# A Novel Energy-Efficient Transmission System and Control Strategy for Hydraulic Machines

Authors: Xiaopeng Yan; Songlin Nie; Hui Ji; Zhonghai Ma; Baijin Chen
Journal: International Journal of Energy Research
 (2023)

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## Abstract

Each part of a hydraulic press dissipates a large amount of energy when energy or power is transmitted. Therefore, this study proposes a 3-D vertical arrangement structure for the hydraulic press to reduce their energy dissipation. In the proposed structure, the aboveground and underground spaces are comprehensively utilized, and then the hydraulic equipment are arranged three-dimensionally and set in layers according to the functional requirements. Some equipment is connected directly to minimize the use of valves and pipelines and reduce the transmission distance of hydraulic energy between the pump station and the actuator. Furthermore, a method for scheduling the drive system in the above structure is presented to share a transmission zone with no conflict and shorten their idle time. The composition of each zone is set to match the power demand of each operation to achieve the scheduling schemes. Finally, the proposed scheme is applied to a 31.5 MN hydraulic press as a case study. Results showed that the energy efficiency increased from 43.95% to 85.03%, demonstrating excellent energy-saving potential.


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## Body

## 1. Introduction

Hydraulic systems, as the core of hydraulic presses (HPs), have been widely recognized for their smooth transmission, ease of control, and small size [1]. However, they are famous for their high installed power and poor efficiency rate as well [2], which not only increases the investment costs and installed capacity but also leads to excessive fluid temperatures and reduced operating stability [3]. In the context of rising energy demands across the globe and the depletion of the Earth’s natural resource, research on energy-conversion methods for HPs could effectually accelerate the technological upgrading of hydraulic equipment, which is crucial for energy-saving and emission reduction targets in various industries in the national economy [4, 5].As illustrated in Figure1, when a hydraulic drive system transmits hydraulic energy to the main working cylinder, each part of the HP generates energy dissipation, and only 7.9% of the input power is transformed and converted into the useful work [6]. For better studying the energy-conversion schemes of HPs, the primary cause for low energy efficiency should be investigated first. For this reason, Huang [7] studied the energy transfer law of HPs and illustrated the power mismatch is a primary reason for serious energy dissipation. As presented in Figure 2, the installed power of a press is designed to satisfy the max load power requirements during PS stage [8]. However, as the same motor pumps also serve other low-load stages, a power mismatch can occur [9]. Furthermore, the HP process is periodic, and the waiting time between two adjacent cycles is almost equal to the time of one operating cycle. Once the HP is in operation, the motors in the drive system will not switch off even if all of the load power is zero and the hydraulic pumps are unloaded, resulting in substantial energy dissipation [10].Figure 1
Analysis of energy dissipation of each energy unit in the forging HP.Figure 2
Distribution of motor power and load power of one press during an operation cycle.In the past several decades, researchers have made considerable efforts to circumvent the mismatch between the installed power and the demand power, thus improving the energy efficiency of HPs. As reported by Lin et al. [11] and Li et al. [12], these matching methods can be classified into the following categories: energy matching and energy recovery.In view of the power mismatch, researchers have made unremitting efforts in the past few decades to circumvent the mismatch between the installed power and the load power [13]. As shown in Figure 3 [14], common energy-matching methods are as follows: adjusting the motor speed to change the output flow; changing the pump displacement to change the output flow; and adjusting the motor speed and the pump displacement simultaneously [15]. In particular, Darko applied a variable-speed motor and a constant-displacement pump in the load sensing system, and then precise control of the system flow was achieved. Compared with the traditional pump control system, high energy efficiency and low energy dissipation were attained [16]. Xu et al. incorporated a variable-frequency control scheme in hydraulic elevators, and the low efficiency of traditional elevators was significantly improved, which provides a reference for increasing the efficiency of other hydraulic machinery [17]. Quan and Helduser proposed a novel variable-displacement pump-controlled single-rod cylinder hydraulic circuit, which was found to not only reduce the energy dissipation of the system but also improve the internal stability of traditional operating circuits [18]. Moreover, Wang et al. [19], Gao et al. [20], and Chaing et al. [21] all studied the control strategies for variable-displacement pumps based on real-time loads and achieved energy efficiency improvement in some scenarios. The above method achieves flow matching by adjusting the state of a single unit but does not consider the impact of some state changes on other units or the overall efficiency. Therefore, Huang et al. [22] and Ge et al. [23] studied the dual variable system, and the match between the output flow and the demanded flow was achieved by adjusting the motor speed and the displacement of the variable-displacement hydraulic pump simultaneously. Based on this system, Johannes proposed an optimal control scheme by which to minimize the energy loss and optimize the operating performance of the hydraulic drive circuit [24].Figure 3
Schematic illustration of the conventional flow matching method.The above pressure and flow matching method, presented in Figure3, can improve the power mismatch in several ways [25]. However, because of the working characteristics of HPs, the demanded power of most operations is much less than the installed power, and motors in the hydraulic drive system idle during most of the process time. This will result in severe idling loss as illustrated in Figure 2, in which the red and blue parts represent useful work and useless work, respectively. Under current technological limitations, those energy dissipations in a hydraulic system are unavoidable [26]. Thus, energy recovery becomes the final means of reducing the installed power and improving energy efficiency [27]. As illustrated in Figure 4, accumulators, flywheels, capacitors, and power grids are common energy-recovery devices. By coordinating the action beat of HPs and the operating characteristics of the energy storage device, the potential energy of the slider and the mechanical energy of the motors are stored in the low-load stage and released during the high-load stage. Lin and Wang [28] utilized hydraulic and electric accumulators and presented a compound energy-recovery system to reduce the energy dissipation of forging hydraulic equipment. The results showed that the compound system increased the energy efficiency by approximately 39%. Dai et al. [29] applied a recovery accumulator to a 16 MN fast forging hydraulic press to reduce the installed power by the absorption of large flow and pressure pulses. Results show that the accumulator has promising energy-saving effects. Waldemar and Jerzy [30] proposed a new type of accumulator energy storage system by combining the features of a mechanical kinetic energy-recovery system (KERS) and conventional hydraulic accumulator. Herein, both potential and kinetic energies are stored at high energy densities. Yan et al. [31] applied the flywheel energy storage system into the hydraulic press, which can store mechanical energy during the low-load and no-load stage and release the stored energy in the high-load stage, thus reducing the installed power and circumventing the power mismatch. Xia et al. [32] proposed an integrated drive and potential energy-recovery system for a hydraulic excavator, which can store the potential energy generated when hydraulic cylinders move back and forth by a three-chamber hydraulic cylinder. Besides, power girds [32] and capacitance recovery system are all employed to realize the potential energy recovery. However, as reported by Li et al. [15], the low use efficiency of the recovered energy is a Gordian knot that should not be ignored. The energy-recovery cycle consists of two subprocesses: recovering operation and reusing operation. It was worth mentioning that the quantity of power transformations increased with an addition of these recovery systems, raising the HP structure complexity and resulting a lower utilization rate [33].Figure 4
Energy-recovery scheme for hydraulic system.In summary, energy efficiency and power mismatches could be significantly improved using the aforementioned energy-matching and energy-recovery methods. However, most of the abovementioned methods only consider the energy efficiency, and the impact of these methods on the press itself is neglected. Therefore, most energy-efficient presses present complex and impractical structures. Furthermore, existing methods only focus on the hydraulic power units and control systems, while neglecting the fact that a hydraulic press is a closed energy-conversion system composed of various energy units. Each of these units may result in significant energy dissipation during the forming process; thus, addressing this issue could enable considerable energy savings.As presented in Figure1, the hydraulic-hydraulic energy unit is the unit that suffers from the highest energy loss during energy conversion and transmission. Traditionally, hydraulic energy is generated at pump stations and transmitted to hydraulic actuators through several hundred meters of transmission pipelines and dozens of control-valve groups. Friction, sudden changes in pipe diameter, flow interception, and other factors can cause severe hydraulic energy losses during transmission. If the transmission distance of hydraulic energy and the use of valve groups can be reduced, the energy efficiency of the system will be improved significantly.To handle this problem, this paper studied the mechanism of energy loss in the H-H unit and proposed a 3-D vertical arrangement structure for the hydraulic press. In the proposed structure, the aboveground and underground spaces are comprehensively utilized, and then the hydraulic equipment are arranged three-dimensionally and set in layers according to the functional requirements, thus reducing the transmission distance of hydraulic energy between the pump station and the actuator as well as the number of control valves. Additionally, a configuration and energy supply strategy for the development of the above system is introduced so as to achieve an orderly and efficient energy supplement, which indicates significant economic and energy-saving potential. Figure5 is the overall research flowchart for the proposed method.Figure 5
The overall research flowchart for the proposed method.


## 2. Method

### 2.1. Energy Dissipation Analysis of PVS

In the traditional HP (THP) transmission system, the installed capacity of a press is high, the number of components of an HP is large, and the mass of each component is also large. The arrangement method for the equipment generally adopts a plane layout. As shown in Figure6, the HP components cover a wide area, the hydraulic pipelines used to connect the HP components are long, and a circuitous phenomenon in the PVS is common. Due to the flow resistance in the transfer process of hydraulic energy, energy dissipation will inevitably occur, and the system efficiency will be reduced. Moreover, due to the unreasonable arrangement of the THP, the positions of each piece of equipment are disordered, which also causes great inconvenience for the maintenance of the equipment.Figure 6
Plane structure for a traditional forging HP.If the THP structure shown in Figure6 can be changed and redesigned, the HP structure can be rearranged in the vertical direction. Additionally, by optimizing the distribution characteristics of the HP components and reducing the length of the hydraulic transmission pipelines and the number of hydraulic valves, a short energy transmission distance between the motor pumps and hydraulic actuators and components with low energy consumption can be achieved, thereby theoretically achieving less energy dissipation and higher transmission efficiency, which is also verified in Section 3.


### 2.2. Structure Optimization Scheme

Therefore, this study proposes a novel 3-D layout strategy for the THP, which arranges the HP components three-dimensionally and sets them up in layers based on their functional requirements. In this new structure, the installation of an HP is divided into two layers: the aboveground layer and the underground layer. Additionally, electronic control circuit, power station, power transmission system, host device, and other systems of the HP are connected to each other in space and arranged in layers.To arrange these units along the vertical direction, the installation space is also subdivided into four floors, which are numbered as follows: the negative first floor, the first floor, the second floor, and the third floor. The underground layer is the negative first floor, and the ground layer is set as the first, second, and third floors according to a certain height from the ground up, respectively. Meanwhile, each layer is offered with left and right rooms for setting up various pieces of HP equipment, and the negative first floor, first floor, second floor, and third floor left chambers are connected up and down.Figure7 shows the layout scheme of the 3-D arrangement structure for the HP, in which 1 is the host device, 2 represents the electronic control system, 3 denotes the recovery accumulator, 4 is the unloading and recovery system, 5 denotes the auxiliary circuit, 6 denotes the cooling system, 7 denotes the supercharging system, 8 denotes the electric transformer, 9 denotes the current-control cabinet, 10 represents the motor pumps, 11 denotes the monitor cabinet, 12 denotes the fuel tank, a is the negative first floor, b is the first floor, c is the second floor, and d is the third floor. The host device is installed on the left parts of the foundation of floor a, and penetrates through the left chamber of negative first floor a, first floor b, second floor c, and third floor d, simultaneously. The motor-pump station is placed in the right chamber of the basement, that is, the right chamber of the negative first floor. The electric transformer 8 and the current-control cabinet 9 are installed on the right chamber of floor b. The power conversion and supercharging system 7 are installed on the right chamber of the second layer, and the auxiliary equipment and cool system can be installed in the right chamber of the third layer. The aforementioned units are jointed by oil pipelines, while the hydraulic valves are arranged to regulate the direction of the flow.Figure 7
Schematic diagram of the 3-D arrangement structure for the HP.To prevent serious heating due to the long-term operation of the motor pumps, cooling ventilation holes are provided at the top of the right chamber in the layer a. Additionally, a translucent partition wall is installed to separate the left and right chambers of floorb, and the control device for the HP is installed between the host device and the power transformer. The sides of the left and right chambers on the first, second, and third floors are set to be transparent, and the edge of the second and third layers have safety guardrails, as shown in Figure 8.Figure 8
3-D model of the proposed 3-D structure for the HP.Furthermore, when the HP operates at UL stages, working cylinders will instantaneously generate pressure fluctuations as high as tens of MPa, which can have a huge impact on the PVS and the proposed structure. Therefore, to ensure the stability of the vertical structure and to avoid vibrations, a servo proportional valve and an energy-recovery system combined with an unloading system are installed in the three layers of the right chamber; therefore, the pressure generated by the master cylinder can be smoothly unloaded to the energy-recovery device, which can be utilized later.The 3-D vertical structure sorts the complex HP components into categories and then arranges these components organically, which shortens the length of the hydraulic transmission pipeline and solves the problem of substantial energy dissipation caused by the PVS. Based on this structure, the shortest hydraulic transmission pipeline and the best transmission efficiency and responding speed for the HP are all achieved.In terms of cost, the proposed structure has three main advantages over traditional methods. First, because the conventional planar-arrangement structure is based on underground and aboveground space, an excavator is firstly required to clear the underground space during the early equipment-construction stage and then to build different functional rooms for different auxiliary devices. In fact, constructing an underground space with the same area often costs more money than aboveground spaces. Second, the traditional arrangement requires more floor space. The footprint of the press with the same tonnage, based on the traditional arrangement, is approximately three times that of the 3-D arrangement. Therefore, the structure proposed herein can afford considerable savings in terms of installation space. Finally, the hydraulic equipment in the proposed vertical arrangement are directly connected to each other through pipelines, in an attempt to replace traditional valve groups. Consequently, initial equipment investment can be significantly reduced.


### 2.3. Energy Supply Scheme

As presented in Figure2, an operating cycle consists of multiple forming actions, and each action corresponds to a different forming power. Therefore, this section discusses the optimization of the energy supply scheme of the hydraulic system according to the HP components configured under the 3-D structure. This scheme divides the drive system into different zones. Each zone only supplies energy for actions with similar power, thereby reducing the mismatch between the installed power and the load power and further improving the energy efficiency of the HP.As shown in Figure9, the drive system is majorly composed of the following zones: zone 1 (motor pumps (MPU)), zone 2 (supercharging system (SS)), zone 3 (recovery accumulator system (RAS)), and zone 4 (potential energy-recovery and oil-filling system (PROFS)).Figure 9
Energy scheduling scheme for the hydraulic press based on the proposed structure.In the WT and FF stages, contrary to the THP, the motors in the zone 1 are no longer idle but drive the pumps to suck oil from the oil tank, which can be used to fill the recovery accumulator in the zone 3 set on the second floor of the 3-D structure. Then, the stored energy is released to power the supercharger in the zone 2 and hydraulic actuator together with the motor pumps (zone 1) in the PS and FR stages, that is, the effective utilization of the no-load energy, thereby reducing the idling loss of a HP.When the HP operates at the PS stage or the loading power exceeds the installed power of the hydraulic press, the supercharger in the zone 2 starts to work. The oil output from zone 1 and 2 no longer directly drives the main engine to work but is supplied to the main working cylinder after being boosted by the supercharger. At this time, the hydraulic press with the ideal installed power can satisfy the high power demand in the high-load stage, and the energy efficiency problem caused by the power mismatch can be effectively solved.Furthermore, when the HP operates at the UL stage, substantial hydraulic energy and elastic potential energy are stored in the hydraulic cylinder, frame, and pipeline. Widely used HPs do not recover and reuse the above energy but directly release the above energy through an unloading valve, leading to a large amount of energy loss. Therefore, to further improve the energy efficiency, the proportional unloading system and the energy-recovery system (zone 4) in the third floor are combined to restore the potential energy generated at UL stage. Then, the stored energy is released to power the press in FF stage of the next cycle and drives the moving beam to fall down.


## 2.1. Energy Dissipation Analysis of PVS

In the traditional HP (THP) transmission system, the installed capacity of a press is high, the number of components of an HP is large, and the mass of each component is also large. The arrangement method for the equipment generally adopts a plane layout. As shown in Figure6, the HP components cover a wide area, the hydraulic pipelines used to connect the HP components are long, and a circuitous phenomenon in the PVS is common. Due to the flow resistance in the transfer process of hydraulic energy, energy dissipation will inevitably occur, and the system efficiency will be reduced. Moreover, due to the unreasonable arrangement of the THP, the positions of each piece of equipment are disordered, which also causes great inconvenience for the maintenance of the equipment.Figure 6
Plane structure for a traditional forging HP.If the THP structure shown in Figure6 can be changed and redesigned, the HP structure can be rearranged in the vertical direction. Additionally, by optimizing the distribution characteristics of the HP components and reducing the length of the hydraulic transmission pipelines and the number of hydraulic valves, a short energy transmission distance between the motor pumps and hydraulic actuators and components with low energy consumption can be achieved, thereby theoretically achieving less energy dissipation and higher transmission efficiency, which is also verified in Section 3.


## 2.2. Structure Optimization Scheme

Therefore, this study proposes a novel 3-D layout strategy for the THP, which arranges the HP components three-dimensionally and sets them up in layers based on their functional requirements. In this new structure, the installation of an HP is divided into two layers: the aboveground layer and the underground layer. Additionally, electronic control circuit, power station, power transmission system, host device, and other systems of the HP are connected to each other in space and arranged in layers.To arrange these units along the vertical direction, the installation space is also subdivided into four floors, which are numbered as follows: the negative first floor, the first floor, the second floor, and the third floor. The underground layer is the negative first floor, and the ground layer is set as the first, second, and third floors according to a certain height from the ground up, respectively. Meanwhile, each layer is offered with left and right rooms for setting up various pieces of HP equipment, and the negative first floor, first floor, second floor, and third floor left chambers are connected up and down.Figure7 shows the layout scheme of the 3-D arrangement structure for the HP, in which 1 is the host device, 2 represents the electronic control system, 3 denotes the recovery accumulator, 4 is the unloading and recovery system, 5 denotes the auxiliary circuit, 6 denotes the cooling system, 7 denotes the supercharging system, 8 denotes the electric transformer, 9 denotes the current-control cabinet, 10 represents the motor pumps, 11 denotes the monitor cabinet, 12 denotes the fuel tank, a is the negative first floor, b is the first floor, c is the second floor, and d is the third floor. The host device is installed on the left parts of the foundation of floor a, and penetrates through the left chamber of negative first floor a, first floor b, second floor c, and third floor d, simultaneously. The motor-pump station is placed in the right chamber of the basement, that is, the right chamber of the negative first floor. The electric transformer 8 and the current-control cabinet 9 are installed on the right chamber of floor b. The power conversion and supercharging system 7 are installed on the right chamber of the second layer, and the auxiliary equipment and cool system can be installed in the right chamber of the third layer. The aforementioned units are jointed by oil pipelines, while the hydraulic valves are arranged to regulate the direction of the flow.Figure 7
Schematic diagram of the 3-D arrangement structure for the HP.To prevent serious heating due to the long-term operation of the motor pumps, cooling ventilation holes are provided at the top of the right chamber in the layer a. Additionally, a translucent partition wall is installed to separate the left and right chambers of floorb, and the control device for the HP is installed between the host device and the power transformer. The sides of the left and right chambers on the first, second, and third floors are set to be transparent, and the edge of the second and third layers have safety guardrails, as shown in Figure 8.Figure 8
3-D model of the proposed 3-D structure for the HP.Furthermore, when the HP operates at UL stages, working cylinders will instantaneously generate pressure fluctuations as high as tens of MPa, which can have a huge impact on the PVS and the proposed structure. Therefore, to ensure the stability of the vertical structure and to avoid vibrations, a servo proportional valve and an energy-recovery system combined with an unloading system are installed in the three layers of the right chamber; therefore, the pressure generated by the master cylinder can be smoothly unloaded to the energy-recovery device, which can be utilized later.The 3-D vertical structure sorts the complex HP components into categories and then arranges these components organically, which shortens the length of the hydraulic transmission pipeline and solves the problem of substantial energy dissipation caused by the PVS. Based on this structure, the shortest hydraulic transmission pipeline and the best transmission efficiency and responding speed for the HP are all achieved.In terms of cost, the proposed structure has three main advantages over traditional methods. First, because the conventional planar-arrangement structure is based on underground and aboveground space, an excavator is firstly required to clear the underground space during the early equipment-construction stage and then to build different functional rooms for different auxiliary devices. In fact, constructing an underground space with the same area often costs more money than aboveground spaces. Second, the traditional arrangement requires more floor space. The footprint of the press with the same tonnage, based on the traditional arrangement, is approximately three times that of the 3-D arrangement. Therefore, the structure proposed herein can afford considerable savings in terms of installation space. Finally, the hydraulic equipment in the proposed vertical arrangement are directly connected to each other through pipelines, in an attempt to replace traditional valve groups. Consequently, initial equipment investment can be significantly reduced.


## 2.3. Energy Supply Scheme

As presented in Figure2, an operating cycle consists of multiple forming actions, and each action corresponds to a different forming power. Therefore, this section discusses the optimization of the energy supply scheme of the hydraulic system according to the HP components configured under the 3-D structure. This scheme divides the drive system into different zones. Each zone only supplies energy for actions with similar power, thereby reducing the mismatch between the installed power and the load power and further improving the energy efficiency of the HP.As shown in Figure9, the drive system is majorly composed of the following zones: zone 1 (motor pumps (MPU)), zone 2 (supercharging system (SS)), zone 3 (recovery accumulator system (RAS)), and zone 4 (potential energy-recovery and oil-filling system (PROFS)).Figure 9
Energy scheduling scheme for the hydraulic press based on the proposed structure.In the WT and FF stages, contrary to the THP, the motors in the zone 1 are no longer idle but drive the pumps to suck oil from the oil tank, which can be used to fill the recovery accumulator in the zone 3 set on the second floor of the 3-D structure. Then, the stored energy is released to power the supercharger in the zone 2 and hydraulic actuator together with the motor pumps (zone 1) in the PS and FR stages, that is, the effective utilization of the no-load energy, thereby reducing the idling loss of a HP.When the HP operates at the PS stage or the loading power exceeds the installed power of the hydraulic press, the supercharger in the zone 2 starts to work. The oil output from zone 1 and 2 no longer directly drives the main engine to work but is supplied to the main working cylinder after being boosted by the supercharger. At this time, the hydraulic press with the ideal installed power can satisfy the high power demand in the high-load stage, and the energy efficiency problem caused by the power mismatch can be effectively solved.Furthermore, when the HP operates at the UL stage, substantial hydraulic energy and elastic potential energy are stored in the hydraulic cylinder, frame, and pipeline. Widely used HPs do not recover and reuse the above energy but directly release the above energy through an unloading valve, leading to a large amount of energy loss. Therefore, to further improve the energy efficiency, the proportional unloading system and the energy-recovery system (zone 4) in the third floor are combined to restore the potential energy generated at UL stage. Then, the stored energy is released to power the press in FF stage of the next cycle and drives the moving beam to fall down.


## 3. Energy Efficiency Analysis of the 3-D Structure

### 3.1. Energy Dissipation Model of the 3-D Structure

When fluid runs through the PVS, it is subject to friction resistance opposite to the flow direction and is expressed in the form of a pressure drop, resulting in energy dissipation. The resulting power dissipation (PLoss) can be denoted as:
(1)PLoss=∆p×q,where ∆p is the pressure loss generated when passing through the PVS and q is the flow rate passing through the PVS.The pressure losses are classified into local dissipation and pressure dissipation along the path. The local dissipation usually occurs at a place with a sudden change in diameter, i.e., elbows and valves, and can be expressed as:(2)∆pvalve=ρ21/2m×qλAValve1/m,where λ is the local resistance coefficient of the valve, ρ represents the oil density, and AValve and m are the flow area and the throttling coefficient of the valve, respectively.The pressure dissipation generated by the pipeline circuits (∆pPipeline) is part of the pressure loss along the way, which can be expressed as:
(3)∆pPipeline=∑δ×Ld×ρq22Apipeline2,where δ is the resistance coefficient along the hydraulic pipeline, L is the length of the hydraulic pipeline, d is the diameter of the hydraulic pipeline, and Apipeline is the flow area of each pipeline.The damping index in the PVS is constant when the hydraulic system is determined, and the flow rate of the hydraulic oil flowing through the PVS is proportional to the operating speed of the HP actuator; thus, the total power loss generated by the PVS can be expressed as:(4)PLoss=∆p×q=∑M=0iεI×ḣ3+∑N=0lζL×ḣ3,where εI=ρAValve3/2×1/δAValve2, ζL=ρApipeline3/2×L/d×λ/Apipeline2. M and N represent the quantity of the local dissipation and route dissipation components, respectively. ḣ is the descending speed of the movable beam.The PVS discussed above only participates in the transmission of hydraulic power when the hydraulic cylinder moves but does not operate in other stages. Thus, the energy dissipation generated by the PVS (ELoss) during the operating stages can be expressed as:
(5)ELoss=∑i=FF+PS∫∑m=0iεI×ḣ3+∑L=0lζL×ḣ3dt.As presented in Equation (5), the energy dissipation generated by the PVS depends on the PVS parameters themselves, as well as the operating speed of each hydraulic cylinder. The fewer the pipeline–valves and the smoother the pipeline, the smaller the amount of energy dissipation during transmission, and vice versa.Moreover, considering the energy loss of motor pumps, the energy dissipation generated when the electric is converted into hydraulic energy (ELoss1) is obtained as:
(6)ELoss1=∑i∫T0T0+T1p×q1−ηDriveηDrivedt,where T0 represents the starting time of the FF stage, T1 represents the ending time of the PS stage, and ηDrive=0.92×ηv×ηm is the conversion efficiency of the motor pumps, where ηv and ηm are the volumetric and mechanical efficiencies, respectively, which could be obtained by:
(7)ηv=1−1β×60×∆pμ×n,(8)ηm=11+Cr×1/β×μ×n/60×∆p+Cf×2πTc/∆p+1,where β is the elastic modulus of fluid volume, n is the motor-pump speed, μ is the viscous damping coefficient, Tc denotes the motor-pump torque, and Cr and Cf represents the motor rotational efficiency and frictional coefficient, respectively.Combined with the above analysis, the electric energy demanded by an HP to complete the FF and PS operations can be obtained by:(9)Eelectric1=∑i∫T0T0+T1p×q1ηDrivedt.In addition to the electrical energy used during the above two stages, the electric energy consumed by the pressure relief and unloading circuits during the other low-load stages can be denoted by:(10)Eelectric2=∑e∫TmP1dt+∫TnP2dt+∫TiP3dt,where P1, P2, and P3 are the motor’s instantaneous powers at UL, FR, and WT, respectively, and Tm, Tn, and Ti are the durations of these stages, respectively.Since the motor-pump configuration for a THP is consistent with the HP with the proposed 3-D structure, the power consumption in one operating cycle (Eelectric−Total) can be obtained as:
(11)Eelectric−Total=Eelectric1+Eelectric2+ELoss.The hydraulic cylinder outputs forming energy until the end of an operating cycle, so the useful energy in one operating cycle (EForming) is:
(12)EForming=∑i∫iFa×ḣdtwhere Fa represents the force acting on the slider. In summary, the energy efficiency of the HP (ηE−F) is:
(13)ηE−F=EFormingEelectric−Total.In summary, the useful energy consumed by the newly proposed HP to finish the same operation is the same as that of a THP, but an HP with a 3-D structure has a more efficient PVS, thereby leading to higher energy utilization and lower energy losses.


### 3.2. Components Statistics of 3-D Structure

In the first two sections, the feasibility of this new structure has been discussed in detail, and the energy-saving mechanism has been theoretically explained through a mathematical model. To further study the energy efficiency of an HP based on this structure, two rapid-forging HPs with similar tonnages and configurations, which are commonly used in free forging and other forming processes, were chosen as a research object for the efficiency analysis.The maximum forming forces of the two HPs were both 31.5 MN. One of these was built according to the traditional planar structure, and the other was built according to the 3-D vertical structure proposed in this study. Through the experimental measurements and statistical calculations of the structures of the two pieces of equipment, the energy saved by the proposed structure was obtained.The 31.5 MN rapid-forging HP with a 3-D vertical arrangement structure was developed by our research team; the novel HP is mainly made up of motor-pump stations, a main-cylinder circuit, an unloading system, an oil-filling circuit, a direction–speed–pressure control circuit, and an energy-recovery circuit. After component planning in the design process and determining the component statistics during assembly, the statistics of the energy consumption components in each circuit of the two presses were obtained and are shown in Table1.Table 1
Compositions statistics of each press.
HPCircuitDiameter×length (mm×m)ElbowPipe jointValve90°135°150°HP with 3-D structureMotor-pump circuit114×12; 127×10910686Main-cylinder circuit168×12; 194×12261078Unloading circuit133×18; 127×12; 114×10181087Oil-filling circuit168×17; 194×1219868Control circuit219×10; 310×11; 219×13; 168×14; 133×16181086Boosting and recovery circuit194×8; 133×506274THPMotor-pump circuit114×36; 127×28131481416Main-cylinder circuit168×24; 194×2958141410Unloading circuit168×25; 102×24; 127×25; 89×22410121612Oil-filling circuit168×28; 198×24412101211Control circuit219×15; 325×18; 219×15; 114×24; 168×12; 133×24010101610The motor pumps of the THP must pass through 373 m hydraulic pipelines and 59 control valves to input the hydraulic oil to the hydraulic actuator. The friction loss along the way and the local energy loss resulted in considerable energy loss during the transmission of hydraulic energy. However, as presented in Table1, compared with the THP, the press with the new structure was significantly optimized in terms of pipe length, number of pipe joints, and number of valve groups, and only needed 189 m pipelines and 39 control valves.


### 3.3. Theoretical Analysis of Energy Dissipation

The motor-pump circuit for the 31.5 MN HP with a 3-D structure adopted 12 plunger pumps (the motor power and speed were 110 kW and 1480 rpm, respectively) to provide power for the hydraulic system, which outputs 360 L fluid per minute. Moreover, HM46 hydraulic oil was employed to serve the two HPs.ξ and ζ denote the local and the route resistance index, which can be obtained by Equations (3) and (4).This section firstly takes the motor-pump circuit as a case study to analyze the pressure dissipation during an operating cycle, and then the total energy dissipation of the two selected HPs during a cycle is obtained. Taking one of the hydraulic pipelines with a diameter and length of 0.114 m and 12 m as an example, the pressure drop caused by fluid flowing through the hydraulic pipeline can be obtained by Equation (14) as 0.13 MPa.
(14)∆pPipeline=ζ×L/d×pq/2A.For a 90° elbow, the throttling index (m) is 0.5, and the resistance coefficient (ξ) is 0.14, Based on Equation (15), the pressure loss generated by the 90° elbow is 0.003 MPa. Likely, the pressure loss generated by a 135° elbow and a 150° elbow is 0.35 MPa and 0.39 MPa, respectively.
(15)∆pjoint=ξ×ρ/21/2m×q1/m.Although structures and functions of each valve are various while the energy dissipation remains the same, the pressure losses of one valve is obtained as 0.09 MPa according to the following equation.(16)∆pValve=ξ×ρ2×q/A.Therefore, the total pressure loss generated by the motor-pump stations during a forming cycle is 1.67 MPa according to the following equation.(17)∆p1=∑∆pPipeline+m×∆pValve+x×∆p150°+y×∆p135°+z×∆p90°,where m, x, y, and z are the quantity of valves, 150° elbows, 135° elbows, and 90° elbows in a circuit, respectively.Then, the pressure losses of the other circuits can be achieved, as shown in Table2. Combined with the constant-forging process, the processing pressure and the processing cycle are set at 31.5 MPa and 3 s, respectively. Additionally, the flow rates of the motor-pump circuit, the main-cylinder circuit, the oil-filling circuit, the booster and recovery circuit, the unloading circuit, and the control circuit are 4320 L/min, 6612 L/min, 6480 L/min, 750 L/min, 5613 L/min, and 5200 L/min, respectively. Then, according to the pressure loss presented in Table 2 and the above processing parameter, the power dissipation (PPVLoss) and the energy dissipation (EPVLoss) caused by the PVS in the selected HPs can be calculated based on Equations (18) and (19), and the results are shown in Table 3.
(18)PPVLoss=∑i=0n∆pi×qi,(19)EPVLoss=∫tstarttendPPVLossdt.Table 2
Pressure loss of each circuit in the two HPs.
CircuitsHP with 3-D structureTHPMotor-pump circuits1.67 MPa4.24 MPaMain-cylinder circuits1.61 MPa5.12 MPaOil-filling circuits1.52 MPa4.09 MPaUnloading circuits1.33 MPa4.71 MPaControl circuits1.41 MPa3.30 MPaBoosting and recovery circuit1.23 MPa0 MPaTable 3
Energy dissipations of the above circuit in two HPs during one cycle.
Circuits (time/s)HP with 3-D structureTHPMotor-pump circuit (0-0.9)57.38 kJ184.01 kJMain-cylinder circuit (0.9-1.5)78.69 kJ229.01 kJOil-filling circuit (1.5-2.0)59.42 kJ168.29 kJUnloading circuit (2.0-2.8)72.09 kJ204.47 kJControl circuit (2.8-3.00)33.23 kJ102.96 kJBoosting and recovery circuit23.71 kJ0 kJTotal energy dissipation324.52 kJ888.74 kJAs shown in Table3, under the same load conditions, compared with the THP, a press with a 3-D arrangement structure produces less energy dissipation during the hydraulic energy transmission process. During one operating cycle (3 s), the newly proposed HP produced a total energy loss of 324.52 kJ, which was 564.22 kJ lower than that of the THP, showing better energy transmission efficiency.


## 3.1. Energy Dissipation Model of the 3-D Structure

When fluid runs through the PVS, it is subject to friction resistance opposite to the flow direction and is expressed in the form of a pressure drop, resulting in energy dissipation. The resulting power dissipation (PLoss) can be denoted as:
(1)PLoss=∆p×q,where ∆p is the pressure loss generated when passing through the PVS and q is the flow rate passing through the PVS.The pressure losses are classified into local dissipation and pressure dissipation along the path. The local dissipation usually occurs at a place with a sudden change in diameter, i.e., elbows and valves, and can be expressed as:(2)∆pvalve=ρ21/2m×qλAValve1/m,where λ is the local resistance coefficient of the valve, ρ represents the oil density, and AValve and m are the flow area and the throttling coefficient of the valve, respectively.The pressure dissipation generated by the pipeline circuits (∆pPipeline) is part of the pressure loss along the way, which can be expressed as:
(3)∆pPipeline=∑δ×Ld×ρq22Apipeline2,where δ is the resistance coefficient along the hydraulic pipeline, L is the length of the hydraulic pipeline, d is the diameter of the hydraulic pipeline, and Apipeline is the flow area of each pipeline.The damping index in the PVS is constant when the hydraulic system is determined, and the flow rate of the hydraulic oil flowing through the PVS is proportional to the operating speed of the HP actuator; thus, the total power loss generated by the PVS can be expressed as:(4)PLoss=∆p×q=∑M=0iεI×ḣ3+∑N=0lζL×ḣ3,where εI=ρAValve3/2×1/δAValve2, ζL=ρApipeline3/2×L/d×λ/Apipeline2. M and N represent the quantity of the local dissipation and route dissipation components, respectively. ḣ is the descending speed of the movable beam.The PVS discussed above only participates in the transmission of hydraulic power when the hydraulic cylinder moves but does not operate in other stages. Thus, the energy dissipation generated by the PVS (ELoss) during the operating stages can be expressed as:
(5)ELoss=∑i=FF+PS∫∑m=0iεI×ḣ3+∑L=0lζL×ḣ3dt.As presented in Equation (5), the energy dissipation generated by the PVS depends on the PVS parameters themselves, as well as the operating speed of each hydraulic cylinder. The fewer the pipeline–valves and the smoother the pipeline, the smaller the amount of energy dissipation during transmission, and vice versa.Moreover, considering the energy loss of motor pumps, the energy dissipation generated when the electric is converted into hydraulic energy (ELoss1) is obtained as:
(6)ELoss1=∑i∫T0T0+T1p×q1−ηDriveηDrivedt,where T0 represents the starting time of the FF stage, T1 represents the ending time of the PS stage, and ηDrive=0.92×ηv×ηm is the conversion efficiency of the motor pumps, where ηv and ηm are the volumetric and mechanical efficiencies, respectively, which could be obtained by:
(7)ηv=1−1β×60×∆pμ×n,(8)ηm=11+Cr×1/β×μ×n/60×∆p+Cf×2πTc/∆p+1,where β is the elastic modulus of fluid volume, n is the motor-pump speed, μ is the viscous damping coefficient, Tc denotes the motor-pump torque, and Cr and Cf represents the motor rotational efficiency and frictional coefficient, respectively.Combined with the above analysis, the electric energy demanded by an HP to complete the FF and PS operations can be obtained by:(9)Eelectric1=∑i∫T0T0+T1p×q1ηDrivedt.In addition to the electrical energy used during the above two stages, the electric energy consumed by the pressure relief and unloading circuits during the other low-load stages can be denoted by:(10)Eelectric2=∑e∫TmP1dt+∫TnP2dt+∫TiP3dt,where P1, P2, and P3 are the motor’s instantaneous powers at UL, FR, and WT, respectively, and Tm, Tn, and Ti are the durations of these stages, respectively.Since the motor-pump configuration for a THP is consistent with the HP with the proposed 3-D structure, the power consumption in one operating cycle (Eelectric−Total) can be obtained as:
(11)Eelectric−Total=Eelectric1+Eelectric2+ELoss.The hydraulic cylinder outputs forming energy until the end of an operating cycle, so the useful energy in one operating cycle (EForming) is:
(12)EForming=∑i∫iFa×ḣdtwhere Fa represents the force acting on the slider. In summary, the energy efficiency of the HP (ηE−F) is:
(13)ηE−F=EFormingEelectric−Total.In summary, the useful energy consumed by the newly proposed HP to finish the same operation is the same as that of a THP, but an HP with a 3-D structure has a more efficient PVS, thereby leading to higher energy utilization and lower energy losses.


## 3.2. Components Statistics of 3-D Structure

In the first two sections, the feasibility of this new structure has been discussed in detail, and the energy-saving mechanism has been theoretically explained through a mathematical model. To further study the energy efficiency of an HP based on this structure, two rapid-forging HPs with similar tonnages and configurations, which are commonly used in free forging and other forming processes, were chosen as a research object for the efficiency analysis.The maximum forming forces of the two HPs were both 31.5 MN. One of these was built according to the traditional planar structure, and the other was built according to the 3-D vertical structure proposed in this study. Through the experimental measurements and statistical calculations of the structures of the two pieces of equipment, the energy saved by the proposed structure was obtained.The 31.5 MN rapid-forging HP with a 3-D vertical arrangement structure was developed by our research team; the novel HP is mainly made up of motor-pump stations, a main-cylinder circuit, an unloading system, an oil-filling circuit, a direction–speed–pressure control circuit, and an energy-recovery circuit. After component planning in the design process and determining the component statistics during assembly, the statistics of the energy consumption components in each circuit of the two presses were obtained and are shown in Table1.Table 1
Compositions statistics of each press.
HPCircuitDiameter×length (mm×m)ElbowPipe jointValve90°135°150°HP with 3-D structureMotor-pump circuit114×12; 127×10910686Main-cylinder circuit168×12; 194×12261078Unloading circuit133×18; 127×12; 114×10181087Oil-filling circuit168×17; 194×1219868Control circuit219×10; 310×11; 219×13; 168×14; 133×16181086Boosting and recovery circuit194×8; 133×506274THPMotor-pump circuit114×36; 127×28131481416Main-cylinder circuit168×24; 194×2958141410Unloading circuit168×25; 102×24; 127×25; 89×22410121612Oil-filling circuit168×28; 198×24412101211Control circuit219×15; 325×18; 219×15; 114×24; 168×12; 133×24010101610The motor pumps of the THP must pass through 373 m hydraulic pipelines and 59 control valves to input the hydraulic oil to the hydraulic actuator. The friction loss along the way and the local energy loss resulted in considerable energy loss during the transmission of hydraulic energy. However, as presented in Table1, compared with the THP, the press with the new structure was significantly optimized in terms of pipe length, number of pipe joints, and number of valve groups, and only needed 189 m pipelines and 39 control valves.


## 3.3. Theoretical Analysis of Energy Dissipation

The motor-pump circuit for the 31.5 MN HP with a 3-D structure adopted 12 plunger pumps (the motor power and speed were 110 kW and 1480 rpm, respectively) to provide power for the hydraulic system, which outputs 360 L fluid per minute. Moreover, HM46 hydraulic oil was employed to serve the two HPs.ξ and ζ denote the local and the route resistance index, which can be obtained by Equations (3) and (4).This section firstly takes the motor-pump circuit as a case study to analyze the pressure dissipation during an operating cycle, and then the total energy dissipation of the two selected HPs during a cycle is obtained. Taking one of the hydraulic pipelines with a diameter and length of 0.114 m and 12 m as an example, the pressure drop caused by fluid flowing through the hydraulic pipeline can be obtained by Equation (14) as 0.13 MPa.
(14)∆pPipeline=ζ×L/d×pq/2A.For a 90° elbow, the throttling index (m) is 0.5, and the resistance coefficient (ξ) is 0.14, Based on Equation (15), the pressure loss generated by the 90° elbow is 0.003 MPa. Likely, the pressure loss generated by a 135° elbow and a 150° elbow is 0.35 MPa and 0.39 MPa, respectively.
(15)∆pjoint=ξ×ρ/21/2m×q1/m.Although structures and functions of each valve are various while the energy dissipation remains the same, the pressure losses of one valve is obtained as 0.09 MPa according to the following equation.(16)∆pValve=ξ×ρ2×q/A.Therefore, the total pressure loss generated by the motor-pump stations during a forming cycle is 1.67 MPa according to the following equation.(17)∆p1=∑∆pPipeline+m×∆pValve+x×∆p150°+y×∆p135°+z×∆p90°,where m, x, y, and z are the quantity of valves, 150° elbows, 135° elbows, and 90° elbows in a circuit, respectively.Then, the pressure losses of the other circuits can be achieved, as shown in Table2. Combined with the constant-forging process, the processing pressure and the processing cycle are set at 31.5 MPa and 3 s, respectively. Additionally, the flow rates of the motor-pump circuit, the main-cylinder circuit, the oil-filling circuit, the booster and recovery circuit, the unloading circuit, and the control circuit are 4320 L/min, 6612 L/min, 6480 L/min, 750 L/min, 5613 L/min, and 5200 L/min, respectively. Then, according to the pressure loss presented in Table 2 and the above processing parameter, the power dissipation (PPVLoss) and the energy dissipation (EPVLoss) caused by the PVS in the selected HPs can be calculated based on Equations (18) and (19), and the results are shown in Table 3.
(18)PPVLoss=∑i=0n∆pi×qi,(19)EPVLoss=∫tstarttendPPVLossdt.Table 2
Pressure loss of each circuit in the two HPs.
CircuitsHP with 3-D structureTHPMotor-pump circuits1.67 MPa4.24 MPaMain-cylinder circuits1.61 MPa5.12 MPaOil-filling circuits1.52 MPa4.09 MPaUnloading circuits1.33 MPa4.71 MPaControl circuits1.41 MPa3.30 MPaBoosting and recovery circuit1.23 MPa0 MPaTable 3
Energy dissipations of the above circuit in two HPs during one cycle.
Circuits (time/s)HP with 3-D structureTHPMotor-pump circuit (0-0.9)57.38 kJ184.01 kJMain-cylinder circuit (0.9-1.5)78.69 kJ229.01 kJOil-filling circuit (1.5-2.0)59.42 kJ168.29 kJUnloading circuit (2.0-2.8)72.09 kJ204.47 kJControl circuit (2.8-3.00)33.23 kJ102.96 kJBoosting and recovery circuit23.71 kJ0 kJTotal energy dissipation324.52 kJ888.74 kJAs shown in Table3, under the same load conditions, compared with the THP, a press with a 3-D arrangement structure produces less energy dissipation during the hydraulic energy transmission process. During one operating cycle (3 s), the newly proposed HP produced a total energy loss of 324.52 kJ, which was 564.22 kJ lower than that of the THP, showing better energy transmission efficiency.


## 4. Case Study

### 4.1. Experiment Scheme

For analyzing the processing performance and energy utilization rate of this novel HP structure designed in this study, we selected two forging HPs with similar processing tonnages to conduct constant-forging experiments. One press has a traditional planar structure, and the other employs a 3-D vertical structure, as presented in Figure10.Figure 10
31.5 MN fast forging HP: (a) host equipment, (b) 3-D structure, and (c) drive system.
(a)(b)(c)This study adopted the persistent processing method to analyze the energy-saving efficiency of the press. It includes the following five actions: FF (0 s–0.9 s), PS (0.9 s–1.5 s), UL (1.5 s–2.0 s), FR (2.0 s–2.8 s), and SR (2.8 s–3.0 s). Each drive unit in the power system varies the state of its access to the hydraulic system based on the energy required for each processing operation. A power unit, which is ineffective for operation, was set in an unloading state by switching the pressure-relief valve.The working pressure and cycle time were set at 31.5 MPa and 3 s, respectively. The operating speeds in the FF, PF, and FR stages were set at 23 cm/s, 10 cm/s, and 23 cm/s, respectively. Additionally, the pressure of the return cylinder and the balance cylinder were all 160 bar. Through the experimental measurement of the actual electricity consumptions of the two HPs, then energy-saving potential for this structure was achieved. Table4 lists the components of the power system and the number of power units connected during each operation.Table 4
Components of the power system and scheduling scheme in different operations.
StageNovel hydraulic pressTraditional hydraulic pressMPURASSSPROFSMPUFF-12/12+6/6+6/6-1/1-15/15PS-12/12-6/6-6/60/1-15/15UL-12/120/60/6+1/115∗/15FR-12/12-6/60/60/1-15/15SR-12/12-2/60/60/1-15/15A/B (−12/12, et al.): B and A represent the total number of components and the number of components used in this stage, respectively. +: restore energy. −: release energy.The actual electricity dissipation of motors in the drive system was measured to scale the energy consumption of two HPs. Power meters and AD cards were chosen to continuously monitor and accept the power through the motors, respectively. The real-time measured data were then transmitted to the monitoring system after processing. The energy consumed by a THP to complete theith action was determined using the following formula:
(20)Eics=∫ti−Startti−EndPitdt=∑ti−Startti−EndPi×∆t,where i denotes the forming actions mentioned in Figure 2, ti−Start and ti−End are the starting and ending times, respectively, and Pi is the sum of the motor power connected to the drive system under each action.


### 4.2. Actual Efficiency Analysis

According to Equation (20), the actual electricity consumption of the two HPs during one cycle was achieved, as shown in Table 5 and Table 6. As presented in Table 5, the total input energy of the 31.5 MN THP was 8592.72 kJ, of which the energy consumed for the normal operation of the conventional HP system was only 3776.37 kJ, and the remaining energy was dissipated by other hydraulic components during the energy transmission process, especially the PVS. The PVS consumed a total of 937.28 kJ, accounting for 11% of the total input energy, resulting in a 43.95% energy efficiency for THP.Table 5
Energy performance of the traditional HP in one operating cycle.
OperationFFPSULFRSRTotalEnergy (kJ)Time (s)0-0.90.9-1.51.5-2.02.0-2.82.8-3.03.0Input energy1311.594329.38826.411793.56331.788592.72Energy loss of PVS199.01234.21179.47221.58103.01937.28Useful energy11.652834.33190.22664.9875.193776.37Energy efficiency0.89%65.47%23.02%37.08%22.66%43.95%η32.98%Table 6
Energy performance of the proposed HP in one operating cycle.
OperationFFPSULFRSRTotalEnergy (kJ)Time (s)0-0.90.9-1.51.5-2.02.0-2.82.8-3.03.0Input energy1068.271211.56642.111194.74228.314344.99Energy loss of PVS62.5681.6981.3173.8834.21333.65Useful energy932.331126.19487.22929.48219.163694.38Energy efficiency87.27%92.95%75.88%77.80%95.99%85.03%η65.23%However, based on the high-efficiency hydraulic energy transmission structure and the hydraulic system’s energy scheme, the overall energy performance of the proposed HP had been improved compared with the conventional HP. As illustrated in Table6, the total input energy and useful energy for the newly proposed HP were 4344.99 kJ and 3694.38 kJ under the above states, respectively. Meanwhile, the energy dissipated by the PVS was approximately 333.65 kJ, which was 603.63 kJ lower than that of the conventional setup, as shown in Table 7. Moreover, through the effective recovery and reuse of the potential energy generated in the UL stage, the total input energy of the HP was reduced by 4247.73 kJ, the energy efficiency increased from 43.95% to 85.03%, and the forming efficiency also increased by 32.25%, as presented in Figure 11.Table 7
Analysis of experimental results between the selected two HPs.
OperationFFPSULFRSRTotalInput energy difference243.323117.82184.3598.82103.474247.73Dissipation reduction by PVS136.45152.5298.16147.768.8603.63Efficiency improvement86.38%27.48%52.86%40.72%73.33%41.08%Figure 11
Energy performance of the selected two hydraulic presses.Furthermore, combined with the efficient energy supply system, the input energy was significantly decreased in each operation stage, as presented in Table7. That is, the idling loss was also significantly reduced during some no-load and low-load stages. The motors in the hydraulic power system drive the oil pump to suck oil from oil tanks and then store energy for the recovery accumulator instead of idling. The stored energy could be used to power the actuator at high-load stages together with the motor pumps, thus lowering the power burden of the traditional drive system and offering the possibility to reduce the installed power. In fact, the installed power of this new HP was only 1420 kW, while that of the THP with the same tonnage was approximately 3200 kW. 1780 kW installed power difference also demonstrates excellent energy-saving potential.In summary, the theoretical calculation result and the actual experimental result all illustrated that this new structure and its corresponding energy supply scheme proposed in this study have good energy-saving prospects.


### 4.3. Processing Performance Analysis

Stable processing performance is the premise for improving energy efficiency. If an HP with improved efficiency cannot guarantee normal processing, the goal of high energy efficiency will also become difficult to achieve.Therefore, we combined the two selected HPs to verify their processing performance by carrying out simulated disturbance experiments on the presses. The experimental parameters and configurations were consistent with the above settings, and the operating speeds of the actuator in the FF, PS, FR, and SR were set to 23 cm/s, 10 cm/s, 23 cm/s, and 10 cm/s, respectively. Additionally, a displacement sensor and several pressure sensors were employed to monitor and measure the system states, including the hydraulic actuator displacement and each circuit pressure. Finally, the real-time measured data was processed and sent to an upper computer.Based on several experiments, Figure12 shows the operating state parameters of the two HPs in one operating cycle. As illustrated in the figure, an HP with the proposed 3-D structure had the same process characteristics and working performance as those of the THP. The HP with the 3-D structure ran smoothly under a 100% loading state without obvious pressure and flow fluctuations. The actuator in the 3-D HP operated with a setting speed and arrived the processing location at a setting time, showing excellent dynamic performance.Figure 12
Press processing characteristics under 31.5 MPa: (a) displacement, (b) velocity, (c) acceleration, (d) main-cylinder pressure, (e) returning cylinder pressure, and (f) drive system pressure.
(a)(b)(c)(d)(e)(f)In addition, the speed and acceleration of the slider changed uniformly, which almost coincides with the change trajectory of the THP. Moreover, although the structure and energy supply mode were changed, there was no obvious sudden change, ensuring that the processing performance of the novel configuration was almost the same as that of the THP. Thus, this also lays a good foundation for further energy improvements and also verifies the effectiveness of the energy-saving scheme developed in this paper.


## 4.1. Experiment Scheme

For analyzing the processing performance and energy utilization rate of this novel HP structure designed in this study, we selected two forging HPs with similar processing tonnages to conduct constant-forging experiments. One press has a traditional planar structure, and the other employs a 3-D vertical structure, as presented in Figure10.Figure 10
31.5 MN fast forging HP: (a) host equipment, (b) 3-D structure, and (c) drive system.
(a)(b)(c)This study adopted the persistent processing method to analyze the energy-saving efficiency of the press. It includes the following five actions: FF (0 s–0.9 s), PS (0.9 s–1.5 s), UL (1.5 s–2.0 s), FR (2.0 s–2.8 s), and SR (2.8 s–3.0 s). Each drive unit in the power system varies the state of its access to the hydraulic system based on the energy required for each processing operation. A power unit, which is ineffective for operation, was set in an unloading state by switching the pressure-relief valve.The working pressure and cycle time were set at 31.5 MPa and 3 s, respectively. The operating speeds in the FF, PF, and FR stages were set at 23 cm/s, 10 cm/s, and 23 cm/s, respectively. Additionally, the pressure of the return cylinder and the balance cylinder were all 160 bar. Through the experimental measurement of the actual electricity consumptions of the two HPs, then energy-saving potential for this structure was achieved. Table4 lists the components of the power system and the number of power units connected during each operation.Table 4
Components of the power system and scheduling scheme in different operations.
StageNovel hydraulic pressTraditional hydraulic pressMPURASSSPROFSMPUFF-12/12+6/6+6/6-1/1-15/15PS-12/12-6/6-6/60/1-15/15UL-12/120/60/6+1/115∗/15FR-12/12-6/60/60/1-15/15SR-12/12-2/60/60/1-15/15A/B (−12/12, et al.): B and A represent the total number of components and the number of components used in this stage, respectively. +: restore energy. −: release energy.The actual electricity dissipation of motors in the drive system was measured to scale the energy consumption of two HPs. Power meters and AD cards were chosen to continuously monitor and accept the power through the motors, respectively. The real-time measured data were then transmitted to the monitoring system after processing. The energy consumed by a THP to complete theith action was determined using the following formula:
(20)Eics=∫ti−Startti−EndPitdt=∑ti−Startti−EndPi×∆t,where i denotes the forming actions mentioned in Figure 2, ti−Start and ti−End are the starting and ending times, respectively, and Pi is the sum of the motor power connected to the drive system under each action.


## 4.2. Actual Efficiency Analysis

According to Equation (20), the actual electricity consumption of the two HPs during one cycle was achieved, as shown in Table 5 and Table 6. As presented in Table 5, the total input energy of the 31.5 MN THP was 8592.72 kJ, of which the energy consumed for the normal operation of the conventional HP system was only 3776.37 kJ, and the remaining energy was dissipated by other hydraulic components during the energy transmission process, especially the PVS. The PVS consumed a total of 937.28 kJ, accounting for 11% of the total input energy, resulting in a 43.95% energy efficiency for THP.Table 5
Energy performance of the traditional HP in one operating cycle.
OperationFFPSULFRSRTotalEnergy (kJ)Time (s)0-0.90.9-1.51.5-2.02.0-2.82.8-3.03.0Input energy1311.594329.38826.411793.56331.788592.72Energy loss of PVS199.01234.21179.47221.58103.01937.28Useful energy11.652834.33190.22664.9875.193776.37Energy efficiency0.89%65.47%23.02%37.08%22.66%43.95%η32.98%Table 6
Energy performance of the proposed HP in one operating cycle.
OperationFFPSULFRSRTotalEnergy (kJ)Time (s)0-0.90.9-1.51.5-2.02.0-2.82.8-3.03.0Input energy1068.271211.56642.111194.74228.314344.99Energy loss of PVS62.5681.6981.3173.8834.21333.65Useful energy932.331126.19487.22929.48219.163694.38Energy efficiency87.27%92.95%75.88%77.80%95.99%85.03%η65.23%However, based on the high-efficiency hydraulic energy transmission structure and the hydraulic system’s energy scheme, the overall energy performance of the proposed HP had been improved compared with the conventional HP. As illustrated in Table6, the total input energy and useful energy for the newly proposed HP were 4344.99 kJ and 3694.38 kJ under the above states, respectively. Meanwhile, the energy dissipated by the PVS was approximately 333.65 kJ, which was 603.63 kJ lower than that of the conventional setup, as shown in Table 7. Moreover, through the effective recovery and reuse of the potential energy generated in the UL stage, the total input energy of the HP was reduced by 4247.73 kJ, the energy efficiency increased from 43.95% to 85.03%, and the forming efficiency also increased by 32.25%, as presented in Figure 11.Table 7
Analysis of experimental results between the selected two HPs.
OperationFFPSULFRSRTotalInput energy difference243.323117.82184.3598.82103.474247.73Dissipation reduction by PVS136.45152.5298.16147.768.8603.63Efficiency improvement86.38%27.48%52.86%40.72%73.33%41.08%Figure 11
Energy performance of the selected two hydraulic presses.Furthermore, combined with the efficient energy supply system, the input energy was significantly decreased in each operation stage, as presented in Table7. That is, the idling loss was also significantly reduced during some no-load and low-load stages. The motors in the hydraulic power system drive the oil pump to suck oil from oil tanks and then store energy for the recovery accumulator instead of idling. The stored energy could be used to power the actuator at high-load stages together with the motor pumps, thus lowering the power burden of the traditional drive system and offering the possibility to reduce the installed power. In fact, the installed power of this new HP was only 1420 kW, while that of the THP with the same tonnage was approximately 3200 kW. 1780 kW installed power difference also demonstrates excellent energy-saving potential.In summary, the theoretical calculation result and the actual experimental result all illustrated that this new structure and its corresponding energy supply scheme proposed in this study have good energy-saving prospects.


## 4.3. Processing Performance Analysis

Stable processing performance is the premise for improving energy efficiency. If an HP with improved efficiency cannot guarantee normal processing, the goal of high energy efficiency will also become difficult to achieve.Therefore, we combined the two selected HPs to verify their processing performance by carrying out simulated disturbance experiments on the presses. The experimental parameters and configurations were consistent with the above settings, and the operating speeds of the actuator in the FF, PS, FR, and SR were set to 23 cm/s, 10 cm/s, 23 cm/s, and 10 cm/s, respectively. Additionally, a displacement sensor and several pressure sensors were employed to monitor and measure the system states, including the hydraulic actuator displacement and each circuit pressure. Finally, the real-time measured data was processed and sent to an upper computer.Based on several experiments, Figure12 shows the operating state parameters of the two HPs in one operating cycle. As illustrated in the figure, an HP with the proposed 3-D structure had the same process characteristics and working performance as those of the THP. The HP with the 3-D structure ran smoothly under a 100% loading state without obvious pressure and flow fluctuations. The actuator in the 3-D HP operated with a setting speed and arrived the processing location at a setting time, showing excellent dynamic performance.Figure 12
Press processing characteristics under 31.5 MPa: (a) displacement, (b) velocity, (c) acceleration, (d) main-cylinder pressure, (e) returning cylinder pressure, and (f) drive system pressure.
(a)(b)(c)(d)(e)(f)In addition, the speed and acceleration of the slider changed uniformly, which almost coincides with the change trajectory of the THP. Moreover, although the structure and energy supply mode were changed, there was no obvious sudden change, ensuring that the processing performance of the novel configuration was almost the same as that of the THP. Thus, this also lays a good foundation for further energy improvements and also verifies the effectiveness of the energy-saving scheme developed in this paper.


## 5. Conclusions

This study proposes an optimization strategy for the arrangement and structure of the THP. Under the premise of fulfilling the basic processing requirements, a 3-D vertical structure for the THP was developed to improve its energy efficiency and reduce its power loss. In this structure, the aboveground and underground spaces were comprehensively utilized, and the hydraulic components were arranged three-dimensionally and set in layers according to their function requirements. Based on this structure, the energy consumption of the equipment and the energy transmission distance between the drive system and the actuator were all significantly reduced, thereby improving the hydraulic energy transmission loss and the response speed. Additionally, a configuration and energy supply strategy by which to develop the above structure was introduced to achieve an orderly and efficient energy supplement.Finally, the proposed structure was applied into a 31.5 MN HP as a case study. Through theoretical calculations and comparative experiments, the energy transmission efficiency was found to be significantly improved, the total input energy of the HP was decreased by 4247.73 kJ in one cycle (3 s), and the energy utilization efficiency was improved from 43.95% to 85.03%, demonstrating excellent energy-saving potential. Moreover, the forming efficiency was also increased by 32.25%.



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*Source: 6275886-2023-02-09.xml*