Barbara Zardin

The Critical Speed of Slipper Bearings in Axial Piston Swash Plate Type Pumps and Motors

A stationary model is adopted to determine the critical condition for which the slipper moves away from the swashplate in an axial piston machine. The aim of the analysis is to find the critical speed, i.e. the value of the machine speed for which the slipper moves away from the swashplate; usually this condition may determine bad operating behaviour of the machine, although a retainer plate is used to maintain the slipper sufficiently near to the swashplate. The influences of the pressure transition in the cylinder, the swashplate angle and the radial clearance between piston and cylinder on the critical speed are depicted. Successively, the role of the position of the point of application of the resultant force due to the slipper-retaining plate contact is analyzed.Copyright

The Influence of Cavitation and Aeration on Gear Pumps and Motors Meshing Volumes Pressures

The paper describes the influence of the fluid modeling on cavitation and aeration detection in external gear pumps and motors inter-teeth volumes during the gears meshing process, in order to compare the results coming from the use of different physical models of air release/adsorption and cavitation. A simplified cavitation model is firstly involved, and pressure transients are calculated imposing a pressure cut when the fluid vapor pressure (or the dissolved air partial pressure) is reached. After, assuming an equivalent approach able to involve the vapor phase generation, the cavitation phenomena in the meshing volumes are deepened, and the influence of the fluid modeling enhancement on the cavitating machine behavior is highlighted. Then, the equivalent fluid approach is enhanced introducing the air release, and properly coupling the gaseous phases release/adsorption to the Henrys Law for not instantaneous processes. Finally, the influence of the air release/adsorption time constant on meshing volumes pressure transients are detailed, with particular attention devoted to the modification introduced by the cavitation detailing on the gaseous phase void fraction determination and on the angular extension of the cavitation phenomena detection.Copyright

MODELLING AND SIMULATION OF THE HYDRAULIC CIRCUIT OF AN AGRICULTURAL TRACTOR

This work aims to analyse a complete hydraulic system of a medium size agricultural tractor, in order to perform an energy dissipation analysis and to test possible alternative configurations and solutions. The fuel consumption and energy dissipation in off-road vehicles have infact become a key feature, given the great attention devoted to the need of reducing pollutant emissions, in order to satisfy the future emission limits. Standard and alternative architecture configurations are modelled and compared on the basis of the power consumption to perform the same duty cycle. Among the results, it is worth highlighting that a relevant percentage of energy may be saved with simple modifications in the hydraulic power generation group.Copyright

Dynamic Modelling of an Off-Road Vehicle for the Design of a Semi-Active, Hydropneumatic Spring-Damper System

In this paper a tool integrating a multibody full car model of a tractor and the hydraulic model of the suspension system is presented as a virtual tool able to help the designer and also the control tuning of the system. The full car approach is chosen in order to be able to describe all the vehicles movements (roll, yaw, pitch) while the detailed lumped parameters model of the hydraulic suspensions allows to consider the role of the electrohydraulic valves, accumulator, hydraulic actuator on the vehicle dynamic behaviour. The hydraulic model and the multibody model exchange forces and displacements at the joint points: one between actuator and sprung mass (chassis) and the other one between actuator and unsprung mass (front axle and wheels).Experimental test have been performed (suspension leveling maneuvers, tests on ISO 50008 track, bump tests) and the results of the numerical model have been compared with the experimental data, allowing the understanding of the influence of the numerous design and control parameters involved in the determination of the vehicle dynamic behaviour. The influence of mechanical and geometrical parameters on the damping force hysteresis measured during levelling test are shown and described. Finally, the dynamic behavior of the suspension is analyzed making reference to a dynamic test over a bump.Copyright

Axial Balance of External Gear Pumps and Motors: Modelling and Discussing the Influence of Elastohydrodynamic Lubrication in the Axial Gap

This paper focuses on the analysis of hydraulically balanced external gear pumps, in particular on the lubricating gap between the bushes and the gears. This topic is of key importance for the optimization of the machine efficiency because it both influences the mechanical-viscous friction and the volumetric losses.In this paper the intent is to investigate the role of the elastic deformation of the bushes surfaces, to compare the analysis done with rigid and elastic surfaces and finally to correlate the results with the design of the bushes balancing surfaces. A numerical procedure for the determination of the pressure distribution inside the gap bounded by gears sides and the bushes internal surfaces is presented and applied.With respect to past works of the authors, the procedure has been integrated taking into account the elastic deformation of the internal surfaces of the bushes and the variation of the dynamic viscosity of fluid, two well recognized phenomena that can play a key role on the determination of the bushes behaviour and lubricating gap pressure distribution.It is shown that, when the design of the bushes rear surfaces determines a strong balancing thrust, the bushes themselves need to tilt strongly with respect to the gears to generate an opportune widening thrust to avoiding contact with the gears. Useful suggestions for the bushes balancing surface design may be drawn from the analysis of the balancing maps reported in the paper, which illustrate the widening thrust magnitude and position in both the pure hydrodynamic and elasto-hydrodynamic cases, for different tilted positions and operating conditions.Copyright