Main hydraulic oil is broadly classified into following 3 types.
General hydraulic oil
Hydraulic oil specially used in general hydraulic machines.
Antifriction characteristics, shear stability of viscosity, oxidation stability etc. suitable for hydraulic machines.
Wear-resistant hydraulic oil
Hydraulic oil specially suitable for use in hydraulic machines used at high pressure and high temperature.
Equipped with a grill so as to prevent wear.
Flame retardant hydraulic oil
General petroleum-based hydraulic oil used in hydraulic system having risk of ignition or fire. Phosphoric acid ester system, water – glycol system etc. known for use in aircraft are available. Special material, seal packing, paints etc. are used depending upon type of hydraulic oil.
Types and features of flame retardant hydraulic oils
Lubricant oil comes with a problem of “burning”. In case of using mineral oil based hydraulic oil at work places where heat source of high temperature is present, it has the risk of causing fire due to this “burning”. Flame retardant hydraulic fluid that prevents burning of hydraulic oil is used in such situation.
Use of different types of fluids that prevent burning from inside the liquid(synthetic type) and use of water in hydraulic base oil(hydrous type) can be given as examples of method of preventing burning of hydraulic oil. Types of flame retardant hydraulic oils are given in table 1.
Table 1 Types of flame retardant hydraulic oil
Whether phosphoric acid ester conforms to the sealing or packing inside the hydraulic cycle or not must be checked. Moreover, it is widely used as hydraulic oil of aircrafts even if extremely expensive.
Fatty acid ester based have excellent lubricating property and stability and conformity to sealing and packing inside hydraulic cycle is good and change from mineral oil based is simple. However, flame retardation property is poorer than phosphoric acid ester based oils.
Water – glycol system is widely used. More than 30% of water is required for assuring flame retardation property and addition of metal soap used as wear preventing agent and alkalinity adjusting agent for its stability makes it alkaline. Therefore, compatibility with metal seal used inside hydraulic cycle and paints etc. must be checked.
W/O emulsion system is extremely cheap and is excellent in terms of compatibility with hydraulic cycle and in terms of waste treatment. This does not have major problem for practical performance and hence, being used as a substitute for water – glycol system in the scenario where regulation of total quantity of factory water drainage is mandatory.
O/W emulsion system has high water content of 90 ~ 95% which is not used.
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