It is known that the quantity of permanent gases dissolved in a fluid depends upon the pressure which exists at the contact surface between the gas and the fluid. A reduced quantity of such permanent gases in the pressure fluid of a hydraulic system is desirable, so that the pressure fluid is as incompressible as possible and the operation of the elements of the hydraulic system by the fluid is as precise as possible. Furthermore, degassing avoids gas cavitation (pseudocavitation), dieseling effects and accumulations of air bubbles within the hydraulic system.
In a known method and apparatus for degassing the pressure fluid of a hydraulic system of the above-mentioned type (German OS No. 2 221 551), the free space of a reservoir containing the pressure fluid of the hydraulic system is not open to the surrounding atmosphere as is usually the case, but rather is connected to a vacuum pump which produces and maintains a vacuum in the free space above the fluid. As a result, the percentage of permanent gases in the pressure fluid is reduced proportionally to the ratio of the pressure in the reservoir with respect to the atmospheric pressure. This known degassing method, however, was unsuccessful in practice, because the degassing was insufficient compared to the added expense. To avoid evaporation losses of the pressure fluid and the changes in characteristics associated therewith, such as viscosity increase and increase of the setting point, it has been suggested that the vacuum in the reservoir be limited, based on the boiling point of the pressure fluid. This limitation of the vacuum, however, prevents effective degassing of the pressure fluid. For example, it is not sufficient to lower the saturation pressure of the dissolved permanent gases from the normal condition, as from 1 bar to 0.45 bar or in an extreme case to 0.1 bar, as is stated in the technical literature (Kurt Blume, "Luft im Hydrauliksystem", Magazine Oelhydraulik und Pneumatik 16, 1972, No. 9, Pages 389 to 392). Experimental tests have shown that the pressure in the pressure fluid can drop at throttle points and at sharp-edged flow deflections to values of 1 mbar under conditions which exist in almost every conventional hydraulic system. In view of these experiments, in conventional vacuum degassing devices the lowest permissible vacuum of 0.13 bar is induced, then the saturation pressure of the dissolved air or other permanent gases in the pressure fluid of the hydraulic system will still be undesirably high. In spite of the degassing, gas cavitation, dieseling effects and accumulation of air bubbles occur within the hydraulic system and lead to the known disadvantageous consequences. It is clear that with such limited effectiveness, the additional expense for a vacuum degassing device of the conventional type is not practicable. Limiting the vacuum induced and regulating it to a sufficiently constant value, which are necessary in the conventional degassing device, in and of themselves require a considerable construction expense and also introduce additional sources of potential breakdown into the entire system.
Accordingly, it is an object of this invention to provide a method and an apparatus for degassing the pressure fluid of a hydraulic system, in which simple means provide extensive degassing without additional evaporation losses, viscosity increase and other undesired changes in characteristics of the pressure fluid in the hydraulic system.