This paper considers using the cross-angle in variable displacement hydraulic machines. The cross-angle is a fixed displacement angle around the axis perpendicular to the normal displacement direction. The cross-angle changes the angles to the pistons top and bottom dead centres as a function of the fraction of displacement in such a way that the valve plate timing is varied and different pre-compression and decompression angles are obtained. A non-gradient optimisation technique, the Complex method, is used together with a comprehensive simulation model in order to find the optimal cross-angle for a variable displacement hydraulic motor. The paper shows that the cross-angle can be used to reduce noise in variable displacement motors. One issue that makes the motor application more difficult is the increased dependence between outlet and inlet flow ripple which is not found in pump applications. Furthermore, the paper discusses how to use the cross-angle for machines which can work both as a motor and a pump.

Noise is a well known challenge for hydraulic systems and hydrostatic machines is one of the largest noise contributors in a hydraulic system. The noise from the machine originates from flow pulsations in the discharge and suction ports, as well as pulsations in piston forces and bending moments. To the design a quite hydraulic machine is a difficult task where many different objectives need to be considered. This paper presents a generic method for how optimization based on simulation models could be used to design quieter hydraulic machines. In order to stay competitive on a global market an efficient product development process is essential for all manufacturing industries. By using simulation-s tools in the design process, the product can be analysed before the actual product is manufactured. Furthermore, in order to find an optimal design of the machine with respect to noise, a comprehensive dynamic simulation model is needed. The model contains all important noise contributors. In the paper, the simulation models are used together with a non-gradient optimization method in order to find the best possible design. A vital part when using optimization to support design is always to formulate the objective function. As mentioned above, noise is generated from different sources and all these sources need to be considered when the objective function is formulated. For example a design that minimizes flow pulsations in the suction port will surely perform badly in some other objective. Therefore noise minimization could be looked upon as a typical multi-objective optimization problem. It is also not evident how the different objective should be ranked because the observed noise level is strongly depending on the system in which the machine is to be used. The paper also considers whether the objective function should be formulated in time or frequency domain. Traditionally, simulation of machine performance is conducted in the time domain, but the human ear hears noise in the frequency domain and perceives high and low frequencies differently. Furthermore, transformation from piston forces into emitted noise is much higher at high-frequency content than low-frequency content. This makes it natural to formulate the objective function in frequency domain, which raises the question of how the different harmonic should be ranked. In the paper a number of different approaches to formulate the objective function is presented and evaluated. The objectives considered are flow pulsation in both discharge and suction ports, as well as pulsation in piston forces and bending moments. Furthermore, the objectives are studied in both time and frequency domain. The design application is a variable hydraulic machine of bent axis type with nine pistons, which is operated both as a pump and a motor. However, the methods presented in the paper could be applied to other types of hydraulic machines as well. 

Structure borne noise in a machine rises from piston force and bending moments among others. This noise arises directly from the pump shell. In this study, a transfer function methodology is employed for mapping simulated internal pump dynamics, such as piston forces and bending moments, on to structure borne noise. Using these transfer functions, it is possible to predict how, for instance, changed valve plate timing affects simulated piston forces and bending moments and in turn how that will affect audible noise. Hence, it is possible to design an objective function that directly reflects audible noise. The transfer functions are experimentally obtained and are valid for a specific machine shell and to some minor extent the room’s acoustical properties. Also, fluid borne noise is important to consider when designing a quiet machine. Fluid borne noise arises mainly from flow pulsation created inside the machine.

Simulation of the internal pump dynamics, and optimisations, are carried out using a pump model developed in the simulation tool HOPSAN. The design application is a hydraulic machine of bent axis type with seven pistons. The theory outlined and the method proposed in the paper can also be applied to other types of hydraulic machines. The paper shows how both structure borne noise and fluid borne noise can be considered using multi-objective optimisation. The paper shows how different noise reduction features affect the sound pressure level and the flow pulsation. The paper also compare the pump and motor case.

In conventional machines, the pulsations are periodic and originate from the uniform placement of a given number of pistons. This paper discusses the possibilities to introduce non-uniform placement of the pistons. The pulsations periodicity is thus changed, which can have a major impact on the noise level and how the noise is experienced. A number of approaches are presented, evaluated and ranked and the usefulness of the modifications is assessed. This study employs a transfer function methodology to map simulated internal pump dynamics, such as piston forces and bending moments, to audible noise. Using these transfer functions, it is possible for instance to predict how changed valve plate timing affects simulated piston forces and bending moments and in turn how this will affect audible noise. Copyright © 2009 by ASME.

Noise is a well-known challenge in hydraulic systems and hydrostatic pumps are one of the largest noise contributors in a hydraulic system. The existing noise reduction features, such as pressure relief groove and pre-compression filter volume, are more or less dependent on the working condition. It is essential to know the amount of free air when designing a quiet pump; however, it is not evident how much free air the oil contains. The free air content is different if the suction port is boost pressured or self-priming. The amount of free air in a well-designed system can be as low as 0.5% while in others up to 10%.This paper uses the three-transducer method to measure the amount of free air in the oil. The oil's compressibility can be measured for different working conditions and the free air content can then be calculated. The pre-study is performed with an extensive simulation model. Various noise reduction features' sensitivity to free air content is considered.

Noise is a well known challenge in hydraulic systems. Hydrostatic machines are among the largest noise contributors in a hydraulic system.The noise from the machine originates from flow pulsations at the discharge and suction ports, as well as pulsations in piston forces and bending moments.

This article investigates the dynamic behaviour of unsteady flow through a valve plate in an axial piston pump. The proposed extension of the steady state restrictor equation includes a dynamic internal mass term and a resistance. The results from 1D model are validated with a 3D CFD model. Different valve plates’ configurations and pump sizes are easily simulated with the two simulation models. The simulation results show very good comparison with experimental tests. The proposed method is verified with a hydraulic pump application but it can probably also apply for original restrictors too.