Chapter 13 Applying Hydraulic Power

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Chapter 13Chapter 13

Applying Hydraulic PowerApplying Hydraulic Power

Typical Circuits and SystemsTypical Circuits and Systemsyp yyp y

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ObjectivesObjectives

Name common, basic subsystems that compose l h d li t d d ib th icomplex hydraulic systems and describe their

function.

Compare the design and operation of basic pressure control circuits.

Contrast the operating characteristics of meter-

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.3

Contrast the operating characteristics of meter-in, meter-out, and bleed-off flow control circuits.


Compare the design and operation of several t i l fl t l i ittypical flow control circuits.

Compare the design and operation of basic motion control circuits.

Describe the design and operation of rapid-advance-to-work circuits


advance-to-work circuits.

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Describe various hydraulic circuits designed to id f t f t d th t tiprovide for operator safety and the protection

of system components.


Basic Circuits and Their Use in Basic Circuits and Their Use in System Analysis and DesignSystem Analysis and Design

Hydraulic systems involve a group of components that closely work together tocomponents that closely work together to produce a desired performance– Pressure control– Flow control– Motion control

Mi ll f ti


– Miscellaneous functions

A hydraulic system typically includes several of these segments

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Design and Operation of Basic Design and Operation of Basic PressurePressure--Related CircuitsRelated Circuits

Basic pressure-control circuits limit the i ti f h d limaximum operating pressure of a hydraulic

system

Variations of this function include circuits that can:– Control pressure from a remote location


Control pressure from a remote location

– Provide for the selection of multiple pressures in a single circuit


A basic pressure-control circuit


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Remote pressure control in a circuit



Multiple operating pressures can be achieved f t b i d li f l

Design and Operation of BasicDesign and Operation of BasicPressurePressure--Related CircuitsRelated Circuits

for a system by using a compound relief valve

The venting feature is combined with additional low-flow-capacity, direct-operated relief valves


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Multiple pressures in a circuit



Reduced pressure in a circuit


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With open- and tandem-center directional control valves, a spring-loaded check valve may be placed in the return line to create low back pressure– Maintains low system pressure necessary to

operate pilot-controlled functions


p p

– This variation still produces an energy-efficient circuit that is a feature of these valve center designs


Back-pressure check valvevalve


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Design and Operation of Basic Design and Operation of Basic FlowFlow--Related CircuitsRelated Circuits

Three basic flow control circuits are used in h d li thydraulic systems:– Meter in

– Meter out

– Bleed off

These basic circuits meet the varying flow-


These basic circuits meet the varying flowcontrol demands for systems with positive and negative loads


The meter-in flow control design places the fl t l l b t th d thflow control valve between the pump and the inlet of the actuator– Should only be used for positive loads

– Cannot provide accurate control under a negative load condition


– The prime mover is always operating against the maximum pressure setting of the system relief valve

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Basic meter-in circuit



The meter-out flow control design places the fl t l l b t th t t tl tflow control valve between the actuator outlet and the reservoir– Can provide accurate control for positive and

negative loads

– The prime mover is always operating against the


maximum pressure setting of the system relief valve

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Basic meter-out circuit



The bleed-off flow control design places the fl t l l i t i th ki liflow control valve in a tee in the working line between the directional control valve and the actuator inlet– Outlet of the flow control is directly connected to

the reservoir


– Measured flow is diverted to the reservoir while remaining flow operates the actuator

– The prime mover operates against a pressure only high enough to move the load

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Basic bleed-off circuit



Meter-in and meter-out circuits provide the most accurate actuator speeds

Both meter fluid flow delivered directly to or from the actuator

The meter-out circuit is the best method for negative loads that may pull the actuator


negative loads that may pull the actuator

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The bleed-off flow control circuit is less t th ith th t i t taccurate than either the meter-in or meter-out

system– Flow is metered back to the reservoir while the

remaining pump output establishes actuator speed

– The remaining flow can vary because of pump


efficiency and system leakage

– Flow control valve metering accuracy under varying load conditions is also a factor


The bleed-off circuit is the most energy-ffi i t d iefficient design

This is due to the fact the prime mover operates at a pressure only high enough to move the load


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Design and Operation of Other Design and Operation of Other FlowFlow--Related CircuitsRelated Circuits

Several other circuit designs can also be used t t l fl t i it t tto control flow to a circuit actuator– High-low pump circuit

– Regenerative-cylinder-advance circuit

– Rapid-advance-to-work circuit



The high-low pump circuit allows a system to t i t duse two pumps in tandem

– Provides high-volume flow during low-pressure demand

– Provides low-volume flow during high-pressure demand


– Lowers equipment costs by reducing prime mover size and cuts ongoing system energy consumption

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High-low pump circuit



A regenerative-cylinder-advance circuit provides a rapid advance to the point where a load is encountered– Fluid is recirculated from the rod end into the

blind end and coupled with pump output to increase flow into the blind end, thus increasing


cylinder extension speed

– Recirculation is possible due to the difference in area between the cylinder piston ends

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Regenerative-cylinder-advance circuit



During the rapid-extension phase of a ti i it th ff ti f iregenerative circuit, the effective force is

limited to the maximum pressure setting of the system multiplied by the cross-sectional area of the cylinder rod


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A mechanically activated deceleration valve is ft d i id d t k i itoften used in rapid-advance-to-work circuits

This controls the rate at which the advancing cylinder is slowed from the rapid-advance speed to the slower feed rate



Application of a deceleration valve


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Design and Operation of Basic Design and Operation of Basic MotionMotion--Related CircuitsRelated Circuits

Sequence circuits operate actuators in a specific order

Two common methods for actuator sequencing are:– Spring-loaded check valves

Specially designed sequence valves


– Specially designed sequence valves


Check valves are inexpensive

Valves designed for specific sequence control provide more accurate movement


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Sequencing with spring-loaded check valves



Specially constructed pressure control valves can be used to sequentially block fluid flow to actuators– When a preselected pressure is reached, these

valves open to allow fluid flow to the actuator

– This provides the order of operation


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– Check valves allow reverse flow around these pressure control valves

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Sequencing with special valves



Synchronizing the movement of cylinders i t l t i fl id fl i trequires accurately metering fluid flow into

the actuators– Accurate flow control valves

– Other metering devices, such as mechanically connected hydraulic motors


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Synchronization with accurate flow control lvalves



Synchronization with mechanically coupled tmotors


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Design and Operation of Design and Operation of Circuits with Other FunctionsCircuits with Other Functions

Two factors that are basic to motor operation are:– Fluid flow rate establishes motor speed

– Pressure establishes torque output

Other factors must also be considered:– Motor freewheeling


– Motor freewheeling

– Motor braking


Motor control circuits with a three-position di ti l t l l ll h d lidirectional control valve can allow a hydraulic motor to:– Rotate with full speed and torque capability

– Freewheel using the inertia of the load to sustain rotation


– Brake to a stop under controlled conditions

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Circuit allowing a t t b dmotor to be powered,

braked, or to freewheel



Decompression circuits release the pressures f th i d t ti ffrom the expansion and contraction of components and the external loading of actuators – These high pressures can exist even when the

power unit is not operating


– Decompression is for both operator safety and the continued functioning of the system

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Decompression circuits often use staged, il t t d h k lpilot-operated check valves

– Pressurizing the first stage opens a small orifice that quickly reduces the high system pressure

– The pilot mechanism continues to open the check valve, exposing a larger orifice that allows the


rapid movement of system fluid to the reservoir


Decompression circuit on a platen press


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Safety is always a factor when designing h d li i ithydraulic circuits– High pressures

– High fluid flow rated

– Substantial forces

– Rapid movement of system components


Rapid movement of system components

Inappropriate use of any of these elements can produce a dangerous work situation for a machine operator


Safety-related circuits ensure:– Equipment can be operated without injury

– Equipment cannot unexpectedly operate

– Costly equipment is protected against excessive pressure and operating rates


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Design and Operation of Design and Operation of Circuits with Other FunctionsCircuits with Other Functions Two-hand safety circuit for machine operator

safetysafety


Review QuestionReview Question

Remote control of pilot-operated relief valves b il hi d b i thcan be easily achieved by using the _____

feature of the valve.

vent


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A maximum pressure lower than the system li f l tti b bt i d i tirelief valve setting may be obtained in a portion

of a circuit by using a(n) _____ valve.

pressure-reducing


pressure-reducing


Name the three characteristics of basic flow t l i it th t d t b id dcontrol circuits that need to be considered

when selecting a design for use in a hydraulic system.

A. Accuracy of flow control, B. operating


efficiency, and C. ability to control varying loads.

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Which flow control circuit is considered the t ffi i t?most energy efficient?

Bleed-off circuit.



Describe two methods that may be used to h i th t f tsynchronize the movement of two or more

hydraulic cylinders.

A. Use of two hydraulic motor with a mechanical coupling between their output


mechanical coupling between their output shafts and B. use of accurate pressure-compensated flow control valves.

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How is rapid cylinder advance created in a ti li d d i it?regenerative-cylinder-advance circuit?

Fluid from the rod end of the cylinder is directed to the blind end to increase


the amount of fluid flow available to extend the cylinder.


Name three factors that hydraulic safety circuits ensure.

A. Equipment can be operated without injury, B. equipment cannot unexpectedly operate,

d C C l i i d i


and C. Costly equipment is protected against excessive pressure and operating rates.

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GlossaryGlossary

Back-pressure check valve A i l d d h k l d ith il t– A spring-loaded check valve used with some pilot-controlled hydraulic systems to maintain sufficient pressure during the idle portion of the cycle to shift directional control valves.


GlossaryGlossary

Bleed-off circuit– A hydraulic speed-control circuit in which the

required flow rate to the actuator is set by returning unneeded pump output directly to the reservoir through a flow control valve.


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GlossaryGlossary

Deceleration valve– A control valve specifically designed to allow

maximum, deceleration, and feed flow rates. The valves often use mechanical activation and a check valve that allows an unrestricted return flow rate.


GlossaryGlossary

High-low pump circuit– A circuit using two fixed-displacement pumps. The

pumps are connected, with appropriate pressure control valves, to provide a high-volume flow at low pressure and a low-volume flow when high pressure is required to operate a system.


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GlossaryGlossary

Maximum operating pressure– The highest pressure recommended for operation of

a hydraulic or pneumatic system.

Meter-in circuit– A fluid power flow-control circuit in which fluid is

metered into an actuator to control operating speed.


p g p

GlossaryGlossary

Meter-out circuit – A fluid power flow-control circuit in which fluid

is metered out of an actuator to control operating speed.


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GlossaryGlossary

Rapid-advance-to-work circuit– A fluid power circuit design that allows a tool or

other machine element to rapidly move to a preset position, slow to a point where machining is to occur, and then move at an accurate cutting speed. May also allow rapid retraction of the work elements


elements.

GlossaryGlossary

Regenerative-cylinder-advance circuit– A hydraulic circuit design in which fluid discharged

from the rod end of a cylinder combines with pump delivery and is directed to the cylinder blind end. The design results in increased cylinder extension speed, but reduced force capability.


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GlossaryGlossary

Remote location– A location for control components that is beyond

the distance usually used for placement of components on machines.

Sequence– The order in which various valve and actuator


functions occur in a fluid power circuit.

GlossaryGlossary

Synchronization– The timing of one movement or operation with

another, such as the identical movement of two actuators operating in a fluid power circuit.


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GlossaryGlossary

Two-hand safety circuitA fluid power circuit in which machine activation– A fluid power circuit in which machine activation and continued operation requires both hands of the machine operator to continuously depress control valves.


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Chapter 13 Applying Hydraulic Power