Internal Leakage Predicition of Hydraulic Spool valves Based on Acoustic Emission Technology

In order to realize the nondestructive testing (NDT) of the internal leakage fault of hydraulic spool valves, the internal leakage rate must be predicted by AE (acoustic emission) technology. An AE experimental platform of internal leakage of hydraulic spool valves is built to study the characteristics of AE signals of internal leakage and the relationship between AE signals and leakage rates. The research results show the AE signals present a wideband characteristic. The main frequencies are concentrated in 30~50 kHz and the peak frequency is around 40 kHz. When the leakage rate is large, there are significant signal characteristics appearing in the high frequency band of 75~100 kHz. The exponent of the root mean square(RMS) of AE signals is positively correlated with the exponent of the leakage rate only if the leakage rate is greater than 2~3 mL/min. This find could be used to predict the internal leakage rate of hydraulic spool valves.

Digital Twin-Driven Mating Performance Analysis for Precision Spool Valve

The precision spool valve is the core component of the electro-hydraulic servo control system, and its performance has an important influence on the flight control of aviation and aerospace products. The non-uniform surface topography error causes a non-uniform mating gap field inside the spool valve, which causes oil leakage and leads to deterioration of the spool valve performance. However, the current oil leakage calculation method only considers the influence of size errors, which is not comprehensive. Thus, how to characterize the mating behavior of the spool valve and its effect on oil leakage with consideration of surface topography errors is the key to evaluating the performance of the spool valve. This paper proposes a new way of analyzing the mating performance of precision spool valves, which considers the surface topography errors based on digital twin technology. Firstly, a general framework for the analysis of mating performance of precision spool valve based on a digital twin is proposed. Then, key technologies of assembly interface geometry modeling, matching behavior modeling and performance analysis are studied. Finally, a quantitative correlation between the mating parameters and the oil leakage of the precision spool valve is revealed. The method is tested on a practical case. This proposed method can provide theoretical support for the accurate prediction and evaluation of the mating performance of the precision spool valve.

Experimental Methods for Measuring the Viscous Friction Coefficient in Hydraulic Spool Valves

In hydraulic components, nonlinearities are responsible for critical behaviors that make it difficult to realize a reliable mathematical model for numerical simulation. With particular reference to hydraulic spool valves, the viscous friction coefficient between the sliding and the fixed body is an unknown parameter that is normally set a posteriori in order to obtain a good agreement with the experimental data. In this paper, two different methodologies to characterize experimentally the viscous friction coefficient in a hydraulic component with spool are presented. The two approaches are significantly different and are both based on experimental tests; they were developed in two distinct laboratories in different periods of time and applied to the same flow compensator of a pump displacement control. One of the procedures was carried out at the Fluid Power Research Laboratory of the Politecnico di Torino, while the other approach was developed at the University of Parma. Both the proposed methods reached similar outcomes; moreover, neither method requires the installation of a spool displacement transducer that can significantly affect the results.

A Numerical and Experimental Methodology to Characterize the Gaseous Cavitation in Spool Valves with U-notches

The performance of spool valves can deteriorate and noise can occur due to cavitation. The noise sound levels caused by cavitation are influenced by many parameters, among which the most important is not-optimal geometry of components. In this paper, a 2 ways - 2 positions directional control valve was studied using experimental and numerical approaches. Tests were performed on a plexiglass body and steel spool analyzing the cavitating area that develops in U-notches. A dedicated test rig was equipped with a high-speed camera placed directly in front of the area of interest where cavitation occurs. Different working conditions were tested by varying the upstream pressure to encourage the development of cavitation. Images were acquired and post-processed, focusing the contour extraction between the liquid and gaseous phases. The images were compared with results from three-dimensional CFD numerical simulations performed using commercial software. The numerical estimation of flow characteristics corroborated the results from investigations carried out using a fast camera, including periodic cavitation structures. This study demonstrates the importance and usefulness of using a three-dimensional CFD approach during the prototyping phase to create quieter component designs.

Simulation of a high frequency on/off valve actuated by a piezo-ring stack for digital hydraulics

Despite being widely used in several applications, commercially available spool valves, both servovalves and proportional valves, are inefficient components because they cause high power consumption due to the large pressure drops across the metering orifices. A recent research field aims at substituting spool valves with on/off valves having high switching frequency (changing state between open and closed in a few milliseconds or less) and producing low pressure drops, in order to make the so-called digital hydraulics possible. In spite of the advantages that it could provide, digital hydraulics does not have significative industrial applications yet, because of the difficulty in manufacturing such high frequency on/off valves. Hence, this paper performs a feasibility study of an on/off poppet-type valve actuated directly by a commercially available ring stack, which is a multilayer piezo-actuator capable of generating very high actuation forces needed for this application. Modulation of the average flow can be achieved by changing the duty cycle of the pulse width modulation (PWM) signal driving the piezostack. An inertance tube could also be used to smooth flow pulsation. The simulations obtained using a detailed Simulink model show that high switching frequency and very effective flow modulation can be obtained with this valve architecture along with low pressure drops and high flow rates, thus making it potentially suitable for digital hydraulics. The disadvantages of this single stage architecture are the large dimensions of the piezo stacks, and the high current generated because of both the high capacitance of the piezo stack and the high frequency switching. However, large-scale production of these components could help to reduce the costs, and the simulations show that limiting the maximum current to 10 A still provides good regulation.

Modeling and theoretical study of relief chamfer method for reducing the flow ripple of a spool valves distribution radial piston pump

This paper presents a study of flow ripple reduction method of a spool valves distribution radial piston pump (SVDRPP). Relief chamfers are adopted to prolong and moderate the opening processes of the delivery spool valves, thus to relieve the pressure surges as well as the consequent flow ripples, vibrations and noises. The mathematical model of this method is established and multiple numerical simulations are conducted to analyze the mechanism as well as the effectiveness of this method. According to the simulation results, different relief chamfer angles have varying influences on flow ripple reduction. Remarkable reduction of flow fluctuation from 43.4% to 36% could be achieved, when the relief chamfer angle is set around 30°. Comparisons between the relief chamfer method and the time delay method indicate that the former has better compatibility to the load pressure lower than the rated value; while the latter has better compatibility to the higher load pressure.

A 4/3-Way Flat Slide Valve With a Metallic Main Stage

Most proportional directional control valves currently in practical use are piston spool valves. A disadvantage of this type of design is the occurring leakage between spool and sleeve or housing. A second disadvantage is that the service life is limited by wear on the control edges. The ceramic flat slide valve is a promising concept, which allows to reduce the mentioned leakage and to increase service life. In contrast to piston spool valves, the main stage of the flat slide valve consists of three plates. Two control plates and a movable slide plate, which is located in between. An external force is applied to the plates by the so-called pressure compensation, which presses the plates together and thus counteracts a gap expansion and thereby an effective reduction of occurring leakage is accomplished. The plate-by-plate design of the main stage allows the use of technical ceramics, which are more resistance to abrasive wear and can thereby extend the service life of valves. A challenge in the realization of the flat slide valve is the design of the pressure compensation. If the force applied by the pressure compensation is not sufficient to prevent a gap formation between the plates, an increased leakage will occur, which results in a reduction of efficiency. In case the applied force is too high due to the pressure compensation, high frictional forces between the plates occur and an adjustment of the slide plate position through an actuator is no longer possible. Therefore, the design of the pressure compensation is essential for the success of the concept. This contribution presents the results of testing a main stage, which consists out of two metallic control plates and a slide plate made of special brass. The main stage is designed in such a way that a 4/3-way directional control valve with a proportional characteristic can be realized, as it is also achieved by valves currently available on the market. Therefore, in addition to the pressure forces, further influences on the resulting force of the pressure compensation were determined and their influence was estimated based on test bench results. With a program written in MATLAB, the forces prevailing in the valve can be calculated time-efficiently for different geometries and are validated with test bench trials.

Development and Testing of Hydraulic ‘Bharath Valve’ to Control Multiple Actuators

The Research is entitled “Design, Fabrication and Performance Testing of Directional Control Valve for Control of Multiple Actuators”. An indexing unit is a direction control valve used in Hydraulic system. This device shall be named ‘Bharath Valve’ (US10180190), as is the name of its Inventor. This valve can control more than one number of actuators. This valve consists of a hollow outer cylinder and inner cylinder. The inner cylinder fits in the hollow portion of outer cylinder; this inner cylinder is operated manually. The rotation of the inner cylinder inside the outer cylinder changes the direction of flow of liquid through the valve this control the movements of actuator. This Indexing valve can become a highly advantageous replacement of the conventional Spool valves that are used for controlling the actuator. In the present scenario one spool valve can control only one actuator thus the number spool valves used in a hydraulic system is equal to the number of actuators. However ‘Bharath’ valve can control more than one number of actuators, this in turn makes the number of system components less and also reduces the overall cost of hydraulic system. Even the controlling of movement of actuators can be easy. A computer interface if given can make this valve versatile and a very cheap alternative to the existing valves, taking number of components, construction, friction and efficiency into account. This research proves that ‘Bharath’ valve has more advantages than that of existing ones and some of the advantages are listed below,  One valve can control more than one actuators thus reduces the number of valves.  Construction of this indexing valve is simpler and parts involved in friction is less.  Reduction in the cost of manufacturing is highly significant.  This valve makes it possible to actuate the desired cylinder keeping the other at rest.  The modification in operating actuators of system can be easily changed by operating ON/OFF flow valve connected to the inlets of Indexing valve.