Solid particles multiply continuously, particularly in enclosed hydraulic systems.
These particles flow through the hydraulic system under high pressure and at high speed and sandblast its components (pumps, valves, seals and cylinders).
The tiniest particles which are either equal in size or smaller than the critical dynamic clearances (< 5 µ) are the most dangerous . They have the ability to overcome the tightest tolerances and cause damage to one or both surfaces.
Control pumps and servo-valves have critical clearances of 1-10µ and, as a result, are highly sensitive to fine dirt particles. It is, therefore, not surprising that most manufacturers of this type of component state in their literature that the stanbdard of the oil used be grade better than NAS 6 or ISO 15/12, in order to avoid downtime and thus ensure a long life span.
A pump with a flow rate of 200 l/min. and contaminated oil (NAS 12 or ISO 21/18) has to pump more than 3,000 kg of dirt particles per annum. This same pump with a clean oil (NAS 4 or ISO 12/9) only needs to process 6kg.
In addition, metals are highly active catalyst and accelerate the oxidation process of the oil. This oxidation process is neutralised by antioxidation additives which are present in the oil. Through a process of polymerisation they adhere to the metallic particles and thus prevent direct contact between the oil on the one hand, and the metal on the other. However, new metallic particles require constant neutralising, which means that the additives are slowly being depleted.
Small metallic particles play an important part in this process as they represent such an enormous surface area (the smaller the particles, the greater their surface per unit of volume).
Consequence: the higher the pollution, the faster acid develops in the oil and the faster the oil loses all its physical and chemical properties. A change of oil is then the only solution.
Causes of servo-valve malfunctions
Valve jet steering: bigger particles can stick within this narrow passage and cause malfunction of the valve steering.
First stage steering: the ball needle has to steer the positioning of the first stage spool. But very small metal particles( < 10µ ) cause wear of the ball surface.
The edges of a servo valve’s spool have a zero-overlap. By small metallic particles these edges will wear out, resulting in fluid leakage and valve malfunction.
In case of a high resin presence, the resins can completely stick the spool. Also metal particles can stick between the narrow tolerances and block the valve completely.
A = Used oil
B = New oil
C = Oil after filtration
how by - pass filtration works -
The Kleenoil filter cartridge will remove particles down to 1 micron (3 absolute) and totally remove water.
The principle for filtering particulate matter is ‘liquid liquid chromatography’ which is in effect allowing a fluid to drain down a surface which will progressively arrest particles. This is achieved by having the tissue rolled on a core.
Oil is passed up the core of a paper roll where it collects in a cavity between the lid of the filter housing and the paper roll. It is then forced down between the layers of the tissue where particles are adsorbed within the matrix created by millions of cellulose fibres which form the tissue layer. The principle for filtering water is capillary absorption into the hollow vegetable fibres of the cellulose tissue. The molecular structure of the oil is too large to be absorbed by ‘capillary action’ into the fibre's, however the water is absorbed into the fibres and separates from the oil. Water retention is approximately 1 quart per pound of tissue.
As a structure ... The Kleenoil filter cartridge is a tightly would long fibre paper product made from specific trees that allow the absorption action of the long fibres to draw the water from the oil without surrendering the ability to filter the oil through its dense fibres. The paper has only been pulped once to maintain its strength but to allow the cartridge to be manufactured. The cartridge employs a 4 –stage filtration process. The main body of the cartridge is made from the long fibres paper described above which is rated to 3‐microns absolute. Approximately half way down the cartridge, another 1 micron disk is inserted to help even further increase the cartridge efficiency levels. This disk helps to further reduce the ISO cleanliness levels of the oil and is especially helpful in engine applications that have soot issues, like the EGR engines. Once passing through the 1 micron disk the third stage is another 3 micron filtration level (the bottom half of the cartridge) and then finally on the bottom of the cartridge there is another 1 micron disk inserted (being the 4th stage of filtration).
with reference to the question of additives - will it remove them ? .... no - while the filtration unit is extracting the water and the contaminant, it is continuously safeguarding the desirable elements compounded within the actual oil in use. These typically include, dispersants, detergents, oxidation and rust inhibitors, metal de‐activators, pour‐point depressants, viscosity improvers, EP agents, friction modifiers, fungicidal, anti‐foaming and gelling additives. These additives are held in suspension and their levels can be critical if the oil is to maintain its beneficial qualities.