Lecture 5_Hydraulic Systems of Machine Tools

8/10/2019 Lecture 5_Hydraulic Systems of Machine Tools

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Hydraulic Systems ofHydraulic Systems ofMachine ToolsMachine Tools


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Hydraulic Systems of

Machine Tools Hydraulic systems are now ever morewidely used in machine tools as principaland feed movement drives, speed-

changing devices, braking mechanisms,clamping devices, automatic control ofmachining cycle, etc.

Hydraulic systems become the main typeof drive in machine tools like grinders,shapers, copying millers, broachingmachines, etc.


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Machine Tools This extensive use of hydraulic systemsis due to their capability of: providing infinitely variable speed over a

wide range, smooth reversal of moving machine members, automatic overload protection, easy lubrication, etc.Hydraulically controlled machine tools take

less floor space, andtheir parts and units can be easily

standardized.


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Machine Tools mong their shortcomings are: leakage of hydraulic fluid through seals and

gaps,

ingress of air into fluid,

effects of temperature and time on fluidproperties, etc.


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Hydraulic Systems of MachineTools

Energy losses in hydraulic systems aremade up of !olumetric losses due to lea"age of hydraulic

fluid# hydraulic losses due to a drop in pressure# and

mechanical losses due to friction $et%eencontact surfaces&

Total efficiency of the hydraulic system# ordri!e is = vhm

'here#v# h# m are: !olumetric# hydraulic and mechanicalefficiency# respecti!ely&


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)ormal functioning of the hydraulicsystem largely depends on the type of%or"ing fluid employed&

This fluid should $e sufficiently !iscousand uniform*

it should possess good lu$ricity andprotect mechanisms from corrosion*

it should retain its properties %ith

changes in temperature# pressure# speedand direction of mo!ement&

'or"ing fluids inhydraulic systems


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'or"ing fluids in

hydraulic systems The %or"ing fluid should not $ecome o,idi-ed ore!ol!e sediments# e!aporate or inflame&

These re.uirements are $est met $y mineral oils andtheir mi,tures&

The principal property used in selecting andcomparing oils is the !iscosity inde,# %hich sho%s thechange in the !iscosity of an oil %ith its temperature&

The higher the !iscosity inde,# the higher the .ualityof the oil and the higher its refinement&

/il !iscosity inde, 0 is $est suited for hydraulicsystems&


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omponents of the HydraulicSystem of Machine Tools

Typically# the machinetool hydraulicsystem includes an oil tan"*

a pump to deli!er oil into the system* control de!ices !al!es# regulators#

pressuresensiti!e relays# timers# etc&6

to control pressure and !olume of oil inthe system*


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omponents of the HydraulicSystem of Machine Tools

distri$uting de!ices to control the %or"ingcycle* operati!e cylinders for straightmo!ement and hydraulic motors for rotatingmo!ement*

piping to connect all the elements of thesystem&

Hydraulic systems in machine tools operate at anoil pressure of up to 2 M)7m22 "gf7cm26&


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!umps

onstant and !aria$ledisplacementcontinuousaction pumps# such as gearpumps# vane pumps# and radial pistonpumpsare used to deli!er oil underpressure into the machinetool hydraulicsystem

The principal characteristic of thehydraulic pump is the output# or deli!ery&

The rates of pump deli!ery ha!e $een

standardi-ed* from a !alue of 1,143m37min on%ards&


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!umps 8ates of deli!ery constitute a geometric series%ith a ratio of 9 : 1&41&

The standard series of pump deli!eries# in 13

m37min# is&5* 1* 2* "* 5* * 12* 1* 25* 35* 5* +* 1* 14#2* 2* #$$* 5(* * 112* 1(* 225* 32*45* (3* 0* 12&

Minimum pump deli!ery for machinetool hydraulicdri!es is ta"en as ;min: 3,13m37min# andma,imum deli!ery ;ma,:4,13m37min&


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!umps


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!umps

To pro!ide trou$lefree operation of the pumpthe follo%ing points should $e addressed its shaft should $e carefully aligned %ith the

dri!e motor shaft permissi$le misalignment isnot more than &1 mm6# the rotational directionmar"ed or#

the pump casing should $e chec"ed fornonre!ersi$le pumps6# and

motor and pump shafts should $e "ept free of$urrs&

'idely used in machine tools are constant and!aria$ledisplacement pumps %ith different !aluesof deli!ery and pressure&


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@ear pumps

%ear pumps# as a rule# come in a constantdisplacement !ersion& These are used in hydraulicsystems operating at relati!ely lo% pressures&

The gear pump sho%n in Aig& (4comprises driving

gear "and driven gear &mounted in casing '& 'hen the gears rotate# oil is dra%n into suctionzone ($y !acuum generated therein and carriedin the tooth spaces to discharge zone ($# andthen forced into the pipe line&


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)ig. *#.+lear pump


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@ear pumps

The proecting end of shaft #is sealed $ysleeve # %hose end face is pressed to the end ofthe flange $y spring &

The spring $ears against ring *# %hose

displacement is limited $y pin /& The oil escaping through the gaps $et%een theBoint surfaces is directed to the oil tan" along thecorresponding drain groo!es&


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)ig. *#.%ear pump


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@ear pumps

The main re.uirements for gear pumps are minimum clearances in all the fits*

gears centre distance should $e held to strict

tolerances# and minimum $ac"lash in meshing gears should $epro!ided*

pump casing and gears should $e manufactured

from high.uality materials&


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Cane pumps

0ane pumps& onstantdisplacementdou$leacting !ane pump is the mostcommon type in use&

These pumps are dri!en either directly $ya motor or through a mechanicaltransmission&

The pumps of this type ha!e high

efficiency and pro!ide uniform deli!ery ofoil into the pipe line&


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Cane pumps

Cane pumps come in single and duple, types& Duple,type !ane pumps are used for !arious

com$inations of deli!ery and pressures& Highdeli!ery pumps are designed for pressures

of up to 2&5 M)7m2

25 "gf7cm2

6# and lo%deli!erypumps# for pressures of up to (&5 M)7m2(5"gf7cm26&

Cane pumps are e,tensi!ely used in hydraulicmachines %ith flo% control %here operatingmem$ers tra!el at lo% speed and %ith high pullforce&


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Cane pumps

Duple,type pumps are employed# for instance# to pro!ide rapid tra!erse mo!ements along %ith

slo% operati!e mo!ements of some machinemem$ers&

Here# highdeli!ery pumps are used for rapidtra!erse&

The arrangement and principle of operation of a!ane pump are illustrated in Aig& (5&


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)ig. */: 0ane pumps


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0ane pumps

The pump includes castiron casing (# mountedinside it is steel stator '%ith an ellipticalinternal surface# and mounted on shaft *is rotor"carrying blades #&

ron-e $ushings integral %ith discs /closing the%or"ing cham$er at $oth sides ser!e as $earingsfor the rotor&


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0ane pumps Each disc has t%o suction and t%o dischargeports&

The in%ard end faces of the discs ha!e t%o

groo!es each# ser!ing to deli!er some oil from thedischarge cham$ers to underneath the $lades& The suction ports and the discharge ports are

connected in pairs $y castin channels in thecasing# there$y forming the suction cham$er %ith

openings and the discharge cham$er %ithopenings &


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0ane pumps

'hen the rotor rotates# the $lades arepressed to the stator $y centrifugal forceand the additional pressure of the oildeli!ered from the discharge cham$er tounderneath the $lades# as mentionedearlier&


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0ane pumps Fs the $lades pass $y openings # the oil is

forced out through these openings into thedischarge cham$er&

Thus# two full working cyclesare carried out per

pumprotor re!olution# that is %hy this type ofpump is called dou$leacting&

To pre!ent the $lades from Bamming# they areinclined at an angle of 1 2 ("$-(#$&


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8otary !aria$ledisplacementpiston pumps

8otary !aria$ledisplacement piston pumps arecommonly used in the follo%ing applicationsa6 in manually controlled displacement %ith adeli!ery range of ; : 526,1-"m"7min atpressure of p : 2 M)7m'2 "gf7cm'6*$6 in hydraulically controlled displacement %ith adeli!ery range of ; : 326,1-"m"7min atpressure of +&5 M)7m' +5 "gf7cm'6 *c6 in electrohydraulically controlleddisplacement %ith a deli!ery range of ; : 1516 ,1-"m"7min at pressure of p : 1 M)7m'1 "gf7cm'6*


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8otary !aria$le

displacement piston pumpsd6 %ith a deli!ery range of ; : 326,143m37min at pressure of p : +&5 M)7M2+5 "gf7cm26 *

e6 in hydraulic ser!o system controls %itha deli!ery range of ; : 1556&143m37minat pressure of p : 2 M)7m22

"gf7cm26&


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8otary !aria$le

displacement piston pumps 8otary piston pumps %ith re!ersi$le deli!ery aredesigned for the hydraulic systems of hea!ymachine tools ha!ing a high pullcapacity andoperating at high speeds# such as slotting

machines# $roaching machines# planers# etc& 8otary piston pumps may $e of the radial type

radial piston pumps6 and of the a,ial type a,ialpiston pumps6# %ith pistons mounted either in thestator or in the rotor&

Aigure ((sho%s a radial piston pump %ith pistonsmounted in rotor (&


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8otary !aria$le

displacement piston pumps


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8otary !aria$le

displacement piston pumps The rotor rotates a$out its a,is along %ith pistons'& 3tator "# contacting the heads of pistons '# isimmo!a$le# $eing positioned eccentrically %ithrespect to the rotor&

The rotor shaft has t%o separate inner cham$ers#one $eing the suction cham$er and the other# thedischarge&

'hen the rotor rotates through ($4,each

piston mo!es a%ay from the centre to theperiphery along its $ore# suc"ing in oil from thecorresponding cham$er of the shaft&


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8otary !aria$le

displacement piston pumps Aurther rotation of the rotor 5from ($ to "*$46mo!es the pistons $ac" to the centre and forcesthe oil into the discharge cham$er&

Thus# each piston ma"es a dou$le stro"e per rotorre!olution& The length of the stro"e depends on the

eccentricity of the stator to the rotor& F change in the !alue of eccentricity results in

pump deli!ery !ariation&


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8otary !aria$le

displacement piston pumps 'hen the rotor and stator a,es coincide 5e 2$6,pump deli!ery is nil.

The pistons of the radial piston pump are urgedout%ards $y centrifugal force and the additionalpressure of the oil deli!ered from an au,iliarylo%pressure pump&

8adial piston pumps may contain from / to ('*pistons arranged in one# t%o# three or four ro%s#depending on the pump deli!ery&


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8otary !aria$le

displacement piston pumps 8adial piston pumps ha!e delivery ranges of $.'-# litres per minfor smallsi-e models# and (-*$$ litres per minfor largesi-e models# %ith

pressure ranges of up to ./ 789m'

5/kgf9cm' 6 and higherunder continuous operation&


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!ressure- and )low-+ontrolevices

Carious control de!ices are used to control theoilflo% parameters in machinetool hydraulicsystems# such as pressureand volumeof flo%&

More specifically# these are used to maintain the

re.uired pressure %ithin the hydraulic system# toregulate the flo% and also to disconnect certain parts

of the hydraulic circuit from the others&


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!ressure- and )low-+ontrolevices


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Aigure (+ Safety !al!e %ith $all


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Safety !al!e

'hen the oil pressure e,ceeds its predetermined !alue# the $all is lifted up#compressing the spring# and the e,cess oilescapes through the e,haust passages into

the oil tan"# %hereupon the $all returns toits original position and closes the !al!e& The limiting pressure at which the valve

opens is set up by an adusting capwhose displacement changes the force ofthe spring.


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Hydraulic !ower +ylinders and

7otors Hydraulic po%er cylinders are designed to

transform the energy of a fluid flo% into the"inetic energy of a reciprocating machinetooloperati!e mem$er&

There is a great !ariety of hydraulic cylinder

designs and arrangements* ho%e!er# symmetricaland asymmetrical cylinders are the most %idelyemployed&

The former# ha!ing a dou$leend piston rod#pro!ide e.ual speeds of for%ard and return

stro"es at constant pressure# %hile the latter#ha!ing a singleend piston rod# mo!e faster on thereturn than on the for%ard stro"e&

H d li ! + li d d


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Hydraulic !ower +ylinders and7otors

'ith symmetrical cylinders# the rod ofreciprocating piston 2 is connected to ta$le 1#%hile the cylinder itself is stationary Aig& +5a6&

'ith asymmetrical cylinders# it is either thecylinder Aig& +5$6or the piston Aig& +5c6that isstationary&

Gn the latter instance# the ta$le is connected %iththe reciprocating cylinder&

Hydraulic motors# similar in design to hydraulic

pumps mostly of the rotarypiston type6#transform the energy of a fluid flo% into the"inetic energy of the rotating motor shaft&


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Aig& +5


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Directional ontrol De!ices Directional control de!ices# such as spooltype

!al!es# pilots# etc& are used to control thedirection of oil flo% and to connect anddisconnect !arious $ranches of the hydraulicsystem&

Such de!ices are operated manually# mechanically

from trip dogs of the machine tool6# hydraulicallyor electrically&

The hydraulically operated a t%o%ay directionalcontrol !al!e sho%n in Aig& +4is used to re!ersethe motion of machinetool operati!e mem$ers&


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Aig& +4Directional ontrol Cal!e


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Directional ontrol Cal!e connected to the ylinder

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Directional ontrol Cal!e

The t%o e,treme positions of spool (correspond toopposite directions of the controlled unit motion& The spool changes its position under the pressure

of oil directed to its end faces through inlets madein the !al!e flanges&

During the right e,treme position of the spool# assho%n in the figure# li.uid flo%s through the inlet atthe left hand side of the figure# %hile the outlet atthe right hand side acts as the outlet for the li.uidand !ice !ersa# due to the design of the spool asindicated in the figure&

The speed of the spool shifting is regulated $ythrottle "&


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There are t%o adusting screws 'to limitthe tra!el of the spool %hene!er the

fre.uency of re!ersals and theirsmoothness need to $e increased&


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Hydraulic rive 3chemes

0olume9speed control Hydraulic dri!es are classified according to themethod of speed regulation of hydraulic motors&

The speed of a hydraulic motor or cylinder pistondepends on the rate of flo% the !olume of oil per

unit of time6 deli!ered to it& /ne method of changing the rate of flo%# "no%nas !aria$ledisplacement control# in!ol!es the useof a !aria$ledisplacement pump* this method isemployed in hydraulic systems %ith a %ide range

of speeds and loads


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Fnother method# "no%n as flo% control# consistsin the application of throttle !al!es installed inhydraulic systems using constantdisplacementpumps# %hich discharge oil at a constant pressure&

The $asic circuit diagrams of hydraulic dri!es

%ith flo% control of speed are presented in Aig&+( a# $# c&

Gn the circuit sho%n in Aig& +(a#throttle valve 'is located in the discharge line leading out of the

cylinder&

Ai +( ti f th ttl i h d li


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Aig& +(a ocation of throttles in hydrauliccontrol circuits


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onstantdisplacement !anetype pump (suc"s oilfrom the tan" and deli!ers it to the left cham$erof the cylinder&

The right cham$er of the cylinder can $edischarged only through throttle '# %hosepassage area determines the !olume of the

discharged oil# and so the speed of the piston& Since pump deli!ery in flo%controlled dri!es

e,ceeds that needed for pro!iding the ratedpiston speed# the e,cess oil is directed to thetan" through relief valve "#


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Gn the circuit sho%n in Aig& +($# throttle valve 'is located in the deli!ery line leading to thecylinder# and its oil passage area determines thepiston speed&

E,cess oil from pump (is also directed to thetan" through relief valve "&

Ai +($ ti f th ttl s i


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Aig& +($ ocation of throttles inhydraulic control circuits&


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Gn the circuit according to Aig& +(c# the throttle!al!e is located at the $ypass $ranch connectedin parallel %ith the cylinder&

The piston speed is determined $y the difference$et%een the !olume of the oil deli!ered $y the

pump and the !olume of the oil directed to thetan"through the throttle&

Aig& +(c ocation of throttles in


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ghydraulic control circuits&


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Gn hydraulic dri!es arranged according to theschemes sho%n in Aigs +(aand $# the po%erconsumption of the pump is the ma,imum#irrespecti!e of the useful load# %ith the !alue ofpressure# at %hich e,cess oil is drained to the

tan"# set $y relief !al!e 3& Gn these dri!es# ho%e!er# !olumetric losses do notaffect the piston speed.

Gn the dri!e sho%n in the scheme Aig& +(c# thepressure corresponds to the useful load# $ut the

change of this load and also !olumetric lossesaffect the piston speed&

Lecture 5_Hydraulic Systems of Machine Tools

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Transcript of Lecture 5_Hydraulic Systems of Machine Tools