air bubbles, or can affect the controllability of parts of the hydraulic system. The physics behind the gas contamination phenomenon was described in detail in Section 2.7 of this chapter.
For every type of contaminant, three different sources can be distinguished:
1 Native contamination. Contaminant particles can be present in the system as residual from the manufacturing of the components or from the assembly or repair of the system. Residual from machining or welding processes, or excessive sealants, can also be present in a brand‐new hydraulic system before the hydraulic fluid is introduced for the first time. Humidity from the environmental air can also cause undesired amount of water to condense in the components.It is also important to remark that a new hydraulic fluid may not be clean enough for a modern hydraulic system. Manufacturing, handling, and storage processes for the hydraulic fluids need to be strict enough to guarantee that the level of contaminants complies with the requirements of the hydraulic system.
2 Ingressed contamination. All the three types of contaminants (solid, liquid, gas) can enter the hydraulic system from the surrounding environment. The airflow into the reservoir, to compensate the changes in the fluid level during the operation of the system, might bring dirt particles and excessive humidity. The breather caps typically used in hydraulic reservoirs include filters to limit the amount of contaminants entering the system. Another typical source of ingressed contamination can be through hydraulic cylinders. Particularly when the cylinder is worn, the contaminants (dust and dirt particles) can enter from wiper seal during the retraction of the piston rod. This is a typical problem of many off‐road hydraulic applications such as construction equipment.
3 Internally generated contamination. Many hydraulic components are subjected to wear. The most critical components subjected to wear are usually hydraulic pumps and motors. These components have internal parts in relative motion, and consequently, particles can be removed from the interior parts. These particles can further promote wear or cause damage to other surfaces not subjected to wear.
Numerous studies have identified the fluid contamination as the most frequent cause of failure of a hydraulic system. Therefore, it is important to always take all possible precautions against possible cavitation damages when designing or maintaining a hydraulic system.
Necessary precautions against gas cavitation were already described in Section 2.7: the designer has to make every possible effort to eliminate or limit chances of entrained air or fluid pressure falling below the saturation conditions.
As pertains to liquid contamination, usually related to an undesired water content in the oil, it is important to maintain the water level below the saturation conditions. As a matter of fact, like air, water can also dissolve into the hydraulic fluid without interfering with the main fluid properties. For many hydraulic machines, the humidity of the surrounding air does not cause the water level within the working fluid to rise above saturation conditions. However, instances of water ingression from the reservoir or from worn cylinders can cause the water level to rise to levels that hamper the safe operation of the system.
Several techniques are nowadays available for removing water from hydraulic fluids using devices that are generally called “separators.” Separating tanks are a common method based on the gravity and take advantage of the higher density of water with respect to most of the hydraulic fluids. If there is a sufficient resident time for the fluid in the hydraulic reservoir, water tends to settle at the bottom, which makes its removal possible. Other techniques include centrifugal separation, coalescing separation, and absorbent polymer separation [27]. These methods are sometimes used in special filters.
Solid contamination is always unavoidable in a typical hydraulic system due to all the three sources of contamination listed above. For this reason, one or more filter elements that ensure proper cleanliness level of the working fluid need to be present in a well‐designed hydraulic system.
2.8.1 Considerations on Hydraulic Filters
The hydraulic components most affected by solid contaminants are those where mechanical parts are in relative motion with minimum clearances. This is the case of hydraulic pumps and motors, linear actuators, and hydraulic control valves. Ideally, a thin film of fluid is maintained in the gaps between the moving parts to avoid solid‐to‐solid contact. In presence of solid contaminant, the particles can cause erosion or abrasion or can even block the relative motion, as shown in Figure 2.14. The figure shows different particle sizes to make evident that the most dangerous particles are those of a size comparable with the gap height. Particles of the smallest size have high chances of passing through the gap without causing significant damages to the solid surfaces. Particles bigger than the gap height will not enter the gap, although they may cause erosion at the gap entrance regions. Finally, particles similar in size to the gap height will likely enter the gap and engage with frequent contacts with the surfaces. In the worst case they can get stuck between the mechanical parts and leave the gap region after causing severe abrasion damages.
When selecting the features of a hydraulic filter, it is important to keep in mind the geometrical clearances of the components used in each system. Table 2.7 gives a general reference on the typical clearances within hydraulic components. To provide the reader with a tangible reference for these values, it is worth mentioning that the diameter of a human hair ranges from 50 to 150 μm and the diameter of a grain of salt is from 80 to 200 μm. In inches, 10 μm corresponds to 0.000 39 in. Considering that a human cannot see objects smaller than 20–40 μm, the most dangerous particles cannot be seen by a naked eye!
Figure 2.14 Solid particles entering the clearances of a hydraulic control valve.
Table 2.7 Typical clearances in hydraulic components.
Source: Assofluid [11] and Parker Hannifin [28].
| Component | Clearance [μm] |
|---|---|
| Vane pumps (vane tip) | 0.5–1 |
| Gear pumps (side plates) | 0.5–10 |
| Piston pumps (piston to bore) (valve plate) | 5–40 0.5–5 |
| Servo valves (spool to sleeve) | 1–4 |
| Directional valves | 2–20 |
| Cylinders | 50–250 |
ISO standard 4406 [29] defines a method for quantifying the solid contamination level of a hydraulic fluid. The standard is based on the counting, through proper experimental procedures, of the number of particles per milliliter of fluid. The contamination level is expressed by three numerical codes, which are determined according to the convention defined in Table 2.8.
Based on the number of particles counted in each size range, the corresponding codes are assigned according to Table 2.9.
