Hydraulics and Pneumatics

The Forces of Fluid Power 201

The Forces of Fluid Power presents a comprehensive overview of fluid power transmission systems. It offers a broad scope of information, from fluid characteristics and basic energy forms to force multiplication and the effect of fluid flow rate in a system. When pressurized, fluids are able to produce tremendous power with a minimal amount of effort. Maintaining constant fluid flow is essential for any system to work effectively. While the type of fluid in systems differ, the key components of all fluid systems and processes are similar. More importantly, the units of measurement are the same.

Without a full understanding of fluid power and the units used to measure key components of a fluid system, a fluid system may not have the proper pressure, volume, force, or fluid flow rate needed to maintain constant fluid flow. After taking the class, users will be able to better recognize how fluids systems function and explain the variables that affect them.

  • Difficulty Intermediate

  • Format Online

  • Number of Lessons 18

  • Language English


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Course Outline
  • Power Transmission Systems
  • Fluid System Components
  • Fluid Energy
  • Liquid and Gas Fluids
  • Fluid Basics Review
  • The Effects of Viscosity
  • Friction
  • Reducing Friction
  • Friction Review
  • Energy Transfer Overview
  • Energy Transfer in Fluid Systems
  • Work and Power
  • Energy Review
  • Fluid Pressure
  • Force Multiplication
  • Fluid Flow Rate
  • Actuators at Work
  • Fluid Power Variables Review
  • Identify the three major types of power transmission systems.
  • Identify the components of a fluid system.
  • Distinguish between potential energy and kinetic energy.
  • Distinguish between liquid and gas fluids.
  • Explain how viscosity affects fluid flow.
  • Describe friction in a fluid system.
  • Identify ways to decrease friction in fluid systems.
  • Explain the law of conservation of energy.
  • Explain different methods of energy transfer in fluid systems.
  • Define work and power.
  • Explain how pressure affects contained fluids.
  • Explain how multiplication of force occurs.
  • Define fluid flow rate.
  • Identify the two types of motion produced by an actuator.
Vocabulary Term


A component in a fluid system that converts hydraulic and pneumatic energy into mechanical energy. Actuators are also known as effectors.


A device that cools discharged air from an air compressor. An aftercooler also removes moisture and helps minimize vapor lock.

air compressor

A component that pressurizes ambient air and directs it into a pneumatic system. An air compressor decreases the volume of air.


A device that converts mechanical energy into electrical power. An alternator produces an alternating current.

ambient atmosphere

The air immediately surrounding a pneumatic system's intake filter. Ambient atmosphere is pressurized by a compressor for use in a pneumatic system.


The amount of space contained on a surface or within an enclosed object. Area is typically measured in square units.

atmospheric pressure

The pressure exerted by the weight of air in the atmosphere. Atmospheric pressure is also called barometric pressure.


An imaginary straight line passing through the center of an object. A round component such as a gear typically rotates around its axis.


Flow-directing or obstructing plates designed to support tube bundles. Baffles direct the flow of fluids in a heat exchanger shell.


A device that allows one moving part to glide past another moving part without excess wear or friction. Bearings work using a sliding or rolling mechanism.

boundary layers

A thin layer of flowing fluid in contact with a surface. Boundary layers may be laminar or turbulent.


A circular or cylindrical component designed to make contact with another component while rotating. Cams may convert rotational movement into linear movement.


A type of fluid power system in which the fluids flow continuously between the pump and the actuators. Closed-loop systems, also called servo systems, use feedback to maintain consistent input and output.

compressed air

A gas, or combination of gases, that has been squeezed to a greater pressure than the air surrounding it. Compressed air is used in various applications, including tire pumps and jackhammers.


A form of heat transfer. Conduction moves heat from a solid object to another object or area that has a different temperature.


A component such as a pipe, tube, or hose. Conductors convey liquids or gases throughout a fluid system.

control valves

A mechanism that controls fluid in a pneumatic or hydraulic system. Control valves direct fluid movement and regulate the amount of pressure exerted in the system.


A device that connects two shafts in a mechanical system in order to transfer motion.


One complete revolution or rotation of a rotary hydraulic component. A cycle is one complete extension and retraction of a linear hydraulic component.

cylinder face

The top or base of a cylinder. A cylinder face is considered one of two plane surfaces that meet with a cylinder's curved surface.

directional control valves

A mechanism that controls fluid direction by shifting from one position to another. Simple directional control valves, known as check valves, allow fluid to flow in only one direction.


The resistance to air flow. Drag is used to describe the friction of air.

electric motor

A machine that converts electricity into mechanical energy or motion. An electric motor is a type of prime mover for a fluid system.

electrical energy

Energy created by the movement of electrons. Electrical energy can be converted into light, heat, or motion.

electrical power system

A power transmission network that uses the force of flowing electrons to transmit power. Common electrical power system components include the power supply, conduits, and circuit protectors.


A negatively charged particle that orbits the nucleus of an atom. An electron moves between atoms to cause electrical flow.


The ability to do work. Energy may be potential or kinetic and may appear as electrical, mechanical, thermal, or chemical energy, among other forms.


FRL. A device that conditions air for use in pneumatic systems. An FRL unit removes any solid particles that could cause obstruction, controls air pressure, and lubricates the air using a small amount of oil.

fluid flow

The volume of fluid displaced by a pump or compressor into a hydraulic or pneumatic system. The amount of fluid in motion is contingent upon unbalanced forces.

fluid flow rate

The volume of fluid displaced within a given time. Fluid flow rate determines the speed at which work is performed by fluid power systems.

fluid friction

The resistance to an object's motion through a liquid or gas. Motion occurring in a liquid or gas is referred to as viscous resistance or air friction, respectively.

fluid power motors

An actuator that converts energy from fluid flow into continuous mechanical motion. Fluid power motors are used in both hydraulic and pneumatic fluid systems.

fluid power system

A power transmission network that uses the force of flowing liquids or gases to transmit power. Fluid systems are hydraulic or pneumatic.

fluid reservoir

Part of a hydraulic circuit that stores reusable fluid. A fluid reservoir dissipates heat, cleanses fluid, and allows air bubbles to disperse.


A state of matter that has the ability to flow. Fluids can be liquids or gases.


ft-lb. A unit used for measuring work. A foot-pound, the amount of energy needed to move one pound a distance of one foot, is equal to 1.36 Nm.


The push or pull that changes an object’s motion or state of rest. Force is calculated by multiplying the mass of an object by its acceleration.

force multiplication

The exponential increase in available power. Force multiplication is provided by hyrdaulic and pneumatic drives.

forced convection

A device in which hot fluid is made to flow over a surface or in a tube by external means, such as a pump or fan. Forced convection can remove excess heat from fluid systems.


The accumulation of unwanted material on the surface of a component. Fouling reduces flow, may increase flow velocity elsewhere, and may cause flow blockage.


A force that resists motion between two objects that are in contact with each other. Smoother surfaces exhibit less friction, while rougher surfaces exhibit more friction.


foot pounds. A unit used for measuring work. A foot-pound, the amount of energy needed to move one pound a distance of one foot, is equal to 1.36 Nm.


An airlike fluid that expands freely to fill any space available, regardless of its quantity. Gas differs from liquid in that there are no forces keeping gas molecules together.


A round or cylindrical mechanical component with teeth that is used to transmit power. A gear is designed to mesh with another gear and can alter the speed, torque, or direction of mechanical energy.


One of a set of toothed wheels that work together. A gear increases the force needed to push or pull something.

gear pump

A pump that uses the meshing of toothed wheels to move fluid by displacement. Gear pumps are one of the most common types of pumps used in hydraulic fluid power applications.


A device that converts mechanical energy into electrical power. A generator produces a direct current.

gravitational potential energy

Energy an object possesses because of its position in a gravitational field. While all objects may have potential energy, gravitational potential energy is only stored in the height of an object.

gravitational pull

The invisible force on an object located near the earth's surface. The amount of gravitational pull depends on an object's mass.


The force exerted by the Earth on other objects at or near its surface. Gravity pulls on the entire mass of an object.

heat exchanger

A device that allows heat from a fluid to pass to a second fluid without the two fluids coming into direct contact. Heat exchangers may be shell and tube, plate, or air cooled.


A unit of power used to measure mechanical power. One horsepower equals 33,000 foot-pounds (ft-lb) of work per minute, or 746 watts.

hydraulic cylinders

A linear actuator powered by liquid, usually oil. Hydraulic cylinders are capable of consistent, repeatable back-and-forth action.

hydraulic fluid system

A power transmission system that uses liquid conveyed in a confined conduit under pressure to accomplish work. Hydraulic systems differ from pneumatic systems in that they use petroleum- or water-based liquids.

hydro-electric power plant

A facility that uses the movement of water to generate electricity. A hydro-electric power plant operates using stored water in a dam.

inert gas

A type of gas, also referred to as noble gas, that does not react with other elements. Argon and helium are inert gases.

internal friction

The force resisting motion between the elements making up a substance while it undergoes deformation. Internal friction, also called fluid friction, affects the viscosity of a fluid.

kinetic energy

Energy existing due to an object's motion. Kinetic energy is often harnessed, converted to other types of usable energy, or transferred to other objects.

laminar flow

The type of flow in a fluid system characterized by the presence of laminae, or parallel layers of fluid. Laminar flow is considered smooth fluid flow in which very little friction exists between the layers.

law of conservation of energy

The scientific principle stating that energy can neither be created nor destroyed, but can be changed from one form into another form or transferred from one object to another object.


A simple machine consisting of a rigid bar that pivots around a fixed point. Levers are used to transmit motion.

linear actuators

A mechanical device that converts energy to create motion in a straight line. Linear actuators include hydraulic and pneumatic cylinders.

linear motion

Motion that takes place along a straight line. Mechanical energy can take the form of linear or rotary motion.


A fluid with a fixed volume. A liquid has the ability to flow and take the shape of its container.


The weight or source of pressure placed upon an object. A load may be indicated in pounds (lb.) or Newtons (N).


The property that diminishes friction and increases smoothness and slipperiness. An effective lubricant has a high lubricity.


A substance that has mass and exists as a solid, liquid, or gas. Matter can change states based on temperature and pressure.

mechanical energy

Energy that is produced by a force of motion. Mechanical energy results from the physical interaction of instruments or tools.

mechanical power system

A power transmission network that uses forces created by the physical contact of mechanisms to transmit power. Mechanisms may include levers, pulleys, gears, and couplings.

molecular structure

The arrangement of the forces that bind atoms together in molecules. Molecular structure affects fluid flow.


A group of atoms bonded together. Molecules represent the smallest physical unit of an element.

natural convection

A form of heat transfer that occurs as heat moves from components in a hydraulic system into the surrounding atmosphere. In smaller hydraulic systems, heat transfer from the oil to conduits and other component surfaces usually provides sufficient cooling.

newton meters

N-m. A unit for measuring work. One newton meter is the amount of force needed to move one kilogram a distance of one meter.

Newton's first law of motion

A scientific law of inertia developed by Sir Isaac Newton. Newton's first law of motion states that an object will remain at rest or in its original motion until acted upon by an outside force.

newtons per square meter

N/m². A unit of pressure that measures the number of newtons per square meter. Newtons per square meter are also known as pascals, which are derived from the International System of Units (SI).


newton meter. A unit for measuring work. One newton is the amount of force needed to move one kilogram a distance of one meter.


A type of fluid power system in which the fluids return to a reservoir before flowing to the pump inlet. An open-loop system generates less heat than a closed-loop system.

Pascal's Law

A principle that states when pressure is applied to a contained fluid, the force is transmitted equally in all directions. Pascal's Law was established in the 1600s by French scientist Blaise Pascal.


Developed from gaseous, liquid, or solid hydrocarbons naturally occurring beneath the earth's surface. Refined petroleum oil is the most common hydraulic fluid base.


A disk or short cylinder fitting closely within a tube. A piston moves up and down against a liquid or gas.

pneumatic cylinders

A linear actuator powered by compressed air. Like hydraulic cylinders, pneumatic cylinders are capable of consistent, repeatable back-and-forth action.

pneumatic fluid system

A power transmission network that uses the force of flowing gases to transmit power. A pneumatic system employs compressed air or gas.

positive displacement pump

A device that moves liquids by trapping a fixed amount of liquid and forcing it into the discharge conduit. A positive displacement pump moves a constant amount of fluid through each cycle or rotation.

potential energy

Stored power in an object. A component called an actuator converts potential energy to kinetic energy.

pounds per square inch

psi. A unit of pressure that measures the number of pounds per square inch. Pounds per square inch is derived from the English system.


The rate at which work is accomplished. Power is the result of the amount of work accomplished divided by the time it took to do the work.

power transmission

The transfer of energy from a source to a specific location. Power transmission includes mechanical, electrical, and fluid power.


The force per unit area exerted by a fluid against a surface with which it is in contact. Pressure is typically expressed in pounds per square inch (psi).

pressure control valves

A valve used to control pressure in a fluid flow line. In pneumatic systems, pressure control valves regulate the force of the gas and are used in conjunction with flow meters to manage the gas's flow rate.

pressure drop

The difference in pressure between two points of a fluid system. Pressure drop occurs when friction acts on a fluid as it flows through a conduit.


Producing or maintaining a rise in the continuous physical force exerted on or against an object by something in contact with it. Pressurized fluids have a physical force that is produced or maintained artificially.

prime mover

The main component of a hydraulic or pneumatic system that powers a pump or compressor. Electric motors and diesel engines are common prime movers.


A circular device that is used to transmit motion. Pulleys may be movable or fixed.


A mechanical device that moves liquids in a hydraulic system by introducing flow into the system. Common pump types include gear, vane, and piston.


A straight line from the center to the circumference of a circle or sphere. The radius is half the diameter.


A quantity measured with respect to another measured quantity. Rates vary depending on speed.

reaction force

Push or pull that acts in the opposite direction of a downward, upward, or horizontal force. A reaction force is a subject of Newton's third law of motion which states that for every action, there is an equal and opposite reaction.

receiver tank

A storage tank used with pneumatic systems. A receiver tank balances the air compressor capacity with airflow demand.

rotary actuators

A device that directs force in a circular motion. Rotary actuators provide rotational motion of less than 360 degrees.

rotary motion

Spinning action that takes place around an axis without a change in linear position. Mechanical energy can take the form of rotary or linear motion.


A substance that has the qualities of both a fluid and a solid. Semifluid is also referred to as semiliquid.


A coil of wire that converts electrical energy into mechanical energy via magnetic fields to exert a force, typically in a linear direction. Solenoids can be used to activate switches and valves.


The rate at which an object travels. Speed is calculated by dividing the distance traveled by the time elapsed.


The state of an object at rest. A static object may be working while causing no movement.

static state

The state of an object at rest. A static object may be working while causing no movement.

stroke length

The full range of linear motion of a reciprocating part, such as a piston. The stroke length of a piston is the distance marked by the farthest ends of vertical movement of the piston within the cylinder.

surface area

The sum of an exposed area of a solid object. Surface area is expressed in square units.

thermal energy

Energy resulting from the motion of particles. Thermal energy is a form of kinetic energy and is transferred as heat.


Related to or caused by heat. When particles are in motion, thermal energy is transferred as heat.


A force that produces rotary motion. Torque is measured in foot-pounds in the English system and Newton meters in the metric system.


A machine for producing continuous power. A turbine consists of a wheel or rotor made to revolve by a fast-moving flow of water, steam, gas, air, or other fluid.

turbulent flow

The type of flow in a fluid system in which the layers of fluid are mixed together. Turbulent flow differs from laminar flow in that its layers move in a disordered pattern.

unbalanced force

An application of energy to an object that is unequal to another force applied to the same object. An unbalanced force can cause an object to start motion, stop motion, or change direction.


A device that controls the flow of fluid by opening and closing. Valves come in various sizes and types.

vapor lock

The rapid formation of condensation in fluid conductors. Vapor lock can obstruct fluid flow.


The rate of change in an object's position. Velocity has a speed and a direction.


The measure of a fluid's thickness, stickiness, or resistance to flow. Fluids with high viscosity resist flow.


A measurement of the amount of space contained within a three-dimensional shape. Volume is measured in cubic inches (in³)


A measurement of the amount of space contained within a three-dimensional shape. Volume is measured in cubic inches (in.³).


A hydraulic fluid of or primarily consisting of water. Water-based fluids are ideal for hydraulic systems that are used near a fire hazard.


The result of a force applied to an object and the distance through which the force is applied. Work is equivalent to force multiplied by distance.