IT 264 INDUSTRIAL FLUID POWER Chapter 2 Physical Properties of Hydraulic Fluids

INTRODUCTION The single most important material and hydraulic system is the working fluid itself. The fluid characteristics have a crucial effect on equipment performance and life and it is important to use a clean, high-quality fluid in order to achieve efficient hydraulic system operation. A hydraulic fluid as for primary functions:  Transmit power  Lubricate moving parts  Seal clearances between mating parts  Dissipate heat In addition to hydraulic fluid must be inexpensive and readily available

INTRODUCTION CONTINUED To properly cobblers for primary functions and be practical from a safety and cost point of view, he drawing fluid should have the following properties:  Good lubricity  Ideal viscosity  Chemical stability  Compatibility with system materials  High degree of incompressibility  Fire resistance  Good heat transfer capability  Low density  Foam resistance  Non-toxicity  Low volatility

INTRODUCTION CONTINUED No single hydraulic fluid possesses all of the desirable properties therefore fluid power designers must select a fluid it comes the closest to being ideal for a particular application. Additionally hydraulic fluids must also be changed periodically, the frequency depends on the fluid and the operating conditions.

LIQUIDS As discussed in Chapter 1 the term fluid refers to both gases and liquids. A liquid is a fluid that, for a given mass, will have a definite volume independent of the shape of its container. Liquids are considered to be incompressible so there volume does not change with pressure changes.

GASES Gases are fluids that are readily compressible. The volume will vary to fill the vessel containing them. Unlike liquids which have a definite volume for a given mass, the volume of the given mass of gas will increase to fill the vessel that contains. Gases are also greatly influenced by the pressure to which they are subjected. Air is the only gas commonly used in a fluid power system because its inexpensive and readily available, and has the following desirable features:  It is fire resistant  Is not messy  It can be exhausted back in the atmosphere

GASES CONTINUE The disadvantages of using air versus hydraulic oil are:  Due to its compressibility, air cannot be used in applications were accurate positioning or rigid holing is required.  Because it is compressible it tends to be sluggish.  Air can be corrosive since it contains oxygen and water.  A lubricant must be added to air to lubricate valves and actuators.  Air pressures of greater than 250 psi are typically not use due to the explosion dangers involved if components such as air tank should rupture. Due to its compressibility error can store a large amount of energy as it is compressed in a manner similar to that of a mechanical spring

SPECIFIC WEIGHT / SPECIFIC GRAVITY

FORCE AND PRESSURE

ATMOSPHERIC PRESSURE Atmospheric pressure is depression develop the earth surface due to the force of attraction between the atmosphere and the earth. Atmospheric pressure is 14.7 lb/in 2 on earth surface at sea level. This pressure is called atmospheric pressure and this value is referred to as standard atmospheric pressure because atmospheric pressure varies a small amount depending on weather conditions which affect the density of air unless otherwise specified the actual atmospheric pressure will be assumed to be equal to the standard atmospheric pressure.

GAUGE PRESSURE Gauge pressure measured relative to the atmosphere where as absolute pressure is measured relative to a perfect vacuum such as that existing in outer space. For example gauge pressure would read zero pressure instead of 14.7 psi. Pressure readings to Billy are labeled psia for absolute pressure and psig gauge pressure. For example 14.7 psia(Absolute pressure) would equal 0 psig(Gauge pressure).

BULK MODULUS Bulk modulus is the stiffness of a hydraulic fluid system that is directly related to the incompressibility of the oil. Bulk modulus is a measure of this incompressibility the higher the bulk modulus to less compressible or stiffer the fluid. Bulk modulus of an oil changes somewhat with changes in pressure and temperature.

VISCOSITY Viscosity is probably the single most important property of a hydraulic fluid. Disgusting is a measure of a fluid resistance to flow. When the viscosity is low the fluid flows easily and is thin in appearance and when the viscosity is high the fluid flows with difficulty and is thick in a appearance. Viscosity index is a relative measure of an oil's viscosity change with respect to temperature change. And oil having a low viscosity index is one that exhibits a large change in viscosity with temperature change. And oil having a high viscosity index is one that has a relatively stable viscosity which does not change appreciably with temperature