Load analysis and life prediction of key components of high-speed press based on virtual prototyping

3.1. Measuring method of load data

The field test was performed for the J76-125E high-speed press, and then the load data of the J76-125E and the operational information were obtained. The main parameters are measured as follows:

1) Acceleration. The acceleration of the upper die and slider can be measured directly by the acceleration sensor (type A104EX-100G) installed on the slider.

2) Force. The piling force on the upper die is the main force source in operation, especially the instantaneous force by the collision of the upper die and the workpiece. Generally, two tonnage meters (type Sigmar-810 H/500 t) are used to measure the piling force on the upper die. The installation of the acceleration sensor and tonnage meter are shown in Fig. 2.

Fig. 2The installation of acceleration sensor and tonnage meter

The disturbance by the other variables should be reduced as little as possible to make the test data accurate, so the test is conducted by way of piling. The maximum impact force of the upper die is 125 tons and the top blanking speed is 330 r/min, so the piling forces are divided into 0 t, 50 t, 75 t, 100 t and 125 t and the frequencies are divided into 150 r/min, 200 r/min, 250 r/min, 300 r/min, 330 r/min and 350 r/mm (In order to maintain safe operation, a frequency of 350 r/min is only tested under the piling force of 0 t and 50 t). Based on above division, 27 groups of tests are conducted.

3.2. Measured results

Take the test of piling force of 75 t and the frequency of 300 r/min as an example and this test is referred to as 75 t-300 r/min below this paper. The test results are shown in Fig. 3 and Fig. 4.

Fig. 3The piling force under 75 t-300 r/min condition

Fig. 4Slider acceleration under 75 t-300 r/min condition

Fig. 3 and Fig. 4 show that the piling force and the slider acceleration are significantly periodic, and the return cycle is equal to the piling cycle, that is about 0.3 s. The piling force of No. 1 and No. 2 tonnage meter is slightly different, but the total force can reach the set value in the test. The peak acceleration of the slider is about 22500 mm/s² in one cycle and it is slightly different in one working condition during each cycle. The maximum accelerations of the slider are tested in 27 different operating conditions, and shown in Table 2 and Fig. 5.

4.1. Simulation model

Earlier studies show that the high-speed press needs to bear a heavy load in practical working conditions, and that the transmission system has a high rotating speed, which results in a tremendous impact on the load, thus it is difficult to test the practical load. Using the dynamic simulation method, however, can solve this problem. The 3D model of the high-speed press is modelled in a three-dimension solid (CATIA), and then the model is imported into an ADAMS enviroment. The 3D model and ADAMS model are shown in Fig. 6 and Fig. 7 respectively.

Fig. 63D model of the high-speed press

Fig. 7ADAMS model of of the high-speed press

The load of the key parts can be obtained by the dynamic simulation based on the ADAMS software. The stiffness, density and Poisson’s ratio parameters of the materials should be set in the target simulation models. When the target parts are divided into meshes and made into flexible components, the deformation and internal stress approximating true conditions can be simulated through ADAMS. Before the simulation is performed in ADAMS, the established parts should be made into flexible bodies in the format file with a .mnf filename including the modal parameters of the target parts. The flexible bodies of the .mnf filename file are made as shown in Fig. 8. The .mnf filename file should be imported into ADAMS to replace the original rigid body model. The made-flexibe model of the connecting rods and crankshaft are shown in Fig. 9 and Fig. 10 respectively.

Two columns are established in ADAMS in order to simulate the tonnage meter. The position and size parameters are set according to the real high-speed press. After the parts are replaced by the flexible bodies, the parameters of the simulation model are further set, and the final ADAMS model is shown in Fig. 7.

Fig. 8Flowchart for making a .mnf filename file

Fig. 9Connecting rod mesh

Fig. 10Crankshaft mesh

4.2. Parameter setting and simulating

The frequency of the piling is set according to the order of 150 r/min, 200 r/min, 250 r/min, 300 r/min, 330 r/min or 350 r/min, and the piling force is set according to the order of 0 t, 50 t, 75 t, 100 t or 125 t in the virtual prototyping. The frequency of piling can be set by setting initialization speed parameters of the belt wheel. The initial speed of the hinge joint of the belt wheel is set at 1800°/s, 2400°/s, 3000°/s, 3600°/s, 3960°/s or 4200°/s during simulating according to the transmission ratio of the coupling constraint of the belt wheel, flywheel, and the crankshaft rotating speed. The piling force of the press can be adjusted according to the height of the column. As shown in Table 3, the data of the piling force and the tonnage meter height can be obtained based on the parametric analysis function of ADAMS [16].

Considering the data provided in Table 3, the piling force $F$ and the tonnage meter height $d_1$ have the following relationship:

Eq. (3) is the function relationship between the piling force and the tonnage meter height which is represented by $d_1$. Obviously, the tonnage meter height has relationship with the installation of the tonnage meter in practice，what makes Eq. (3) inapplicable. In order to make Eq. (3) more applicable, we suppose the piling contact area is $S$ and the piling penetration depth is $d_2$. The piling contact area in simulation is 31416 mm² and the distance between the bottom of tonnage meter and the lower dead point is 299 mm, then $d_1=\left(S/31416\right)d_2+299$.

Eq. (3) can be changed into the following one:

Simplifying the above equation:

where $F$ represents the piling force and its unit is N; $d_2$ represents the piling penetration depth in simulation and its unit is mm; $S$ represents the piling contact area and its unit is mm².

The design values of the penetration depth and the height of the tonnage meter column under different piling forces obtained according to Eq. (3) and (4) are shown in Table 4 (half of the total piling force is the measured value of a single tonnage meter). After parameters are set, simulation should be performed under different work conditions. The results are summarized and analyzed in Section 5.

5.1. Data statistics

The load data obtained by simulating should be exported for further statistical analysis. The maximum stress and strain of the two kinds of components in 30 simulation working conditions are shown in Table 5 to Table 8. The data indicates that different piling frequencies have little impact on the maximum stress and strain under the same piling force. When the high-speed presses are not in loading, the maximum stress and strain increase with the increasing of frequency of piling. When the high-speed presses are in loading, the maximum stress and strain keep nearly unchanged with increasing frequency of piling, or even at a slight reduction. Taking the 300 r/min frequency of the piling as an example, the changes of the maximum value stress and strain of two components with the changes of the piling forces are shown in Fig. 11 and Fig. 12. From Fig. 11 and Fig. 12, we can see that the maximum stress and strain of components are not affected by the piling frequency, so the tendencies of the maximum stress and strain with the changes of the piling force under the other piling frequencies are similar to what is shown in Fig. 11 and Fig. 12.

The relationship between the maximum stress and the piling force can be obtained from Fig. 11 and Fig. 12 and the coefficients of Eq. (5) are listed in Table 9.

where $x$ represents the piling force in units of t; $y$ represents the maximum value stress and strain on the component in units of MPa.

According to Eq. (5), the maximum piling force the high-speed press bear can be obtained under the permissible stress, and then the maximum strain when the high-speed press are under the maximum piling force condition can be calculated. Namely, Eq. (5) can be used to calculate the maximum piling force and the maximum strain which are useful in the design of high-speed press.

Fig. 11Maximum value of components stress under different piling forces

Fig. 12Maximum value of strain of components under different piling forces

5.2. Results comparison

The aim of the multibody dynamic simulation is to get the accurate load data, therefore how well the simulation model fits the real condition is of great importance. Some experimental data were measured during the high-speed press operation, and then this section presents the contrast between the measured data and the simulation data to check the accuracy of the simulation model.

5.2.1. Comparison of the piling force

The piling force of the simulation model is generated due to the collision of the model. It can be observed that the simulation piling force has a high similarity to the measured piling force by the comparison between Fig. 3 and Fig. 13 in 75 t-300 r/min working conditions. The forces of two tonnage meters may be different in the field test, but they are the same in simulation. Comparison of the piling force between simulation and measured data obtained under all working conditions is conducted, and it shows that they match well.

Fig. 13Piling force of tonnage meter in 75 t-300 r/min condition (simulation)

5.2.2. Comparison of the slider acceleration

The experimental curve of Fig. 4 is compared with the simulation curve of Fig. 14 in the 75 t-300 r/min working condition. The real frequency of piling is not very precise when experimenting, and there is a deviation between the set speed and the real speed. To compare and observe two curves in detail, the frequency of the curve in Fig. 4 is slightly adjusted to match the frequency of the simulation and keep the phasees of two curves the same. The two curves are shown in Fig. 15.

The two curves have the same trend in one cycle. From the cure, two peaks occur when the slider makes contact with the tonnage meter, and they exit at the time the slider contacts the tonnage meter and leaves from the tonnage meter respectively. The first peak is relatively bigger and the second peak is smaller. The experimental curve and simulation curve of the slider acceleration are approximately the same for both the trend and the peak extreme in all other working conditions. This indicates that the simulation model can fit the piling dynamic features of the slider accurately. The rest comparison diagrams are not listed one by one.

There are several peaks in the experimental curve and simulation curve in one cycle, which obviously embody the “viscous effect.” The explanation is that the tonnage meter is not fully fixed on the base, but placed on the base by its gravity in the experiment. After the slider generates a huge impact force on the tonnage meter in the case of piling, the slider would leave from the tonnage meter, and then the tonnage meter bounces up and collides with the slider, so the change of acceleration leads to the viscous effect.

Fig. 14Slider acceleration in the 75 t-300 r/min condition (simulation)

Fig. 15Comparison of slider acceleration between experiment and simulation in the 75 t-300 r/min

5.3. Life prediction

MSC. Fatigue is a function module, which is used to analyze the material fatigue in Patran software. It can read the load file of the flexible body established by ADAMS and predict the fatigue life of the flexible body. The fatigue life of the connecting rod and crankshaft can be predicted by the S-N (stress or strain life) curve method in MSC. Fatigue. The following part analyzes the data in the extreme working conditions, over-limit working conditions and real working conditions.

The material of connecting rod and crankshaft is 42CrMo, its elastic modulus $E$ is 212 GPa, the Poisson ratio $\mu$ is 0.28, and the density is 7.85×10^-6 kg/mm³. The connecting rod and crankshaft are divided into meshes in Patran software. There are 14834 units, 23492 nodes and 29501 units and 44917 nodes, respectively.

5.3.1. Extreme working condition

Fig. 16 shows that the weakest point of the connecting rod is located at the number 6495 node on the column‘s inner surface. The life of the number 6495 node is 10^7.59 cycles. Based on the parameter setting in Section 3, one simulation cycle requires 3 s, then the life is 32420.5 hours, in other words, 45 months. Besides the fact that the fatigue life of the points on the column‘s inner surface with unequal forces is shorter, the circulation life in the narrow area of the column above the connecting rod hinge is less than 10^8.42 cycles, namely 219189 hours, about 304.5 months. When the press is operating in real time, wearing and pitting risks exist in the contact inner surface of the column between the connecting rod and bearings in several unequal surface areas during a short period of time (4 years). The narrow area above the ball head connector is easy to crack after a longer period of time (25 years).

Fig. 17 shows that the minimal life point of the crankshaft is number 39023 node. Its life is 10^9.04, namely 913732 hours, or 1269 months. The crankshaft bears force uniformly, and its fatigue life is longer than that of the connecting rod, therefore it is safe in the operation of the press.

Fig. 16Life prediction cloud diagram of connecting rod under 125 t-350 r/min

Fig. 17Life prediction cloud diagram of crankshaft under 125 t-350 r/min

5.3.2. Over-limit working conditions

The load of simulation of the connecting rod and crankshaft are exported into MSC. Fatigue, then calculate the fatigue. New life cloud diagrams are shown in Fig. 18 and Fig. 19.

Fig. 18Life prediction cloud diagram of connecting rod under 250 t-350 r/min

Fig. 19Life prediction cloud diagram of crankshaft under 250 t-350 r/min

Fig. 18 and Fig. 19 show that the life of two components are almost similar in distribution but change only in their values. The position of the shortest life point of the connecting rod does not change, but its life reduces to 10^6.19 under over-limit working conditions, namely 1291 hours, or 1.8 months. The area above the ball hinge, which has a life span of less than 10^8.42, expands greatly. The shortest life point of the crankshaft is number 12041 node changing from the left to the right. The life span reduces to 10^7.09, namely 10252 hours, or 14 months.

All of this indicates that the load form of the high-speed press does not change. When the load increases, the life distribution of the parts continues to be roughly stable but life of almost all areas reduce. The position of the shortest life point may change.

5.3.3. True working conditions

Import the load data gained from the simulation method into MSC. Fatigue, and then calculate the fatigue life of the connecting rod and the bent axle. The new life cloud diagrams are shown in Fig. 20 and Fig. 21.

Fig. 20 and Fig. 21 show that the shortest life of the connecting rod is 10^12.4 cycle time (3 s for one cycle) when the high-speed presses are under the actual operation, namely 2.09×10⁹ hours, or about 2.9×10⁶ years. The shortest life of the crankshaft is 10^14.1, namely 1.05×10¹¹ hours, or about 1.2×10⁷ years. The life of the two components are very close, so the risks of fatigue failure of the two componentsis are very low.

Fig. 20Life prediction cloud diagram of connecting rod under 25 t-200 r/min

Fig. 21Life prediction cloud diagram of crankshaft under 25 t-200 r/min

As we know that the virtual prototyping simulation model of the high-speed press can conduct the multibody dynamic simulation and get the load data of the key structural components. Then, the fatigue life of the high-speed press’ key structural components can be predicted under the load spectrum obtained by the corrected virtual prototyping simulation model and then the high-speed press can be designed optimally. Furthermore, the load spectrum of the key structural components can be a reference for the reliability design of the key structural components of the high-speed press in similar size.