Force analysis and modification of connecting rod

2022-08-15
  • Detail

Stress analysis and improvement of connecting rod big end of 240/275 diesel engine

abstract the improved G-type connecting rod of 240/275 diesel engine still produces micro cracks on the outermost tooth surface of the short arm in the oblique section of the connecting rod big end when the locomotive is overhauled. This paper makes a theoretical analysis on the stress condition of the connecting rod big end, finds out the main reason for the micro cracks on the outermost tooth surface of the short arm at the big end, and makes an improved design, The fatigue test of the connecting rod was carried out by comparing the new and old schemes, and the line operation examination test was carried out in 1996

key words: improved design of micro cracks on the tooth surface of the big end of the connecting rod

1 introduction

the market of PAN based carbon fiber composites is 165.3 billion yen (about 8.6 billion yuan)

the connecting rod is one of the important moving parts of the diesel engine. For the four stroke diesel engine, during the expansion stroke, it will transform the high gas force acting on the piston into the rotating torque of the crankshaft. At the end of the exhaust stroke and the beginning of the suction stroke, It will bear a lot. Its main characteristics are: ① adopt the reciprocating inertia force of a single 5V power supply (especially the big end), and the alternation is very frequent in operation, so there are many faults. The connecting rod of 240/275 diesel engine has experienced the fracture of connecting rod screws, connecting rod caps and cracks on the side teeth of the big end short arm. After several improvements, our factory has evolved into the current G-type connecting rod. It should be said that the G-type connecting rod is significantly improved compared with the early connecting rod. However, during the overhaul, some connecting rods were still found to have micro cracks on the outermost tooth surface of the short arm of the big end oblique section. Therefore, we theoretically analyzed the stress condition of the big end of the connecting rod in operation, found out the main reason for the formation of the micro cracks on the outermost tooth surface of the short arm at the big end, and improved the reason for use

2 improvement of the big end of the G-type connecting rod of 240/275 diesel engine

2.1 repair the cracked G-type connecting rod

although the G-type connecting rod has been used for about an overhaul period, it has produced micro cracks on the outermost tooth surface of the big end short arm, but it has not brought major problems to the operation. Considering that a large number of G-type connecting rods have been installed on 16V240ZJB and C-type diesel engines, if certain measures are taken for the connecting rods with cracks on the outermost tooth surface of the big end short arm, so that they can continue to be used safely for a period of time, it is still of great significance to users. Therefore, we have carried out the following Feasibility Exploration on the renovation of the cracked G-shaped connecting rod

1) for the G-type connecting rod with cracks found in the overhaul, according to the suggestions of Ricardo company, shave off the outermost teeth of the connecting rod cap and eliminate the cracks. The test is carried out at 1.1 times the load of the operating condition of the C-type diesel engine × 107 cycle fatigue tests. Through flaw detection after the test, no cracks and other abnormal phenomena were found on the tooth surface (see the fatigue test report of 16V240ZJ diesel engine G-type connecting rod)

2) after the running in adjustment, comprehensive performance test, 100h and 360h test of the E-type diesel engine prototype (with the rated speed of 1000r/min and the rated power of 3680kw) for about 550h, 13 G-type connecting rod tooth surfaces have been found to have cracks through physical examination. We will not deal with the two cracked connecting rods and continue to install them; The remaining 11 pieces will be installed after the cracks are eliminated. After that, the E-type prototype successfully completed more rigorous assessment tests, including 6000 times of thermal shock test from idle speed to full load, 50h110% load test (1h of which is over speed), five 10h simulated locomotive cycle load tests, 30h idle speed test and performance review test, with a total of about 550h. After the test, the disassembly inspection found that the three connecting rods without cracks also had cracks; Two of the 11 connecting rods with cracks eliminated have cracks again; Although the crack on the tooth surface of the two connecting rods without any treatment expanded, there was no large damage on the side tooth of the big end short arm

through the above exploration, it shows that the G-shaped connecting rod with cracks, if the outermost tooth of the short arm of the connecting rod cap is shaved and the cracks are eliminated, it can still be installed and used for a period of time (at least one intermediate repair period), without serious consequences for operation

2.2 discussion on the new scheme of connecting rod

although the cracked G-shaped connecting rod can continue to be installed and used for a period of time after proper repair, it is not ideal from the design point of view. Therefore, we theoretically analyzed the stress condition of the big end of the connecting rod in operation, found out the main reason for the formation of the micro cracks on the outermost tooth surface of the short arm at the big end, and improved the reason

(1) cause analysis of fretting wear and micro cracks on the tooth surface

in order to find out the root cause of cracks, we calculated the load on each key radial section of the big end of the G-shaped connecting rod under the action of inertia force and gas force (for the calculation method, see "force analysis of the big end of connecting rod and determination of screw preload")

Table 1 lists the loads on these sections. Figure 1 and Figure 2 respectively show the positions of the key radial sections at the big end and the bending moment distribution diagram of each radial section at the big end under the action of inertia force and gas force

it can be seen from table 1, figure 1 and Figure 2:

1) whether under the action of inertia force or gas force, the bending moment borne by the joint surface at the short arm side is much greater than that borne by the joint surface at the long arm side in the promotion process of all projects

2) for the short arm side, the bending moment under inertia force is much larger than that under gas force, and the direction is opposite

3) for the connecting rod with vertical notch at the big end, the bending moment borne by the joint surface of the body and cover is much smaller than that borne by the joint surface of the short arm side of the inclined notch connecting rod, whether under the action of inertia force or gas force

Figure 1 bending moment distribution diagram of each radial section at the big end of the connecting rod under the action of inertia force

Figure 2 bending moment distribution diagram of each radial section at the big end of the connecting rod under the action of gas force

for construction machinery, bending moment is often the main factor causing damage to parts. Under the action of inertia force and gas force, the short arm side of the big end of the G-shaped connecting rod bears bending moments in opposite directions respectively. Under the action of inertia force, the direction of bending moment is tension on the outside and compression on the inside; Under the action of gas force, the direction of bending moment is external compression and internal tension. If the structural design is unreasonable or (and) the pre tightening force of the screw is insufficient, there will be strong fretting friction on the outer tooth surface of the short arm, resulting in fretting wear, and micro cracks will appear on the tooth surface over time

Table 2 lists the screw preload required to ensure that the joint surface on the side of the short arm does not disengage under the action of inertia force and gas force, as well as the current actual screw preload. It can be seen that the screw preload of the G-type connecting rod is insufficient, resulting in fretting wear of the tooth surface in application. Finite element analysis also confirmed this point. This is the main reason for fretting wear and micro cracks on the outermost tooth surface of the big end short arm

(2) design of new connecting rod scheme

according to the above analysis, we have put forward a total of 6 improvement schemes, together with the improvement suggestions of Ricardo company, a total of 12 schemes. Each scheme has carried out finite element calculation, analyzed and compared the calculation results, and selected the best scheme as the formal new connecting rod improvement scheme. The new scheme for the final improvement of the connecting rod has the following structural characteristics:

1) the diameters of the connecting rod big end hole and the crankshaft connecting rod journal remain unchanged at ∅ 205 and ∅ 195 respectively, which has good versatility and is very important to users

2) adjust the stiffness of both sides of the long and short arms at the big end of the connecting rod to make it symmetrical according to the axis of the connecting rod as far as possible, so as to meet the design principle of two force components

3) appropriately reduce the thickness of the back of the long arm and increase the width of the joint surface of the short arm to increase the bending modulus of the joint surface

4) the connecting rod screw should be moved outward properly to make full use of the screw preload to enhance the bending resistance of the joint surface

5) increase the pre tightening force of the connecting rod screw, overcome or minimize the fretting friction of the outer tooth surface of the big end short arm during operation, and prevent fretting wear and micro cracks

6) the threaded hole on the side of the long arm is changed into a through hole, which is conducive to the processing technology

7) the mass of the connecting rod is about 3kg lighter than the original G-type connecting rod, which reduces the inertia force and is beneficial to the big end of the connecting rod and other moving parts of the diesel engine

in addition, there is also the problem of scratch on the shoulder surface of the screw head in the application of the G-type connecting rod. The new connecting rod scheme has also made the following improvements: the circular height of the connecting rod screw head is increased to reduce the deformation of the screw head support table during tightening; According to the actual effect of the round root rolling of the screw head is not great, and the fatigue test results of the round root non rolling screw, the rolling of the round root of the screw head is cancelled to eliminate the deformation caused by the rolling on the support table

(3) comparison of new and old schemes

1) comparison of the slip condition of the tooth surface contact point pair

Table 3 lists the average slip ratio of the tooth surface on the short arm side of the new connecting rod scheme and the original G-type connecting rod. It can be seen that the slip condition of the tooth surface on the big end short arm side of the new scheme has been greatly improved: under the tensile working condition, the slip ratio of the outermost tooth surface is only 60% of the G-type connecting rod, and the second tooth surface (the inner tooth surface of the outermost tooth) is 36%, The third tooth surface (the outer tooth surface of the outer second tooth) is 89%; Under the compression condition, the slip ratio of the outermost tooth surface is larger than that of the original G-type connecting rod, but the value of the slip ratio is smaller, the second tooth surface is 62%, and the third tooth surface is 28%

2) comparison of the deformation of the big end hole

Table 4 lists the deformation of the big end hole of the new connecting rod scheme and the original G-type connecting rod under tension and compression conditions. In terms of deformation: under tension condition, the longitudinal and transverse deformation of the new scheme is basically the same as that of the G-shaped connecting rod. Under compression condition, the longitudinal shrinkage of the big end hole of the new scheme is slightly larger (0.014mm) than that of the original G-shaped connecting rod, but it is still within the acceptable range

from the node displacement calculated by the finite element method, the deformation of the big end hole of the new scheme is significantly improved than that of the original G-shaped connecting rod, basically along the axis of the connecting rod, indicating that the stiffness on both sides of the long and short arm is close to symmetry, which conforms to the design principle of two force components, and is beneficial to reducing the tooth sliding of the side joint surface of the short arm

3) comparison of stress safety factors at key parts of the big end

Table 5 and table 6 list the safety factors of the new connecting rod scheme and the original G-type connecting rod big end short arm side tooth and other parts respectively, and the tooth node number is shown in Figure 3. From the stress safety factor of the tooth and other parts of the big end, the minimum safety factor of the new scheme connecting rod and the original G-type connecting rod is greater than the allowable minimum safety factor of 1.5; According to the improvement scheme proposed by Ricardo company (completely shaving the outermost teeth of the connecting rod cap short arm of the G-shaped connecting rod), the safety factor at node 1144 is only 1.25, which is less than the allowable minimum safety factor of 1.5. The reason is that the bending moment bearing capacity of the joint surface of the short arm is not improved, but decreased

Fig. 3 node position and number of the big end tooth of the connecting rod

4) test of the new scheme of the connecting rod

after the new scheme of the connecting rod was determined, 20 samples were quickly produced. One of them was tested on the fatigue testing machine of Xi'an Jiaotong University at 110% of the rated load of the E-type machine × 107 cycles of fatigue test (see "durability fatigue test report of new scheme G-type connecting rod"). The test results show that no micro cracks are found on the side tooth surface of the short arm of the new connecting rod scheme through flaw detection; Compared with the original G-type connecting rod, the stress condition of the side tooth of the short arm is improved, and there is no abnormal condition in the rest of the big end

another 16 sample new connecting rods have been installed in the C-type machine and left the factory in December 1996, and the line operation assessment test is being carried out

3 conclusion

the stress condition of connecting rod in operation is relatively complex, and the application of locomotive diesel engine

Copyright © 2011 JIN SHI