft is bi-directional bending fatigue at the journal fillet, resulting in the fracture of the connecting part between the connecting rod journal fillet and the crank wall. As shown in Fig. 2-12, it is a special example of crankshaft fracture in t Fig. 2-13.
The stress on the crankshaft of second-hand semi-trailer is very complex. It bears the explosive force of gas and the inertia of moving parts
Force, various vibrations and reaction forces related to constraints such as body bearings, reaction forces with driven devices, etc. When the crankshaft works, the journal is mainly twisted and the crank arm is mainly bent. These loads change periodically and can be regarded as pulsating cyclic loads approximately. When the crank bears the maximum burst pressure near the top dead center, the crank arm bears the maximum bending moment, resulting in great stress at the transition fillet between the crank arm and the journal. When this stress exceeds the fatigue limit of the crankshaft, it will lead to bending fatigue cracks. This is because the cross-sectional area resisting bending moment is the smallest at these positions, where Yiyi bears the stress peak formed by the maximum bending moment.
Because the connecting rod journal is thinner than the main journal, the relative rigidity of the connecting rod journal fillet transition is poor, and the fatigue source is more likely to occur first at the connecting rod journal fillet transition. When the fatigue source occurs in Hi, with the expansion of the fatigue zone, greater stress is borne at the fillet of the opposite journal.
Due to the rapid increase of stress concentration, when the stress exceeds the fatigue limit, secondary fatigue sources will occur and expand. Usually, the crack expands gradually in operation and will suddenly break when it reaches a certain time. When observed on the fracture surface, the dark brown part is often found, although it is an old crack. The shiny tissue is the trace of sudden fracture after development.
The crack on the crankshaft is fine and difficult to see with the naked eye. The magnetic flaw detector can be used for inspection. When the conditions are not available, the easiest inspection method is oil overflow hammer method. The method is to diffuse the crankshaft into kerosene for a moment, take it out and wipe it clean, and then sprinkle with white chalk. Then use the hand hammer to knock on the crankshaft arm in sections. Due to the vibration, the kerosene in the crack will seep out, and the end of the white chalk will show oil stains as yellow lines. Therefore, the crack position and length can be determined.
Practice has proved that the crankshaft can still be used if longitudinal cracks are found on the surface of the journal before it is broken and can be eliminated in time after grinding. If a transverse crack is found, especially along the oil hole, the crack will expand quickly, so the crankshaft should be replaced in time.
(2) common causes of broken crankshaft of second-hand semi-trailer
1) Poor lubrication when the oil circuit of the diesel engine lubrication system is not smooth, so that the connecting rod bearing bush and journal are in a semi friction state, that is, the Bush burning and shaft holding fault may occur. Due to the weakening of the supporting force at the ablated part, the radial runout of the shaft will be frequent, and the stress will change after root ablation, which will lead to fracture failure between the root of crankshaft journal and crank arm.
2) poor operation of bearings. Take 6135 diesel engine as an example. If the rolling bearings for main bearings have rolling turns, or the clearance is too large and loose for a long time, resulting in excessive radial circular runout of different shafts of main bearings, the crankshaft will be subjected to uneven stress due to frequent radial circular runout of crankshaft, certain weight of diesel engine flywheel itself and working impact of each cylinder. In addition, if the assembly clearance of connecting rod bearing bush is too large or the alloy falls off, it will also increase the impact load and lead to the early fracture of crankshaft.
The loosening of the main bearing cap nut makes the crankshaft lose its due support, or the coaxiality of each bearing seat hole installed on the crankshaft deviates, which is also one of the important reasons for the early fatigue fracture of the crankshaft. Taking 6105 Diesel engine as an example, if the nut of a main bearing is loose, the fit clearance between the shaft and bearing bush must exceed the allowable range, so that it will lose its supporting effect on the main journal. If the concentricity deviation occurs due to the bearing seat holes at the front end and the last end, when the crankshaft rotates, the crankshaft fracture accident will be caused due to the different loads borne by the relevant cranks on the crankshaft.
3) Excessive axial movement of crankshaft and excessive axial clearance of crankshaft are the main causes of axial movement of crankshaft. Take 6105 Diesel engine as an example, Both sides of the fourth main bearing are integrated with the thrust plate (commonly known as combined flanging pad) to position the crankshaft axially and ensure a certain axial clearance. When its displacement reaches 0.5 ~ 0.6mm or more, the consequence is that when the diesel engine works with load, the counterattack force on the end face of the crankshaft crank arm will be more serious. Due to stress concentration, it is possible to accelerate the progress of crankshaft fracture.
4) If the flywheel bolt is loose, the crankshaft assembly rotating at high speed will lose its original dynamic balance and produce a large inertial force, resulting in crankshaft fatigue and easy to break at the tail end. Therefore, the crankshaft and flywheel are fitted with tapered holes (e.g. S195 diesel engine). If the fitting area of the two is less than 75%, the flywheel will also loosen during the operation of the diesel engine, which will impact the crankshaft, resulting in cracks in the part fitted with the keyway and breaking the crankshaft.
5) During the maintenance of the second-hand semi-trailer power imbalance, if the crankshaft is replaced with a balance iron that does not meet the requirements, or the original balance is damaged due to the dislocation of the balance iron, it is easy to make the crankshaft bear a great inertial impact force. In addition, if the crank arm and crank radius of the repaired crankshaft are offset, the unbalance will greatly exceed the allowable value. The resulting unbalanced centrifugal force will overload the bearing and cause strong vibration of the diesel engine. After a long time, it is easy to fatigue fracture the crankshaft.
6) The most important factor affecting the bending fatigue strength of crankshaft is the transition fillet between connecting rod journal and crank arm, and the transition fillet between main journal and crank arm. Therefore, when grinding the crankshaft, if only the coaxiality, parallelism, crank radius, roughness of journal shoulder and other repair surfaces of the crankshaft and connecting rod journal, as well as the included angle between each connecting rod journal are inspected, the size and geometric shape of the transition fillet are ignored. As a result, the fillet of the grinding wheel is improperly trimmed when grinding the crankshaft journal. Because the fillet radius is not correctly selected or grinded into a right angle, it is easy to produce stress concentration at the crankshaft fillet, resulting in cracks due to reduced strength.
7) parts with rhyme difference take 6110 Diesel engine as an example. The pistons are divided into three groups according to mass. The pistons installed on the same diesel engine shall be grouped by the same mass (a, B and C), and the mass difference of the same group shall not exceed 8g. The connecting rod is divided into four groups. The connecting rod optional on the same diesel engine shall also be the same mass group (red, blue, white and yellow), and the difference in the total mass of the piston connecting rod group assembly installed on the same diesel engine shall not exceed 30g. Otherwise, it means
Therefore, it is easy to make the explosive force and inertial force of each cylinder inconsistent with each other. As a result, the force on each journal of the crankshaft is uneven. When the crankshaft is impacted by vibration, it may also cause crankshaft fracture.
8) When the crankshaft is bent due to improper straightening process, when the cold straightening method is used for repair, the straightening amount can often range from a few mm to a few mm. However, because cold straightening is a plastic deformation method, sometimes the impact load is large, which will produce large additional residual stress and greatly reduce the strength of the crankshaft. Therefore, the crankshaft will fracture due to fatigue under long-term alternating load.
9) In case of overspeed operation or speeding diesel engine often operates above the rated speed, or in case of speeding accident, excessively violent emergency braking measures shall be taken to make the diesel engine cease fire. As a result, the crankshaft is excessively stressed and the stress increases greatly, which may also lead to the accidental fracture of the crankshaft of the second-hand semi-trailer.
