Connecting rods are extremely critical components in gasoline engines and diesel engines, with a wide variety and huge demand, among which automobile engines have the largest demand. Today, Xiaogong takes you to understand the relevant knowledge of connecting rod manufacturing.
The structure and function of connecting rod
The connecting rod is a relatively slender non-circular rod with variable cross-section, and the cross-section of the rod body gradually decreases from the large end to the small end to adapt to the rapidly changing dynamic load during work. It is composed of the big end of the connecting rod, the rod body and the small end of the connecting rod. The big end of the connecting rod is separated, half is integrated with the rod body, and the other half is the connecting rod cover. The connecting rod cover is assembled with the crankshaft main journal with bolts and nuts. together.
The connecting rod connects the piston and the crankshaft, and transmits the force on the piston to the crankshaft, converting the reciprocating motion of the piston into the rotational motion of the crankshaft. It is one of the main transmission components of the automobile engine. It transmits the pressure of the expanding gas acting on the top of the piston to the crankshaft, so that the reciprocating linear motion of the piston becomes the rotary motion of the crankshaft to output power.
Workpiece material and blanks
Most of the connecting rod materials are selected high-strength 45 steel, 40Dr steel, etc., and are quenched and tempered to improve cutting performance and impact resistance. The hardness requires 45 steel to be HB217~293, and 40Dr to be HB223~280. There are also those that use ductile iron and powder metallurgy technology, which can reduce the cost of blanks.
The blanks of steel connecting rods are generally produced by forging, and there are two forms of blanks: one is to forge the body and cover separately; The breaking process will swell it. In addition, in order to avoid defects in the blank, 100% hardness measurement and flaw detection are required.
Connecting rod machining process
1. Positioning and clamping 1) The correct selection of the rough datum and the rational design of the initial positioning fixture are crucial issues in the processing technology. When pulling the positioning surfaces of the large and small heads of the connecting rod, the reference end face of the connecting rod and the three-point outer circle of the small end blank and the two points of the outer circle of the large end blank are used for rough reference positioning. In this way, the machining allowance of the large and small head holes and the processing surfaces of the cover is uniform, and the weighing and deduplication of the large end of the connecting rod is ensured, and the final shape and position of the part assembly are ensured.
2) In the processing of the connecting rod and the assembly, the processing and positioning methods of the rod end face, the top surface and side of the small head, and the side of the large head are adopted. In the machining of the connecting rod cover in the machining process from the bolt hole to the spigot, the machining and positioning method of its end face, the two bolt seating faces, and the side face of one bolt seating face is adopted. This kind of positioning and clamping method with high repeated positioning accuracy, stable and reliable positioning, small deformation of parts, convenient operation, and can be used in various processes from roughing to finishing. Since the positioning reference is unified, the size and position of the positioning points in each process are also kept the same. All these provide good conditions for stabilizing the process and ensuring the machining accuracy.
2. Arrangement of processing sequence and division of processing stages
The dimensional accuracy, shape accuracy and position accuracy of the connecting rod are very high, but the rigidity is poor and it is easy to deform. The main machining surfaces of the connecting rod are the big and small head holes, the two end faces, the joint surface between the connecting rod cover and the connecting rod body, and the bolts. The secondary surfaces are oil holes, locking grooves, etc. There are also processes such as weighing and de-weighting, inspection, cleaning and deburring. The connecting rod is a die forging, and the machining allowance of the hole is large, and it is easy to generate residual stress during cutting. Therefore, when arranging the process, the rough and finishing processes of each main surface should be separated. In this way, the deformation caused by roughing can be corrected in semi-finishing. The deformation generated in the semi-finishing process can be corrected in the finishing process, and finally the technical requirements of the part are met and the positioning datum is processed first in the process arrangement.
The connecting rod process can be divided into the following stages:
1) Rough machining stage The rough machining stage is also the processing stage before the connecting rod body and the cover are combined: mainly the processing of the datum plane, including the auxiliary datum plane processing, and the preparation for the combination of the connecting rod body and the cover, such as the opposite surfaces of the two. milling, grinding, etc.
2) Semi-finishing stage The semi-finishing stage is also the processing after the connecting rod body and the cover are combined, such as fine grinding of two planes, semi-finished big head hole and hole chamfering, etc. In short, it is the stage of preparation for finishing large and small head holes.
3) Finishing stage The finishing stage is mainly to ensure that all the large and small holes on the main surface of the connecting rod meet the requirements of the drawing, such as honing the big end hole, fine boring the small end bearing hole, etc.
4) Process flow table of connecting rod processing
What kind of connecting rod is a good connecting rod?
The small end of the connecting rod is connected with the piston through the piston pin, and the large end is connected with the journal of the crankshaft. The size of the large and small ends depends on the pressure bearing area. The working temperature of the connecting rod is 90～100℃, and the running speed is 3000～5000r/min. In order to ensure the smooth entry of the connecting rod forgings into the automatic precision machining production line and the assembly accuracy of the finished parts in the engine, and at the same time, in order to maintain the high frequency of tension and compression alternating stress during high-speed operation, the crankshaft is always in balance. condition, requiring high strength and fatigue life of connecting rod forgings.
On the premise of meeting the dimensional accuracy of the drawings, the connecting rod forgings should also meet the following technical and quality requirements:
1. The uninjected forging slope is between 3° and 5°, and the uninjected fillet radius R is between 2 and 5mm.
2. The non-machined surface should be smooth, and no defects such as cracks, folds, scars, and oxide scales (pits with a depth of >1mm) are allowed.
3. The width of residual flash on the parting surface is less than or equal to 0.8mm.
4. The direction of the metal fibers in the longitudinal section should be along the direction of the center line and in line with the shape. There should be no disorder and discontinuity, and no defects such as porosity, folding and non-metallic inclusions are allowed.
5. The hardness of quenching and tempering treatment is between 220 and 270HB.
6. Forgings should be inspected for flaw detection.
7. Repair welding is not allowed for defects on forgings.
8. The quality deviation of each batch of forgings is less than or equal to 3%.
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