abstract

Connecting rod is one of the main transmission parts of diesel engine. This paper mainly discusses the processing technology and fixture design of connecting rod. Connecting rods require high dimensional accuracy, shape accuracy and position accuracy, while the rigidity of connecting rods is relatively poor and easy to deform. Therefore, when arranging the process flow, it is necessary to separate the rough machining and finish machining processes of each main surface. Gradually reduce the influence of machining allowance, cutting force and internal stress, and correct the deformation after machining, and finally meet the technical requirements of the parts.

Chapter I Processing Technology of Automobile Connecting Rod

1. 1 Structural characteristics of connecting rod

Connecting rod is one of the main transmission parts of automobile engine. In a diesel engine, it transfers the expansion pressure acting on the top surface of the piston to the crankshaft, which drives the piston to compress the gas in the cylinder. The connecting rod bears a rapidly changing dynamic load in its work. The connecting rod consists of a connecting rod body and a connecting rod cover. The connecting rod body and the large hole on the connecting rod cover are assembled with the crankshaft through bolts and nuts. In order to reduce wear and facilitate maintenance, a thin-walled metal bearing bush is installed in the big end hole of the connecting rod. The bottom of the bearing bush is made of steel, and the inner surface of the bottom is poured with a layer of wear-resistant babbitt bearing bush metal. There is a set of gasket between the big end of the connecting rod body and the connecting rod cover, which can be used to compensate the wear of the bearing bush. The small end of the connecting rod is connected with the piston through the piston pin. The bronze bushing is pressed into the small head hole, which reduces the wear between the small head hole and the piston pin, and is convenient for maintenance and replacement after wear.

During the working process of the engine, the connecting rod is subjected to the alternating pressure and inertia force of the inflation gas. The connecting rod itself should not only have enough strength and rigidity, but also reduce the inertia force as much as possible. The connecting rod shaft generally adopts an I-shaped cross-section shape that gradually decreases from the big end to the small end. In order to ensure the balanced operation of the engine, the mass of each connecting rod in the same engine should not be too different. Therefore, bumps for eliminating unbalanced mass are arranged at both ends of the connecting rod parts, so as to eliminate unbalanced mass after weighing. The large and small ends of the connecting rod are symmetrically distributed on both sides of the middle section of the connecting rod. Considering the requirements of clamping, placing and handling, the thickness of the big and small ends of the connecting rod is equal (the basic dimensions are the same). There is an oil hole (or oil groove) at the top of the connecting rod small head. When the engine is working, depending on the high-speed rotation of the crankshaft, the lubricating oil at the lower part of the cylinder splashed into the oil hole at the top of the small head, lubricating the oscillating motion pair between the connecting rod small head bushing and the piston pin.

The function of the connecting rod is to connect the piston with the crankshaft, so that the reciprocating linear motion of the piston can be transformed into the rotary motion of the crank to output power. Therefore, the machining accuracy of connecting rod will directly affect the performance of diesel engine, and the selection of technology is the main factor that directly affects the accuracy. There are five main parameters reflecting the accuracy of the connecting rod: (1) the symmetry of the central plane of the big end and the central plane of the small end of the connecting rod relative to the central plane of the connecting rod shaft; (2) the dimensional accuracy of the center distance between the large and small head holes of the connecting rod; (3) Parallelism of large and small head holes of connecting rod; (4) the dimensional accuracy and shape accuracy of the large and small head holes of the connecting rod; (5) Verticality of connecting rod bolt hole and joint surface.

1.2 main technical requirements of connecting rod

The main surfaces to be machined on the connecting rod are: large and small head holes and their two end faces, the joint surface of connecting rod body and connecting rod cover, and connecting rod bolt positioning holes. The main technical requirements of the connecting rod assembly (Figure 1- 1) are as follows.

Connecting rod assembly drawing (1— 1)

1.2. 1 dimensional accuracy and shape accuracy of large and small head holes

In order to make the big head hole and bearing bush, crankshaft and small head hole and piston pin fit closely, reduce the adverse impact of impact and facilitate heat transfer. The tolerance grade of the big hole is IT6, and the surface roughness Ra should not be greater than 0.4μ m; The cylindricity toleRance of the big hole is 0.0 12 mm, the tolerance grade of the small hole is IT8, and the surface roughness ra should not be greater than 3.2 microns ... The cylindricity tolerance of the bottom hole of the small head press sleeve is 0.0025 mm, and the plane straight line parallelism tolerance is 0.04/ 100 mm.

1.2.2 Parallelism of large and small hole axes in two mutually perpendicular directions

The parallelism error of two holes in the connecting rod axis direction will make the piston tilt in the cylinder, resulting in uneven wear of the cylinder wall and edge wear of the crankshaft connecting rod journal, so the parallelism tolerance of the two holes in the connecting rod axis direction is small; However, the parallelism error of two hole axes in the direction perpendicular to the connecting rod axis has little influence on uneven wear, so its tolerance value is large. The parallelism of the two axial holes of the connecting rod is 0.04 mm; The length is 100 mm; Parallelism perpendicular to the connecting rod axis has a tolerance of 0.06 mm on the length of100 mm.

1.2.3 Center distance of large and small holes

The center distance between the large and small head holes affects the compression ratio of the cylinder, that is, the efficiency of the engine, so a higher requirement is stipulated:190 0.05 mm.

1.2.4 verticality of both ends of the big end hole of the connecting rod facing the center line of the big end hole.

The perpendicularity between the two ends of the big end hole of the connecting rod and the center line of the big end hole affects the installation and wear of the bearing bush, and even causes burns; Therefore, some requirements are put forward: the verticality tolerance level should be not lower than IT9 (the verticality of both ends of the big head hole facing the axis of the big head hole is 0.08 mm within the length of 100 mm).

1.2.5 technical requirements for both ends of large and small holes.

The basic dimensions of the distance between the two end faces of the large and small end holes of the connecting rod are the same, but the technical requirements are different. The dimension tolerance grade of the two end faces of the big end is IT9, the surface roughness Ra is not more than 0.8μm, the dimension tolerance grade of the two end faces of the small end is IT 12, and the surface roughness Ra is not more than 6.3 μ m ... This is because the two end faces of the connecting rod big head have matching requirements with the two shoulder end faces of the crankshaft connecting rod journal, but the two end faces of the connecting rod small head have no matching requirements with the internal gear of the piston pin hole seat. The distance tolerance zone between the big end faces of the connecting rod just falls within the distance tolerance zone between the small end faces of the connecting rod, which brings a lot of convenience to the processing of the connecting rod.

1.2.6 technical requirements for bolt holes

As mentioned above, the connecting rod will be subjected to severe dynamic load in the working process. This dynamic load is transferred to the two bolts and nuts of the connecting rod body and the connecting rod cover. Therefore, these two power bolts not only have high technical requirements for bolts and nuts, but also have certain requirements for installation holes and end faces. It is stipulated that bolt holes should be processed according to the tolerance grade of IT8, and the surface roughness Ra should not be greater than 6.3 μ m; The symmetrical tolerance of the two bolt holes on the split face of the big head hole is 0.25mm. ..

1.2.7 technical requirements for joint surface

When the connecting rod is subjected to dynamic load, the skew of the joint surface makes the connecting rod cover and the connecting rod body relatively dislocate along the middle plane, which affects the poor combination between the connecting rod journal and the crankshaft bearing and causes uneven wear. The parallelism of the joint surface will affect the tightness of connecting rod body, connecting rod cover and gasket, thus affecting the stress of bolt and the wear of crankshaft and bearing bush. For this kind of connecting rod, the flatness tolerance of the joint surface is required to be 0.025mm. ..

1.3 connecting rod material and blank

Connecting rod bears multi-directional alternating load in work, which requires high strength. Therefore, connecting rod materials generally adopt high-strength carbon steel and alloy steel; Such as 45 steel, 55 steel, 40Cr, 40CrMnB, etc. In recent years, ductile iron has also been used. Powder metallurgy parts have high dimensional accuracy, less material loss and low cost. With the appearance and application of powder metallurgy forging process, the density and strength of powder metallurgy parts are greatly improved. Therefore, powder metallurgy is a promising manufacturing method for connecting rods.

The selection of the manufacturing method of connecting rod blank mainly depends on the production type, the manufacturability (plasticity and ductility) of the material, the requirements of the parts for the organization and performance of the material, the shape and external dimensions of the parts, the existing production conditions in the blank workshop and the possibility of adopting advanced blank manufacturing methods to determine the manufacturing method of the blank. According to the production procedure, for mass production, connecting rods are often forged into blanks. There are two forms of connecting rod die forging, one is to forge the body and cover separately, and the other is to forge the body and cover into a whole. In the subsequent processing, the whole forging blank needs to be cut. In order to ensure the uniformity of rough boring allowance after cutting, it is best to forge the big end hole of the whole connecting rod into an ellipse. Compared with split forging, integral forging has some problems, such as high power required by forging equipment and metal fiber being cut off. However, due to the advantages of less material loss, short forging time and fewer dies, integral forging is increasingly adopted and becomes the main form of connecting rod blank. In a word, the choice of blank type and manufacturing method should reduce the total production cost and improve the performance of parts.

At present, some domestic factories producing connecting rods have adopted the connecting rod roll forging process. Fig. (1-2) is a schematic diagram of connecting rod roll forging. After the blank is heated, the hairiness is plastically deformed through the grooves of the upper forging roll die 2 and the lower forging roll die 4, thus obtaining the required shape. The surface quality, internal metal structure, metal fiber direction and mechanical strength of connecting rod forgings produced by roll forging can reach the level of die forging, and the equipment is simple, the working conditions are good, the productivity is high, it is convenient to realize mechanization and automation, and it is suitable for mass production. Roll forging needs multiple step-by-step forming.

Fig. (1-2) Schematic diagram of connecting rod roll forging.

Figures (1-3) and (1-4) show the forging process of the connecting rod. The bar is heated to1140 ~1200c0 in the furnace, and then it is rolled and forged with four grooves on the roll forging machine, as shown in Figure (65438+). After forging, the connecting rod blank needs to be quenched and tempered to obtain fine and uniform tempered sorbite structure, so as to improve the performance and reduce the internal stress of the blank. In order to improve the accuracy of the blank, the connecting rod blank needs to be thermally corrected.

Connecting rods must go through appearance defects, internal flaw detection and comprehensive inspection of blank size and quality before entering the machining production line.

Processing technology of 1.4 connecting rod

From the analysis of the above technical conditions, it can be seen that the dimensional accuracy, shape accuracy and position accuracy of the connecting rod are very high, but the rigidity of the connecting rod is relatively poor and it is easy to deform, which brings many difficulties to the mechanical processing of the connecting rod and must be paid full attention to.

The machining process of connecting rod is shown in the following table (1-1):

Process process name process content process equipment

1 Milling two planes of connecting rod's big end and small end, with grinding amount of 0.5mm X52K on each side.

2. Roughly grind one large plane, and then grind another large plane to ensure the symmetry of the center line. The unmarked surface is called the base surface. (the same below) M7350

3 drilling and base plane positioning, drilling, expanding and reaming small head hole Z3080

4. Milling should be based on the base surface and large and small head holes, and both sides of the workpiece with the milling size of mm should be clamped to ensure symmetry (this plane is the process reference surface). X62W modular machine tool or special fixture.

5. Expand the base plane positioning, use small head hole positioning, and expand the head hole to φ 60 mm Z 3080.

6. Milling is to clamp and cut the workpiece, number the shaft and mark the upper cover respectively based on the base surface and the large and small head holes. X62W modular machine tool or special fixture saw blade milling cutter is 2mm thick.

7. Milling the bottom surface and one side surface to position and clamp the workpiece, milling the joint surface of the connecting rod body and the cover, and measuring the depth in the gage direction with a 27.5mm X62 combined fixture or special fixture.

8 Grind the bottom surface and one side surface of the workpiece, and grind the joint surface of connecting rod and cover M7350.

9. Milling the base surface and joint surface of workpiece, and milling the mm 8mm chute of connecting rod body and cover.

X62 combination fixture or special fixture

10υ Position and clamp the workpiece with the base surface, joint surface and one side surface. υ The seating surfaces of the two bolts are mm, R 1 1mm, and the size is guaranteed to be mm..

X62W

1 1 Drill 2 2- 10/0mm bolt holes.

Z3050

12 enlarge the bolt hole by 2- 12 mm, then enlarge the bolt hole by 2- 13 mm, and chamfer the Z3050.

13 hinge 2-12.2mm bolt hole

Z3050

14 connecting rod body and connecting rod cover are assembled into a connecting rod assembly with special pliers screws, and the torque is100-120 n.m.

15 Boring Rough Boring Big Head Hole T6 8

16 chamfering at both ends of large end hole X62W

17 grind both ends of the big and small heads to ensure that the thickness of the big end face is mm.

M7 130

18 boring is based on the base surface and one side surface, semi-finish boring large holes, and finish boring small holes to the drawing size, with the center distance of mm.

Adjustable biaxial boring

19 boring the big hole to the size of T2 1 15.

20 Weighing Unbalanced Mass Spring Scale

2 1 pliers should be weighed according to the specified value.

22 Drill the small oil hole of connecting rod body 6.5 mm,10 mm.

Z3025

Copper sleeve double-sided pneumatic press

24 press for extruding copper sleeve hole

25 chamfer the small head holes at both ends of Z3050

26. Semi-finish boring and finish boring small copper sleeve hole T2 1 15

Honing machine tool for 27-hole honing

Check the size and accuracy of each part.

Non-destructive testing and hardness testing

30 storage

The main machining surfaces of the connecting rod are large and small head holes and two end faces, the more important machining surfaces are the combination surface of the connecting rod body and the cover and the positioning surface of the connecting rod bolt hole, and the secondary machining surfaces are the locking groove of the bearing bush, the oil hole, the two sides of the big head and the bolt seat surface of the body and the cover.

The machining route of the connecting rod is arranged around the machining of the main surface. The machining route of connecting rod can be divided into three stages according to the separation and combination of connecting rod: the first stage is the machining of connecting rod body and cover before cutting; The second stage is the processing of the connecting rod body and the cover after cutting; The third stage is the processing of connecting rod body and cover after assembly. The first stage of processing is mainly to prepare fine datum (end face, small head hole and outside of big head) for its subsequent processing; The second stage is mainly to process other surfaces except precision datum, including rough machining of big head hole, rough machining of bolt hole and joint surface for assembly, and machining of locking groove of bearing bush. The third stage is mainly to ensure the processing of the technical requirements of the connecting rod, including the semi-finishing of the large end hole and the finishing of the end face after the connecting rod is assembled, and the finishing of the large and small end holes. If the connecting rod is assembled before and after, the technological route before assembly belongs to the rough machining stage of the main surface, and the technological route after assembly belongs to the semi-finish machining and finish machining stage of the main surface.