One of the keys to maximizing the service life of hydraulic components is to ensure that internal sliding parts are adequately lubricated to minimize friction and wear. Depending on the nature of the lubricated contacts, “adequate lubrication” ideally involves separation of the two sliding surfaces by a film of oil, known as full-film, or hydrodynamic lubrication. Forming and maintaining hydrodynamic lubrication depends on surface geometry, speed, load, and the viscosity of the lubricant.

Figure 1. Hydrostatically balanced cylinder loading two lubricated surfaces.

On the other hand, if oil film thickness does not exceed the combined roughness of the two lubricated surfaces, reducing friction and wear depends on the chemical characteristics of the lubricant. This condition is called boundary lubrication. Boundary lubrication is undesirable and should be avoided, where possible, by maintaining the oil viscosity required for full-film lubrication. Otherwise, chemicals must be added that react with component surfaces to form soft, soap-like films. These chemicals are known as anti-wear additives.

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

Balancing hydrostatic force
Hydraulic components are unique in that it is often possible to offset or balance hydrostatic forces to reduce loads on lubricated surfaces. Reducing surface loading improves full-film lubrication, and, therefore, boundary lubrication conditions are less likely to occur.

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

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

The rod-side area of the piston, area B, is 80% of area A. This means 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 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 4. Cross-section of axial piston showing blocked balance drilling.

The same principle applies to a typical axial-piston pump or motor, as 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 swashplate. System pressure also acts on area B — the balancing area of the slipper — via the channel through the center of the piston. Area C is the sliding area of the lubricated 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 the force transmitted to area C is half of the force developed by area A and is spread over 1.4 times the area, further reducing the load on the lubricated surfaces.

If the hydrostatic balancing force is lost (no pressuring is 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 depends on the hydrostatic balance of the cylinder, boundary lubrication conditions will occur, and two body abrasion will be likely.

Figure 5. Mushrooming of slipper surface reduces effective balancing area.

Applied to an axial piston, this is equivalent to blocking the balance channel by contamination, Figure 4. As a consequence, all of the force developed by the pressure acting on area A is transferred to area C, almost certainly resulting in boundary lubrication and two-body abrasion between slipper and swashplate.

Steady hydrostatic balance
Because hydrostatic force is a product of pressure and area (F = P × a), hydrostatic balance is affected by changes in either pressure or area. As illustrated in Figures 3 and 4, blockage of balance channels by contamination results in loss of hydrostatic balance. Wear caused by two-body and three-body abrasion can also affect hydrostatic balance by altering the areas on which balancing pressure acts or by reducing the effective balancing pressure.

Figure 6. Advanced ‘mushrooming’ and wear of piston slipper.

To illustrate this, consider the sliding surface of an axial-piston slipper. The inner edge of the slipper’s sliding surface is a load concentration point. Deformation of this area can result in localized contact (two-body abrasion) between slipper and swashplate. This causes the slipper’s surface to mushroom, Figure 5, which creates an area that acts to increase the hydrostatic force, and therefore load, on the lubricated surfaces of the slipper and swashplate. Once the slipper begins to mushroom, a cycle of increased load and wear ensues, Figure 6, leading to eventual loss of hydrostatic balance and slipper failure.

Figure 7. Heavy scoring of piston slipper.

The slipper’s sliding surface acts as a seal for hydrostatic balancing pressure. If this surface becomes severely scored from three-body abrasion, Figure 7, leakage can increase to the point where a pressure drop develops between the piston area and the slipper’s balancing area. This reduces the hydrostatic balancing force, and, therefore, increases the load on the lubricated surfaces of the slipper and swashplate. The cycle of increased load and wear that ensues, leads eventually to loss of hydrostatic balance and slipper failure, Figure 8.

Figure 8. Failure of piston slipper caused by loss of hydrostatic balance.

To learn more about the construction of hydraulic components, their modes of failure and how to prevent them, read the author’s book ‘Preventing Hydraulic Failures’ available at: www.PreventingHydraulicFailures.com.

Brendan Casey has more than 20 years experience in the maintenance, repair, and overhaul of mobile and industrial hydraulic equipment. For information, visit www.hydraulicsupermarket.com.