On February 27, 2008, our company accepted an accident of connecting rod bolt fracture during running in after engine overhaul in Shenyang Steyr Hongyan Auto Parts Co., Ltd. According to the actual damage and photos fed back by customers; Organize relevant technicians to analyze the cause of the accident in combination with the damaged connecting rod and connecting rod bolt:
The upper and lower cylinder blocks of the engine are damaged, and the lower cylinder block is pierced by the bearing cap of the connecting rod
It can be seen from the figure that there is extrusion damage at the lower part of the upper cylinder block and collision damage at the lower cylinder block. According to the working principle of the engine, the damage position should be on the right side of the cylinder block.
The deformation of connecting rod big end and bearing cap is relatively small (low engine speed)
The connecting rod bolts are broken at the screw thread position of the bolts, and the residual parts are left on the connecting rod pad cover
It can be seen from the above figure that there is serious wear at the joint between the connecting rod bolt and the shoulder bearing surface of the connecting rod bearing cap, which is usually due to burr or unevenness on the shoulder bearing surface of the connecting rod bearing cap; Or the gap between the connecting rod bolt and the bearing surface of the pad cover shoulder will appear during operation.
It is rare that brittle fracture and ductile fracture occur successively.
The fracture of the connecting rod bolt on the left in the figure is distributed in fibrous shape, the fracture is relatively flat, metallic luster, perpendicular to the normal stress, and there are herringbone patterns on the section. There are obvious wear steps at the joint between the bolt head and the shoulder bearing surface of the connecting rod bearing cap (compared with the right figure). It is the connecting rod bolt that breaks first.
The connecting rod bolt on the right side of the figure shows necking and bending, and the section forms an angle of 45 ° with the principal stress (about), which is a typical macro feature of ductile fracture.
The positioning guide necks of the two connecting rod bolts are worn to varying degrees. The wear of the positioning guide neck of the brittle fracture connecting rod bolt is very serious, which indicates that the positioning guide neck and the connecting rod pad cover move relatively under the working state of the connecting rod bolt. This explains the formation of the wear boss in (analysis 2).
Connecting rod bolt is one of the important parts of the engine, which bears great alternating impact load during engine operation. During the maintenance process, many maintenance personnel do not pay attention to checking the fit between the connecting rod bolt and the shoulder bearing surface of the connecting rod bearing cap; In terms of pre tightening force control of connecting rod bolts, many repairmen do not use special tools for pre tightening, and the pre tightening torque usually does not meet the torque specified by the manufacturer.
The bolt fracture is actually an overload fracture caused by external stress overload.
① During maintenance, the tightening torque of the button is insufficient, there is a gap on the connecting rod joint surface during work, and the bolt is subjected to large additional impact force.
② The connecting rod bolt and the bearing surface of the connecting rod end cap shoulder are poorly fitted, and the connecting rod bolt is subjected to large additional torque.
(relevant maintenance data shall be confirmed by call recording)
To sum up, the connecting rod bolt fracture accident of this engine does not belong to the quality problem of the bolt itself!
For the corresponding connecting rod material report, please contact the manufacturer or relevant technical appraisal organization directly.
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