液压与气压传动(英汉双语)第2版-教学ppt课件---第2章-Chapter.ppt

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1、Hydraulic and pneumatic pressure transmissionChapter 2 Fundamental Hydraulic Fluid MechanicsChapter 2 Fundamental Hydraulic Fluid Mechanics 22.1 Performances of the Hydraulic Oil 2.2 Hydrostatics 2.3 Hydrodynamics 2.4 Characteristics of Fluid Flow in Pipeline2.5 Flow Rate and Pressure Features of Or

2、ifice 2.6 Hydraulic Shock and Cavitation Chapter listChapter 2 Fundamental Hydraulic Fluid Mechanics 32.1.1 The Main performances 2.1.2 The requests and choice of hydraulic oil 2.1 Performances of the Hydraulic Oil Chapter 2 Fundamental Hydraulic Fluid Mechanics 41.Density(kg/m3)2.2.Compressibility

3、2.1.1 The Main performances the coefficient of compressibility ,is the bulk modulus of elasticity(2-2)(2-1)is defined as the ratio of the change in pressure()to relative change in volume()while the temperature remains constant.Chapter 2 Fundamental Hydraulic Fluid Mechanics 53.3.Viscosity The experi

4、ments have proved that friction force between the two fluid molecules can be described asWhere is viscosity coefficient,also kinematic viscosity.Fig.2-1 The sketch of viscosity The sketch of viscosity is illustrated by Fig.2-1.(2-3)Cohesion between two moleculesChapter 2 Fundamental Hydraulic Fluid

5、Mechanics 6There are three methods to describe the viscosity:absolute viscosity,Kinematic viscosity and relative viscosity.(1)Dynamic viscosity or absolute viscosity(1)Dynamic viscosity or absolute viscosity(Pas)or(N s/m2)(2)(2)KinematicKinematic viscosity viscosity (mm(mm2 2/s)/s)(2-4)(3)(3)Relativ

6、e viscosity(conditionalRelative viscosity(conditional viscosity)The relative viscosity which used in China is tested by the viscometer,such as Fig.2-2.Chapter 2 Fundamental Hydraulic Fluid Mechanics 7Fig.2-2 Principle of viscometer Chapter 2 Fundamental Hydraulic Fluid Mechanics 8Take the note descr

7、ibes the viscosity:The conversion formula between the and kinematic viscosity is (m2/s)(4)Viscosity-temperature(4)Viscosity-temperature:For the viscosity less than 15 and the temperature 30 150,the viscosity-temperature formula is describe as following(We can also look up from Fig.2-3):(2-5)(2-6)(2-

8、7)(5)(5)Viscosity-pressure(2-8)(6)Others performances(6)Others performances:physical and chemical,such as anti-inflammability,anti-oxygenation,anti-concreting,anti-foam and anti-corrosion etc.Chapter 2 Fundamental Hydraulic Fluid Mechanics 9Fig.2-3 The viscosity-temperature of homemade oils Chapter

9、2 Fundamental Hydraulic Fluid Mechanics 102.2.Choice The hydraulic oil in a hydraulic system at is recommended generally.1.Request The oil plays two roles of transmission energy and lubrication on the surfaces of working interaction.The requests for the hydraulic fluids are:appropriate viscosity,the

10、 good in property of favorable viscositytemperature,a good lubricity,chemically and environmentally stabilities,compatible with other system materials and so on.2.1.2 The requests and choice of hydraulic oil The hydraulic oil should be chosen in according to the request of hydraulic pump.The hydraul

11、ic oil viscosity adapted for different hydraulic pumps is listed in Tab.2-2.Chapter 2 Fundamental Hydraulic Fluid Mechanics 11Tab.2-2 The range of viscosity of hydraulic oil adapted to pumpsTypesviscosities(10-6 m2/s)TypesViscosities(10-6 m2/s)54040805404080Vane PumpsPCr，neglect Cr,the flow rate as

12、The flow rate coefficient can be obtained by Fig.2-22,the Reynolds number can be calculated by following,(2-44)(2-45)(2-46)Chapter 2 Fundamental Hydraulic Fluid Mechanics 50 For a hydraulic valve whatever flowing in or out,is the angle between streamline and spool line and is called speed direction

13、angle,it is usually .Fig.2-22 Flow coefficient on the orifice of spool valve Chapter 2 Fundamental Hydraulic Fluid Mechanics 51The poppet valve orifice is shown in Fig.2-23,When poppet moves up a distance of ,the average diameter of ,then the flow rate is Fig.2-23 Orifice shape of poppet valveFig.2-

14、24 Flow coefficient of poppet valve orifice(2-47)Where the flow rate coefficient can be obtained by Fig.2-24Chapter 2 Fundamental Hydraulic Fluid Mechanics 522.5.2 2.5.2 Stubby orifice or slot orifice Fig.2-25 Flow rate coefficients in Stubby orifice The flow rate equation for slot orifice obeys the

15、 formula(2-31),i.e.The flow rate equation for the stubby orifice is the same as formula(2-42),but the flow rate coefficient can be obtained from the curve in Fig.2-25.The stubby orifice is defined as ，slot orificeChapter 2 Fundamental Hydraulic Fluid Mechanics 532.5.3 Plate clearanceFig.2-26 Flow in

16、 parallel plain clearanceThe flow rate fluid flow through the plain plate clearance is （2-48）The formula（2-48）has two statuses:1）Fluid flow at pressure differential:（2-49）2）Fluid flow by viscosity shear：（2-50）The fluid flows under pressure differential and velocity as shown in Fig.2-26.Chapter 2 Fun

17、damental Hydraulic Fluid Mechanics 542.5.4 Cylinder annular clearance 1.The flow rate equation in a concentric annular orificeFig.2-27 shows a sketch of concentric clearance flow Fig.2-27 Sketch of concentric clearance flowLets consider annular clearance expanded along the length direction is the sa

18、me as a plain plate clearance,so substituting into formula(2-48)If the motion direction of cylinder is the same as the direction of pressure differential,the symbol in(2-51)chooses“+”,otherwise“”.the flow rate is(2-51)(2-52)Chapter 2 Fundamental Hydraulic Fluid Mechanics 55 as shown in Fig.2-28,we c

19、an abtain For very small clearances,is very small and ,thenBecause of small clearance，can be considered as Plates clearance flow,the incremental flow is where2.The flow rate equation in eccentric annular orifice Fig.2-28 Eccentric annular orifice(2-53)(2-54)(2-55)Chapter 2 Fundamental Hydraulic Flui

20、d Mechanics 56If e=h0，the flow is greater than it would be indicated by the use of equation(2-51).Substitute（2-54）into（2-55）(2-56)(2-57)Integrating:Or (2-58)Chapter 2 Fundamental Hydraulic Fluid Mechanics 573.The flow rate through a conical annular clearance Because of machining irregularities,such

21、as piston or bore,valve core or seat core,some degree of conic must always be expected,as shown in Fig.2-29.Fig.2-29 Fluid flow through a conical annular clearancea)Converse cone b)Sequence cone When it is called inverse degree of conic as shown in Fig.2-29 a;otherwise sequence degree of conic as sh

22、own in Fig.2-29 b Chapter 2 Fundamental Hydraulic Fluid Mechanics 58For the status of Fig.2-29 a,substituting into formula(2-51),Because h=h1+xtan,substituting into formula(2-59)：Integrating and substituting into We obtain the flow rate as (2-59)(2-60)(2-61)(2-62)Chapter 2 Fundamental Hydraulic Flui

23、d Mechanics 59When ，flow rate is Integrating formula(2-61)the pressure distribution in this clearance flowing,and substituting the boundary condition at h=h1，p=p1,we obtainSubstituting formula(2-62）and into(2-64)，,When u0=0,we have (2-64)(2-63)(2-65)(2-66)Chapter 2 Fundamental Hydraulic Fluid Mechan

24、ics 60 For the status of Fig.2-29b,the sequence degree of conic the flow rate formula is the same as the formula(2-62),but pressure distribution when is or(2-67)(2-68)Chapter 2 Fundamental Hydraulic Fluid Mechanics 614.4.Hydraulic lock and force If there is a eccentricity“e”between spool core and se

25、at due to setting,as shown in Fig.2-30.a)Eccentric with inverse order conical annularb)Eccentric with in order conical annularc)Section figure at any pointd)Spool core notched balance pressureFig.2-30 Fluid flow through a conical annular clearance with eccentricChapter 2 Fundamental Hydraulic Fluid

26、Mechanics 62The value of“h”at any point is(2-69)（1）For the case of eccentric with inverse order conical annular clearance(Fig.2-30a):the side force F will enlarge the eccentric value“e”(Fig.2-30a)to make the spool core locked on the wall of seat.Since ，from the equation(2-66),the pressure at point o

27、f is less than that at ，i.e.(Fig.2-30c).Chapter 2 Fundamental Hydraulic Fluid Mechanics 63（2）For the case of eccentric with inverse order conical annular clearance(Fig.2-30b)：the side Force F will be created and it will reduce the eccentric value“e”(Fig.2-30b)to make the spool core centered in the s

28、eat.In fact,the hydraulic lock is objective and what we can do is to try to eliminate it.The most simple method is balance notches on the spool core.Chapter 2 Fundamental Hydraulic Fluid Mechanics 642.6.1 2.6.1 Hydraulic shock 2.6.2 2.6.2 Cavitation 2.6 2.6 Hydraulic Shock and CavitationHydraulic Sh

29、ock and Cavitation Chapter 2 Fundamental Hydraulic Fluid Mechanics 652.6.1 Hydraulic ShockHydraulic shock and damages1.1.The types of hydraulic shockThe types of hydraulic shock(1)That occurs due to the sudden reduction of the across-section of the orifice or change of the flow direction;(2)That res

30、ults from the inertia of the high speed working components suddenly braking or changing direction.Chapter 2 Fundamental Hydraulic Fluid Mechanics 66 1 1）Hydraulic shock results from fluid flow stopping Hydraulic shock results from fluid flow stopping suddenlysuddenly Fig.2-31 Hydraulic impact(2-70)T

31、he fluid flow is stopped suddenly when the check valve is closed suddenly.According to conservation of energy,So Chapter 2 Fundamental Hydraulic Fluid Mechanics 67Spread speed of shock wave in pipe can be calculated by (2-71)Formula(2-72)is initituled the whole hydraulic shock situation.The pressure

32、 peak value at non-whole hydraulic shock is less than the whole hydraulic shock and it can be calculated by (2-73)Formula(2-70)is only used to the closed pipe,i.e.,the time t of the check valve closed is less than the time (time of critical close).(2-72)Chapter 2 Fundamental Hydraulic Fluid Mechanic

33、s 682)Hydraulic shock resultes from the motion parts braked2)Hydraulic shock resultes from the motion parts braked according to the conservation of momentum,the system shock pressure can be calculated:Example 2-7.If the inner diameter of pipe is d=200mm,the wall thickness 10mm,the initialization spe

34、ed ,pressure ,the bulk modulus of elasticity of fluid MPa,the modulus of elasticity of pipe material MPa.Calculate the maximum pressure drop when the check valve is closed suddenly.Chapter 2 Fundamental Hydraulic Fluid Mechanics 692.Measures to reduce the hydraulic shock1)Prolong the time closing ch

35、eck and motion components.2)Valve orifice of motion working-piece is designed accurately making speed uniformity change.3)Expand properly the diameter of pipe.4)Try to shorten the length of pipe.5)Utilize rubber tube or accumulator at point of hydraulic shock.Chapter 2 Fundamental Hydraulic Fluid Me

36、chanics 702.6.2 2.6.2 Cavitatation 1.The principle and harm of cavitation The principle of cavitation：The phenomenon of cavitation happens easily in suction of pump and valve orifice.harm：air-corrosion which will damage working-pieces of hydraulic machinery and shorten the machines life.Chapter 2 Fu

37、ndamental Hydraulic Fluid Mechanics 712.The measure to decrease the Cavitation 1)Reducing the pressure drop,usually the special pressure is controlled p1p23.5.2)When design it is better for trying to avoid narrow,elbow or sudden direction changed.3)Try to reduce the suction height of pump.4)Good sea

38、l parts to avoid air entrance.5)Resist corrosive material used try to keep the element rigidity and increasing exterior roughness for withstanding corrosion.Chapter 2 Fundamental Hydraulic Fluid Mechanics 72your suggestions are welcome!The EndChapter 2 Fundamental Hydraulic Fluid Mechanics 73Fig.2-2

39、 Principle of Fig.2-2 Principle of 0 0E viscometerE viscometerChapter 2 Fundamental Hydraulic Fluid Mechanics 74Fig.2-16 hydraulic dynamic on the spool valve Chapter 2 Fundamental Hydraulic Fluid Mechanics 75Fig.2-17 hydraulic dynamic on the poppet valve Chapter 2 Fundamental Hydraulic Fluid Mechanics 76Fig.2-18.The setup of Reynolds testFig.2-18.The setup of Reynolds test