Understanding hydrostatic balance

Hydraulic components are unique in that it is often possible to offset or balance hydrostatic forces to reduce loads on lubricated surfaces. By reducing surface loading, the maintenance of full-film (hydrodynamic) lubrication is improved and therefore boundary lubrication conditions are less likely to occur.

Hydrostatic force is the product of pressure and area. Expressed mathematically: F = P x a. The balancing or offsetting of hydrostatic force is achieved by exposing opposing areas to the same pressure. The double-acting cylinder in Figure 1 illustrates this concept.

Figure 1. Hydrostatically balanced cylinder loading two lubricated surfaces.

The rod-side area of the piston, area B, is 80% of area A. This means that the force exerted on the lubricated surfaces at the end of the cylinder rod is 20% of the force developed by the pressure acting on area A. This is due to the balancing or offsetting force developed by the same pressure acting on area B. Assuming the speed of the rotating surface (C) and fluid viscosity are adequate, full-film lubrication of the sliding surfaces is achieved.

Figure 2. Typical cross-section of an axial design piston.

The same principle applied to a typical axial design piston is illustrated in Figure 2. Area A is exposed to system pressure during outlet (pump) or inlet (motor) and the force developed is transmitted to the lubricated surfaces of the slipper and swash plate. System pressure also acts on area B, the balancing area of the slipper, via the drilling through the center of the piston. Area C is the sliding (lubricated) area of the slipper. While the ratio of these three areas varies, in this particular piston, area B is 50% of area A, and area C is 140% of area A. This means that the force transmitted to area C is half that developed by area A and is spread over 1.4 times the area, further reducing the load on the lubricated surfaces.

Figure 3. Loss of hydrostatic balance increases load on the lubricated surfaces.

If the hydrostatic balancing force is lost, that is there is no pressuring acting on area B (Figure 3), the force exerted on the lubricated surfaces at the end of the cylinder rod will be 100% of the force developed by the pressure acting on area A. If full-film lubrication was dependent on the hydrostatic balance of the cylinder, boundary lubrication conditions will eventuate and two body abrasion is likely. To learn more about the construction of hydraulic components, their modes of failure and how to prevent them, read Preventing Hydraulic Failures.

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