Chapter 2

Learning Objectives 1. Explain the primary functions of a hydraulic fluid. 2. Define the term fluid. 3. Distinguish between a liquid and a gas. 4. Appreciate the properties desired of a hydraulic fluid 5. Define the terms specific weight, density, and specific gravity. 6. Understand the terms pressure, head, and force 7. Differentiate between gage pressures and absolute pressures. 8. Calculate the force created by a pressure 9. Understand the terms kinematics viscosity and absolute viscosity. 10. Convert viscosity from one set of units to another set of units. 11. Explain the difference between viscosity and viscosity index.

Primary functions of hydraulic fluid 1. Transmit power 2. Lubricate moving parts 3. Seal clearances between moving parts 4. Dissipate heat

Why do we study the properties of fluid

- To improve efficiency of fluid power system - To determine the necessary maintenance - To select the desired fluid - System Calculation - Load collection - Fluid improvement

(challenging list ) 1. Good lubricity 2. Ideal viscosity 3. Chemical stability 4. Compatibility with system materials 5. High degree of incompressibility 6. Fire resistance 7. Good heat-transfer capability 8. Low density 9. Foam resistance 10. Nontoxicity 11. Low volatility

Define the term fluid. FLUIDS GASES LIQUIDS incompressible compressible

Physical differences between liquids and gases. Parameter Liquid Gas Has its own volume Volume is determined by container Volume Takes shape of container but only to its volume Expands to completely fill and take the shape of the container Shape Incompressible for most engineering applications Readily compressible Compressibility

Air features Advantages 1. It is fire resistant. 2. It is not messy. 3. It can be exhausted back into the atmosphere. Disadvantages of using air versus using hydraulic oil are: 1. It cannot be used in in applications where accurate positioning or rigid holding is required. 2. Because air is compressible, it tends to be sluggish 3. Air can be corrosive, since it contains oxygen and water. 4. A lubricant must be added to air to lubricate valves and actuators. 5. Air pressures of greater than 250 psi are typically not used due to the explosion dangers involved if components such as air tanks should rupture. This is because air (due to its compressibility) can store a large amount of energy as it is compressed in a manner similar to that of a mechanical spring.

The most important fluid properties are Density Pressure Viscosity

Appreciate the properties desired of a hydraulic fluid. Weight Versus Mass All objects, whether solids or fluids, are pulled toward the center of the earth by a force of attraction. This force is called the weight of the object and is proportional to the object’s mass, as defined by (2-1) where, in the English system of units we have F = force in units of lb. W = weight in units of lb, m = mass of object in units of slugs. g= proportionality constant called the acceleration of gravity, which equals 32.2 ft/s2 at sea level.

EXAMPLE 2.1 Find the weight of a body having a mass of 4 slugs Solution Substituting into Eq (2-1) yields W=mg= 4 slugs ×32·2 ft/s2= 129lb

Specific Weight Since the container has the shape of a rectangular solid, its volume can be calculated using Eq. (2-2). Substituting values yields

It has been found by measurement that 1 ft3 of water weighs 62. 4 lb It has been found by measurement that 1 ft3 of water weighs 62.4 lb. Specific weight is defined as weight per unit volume. Stated mathematically, we have or (2-3) where  = specific weight (lb/ft3), W = weight (lb), V = volume (ft3).

Specific weight of water specific weight of water in units of lb/ft3, specific weight of water in units of lb/in3,

EXAMPLE 2-2 If the body of Example 2-1. has a volume of 1.8 ft, find its specific weight. Solution Using Eq. (2-3) we have

Specific Gravity The specific gravity (SG) of a given fluid is defined as the specific weight of the fluid divided by the specific weight of water. Therefore, the specific gravity of water is unity by definition. The specific gravity of oil can be found using Substituting the most typical value of specific weight for oil we have

EXAMPLE2-3 Air at 8°F and under atmospheric pressure has a specific weight of 0.0752 lb/ft3. Find its specific gravity Solution

Density Density is defined as mass per unit volume: where  = density (slugs/ft3), M= mass (slugs), V = volume (ft3).

Obtaining SG by the means of density of the given fluid

EXAMPLE Find the density of the body of Examples 2-1 and 2-2 Solution Using

FORCE, PRESSURE, AND HEAD

Pressure - It is defined as force per unit area - It is the amount of force acting over a unit area where p — pressure, F force, A area.

Units of pressure 1. [lb/ft2] if F and A have units of lb and ft2 2. [lb/in2] changing the units of A from ft2 to in2 3. [psf] knowing that the total force acting at the bottom equals the 62.4-lb weight of the water: 4. [psi] 1 ft2 = 144 in2, The pressure at the bottom of the container can be found in units of lb/in2 as follows

Pressure Head It is a 1-ft column of water develops at its base a pressure of 0.433 psi. What happens if the column height is not 1 ft? What happens to the pressure if the fluid is not water?

What happens if the column height is not 1 ft? Each foot of the 10-ft head develops a pressure increase of 0.433 psi from its top to bottom.

Discussion The pressure at the base is Remember 10 ft3 of water and each cubic foot weighs 62.4 Ib, the total weight of water is 624 lb.

What happens to the pressure if the fluid is not water?

Assuming a weight density of 57 lb/ft3, the pressure at the base is The specific weight of oil is somewhat less than that for water

Calculation of the pressure developed at the bottom of a column of any liquid. Where p- pressure at bottom of liquid column,  - specific weight of liquid, H - liquid column height or head. units for pressure:

Pressure Categories 1. Absolute pressure – pressure at a point in a fluid relative to a vacuum (absolute zero of pressure) 2. Gauge Pressure – pressure relative to local atmospheric pressure. 3. Differential Pressure – difference between two unknown pressures, neither of which is atmospheric pressure.

Forms of pressure Differential pressure Absolute pressure Gauge pressure

Relationship between absolute and gauge pressures Pabs=Pgauge+Patm

Atmospheric Pressure

Understanding atmospheric Pressure It is a pressure produced by air column weighs 14.7 lb by in2 Standard atmosphere pressure the value of 14.7 lb/in2

Gage and Absolute Pressure Gage pressures Absolute pressures they are measured relative to the atmosphere They are measured relative to a perfect vacuum. They are labeled psig, or simply psi They are labeled psi (abs), or simply psia

Measuring atmospheric pressure The specific weight of mercury is 0.490 lb/in3: This means that : a column of mercury equal to 30.0 in equals to head produces a pressure of 14.7 psi.

BULK MODULUS It is used to express incompressibility. The higher the bulk modulus, the less compressible or stiffer the fluid. The minus sign indicates that as the pressure increases on a given amount of oil, the oil’s volume decreases, and vice versa.

Self study!!! Comparisons Temperature Comparisons Pressure Comparisons Length. Mass, and Force Comparisons with English System Self study!!!

Viscosity

-The top plate is moving with velocity v relative to the bottom plate, which is stationary. The fluid molecules in contact with the bottom plate are at rest, and those in contact with the top plate are moving at velocity v. In between, a velocity profile is established. If y is the distance between the plates, the slope of the velocity profile is - Suppose that the moving plate has an area A and it requires a force F to keep it moving at velocity v. Shear stress in the fluid between the plates is

Dynamic and Kinematic viscosities Dynamic viscosity (or absolute viscosity) It is the ratio between the shear stress and the slope. Kinematic viscosity It is simply the dynamic viscosity divided by the fluid density measured at the same temperature as the dynamic viscosity measurement. where μ - dynamic viscosity ρ - density

Checking units for p in the English system Viscosity is often expressed in the CGS (centimeter-gram-second) metric system. in the CGS metric system A more convenient unit is the centipoise, abbreviated cP.

Units for kinematic viscosity are given as follows: English: ft2/s, SI metric: m2/s, and CGS metric: cm2/s. A viscosity of 1 cm2/s is called a stoke. Centistokes viscosity is typically reported in centistokes (CS).

The use of an oil with too low a viscosity can lead to several problems. 1. It can result in a loss of pump (and motor) efficiency due to increased internal leakage. (Clearances are not sealed.) 2. It can cause increased component wear due to breakdown of the lubrication film. 3. At high operating speeds and high operating pressures, the lubrication film can breakdown completely, which will cause the moving parts to “spot weld” together and ultimately cause a complete failure. The use of an oil with too high a viscosity can cause the following problems: 1. Pump cavitation—the oil is so “thick” that it does not flow readily into the pump. The pump is filled partly with oil and partly with air, a condition known as cavitation . 2. High pressure drops occur due to friction in the lines.

Saybolt Viscometer

It consists of a. an inner chamber containing the sample of oil to be tested. b. A separate outer compartment, which completely surrounds the inner chamber, Contains a quantity of oil whose temperature is controlled by an electrical thermostat and heater. 3. A standard orifice is located at the bottom of the center oil chamber. Operation When the oil sample is at the desired temperature, the time it takes to fill a 60-cm3 container through the metering orifice is then recorded. The time, (t), measured in seconds, is the viscosity of the oil in official units called Saybolt Universal Seconds (SUS). Since a thick liquid flows slowly, its SUS viscosity value will be higher than that for a thin liquid.

Calculations Dynamic viscosity A relationship exists between the viscosity in SUS and cS. This relationship is provided by the following empirical equations: Kinematic viscosity it is common practice in the fluid power industry to use viscosity expressed in units of SUS or cS.

Capillary Tube Viscometer

VISCOSITY INDEX Rules 1. Oil becomes thicker as the temperature decreases and thins when heated. 2. The viscosity of a given oil must be expressed at a specified temperature. 3. It is a general rule of thumb that the viscosity should never fall below 45 SUS or rise above 4000 SUS regardless of the temperature.

What is Viscosity index (VI)? it is a relative measure of an oil’s viscosity change with respect to temperature change. An oil having a low VI is one that exhibits a large change in viscosity with temperature change. A high-VI oil is one that has a relatively stable viscosity, which does not change appreciably with temperature change. Oils exist with VI values well above 100. A high-VI oil is a good all-weather-type oil

The VI of any hydraulic oil can be found by using Typical curves for a viscosity index test.

ASTM Standard Viscosity-Temperature Chart for Liquid Petroleum