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 SO (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−SO curve irrespective of the operating conditions such as the supply pressure or the rotation speed.

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