## 4 Case Study
### Processing Performance Analysis  
The forging frequency was set to 90 times per minute and 20 times per minute, respectively. The nominal pressure was set to 35 MPa. The relationships between TPU pressure and prefill pressure are shown in Figures 6 and 7. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Process Type** & **Forging Frequency** & **Load Pressure** & **Main Pump Unit Power** & **Displacement** \\ \hline Regular forging & 20 times per minute & 35 MPa & 810 kW & 450 mm \\ Fast forging & 90 times per minute & 35 MPa & 810 kW & 200 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of the upsetting process. Figure 5: Photograph of the energy-saving buffering system. As it can be seen in Figures 6 and 7, the hydraulic press ran smoothly (without vibration) in the no-load and low-load stages. The TPU was used to supply energy to maintain the normal operation of the press, while the servo valve outside the liquid-providing tank was closed so that the output pressure and flow of the liquid-filling system was approximately zero. When the hammer of the hydraulic press contacted the forging workpiece, the load power rose and exceeded the nominal power supplied by the TPU; the servo valve outside the PFS opened with high accuracy, and the output power and flow of the PFS were used in conjunction with those of the TPU as the load pressure was reaching its maximum pressure. In the fast forging process, the servo valve remained open in order to ensure timely rehydration, since the timespan of one fast forging cycle was extremely short (see Figure 6). When the press operated at the unloading stage, the traditional hydraulic system generated an impact pressure of up to 20 MPa in 0.04 s, causing a great impact on hydraulic pipes and beams. Many oil pipe ruptures and beam bending deformations are caused by this phenomenon, seriously affecting machining accuracy and the service life of the hydraulic press. Figures 8 and 9 show that the pressure-relief impact was well improved by integrating a buffer system. Since the PFS and the buffer system were installed close to the hydraulic cylinder, the high-pressure oil generated at the unloading stage directly entered the liquid-filling tank and the buffer tank without traveling a long distance along the hydraulic pipelines. Therefore, hydraulic impact and oil leakage could be reduced to a large extent, so as to improve the hydraulic working environment and reduce environmental  
Figure 6: Output pressure of the prefill system and the main power unit during fast forging. Figure 7: Output pressure of the prefill system and the main power unit during constant forging. pollution. According to experimental statistics, regardless of the forging method, adding the proposed system to a hydraulic press can reduce hydraulic impact by at least 9.2 MPa. Through several test experiments, the experimental data from Figures 6-9 are summarized in Tables 2 and 3, in which Tables 2 and 3 are the results of the experiments at the PS and unloading stage for different forging methods, respectively. When the forging hydraulic press operates at the low-load or no-load stage, the energy supplied by the TPU is sufficient for the hydraulic press to operate normally, and the PFS does not work. As seen from Figures 6 and 7 and Table 3, when the hydraulic press operates at Stage 3, the load power rises and exceeds the maximum power the TPU could output, the servo valve outside the PFS opens, and then the PFS outputs high-pressure oil and energizes the main working cylinder together with the TPU; the maximum output pressure and flow rate of the TPU are 28.1 MPa and 2900 L/min, respectively, while the load pressure and flow rate of the press are 35 MPa and 4400 L/min. The difference between the load pressure and the output pressure of the TPU is the energy provided by the PFS, which are 5.4 MPa and 1480 L/min, respectively.