Transcript Viscosity

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
LIQUIDS
incompressible
GASES
compressible
Physical differences between liquids and gases.
Parameter
Volume
Shape
Compressibility
Liquid
Has its own volume
Takes shape of container but
only to its volume
Incompressible for most
engineering applications
Gas
Volume is determined by
container
Expands to completely fill and
take the shape of the container
Readily compressible
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. Specific
weight is defined as weight per unit volume. Stated mathematically, we have
or
where
 = specific weight (lb/ft3),
W = weight (lb),
V = volume (ft3).
(2-3)
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
W 129 lb
3



71
.
6
lb/ft
V 1.8 ft 3
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
( SG ) air
 air
0.0752 lb/ft 3


 0.00121
3
 water
62.4 lb/ft
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
W  m g
m  ρV
EXAMPLE
Find the density of the body of Examples 2-1 and 2-2
V  1.8 ft 3
Solution
Using
4slugs
ρ
 2.22 slugs/ft
3
1.8ft
m  4 sulgs
3
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
Gauge pressure
Absolute 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
they are measured relative to
the atmosphere
They are labeled psig, or simply psi
Absolute pressures
They are measured
relative to a perfect
vacuum.
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.
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 60cm3 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