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1-3. Flow

Flow is the movement of a hydraulic fluid caused by a difference in the pressure at two points. In a hydraulic system, flow is usually produced by the action of a hydraulic pump-a device used to continuously push on a hydraulic fluid. The two ways of measuring flow are velocity and flow rate.

a. Velocity. Velocity is the average speed at which a fluid's particles move past a given point, measured in feet per second (fps). Velocity is an important consideration in sizing the hydraulic lines that carry a fluid between the components.

b. Flow Rate. Flow rate is the measure of how much volume of a liquid passes a point in a given time, measured in gallons per minute (GPM). Flow rate determines the speed at which a load moves and, therefore, is important when considering power.

1-4. Energy, Work, and Power

Energy is the ability to do work and is expressed in foot-pound (ft lb). The three forms of energy are potential, kinetic, and heat. Work measures accomplishments; it requires motion to make a force do work. Power is the rate of doing work or the rate of energy transfer.

a. Potential Energy. Potential energy is energy due to position. An object has potential energy in proportion to its vertical distance above the earth's surface. For example, water held back by a dam represents potential energy because until it is released, the water does not work. In hydraulics, potential energy is a static factor. When force is applied to a confined liquid, as shown in Figure 1-4, potential energy is present because of the static pressure of the liquid. Potential energy of a moving liquid can be reduced by the heat energy released. Potential energy can also be reduced in a moving liquid when it transforms into kinetic energy. A moving liquid can, therefore, perform work as a result of its static pressure and its momentum.

b. Kinetic Energy. Kinetic energy is the energy a body possesses because of its motion. The greater the speed, the greater the kinetic energy. When water is released from a dam, it rushes out at a high velocity jet, representing energy of motion-kinetic energy. The amount of kinetic energy in a moving liquid is directly proportional to the square of its velocity. Pressure caused by kinetic energy may be called velocity pressure.

c. Heat Energy and Friction. Heat energy is the energy a body possesses because of its heat. Kinetic energy and heat energy are dynamic factors. Pascal's Law dealt with static pressure and did not include the friction factor. Friction is the resistance to relative motion between two bodies. When liquid flows in a hydraulic circuit, friction produces heat. This causes some of the kinetic energy to be lost in the form of heat energy.

Although friction cannot be eliminated entirely, it can be controlled to some extent. The three main causes of excessive friction in hydraulic systems are:
  • Extremely long lines.
  • Numerous bends and fittings or improper bends.
  • Excessive velocity from using undersized lines.

In a liquid flowing through straight piping at a low speed, the particles of the liquid move in straight lines parallel to the flow direction. Heat loss from friction is minimal. This kind of flow is called laminar flow. Figure 1-8, diagram A, shows laminar flow. If the speed increases beyond a given point, turbulent flow develops. Figure 1-8, diagram B, shows turbulent flow.

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Figure 1-9 shows the difference in head because of pressure drop due to friction. Point B shows no flow resistance (free-flow condition); the pressure at point B is zero. The pressure at point C is at its maximum because of the head at point A. As the liquid flows from point C to point B, friction causes a pressure drop from maximum pressure to zero pressure. This is reflected in a succeedingly decreased head at points D, E, and F.
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d. Relationship Between Velocity and Pressure. Figure 1-10  explains Bernouilli's Principle, which states that the static pressure of a moving liquid varies inversely with its velocity; that is, as velocity increases, static pressure decreases. In the figure, the force on piston X is sufficient to create a pressure of 100 psi on chamber A. As piston X moves down, the liquid that is forced out of chamber A must pass through passage C to reach chamber B. The velocity increases as it passes through C because the same quantity of liquid must pass through a narrower area in the same time. Some of the 100 psi static pressure in chamber A is converted into velocity energy in passage C so that a pressure gauge at this point registers 90 psi. As the liquid passes through C and reaches chamber B, velocity decreases to its former rate, as indicated by the static pressure reading of 100 psi, and some of the kinetic energy is converted to potential energy.

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Figure 1-11 shows the combined effects of friction and velocity changes. As in Figure 1-9 pressure drops from maximum at C to zero at B. At D, velocity is increased, so the pressure head decreases. At E, the head increases as most of the kinetic energy is given up to pressure energy because velocity is decreased. At F, the head drops as velocity increases.

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e. Work. To do work in a hydraulic system, flow must be present. Work, therefore, exerts a force over a definite distance. It is a measure of force multiplied by distance.

f. Power. The standard unit of power is horsepower (hp). One hp is equal to 550 ft lb of work every second. Use the following equation to find power:

P = f x d/t


P = power, in hp
f = force, in GPM
d = distance, in psi
t = time (1,714)

Primary content provider: U.S. Army
Web developer: David L. Heiserman
Publisher: SweetHaven Publishing Services

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