Product structure characteristics of commercial vehicle frame longitudinal beam

Compared with other stamping parts, the longitudinal beam has the following characteristics:

(1) The shape is long and the sheet is thick. Depending on the function of the model, the length of the longitudinal beam is about 5-12m, and the thickness is 4-10mm. For example, the length of the longitudinal beam of the tractor frame is generally 6 to 8 m, and the length of the longitudinal beam of the cargo dump frame is generally 8 to 12 m.

(2) The number of holes is large. More than 80% of the important assemblies and components of commercial vehicles are assembled on the frame rails. For example, the number of longitudinal beam holes is generally 200 to 400 for engines, gearboxes, axles, cabs and external accessories.

(3) The structure is not complicated but the precision is high. The U-shaped structure is mainly used. Since most of the holes and profiles of the longitudinal beams are assembly holes and assembly surfaces, the dimensional accuracy requirements are high.

(4) High material strength. The yield strength δs of the steel sheet is 500 to 700 MPa.

The longitudinal beam and inner reinforcement plate of commercial vehicle frame generally have three main structural forms: single-layer plate, double-layer plate and triple-layer plate. Generally, the thickness of the inner reinforcement plate is not greater than that of the longitudinal beam. According to the load-bearing needs of different models, the reinforcement plates of different levels and plate thicknesses are configured. According to the section, there are two kinds of constant section and variable section, and according to the ventral surface, there are two kinds of straight and curved.

Processing technology of commercial vehicle longitudinal beam

Traditional stamping process

The large-scale press combined with the mold for cold stamping is the production process adopted by the major domestic truck manufacturers in the initial production of longitudinal beams. This production mode is suitable for the production of small quantities and large batches. From the 1960s to the 1990s, there were few types of truck products, large production batches, high efficiency and good stability of die stamping production. Jiefang Company Truck Factory used this process to produce longitudinal beams until 2004.

The processing process is: double-sized steel plate → pickling → die blanking and punching → die bending.

Its advantages are:

(1) The mass production efficiency of a single variety is high.

(2) The production area is small and the logistics links are few.

(3) Blanking and punching, high precision and good consistency of the shape and size of the hole.

(4) The product has strong adaptability, and both straight grooved longitudinal beams and variable-section longitudinal beams can be produced.

Weakness:

(1) The mold investment is large and the manufacturing cycle is long. A set of blanking and punching dies and bending dies for a commercial vehicle longitudinal beam cost about 5 million in total, and the manufacturing cycle of the dies is 8 to 12 months. When new products are prepared, they cannot meet the needs of rapid production.

(2) The utilization rate of the closed blanking material of the double-sized material is low. When closing the blanking and punching, the lap value of both sides is 30mm. Compared with the net material, each piece weighs about 15kg. Based on the monthly production of 10,000 vehicles, nearly 300t of waste is generated every month.

(3) Low degree of flexibility and poor product adaptability. When any parameter of product hole position, groove width and thickness changes, it is necessary to modify the mold. The cycle of modifying the mold cannot meet the cycle of product change, which will lead to the failure of new products to be put into production on schedule.

(4) Stamping and forming springback is difficult to control. Due to the existence of elastic deformation during stamping forming, the longitudinal beam springs back after forming. The main defects of the longitudinal beam in the stamping forming process are: airfoil angle inward or outward expansion, twisting at the variable section, warping of the ventral surface, etc., as shown in Figure 2. Although some springback compensation measures can be taken on the forming die, due to many factors affecting the springback of the longitudinal beam, it is difficult to control the springback in actual production. Longitudinal springback (warping) is the most severe and difficult to control due to the longer stringers. As the strength of the longitudinal beam material increases, the springback problem of the longitudinal beam becomes more and more prominent.

In addition, in the stamping production process, due to the deviation of the material thickness and the poor surface quality of the material, it is easy to cause the surface of the die of the mold to wear, resulting in the formation of tension marks on the airfoil of the longitudinal beam, which seriously affects the surface quality of the longitudinal beam.

(5) The pickling process pollutes the environment and affects the health of operators.

Stamping combined with CNC punching and cutting process

After 2004, with the rapid development of CNC technology, flexible equipment such as CNC punching machines and robotic plasma cutting machines have been successively put into the production process of stringers. The CNC punching technology for clean-size materials and the robotic plasma cutting machine replace the closed blanking punching for double-size materials, and the pickling process is replaced by the shot blasting process.

Processing process: clean-length steel plate → CNC punching → robot plasma shape cutting → die bending forming.

The advantages are: the hole processing and the shape processing realize the rapid conversion, adapt to the multi-variety and small batch production, and save the mold cost.

Weakness:

(1) The efficiency is low, and the one-time input cost is high. The cycle time of CNC punching and robot plasma cutting is 4-6min/piece, which is about 1/8 of the punching cycle of the die. In order to solve the cycle problem, it is necessary to invest in multiple equipment to produce at the same time.

(2) The hole precision is low. The hole precision of die punching is mainly guaranteed by the die precision, while the hole precision of CNC punching is determined by the precision of CNC punching equipment. CNC punching equipment is composed of servo motors, robots, high-precision screws, racks and pinions, etc. Compared with traditional presses, its structure is more complex, the accuracy decays quickly, and maintenance is difficult, and it is not easy to solve problems when there are problems. The punching accuracy is also affected by the dimensional accuracy of the incoming material.

(3) The combination of CNC equipment and stamping equipment has complicated production logistics, and it is impossible to achieve "one-stream" production. After the CNC process, a large number of semi-finished products are buffered to be processed, which greatly increases the difficulty of workshop production organization and material management.

Rolling combined with CNC punching and cutting process

After 2008, the market demand for trucks has increased, and users' requirements for truck functions have also increased. The main changes are concentrated in the hole position of the longitudinal beam and the width of the groove. All automobile factories basically focus on the production mode of multi-variety and small batches. Although the input of numerical control equipment can meet the flexible production of hole processing, die bending is still the bottleneck restricting production. In addition, with the lightweight requirements, the strength of the steel plate gradually increases, and the quality of the die bending is difficult to meet the design requirements. In view of the problem of bending and forming of longitudinal beams, after comparing the manufacturing processes of longitudinal beams of well-known domestic and foreign manufacturers, it is found that "Volvo", "Man" and China Dongfeng Motor all use roll forming combined with CNC machining technology.

Processing process: steel coil → roll forming → CNC punching → robot plasma profile cutting → shot blasting → ventral bending (optional).

Advantages:

(1) High degree of flexibility, less production auxiliary time, and high material utilization rate. When the product is switched to production, just enter the name of the part, the equipment will automatically adjust the parameters to the processing size of the part, and the adjustment time for product groove width changes is only 3 minutes, and the rest of the CNC equipment can switch products at any time. The entire production line adopts automatic transmission equipment for online production, the rhythm between each process is reasonably matched, and the number of material buffers is low. In the production process, the production of products can be reasonably allocated according to the length of the coil, and the utilization rate of the steel coil can reach up to 98%.

(2) The processing precision of the roll shape is high. The roll press adopts 16 sets of rollers to gradually form, which can effectively remove the residual stress of the steel plate during the forming process and reduce the springback. After the equipment is formed, a correction device is installed, which can effectively control defects such as distortion of the longitudinal beam and out-of-tolerance straightness. After correction, the straightness accuracy of the longitudinal beam's ventral surface can reach 5mm/12m, the verticality accuracy of the airfoil can reach ±0.5mm, the distortion is less than 4mm, and the height accuracy of the airfoil is ±1mm. The simulation analysis of the rolling process and the equipment structure are shown in Figure 3. Since the holes are punched after roll forming, the roll shape precision is high, making the hole precision easier to ensure.