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From the October 1967 issue of Hydraulics & Pneumatics.
One of the greatest attributes of hydraulics as a method of power transmission is great stiffness, which gives a hydraulic system instant, accurate response. Therefore, one of our chief concerns is to make certain that there is no elastic, power absorbing component in a hydraulic system.
Problems resulting from air
Air in the system has the following major effects:
Spongy control - Because fluids are considered to be basically incompressible, we expect great stiffness in a hydraulic system. That is, the positioning of an actuator should be immediate (rapid response) and precise. The larger the amount of free or entrained air, the spongier (softer, less stiff) the system.
Loss of horsepower - When an air pocket is present in an actuator, it is alternately compressed and relaxed as the actuator is cycled. Since the air pocket must first be compressed before the fluid can cause the actuator to move, power is consumed. Upon relaxation, the air pocket expands and rives fluid out. The stored power, therefore, is expended in driving fluid back into the reservoir and not in moving the actuator.
Loss of bulk modulus - Free or entrained air in the hydraulic system reduces substantially the effective bulk modulus of the system. That is, an air-oil mixture appears to increase the compressibility of the fluid, making the system spongy.
Test data seems to indicate that dissolved air has no effect on bulk modulus, providing the air is in solution. These facts, at first, appear paradoxical. However, if one visualizes a container filled to the brim with marbles (which represent the oil molecules) it is possible to pour in a fluid (representing air) around them, or remove the fluid with no change in volume. The weight of the container changes but not the volume.
Loss of system fluid - One of the most serious conditions that can occur in a hydraulic system is the loss of reservoir fluid. The fluid level must be kept high enough to insure enough fluid for the pump intake, otherwise cavitation begins.
A drop in the reservoir level can occur if a large quantity of air, not initially flushed from the lines, makes its way back tot eh reservoir. The reservoir...