Hydraulic Systems Basic Principles types of fluids properties of fluids selection Hydraulic fluids primary task Power transmission Secondary tasks o Lubrication of rotating and translating components to avoid friction and wear o Heat transport away from the location of heat generation usually into the reservoir o Transport of particles to the filter o Protection of surfaces from chemical attack especially corrosion If you use the wrong oil it will shorten the lifespan of your fluid system s parts Air content in oil is harmful Never mix old and new oil Synthetic oil is a man made lubricant that consists of artificially made chemical compounds hydraulic fluid hydraulic oil can be synthetic or mineral based Types of fluids 1 Mineral hydraulic oil petroleum base 2 Phosphate ester based synthetic hydraulic fluids 3 Vegetable hydraulic oils 4 Water glycol synthetic hydraulic fluids Mineral oils are refined petroleum based hydrocarbons Mineral oils are combinations of paraffins napthenes and aromatic oils A wide variety of mineral oil compositions are employed in cutting and grinding fluids The lubrication properties are modified for particular applications using additives DIN german institute for standardisation Hydraulic fluid types Water Water The least expensive hydraulic fluid is water Water is treated with chemicals before being used in a fluid power system This treatment removes undesirable contaminates Advantages o No environmental pollution o No disposal effort o Cheap o No fire or explosion danger o Available everywhere o 4 times larger heat conduction coefficient than mineral oils 2 times higher compression module than mineral oils o o Viscosity does not depend strongly on temperature Disadvantages o Bad lubrication characteristics o Low viscosity o Corrosion danger o Cavitation danger Petroleum Oils Mineral oil The characteristic of petroleum based hydraulic oils are controlled by the type of crude oil used Naphthenic oils have low viscosity index so it is unsuitable where the oil temperatures vary too widely Paraffinic oils have a high viscosity index and they are more suitable for the system where the temperature varies greatly Without additives With additives o Always mixtures of different oils often with additives Additives decrease corrosion increase life duration improve temperature dependence of viscosity improve particle transport Advantages o Good lubrication o High viscosity good for sealing bad for losses o Cheap o Excellent lubricity Reasonable cost Non corrosive Disadvantages o Inflammable o Environmental pollution o Tendency to oxidize rapidly Not fire resistance Water Glycols These are solutions contains 35 to 55 water glycol and water soluble thickener to improve viscosity Additives are also added to improve anticorrosion anti wear and lubricity Advantages Better fire resistance Less expensive Compatible with most pipe properties compounds and seals Disadvantage Low viscosity Poor corrosion resistance not suitable for high loads Water Oil Emulsions These are water oil mixtures The oil in water emulsion has water as the continuous base and the oil is present in lesser amounts as the dispersed media Advantages High viscosity index Oxidation stability Film strength Disadvantage Depletion of water due to evaporation decreases fire resistance Phosphate Ester It results from the incorporation of phosphorus into organic molecules They have high thermal stability They serve as an excellent detergent and prevent building up of sludge Advantages Excellent fire resistance Good lubricity Non corrosive Disadvantage Not compatible with many plastics and elastomers Expensive Properties of Liquids Have no Shape incompressible Transmit force in all Directions Equally Multiply Force required properties best lubrication ability least corrosion problems environmental stability Higher viscosity index Protection against rust Good sealing characteristics Easy dissipation of heat Easy cleaning by filtration Non compressible Fire resistance Anti wearing to its system Tolerance to water resistance to water contamination disadvantage of fluids is their flammability They pose fire hazards mainly from the leakages in high temperature environments properties of a hydraulic fluid i Viscosity ii Viscosity Index iii Oxidation stability iv Demulsibility v Lubricity vi Rust prevention vii Flash point and fire point viii Neutralization Number 1 Viscosity It is a measure of the fluid s internal resistance offered to flow If the viscosity of the hydraulic oil is higher than recommended the system will be affected in the following manner 1 The viscous oil may not be able to pass through the pipes 2 The working temperature will increases because there will be internal friction 3 The consumption of power will increase If the viscosity of the oil is lesser than recommended then 1 The internal and external leakage will increase 2 It cannot lubricate properly and will lead to rapid wear of the moving parts 2 Viscosity Index This value shows how temperature affects the viscosity of oil The viscosity of the oil decreases with increase in temperature and vice versa The rate of change of viscosity with temperature is indicated on an arbitrary scale called viscosity index VI temperature and vice versa The lower the viscosity index the greater the variation in viscosity with changes in temperatures oil products sluggish 3 Oxidation Stability Oil mixes with O2 and can create by products that are fairly acidic These will create oxidation so you want to look for oil that will stay fairly stable even at high The oxidation will be affected by water and wear metals as well as the temperature The most important property of an hydraulic oil is its oxidation stability Oxidation is caused by a chemical reaction between the oxygen of the dissolved air and the The oxidation of the oil creates impurities like sludge insoluble gum and soluble acidic The soluble acidic products cause corrosion and insoluble products make the operation System design should minimize these thermal problems and the fluid should have additives that exhibit good thermal stability inhibit oxidation and neutralize acids 4 Demulsibility The ability of a hydraulic fluid to separate rapidly from moisture and successfully resist emulsification is known as Demulsibility If oil emulsifies with water the emulsion will promote the destruction of lubricating value and sealant properties Highly refined oils are basically water resistance by nature