Fluid Power System Principles Chapter 2 Fluid Power System Principles Energy • Fluid Power Energy Transmission • Fluid Power System Variables

In fluid power systems, total energy is the sum of static energy, kinetic energy, and thermal energy. Total energy is the combined forces of different forms of energy. In fluid power systems, total energy is the sum of static energy, kinetic energy, and thermal energy. See Figure 2-1. The main forms these energies are transformed into are thermal, electrical, mechanical, and sound energy.

Rotating mechanical energy is transferred to a hydraulic pump, which supplies hydraulic energy to the system in the form of kinetic energy (fluid flow). Hydraulic pressure is created when the fluid flow created by a hydraulic pump meets resistance. The transmission of energy throughout a hydraulic system begins when electrical energy is converted into rotating mechanical energy by the electric motor. The rotating mechanical energy of the motor is transferred to the hydraulic pump, which supplies hydraulic energy to the system in the form of moving fluid (kinetic energy). See Figure 2-2. Ultimately, the kinetic energy is converted back into mechanical energy by hydraulic cylinders and motors.

Electric energy is changed into rotating mechanical energy by the electric motor and is transmitted to the conveyor belt by a belt drive system. Mechanical energy is machine energy. Mechanical energy is energy produced and transferred using gears, pulleys, or belts. Mechanical energy is used to transmit energy from one solid object to another solid object. For example, a conveyer line uses mechanical energy because an electric motor rotates a belt-drive system that is connected to a conveyor belt. The rotating energy from the motor transforms into mechanical energy, causing the conveyor belt to move. See Figure 2-3.

When making comparisons between hydraulics and pneumatics, there are differences in system variables that must be taken into consideration. The transmission of energy using fluid power is accomplished by either hydraulic or pneumatic systems. Hydraulic systems use liquid to transmit energy. Pneumatic systems use gas to transmit energy. Air is the most common fluid used with pneumatics. Although water was the first liquid to be used with hydraulic systems, modern hydraulic equipment uses petroleum-based oil as the main method of hydraulic energy transmission. When comparing hydraulic systems and pneumatic systems, differences in system variables must be taken into consideration. See Figure 2-4.

Heat from friction is generated through different actions in a fluid power system, such as fluid flowing through a hose, or a piston moving against a cylinder body. Resistance is the force that stops, slows, or restricts the movement of fluid or devices in a fluid power system. The most common type of resistance in a fluid power system is friction. Friction is the resistance to movement between two mating surfaces. Friction between two mating surfaces turns the energy that is being used to move the surfaces into heat. Heat from friction is generated by different actions in a fluid power system, such as fluid flowing through a pipe or a piston moving against a cylinder body. See Figure 2-5.

Fluid power systems complete work using actuators such as cylinders, motors, and oscillators. Work is the movement of an object (in lb) through a distance (in ft). Force is anything that changes or tends to change the state of rest or motion of a body. Work is accomplished when a force overcomes a resistance. All fluid power systems are meant to accomplish some type of work. In most applications, fluid power systems accomplish work by moving something, such as large amounts of backfill material, concrete, or cartons of product. Also, fluid power systems accomplish mechanical work by using actuators such as cylinders, motors, and oscillators. See Figure 2-6.

The amount of work produced is calculated by multiplying the force that must be overcome by the distance over which it acts. The amount of work (W) produced is calculated by multiplying the force (F) that must be overcome (in lb) by the distance (d) (in ft) over which it acts. See Figure 2-7. Foot-pounds (ft-lb) are a measure of work. The amount of work produced is calculated by applying the following formula: W = F × d where W = work (in ft-lb) F = force (in lb) d = distance (in ft)

Power indicates the rate that work is done. Power is the amount of work accomplished over a specific period of time. While work indicates how much energy is needed to move an object, power indicates the rate that work is done. See Figure 2-8. Power (P) is measured in foot-pounds per second (ft-lb/sec). Power is calculated by applying the following formula: P = W / t where P = power (in ft-lb/sec) W = work (in ft-lb) t = time (in sec)

Horsepower is a mechanical unit of measure equal to the force required to lift 550 lb, 1 ft in 1 sec. In a fluid power system, power is commonly expressed in units of horsepower. Horsepower (HP) is a mechanical unit of measure equal to the force required to move 550 lb, 1 ft in 1 sec. See Figure 2-9. Horsepower is calculated by applying the following formula: HP = P / 550 where HP = horsepower P = power (in ft-lb/sec) 550 = constant

Pascal’s law states that an applied force placed on a fluid will transmit undiminished in all directions. The operation of fluid power systems is based on different interrelated variables. These variables are force, area, and pressure and are based on basic principles derived from Pascal’s law. Pascal’s law is a fluid power law that states that when a force is applied to a confined fluid, the force is felt throughout the fluid undiminished. This means that if fluid trapped in a cylinder has a pressing force on it, that force is distributed equally in all directions within that cylinder. See Figure 2-10. The fluid power circle is a visual representation of the force, area, and pressure relationship.

In hydraulic systems, area typically refers to diameter of the piston face. In hydraulic systems, area typically refers to the piston face, which is circular in shape. The piston face is the surface that contacts pressure in the fluid. The opposite piston face is ring-shaped because it has a rod attached to it. See Figure 2-11. The area of a piston can be determined by using diameter or radius, so it is important to be able to determine the area of a circle by using either measurement.

Pressure is determined by the area and force present. Pressure is the resistance to flow. Working pressure is the measure of the force applied to a given area. Pressure in a hydraulic system is the resistance to fluid flow. Pressure is determined by how much area is available and how much load (in lb) is present on the actuator. These two variables determine the amount of pressure a system can produce. Pressure is measured in pounds per square inch (psi), which translates to how many pounds are applied per a given area in square inches. See Figure 2-12. Working pressure is calculated by applying the following formula: p = F / A where p = working pressure (in psi) F = force (in lb) A = area (in sq in.)

The fluid power (Pascal’s law) circle is a visual representation of how the formulas for pressure, force, and area are interrelated in a fluid power system. The fluid power (Pascal’s law) circle is a visual representation of how the formulas for pressure, force, and area are interrelated in a fluid power system. See Figure 2-13. To use the fluid power circle, two of the three variables represented must be known. When two of the three variables are known, the unknown variable can be calculated.

Less pressure is required to extend a rod than to retract one due to more surface area. All hydraulic cylinders have a piston with a rod attached to one side. In a hydraulic cylinder, the surface area acted on during extension includes the entire surface of the piston and fastener. A fastener is a mechanical device used to attach two or more members in position, or join two or more members. The surface area during retraction is reduced by the area of the rod. This means that the amount of pressure required to move a hydraulic cylinder with an attached load is different for extension than retraction. For example, the amount of pressure required for a hydraulic cylinder to retract a splitting wedge in a log splitter is different from the pressure required to extend a splitting wedge in a log splitter. See Figure 2-14.

Atmospheric pressure is the pressure created by the weight of the atmosphere at sea level under standard conditions. Atmospheric pressure is the pressure created by the weight of the atmosphere at sea level under standard air conditions. A 1 sq in. column of air extending from sea level to the top of the atmosphere weighs 14.7 lb (1.01 bar, 101 kPa). Atmospheric pressure is placing pressure on everything all the time. For example, a hydraulic reservoir (oil tank) is considered to have only atmospheric pressure applied to it. See Figure 2-15.

The difference between gauge pressure and absolute pressure is 14 The difference between gauge pressure and absolute pressure is 14.7 psi, or atmospheric pressure. Absolute pressure is the sum of gauge pressure and atmospheric pressure. See Figure 2-16. Absolute pressure is expressed in pounds per square inch absolute (psia). Absolute pressure can be used in systems that have vacuums to avoid using a negative number. For example, a vacuum reading in psi would be –10 psig, but in psia, it would be 4.7 psia. In theory, a perfect vacuum is 0 psia, although it is not possible to get to 0 psia because there will always be a small amount of air inside a container.

A mercury barometer indicates atmospheric pressure with a column of mercury. A mercury barometer is an instrument used to measure atmospheric pressure using a column of mercury (Hg). A mercury barometer consists of a glass tube that is completely filled with mercury and closed on one end. See Figure 2-17.

A Bourdon tube pressure gauge indicates pressure using the movement of a Bourdon tube. A Bourdon tube pressure gauge is a measurement device that is used to register and measure pressure in fluid power systems. A Bourdon tube is a curved device that straightens as pressure increases and bends as pressure decreases. See Figure 2-18. Bourdon tube pressure gauges can take readings in psi, bars, and/or kilopascals.

A spring-loaded piston gauge uses fluid pressure to push a piston against a compression spring that is attached to a pointer. A Schrader gauge is a pressure gauge that uses fluid pressure to push a piston against a compression spring that is attached to a pointer. See Figure 2-19. Schrader gauges are also known as spring-loaded piston gauges. The higher the pressure in the system, the more the spring is compressed. Schrader gauges are mostly used in hydraulic systems as an economical method to read hydraulic pressure. Schrader gauges are accurate to about ±10%. Therefore, they should not be used for taking precision measurements.

A digital pressure gauge converts fluid pressure into an electrical signal and has an easy-to-read digital display. A digital pressure gauge is a pressure gauge that converts fluid pressure into an electrical signal. In fluid power systems, fluid flow is converted to electrical current corresponding to the amount of pressure applied. The electrical current is interpreted by an electronic circuit board, which indicates the amount of pressure in the system on a digital display. A computer interface can be used to transmit data to a computer screen. See Figure 2-20.

Common vacuum gauge readings range from –1. 47 in. Hg to –14. 7 in Common vacuum gauge readings range from –1.47 in. Hg to –14.7 in. Hg, and –0.10 bar to –1.01 bar. Vacuum is used in hydraulic systems to allow fluid from the reservoir to travel into the pump. The strength of a vacuum depends on how far below 14.7 psia it is. The amount of vacuum present typically ranges from 3.0 in. Hg to 29.92 in. Hg. Bourdon tubes are commonly used for vacuum gauges and can take measurements in inches of mercury, bars, psig (or psi) with an accuracy of ±1.5%. Common vacuum gauge readings range from 0 in. Hg to 30 in. Hg, 0 bar to –1 bar, and 0 psig to –14.7 psig. See Figure 2-21. Vacuum ratings of pumps depend on the design of the pump and the rating specification assigned by the pump manufacturer.