Comprehensive Study Guide for Fluid Power and Hydraulic Systems and Fluid Power

Course Outcomes and Introduction to Fluid Power

• The study of Fluid Power includes the following primary outcomes:
  • 1. Introduction to Fluid Power.
  • 2. Hydraulic Circuits Main Components.
  • 3. Hydraulic Circuits Accessories.
  • 4. Hydraulic Circuits Design and Analysis.

Fundamental Units and Dimensions in Engineering

• All quantities used in engineering can be reduced to four basic dimensions, termed the M-L-T-$\theta$ system:
  • Mass (M): Measured in kilograms ($kg$).
  • Length (L): Measured in meters ($m$).
  • Time (T): Measured in seconds ($s$).
  • Temperature ($\theta$): Measured in Kelvin ($K$).
• All other engineering quantities can be defined in terms of these four basic dimensions. Utilizing M, L, T, and $\theta$ allows for a universal reference independent of any specific measurement system.

Derived and Extracted Engineering Units

• Volume ($V$): Calculated as Area $\times$ Height. Unit: $m^3$.
• Velocity ($v, u$): Defined as the rate of change of distance. Unit: $m/s$.
• Acceleration ($g, a$): Defined as the rate of change of velocity. Unit: $m/s^2$.
• Force ($F$): Defined as the quantity needed to accelerate a mass. $1\,N$ is required to accelerate $1\,kg$ at $1\,m/s^2$. Unit: Newton ($N$).
• Pressure ($P$): Defined as force per unit area. Unit: $N/m^2$ (Pascal, $Pa$).
• Density ($\rho$): Defined as mass per unit volume. Unit: $kg/m^3$.
• Area ($A$): Calculated as Width $\times$ Height. Unit: $m^2$.
• Work ($W$): Force applied over a distance. Unit: $N \cdot m$ (Joule, $J$).
• Power ($P$): The rate of work done. Unit: $N \cdot m/s$ (Watt, $W$).

Physical Properties of Fluids: Density

• Definition: Density is defined as mass per unit volume. It is a property of matter that indicates how much mass is packed into a specific volume.
• Mathematical Formula: $\rho = \frac{m}{V}$.
• Typical Values:
  • Water: $1000\,kg/m^3$.
  • Air: $1.23\,kg/m^3$.
• Factors Affecting Density: It is slightly affected by changes in pressure and temperature.
• Specific Gravity (SG) or (\gamma): Defined as the ratio of the density of a fluid to the density of water: $\gamma = \frac{\rho}{\rho_w}$.
• Specific Volume: Defined as the reciprocal of density ($1 / \rho$).
• Temperature Relation for Water: The density of water is exactly $1000\,kg/m^3$ at a temperature of $4^\circ C$. As temperature increases beyond this point (up to $100^\circ C$), the density gradually decreases from $1000\,kg/m^3$ toward approximately $958\,kg/m^3$.

Fluid Viscosity and Internal Friction

• Viscosity (\mu): A measure of resistance to fluid flow resulting from intermolecular cohesion. It represents internal friction which leads to energy loss during motion.
• Shear Stress (\tau): Different fluids deform at different rates under the same shear stress. The ease of pouring is an indicator of viscosity (e.g., syrup vs. water).
• Units for Dynamic Viscosity: $N \cdot s/m^2$ or $kg/(m \cdot s)$.
• Typical Values:
  • Water: $1.14 \times 10^{-3}\,kg/(m \cdot s)$.
  • Air: $1.78 \times 10^{-5}\,kg/(m \cdot s)$.
• Temperature Effects on Viscosity:
  • Liquids: Viscosity decreases as temperature increases because heat breaks the bonds between molecules.
  • Gases: Viscosity increases as temperature increases. Heat provides molecules with more kinetic energy, leading to more random collisions and making smooth flow in a specific direction more difficult.

Society of American Automotive Engineers (SAE) Grades

• Mineral oil is derived from petroleum through refining.
• Base oils are tested for viscosity at different temperatures and loads and assigned a "Grade."
• Common grades include 5W, 30, 40, and multi-grade oils like 5W-40. The "W" stands for winter or low-temperature performance.

Kinematic Viscosity

• Definition: The ratio of the dynamic viscosity to the density of the fluid.
• Mathematical Formula: $\nu = \frac{\mu}{\rho}$.
• Units: $m^2/s$.
• Stoke Unit: $1\,Stoke = 10^{-4}\,m^2/s$.
• Typical Values:
  • Water: $1.14 \times 10^{-6}\,m^2/s$.
  • Air: $1.46 \times 10^{-5}\,m^2/s$.

Definitions of Fluids and Fluid Power Technology

• Fluid: A substance that take the shape of the holding container or any solid body with which it comes into contact. It is defined as a substance that continually deforms (flows) under an applied shear stress, regardless of the magnitude of that stress.
• Fluid Media: Typically categorized into Liquids (e.g., Water, Oil) and Gases (e.g., Air).
• Fluid Power: Technology that deals with the generation, control, and transmission of power using pressurized fluids.

Historical Development of Fluid Power

• Ancient History: Water wheels were used for centuries to produce power via kinetic energy. Windmills used air to propel ships and perform work.
• Hydrodynamic Devices: Devices that use the impact or kinetic energy in a liquid to transmit power.
• Scientific Foundations:
  • 1650: Discovery of Pascal's Law: "Pressure is transmitted undiminished in a confined body of fluid."
  • 1750: Bernoulli developed the Law of Conservation of Energy for a fluid flowing in a pipeline.
• Early Industrial Implementation: By 1870, fluid power was used for cranes, presses, winches, jacks, and riveting machines. These systems used steam engines to drive hydraulic water pumps.
• The Modern Era (1906): A significant milestone occurred when a hydraulic system was developed to control guns on the battleship USS Virginia. This system used oil instead of water, which solved major sealing and corrosion problems.
• World War II: Aviation and aerospace needs drove advancements like hydraulic-actuated landing gears, cargo doors, and flight control surfaces (rudders, ailerons, elevons).

Categories of Hydraulic Devices

• Hydrodynamic Device: Uses the impact or kinetic energy (moving water) to perform work.
• Hydrostatic Device: Operates by applying force to a confined liquid. Power is transmitted via pressure (force distributed over an area), expressed as force per unit area.

Fluid Media Varieties

• Hydraulics: In modern terminology, this implies a circuit using mineral oil. Water was used historically.
• Pneumatics: Uses compressed air (atmospheric air compressed 7 to 10 times). High flowability through pipes and tubes.
• Specialized Gases: Nitrogen or Argon can be used but are generally too expensive for standard industrial processes.

Industrial Applications of Fluid Power

• Transportation: Automobiles (Hydraulic brakes, power steering, transmissions, AC, lubrication, coolant pumping), Airplanes (Airbus A380), Boats, and Missiles.
• Construction and Earthmoving: Cranes, Tractors (John Deere), Caterpillar equipment (CAT 972M, 365C LP), and industrial trucks/trailers.
• Automation and Industry: Factory automation (Vimto bottling), food processing (Schrader Bellows), machine tools, and robots.
• Precision Control: Power steering pumps, valve body units, and master cylinders in braking systems.
• Miscellaneous: Amusement rides, displays, and pneumatically controlled dextrous hands (Utah/MIT).

Physics of Pressure

• Definition: Pressure is the force per unit area applied perpendicular to the surface of an object ($P = F/A$).
• Units:
  • British: PSI ($lb/in^2$).
  • Metric: $kg/cm^2$.
  • S.I.: $N/m^2$ (Pascal).
• Pascal’s Law: The pressure at any point in a confined fluid is the same in all directions and acts normal to any surface.
• Hydrostatic Pressure: Pressure at a given depth in a static liquid is caused by the weight of the liquid acting on a unit area at that depth.
  • $P = h \rho g$
  • $h$ = depth ($m$)
  • $\rho$ = density ($kg/m^3$)
  • $g$ = gravitational constant ($9.81\,m/s^2$).

Atmospheric Pressure

• Definition: The pressure exerted by the weight of the air above a point.
• At Sea Level: Atmospheric pressure is $14.7\,psia$ or approximately $1\,bar$.
• Altitude Variation: Atmospheric pressure decreases by approximately $1.0\,psi$ for every $2343$ feet ($71\,cm$) of ascent.
• General Rule: Pressure halves for every $5.5\,km$ of altitude increase.
  • Sea Level: $1\,bar$
  • $5.5\,km$: $0.5\,bar$
  • $11\,km$: $0.25\,bar$ (Commercial aircraft fly at approximately $10.5$ to $12.8\,km$).

Pressure Relationships and Types

• Absolute Pressure ($P_{abs}$): The total pressure referenced to a perfect vacuum ($0\,Pa$).
  • $P_{abs} = P_{atm} + P_g$
• Gauge Pressure ($P_g$): Pressure referenced to one atmosphere. A reading of zero means the pressure is equal to atmospheric pressure.
• Vacuum Pressure ($P_{vac}$): Pressure below atmospheric levels.
  • $Vacuum = P_{atm} - P_{abs}$
• Reference Values:
  • $0\,PSIA = -14.7\,PSIG$ (Perfect vacuum).
  • $14.7\,PSIA = 0\,PSIG$.
  • $29.92\,inHg = 0\,PSIA$.

Vapor Pressure and Cavitation

• Vapor Pressure: The partial pressure produced by fluid vapor in a container when in equilibrium. It is dependent solely on temperature in closed containers.
• Boiling: Occurs when absolute fluid pressure is reduced below its vapor pressure for a specific temperature.
• Cavitation: The formation of vapor bubbles in a pipe or pump (often at the suction section) when the local pressure drops below the fluid's vapor pressure. When these bubbles collapse, they cause noise and damage.
• Suction Vacuum and Pump Health Table:
  • 0 to -0.17 bar (0 to -12.7 cmHg): Ideal. Smooth flow, maximum pump lifespan.
  • -0.2 to -0.35 bar (-15 to -26 cmHg): Warning Zone. Beginning of gaseous cavitation; increased noise.
  • -0.4 bar (-30 cmHg): Critical Limit. Maximum allowable limit; indicates mechanical wear.
  • -0.5 to -0.8 bar (-37.5 to -60 cmHg): Danger Zone. Severe cavitation; "gravel-like" noise; gear destruction.
  • Approaching -0.9 bar (-67.5 cmHg): Vapor Pressure Point. Fluid turns to vapor at pump inlet.

Principles of Fluid Flow

• Types of Flow: Laminar and Turbulent.
• Discharge/Flow Rate ($Q$): The volume of fluid moving through a section per unit of time.
  • $Q = A \times v$
  • Unit: $GPM$ (Gallons per minute) or $L/min$.
• Relation to Speed: Increasing the flow rate increases the speed of the actuator/system.
• Continuity Equation: $\rho A_1 v_1 = \rho A_2 v_2$ (For incompressible fluids, $A_1 v_1 = A_2 v_2$).
• Bernoulli’s Principle: $P_1 + \frac{1}{2} \rho v_1^2 + \rho g z_1 = P_2 + \frac{1}{2} \rho v_2^2 + \rho g z_2$.
  • This proves that with a constant flow rate, the sum of pressure and kinetic energy is constant.
  • If velocity increases (e.g., in a narrow pipe), pressure must decrease.

The Venturi Effect

• Definition: The reduction in fluid pressure that results when a fluid flows through a constricted section of pipe.
• Application (Gasoline Engine Carburetor):
  • 1. Air volume is determined by a butterfly valve.
  • 2. At the venturi throat (narrowest point), air speeds up and pressure drops (Maximum Velocity, Minimum Pressure, Minimum Temperature).
  • 3. Higher pressure in the fuel bowl pushes fuel into the low-pressure air stream in the throat.
• Duct Behavior:
  • Convergent Duct: Velocity increasing, Pressure decreasing, Temperature decreasing.
  • Divergent Duct: Velocity decreasing, Pressure increasing, Temperature increasing.