Chapter 10

Fluid Power Systems

Objectives

• Advantages of hydraulic and pneumatic power systems.

• Safety concerns when working with liquids and gases under pressure.

• Mechanical advantage created by using liquids under pressure.

What are Fluid Power Systems

• Fluid power systems include both hydraulic and pneumatic systems.

• Hydraulic systems control and transmit energy through the use of liquids such as oil. Components include cylinders and pumps.

• In pneumatic systems gas is used instead of liquids.

Why Use Fluid Power?

• The primary advantage of fluid power is the ability to multiply force in order to generate strength.

• The components used in a fluid power system experience less wear than purely mechanical systems.

• Gases under pressure have a natural springiness which produces a cushioning effect.

The Physics of Fluid Power Systems

• Fluids exert pressure: Because pressure is caused by the weight of liquid in a container, more pressure is exerted on its lower sides.

• How fluids act: Fluids will flow from an area of high pressure to an area of low pressure.

• Measuring effort and rate in fluid systems: A pressure gauge measures the difference between pressure within the fluid circuit and the pressure of the surrounding atmosphere in pounds per square inch (psi). The rate of flow is measured with a flowmeter in gallons per minute (GPM) for liquids and cubic feet per minute (CFM) for gases.

• Viscosity of liquids: Viscosity is the resistance of a fluid to flow. • Opposition to flow in fluid systems: include friction and

turbulence.

Components of a Fluid Power System

• Hydraulic pumps: Convert mechanical energy into fluid power, creating the necessary flow in the system.

• Gear pumps: create the pressure needed to operate many hydraulic systems.

• Centrifugal pumps: use centrifugal force to move fluids in a system.

• Reciprocating pumps: use a piston that moves back and forth in a cylinder to move hydraulic fluid.

• Air Compressors: convert mechanical energy input into pneumatic power. Compressors are teamed up with reservoirs or pressure tanks to store and transmit compressed air to the power system whenever it is needed. The most common air compressors are the reciprocating type.

Controlling Fluid Power

• Flow control valves: A valve to start or stop flow of fluid in a system.

• Pressure control valves: Pressure reducing valves reduce pressure within a fluid circuit to levels suitable for use.

• Pressure relief valves: If pressure increases to dangerous levels, relief valves automatically open to reduce it.

• Directional Control Valves: They are used to control which path a fluid takes in a circuit.

Sizing Fluid Power Conductors

• Flow considerations: As the diameter is doubled, the cross sectional area increases four fold.

• Velocity considerations: A conductor with a larger diameter allows greater flow capacity, but the velocity decreases.

• Pressure considerations: Hoses should be designed to withstand not only working pressure, but also excess pressure.

Making Fluid Power Work

• Actuators: Actuators are devices that convert fluid power to mechanical power.

• Fluid motors: convert fluid power into rotary mechanical motion.

• Storing Fluid Power: pressure tanks typically include a switch that automatically turns the compressor motor on when the pressure in the tank drops below a preset level.

Working Safely with Fluid Power

• 1. Always adjust pressure relief valves to provide a safe operating pressure.

• 2. Always wear safety glasses to protect your eyes from unexpected hazards. Hoses can burst from their fittings causing them to whip violently in the air until the pressure is released or shut down.

• 3. Make necessary adjustments and changes to fluid circuits when they are not under pressure.

Comparing Liquids and Gases

• Gases can be compressed, liquids cannot.• Another difference is that pneumatic power can

be an open system, while as hydraulic power must be a closed system.

• The properties of confined fluids make hydraulic devices capable of increasing mechanical advantage (hydraulic jacks are good examples).

Summary• The primary advantage of fluid power is the ability to multiply force in order to

generate strength.• The components used in a fluid power system experience less wear than purely

mechanical systems.• Gases under pressure have a natural springiness which produces a cushioning

effect.• The rate of flow is measured with a flowmeter in gallons per minute (GPM) for liquids

and cubic feet per minute (CFM) for gases.• Hydraulic pumps: Convert mechanical energy into fluid power, creating the

necessary flow in the system.• Air Compressors: convert mechanical energy input into pneumatic power.• Flow control valves: A valve to start or stop flow of fluid in a system.• Always adjust pressure relief valves to provide a safe operating pressure. Always

wear safety glasses to protect your eyes from unexpected hazards.• The properties of confined fluids make hydraulic devices capable of increasing

mechanical advantage.

Home Work

• 1. What are the advantages of fluid power?• 2. What is the advantage of gas power?• 3. Give an example of why we need to wear

safety glasses when working with fluid power?