Basic Hydraulics Hydr 1305 Chapter 3 Petroleum Based

Basic Hydraulics Hydr 1305 Chapter 3 Petroleum Based Hydraulic Fluid

Petroleum Based Hydraulic Fluid o Hydraulic fluid is used to transmit energy from a pump to an actuator or MACHINED STEEL SURFACE MAGNIFIED hydraulic system. o Hydraulic fluid also provides lubrication for the moving parts within a hydraulic system. § To reduce friction in hydraulic systems lubrication occurs when the hydraulic fluid forms a layer of oil between two surfaces. • Referred to as hydrodynamic lubrication. § This prevents the surfaces from making direct contact. § Even though surfaces may appear smooth they are very rough when highly magnified and without lubrication the peaks of one surface will plough through the peaks of the other surface.

Petroleum Based Hydraulic Fluid • Lubricity o As unlubricated surfaces slide over each other heat is built up and may cause the two surfaces to weld to each other. o Thus to reduce friction a lubricant is used to prevent contact between the surfaces. o Lubricity is the lubricating film’s ability to form a durable film between two surfaces. o The ability of the film to provide a durable lubricating surface depends on: § the fluid’s natural film thickness § the fluid’s tendency to adhere to a surface

Petroleum Based Hydraulic Fluid • Lubricity o Petroleum oil has good lubricity. § When petroleum oil is poured on the surface of a steel plate, it will wet the surface, adhere to the surface, and form a durable film. § When water is poured on the surface of a steel plate, it will wet the surface too, but can be easily penetrated, making it have poor lubricity. o A liquid with good lubricity adheres to a surface and develops a substantial continuous film. § Petroleum oils have possess these characteristics. o Both energy transmission of a fluid and its lubricity are dependent upon the fluid’s viscosity. § Fluid viscosity is one of the most important characteristics of a petroleum based fluid.

Petroleum Based Hydraulic Fluid • Viscosity o Both energy transmission of a fluid and its lubricity are dependent upon the fluid’s viscosity. § Fluid viscosity is one of the most important characteristics of a petroleum based fluid. o Viscosity is related to the resistance of a fluid sliding over itself. § Viscosity is the internal friction of a fluid's resistance to flow; that is how thick the oil is. § The greater the viscosity, the greater the resistance to flow. o Viscosity can be described as how strong an affinity the molecules have to be attracted to each other. § Mutual affinity of liquid molecules is much stronger than the affinity of gaseous molecules, but not as strong as the affinity of solid molecules. § Liquid molecules, while having a strong affinity for other molecules of the liquid, they move easily over each other.

Petroleum Based Hydraulic Fluid • Viscosity - Temperature o A bottle of molasses taken from the refrigerator has very slowly moving molecules. § The cold molasses has a high resistance to flow, it pours slowly. o Heating the molasses in a sauce pan adds energy to its molecules and decreases the attractive forces between its molecules. § With less resistance to flow, the molasses pours much more freely. o Generally, as temperature increases, viscosity decreases. • Viscosity - Pressure o As pressure increases, viscosity increases. o In common industrial hydraulic fluids, viscosity increases by as much as 40 percent when pressure is increased from 0 to 3000 psi.

Petroleum Based Hydraulic Fluid • Viscosity – Affects Heat Generation o Fluid flowing with a higher viscosity will cause more heat to be generated than fluid flowing with a lower viscosity fluid. § This is because of the internal friction of a fluid's resistance to flow.

Petroleum Based Hydraulic Fluid • Saybolt Universal Second o The Saybolt Universal Second (SUS or SSU) is one measure of viscosity. o The SI unit of viscosity is the centistoke (CST). o To measure viscosity a Viscosimeter is used. o The technique evolved liquid in a container with a corked bottom hole of specific diameter. § The cork is pulled and the time required for 60 milliliters of fluid to drain out is measured in seconds. o If an oil at 100 F required 143 seconds to drain 60 ml, then the SUS rating would be 143 SUS @ 100 F.

Petroleum Based Hydraulic Fluid o In many hydraulic systems, the fluid viscosity is 150 – 250 SUS VISCOSIMETER

Petroleum Based Hydraulic Fluid • Viscosity Affects Hydrodynamic Lubrication o An important effect of viscosity is its ability to lubricate between close-fitting moving parts. o Frequently, the seal in many hydraulic systems, is metal to metal with a continuously lubricating fluid leaking between them. o Locations of lubricating leaking fluid. § § § between pistons and the cylinder bore vane pumps and the vane tips between gears and the housing in gear pumps between pistons rings and cylinder bores between spools and bodies in control valves o The clearances should be very small, but still allow fluid to pass for lubrication.

Petroleum Based Hydraulic Fluid SLIDING SURFACES ROTATING SLIDING SURFACES

Petroleum Based Hydraulic Fluid • Viscosity Affects Clearance Flow o Hydrodynamic lubrication makes use of the viscosity of the fluid acting as a resistance (to not getting out of the way) causing a solid object to lift and be propelled over the surface of the fluid. § When surfaces are moving, they are lubricated by hydrodynamic lubrication which causes a fluid wedge to develop between them. § The greater the viscosity, the larger the wedge, but the effect has to be balanced with the heat generation of the fluid. o As long as the pressure on the upper body does not become excessive, the fluid wedge resists attempts at penetration. o However, at start up or at too heavy a load, the fluid wedge could be lost. § There is no wedge before two objects begin sliding against other but there is a thin film of oil. § Too heavy of a load will collapse the wedge resulting in metal to metal contact.

Petroleum Based Hydraulic Fluid • Viscosity Affects Clearance Flow o If the fluid has the appropriate viscosity, lubrication will be adequate. o Too much fluid leakage can be harmful to the system, however, too little leakage can limit the lubrication of the system.

Petroleum Based Hydraulic Fluid • Viscosity Affects Clearance Flow o If the fluid has too high of a viscosity, lubrication flow through the clearances will be reduced which could cause damage to the surfaces. o If the fluid has too low of a viscosity, excessive fluid will pass through the component’s metal to metal seals causing a reduction in the fluid flowing through the system and a build up of heat.

Petroleum Based Hydraulic Fluid • Viscosity Index o About the viscosity of hydraulic fluid: § viscosity of hydraulic fluid does change with temperature § viscosity is important in the performance of a hydraulic system § hydraulic systems are not and can not be maintained at constant temperature o Therefore a hydraulic fluid whose viscosity remains relatively constant with temperature is desirable. o The viscosity index of an oil indicates how the viscosity of the oil changes with temperature.

Petroleum Based Hydraulic Fluid • Viscosity Index o The American Society for Testing Materials (ASTM) provides a method for assessing the viscosity of a fluid according to its viscosity and temperature. o First the viscosity is measured at one temperature and then at a different temperature. § A line is drawn between these to points which provides the viscosity of the fluid at any temperature between the two.

Petroleum Based Hydraulic Fluid • Viscosity Index o If for Oil A, the viscosity at 100 F was determined to be 153 SUS and at 210 F was determined to be 44 SUS then these two points are plotted on semi-log paper and a straight line drawn between them. o For Oil B, the viscosity at 100 F was determined to be 165 SUS and at 210 was determined to be 42 SUS and these two points also plotted. o The comparison of the two plots reveals that Oil A is less sensitive to temperature changes than Oil B.

Petroleum Based Hydraulic Fluid

• Friction is a force that can _____ a moving body The best answer/response is: a) energize b) stop c) retard d) all of the above e) both a) and b) but not c) f) both b) and c) but not a)

• Some surfaces appear to be very smooth when viewed with the naked eye, but are shown to be very rough and with peaks when viewed under high magnification The best answer/response is: a) True b) False

• Friction occurs because of the roughness of the surface and minute welding that occurs when peaks are sheared and expose fresh material on each surface The best answer/response is: a) True b) False

• Surface roughness when viewed under high magnification reveals asperities that appear somewhat like mountain peaks The best answer/response is: a) True b) False

• When two apparently smooth surfaces are caused to slide over each other, the peaks (asperities) of one surface ploughs through the opposite surface causing wear – this gouging is a principal cause of friction The best answer/response is: a) True b) False

• When the sliding surface asperities of two mating surfaces expose new surface, small microwelds can occur that must be broken for continued motion and this breaking of asperity welds is a cause of friction The best answer/response is: a) True b) False

• To reduce friction between two mating surfaces, a fluid film is caused to be place between the surfaces The best answer/response is: a) True b) False

• Lubricating films placed between two mating surfaces in itself causes friction by is resistance to slide over itself – that sliding fluid resistance is called _____ The best answer/response is: a) specific gravity b) organic bonding c) density d) viscosity e) inorganic bonding f) none of the above

• Introducing a fluid film between to mating solid surfaces by itself does little to reduce the friction between the solids The best answer/response is: a) True b) False

• A film of water between two mating solid surfaces will form a very durable agent for separation of the solid surfaces The best answer/response is: a) True b) False

• Every liquid can form a separating film between mating solid parts The best answer/response is: a) True b) False

• Only a few petroleum oils have been found to form a durable separating film between two solid mating parts The best answer/response is: a) True b) False

• The term “lubricity” means the ability to form a durable film between two mating solid surfaces The best answer/response is: a) True b) False

• The ability of a film to form a durable lubricating surface depends on _____ The best answer/response is: a) the fluid’s natural film thickness b) the fluid’s tendency to adhere to a surface c) both a) and b) d) neither a) nor b)

• When petroleum oil is poured on a metallic surface is generally beads up and only partially wets the surface The best answer/response is: a) True b) False

• When water is poured on a surface of steel, it flows out easily and wets the entire surface, but has poor resistance to penetration making it have poor lubricity The best answer/response is: a) True b) False

• A liquid with good lubricity will adhere to the surface and develop a substantial continuous film The best answer/response is: a) True b) False

• A petroleum based hydraulic fluid has two important functions, these are _____ The best answer/response is: a) act as a medium for transmitting energy and provide good lubricity b) wet the surface and transmit heat c) provide good surface tension and transmit heat d) all of the above e) none of the above

• Both energy transmission and lubricity of a fluid are dependent on the _____ of the fluid The best answer/response is: a) specific gravity b) molecular weight c) wettabilty d) viscosity e) both a) and b) f) both c) and d)

• Fluid viscosity is one of the most important characteristic of a petroleum based fluid The best answer/response is: a) True b) False

Petroleum Based Hydraulic Fluid • Viscosity Index o When first adopted an oil with a very high change with temperature was given an index of 0 and the one with the lowest change with temperature was given the value of 100. § The higher the index number, the less the oil's viscosity changes with temperature o With enhanced refinery practices, oils can now have an index value greater than 100.

Petroleum Based Hydraulic Fluid • Oil Operating Range o Petroleum oil is an excellent lubricant for a hydraulic system, but not at all viscosities. o If the viscosity of the oil is too low, the fluid film would be too thin and not be as good a lubricant. o If the viscosity is too high, the fluid film would be too thick to flow through the bearings and not be a good lubricant. o In selecting an oil the operating temperature range and the viscosity must be matched.

Petroleum Based Hydraulic Fluid • Pour Point o Pour point is the lowest temperature at which petroleum oil will flow. o At extremely low temperatures, petroleum oil does not flow – a paraffin wax structure forms and hinders the flow o The pour point for various oils should be at least 20 F below the lowest temperature anticipated in use.

Petroleum Based Hydraulic Fluid • Oil Problems and Additives o Additives have been developed to respond to problems that can occur in routine operation of a hydraulic system: § high pressure lubrication § oil oxidation § oil contamination with water § air bubbles § dirt o Additives cannot solve every fluid problem and some additives are not compatible with others.

Petroleum Based Hydraulic Fluid • Antiwear Additives o Antiwear (AW) and Wear Resistant (WR) Additives can be divided into three types. § oiliness or lubricity agent § combination with the metal surface agent § extreme pressure (EP) additive o Oiliness additives attach to the surface of the metal and create a chemical film that acts like a solid that supports the load and allow the moving part slide. o The Metal Adhering Additives combine with the metal to form a protective film and smooth's out the surface. o Extreme Pressure Additives form a film on the metal surface as high frictional heat is generated. § Under high heat the metal surfaces attempt to weld to each other. § Under high heat the EP additive becomes effective.

Petroleum Based Hydraulic Fluid • Antiwear Additives o All three basic types of additives are not used in the same fluid at the same time. o When oiliness agents are used, they are generally at low pressure, generally below 1000 psi. o When extreme pressure (EP) additive are added, they are generally at pressures above 3000 psi. o Manufacturer’s product sheets tell if the lubricant is designed for high pressure. § Gulf Oil Company – “Harmony 48 AW” § Sun Oil Company – “Sunvis 816 WR” o If excessive wear has been a problem, the switching to an oil with antiwear additives will probably help the problem.

Petroleum Based Hydraulic Fluid • Oil Oxidation o Oxidation is a process in which oxygen combines chemically with a material – including oil. o Oxidation of oil occurs and can be pinned down to two system locations. § reservoir outlet § pump inlet o In the reservoir, free oil reacts with oxygen in the air which produces weak acids and soaps. § Acids will cause pitting and weaken component surfaces. § Soaps coat surfaces and can plug pressure-sensing orifices and lubrication paths. o Heat is a major contributor to oxidation in the reservoir o As a rule of thumb oil oxidation rates double for every 18 -20 F above a reservoir temperature of 130 F.

Petroleum Based Hydraulic Fluid • Oil Oxidation o Oxidation can occur if air bubbles are present at the pump suction line as a result of an air leak or the returning fluid is churning up in the reservoir. o The air bubbles suddenly collapse at the pump outlet and temperature can rise to 2100 F locally when the bubble is compressed from 0 to 3000 psi. o The high temperature fries the oil forming resinous products and caused the oil to acquire a characteristic burnt odor. o When a hot surface is encountered, such as the pump rotor or the pressure relief valve, the resin comes out of solution and coats the component forming a varnish or lacquer on the surface. o This coating can cause moving parts to stick.

Petroleum Based Hydraulic Fluid • Oil Oxidation o This varnish coating can cause moving parts to stick. o The resinous material can also form a sludge by combining with dirt and float around the system and plug filters and small openings, and reduce heat transfer to walls. o Collapsing air bubbles is a major cause of rapid oil degradation (cavitation). o Check for oxidized oil by comparing the suspect oil with new oil out of the drum. § New oil will stick to your fingers and feel very slippery. § Oxidized oil feels very much like water – it exhibits little body and small tendency to adhere. § Oxidized oil may have a pungent odor o If an oil is suspect, send it to the lab for examination or simply replace it with fresh oil.

• Molecules of a liquid show a strong affinity to be attracted to each other The best answer/response is: a) True b) False

• Mutual affinity of liquid molecules is much stronger than the affinity of gaseous molecules The best answer/response is: a) True b) False

• Viscosity is a type of internal resistance of liquid molecules to flow or slide paste each other The best answer/response is: a) True b) False

• Generally, as temperature increases, viscosity decreases The best answer/response is: a) True b) False

• The Seybolt Universal Second (SUS) is a measure of viscosity used in the U. S. Customary System of units while the centistoke (CST) the measure of viscosity in the SI units of measure The best answer/response is: a) True b) False

• Generally, as pressure increases, viscosity decreases The best answer/response is: a) True b) False

• High viscosity fluids are useful in separating mating surfaces, but they generate more heat when in motion The best answer/response is: a) True b) False