This document discusses lubricants used in hydraulic systems. It covers the key properties of hydraulic oil including viscosity, lubricity, and viscosity index. Viscosity determines a lubricant's ability to form protective films and is affected by temperature and pressure. Additives are used to improve oxidation resistance, anti-wear properties, and prevent foaming. Proper lubricant selection and maintenance like regular filter changes are important to ensure optimal system performance.
This document discusses lubricants used in hydraulic systems. It covers the key properties of hydraulic oil including viscosity, lubricity, and viscosity index. Viscosity determines a lubricant's ability to form protective films and is affected by temperature and pressure. Additives are used to improve oxidation resistance, anti-wear properties, and prevent foaming. Proper lubricant selection and maintenance like regular filter changes are important to ensure optimal system performance.
This document discusses lubricants used in hydraulic systems. It covers the key properties of hydraulic oil including viscosity, lubricity, and viscosity index. Viscosity determines a lubricant's ability to form protective films and is affected by temperature and pressure. Additives are used to improve oxidation resistance, anti-wear properties, and prevent foaming. Proper lubricant selection and maintenance like regular filter changes are important to ensure optimal system performance.
This document discusses lubricants used in hydraulic systems. It covers the key properties of hydraulic oil including viscosity, lubricity, and viscosity index. Viscosity determines a lubricant's ability to form protective films and is affected by temperature and pressure. Additives are used to improve oxidation resistance, anti-wear properties, and prevent foaming. Proper lubricant selection and maintenance like regular filter changes are important to ensure optimal system performance.
Wipulasundara B.Sc. Eng (Hons)UOM, PG Dip UOM, AMIESL Dept. of Mechanical Engineering, Sri Lanka Institute of Information Technology HYDRAULIC OIL
Hydraulic oil is the main medium of energy transfer
in hydraulic systems. Tasks of the oil include; • Lubrication of the interior of the hydraulic components • Prevention of metal parts from corrosion • Dissipation of heat • Wash away particles from the components LUBRICATION • Process of reducing friction between relatively moving surfaces • The principal of supporting a sliding load on a friction reducing film is lubrication • Without lubrication system components will wear excessively • Lubrication will cause heat reduction • Water is a lubricant too • But petroleum oil is better as it forms a durable fluid film. LUBRICITY • The liquid’s ability to form a durable fluid film. • The ability is related to natural film thickness and tendency to adhere to a surface. • Petroleum has high lubricity; if poured on a steel plate, would wet or adhere to the surface with a substantial fluid film. • Water would also wet, but, film too thin. • Mercury has a thick film, but less tendency to adhere. VISCOSITY • Liquid molecules are free and therefore in constant motion. • Viscosity is the resistance of liquid molecules to slide past each other. • A form of internal friction. • The viscosity of honey is high and water is low. • Absolute viscosity is kinematic viscosity corrected to overcome differences in specific gravity. • Centipoise/ Specific Gravity = Centistoke (The SI unit of viscosity) MEASURING VISCOSITY • In memory of George M. Saybolt, who developed a viscosimeter for US Bureau of Standards, Saybolt Universal Second (SUS) is used. • A liquid is heated to a specific temperature • The time taken for it to fill a 60ml container is the SUS • Eg: 143 seconds or 143 SUS @37.7C • Many hydraulic oils have 150 SUS – 250 SUS @37.7C • Viscosity is always associated with temperature. MEASURING VISCOSITY CNTD… VISCOSITY AFFECTED BY TEMPERATURE • Heating molasses adds energy to molecules and reduces attraction between molecules • This reduces the viscosity of molasses • Generally, viscosity reduces with temperature rise. VISCOSITY AFFECTED BY PRESSURE • Viscosity increases with the rise of pressure. AFFECTS OF VISCOSITY • A high viscosity liquid with more internal resistance to flow, will cause more heat to be generated, than a low viscous liquid. • Higher the viscosity, thicker will be the oil film. This affects lubricity. • As a metal block moves, over a lubricant, the leading edge rises, due to oil resistance, forming a wedge. • The wedge would ward off attempts at penetration, “Hydrodynamic Lubrication.” • Viscosity affects hydrodynamic lubrication. AFFECTS OF VISCOSITY CNTD… AFFECTS OF VISCOSITY CNTD… • Viscosity reduces leakage between clearances of close fitting parts. • Small portions of fluid continuously move in through metal to metal seals and lubricate. • Eg: between piston and bore, spool and valve body, gears and housing. • For the best seal, clearance is minimum, if too little, less lubrication will cause heating, compromise between sealing and lubrication, lubricant viscosity must be appropriate. VISCOSITY INDEX • The relationship between temperature and viscosity are plotted according to ASTM (American Society of Testing Materials) standard. • If a rapid change in viscosity is recorded with temperature change, then the “Viscosity Index” is low. (minimum 0). • When fluctuations are minimum, the Index is high. (Highest 100) • For hydraulic applications, the index must exceed 90. VISCOSITY INDEX CNTD… OIL OPERATING RANGE • In situations where temperature fluctuations are high, the operating range of oils becomes important. • Too high a viscosity will prevent the lubricant entering clearances. • Too low a viscosity will cause the film to be too thin. • Therefore the operating range needs to be selected according to the application. OIL OPERATING RANGE CNTD… POUR POINT • Wax structures form, and petroleum oil does not flow at very low temperatures. • “Pour Point” is the lowest temperature at which the oil will pour under ASTM conditions. • For low temperature operating conditions, the pour point must be at least 20 F below the lowest expected temperature. OIL PROBLEMS AND ADDITIVES • High pressure lubrication, oil oxidation and contamination cause problems. • At high pressure, hydrodynamic wedge may break down. Additives are added to oil to prevent this. These are anti- wear additives. • Oils may oxidize (combine with oxygen), and form weak acids and soaps. Acids weaken surfaces and soap blocks clearances. • Oxidized oil adheres less, and feels like water when touched between fingertips. • Rust and oxidation inhibitors are also additives. Oils are then referred to as R and O oils. OIL PROBLEMS AND ADDITIVES CNTD… • When air bubbles entrained in oil are not released when entering the reservoir, foaming occurs. • This oil causes noise and oxidation when re-circulated. • Avoiding leaks in the system is one solution. Using chemicals is another. • Anti foam additives casue bubbles to join, become larger and burst on oil surface. • Air bubbles can be visually detected. An erratic high pitch noise in the pump is another indication, like fire crackers being lit in the pump. Erratic actuator movements and gauge readings may result. DIRT IN OIL • Water, air and dirt are contaminants of oil. • Dirt is insoluble and maybe produced as a result of internal moving parts. • Accumulate due to lack of servicing. • Filters and regular maintenance will reduce dirt. • When filter indicators show “Need service” maintenance should be performed. • If limits exceed, oil is made to by pass filter.` MAINTENANCE • Oil supply drums should be stored in clean dry places with no rain water leaks. • Wipe the drum cover before unplugging and confirm the oil is correct before pouring. • Regularly check oil level (minimum reached), leaks, filters. • Filters must be cleaned. Washing in hot soapy water and blowing off with clean air. A soft (new) paint brush for rubbing. • Filters can be placed within an ultrasonic device. Recommended for filters below 40 micrometers