Axial Piston Pump Research Articles

Thermal-Hydraulic Modeling of Oil-Immersed Motor Pump

The integrated design of the motor and axial piston pump eliminates the coupling structure, resulting in a compact and lightweight motor-pump structure. The challenge of motors overheating has always been a major concern. To address this issue, the hydraulic oil throughout the motor pump is utilized for cooling the high-speed motor, effectively improving the power density and heat dissipation capability of the hydraulic power unit. This integrated design approach has successfully resolved the significant issue of overheating motors, leading to enhanced performance of the hydraulic power unit. To address this concern, the entire motor pump’s oil is utilized to cool the high-speed motor. Consequently, the thermodynamic prediction of high-speed motor pumps has become increasingly important. In this study, the impact of motor heat generation on hydrodynamics is analyzed, and the heat transfer of the motor pump is investigated using the control volume method. Furthermore, thermodynamic models of hysteresis loss, eddy current loss, alternating current loss, churning loss, and throttling loss are established for the oil-immersed motor pump. The change in oil viscosity is also considered. The instantaneous temperature change rule of the oil within the oil-immersed motor pump is derived. Additionally, the influence of various working conditions such as pressure and speed on the temperature of the motor pump’s key node is examined. The experimental results indicate the accuracy of the thermodynamic calculation, and the significant effect of motor loss on the leakage temperature.

Modeling and analysis of the leakage performance of the spherical valve plate pair in axial piston pumps

Axial piston pumps are widely used in construction machinery due to the high power density. The leakage performance of the spherical valve plate pair is of great significance to the large displacement pumps' volumetric efficiency. However, the model and calculation of leakage are unclear. This paper proposed a cylinder block dynamic and oil film lubrication based leakage model of the spherical valve plate pair for axial piston pumps. Numerical solutions on the leakage performance under different working conditions were carried out. The tilt behavior of the cylinder block, thickness, and longitude velocity of the oil film were solved. In addition, the effects of central spring stiffness on leakage performance were analyzed and experiments were conducted. Results show that the pump is with poor volumetric efficiency for operation at low speed and high pressure, and ± 5% adjustment of spring stiffness leads to a 237% maximum leakage gap for spherical valve plate pair.

Tribological characteristics of hard-to-hard matching materials of cylinder block/valve plate interface in electro-hydrostatic actuator pumps

Axial piston pumps, serving as the essential power component in electro-hydrostatic actuators (EHA), play a crucial role in achieving advanced actuation by ensuring high efficiency and reliability. The cylinder block/valve plate interface, which is the greatest contact area inside the axial piston pump, significantly affects both leakage and energy dissipation. The interface experiences significant friction loss and severe wear in aerospace applications, primarily due to the rapidly changing mixed and boundary lubrication conditions under a wide range of speed and pressure. Therefore, to improve the efficiency and reliability of electro-hydrostatic actuators, strengthening the cylinder block/valve plate interface is a key problem to overcome. Previous studies have demonstrated that the utilization of soft-to-hard matching materials enhances the friction characteristics of the interface, but it also results in relatively severe wear on the soft material. Therefore, in this study, we propose the concept of employing hard-to-hard matching materials as a strategy to mitigate wear in axial piston pumps. An optimized friction simulation test apparatus is adopted, which contains a simulated piston structure to take the piston stirring effect into consideration. Experimental results show that the implementation of hard-to-hard materials leads to a substantial improvement in the wear resistance of the interface under a wide range of rotational speed (300–6000 rpm). The wear depths of the TiAlN/TiAlN (hard-to-hard) matching material are only 28.6%~34.7% of the tin bronze/TiAlN (soft-to-hard) matching material. Besides, compared with traditional tin bronze/nitrided (soft-to-hard) matching material, the hard-to-hard matching materials improve both friction and wear performance. The hard-to-hard matching materials provide a novel approach to reinforce the cylinder block/valve plate interface, and thus to promote volumetric efficiency and extend the service life of the axial piston pump.

Axial-piston pump optimal multivariable PID controller tunning

The aim of the present work is the synthesis and optimal tunning of a PID controller designed to control the displacement volume of an axial-piston pump intended for hydraulic systems with open circulation. The pump is controlled in real-time through a system developed by the authors based on an electro-hydraulic proportional valve (replacing the conventional hydro-mechanical controller) and an industrial controller. The experimental system was initially used to obtain a data set that is used in a system identification procedure to obtain a model. The obtained state space model was used for the synthesis of the PID controller. The optimal tunning of controller parameters is performed using three modern methods: genetic algorithm, simulated annealing and pattern search. The resulting three PID controllers with different settings depending on the used methods are compared in simulation and experimental conditions. Based on the results, an analysis of the control performance was carried out.

Effect of Material Selection and Surface Texture on Tribological Properties of Key Friction Pairs in Water Hydraulic Axial Piston Pumps: A Review

A water hydraulic axial piston pump has become the preferred power component of environmentally friendly water hydraulic transmission systems, due to its advantages of a compact structure, high power density, and so on. The poor friction and wear performance in the water medium, especially under extreme conditions of high speed and high pressure, limit the engineering application of the water hydraulic axial piston pump. In this review, the research progress for key friction pair materials (such as special corrosion-resistant alloys, engineering plastics, and engineering ceramics) for water hydraulic axial piston pumps is, firstly, summarized. Secondly, inspired by nature, the processing methods, lubrication drag-reduction mechanism, and tribological properties of the biomimetic surface textures are discussed. The effects of the surface texture shape, equivalent diameter, depth, and arrangement on the pump’s tribological properties are reviewed in detail. Finally, the application status of, and problems with, surface texture technology in water hydraulic axial piston pumps are summarized. It is suggested that future studies should focus on the multi-field coupling lubrication anti-friction mechanism of the multi-type composite texture under extreme conditions and mixed lubrication; and the anti-wear performance of the texture coupled with a coating modification, to further promote the surface texture in the field of lubrication antifriction engineering applications.

Digital Control Method and Performance Analysis of the Double-Compound Axial Piston Pump.

The flow control range of the double-compound axial piston pump with the traditional mechanical-hydraulic feedback servo control is limited and the accuracy is poor. Accordingly, this paper proposes a digital control scheme and its control strategy using a linear stepper motor direct drive servo valve for the precise control and double pumps cooperation of the double-compound axial piston pump. A numerical model of the digital control double-compound axial piston pump is established, and the validity of the model is verified by experimental tests. The performance advantages of the digital control method relative to the mechanical-hydraulic feedback servo control method are analyzed, as is the performance of the control strategy for double pumps. The results show that the digital control method can achieve a wider range of flow control than the traditional mechanical-hydraulic feedback servo control method and avoid the torque impact on the prime mover caused by the active control. The combination of the flow control and the power control including four control modes can meet the performance requirements of the double-compound axial piston pump. The highest priority is given to the energy-saving control, which can reduce the displacement of the main pump in the nonworking state to reduce the additional power loss. The study provides a basis for the accurate matching and optimization of power to load and flow to operating speed of the double-compound axial piston pump.

Research on the improvement of tilt characteristics for the axial piston pump cylinder block based on the magnetic auxiliary support for distributing pair

Due to the unbalanced effect of force and torque on the axial piston pump cylinder block, a wedge angle of oil film exists between the valve plate and the cylinder block. The cylinder block tilt seriously affects the oil film and the distributing pair lubrication state, leading to boundary lubrication appears in some cases. To reduce the impact of the tilt on the cylinder block of axial piston pump, the magnetic auxiliary support (MAS) for the pair is proposed. The MAS reduces the overturning torque on the cylinder block by the auxiliary magnetic support. Then the tilt characteristics of the cylinder block are improved. In this paper, the cylinder block overturning torque equations under the support of MAS are established, and the improvement of overturning torque is analyzed. An experimental platform was constructed to measure the thickness of the oil film of the pair and test the tilt of the cylinder block. The experimental results showed that the MAS can effectively improve the tilt of the cylinder block and the performance of the axial piston pump. This method has certain reference significance for the research of axial piston pumps.

Numerical study on the lubrication performance of oil films in textured piston/cylinder pairs

The piston/cylinder pair is one of the most important friction pairs in the axial piston pump; reducing the friction of it can improve the axial piston pump efficiency. In this paper, the Reynolds equation coupled with the Half–Sommerfeld boundary condition is applied to predict the dynamics of the textured piston/cylinder pairs, and the characteristic function is used to predict the optimal geometry parameters. The analysis results show that the combination of the shearing coefficient and the characteristic function can predict the cavitation region and the critical inclination angle. The results of this paper provide theoretical guidance for the surface texture design on piston/cylinder pairs.

The Influence of Temperature on the Mechanical Properties of the Surface Layer of a Radiationally Strengthened Fluoropolymer

This article discusses the influence of temperature on the mechanical properties of a fluoropolymer treated by λ irradiation above the melting point. The statement of the high efficiency of this strengthening technology on the tribological properties of the fluoropolymer–steel friction pair is substantiated. The changes in mechanical properties in a thin surface layer of the fluoropolymer as a consequence of heating to 400°C are studied using kinetic microindentation. The material hardness, elasticity modulus, total work of indentation, and its elastic constituent are analyzed. It is proposed to apply friction pairs containing radiationally strengthened fluoropolymer parts in the axial piston pumps of hydraulic systems in order to increase the probability of faultless operation.

Dynamic Response Analysis of the Bi-Tandem Axial Piston Pump with Dual-Loop Positive Flow Control under Pressure Disturbance

The bi-tandem axial piston pump is an indispensable powerhouse in high-pressure and high-power engineering hydraulic systems, with its output flow response characteristics under pressure disturbance exerting a significant influence on the working process of double pumps. Unfortunately, the stability of the original single-loop mechanical–hydraulic servo control system is sensitive to unpredictable interference. To alleviate this quandary, this paper proposes a dual-loop positive flow control method for the flow control of the bi-tandem axial piston pump, establishes a mathematical model of the bi-tandem axial piston pump with dual-loop positive flow control, and establishes a simulation model based on Simulink. The validity of the model is verified by experiments. The performance advantages of the dual-loop positive flow control method relative to the single-loop positive flow control method are analyzed. The results show a faster response speed and smaller steady-state error with the dual-loop method, which performs better than the original single-loop positive flow control. Furthermore, the study examines the influence of different forms, degrees, and directions of pressure disturbance on the dynamic response characteristics of the bi-tandem axial piston pump. Symmetric pressure disturbance results in an increase in the maximum relative error of the output flow proportional to its degree. Notably, the influence of asymmetric pressure disturbance on the output flow of the double pumps possesses characteristics of a superimposable nature, and the steady-state value of the output flow is highly dependent on superimposed pressure disturbance and less affected by the action time point of asymmetric pressure disturbance. Further, the unloading pressure disturbance exerts less influence on the system compared to the loading pressure disturbance. This paper provides valuable insights into improving the response speed and control accuracy of bi-tandem axial piston pumps equipped with positive flow control.

A Data-Driven Diagnosis Scheme Based on Deep Learning toward Fault Identification of the Hydraulic Piston Pump

The piston pump is the significant source of motive force in a hydraulic transmission system. Owing to the changeable working conditions and complex structural characteristics, multiple friction pairs in the piston pump are prone to wear and failure. An accurate fault diagnosis method is a crucial guarantee for system reliability. Deep learning provides a great insight into the intelligent exploration of machinery fault diagnosis. Hyperparameters are very important to construct an effective deep model with good performance. This research fully mines the feature component from vibration signals, and converts the failure recognition into a classification issue via establishing a deep model. Furthermore, Bayesian algorithm is introduced for hyperparameter optimization as it considers prior information. An adaptive convolutional neural network is established for typical failure pattern recognition of an axial piston pump. The proposed method can automatically complete fault classification and represents a higher accuracy by experimental verification. Typical failures of an axial piston pump are intelligently diagnosed with reduced subjectivity and preprocessing knowledge. The proposed method achieves an identification accuracy of more than 98% for five typical conditions of an axial piston pump.

Research on the High Speed of Piston Pumps Based on Rapid Erecting of Launch Vehicles

Rapid erection is a key technology for modern warfare in vehicle weapon launch systems. It is challenging to attain rapidity with the hydraulic erecting system because of the intensified cavitation in the piston chamber at high speeds, which reduces volumetric efficiency and increases flow pulsation in the typical high-pressure axial piston pump. In this paper, an improved scheme for the cylinder window area overflow surface was proposed to solve this problem. Based on the full cavitation model and the compressible model, a numerical model of the internal flow in the piston pump was developed, and the effect of rotational speeds on the flow and cavitation characteristics of the pump was analysed. The results show that after the improvement, the maximum flow of the pump is increased from 1.765 kg/s to 2.295 kg/s, an increase of 30.028%, and the maximum speed corresponding to the volumetric efficiency of more than 90% is increased from 1500 rpm to 2100 rpm. At high speeds, the improved block can effectively suppress the cavitation and backflow in the piston chamber, improve the volumetric efficiency of the piston pump and reduce the flow pulsation, which is conducive to reducing the vibration and noise of the pump body.

Multiple-signal defect identification of hydraulic pump using an adaptive normalized model and S transform

Axial piston pump plays a pivotal role in a hydraulic transmission system since it can supply the core power source. The complexity of structure and the invisibility of failure feature bring more difficulties and challenges to fault identification of an axial piston pump. It is of great meaning to exploit an effective and feasible fault identification method for the safe and stable system operation. An improved convolutional neural network (CNN) is constructed by mining and utilizing the interdisciplinary advantages of artificial intelligence and machinery engineering. First, the modeling takes the batch normalization strategy into account due to its capability of decreasing the data distribution differences. Second, Bayesian algorithm is used for intelligent tuning of hyperparameters. Third, the improved CNN is applied to fault identification based on the S transform of multiple source signals. The performance of the proposed method is demonstrated by the experiments on an axial piston pump. The identification accuracies based on three signals reach 88.17%, 95.94%, and 99.44%. Results show that it can automatically recognize typical faults of the axial piston pump, and present the superiority to common CNNs.

Simulation-driven bandpass filter in association with an optimal bandwidth to detect faults in axial piston pumps

Due to the special working environments of axial piston pumps in engineering, fault features are disrupted by the natural periodic impulses. A finite element method (FEM) simulation-driven bandpass filter (BPF) is provided for detecting bearings. However, the bandwidth of the BPF is designed empirically through human experience with uncertainty. To overcome the associated limitations, a bandwidth optimization strategy of FEM simulation-driven BPF is proposed by using an integrated kurtosis, which is a combination of two kinds of kurtosis indices. The new index is used as a discrimination value for a success–failure algorithm to iteratively determine the optimal bandwidth of the BPF. Finally, compared to the original BPF and its improved version, experimental results of faulty bearings in an axial piston pump verify the fault feature extraction ability for the reciprocating motion machine under heavy impact-induced natural periodic impulses.

Squeeze Film Effect and Its Influence on the Lubrication Characteristics of Slipper Oil Film under Pressure Pulse

This study aims at the lubricant film between the slipper and swashplate of an axial piston pump. Based on a single piston/slipper assembly model, a mathematical model for the squeeze film effect is developed according to the pressure-flow transfer process as well as the dynamics of the piston/slipper assembly. The squeeze film effect on the slipper lubricant film has been predicted by nonlinear numerical calculations and computational fluid dynamic (CFD). The slipper lubricant film thickness and leakage flow, sealing land pressure distribution, and load carrying capacity are evaluated as important parameters to indicate the performance of slipper oil film. The influence of piston/slipper assembly geometric parameters, oil compressibility, and oil viscosity on the squeeze film effect under pressure pulse has been studied, and the effect of squeeze film speed on the support force of slipper lubricant film has been given out.

ANALYSIS OF TECHNICAL CHARACTERISTICS OF HIGH TORQUE HYDROMOTORS WITH LIMITATIONS BASED ON DYNAMIC CALCULATIONS

The goal is to study the technical characteristics of the world's largest modern high-torque multi-acting hydraulic motor from the point of view of ensuring reliable operation by the manufacturer at elevated pressures, speeds and depending on the type of working fluid used. Analysis of the influence of dynamic loads on fluctuations in pressure and speed of the hydraulic motor in the modes of starting and braking, taking into account changes in the pump flow rate and inertial loads. Method. Dynamic calculation of a hydraulic fluid power with a high-torque hydraulic motor based on studies of external loads, changes in the pump feed time, providing the maximum speed. To solve nonlinear differential equations when calculating the pressure in the hydraulic drive and the rotational speed of the hydraulic motor, the VisSim application software package was used. The modes of change in the operation of the hydraulic motor over time in terms of rotational speed, pressure in the hydraulic fluid power, output power and hydromechanical efficiency, depending on the external load and pump flow, are presented on the corresponding oscillograms. The mathematical model of the hydraulic fluid power includes simulation modules for controlling the supply of an axial piston pump and changing the moment of resistance on the hydraulic motor. Results. Calculations of the dynamics of a hydraulic fluid power with a high-torque multi-acting radial piston hydraulic motor showed the need for preliminary modeling of its operation modes, taking into account the nature of the external loading, the moment of inertia and the intensity of the pump flow. Conclusion. The conducted studies can be used in the development of hydraulic fluid power with high-torque hydraulic motors to clarify the technological processes of work during the operation of a hydraulic machine.

Effect of Dynamic Swiveling Torque and Eccentricity on the Design of Compensator Cylinders for a Variable Displacement Axial Piston Pump–Modelling & Simulation

Effect of Dynamic Swiveling Torque and Eccentricity on the Design of Compensator Cylinders for a Variable Displacement Axial Piston Pump–Modelling & Simulation

Wear prediction of YN/Si3N4 tribopair lubricated with seawater based on time-series prediction Group Method of Data Handling (GMDH) method

The time-series prediction Group Method of Data Handling (GMDH) method is proposed to predict the friction coefficient of YN9X/Si3N4 tribopair of seawater hydraulic axial piston pump based on wear experiment. The effects of surface roughness, YN nickel content, load, sliding speed and temperature on the friction and wear interaction and sensitivity of YN/Si3N4 tribopair in seawater were studied by orthogonal test. The influence of corrosion wear on the tribological properties of YN material was quantitatively analyzed. The experimental results show that YN9X has better tribological properties than other materials in artificial seawater. Raman spectrum analysis proves that there is tribological reaction between the friction pairs, which can effectively improve the tribological properties of the materials.

Numerical and experimental analysis of rotor-bearing system for axial piston pump with misalignment–rubbing coupling fault

Rotor-bearing system is widely utilised in axial piston pumps in which misalignment–rubbing mixed faults commonly occur, as rubbing and misalignment are both frequent and can occur simultaneously owing to the different working conditions. The dynamic behaviour of a rotor-bearing system misalignment–rubbing coupling defect supported by a ball bearing is complex. To examine the vibration characteristics of such faults, a rotor-bearing system model is established, and the dynamic responses of the rotor-bearing system with a misalignment–rubbing mixed fault are investigated under different parameters. The effects of the rotation speed, tilt misalignment, rubbing clearance, and rubbing stiffness on the rotor-bearing system with a misalignment–rubbing defect are examined via numerical simulations. Additionally, validation studies are conducted, and the results indicate that the distinctive frequency components may be useful for the detection of hybrid faults in rotor-bearing system. The degree of tilt misalignment has little influence on the rubbing frequencies. In the coupling fault, the rubbing fault is prominent. The frequency components of the system are dominated by high frequencies as the speed increases. The major fault feature in the frequency band is a double frequency. The numerical results are in agreement with the experiment. These results reveal the vibration characteristics of the rotor-bearing system for axial piston pump with misalignment-rubbing coupling fault.

Condition Monitoring of an Axial Piston Pump on a Mini Excavator

The focus of this paper is to show the process of developing a condition monitoring system for an axial piston pump mounted on a mini excavator. This work outlines some previous condition monitoring work on axial piston pumps but addresses the lack of research conducted on mobile hydraulics. The valve plate of the pump is chosen as a case study to demonstrate varying degrees of wear and damage to represent healthy and faulty pump conditions. The wear and damage of these valve plates is measured using an optical profilometer, and efficiency measurements were conducted to characterize the fault levels. Once the faults were characterized, the mini excavator was introduced and instrumented to demonstrate what parameters were being considered. Next, three duty cycles were introduced: controlled, digging, and different operator cycles. The controlled cycles are a very repeatable condition that eliminated the need of an operator. The digging cycle was more of a realistic cycle where an operator dug into a loose pile of soil. The different operator cycle is the same as the digging cycle, but a different operator was employed. The sensors that proved to be the most useful in detecting valve plate faults were the drain pressure, pump port pressures, engine speed, and pump displacement. Fault detectability accuracies of 100% were achievable under the controlled cycle utilizing the Fine KNN classification machine learning algorithm. The digging cycle could achieve a fault detection accuracy of 93.6% using the same algorithm and sensors. Finally, the cross-compatibility between a model trained under once cycle and using data from another cycle as an input was investigated. This study showed that a model trained under the controlled duty cycle does not give reliable and accurate fault detectability for data run in a digging cycle, below 60% accuracies. However, cross-compatibility may be achievable if more extreme faults are present. This work concluded by recommending a diagnostic function for mobile machines to perform a preprogrammed operation to reliably and accurately detect pump faults.