The quality of a hydraulic system depends not only on the rationality of its design and the performance of its components, but also on the measures taken for contamination protection and control. System contamination directly impacts the operational reliability of the hydraulic system and the service life of its components. Statistics indicate that approximately 70% of hydraulic system failures-both domestically and internationally-are caused by contamination.
Fluid Contamination
The primary hazards of fluid contamination to the system are as follows:
1. Contamination-Induced Component Wear
Various contaminants within the hydraulic fluid induce different forms of wear on components. When solid particles enter the clearances between moving parts, they cause cutting wear or fatigue wear on the component surfaces. Solid particles carried by high-velocity fluid flows can impact component surfaces, resulting in erosion wear. Furthermore, water present in the fluid, along with byproducts resulting from fluid oxidation and degradation, exert corrosive effects on components. Additionally, air entrained within the system fluid can trigger cavitation, leading to the pitting and destruction of component surfaces.
2. Component Clogging and Seizing Failures
Solid particles can clog the clearances and orifices within hydraulic valves, causing valve spools to jam or seize. This impairs operational performance and can even lead to serious accidents.
3. Accelerated Degradation of Fluid Properties
The presence of water and air in the fluid-along with thermal energy-constitutes the primary conditions for fluid oxidation. Moreover, microscopic metal particles within the fluid act as significant catalysts for this oxidation process. Additionally, water and suspended air bubbles in the fluid significantly reduce the strength of the lubricating film between moving parts, thereby diminishing lubrication performance.
Types of Contaminants
Contaminants are substances present in hydraulic fluid that exert harmful effects on the system. They exist in the fluid in various forms; based on their physical state, they can be categorized into solid contaminants, liquid contaminants, and gaseous contaminants.
Solid contaminants can be further classified into *hard contaminants*-such as diamond particles, machining chips, silica sand, dust, wear metals, and metal oxides-and *soft contaminants*-such as additives, condensed water droplets, fluid decomposition products and polymers, and cotton fibers or lint introduced during maintenance.
Liquid contaminants typically consist of fluids that do not meet system specifications (e.g., incompatible oils), water, paint residues, chlorine, and various halides. These substances are often difficult to remove once introduced; therefore, when selecting hydraulic fluid, it is essential to choose a product that strictly adheres to the system's standards to prevent unnecessary failures. Gaseous contaminants primarily consist of air entrained within the system.
These particles are often so minute that they fail to settle out, remaining suspended within the hydraulic fluid; ultimately, they become lodged within the clearances of various valves. For a reliable hydraulic system, these clearances are of paramount importance-critical for achieving precise and effective control.
Sources of Contaminants:
The primary pathways through which contaminants enter the system fluid can be categorized into the following aspects:
1. Externally Ingression: External contaminants primarily consist of atmospheric grit or dust, which typically enter the system through the reservoir's air vents, cylinder rod seals, and the shafts of pumps and motors. This type of contamination is predominantly a consequence of the operating environment.
2. Internal Contaminants: These are contaminants remaining from various stages of component processing, assembly, testing, packaging, storage, transportation, and installation. While the occurrence of such residues during these processes is unavoidable, it can be minimized; indeed, certain specialized components require assembly and testing to be conducted within a cleanroom or on a clean bench environment.
3. System-Generated Contaminants: These include particles generated during system operation due to component wear; sand grains dislodged from castings; metal particles flaking off pumps, valves, and fittings; rust flakes peeling from internal piping surfaces; and particles or gummy residues resulting from the oxidation and decomposition of the hydraulic fluid itself. A particularly severe issue arises when system piping-prior to being formally commissioned for operation-has not undergone flushing, resulting in the retention of substantial quantities of impurities.
System Maintenance
Prior to formal commissioning, a hydraulic system typically undergoes a flushing procedure. The objective of this flushing is to purge any contaminants, metal shavings, fibrous compounds, core residues, and other debris remaining within the system. Failure to remove such impurities can-even if it does not result in immediate, catastrophic system failure-trigger a series of malfunctions during the initial two hours of operation. Therefore, the system's oil circuit should be cleaned by following the steps outlined below:
1. Clean the hydraulic reservoir using a fast-drying cleaning solvent, then use filtered compressed air to remove any residual solvent.
2. Thoroughly clean all system piping; in certain instances, it may be necessary to immerse the pipes and fittings in a cleaning solution.
3. Install oil filters within the piping network to protect the supply and pressure lines leading to the valves.
4. Install a flushing plate onto the manifold block to temporarily replace precision valves-such as electro-hydraulic servo valves-during the cleaning process. 5. Verify that all piping dimensions are appropriate and that connections are correctly made.
If the system utilizes an electro-hydraulic servo valve, allow me to offer a brief additional note: the servo valve's flushing plate must be configured to allow hydraulic fluid to flow from the supply line to the manifold and return directly to the reservoir. This facilitates continuous fluid circulation, thereby flushing the system and enabling the oil filter to capture solid particulates. During the flushing process, inspect the oil filter every 1 to 2 hours to prevent it from becoming clogged with contaminants; during this time, the filter bypass valve must remain closed. If the oil filter shows signs of clogging, replace it immediately.
The duration of the flushing cycle should be determined based on the system's configuration and its level of contamination. Once a sample of the filtration medium reveals few to no foreign contaminants, install a new oil filter, remove the flushing plate, and install the servo valve to resume normal operation.
Planned Maintenance: Establish a routine maintenance schedule for the system. The following recommendations are suggested for the effective maintenance and upkeep of the hydraulic system:
1. Inspect and replace the hydraulic fluid at intervals of no more than 500 operating hours or three months.
2. Periodically flush the inlet filter of the hydraulic pump.
3. Check the hydraulic fluid for signs of acidification or contamination by other foreign substances; the fluid's odor can serve as a rough indicator of whether it has degraded.
4. Repair any leaks present within the system.
5. Ensure that no foreign particles enter the reservoir through the breather cap, the filter housing plugs, the return line gaskets, or any other openings in the reservoir.
