With the same test apparatus as in a previous report (1) the bearing/seal characteristics of the liquid film between a valve plate and a cylinder block of axial piston pumps and motors were experimentally studied in the case of a solution type, high water base fluid. The kinematic viscosity was 2.30 mm²/s at a test inlet temperature of 40°C.
The results obtained are as follows.
(1) The difference in viscosity on the sliding part between the petroleum base hydraulic fluid and the high water base fluid can become smaller than that for an inlet temperature for both fluids. This is due to the difference in the properties concerning to heat transfer and in the leakage flow rate.
(2) In the case of the high water base fluid, the reduction in the load-carrying capacity is compensated for the increase in both the inclination of the wedge film and in the amplitude of the squeeze film. Then, generally, the fluid film can be as thick as the hydraulic fluid.
(3) The lost power due to the frictional torque is smaller in the case of the high water base fluid, but that due to the leakage flow in the case of the hydraulic fluid. Since the latter is the dominating cause for this test, the total lost power is smaller in the case of the hydraulic fluid.
(4) In the case of the high water base fluid, the tolerance of fitting and the existence of the absorbed substances have a more considerable affect on the characteristics than the hydraulic fluid, and cavitation erosion and wear are also more severe.

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In this report, some effects of the water contents of water-glycols, which are typical water-base fire-resistant hydraulic fluids, on the cavitation characteristics at a circular orifice are mainly presented. All the results were obtained experimentally. Tap water and petroleum-base hydraulic fluid were also tested to compare the experimental results with the water-glycols. The water contents of the water-glycols tested in this experiment were from 46 wt% to 90 wt%, the diameter and length of the orifice were 2 mm and 1.5 mm, respectively. The pressures in the higher reaches, P_ui and in the lower reaches, P_u, in this experiment were 1 MPa <.P_u<6MPa and 0.1 MPa <P_i <3 MPa, respectively.
The information below is some of the results of this report.
(1) When the water-glycols with a higher water content were tested at the temperature T = 20°C and the pressure P_u= 2.1 MPa, flow coefficients at the orifice clearly reached their local maximums near the points of cavitation inceptions.
(2) When the water-glycols with a lower water content were tested at the temperature T = 20°C, cavitation coefficients at the points of cavitation inceptions increased with an increase in the pressure P. (3) When the water-glycol with a water content of 90 wt% were tested at the temperature T = 20°C, cavitation coefficients at the points of cavitation inceptions decreased with an increase in the pressure P_u.

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