Chapter 12

Fluid Power

Objectives

• Upon completing this chapter, you should be able to:– Discuss some fluid power fundamentals– Explain psi, psig, psia, and inches of mercury

("Hg) – Understand how force is transmitted through

a hydraulic system

Objectives (cont’d.)

– Understand the effects of compressing air for a fluid power system

– Learn how to recognize the different valves that may be used in a fluid power system

– List and explain at least eight methods of valve actuation

– Discuss the operation of different actuators

Fluid Power Fundamentals

• Fluid power has several advantages over mechanical power: – Ease of control – Accuracy – Ability to multiply force – Constant force – Constant torque – Instantly reversible

Fluid Power Fundamentals (cont’d.)

• Pressure: restriction against the flow of liquid– Responsive to the amount of restrictions

present in that system– The total pressure will be just the amount

necessary to overcome the restrictions that are present in the system

Hydraulics

• Hydraulics: the science of transmitting power through a liquid– The liquid used most commonly is hydraulic

oil

Oil Temperature

• As oil is pumped, pressure will begin to rise if there are restrictions in the system– As the pressure begins to rise, the

temperature of the oil begins to rise

Static Head Pressure

• The amount of pressure that is developed for every inch of rise in elevation above the point of measurement

Figure 12-1: Static head pressure. Delmar, Cengage Learning 2013.

Hydraulic Operation

• When pressure is applied to a fluid, the pressure is transmitted through the fluid

• Power in should equal power out – Except in the case where losses exist in a

circuit, such as from mechanical friction and flow friction

Hydraulic Operation (cont’d.)

• The force of a piston can be calculated by multiplying the pressure times the area (F = P x A)

Figure 12-2: Hydraulic power is transferred through the fluid. Delmar, Cengage Learning 2013.

Pneumatics

• Pressure is created as soon as the air is compressed

• Pneumatic energy is often referred as “air”• A pneumatic system will have a method of

storing compressed air

Fluid Conditioners

• These devices include:– Filters– Air lubricators

• Typically installed along with a pressure regulator– Trio assembly or filter-lubricator-regulator

(FLR)

A Vacuum

• Vacuum fluid power may be described as deriving its force from the weight of the atmosphere

• Atmospheric pressure (at sea level) is 14.7 psi

• Vacuum measured in inches of mercury ("Hg)

Figure 12-7: An example of using a vacuum to do work.Delmar, Cengage Learning 2013.

Valves

• Used in a fluid power system for one of the following purposes:– Flow control– Directional control– Pressure control

• Open: fluid can flow through it• Closed: fluid cannot flow through it

Valves

• Many types of valves available:– Spool valves– Poppet valves– Gate valves– Ball valves– Needle valves

Linear Actuators

• Cylinders– Refers to all of the components that are

housed within the cylinder, including the piston

– The piston does the work because the shaft of the piston is connected to the load

Cylinders

• Single-acting – Actuated by fluid, and then returns to its

original state using a spring return• Double-acting

– Moved by fluid in both directions

Figure 12-21: The single-acting cylinder principle. Delmar, Cengage Learning 2013.

Rotary Actuators

• Hydraulic and pneumatic motors are referred to as rotary actuators

• Rotary actuator – A device that has a rotary movement instead

of a linear movement as for the cylinder

Flow Control Circuitry

• Three types of flow control circuitry are used in a linear hydraulic system:– The meter-in circuit– The meter-out circuit– The bleed-off circuit

Accumulators

• Accumulators are used in hydraulic systems to maintain pressure within the system

• Refer to Figure 12-27

Pneumatic Directional ControlCircuitry

• The most common pneumatic control circuit - the flip-flop circuit– Uses a double-end rod, double-acting cylinder

• Referred to as an oscillator