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 then depletes itself of enough fluid to fill the original air cavity. If the reservoir volume is small and the air cavity is large, the reservoir would become empty or near empty. The discovery of oil depletion may or may not be made in time to protect the pump and prevent aeration of the lines, depending upon reservoir design.

Foaming — Foaming normally occurs in the reservoir because of liquid impinging on the fluid surface, entraining air bubbles. Foaming also results from dissolved air being released because of an increase in fluid temperature. Foaming affects system performance because fluid entering the pump is no longer “solid” but an air-oil mixture, which causes cavitation and also results in a spongy system.

Temperature Effects — A temperature rise in a system containing air affects the bulk of modulus of the air-oil mixture. The bulk modulus, which may be much lower than, desired because of free and entrained air, new drops further. (See graph) An increase in temperature also tends to liberate more dissolved air. This effect is usually evidenced by the increased amount of foaming in the reservoir.

Erosion and cavitation — Cavitation is the formation of a cavity, as a partial vacuum in a fluid or as a gas-filled space in a liquid. One need only look at the gears or pistons of a hydraulic pump, which has been starved of fluid. The result is usually severe surface eroding or pitting of even the hardest materials. The actual mechanics of this erosion are not too well understood. There are theories about “vapor bubble implosion” and “accelerated oxidation”. Regardless of the physics that cause the cavitation, it is something to avoid. To prevent it, a “solid” system is necessary.

Extensive studies of the actual mechanics of erosion are being conducted in several major laboratories. Preliminary data seems to indicate that the presence of dissolved air in system fluid accelerates erosion.

The following definitions will help one understand the problems resulting from air in hydraulic systems.

Free air is that trapped in a system, but not totally in contact with a fluid. It is neither entrained nor dissolved; it is entrapped. An example of free air is an air pocket in a system; it can be removed by bleeding.

Entrained air is that suspended in a fluid and normally exists the form of small bubbles. Filters or screens that have a low bubble point can remove entrained air

Dissolved air is that in solution in a fluid. Because it is neither free nor entrained air, it does not behave according to Boyle’s law. It does, however obey Henry’s law: the weight of gas dissolved in a liquid is proportional to the pressure of the gas. It can be removed by two methods: subjecting the fluid to a reduced pressure and/or raising the fluid temperature. Its presence or absence does not affect fluid volume.