Absolute filtration

    Defines the minimum size of particles that the filter is capable of holding 100%, in microns.

    Absolute pressure

    Psia, At sea level is 14.5 psi, 0 psi is maximum vacuum.


    This Hydraulic component is shaped like a bowl with the purpose to collect or store the hydraulic energy for Later use. Stores a certain amount of pressure to be released at will by creating a flow, depending on the case it may be very high. There are basically 3 types: Diaphragm, Piston and bladder.

    Air bleeder

    Accessory connector is used to remove air trapped in hydraulic lines or components.


    Element that amplifies the input signal giving a modified higher lever output. Today it is an element widely used in Hydraulic Control Systems. It helps to control Position, Speed, Hydraulic Pressure in linear and rotary actuators. Generally used to control the opening of proportional valves or servo.


    Said of a variable that can have infinite number of different values.

    Annular area

    Is the actual working Area of a hydraulic cylinder on the side of the Rod. It is calculated as the piston area minus the rod area. It is the Area taken into account when calculating as the Rod is entering.

    Anti-extrusion ring

    Ring-shaped washer made of hard plastic that is used to reduce the space between surfaces using an O-ring ("O" ring) to seal. It prevents the "O" ring to be pushed into the gap by the pressure.


    A measure of Pressure close to 14.7 Psi (0.1013 Mpa).


    Back pressure

    Existing or created pressure for some reason in the return line that can adversely affect system regulations. In some other cases it may be a desired condition.


    A backhoe loader is machine that can be used as an excavator and as a loader.


    It is defined as the space between the tooth surfaces of pinions meshing in gear boxes, PTO, hydraulic  gear pumps and  motors.

    Backup ring

    Also known as "anti-extrusion ring", used to fill the gap and prevent "o" ring from cold flow when under pressure.


    Unit of measure for pressure close to 14.5 Psi


    Also known as the Pistons barrel. Cylindrical block is where the pistons move in and out to generate the pumping in axial piston pumps. The term also applies to axial piston motors.

    Base oil

    It refers to the component of the oil without additives.

    Bernoulli's principle

    In an ideal fluid (no viscosity or friction) that is circulating in a closed conduit, the energy that it holds remains constant throughout the course.


    Separator or diaphragm used to separate the two media present in a hydraulic accumulator (oil and Nitrogen). Usually made of materials such as synthetic rubber and other materials compatible with other fluids.


    In hydraulic terms, is to take a very small flow toward the return from a part of the circuit that is under high pressure.

    Boundary lubrication

    This condition occurs when the lubricant film is so thin that the tips of the rough edges of the pieces start to collide. It may be the case when the load is excessive, high temperature or partial absence of the lubricant.

    Boyle's law

    Also referred to as the Ideal Gas Law. Describe the inverse relationship between the absolute pressure and gas volume while the temperature remains constant in a closed system. Necessary to make some calculations with hydraulic accumulators.

    Bulk's Modulus

    Is defined as the resistance to compression of a liquid. It relates the pressure needed to decrease certain volume. Units are: Pa (pascals) or Psi (Pounds per square inch)


    Is a secondary path for the flow.



    A set of pieces that make up a basic hydraulic element. This item can be removed without dismantling the main completely. It can refer to cartridge valves that are inserted or screwed into another, or the rotating group of some hydraulic pumps or motors.


    Phenomenon that occurs when a liquid is subjected to a vacuum so high that it evaporates. Condition that is generated in the suction of the pumps when there is a restriction. Generates, if not corrected soon wear on the pump, fluctuations in the flow and pressure and especially noise.

    Centipoise, eng

    A measure of the dynamic viscosity. (cP) it takes as parameter the viscosity of water that has a value of about 1 cP. For more viscous liquids the value is greater. ie: the honey is 2000 cP.

    Centistoke, eng

    A measure of kinematic viscosity. cSt. Water has a viscosity of 1 cSt kinematic viscosity  at 20 degrees centigrade.


    It refers to the internal cavities in Hydraulic Elements, usually sealed tightly.  Multiple Chambers can be composed independently as in pumps and motors (piston, gears, vanes).

    Charles's law

    In an ideal gas, the volume of a fixed mass of gas varies in proportion to the absolute temperature, if the pressure remains constant. Is applied to Accumulator Calculations.


    Term used when there is an abnormal vibration, causing amplified noise. Caused by problems in pumps, pipes poorly secured, relief valves with similar adjustments are opened and closed by resonance.

    Check valve

    Directional control valve that allows flow only in one direction. Ball or spool type.

    Closed center

    Condition in a Hydraulic System where there is not a free path (no pressure, no restriction) for the flow from the pump to the tank. It refers to the neutral condition of a directional valve. The only option for the pump flow is directed to the tank by opening the relief valve at high pressure.

    Component enclosure

    Dotted rectangle that is drawn around the graphic symbols of one or more hydraulic elements to indicate the limits of an assembly.

    Compressed air

    It is Air under pressure that is usually used for automation of Machinery using pneumatic elements.


    Measures the change in Volume of a fluid when the pressure changes. Standard unit are in./Lb.


    Mechanical device that compresses the air to be used for automation with pneumatic components.


    Is any element that does not make up or is undesirable in the fluid. It can be: water, air, any liquid different than the fluid, solid particles , etc.


    Device used to regulate the output of a unit.

    Counterbalance valve

    Pressure control valve that is used to balance loads that tend to lower the cylinder out of control due to gravity. It acts  placing a back Pressure in the output port that generates a force that balance gravity allowing easy control of the downward movement.

    Cracking pressure

    Is the pressure needed to open the relief valve.


    Pipe element that has four connections in a cross shape.


    The Hydraulic Cylinder is the component of a Hydraulic System that convert Hydraulic Energy into Mechanical Linear Force and Motion.

    Depending on the type, a Hydraulic Cylinder has one or two chambers. When the Fluid is pushed into the chamber, a linear motion is created when the Rod extends. The pressure at the inlet depends on the force load in the Rod. The linear speed of the Rod depends on the flow supplied at the inlet.

    Cylinder rod

    Cylindrical bar that is connected to the cylinder piston that performs the function of pushing or pulling the load.


    Deceleration valve

    Control valve flow control mechanism used to slow hydraulic cylinders down when they are near the end of the stroke. The spool of this valve is actuated by a cam that is coupled to the moving frame of the machine.


    Operation in which the pressure of an element is released gradually and safely from high to low in a certain amount of time.


    Is the relation between the Mass of a substance and its Volume.

    Directional valve

    Hydraulic Directional Valve is the component of the hydraulic system which diverts or changes the direction of flow with different purposes. Each time the valve changes state, the flow changes direction because the path is changed internally. Directional valves may have two, three or more positions or states and three, four or more internal passages. The most common application is the control of hydraulic actuators to change the direction of motion.


    Volume of fluid that is delivered by a pump or a hydraulic motor per shaft revolution. Given in cubic inches / revolution or cubic centimeters / revolution.


    Internal and external ducts which carry the fluid used for lubrication or internal leakage that is not used anymore. Usually connected by separate lines to the tank without any pressure.



    Relationship between input and output values ​​expressed in percentage. Used to show or verify power or volumetric performance in hydraulic pumps and motors.


    Pipe element that forms a 90o angle between the inlet and the outlet. There 30o and 45o elbows also.


    Excavators are mobile machines which are primarily used for the purpose of digging trenches, holes or foundations.



    Hydraulic component that aims to retain the pollutant particles to ensure the purity of the Fluid and prolongs the life of the different components of the hydraulic system.

    Filter element

    Cartridge-shaped element inside the filter housing containing the medium used to filter the fluid that has to be changed regularly or when full of contaminants.

    Filter media

    It refers to the porous material used in the filter element of a filter. There are many different materials and pore sizes.

    Fixed displacement motor

    Is a Hydraulic motor which displacement can not be varied.


    Flat connection system for Hoses or pipes. It connects to its counterpart with bolts. Leak proof.


    Flow, or Flow rate is the amount of fluid in motion, the volume of the fluid per unit of time. In standard units it is Galons Per Minute (GPM). In ISO units is expressed as Liters Per Minute (LPM).

    Flow control valve

    The Hydraulic Flow Control Valve is the component of a Hydraulic System that reduces or controls the amount of flow that any hydraulic component is needing, like the Motor or the Cylinder or another valve. Basically it is a valve that closes the opening inside of it using a screw and a spool. There are two different types, regular and Pressure Compensated. If the flow has to be controlled regardless of the pressure fluctuations in the inlet or outlet, the second one is a better option.

    Flow divider valve

    Valve that splits the input stream in two, sending half the flow to each one of the output ports. There are valves that divide the flow with different percentages: 60/40, 70/30, etc. The pressure variations have little influence on this regulation.

    Flow switch

    It is an electric switch that sends a signal (on/off) to control or monitor the flow in  a line of a hydraulic system.


    Is a device that measures the flow in a Hydraulic System. It is connected in-line. Turbine, differential area type are the most common. The output signal can be analog, digital or electronic to be used by a computer or data acquisition devices.


    The Fluid is the media that the Hydraulic System uses to transfer the Power.

    There are many different Hydraulic Fluids, the most common is the Petroleum oil. there are also, fire resistant fluids, synthetic fluids, bio-degradable fluids, water-oil and water-glicol emulsions and Phosphate ester fluids. Among the most important properties of the hydraulic fluid are: Lubrication, cooling, sealing and contamination removal.


    Condition of the hydraulic system in which the oil is filled with foam (air) and in some cases the tank overflows. Due to the indiscriminate entry of air into the system generally in the pump suction or sometimes in return lines.


    In Hydraulic terms, force is equal to the pressure x area of ​​application. Given in pounds or kilograms.

    Four way valve

    Directional valve with four internal passages connecting alternatively the ports Pressure and Tank to a and b. Widely used with double acting cylinders, as with hydraulic motors when reversibility is required.


    Is the force that opposes the relative motion due to contact between the surfaces of two bodies. Also applies to the layers of any fluid, liquid or gas.

    Friction, coefficient

    A measure of the relationship between the friction force and the clamping force between two bodies sliding against each other.


    Gauge Pressure

    Pressure due to the weight of the atmosphere that is based or zero pressure at sea level.


    Heat exchanger

    Hydraulic element that is used to heat or cool the hydraulic fluid, using different media, eg water or air. They are called: Hydraulic Heater or Hydraulic Cooler.

    Horse power

    Is the Work per unit of time. means the force applied for a distance per unit of time: F x D / t.
    In Hydraulics is Pressure x Flow / 1714 (horse power).


    Technology consisting of two or more fully integrated different technologies like mechanical + electrical or mechanical + hydraulic or electrical + hydraulic.

    Hydraulic circuit

    Is a representation of the whole Hydraulic System showing all the components with their connections.


    Is the branch of engineering and physics that studies the properties and behavior of liquid fluids. In our case studies the properties and behavior of fluids as a means to transmit power.


    Is the branch of engineering and physics that studies the behavior of liquids in motion.


    Is the branch of physics and engineering that studies the behavior of liquids at rest.



    Hydraulic device that is used to multiply pressure.It is an arrangement of two cylinders, one pushing the other. The sectional area of the first piston is several times bigger than the second one creating the increase in pressure in same ratio.




        Laminar flow

        Condition of flow in a pipe where the speed is low enough that the fluid moves in parallel layers. Friction against the walls kept to a minimum.

        Level switch

        It is an electric switch that sends a signal (on/off) to control the fluid level in the hydraulic reservoir.

        Lever arm

        The perpendicular distance between the line of action of a force and a pivot when Torque is generated.


        In Hydraulics is any Hose, Rigid or Flexible Tubing used to conduct the fluid between hydraulic components.

        Linear actuator

        Is the Hydraulic element that converts Hydraulic Energy into Mechanical Energy over a straight line. The hydraulic Cylinder applies force and movement in a straight line.

        Linear motion

        Motion generated in the rod of a hydraulic cylinder when hydraulic fluid is fed. Occurs in a straight line.

        Load pressure

        Is the pressure needed to move the load in cylinders and hydraulic motors.


        Property that the hydraulic fluid must have to protect the internal parts of the hydraulic components from wear and lower the friction losses.



        Hydraulic Element used as a base to all kind of valves like directional, pressure and fluid forming a very compact sub-system.

        Maximum pressure

        Is the maximum pressure with no overhead in the power source (electric motor, etc).


        Millionth of a meter, or thousandth of a millimeter.


        Millionth of a meter, or thousandth of a millimeter.


        The hydraulic motor is the component of a Hydraulic System that convert hydraulic Energy from the hydraulic system into mechanical rotational Energy by rotating the shaft.

        The hydraulic motor has 2 ports, inlet, outlet and a drive shaft that is connected to the load. When hydraulic fluid is pushed into the inlet port the internal chambers that are connected to the shaft transport the fluid to the outlet port creating the rotating motion. Once the oil leave the motor, it is sent back to the reservoir in the return line. The Pressure needed at the inlet of the motor is determined by the Torque load in the shaft. The Flow comming into the motor determines the speed of rotation (Revolutions Per Minute).



        A measure of acidity or alkalinity of the hydraulic fluid. (ASTM D974-58T).


        O ring

        Hydraulic component that is used to seal metal surfaces of internal hydraulic components. Round cross section ring.

        Open center

        Condition in a Hydraulic System where the flow from the pump is sent freely without any restriction back to the tank.


        Calibrated inner hole that is used to limit the pilot flow in hydraulics pumps,motors and valves.



        Same as Torque. "Rotational force" capacity generated in the motor shaft when hydraulic fluid is supplied at the input.


        system in which two or more events happen simultaneously. The flow of the pump has several branches to reach the actuator. In a hydraulic hybrid vehicle, power to the wheel will come from two possible sources, the mechanical transmission or the Accumulator system.

        Pascal's law

        Pressure applied on a confined fluid is transmitted undiminished in all directions, acts with equal force on equal areas, and at right angles to them.


        Internal conducts of Hydraulic valves that allow fluid to flow according to specific function.


        Power Drive Unit

        Peak pressure

        Maximum pressure generated instantly (thousandths of seconds) due to a closing or opening of a valve and other hydraulic directional device.

        Pilot circuit

        Circuit used to control other circuit (s) or Hydraulic element (s). They use very little flow and for that reason all the elements in the circuit are small.

        Pilot pressure

        Is used to control hydraulic components such as pumps, valves of all types and hydraulic motors.


        Cylindrical part That is placed inside a cylinder to do linear force when hydraulic fluid is applied to it. A hydraulic piston motor generates torque and rotational movement. In a piston pump, when the shaft rotates, the flow is generated.


        Is the external connection of a valve, pump, hydraulic cylinder or motor.

        Positive displacement

        Characteristic of the pumps and motors where the inlet and outlet are not interconnected. Preventing the fluid to recirculate internally.

        Power unit

        It refers to the group containing Prime mover(Electric motor or Gas, Diesel Engines), reservoir, pumps, all kinds of valves, accessories that sends Hydraulic power to the actuators (Cylinders and Motors).

        Pre-fill valve

        Valve used to fill a big cylinder directly from the tank by gravity or the motion of auxiliary cylinders. Applied to large cylinders reducing the need for flow from the pump.


        The Hydraulic Pressure is the Force per Area unit, usually expressed in Pound per Square inch (PSI) or Kilograms per Square cm (Kg/cm2) or bars.

        The Fluid that is under pressure is capable of transmit power. In a Hydraulic System The pressure acts with equal force on all equal areas of the confining walls and perpendicular to them (Pascal's Law).

        Pressure diferential

        Is the difference in pressure between two points that share the same stream of fluid.

        Pressure gauge

        Device used to measure the hydraulic pressure at any point. Psi, Atm, Bar, Kg/cm2

        Pressure line

        Is the fluid conductor connecting the output of the hydraulic pump with all the other elements up to the hydraulic actuator. It carries the maximum pressure of the system.

        Pressure override

        The difference between the pressure needed to open the relief valve and the pressure achieved when all the flow is passing through it.

        Pressure reducing valve

        Pressure control valve that reduces the output pressure to a preset value no matter how high is the pressure at the entrance.

        Pressure switch

        Electrical device that opens or closes an electrical contact when a pressure value is reached.


        Is to apply a pressure greater than atmospheric pressure.

        Proportional directional valve

        Valve that changes the direction and regulates the output flow in proportion to an input signal, which has polarity + / - depending on the desired direction. Controls the movement of cylinders and hydraulic motors in both directions of movement.

        Proportional flow valve

        Valve that regulates the output flow proportionally to the input voltage. Usually used to control the speed of the cylinders and hydraulic motors using an electronic signal that can come from a PLC or computer or electronic control.

        Proportional pressure valve

        Valve that regulates the output pressure in proportion to input voltage. Generally used as a pilot valve to control larger relief valves, pressure reducing or variable flow pumps electronically.

        Proportional valve

        Valve that regulates the output variable proportionally to the input voltage. The Output variable Can be pressure, flow or direction.


        The Hydraulic Pump is the component of a Hydraulic System that convert mechanical Energy from the Primer mover into Hydraulic Energy by sending Flow into the System.

        The Hydraulic Pump has two ports, Inlet, Outlet and a drive Shaft that is connected to the prime mover. When the shaft of the pump is rotated and the inlet is connected to the reservoir, Hydraulic fluid is transported from the reservoir to the inlet and then to the outlet of the pump that sends the fluid to the system at a certain rate (flow) and (Pressure) depending on the load. The rate of fluid or Flow depend on the displacement of the internal chambers of the pump.



          Regenerative circuit

          Circuit which adds the Flow coming from the rod end of a double acting Cylinder to the flow from the pump and delivers both of them to the head end, increasing the speed of the Rod coming out.

          Relief valve

          The Hydraulic Pressure Relief Valve is the component of a Hydraulic System that limits the Pressure that the system is using. Due to the fact of the incompressibility of the fluid and the type of pumps used (Positive Displacement), the relief valve serves as the Protection for overpressure. The relief Valve has two ports, inlet and outlet. When the pressure in the inlet is high enough to open it the flow is sent to the outlet and then back to the reservoir. There are many ways to adjust the pressure, manual, external, electronically and some other more.


          The Hydraulic Reservoir is the component that is used mainly to store the fluid that the Hydraulic System is using to operate. The Reservoir helps to dissipate the heat through its walls, it helps to dissipate the air trapped into the fluid before it reaches the Pump, it helps the contaminants to settle at the bottom and usually to support the Hydraulic Components like pump, motor and valves. It is the best place for filters, oil coolers and other accessories.


          A reduced area in a passage or line that produces a Pressure drop.


          Reduccion de la seccion transversal de un conductor de flujo o interno en algun componente hidraulico para restringir el flujo, o suavizar cambios bruscos de presion piloto.

          Return line

          Is the conductor that connects any component fluid and the tank. Is the line that leads the fluid that returns to the tank after doing work in the actuator. It passes through other components (directional valve) after exiting  the actuator before getting to the reservoir. It carries low pressure, the same as the tank.

          Ridgid pipes

          Rigid steel conductor used to drive the flow of a hydraulic element to another. It can not be bent.

          Rotary actuator

          Is the hydraulic element that converts the hydraulic power in the form of flow and pressure into mechanical power and torque as a continuous rotary motion. A Hydraulic motor makes this work.

          Rotary motion

          Motion generated in the hydraulic motor shaft when it is powered by hydraulic fluid.



          Elastic elements that are used for sealing between metal elements internal hydraulic components. Made of the most extensive range of materials such as rubber, plastic, paper, cork and others.


          Movements that follow a specific order.

          Secuence valve

          Is the Pressure Control Valve that opens when the inlet pressure reaches a certain value opening the way for a secondary circuit.


          Analog device that converts a physical variable into an electronic signal voltage or current.


          Hydraulic elements which are connected consecutively forming a chain. For example from the pump , the flow is directed to the directional valve, then to the flow control, then to the pilot check to reach the cylinder. In a vehicle the system transmits power from the engine to the transmission, then to the hydraulic pump, then to the hydraulic motor to reach the wheel.


          Automatic control system with feedback.

          Servo circuit

          Automatic Control of the actuator that uses sensors of the type of the variable to be controlled. If position, force, or speed or combination of them.Today is very common to see computer-controlled Servo Circuits Using programmed control algorithms.

          Servo cylinder

          Is a variety of hydraulic cylinder with very little friction that has position and speed sensor that incorporates a servo valve mounted on the back cover.


          Hydraulic Servoactuators are used as a mechanism to induce or control motion in Hydraulic systems. They are devices which transform an input signal (usually electrical) into motion. Linear with a servo cylinder and rotational with a servo motor.

          Shuttle valve

          Valve that connects the highest between two pressures with the output port. Widely used in directional valves that use the system "load sensing" in conjunction with piston pumps.

          Specific gravity

          In a liquid, it is the ratio of the weight of a given volume of fluid to the weight of the same volume of water.


          Cylindrical piece that moves inside of a hydraulic Element to change the path of the Flow.


          Hydraulic elements that control the direction of a vehicle. These elements can be: The steering pump, steering unit or valve and the steering cylinder. 


          Device consisting of a wire mesh that is placed on the suction of the pump inside the tank to retain large particles and thus avoid damaging the pump. The space between the wires determines the size of the particles that can be trapped. 150 micron is a very common size.


          The maximum travel of the Rod or the Piston in a Hydraulic Cylinder.


          Metal block used to mount hydraulic valves with bolts.

          Suction line

          Is the fluid conductor that connects the tank to the suction of the hydraulic pump. It carries vacuum pressure.

          Surface tension

          It is the cohesive force in the surface of a liquid that prevents an object to enter to it or to exit from it. It is due to the attraction forces among the liquid molecules.It is expressed in pounds per foot.

          Synthetic fluid

          Fluid other than petroleum on which has been artificially compounded for use in a hydraulic system.



          Device used to measure the rotational speed RPM. There are some optical, mechanical, electronic and some that can be connected to computer.

          Three way valve

          Directional valve with three passages. Basically used to pressurize the output port and connect it to the tank alternatively. Very common when controling a single acting cylinder.

          Torque motor

          Electromagnetic device consisting of coils and ferrite cores used to actuate servo valve.


          Steel conductor that can be bent used to drive the flow from a hydraulic element to another. It can be formed to fit the shape of the machine.

          Turbulent flow

          Condition of flow in a pipe where the speed is high enough so that the fluid muve in absolute disorder. Friction against the walls is very high.

          Two way valve

          Directional valve that has two flow passages. Normally open or closed. Opens or closes a passage.


          Unload valve

          Condition in a circuit when the flow from the pump is sent to the tank without pressure (freely).


          Condition in a circuit when the flow from the pump is sent to the tank without pressure (freely).



          The pressure value less than atmospheric pressure. Usually expressed in inches or centimeters of Hg.


          The Hydraulic Directional Valve is the component of a Hydraulic System that diverts or changes the direction of the Flow for different purposes. Every time that this valve changes its State, the flow changes the direction changing the flow paths internally. Directional valves can have Two, Three or more Positions or States and Three, four or more internal Flow Paths. The most common application is to control Hydraulic Actuators to change rotational or linear motion direction.

          Variable displacement motor

          Is a hydraulic motor which its internal displacement can be varied when the volume of the internal chambers, by external or internal means is changed.


          The effect of removing the ability to control pressure. If in a pilot operated relief valve the pilot control line is connected to the tank, the main valve opens at a minimum pressure. The hydraulic system is unable to do the job.


          A measure of the resistance to flow of a fluid.

          Viscosity index

          Measurement of the dependence of viscosity with temperature changes. The higher the index, the less the viscosity is affected.


          Measurement of the dependence of viscosity with temperature changes. The higher the index, the less the viscosity is affected.


          Working pressure

          Is the minimum pressure needed to do the job including all the losses from the pump to the actuator and load fluctuations.