The piston/cylinder interface of swash plate–type axial piston machines represents one of the most critical design elements for this type of pump and motor. Oscillating pressures and inertia forces acting on the piston lead to its micro-motion, which generates an oscillating fluid film with a dynamically changing pressure distribution. Operating under oscillating high load conditions, the fluid film between the piston and cylinder has simultaneously to bear the external load and to seal the high pressure regions of the machine. The fluid film interface physical behavior is characterized by an elasto-hydrodynamic lubrication regime. Additionally, the piston reciprocating motion causes fluid film viscous shear, which contributes to a significant heat generation. Therefore, to fully comprehend the piston/cylinder interface fluid film behavior, the influences of heat transfer to the solid boundaries and the consequent solid boundaries’ thermal elastic deformation cannot be neglected. In fact, the mechanical bodies’ complex temperature distribution represents the boundary for nonisothermal fluid film flow calculations. Furthermore, the solids-induced thermal elastic deformation directly affects the fluid film thickness. To analyze the piston/cylinder interface behavior, considering the fluid-structure interaction and thermal problems, the authors developed a fully coupled simulation model. The algorithm couples different numerical domains and techniques to consider all the described physical phenomena. In this paper, the authors present in detail the computational approach implemented to study the heat transfer and thermal elastic deformation phenomena. Simulation results for the piston/cylinder interface of an existing hydrostatic unit are discussed, considering different operating conditions and focusing on the influence of the thermal aspect. Model validation is provided, comparing fluid film boundary temperature distribution predictions with measurements taken on a special test bench.

Heat Transfer and Thermal Elastic Deformation Analysis on the Piston/Cylinder Interface of Axial Piston Machines

This paper presents a method for evaluating the lubrication characteristics between the piston and cylinder in a swash-plate type axial piston pump-motor under mixed lubrication conditions. A numerical analysis is carried out in order to obtain the metal contact force between the piston and cylinder, and the contact ratio λ is shown to represent the mixed lubrication condition both for the pump and the motor strokes. The contact ratio, λ, is also obtained experimentally by detecting electric resistance between the piston and cylinder in a practical swash-plate type machine. The experimental result is expressed by a relationship between λ and S 0 (the ratio of dynamic pressure to supply pressure) as indicated by the numerical analysis. In conclusion, the mixed lubrication characteristics between the piston and cylinder in a swash-plate type pump-motor is expressed by λ − S 0 curve irrespective of the operating conditions such as the supply pressure or the rotation speed

Mixed Lubrication Characteristics Between the Piston and Cylinder in Hydraulic Piston Pump-Motor

Tribology, lubricants and lubrication engineering — a review

Bearing and sealing gaps are one of the central elements of displacement machines and other hydraulic components.The gap macro and micro geometry as well as the chosen material determine the leakage and friction losses of thedisplacement machines. Besides this the achievable technical parameters of the machine depends deeply on the qualityof the gap design. This paper introduces a new method for the non-isothermal gap flow simulation for self-adjustinggaps. The simulation models have been verified with the help of two basic test rigs for the measurement of friction andtemperature behaviour in the gap between piston and cylinder of an axial piston machine under real operating conditions.

A new approach to the design of sealing and bearing gaps of displacement machines

The design of individual lubricating gaps with their bearing and sealing function mainly determines the loss behaviour, life time, reliability and achievable technical parameters of displacement machines. The gap flow conditions are influenced by the operating parameters, the gap macro geometry, the surface quality and by the chosen surface material. Therefore the determination of the time dependent gap height requires very complex simulation models considering the motion of moveable parts, the interaction between machine parts, the dependency on design and operating parameters and the energy dissipation within the gaps as well as changes of the gap height caused by the flexibility of the applied surface material. This paper introduces a new method for complex gap flow simulation of connected and self-adjusting gaps of axial piston machines, where the model for the piston/cylinder pair was extended to consider elastohydrodynamic effects. The simulation model has been verified with the help of friction force measurements on the piston/cylinder assembly.

Investigation of the gap flow in displacement machines considering elastohydrodynamic effect

Conventional high-pressure hydraulic piston pumps require a delicate balance between clearance, surface finish, and pump materials if a long, efficient life is to be maintained. A cam-operated single cylinder pump has been used to test several pump material combinations using two chemically different hydraulic fluids. The main parameters monitored were the degree of mutual electrical insulation attained and the physical drag of the piston on the cylinder due to both viscous shear and asperity interaction. It was found that for hardened steel pistons and cylinders an optimum surface finish of approximately 8 μinches CLA gave the best results with both fluids. Results are explained on the basis of micro-asperity hydrodynamic lift. Presented at the 29th Annual Meeting in Cleveland, Ohio, April 28–May 2, 1974

F. T. Barwell

Papers

Tribological Interaction between Piston and Cylinder of a Model High Pressure Hydraulic Pump