applied hydraulics and pneumatics notes.doc

8/11/2019 applied hydraulics and pneumatics notes.doc

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Nehru Institute of Technology

Department Of Mechanical Engineering

APPLIED !D"A#LI$% AND PNE#MATI$%

!ear& III %emester '

UNIT I FLUID POWER SYSTEMS AND FUNDAMENTALS

UNIT II HYDRAULIC SYSTEM & COMPONENTS

UNIT III DESIGN OF HYDRAULIC CIRCUITS

UNIT IV PNEUMATIC SYSTEMS AND COMPONENTS

UNIT V DESIGN OF PNEUMATIC CIRCUITS


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UNIT I FLUID POWER SYSTEMS AND FUNDAMENTALS

What is fluid !"#$%

)lui* po+er is energy transmitte* an* controlle* ,y means of a pressur i-e* flui*.either li/ui* or gas0 The term flui* po+er applies to ,oth hy*raulics an* pneumatics0

y*raulics uses pressuri-e* li/ui*. for eample. oil or +ater2 pneumatics uses

compresse* air or other neutral gases0 )lui* po+er can ,e effecti3ely com,ine* +ith

other technologies through the use of sensors. trans*ucers an* microprocessors0

H!" fluid !"#$ "!$s

Pascal4s La+ epresses the central concept of flui* po+er& Pressure eerte* ,ya confine* flui* acts un*iminishe* e/ually in all *irections0

An input force of 16 poun*s 8809 N: on a 1;s/uare;inch


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flui* po+er manifol*s an* 3al3es0


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Hi)h h!$s#!"#$- l!" "#i)ht $ati!+Pneumatic components are compact an*light+eight0 !ou can hol* a fi3e horsepo+er hy*raulic motor in the palm of yourhan*0

L!" s##d t!$.u#+#nli>e electric motors. air or hy*raulic motors can pro*uce largeamounts of tor/ue t+isting force: +hile operating at lo+ spee*s0 %ome hy*raulic an*

airmotors can e3en maintain tor/ue at -ero spee* +ithout o3erheating0 C!(sta(t f!$*# !$ t!$.u#+This is a uni/ue flui* po+er attri,ute0

Saf#t, i( ha/a$d!us #('i$!(0#(ts+)lui* po+er can ,e use* in mines. chemical

plants. near eplosi3es an* in paint applicat ions ,ecause it is inherent ly spar>;free an*

can tolerate high temperatures0 Esta1lish#d s t a (d a $ d s a(d #()i(##$i()+The flui* po+er in*ustry has esta,lishe* *esign

an* performance stan*ar*s for hy*raulic an* pneumatic pro*ucts through N)PA.

the National )lui* Po+er Association an* I%O. the International Organi-ation for

%tan*ar*i-ation0

Fluid !"#$ ali*ati!(s

M!1il#2 ere flui* po+er is use* to transport. eca3ate an* lift materials as +ell as

control or po+er mo,ile e/uipment0 En* use in*ustries inclu*e construction.

agriculture. marine an* the military0 Applications inclu*e ,ac>hoes. gra*ers. tractors.

truc> ,ra>es an* suspensions. sprea*ers an* high+ay maintenance 3ehicles0

I(dust$ial2 ere flui* po+er is use* to pro3i*e po+er transmission an* motion

control for the machines of in*ustry0 En* use in*ustries range from plastics +or>ing

to paper pro*uction0 Applications inclu*e metal+or>ing e/uipment. controllers.

automate* manipulators. material han*ling an* assem,ly e/uipment0

A#$!sa*#2 )lui* po+er is use* for ,oth commercial an* military aircraft. spacecraftan* relate* support e/uipment0 Applications inclu*e lan*ing gear. ,ra>es. flight

controls. motor controls an* cargo loa*ing e/uipment0

Fluid !"#$ $!du*ts

) lu i* p o + e r p r o *u c t s are sol* as in*i3i*ual components or as systems for the original

e/uipment manufacturing. maintenance. repair an* replacement mar>ets0

A typical flui* po+er system inclu*es the follo+ing components&

y*raulic pump or air compressor. +hich con3erts mechanical po+er to flui* po+er0

$ylin*er or motor. +hich con3erts flui* po+er to linear or rotary mechanical po+er0

'al3es. +hich control the *irection. pressure an* rate of flo+0 )ilters. regulators an* lu,ricators. +hich con*ition the flui*0

Manifol*s. hose. tu,e. fittings. couplings. etc0. +hich con*uct the flui*

,et+een components0

%ealing *e3ices. +hich help contain the flui*0

Accumulators an* reser3oirs. +hich store the flui*0

Instruments such as pressure s+itches. gauges. flo+ meters. sensors an*

trans*ucers. +hich are use* to help monitor the performance of a flui* po+er
http://www.nfpa.com/Standards/Standards_Overview.asphttp://productlocator.nfpa.com/productlocator.asphttp://productlocator.nfpa.com/productlocator.asphttp://www.nfpa.com/Standards/Standards_Overview.asp


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system0


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FLUID POWER SYM3OLS4

Li(#s

;continuous line ; flo+ line

;*ashe* line ; pilot. *rain

;en3elope ; long an* short *ashes aroun* t+o or more

component sym,ols0

Ci$*ula$

S.ua$#

Dia0!(d

Mis*#lla(#!us S,01!ls

T$ia()l#

;large circle ; pump. motor

;small circle ; Measuring *e3ices

;semi;circle ; rotary actuator

;one s/uare ; pressure control function;t+o or three a*Bacent s/uares ; *irectional control

;*iamon* ; )lui* con*itioner filter. separator. lu,ricator.

heat echanger:

;%pring

;)lo+ "estriction

;soli* ; Direction of y*raulic )lui* )lo+


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3ASICS OF HYDRAULICS

Chat is hy*raulics

;open ; Direction of Pnematic flo+

y*raulics is the transmission an* control of forces an* motions through the me*ium of

flui*s0 %hort an* simple0

y*raulic systems an* e/uipment ha3e +i*e;sprea* application throughout in*ustry0

)or eample&

; machine tool manufacturing

; press manufacturing

; plant construction

; 3ehicle manufacturing

; aircraft manufacturing

; ship,uil*ing

; inBection mol*ing machines

y*raulic to Electrical Analogy

y*raulics an* electrics are analogous. ,ecause they ,oth *eal +ith flo+. pressure an* loa*0

The components in each type of circuit perform similar functions an* therefore can ,e relate*.

a fe+ eamples are liste* ,elo+&


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'arious forms of energy are con3erte* to accomplish mechanical mo3ement in the inBection

mol*ing machine0 Electrical energy is con3erte* to mechanical energy. +hich in turn

is con3erte* to hy*raulic energy to operate an* control the mo3ing components of the

machine0 The hy*raulic energy is con3erte* to mechanical energy to achie3e the final *esire*result. +hich may ,e mol* clamping pressure or material inBection0 The figure a,o3e

summari-es the energy con3ersions for an inBection mol*ing machine0 $lic> on the thum,nail

for a larger 3ie+0


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Pas*al5s La"

Pascal4s La+ states that a pressure acting on a confine* flui* is transmitte* e/ually an*

un*iminishe* in all *irections0 In the figure ,elo+. a 16 poun* force acting on a 1 s/uare

inch area generates a pressure of 16 poun*s per s/uare inch psi: throughout the container

acting e/ually on all surfaces0

This principle is important to remem,er. that the pressure in any portion of an hy*raulic system

is e/ual throughout that system0 This statement is 3ali* +ith the omission of the force of

gra3ity. +hich +oul* ha3e to ,e a**e*. accor*ing to the flui* le3el0 Due to the pressures that

hy*raulic systems operate at. this smaller

amount nee* not ,e consi*ere* e0g0 a ( foot hea* of +ater approimately e/uals 1805 psi0 a

16 meter hea* of +ater approimately e/uals 1 ,ar0:


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Force Transmission in Hydraulics

One of the mainadvantages on the use of hydraulicto power our moldingmachinery is

the efficienttransmission of forces.

If force F1 isexerted on surface A1,pressure p is created. (p = 1!.A1

"incepressure affects all parts of the system e#ually,therefore pressure p is appliedto

surface area A$.

Force F$ wille#ual pressure p x surface areaA$ (F$ = p xA$!,and transposing that formulafor p E F$

A$.

"ince p = 1 therefore F$ = 1A1 A$ A1.

In the diagram%elow,the followingrelationshipshold& hl"$

EA$A1

EF$F1

'here " E piston stro(eA= piston area

F = force

F1 F2

451

A6

If A1 = 1 s#uare inch and A$ = 1) s#uare inches,then a force of F1 = 1) pounds can supporta force of F$ E 1)) pounds.

*owever, the stroes of the pistons are inverselyproportional to their surface areas .

If the smaller pistonwere moved inthe directionof "1%y 1) inches,then the largerpiston will

only move 1 inch in the directionof "$.


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A$#a a(d F!$*#

As the clamp piston is mo3e* for+ar* *uring the clamp close function. the pressure

*e3elope* acts upon the clamping piston +hich has a certain si-e or area0

A ,asic formula in hy*raulics states that pressure multiplie* ,y area to +hich that pressure

is applie* e/uals force0 i0e0 pressure area force

p A )

The formula can ,e manipulate* to calculate any one of the three 3aria,les p. A or ). if any

of the other t+o 3aria,les are >no+n0

As follo+s&p A )

) ? p A

) ? A p

P$#ssu$#

y*raulic pressure is generate* +hen a flo+ing flui* meets resistance +hich is generally

relate* to the loa* that is ,eing mo3e*0


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A force is applie* 3ia the le3er to pro*uce system pressure p )?A or ) p A:0

If more force is applie*. the system pressure rises until the loa* mo3es. if the loa* remains

constant the pressure +ill increase no further0 The loa* can therefore ,e mo3e* if the

necessary pressure is generate*0 The spee* at +hich the loa* mo3es +ill ,e *epen*ent upon

the 3olume of flui* +hich is fe* to the loa* cylin*er0 )or eample. as the mol* is opening or

closing. the pressure generate* in the system represents the resistance of the toggle le3er to

mo3ement0 A**ing to that resistance +oul* ,e the +eight i0e0 mass: of the mol* an* toggle

le3er an* also the friction ,et+een the toggle le3er ,ushings an* the tie,ars0 Chen the t+o

mol* hal3e s touch an* the toggle ,egins to straighten out. the increasing pressure

represents that +hich is re/uire* to stretch the tie,ars in the generation of a particular clamp

force0 %imilarly +hen inBecting material into the mol* the pressure generate* in the inBection

system represents the resistance of the inBection ram to mo3ement0 A**ing to that resistance

+oul* ,e the mass of the inBection ram an* scre+. the friction ,et+een all mo3ingcomponents an* the resistance of the plastic melt as it is force* /uic>ly into the mol* ca3ity0

P$#ssu$# C!(t$!l

In or*er to safeguar* the system. pressure relief 3al3es are installe*0 The 3al3es ser3e to limit

the amount of pressure that can *e3elop in the hy*raulic system since the 3arious hy*raulic

components are epensi3e an* they are su,Bect to pressure limitations ,efore failure occurs0

One characteristic of flui* flo+ that is important to note here is that flo+ occurs al+ays in the

path of least resistance0 Pressure +oul* continue to rise in the circuit consistent +ith the loa*

,eing mo3e*0 The pressure relief 3al3e is al+ays set to allo+ flo+ to tra3el through the relief

3al3e+ell ,efore pressure rises a,o3e safe le3els an* causes *amage to the system an* its

components0 In other +or*s. the path of least resistance is employe* here to safeguar* the

system after the other mo3ements ha3e ta>en place0


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P$#ssu$# O'#$$id#An etremely important concept to un*erstan* a,out pressure relief 3al3es is their pressure

o3erri*e characteristics0 Pressure o3erri*e is the *ifference ,et+een the pressure at +hich the

relief 3al3e Bust starts to crac> open an* the pressure at the full open position0 )or *irect

acting pressure relief 3al3es this pressure *ifferential can ,e as high as 6F an* proportional

pressure relief 3al3es range from

16F ; (6F0


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P$#ssu$# I(t#(sifi*ati!(

Another important concept to >eep in min* is that of pressure intensification0 This la+

of hy*raulics is often forgotten +hen trou,leshooting hy*raulic circuits0

)or eample. if t+o pistons of *ifferent si-e are connecte* ,y a ro*. the pressure eisting on thesmaller area +ill al+ays ,e greater0 This principle also applies to the cap si*e an* the ro* si*e

of a normal *ou,le acting piston0

If P1 1.666 psi an* A1 16 s/uare inches. then )1 16.666 poun*s of force0

If )1 16.666 poun*s of force an* if A( 5 s/uare inches. then P( (.666

psi0


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S##d i( H,d$auli*s

The spee* of a hy*raulic component can ,e calculate* ,ase* on the formula ,elo+&

)or eample. gi3en the con*itions ,elo+ the inBection piston. therefore the scre+. +ill mo3e at095 inches per secon*0 o+e3er. this spee* +ill not ,e possi,le if the pressure relief

3al3e opens0

H,d$!d,(a0i*s

As +ell as un*erstan*ing the concept of spee* in hy*raulics. it is also important to ha3e some

insight into flo+ characteristics0 )or eample. the *ra+ing ,elo+ sho+s that +hen oil is

flo+ing through *ifferent *iameter pipes an e/ual 3olume flo+s in an e/ual unit of time0 If that

is true an* if the sha*e* /uantity G1 e/uals the sha*e* /uantity G(. then 3elocity '( must ,e

greater than 3elocity '10


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One goal in the initial *esign of hy*raulic po+er transmission systems is to encourage

laminar flo+ as much as possi,le since an increase in tur,ulence +ill increase flo+ resistance

an* hy*raulic losses as +ell0 The *iagram ,elo+ illustrates the concept of tur,ulent flo+0

Although tur,ulent flo+ is +asteful in most hy*raulic applications. it is *esira,le to ha3e

tur,ulence in the oil flo+ as it tra3els through the heat echanger for cooling purposes0 If

tur,ulence eists as the oil flo+s through the heat echanger. more of the oil molecules

come into contact +ith the heat echanger cooling tu,es an* more efficient cooling is the

result0

Di$#*ti!(al C!(t$!l

One of the main a*3antages of hy*raulic ,ase* systems is that the oil flo+ *irection is easily

controlle*0 The *ra+ing ,elo+ sho+s a piston ,eing eten*e*. hel* stationary an* thenretracte*. simply ,y changing the position of a *irectional 3al3e0 E3en though the *ra+ing is

simple in nature. it still *emonstrates the principle in3ol3e* in *irectional control0 In a**ition to

simple *irectional control 3al3es. +e also employ proportional *irectional control 3al3es on

some machines to control the clamp opening an* closing function0


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R#,(!ld7s (u01#$

The "eynol*s Num,er is a non *imensional parameter *efine* ,y the ratio of

*ynamic pressure H u(: an* shearing stress u ? L: an* can ,e epresse* as

"e H u(: ? u ? L:

H u L ?

u L ? J 1:

+here

"e "eynol*s Num,er non;*imensional:

H * e n s it y >g?m. l,m?ft

:

u 3elocity m?s. ft?s:

* yn a m ic 3 is c o s it y Ns?m(. l,m?s ft:

L characteristic length m. ft:

J >ine mat ic 3is co s it y m(?s. ft

(?s:

Da$*,7s E.uati!(

The frictional hea* loss can ,e calculate* using a mathematical relationship that is

>no+n as DarcyKs e/uation for hea* loss0 The e/uation ta>es t+o *istinct forms0 The first form
http://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.htmlhttp://www.engineeringtoolbox.com/dynamic-absolute-kinematic-viscosity-d_412.htmlhttp://www.engineeringtoolbox.com/dynamic-absolute-kinematic-viscosity-d_412.htmlhttp://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.htmlhttp://www.engineeringtoolbox.com/dynamic-absolute-kinematic-viscosity-d_412.htmlhttp://www.engineeringtoolbox.com/dynamic-absolute-kinematic-viscosity-d_412.html


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of DarcyKs e/uation *etermines the losses in the system associate* +ith the length of the pipe0

;18:

f f L 3(

D ( g

+here& f friction factor unitless:

L length of pipe ft:

D *iameter of pipe ft:

3 flui* 3elocity ft?sec:

g gra3itational acceleration ft?sec(:

Eample& DarcyKs ea* Loss E/uation A pipe 166 feet long an* (6 inches in *iameter contains+ater at (66) flo+ing at a mass flo+ rate of 766 l,m?sec0 The +ater has a *ensity of


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UNIT II HYDRAULIC SYSTEM & COMPONENTS

Pu0 t,#s

There are essentially three *ifferent types of positi3e *isplacement pump use* in

hy*raulic systems0

Gear pumps

The simplest an* most ro,ust positi3e *isplacement pump. ha3ing Bust t+o mo3ing parts.is the gear pump0 Its parts are non;reciprocating. mo3e at constant spee* an* eperience auniform force0 Internal construction. sho+n in )igure (07. consists of Bust t+o close meshinggear +heels +hich rotate as sho+n0 The *irection of rotation of the gears shoul* ,e carefullynote*2 it is the opposite of that intuiti3ely epecte* ,y most people0

As the teeth come out of mesh at the centre. a partial 3acuum is forme* +hich *ra+s flui*

into the inlet cham,er0 )lui* is trappe* ,et+een the outer teeth an* the pump housing. causinga continual transfer of flui* from inlet cham,er to outlet cham,er +here it is *ischarge* tothe system0

Pump *isplacement is*etermine* ,y& 3olume of flui*,et+een each pair of teeth2 num,erof teeth2 an* spee* of rotation0Note the pump merely *eli3ers afie* 3olume of flui* from inlet

port to outlet port for eachrotation2 outlet port pressure is*etermine* solely ,y *esign of therest of the system0

Performance of any pump islimite* ,y lea>age an* the a,ilityof the pump to +ithstan* thepressure *ifferential ,et+een inletan* outlet ports0 The gear pumpo,3iously re/uires closely meshing gears. minimum clearance ,et+een teeth an* housing.an* also ,et+een the gear face an* si*e plates0 Often the si*e plates of a pump are *esigne* as*eli,erately replacea,le +ear plates0 Cear in a gear pump is primarily cause* ,y *irt particles

in the hy*raulic flui*. so cleanliness an* filtration are particularly important0

The pressure *ifferential causes large si*e loa*s to ,e applie* to the gear shafts at 85 tothe centre line as sho+n0 Typically. gear pumps are use* at pressures up to a,out 156 ,ar an*capacities of aroun* 156 gpm


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There are some 3ariations of the ,asic gear pump0 In )igure (09. gears ha3e ,een

replace* ,y lo,es gi3ing a pump calle*. not surprisingly

The )igure is another 3ariation calle* theintemal gear pump. +here an etemal *ri3engear +heel is connecte* to a smaller internalgear. +ith flui* separation as gears *isengage,eing performe* ,y a crescent;shape*

moul*ing0 !et another 3ariation on thetheme is the gerotor pump of )igure (0=,.+here the crescent moul*ing is *ispense*+ith ,y using an internal gear +ith one lesstooth than the outer gear +heel0 Internal gearpumps operate at lo+er capacities an*pressures typically 76 ,ar: than other pumptypes0


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Vane pumps

The maBor source of lea>age in a gear pump arises from the small gaps ,et+een teeth.an* also ,et+een teeth an* pump housing0 The 3ane pump re*uces this lea>age ,y using springor hy*raulic: loa*e* 3anes slotte* into a *ri3en rotor. as illustrate* in the t+o eamples of)igure(0160 In the pump sho+n in )igure (016a. the rotor is offset +ithin the housing. an* the3anes constraine* ,y a cam ring as they cross inlet an* outlet ports0 ecause the 3ane tipsare hel* against the housing there is little lea>age an* the 3anes compensate to a large *egreefor +ear at 3ane tips or in the housing itself0 There is still. ho+e3er. lea>age ,et+een rotorfaces an* ,o*y si*es0 Pump capacity is *etermine* ,y 3ane thro+. 3ane cross sectionalarea an* spee* ofrotation0

The *ifference in pressure ,et+eenoutlet an* inlet ports cr eates a se3er e loa* o n t he 3anes an* a large si*e lo a* o nthe rotor shaft +hich can lea* to ,earingfailure0 The pump in )igure (016a isconse/uently >no+n as an un,alance* 3anepump0 )igure(016, sho+s a ,alance* 3ane pump0 Thisfeatures an elliptical cam ring together +itht+o inlet an* t+o outlet ports0 Pressureloa*ing still occurs in the 3anes ,ut the t+oi*entical pump hal3es create e/ual ,utopposite forces on the rotor. lea*ing to -ero net force in the shaft an* ,earings0 alance*3ane pumps ha3e much impro3e* ser3ice li3es o3er simpler un,alance* 3ane pumps0

$apacity an* pressure ratings of a 3ane pump are generally lo+er than gear pumps.,ut re*uce* lea>age gi3es an impro3e* 3olumetric efficiency of aroun* =5F0

In an i*eal +orl*. the capacity of a pump shoul* ,e matche* eactly to loa*re/uirements0 Epression (0( sho+e* that input po+er is proportional to system pressure an*3olumetric flo+ rate0 A pump +ith too large a capacity +astes energy lea*ing to a rise influi* temperature: as ecess flui* passes through the pressure relief 3al3e0

Pumps are generally sol* +ith certain fie* capacities an* the user has to choose the netlargest si-e0 )igure (011 sho+s a 3ane pump +ith a*Busta,le capacity. set ,y the positionalrelationship ,et+een rotor an* inner casing. +ith the inner casing position set ,y an eternal

scre+0

Piston pumps

A piston pump is superficially similar to a motor car engine. an* a simple single cylin*er

arrangement +as sho+n earlier in )igure (0(,0 %uch a simple pump. ho+e3er. *eli3ering a

single pulse of flui* per re3olution. generates unaccepta,ly large pressure pulses into the

system0


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Practical piston pumps therefore employ multiple cylin*er an* pistons to smooth out flui*

*eli3ery. an* much ingenuity goes into *esigning mult icylin*er pumps +hich aresurprisingly compact0

)igure (01( sho+s one form of ra*ial piston pump0 The pump consists of se3eralhollo+ pistons insi*e a stationary cylin*er ,loc>0 Each piston has spring;loa*e* inlet an* outlet3al3es0 As the inner cam rotates. flui* is transferre* relati3ely smoothly from inlet port to theoutlet port0


24/70

Actuators

A hy*raulic or pneumatic system is generally concerne* +ith mo3ing. gripping orapplying

force to an o,Bect0 De3ices +hich actually achie3e this o,Becti3e are calle* actuators. an* can

,e split into three ,asic types0

Linear actuators. as the name implies. are use* to mo3e an o,Bect or apply a force ina straight line0 "otary actuators are the hy*raulic an* pneumatic e/ui3alent of an electricmotor0 This chapter *iscusses linear an* rotary actuators0

The thir* type of actuator is use* to operate flo+ control 3al3es for process control ofgases. li/ui*s or steam0 These actuators are generally pneumatically operate* an* are*iscusse* +ith process control pneumatics in $hapter 70

Li(#a$ a*tuat!$s

The ,asic linear actuator is the cylin*er. or ram. sho+n in schematic form in )igure 5010Practical constructional *etails are *iscusse* later0 The cylin*er in )igure 501 consists ofa piston. ra*ius ". mo3ing in a ,ore0 The piston isconnecte* to a ro* of ra*ius r +hich *ri3es the loa*0O,3iously if pressure is applie* to port +ith port !3enting: the piston eten*s0 %imilarly. if pressureis applie* to port ! +ith port 3enting:. the pistonretracts0

The force applie* ,y a piston *epen*s on ,oth thearea an* the applie* pressure0 )or the eten* stro>e. area A isgi3en ,y A4"

(0 )or a pressure P applie* to port . the

eten* force a3aila,le is&

)c; P 7r "e0 501: Fig A mass supported by a

cylinder


25/70

The units of epression 501 *epen* on the system ,eing use*0 If %I units are use*. theforce is in ne+tons0

Epression 501 gi3es the maimum achie3a,le force o,taine* +ith the cylin*er in astalle* con*ition0 One eample of this occurs +here an o,Bect is to ,e grippe* or shape*0

In )igure 50( an o,Bect of mass M is lifte* at constant spee*0 ecause the o,Bect is notaccelerating. the up+ar* force is e/ual to Mg ne+tons in %I units: +hich from epression501 gi3es the pressure in the cylin*er0 This is lo+er than the maimum system pressure2 thepressure *rop occurring across flo+ control 3al3es an* system piping0 Dynamics of systemssimilar to this are *iscusse* later0

There are fi3e ,asic parts in a cylin*er2 t+o en* caps a ,ase cap an* a ,earing cap: +ithport connections. a cylin*er ,arrel. a piston an* the ro* itself0 This ,asic construction allo+sfairly simple manufacture as en* caps an* pistons are common to cylin*ers of the same*iameter. an* only relati3ely: cheap ,arrels an* ro*s nee* to ,e change* to gi3e *ifferentlength cylin*ers0 En* caps can ,e secure* to the ,arrel ,y +el*ing. tie ro*s or ,y threa*e*connection0 asic constructional *etails are sho+n in )igure 50=0

The inner surface of the ,arrel nee*s to ,e 3ery smooth to pre3ent +ear an* lea>age0Qenerally a seamless *ra+n steel tu,e is use* +hich is machine* hone*: to an accurate finish0In applications +here the cylin*er is use* infre/uently or may come into contact +ithcorrosi3e materials. stainless steel. aluminium or ,rass tu,e may ,e use*0

Pistons are usually ma*e of cast iron or steel0 The piston not only transmits force to thero*. ,ut must also act as a sli*ing ,eating in the ,arrel possi,ly +ith si*e forces if the ro* issu,Bect to a lateral force: an* pro3i*e a seal ,et+een high an* lo+ pressure si*es0 Pistonseals are generally use* ,et+een piston an* ,arrel0 Occasionally small lea>age can ,etolerate* an* seals are not use*0 A ,eating surface such as ,ron-e: is *eposite* on to thepiston surface then hone* to a finish similar to that of the ,arrel0

The surface of the cylin*er ro* is epose* to the atmosphere +hen eten*e*. an* hencelia,le to suffer from the effects of *irt. moisture an* corrosion0 Chen retracte*. theseantisocial materials may ,e *ra+n ,ac> insi*e the ,arrel tocause pro,lems insi*e the cylin*er0 eat treate*chromium alloy steel is generally use* forstrength an* to re*uce effects of corrosion0

A +iper or scraper seal is fitte* to the en*cap +here the ro* enters the cylin*er to remo3e*ust particles0 In 3ery *usty atmospheres eternalru,,er ,ello+s may also ,e use* to eclu*e *ust

)igure 50=a: ,ut these are 3ulnera,le topuncture an* splitting an* nee* regular inspection0The ,eating surface. usually ,ron-e. is fitte*,ehin*the +iper seal0 Fig Single-actingcylinder

An internal sealing ring is fitte* ,ehin* the ,eating to pre3ent high pressure flui* lea>ingout along the ro*0 The +iper seal. ,earing an* sealing ring are sometimes com,ine* asa cartri*ge assem,ly to simplify maintenance0 The ro* is generally attache* to the piston


26/70

3ia a


27/70

threa*e* en* as sho+n in )igures 50=, an* c0 Lea>age can occur aroun* the ro*. so sealsare again nee*e*0 These can ,e cap sea ls as in )igure 50=,: +hich com,ine the roles ofpiston an* ro* seal. or a static O ring aroun* the ro* as in )igure 50=c:0

En* caps are generally cast from iron or aluminium: an* incorporate threa*e* entriesfor ports0 En* caps ha3e to +ithstan* shoc> loa*s at

etremes of piston tra3el0 These loa*s arise notonly from flui* pressure. ,ut also from >ineticenergy of the mo3ing parts of the cylin*er an*loa*0

These en* of tra3el shoc> loa*s can ,ere*uce* +ith cushion 3al3es ,uilt into the en*caps0 In the cylin*er sho+n in )igure 5016. foreample. ehaust flui* flo+ is unrestricte*until the plunger Fig A simple cylinder

Fig Cylinder cushioning


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Fig Two-stage telescopic piston

The stro>e of a simple cylin*er must ,e less than ,arrel length. gi3ing at ,est an

eten*e*?retracte* ratio of (&10 Chere space is restricte*. a telescopic cylin*er can ,e use*0

)igure 501( sho+s the construction of a typical *ou,le;acting unit +ith t+o pistons0 To

eten*. flui* is applie* to port A0 )lui* is applie* to ,oth si*es of piston 1 3ia ports an* !.

,ut the *ifference in areas ,et+een si*es of piston 1 causes the piston to mo3e to the right0

To retract. flui* is applie* to port 0 A flei,le connection is re/uire* for this port0 Che n piston

( is *ri3en fully to the left. port ! is no+ connecte* to port . applying pressure to the fight

; han* si*e of piston 1 +hich then retracts0

The construction of telescopic cylin*ers re/uires many seals +hich ma>es maintenance

comple0 They also ha3e smaller force for a gi3en *iameter an* pressure. an* can only tolerate

small si*e loa*s0

Pneumatic cylin*ers are use* for metal forming. an operation re/uiting large forces0 Pressures

in pneumatic systems are lo+er than in hy*raulic systems. ,ut large impact loa*s can ,e

o,taine* ,y accelerating a hammer to a high 3elocity then allo+ing it to stri>e the target0

%uch *e3ices are calle* impact cylin*ers an* operate on the principle illustrate* in )igure

5010 Pressure is initially applie* to port


29/70

Fig An impact cylinder

to retract the cylin*er0 Pressure is then applie* to ,oth ports A an* . ,ut the

cylin*er remains in a retracte* state ,ecause area is less than area !0 Port is then 3ente*

rapi*ly0 Imme*iately. the full piston area eperiences port A pressure0 Cith a large

3olume of gas

store* ,ehin* the piston. it accelerates rapi*ly to a high 3elocity typically 16m s ;l:0

R!ta$, a*tuat!$s

"otary actuators are the hy*raulic or pneumatic e/ui3alents of electric motors0 )ora gi3en tor/ue. or po+er. a rotary actuator is more compact than an e/ui3alent motor. cannot,e *amage* ,y an in*efinite stall an* can safely ,e use* in an eplosi3e atmosphere0 )or3aria,le spee* applications. the compleity an* maintenance re/uirements of a rotaryactuator are similar to a thyristor;con;trolle* D$ *ri3e. ,ut for fie* spee* applications. theA$ in*uction motor +hich can. for practical purposes. ,e fitte* an* forgotten: is simpler to

install an* maintain0

A rotary actuator or. for that matter. an electric motor: ca n ,e *efine* in terms ofthe tor/ue it pro*uces an* its running spee*. usually gi3en in re3s per minute rpm:0Definition of tor/ue is illustrate* in )igure 50((. +here a rotary motion is pro*uce* againsta force of ) ne+tons acting at a ra*ial *istance * metres from a shaft centre0 The*e3ice is then pro*ucing a tor/ue T gi3en ,y the epression

Fig Rotary actuator symbols


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Fig A gear motor

Fig A vane motor


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Fig Vane operation in hydrauic motor

Fig !imited motion rotary actuators

Ali*ati!(

The operational spee* of an actuator is *etermine* ,y the flui* flo+ rate an* the actuator area

for a cylin*er: or the *isplacement for a motor:0 The physical *imensions are generally fie*for an actuator. so spee* is controlle* ,y a*Busting the flui* flo+ to or restricting flo+ from: the


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actuator0 "otary actuator spee* can also ,e controlle* ,y altering s+ash plate angle0 Thecompressi,ility of air. normally a*3antageous +here smooth operation is concerne*. ma>es

flo+ control more *ifficult for pneumatic than hy*raulic systems0 Although techni/ues

*escri,e* ,elo+ can ,e applie* in pneumatics. precise slo+;spee* control of a pneumatic

actuator is achie3e* +ith eternal *e3ices *escri,e* later0There are essentially four +ays in

+hich flui*flo+ can ,e controlle*0The first is sho+n in )igure 506. +here a pump *eli3ers a flui* 3olume '

per minute0 ecause the pump is a fie* *isplacement *e3ice this 3olume of flui* must goeither ,ac> to the tan> or to the actuator0 Chen the control 3al3e mo3es from its centre

position. the actuator mo3es +ith a 3elocity&

+here A is the piston area0 If pump *eli3ery 3olume ' can ,e a*Buste* ,y altering s+ash

plate angle. say.: and the pump fee*s no other *e3ice. no further spee* control is nee*e*0

Fig Speed control by pump volume


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UNIT III DESIGN OF HYDRAULIC CIRCUITS

Types of control 3al3e

There are essentially three types of control 3al3e2 poppet 3al3es spool 3al3es an* rotary 3al3es0

POPPET VALVES

In a poppet 3al3e. simple *iscs. cones or ,alls are use* in conBunc;tion +ith simple

3al3e seats to control flo+0 allo+s flui* to flo+ from port P to port A0 Chen the ,utton is

release*. spring an* flui* pressure force the ,all up again closing the 3al3e0 )ig sho+s the

construction an* sym,ol of a *isc seal ?( poppet0 Cith the push,utton release*. ports A an* "

are lin>e* 3ia the hollo+ push,utton stem0 If the push,utton is presse*. port " is first seale*.

then the 3al3e *isc pushe* *o+n to open the 3al3e an* connect ports P an* A0 As ,efore.

spring an* flui* pressure from port P closes the 3al3e0 The 3al3e construction an* sym,ol

sho+n in )ig is a poppet changeo3er 8?( 3al3e using t+o stems an* *isc 3al3es0 Cith the

push,utton release*. ports A an* " are lin>e* 3ia the hollo+ left;han* stem an* ports P an*

lin>e* 3ia the normally;open right han* *isc 3al3e0

Chen the push,utton is presse*. the lin> ,et+een ports A an* " is first close*. then

the lin> ,et+een P an* close*0 The lin> ,et+een A an* P is net opene*. an* finally the

lin> ,et+een an* " opene*0 Chen the push,utton is release*. air an* spring pressure puts the

3al3e ,ac> to its original state0 Poppet 3al3es are simple. cheap an* ro,ust. ,ut it is generally

simpler to manufacture 3al3es more complicate* than those sho+n in )igure 8011 ,y using

spool 3al3es0 )urther. a maBor *isa*3antage of poppet 3al3es is the force nee*e* to operate

them0 In the poppet 3al3e of )igure 8016. for eample. the force re/uire* on the push,utton to

operate the 3al3e is P a ne+tons0 Large capacity 3al3es nee* large 3al3e areas. lea*ing to

large operating force0 The high pressure in hy*raulic systems thus ten*s to pre3ent use of

simple poppet 3al3es an* they are. therefore. mainly foun* in lo+ pressure pneumatic systems0


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SPOOL VALVES

%pool or sli*e: 3al3es are constructe* +ith aspool mo3ing hori-ontally +ithin the 3al3e,o*y. as sho+n for the 8?( 3al3e in)igure801(0 "aise* areas calle* 4lan*s4 ,loc> oropen ports to gi3e the re/uire* operation0

The operation of a spool 3al3e is generally

,alance*0 In the 3al3e construction in

)igure

801(,. for eample. pressure is applie* to

opposing faces D an* E an* lo+ tan>

pressure to faces ) an* Q0 There is no net

force on the spool from system pressure.

allo+ing the spoolto ,e easily mo3e*

Fig A $" poppet valve


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Fig Four-way spool valve

Fig Four-way spool valve

)ig is a changeo3er 8?( spool 3al3e0 $omparison of the 3al3es sho+n in )igures 801( an*

801 sho+s they ha3e the same ,o*y construction. the only *ifference ,eing the si-e an*

position


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38/70

Fig Three position %our-way valves

ROTARY VALVES

"otary 3al3es consist of a rotating spool +hich aligns +ith holes in the 3al3e casing to gi3e

the re/uire* operation0 )ig sho+s the construction an* sym,ol of a typical 3al3e +ith centreoff action0 "otary 3al3es are compact. simple an* ha3e lo+ operating forces0 They are.

ho+e3er.

lo+ pressure *e3ices an* are conse/uently mainly use* for han* operation in pneumatic systems0

Fig Rotary Valves

PILOT-OPERATED VALVES

Cith large capacity pneumatic 3al3es particularly poppet 3al3es: an* most hy*raulic 3al3es.

the operating force re/uire* to mo3e the 3al3e can ,e large0 If the re/uire* force is too large for

a solenoi* or manual operation. a t+o;stage process calle* pilot operation is use*0


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The principle is sho+n in )igure 801


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Fig Construction o% a pilot-operated #" valve

Fig Pilot-operated valveFig Chec& valves


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Fig Chec& valve symbols

illustrates se3eral common applications of chec> 3al3es0 )ig sho+s a com,ination pump.

use* +here an application re/uires large 3olume an* lo+ pressure. or lo+ 3olume an* high

pressure0 A typical case is a clamp re/uire* to engage /uic>ly high 3olume an* lo+ pressure:

then grip minimal 3olume ,ut high pressure:0 Pump 1 is the high 3olume an* lo+ pressure

pump. an* pump ( the high pressure pump0 In high 3olume mo*e ,oth pumps *eli3er to the

system. pump 1 *eli3ering through the chec> 3al3e ' 0 Chen high pressure is re/uire*. line

pressure at rises operating unloa*ing 3al3e ' 1 3ia pilot port ta>ing pump 1 off loa*0 Pump

( *eli3ers the re/uire* pressure set ,y relief 3al3e ' (.+ith the chec> 3al3e pre3enting flui*

lea>ing ,ac> to pump 1 an* '10)ig sho+s a hy*raulic circuit +ith a pressure storage

*e3ice calle* an accumulator *escri,e* in a later chapter:0 ere a chec> 3al3e allo+s the

pump to unloa* 3ia the pressure regulating 3al3e. +hile still maintaining system pressure from

the accumulator0

A spring;operate* chec> 3al3e re/uires a small pressure to open calle* the crac>ing

pressure: an* acts to some etent li>e a lo+ pressure relief 3al3e0 This characteristic can ,e use*

to a*3antage0 In )ig pilot pressure is *eri3e* ,efore a chec> 3al3e. an* in )ig a chec> 3al3e

is use* to protect a ,loc>e* filter ,y *i3erting flo+ aroun* the filter +hen pressure rises0 A

chec> 3al3e is also inclu*e* in the tan> return to pre3ent flui* ,eing suc>e* out of the tan>

+hen the pump is turne* off0

PILOT-OPERATED CHECK VALVES

The cylin*er in the system in )ig shoul*.

theoretically. hol* position +hen the

control 3al3e is in its centre. off. position0 In

practice. the cylin*er +ill ten* to creep,ecause of lea>age in the control 3al3e0 $hec>

3al3es ha3e ecellent sealage in the close*

position. ,ut a simple chec> 3al3e cannot ,e

use* in the system in )ig ,ecause flo+ is

re/uire* in ,oth *irections0 A pilot;operate*

chec> is similar to a ,asic chec> 3al3e ,ut

can ,e hel* open


42/70

permanently ,y application of an eternal pilot pressure signal0 There are t+o ,asic formsof


43/70

pilot;operate* chec> 3al3es. sho+n in )ig They operate in a similar manner to ,asic

chec> 3al3es. ,ut +ith pilot pressure *irectly opening the 3al3es0 In the 8$ 3al3e sho+n in

)ig inlet pressure assists the pilot0 The sym,ol of a pilot;operate* chec> 3al3e is sho+n

in )ig The cylin*er application of )igure 80(( is re*ra+n +ith pilotoperate* chec> 3al3es in

)ig The pilot lines are connecte* to the pressure line fee*ing the other si*e of the cylin*er0 )or

any cylin*er mo3ement. one chec> 3al3e is hel* open ,y flo+ operating as a normal chec>

3al3e: an* the other is hel* open ,y pilot pressure0 )or no re/uire* mo3ement. ,oth chec>

3al3es are close* an*

the cylin*er is loc>e* inposition0

Fig Chec& valve applications

Fig System re'uiring a chec& valve( )n the o%% position the load*creeps*

RESTRICTION CHECK VALVES

The spee* of a hy*raulic or pneumatic actuator can ,e controlle* ,y a*Busting the rate at +hicha flui* is a*mitte* to. or allo+e* out from. a *e3ice0 This topic is *iscusse* in more *etail in$hapter 5 ,ut a spee* control is often re/uire* to ,e *irection;sensiti3e an* this re/uires the


44/70

inclusion of a chec> 3al3e0


45/70

A restriction chec> 3al3e often calle* a throttle relief

3al3e in pneumatics: allo+s full flo+ in one *irectio n

an* a re*uce* flo+ in the other *irection0 )igure 80(8asho+s a simple hy*raulic 3al3e an* )igure 80(8, a

pneumatic 3al3e0 In ,oth. a nee*le 3al3e sets restricte* flo+

to the re/uire* 3al3e0 The sym,ol of a restriction chec>3al3e is sho+n in )ig sho+s a typical application in +hich

the cylin*er eten*s at full spee* until a limit s+itch ma>es.

then eten* further at lo+ spee*0 "etraction is at full spee*0A restriction chec> 3al3e '( is fitte* in one leg of the

cylin*er0 Cith the cylin*er retracte*. limit;operate* 3al3e '

is open allo+;ing free flo+ of flui* from the cylin*er as iteten*s0 Chen the stri>er plate

on the cylin*er ram hits the limit. 3al3e ' closes an* flo+ out of the cylin*er is no+

restricte* ,y the nee*le 3al3e setting of 3al3e '(0 In the re3erse *irection. the chec> 3al3eon 3al3e '( opens gi3ing full spee* of retraction0

Fig Pilot-operated chec& valves


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Fig Restriction chec& valve

SHUTTLE AND FAST EXHAUST VALVES

A shuttle 3al3e. also >no+n as a *ou,le chec> 3al3e. allo+s pres;sure in a line to ,e

o,taine* from alternati3e sources0 It is primariSly a pneumatic *e3ice an* is rarely foun* in

hy*raulic circuits0$onstruction is 3ery simple an* consists of a ,all insi*e a cylin;*er. as sho+nin )igure 80(5a0 If pressure is applie* to port . the ,all is ,lo+n to the fight ,loc>ing port !

an* lin>ing ports an* A0 %imilarly. pressure to port ! alone connects ports ! an* A an*

,loc>sport 0 The sym,ol of a shuttle 3al3e is gi3en in )ig A typical applicat ion is gi3en in )ig +here a

spring return cylin*er is operate* from either of t+o manual stations0 Isolation ,et+een thet+o stations is pro3i*e* ,y the shuttle 3al3e0 Note a simple T;connection cannot ,e use* as


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each


48/70


49/70

SEQUENCE VALVESThe se/uence 3al3e is a close relati3e of the pressure relief 3al3e an* is use* +here a set

of operations are to ,e controlle* in a pressure relate* se/uence0 )igure 80(7 sho+s atypical eample +here a +or>piece is pushe* into position ,y cylin*er 1 an* clampe* ,y

cylin*er (0 %e/uence 3al3e ' ( is connecte* to

the eten* line of cylin*er 10 Chenthis cylin*er is mo3ing the

+or>piece. the line pressure is lo+.

,ut rises once the +or>piece hits theen* stop0 The se/uence 3al3e opens

once its inlet pressure rises a,o3e a

preset le3el0 $ylin*er ( then operatesto clamp the +or>piece0 A chec>

3al3e across ' ( allo+s ,oth

cylin*ers to retract together(

TIME DELAY VALVES

Pneumatic time *elay 3al3es are use* to *elay operations +here time;,ase* se/uencesare

re/uire*0 )ig sho+s construction of a typical 3al3e0 This is similar in construction to a ?( +ay

pilot;operate* 3al3e. ,ut the space a,o3e the main 3al3e is comparati3ely large an* pilot air is

only allo+e* in 3ia a flo+re*ucing nee*le 3al3e0 There is thus a time *elay ,et+een applicationof pilot pressure to port an* the3al3e

operation. as sho+n ,y the timing *iagram in)igure 80(9,0 The time *elay is a*Buste* ,y thenee*le 3al3e setting0 The ,uilt;in chec> 3al3ecauses the reser3oir space a,o3e the 3al3e to3ent /uic>ly +hen pressure at is remo3e* togi3e no *elay off0

The 3al3e sho+n in )ig is a normally;close* *elay;on 3al3e0 Many other time *elay3al3es *elay;off. *elay on?off. normally;

open: can ,e o,taine*0 All use the ,asicprinciple of the air reser3oir an* nee*le 3al3e0The sym,ol of a normally;*ose* time *elay3al3e is in )ig


50/70

Fig Pneumatic time delay valve

PROPORTIONAL VALVES

The solenoi* 3al3es *escri,e* so far act. to some etent. li>e an electrical s+itch. i0e0 they can,e

On or Off0 In many applications it is re/uire* to remotely control spee*. pressure or force 3iaan

electrical signal0 This function is pro3i*e* ,y proportional 3al3es0 A typical t+o

position solenoi* is only re/uire* to mo3e the spool ,et+een 6 an* 166F stro>e against the

restoring force of a spring0 To ensure pre*icta,le mo3ement ,et+een the en* positions thesolenoi* must also increase its force as the spool mo3es toensure the solenoi* force is larger

than the increasing opposing spring force at all positions0A proportional

3al3e has a

*ifferent *esign

re/uirement0

The spoolposition can ,e

set any+here,et+een 6F

an* 166F

stro>e ,y

3arying thesolenoi*current0 To gi3e

a pre*icta,leresponse the

solenoi* mustpro*uce a force+hich is*epen*ent solely on the The relationship ,et+een coil current force an* stro>e for aproportional

3al3e solenoi*0 Note the flat part of the cur3e an* the linear relationship ,et+een currentan* force current an* not on the spool position. i0e0 the force for a gi3en current must ,econstant o3er the full stro>e range0 )urthermore. the force must ,e proportional to the current0)ig sho+s a typical response0 The force from the solenoi* is oppose* ,y the force from a


51/70

restoring spring. an* the spool +ill mo3e to a position +here the t+o forces are e/ual0 Cith acurrent of 6075 A. for eample. the spool +ill mo3e to 75F of its stro>e0 The spoolmo3ement in a proportional


52/70

3al3e is small2 a fe+ mm stro>e is typical0 The 3al3es are therefore 3ery 3ulnera,le to

stiction. an* this is re*uce* ,y using a 4+et4 *esign +hich immerses the solenoi* an* its core in

hy*raulic flui*0A proportional 3al3e

shoul* pro*uce a flui* flo+

+hich is proportional to thespool *isplacement0 The spools

therefore use four triangularmetering notches in the spool

lan*s as sho+n on )ig0 As the

spool is mo3e* to the right. port

A +ill progressi3ely lin> to thetan> an* port to the pressure

line0 The sym,ol for this 3al3e is

also sho+n0 Proportional 3al3esare *ra+n +ith parallel lines on

the connection si*es of the 3al3e,loc> on circuit *iagrams0 )iggi3es e/ual flo+ rates to ,oth A

an* ports0$ylin*ers ha3e

*ifferent areas on the full ,orean* annulus si*es

Fig Construction and symbol %or a proportional valve( ,hen used with a cylinder with

". %ull bore to annulus area ratio/ hal% the V cutouts will be provided on one o% the P lands

To achie3e e/ual spee*s in ,oth *irections. the notches on the lan*s must ha3e *ifferent

areas0 Cith a (&1 cylin*er ratio. half the num,er of notches are use* on one si*e0 )igure 801

sho+s the construction an* sym,ol for a restricte* centre position 3al3e0 ere the eten*e*

notches pro3i*e a restricte* typically F: flo+ to tan> from the A an* ports +hen the 3al3e

is in the centre position0 %o far +e ha3e assume* the spool position is *etermine* ,y the

,alance ,et+een the force from the solenoi* an* the restoring force from a spring0 Chilst this

+ill +or> for simple applications. factors

such as hy*raulic pressure on the spool an* spring ageing mean the repeata,ility is poor0 Direct

solenoi*?spring ,alance is also not feasi,le +ith a pilot?main spool 3al3e0 Chat is reallyre/uire* is some metho* of position control of the spool0 To achie3e this. the spool position

must ,e easure*0 Most 3al3es use a *e3ice calle* a Linear 'aria,le Differential Transformer or

L'DT: sho+n on )ig The L'DT consists of a soft iron core +hose position is to ,e measure*

surroun*e* ,y three electrical +in*ings0 A high fre/uency typically a fe+ >-: A$ signal is

applie* to the centre +in*ing +hich in*uces 3oltages into the other t+o +in*ings0 Chen the core


53/70

is central. ' 1 an* ' ( are e/ual ,ut opposite in phase gi3ing -ero 3olts at '

0

F)G Construction and symbol %or a proportional valve with A and 0 ports lin&ed to tan& inthe null position

HYDRAULIC ACCUMULATORSIn a simple hy*raulic system. the pump si-e *eli3ery rate an* hence motor po+er: is

*etermine* ,y the maimum re/uirementsof the actuators0 In )ig a system operates

intermittently at aFig a simple system with uneven

demands( To supply this without anaccumulator a .11 . min -2 is re'uired

although the mean %low is only .3 .minpressure of ,et+een 156 an* (66

,ar. nee*ing a flo+ rate of 166 1 min ;1 for 16 s at a repetition rate of 1 minute0 Cith a simple

system pump. pressure regulator an* loa*ing 3al3e: this re/uires a (66 ,ar. 166 1 min ;1pump *ri3en ,y a,out a 56 hp motor: +hich spen*s aroun* 95F of its time unloa*ing totan>0 In )igure


54/70

,et+een


55/70

times $ an* D0 This *ra+s flui* from the accumulator causing a fall of system pressure0

The pressure s+itch on the accumulator puts the pump on loa* again ,ut it ta>es until time E

,efore the accumulator is charge* rea*y for the net actuator mo3ement at time E Anaccumulator re*uces pump re/uirements0 The original system re/uire* a 166 1 min ;1 pump0

Cith an accumulator. ho+e3er. a pump only nee*s to pro3i*e 17 1 min ;1 that is. 166 1 min

;1 for 16 secs e3ery minute:0 Pump si-e. an* hence motor si-e. ha3e ,een re*uce* ,y a factorof si +ith o,3ious cost an* space sa3ings. plus gains in ancillary e/uipment such as motor

starters an* ca,ling0 There is no gain in the energy use*2 +ith the simple system a 56 hp motor

loa*s for 17F of the time. +ith an accumulator a 16 hp motor loa*s for a,out =6F of the time0

Fig System with an accumulator

Most accumulators operate ,y compressing a gas although ol*er an* smaller accumulators

may +or> ,y compressing a spring or lifting a +eight +ith a cylin*er:0 The most common form

is the gasfille* ,la**er accumulator sho+n in )ig Qas is precharge* to some pressure +ith theaccumulator empty of flui* +hen the +hole of the accumulator is fille* +ith gas0 A poppet

3al3e at the accumulator ,ase pre3ents the ,la**er etru*ing out into the piping0Accumulators are si-e* ,y oyle4s la+ an* a >no+le*ge of the *eman*s of the actuators0 )or the eample system of )ig

Fig The accumulator


56/70

Assuming a precharge of 1(6 ,ar. a charge* accumulator pressure of 196 ,ar an* a fall to a

pressure to 1


57/70

+ith the flat of the han*0 There shoul* al+ays ,e a significant temperature *ifference ,et+een

the gas


58/70

an* the hy*raulic oil an* the oil?gas split can ,e *etecte* ,y the temperature change on the

,o*y of the accumulator0 If the +hole ,o*y is the same temperature something has gone

se3erely +rong +ith the gas ,la**er0 An accumulator is a pressurise* 3essel an* as such

re/uires certification if it contains more than (56 ,ar0litres0 It +ill re/uire a recor*e* epert

3isual inspection e3ery fi3e years an* a full 3olumetric pressure test e3ery ten years0

HYDRAULIC COOLERS AND HEAT EXCHANGERS

Despite the occasional use of heaters mentione* earlier. the pro,lem +ith oil

temperature is usually >eeping it down to the re/uire* 56 In small systems. the heat lost

through reser3oir +alls is sufficient to >eep the oil cool. ,ut in larger systems a**itional

cooling is nee*e*0 Ta,le


59/70

UNIT IV PNEUMATIC SYSTEMS AND COMPONENTS

COMPRESSOR TYPES

Li>e hy*raulic pumps. air compressors can ,e split into positi3e *isplacement *e3ices

+here a fie* 3olume of air is *eli3ere* on each rotation of the compressor shaft: an* *ynamic

*e3ices such as centrifugal or aial ,lo+ers0 The 3ast maBority of air compressors are of

the positi3e *isplacement type0 A compressor is selecte* ,y the pressure it is re/uire* to +or>

at an* the 3olume of gas it is re/uire* to *eli3er0 As eplaine* in the pre3ious section.

pressure in the recei3er is generally higher than that re/uire* at the operating position. +ith

local pressure regulation ,eing use*0 Pressure at the compressor outlet +hich for practical

purposes +ill ,e the same as that in the recei3er: is calle* the +or>ing pressure an* is

use* to specify the compressor0 Pressure at the operating point is calle*. not surprisingly. the

operating pressure an* is use* to specify 3al3es. actuators an* other operating *e3ices0

$are shoul* ,e ta>en in specifying the 3olume of gas a compressor is re/uire* to *eli3er0

Epression 01 sho+s the 3olume of a gi3en mass of gas to ,e highly *epen*ent on pressure

an* temperature0 Deli3ery 3olume of a compressor is *efine* in terms of gas at normal

atmospheric con*itions0 T+o stan*ar*s >no+n as stan*ar* temperature an* pressures %TP: are

commonly use*. although *ifferences ,et+een them are small for in*ustrial users0

The technical normal condition is&

P 60=9 ,ar a,solute. T (6

an* thephysical normal condition is&

P 1061 ,ar a,solute. T 6


60/70

The term normal temperature and pressure NTP: is also use*0"e/uire* *eli3ery 3olume of

a compressor in M min ;1 or ft min ;1. accor*ing to the units use*: may ,e calculate* for

the actuators at the 3arious operating positions +ith healthy safety margins to allo+ for

lea>age: ,ut care must ,e ta>en to ensure this total 3olume is con3erte* to %TP con*ition

,efore specifying the re/uire* compressor *eli3ery 3olume0 A compressor *eli3ery 3olume

can ,e specifie* in terms of its theoretical 3olume s+ept 3olume multiplie* ,y

rotational spee*: or effecti3e 3olume +hich inclu*es losses0 The ratio of these t+o 3olumes

is the efficiency0 O,3iously the effecti3e 3olume shoul* ,e use* in choosing a compressor

+ith. again. a safety margin for lea>age:0 "e/uire* po+er of the motor *ri3ing the compressor

is *epen*ent on +or>ing pressure an* *eli3ery 3olume. an* may ,e *etermine* from

epressions (0( an* (050 Allo+ance must ,e ma*e for the cyclic on?off operation of the

compressor +ith the motor ,eing si-e* for on loa* operation an* not a3erage* o3er a perio*

of time0

PISTON COMPRESSORS

Piston compressors are ,y far the most common type of compressor. an* a ,asic

single cylin*er form is sho+n in )igure 00 As the piston *escen*s *uring the inlet stro>e

)igure

0a:. the inlet 3al3e opens an* air is *ra+n into the cylin*er0 As the piston passes the ,ottom

of the stro>e. the inlet 3al3e closes an* the ehaust 3al3e opens allo+ing air to ,e epelle* as

the piston rises )igure 0,: )igure 0 implies that the 3al3es are similar to 3al3es in an

internal com,ustion engine0 In practice. spring;loa*e* 3al3es are use*. +hich open an* close

un*er the action of air pressure across them0 One common type uses a 4feather4 of spring steel

+hich mo3es a,o3e the inlet or output port. as sho+n in )ig

A single cylin*er compressor gi3es significant pressure pulses at the outlet port0 This

can ,e o3ercome to some etent ,y the use of alarge recei3er. ,ut more often a multi

cylin*er compressor is use*0 These are

usually classifie* as 3ertical or


61/70

Fig Single cylinder compressor


62/70

hori-ontal in;line arrangements an* the more compact '. ! or C constructions0 A compressor

+hich pro*uces one pulse of air per piston sto>e of +hich the eample of )igure 0 is

typical: is calle* a singleacting compressor0 A more e3en air supply can ,e o,taine* ,y the

*ou,le acting action of the compressor in )igure 08. +hich uses t+o sets of 3al3es an* a

crosshea* to >eep the piston ro* s/uare at all times0 Dou,le;acting compressors can ,e foun* in

all configurations*escri,e* earlier0

Fig 6ouble-acting compressor

Piston compressors *escri,e* so far go *irect from atmospheric to re/uire* pressure in

a single operation0 This is >no+n as a single stage compressor0 The general gas la+

epression

101=: sho+e* compression of a gas to ,e accompanie* ,y a significant rise in gas temperature0

If the eit pressure is a,o3e a,out 5 ,ar in a singleacting compressor. the compresse*

air temperature can rise to o3er (66 an* the motor po+er nee*e* to *ri3e the compressor rises

accor*ingly0 )or pressures o3er a fe+ ,ar it is far more economical to use a multistage

compressor +ith cooling ,et+een stages0 )ig sho+s an eample0 As cooling un*erta>en ,y

a *e3ice calle* an intercooler: re*uces the 3olume of the gas to ,e compresse* at the secon*

stage there is a large energy sa3ing0 Normally t+o stages are use* for pneumatic pressures of

16 to 15 ,ar. ,ut multistage compressors are

a3aila,le for pressures up to aroun* 56

,ar0 Multistage compressors can ,e

manufacture* +ith multicylin*ers as

sho+n in )ig or. more compactly. +ith a

single cylin*er an* a *ou,le *iameterpiston as sho+n in )ig There is contact

,et+een pistons an* air. in stan*ar*

piston compressors. +hich may

intro*uce small amounts of lu,rication

oil Fig Two-stage compressor


63/70

from the piston +alls into the air0 This 3ery small contamination may ,e un*esira,le in foo*

an* chemical in*ustries0 )ig sho+s a common +ay of gi3ing a totally clean supply ,y

incorporating a flei,le *iaphragm ,et+een piston an* air0

Fig Combined two-stage compressor

Fig 6iaphragm compressor/ used where air must not becontaminated

SCRE COMPRESSORS

Piston compressors are use* +here high pressures (6 ,ar: an* relati3ely lo+ 3olumes

U 16.666 m hr ;1: are nee*e*. ,ut are mechanically relati3ely comple +ith many

mo3ing parts0 Many applications re/uire only me*ium pressure U 16 ,ar: an* me*ium flo+s

aroun*

16.666 m hr;m:0 )or these applications. rotary compressors ha3e the a*3antage of


64/70

simplicity.

+ith fe+er mo3ing parts rotating at a constant spee*. an* a stea*y *eli3ery of air

+ithout


65/70

pressure pulses0 One rotary compressor. >no+n as the *ry rotary scre+ compressor. is sho+n in

)ig an* consists of t+o intermeshing rotating scre+s +ith minimal aroun* 6065 mm:

clearance0 As the scre+s rotate. air is *ra+n into the housing. trappe* ,et+een the scre+s an*

carrie* along to the *ischarge port. +here it is *eli3ere* in a constant

pulse;free stream0 %cre+s in this compressor can ,e

synchroni-e* ,y eternal t iming gears0 Alternati3ely one

scre+ can ,e *ri3en. the secon* scre+ rotate* ,y contact

+ith the *ri3e scre+0 This approach re/uires oil lu,rication

to ,e spraye* into the inlet air to re*uce friction ,et+een

scre+s. an* is conse/uently >no+n as a +et rotary scre+

compressor0 Cet scre+ construction though. o,3iously

intro*uces oil contamination into the air +hich has to ,e

remo3e* ,y later

oil separation units0 Fig 6ry screw rotarycompressor

ROTARY COMPRESSORS

The 3ane compressor. sho+n in )ig operates on similar

principles to the hy*raulic 3ane pump *escri,e* in $hapter (.

although air compressors ten* to ,e physically larger than

hy*raulic pumps0 An un,alance* *esign is sho+n. ,alance*

3ersions can also ,e constructe*0 'anes can ,e force* out ,y

springs or. more commonly. ,y centrifugal force0 Asingle stage 3ane compressor can *eli3er air at up to ,ar.

a much lo+er pressure than that a3aila,le +ith a scre+ or

piston compressor0 A t+o;stage 3ane compressor +ith large

lo+ pressure an* smaller high pressure sections lin>e* ,y an

intercooler allo+s pressures up to 16 ,ar to ,e o,taine*0

)ig Vane compressor


66/70

Fig !i'uid ring compressor


67/70

)ig sho+s a 3ariation on the 3ane compressor calle* a li/ui* ring compressor0 The *e3ice uses

many 3anes rotating insi*e an eccentric housing an* contains a li/ui* usually +ater: +hich

is flung out ,y centrifugal force to form a li/ui* ring +hich

follo+s the contour of the housing to gi3e a seal +ith no

lea>age an* minimal friction0 "otational spee* must

,e high typically 666 rpm: to create the ring0 Deli3ery

pressures are relati3ely lo+ at aroun* 5 ,ar0 The lo,e

compressor of )igure 011 often calle* a "oots ,lo+er:

is often use* +hen a positi3e *isplacement compressor

is nee*e* +ith high *eli3ery 3olume ,ut lo+ pressure

typically 1;( ,ar:0 Operating pressure is mainly limite*

,y lea>age ,et+een rotors an* housing0 To operate

efficiently. clearances must ,e 3ery small. an* +ear

lea*s to a rapi* fall in efficiency0

Fig !obe compressor

DYNAMIC COMPRESSORS

A large 3olume of air up to 5666 m min ;1: is often re/uire* for applications such as

pneumatic con3eying +here po+*er is carrie* in an air stream:. 3entilation or +here air itself

is one component of a process e0g0 com,ustion air for gas?oil ,urners:0 Pressure in these

applications is lo+ at most a fe+ ,ar: an* there is no nee* for a positi3e *isplacement

compressor0 Large 3olume lo+ pressure air is generally pro3i*e* ,y *ynamic compressors

>no+n as ,lo+ers0 They can ,e su,*i3i*e* into centrifugal or aial types. sho+n in )igure

01(0 $entrifugal ,lo+ers )igure 01(a: *ra+ air in then fling it out ,y centrifugal force0 Ahigh shaft rotational spee* is nee*e* an* the 3olume to input po+er ratio is lo+er than any

other type of compressor0 An aial compressor comprises a set of rotating fan ,la*es as sho+n

in )igure

01(,0 These pro*u ce 3ery large

3olumes of air. ,ut at lo+ pressure

less than one ,ar:0 They are

primarily use* for 3entilation.

com,ustion an* process air0 Output

pressures of ,oth types of *ynamic

compressor can ,e lifte* ,y

multistage compressors +ith

intercoolers ,et+een stages0 Diffuser

sections re*uce air entry 3elocity to

su,se/uent stages. there,y

con3erting air >inetic energy to

pressure energy0 Fig 7on-positive displacement compressors


68/70

80lowers


69/70

AIR RECEIVERS AND COMPRESSOR CONTROL

An air recei3er is use* to store high pressure air from the compressor0 Its 3olume

re*uces pressure fluctuations arising from changes in loa* an* from compressor s+itching0

Air coming from the compressor +ill ,e +arm if not actually hotR: an* the large surface area

of the recei3er *issipates this heat to the surroun*ing atmosphere0 Any moisture left in the air

from the compressor +ill con*ense out in the recei3er. so outgoing air shoul* ,e ta>en from

the recei3er top0 )ig sho+s essential features of a

recei3er0

Fig Compressed air receiver

They are usually of cylin*rical construction for strength. an* ha3e a safety relief 3al3e to guar*

against high pressures arising from failure of the pressure control scheme0 Pressure

in*ication an*. usually. temperature in*ication are pro3i*e*. +ith pressure s+itches for control

of pressure an* high temperature s+itches for remote alarms0 A *rain coc> allo+s remo3al

of con*ense* +ater. an* access 3ia a manhole allo+s cleaning0 O,3iously. remo3al of a

manhole co3er is ha-ar*ous +ith a pressurise* recei3er. an* safety routines must ,e

*efine* an* follo+e* to pre3ent acci*ents0 $ontrol of the compressor is necessary to

maintain pressure in the recei3er0 The simplest metho* of achie3ing this is to start the

compressor +hen recei3er pressure falls to some minimum pressure. an* stop the compressor

+hen pressure rises to a satisfactory le3el again. as illustrate* in )ig In theory t+o pressure

s+itches are re/uire* +ith t he motor start pressure lo+er than the motor stop pressure:

,ut. in practice. internal hysteresis in a typical s+itch allo+s one pressure s+itch to ,e

use*0 The pressure in the recei3er cycles ,et+een the start an* stop pressure settings0


70/70

Fig Receiver pressure control via motor startstop

$ompressors can also ,e controlle* on the inlet si*e0 In the eample of )ig an inlet

3al3e is hel* open to allo+ the compressor to operate. an* is close* +hen the air recei3er

hasreache* the *esire* pressure. the compressor then forms a near 3acuum on its inlet si*e:0

The 3al3es in )igures can ,e electrically;operate* solenoi* 3al3es controlle* ,y pressure

s+itches. or can ,e pneumatic 3al3es controlle* *irectly ,y recei3er pressure0 The control

metho* is largely *etermine* ,y flo+ rates from recei3er to the loa*s: an* the capacity of the

compressor0 If the compressor has significant spare capacity. for eample. start?stop control is

commonly use*0 If compressor capacity an* loa* re/uirements are closely matche*. on the

other han*. short start?stop cycling may cause premature +ear in the electrical starter for the

compressor motor0 In this situation. ehaust or inlet regulation is preferre*0 Air recei3er si-e is

*etermine* ,y loa* re/uirements. compressor capacity. an* allo+a,le pressure *e3iations in

the recei3er0 Cith the compressor stoppe*. oyle4s la+ epression 1017: gi3es the pressure

*ecay for a gi3en 3olume of air *eli3ere* from a gi3en recei3er at a >no+n pressure0 )oreample. if a recei3er of 16 cu,ic metres 3olume an* a +or>ing pressure of 9 ,ar *eli3ers (5

cu,ic metres of air at %TP: to a loa*. pressure in the recei3er falls to approimately 505 ,ar0

Cith the compressor starte*. air pressure rises at a rate again gi3en ,y epression 1017 +ith

the air mass in the recei3er ,eing increased,y the *ifference ,et+een the air *eli3ere* ,y

the compressor an* that remo3e* ,y the loa*:0 These t+o calculations gi3e the cycle time of

the compressor +hen com,ine* +ith setting s of the cut;in an* *rop;out pressure s+itches0 If

this is unaccepta,ly rapi*. say less than a fe+ minutes. then a larger recei3er is re/uire*0

Manufacturers of pneumatic e/uipment pro3i*e nomographs +hich simplify these

calculations0 An air recei3er is a pressure 3essel an* as such re/uires regular 3isual an*

3olumetric pressure tests0 "ecor*s shoul* ,e >ept of the tests0

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applied hydraulics and pneumatics notes.doc