McGraw-Hill CreateTM ReviewCopy forInstructorNicolescu.Notfor distribution.124MeasurementSystems468CHAPTER10Actuators10.8HYDRAULICSHydraulic systems are designed to move large loads by controlling a high-pressurefluid in distribution lines and pistons with mechanical orelectromechanical valves.A hydraulic system, illustrated in Figure 10.34, consists of a pump to deliver high-pressure fluid, a pressure regulator to limit the pressure in the system, valves to con-trolflowratesandpressures,adistribution systemcomposed ofhosesorpipes,andlinear or rotaryactuators.The infrastructure,which consists of the elements con-tained in the dashed box in the figure, is typicallyused to power many hydraulicvalve-actuatorsubsystems.Ahydraulic pump is usually driven by an electricmotor (e.g.,a large AC induc-tion motor)or an internal combustion engine.Typical fluid pressures generated bypumps used in heavy equipment (e.g., construction equipment and large industrialmachines) are in the 1000 psi (6.89 MPa) to 3000 psi (20.7 MPa) range. The hydrau-lic fluid is selected to have the following characteristics: good lubrication to preventwear in moving components (e.g.,between pistons and cylinders),corrosion resis-tance,and incompressibilitytoproviderapidresponse.Mosthydraulicpumpsactbypositive displacement, which means they deliver a fixed volume of fluid with eachcycle or rotation of the pump.The three main types of positive displacement pumpsusedinhydraulic systemsaregearpumps,vanepumps,andpistonpumps.Anexam-ple of a gear pump, which displaces the fluid around a housing between teeth ofmeshinggears, is shown inFigure10.35.Note that themeshing teeth provide a seal, infrastructurecontroloresstrefilterregulatorvalvePAcylinderFigure10.34Hydraulicsystemcomponentsoutlet (P)shaft rotatiofluidcarriedbetweenteethinlet (7)Figure10.35Gearpump
Confirming Pages 468 C H A P T E R 10 Actuators 10.8 HYDRAULICS Hydraulic systems are designed to move large loads by controlling a high-pressure fluid in distribution lines and pistons with mechanical or electromechanical valves. A hydraulic system, illustrated in Figure 10.34 , consists of a pump to deliver highpressure fluid, a pressure regulator to limit the pressure in the system, valves to control flow rates and pressures, a distribution system composed of hoses or pipes, and linear or rotary actuators. The infrastructure, which consists of the elements contained in the dashed box in the figure, is typically used to power many hydraulic valve-actuator subsystems. A hydraulic pump is usually driven by an electric motor (e.g., a large AC induction motor) or an internal combustion engine. Typical fluid pressures generated by pumps used in heavy equipment (e.g., construction equipment and large industrial machines) are in the 1000 psi (6.89 MPa) to 3000 psi (20.7 MPa) range. The hydraulic fluid is selected to have the following characteristics: good lubrication to prevent wear in moving components (e.g., between pistons and cylinders), corrosion resistance, and incompressibility to provide rapid response. Most hydraulic pumps act by positive displacement, which means they deliver a fixed volume of fluid with each cycle or rotation of the pump. The three main types of positive displacement pumps used in hydraulic systems are gear pumps, vane pumps, and piston pumps. An example of a gear pump, which displaces the fluid around a housing between teeth of meshing gears, is shown in Figure 10.35 . Note that the meshing teeth provide a seal, tank filter motor pump pressure regulator control valve cylinder P A T B infrastructure Figure 10.34 Hydraulic system components. Figure 10.35 Gear pump. shaft rotation outlet (P) inlet (T) fluid carried between teeth alc80237_ch10_431-477_sss.indd 468 lc80237_ch10_431-477_sss.indd 468 10/01/11 10:24 PM 0/01/11 10:24 PM 124 Measurement Systems McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution
McGraw-Hill CreateTM ReviewCopyforInstructorNicolescu.NotfordistributionIntroductiontoMechatronicsandMeasurementSystems,FourthEdition12510.8469Hydraulicsspring orvanehydraulicpressureslotted rotorvane guidoinletoutietmotor shaftFigure 10.36VanepumpcylindersectionviewblockinletRinput shaftinlet/outletmanifoldsadjustableoutleangle, fixedswash platepistonanVeWoutletmanifoldcylindersinletmanifoldFigure10.37Swashplatepistonpumpand thefluid is displaced from the inlet to the outlet along the nonmeshing side of thegears. Video Demos 10.22 and 10.23 show and describe various types of gear pumps.Figure10.36illustratesavanepump,whichdisplacesthefluidbetweenvanesguided in rotor slots riding against the housing and vane guide. The vane guide sup-VideoDemoports the vanes from one side of the housing to the next and is constructed to allow10.22Gear pumpsthefluidtopass.Theoutputdisplacementcanbevaried(withaconstantmotor10.23Hydraulicspeed)bymovingthe shaftverticallyrelativetothehousinggearpumpsFigure 10.37 illustrates a piston pump.The cylinder block is rotated by theinput shaft, and thepiston ends aredriven in and out as theyride in thefixed swashplate slot, which is angled with respectto the axis of the shaft.Apiston drawsfluidfrom an inlet manifold overhalf theswash plate and expelsfluid intothe outletmanifold duringthe otherhalf.The displacement of thepump canbe changed sim-ply by changing the angle of the fixed swash plate.Table 10.5 lists and compares thegeneral characteristics of thedifferentpumptypes.Since positive displacement hydraulic pumps provide a fixed volumetric flowrate, it is necessary to include a pressure relief valve, called a pressure regulator
Confirming Pages Figure 10.36 Vane pump. inlet outlet vane spring or hydraulic pressure slotted rotor vane guide motor shaft and the fluid is displaced from the inlet to the outlet along the nonmeshing side of the gears. Video Demos 10.22 and 10.23 show and describe various types of gear pumps. Figure 10.36 illustrates a vane pump, which displaces the fluid between vanes guided in rotor slots riding against the housing and vane guide. The vane guide supports the vanes from one side of the housing to the next and is constructed to allow the fluid to pass. The output displacement can be varied (with a constant motor speed) by moving the shaft vertically relative to the housing. Figure 10.37 illustrates a piston pump. The cylinder block is rotated by the input shaft, and the piston ends are driven in and out as they ride in the fixed swash plate slot, which is angled with respect to the axis of the shaft. A piston draws fluid from an inlet manifold over half the swash plate and expels fluid into the outlet manifold during the other half. The displacement of the pump can be changed simply by changing the angle of the fixed swash plate. Table 10.5 lists and compares the general characteristics of the different pump types. Since positive displacement hydraulic pumps provide a fixed volumetric flow rate, it is necessary to include a pressure relief valve, called a pressure regulator, Figure 10.37 Swash plate piston pump. end views cylinders outlet manifold inlet manifold adjustable angle, fixed swash plate inlet outlet input shaft piston cylinder block inlet/outlet manifolds section view Video Demo 10.22 Gear pumps 10.23 Hydraulic gear pumps 10.8 Hydraulics 469 alc80237_ch10_431-477_sss.indd 469 lc80237_ch10_431-477_sss.indd 469 10/01/11 10:24 PM 0/01/11 10:24 PM Introduction to Mechatronics and Measurement Systems, Fourth Edition 125 McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution
McGraw-HillCreateTM ReviewCopyforlnstructorNicolescu.Notfordistribution.126MeasurementSystems470CHAPTER10ActuatorsTable10.5ComparisonofpumpcharacteristicsTypicalPumptypeCostDisplacementpressure (psi)Fixed2000LowGearVaneVariable3000MediumPistonVariable6000Highto prevent the pressure from exceeding design limits. The simplest pressure regu-lator is the spring-ball arrangement illustrated in Figure 10.38.When the pressureforce exceeds the spring force, fluid is vented back to the tank, preventing a fur-ther increase in pressure. The threshold pressure, or cracking pressure, is usuallyadjusted by changing the spring's compressed length and therefore its resisting force.10.8.1HydraulicValvesThere are two types of hydraulic valves: the infinite position valve that allows anyposition between open and closed tomodulate flow or pressure,and the finite posi-tion valve that has discrete positions, usually just open and closed, each providinga differentpressureandflow condition.Inlet and outlet connections to a valve arecalledports.Finitepositionvalvesarecommonlydescribedbyanx/ydesignation,wherexisthenumberofportsandyisthenumber ofpositions.As an example,a 4/3valve,with4ports and3positions,is illustrated inschematicform inFigure10.39Inposition1,systempressureis vented totank;inposition2,outputportAispres-surized and portBisvented totank; and inposition3,outputportB is pressurizedand port A is vented to tank.As illustrated in Figure 10.40, this particular valve isuseful in controlling a double-acting hydraulic cylinder where ports A and B connect-system pressure (P)ballspringadjustablesupportreturn to tank (7)Figure10.38Pressure regulator.Bposition 1position 2position 3Figure10.394/3valveschematic
Confirming Pages 470 C H A P T E R 10 Actuators to prevent the pressure from exceeding design limits. The simplest pressure regulator is the spring-ball arrangement illustrated in Figure 10.38 . When the pressure force exceeds the spring force, fluid is vented back to the tank, preventing a further increase in pressure. The threshold pressure, or cracking pressure, is usually adjusted by changing the spring’s compressed length and therefore its resisting force. 10.8.1 Hydraulic Valves There are two types of hydraulic valves: the infinite position valve that allows any position between open and closed to modulate flow or pressure, and the finite position valve that has discrete positions, usually just open and closed, each providing a different pressure and flow condition. Inlet and outlet connections to a valve are called ports. Finite position valves are commonly described by an x / y designation, where x is the number of ports and y is the number of positions. As an example, a 4/3 valve, with 4 ports and 3 positions, is illustrated in schematic form in Figure 10.39 . In position 1, system pressure is vented to tank; in position 2, output port A is pressurized and port B is vented to tank; and in position 3, output port B is pressurized and port A is vented to tank. As illustrated in Figure 10.40 , this particular valve is useful in controlling a double-acting hydraulic cylinder where ports A and B connect Table 10.5 Comparison of pump characteristics Pump type Displacement Typical pressure (psi) Cost Gear Fixed 2000 Low Vane Variable 3000 Medium Piston Variable 6000 High Figure 10.38 Pressure regulator. return to tank (T) adjustable support system pressure (P) ball spring Figure 10.39 4/3 valve schematic. P T A B P T A B P T A B position 1 position 3 position 2 alc80237_ch10_431-477_sss.indd 470 lc80237_ch10_431-477_sss.indd 470 10/01/11 10:24 PM 0/01/11 10:24 PM 126 Measurement Systems McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution
McGraw-Hill CreateTM Review Copyfor lnstructor Nicolescu.Not fordistributionIntroduction to Mechatronics and Measurement Systems,Fourth Edition12710.8Hydraulics471to oppositeends of the cylinder,applyingor ventingpressure on opposite sides ofthe piston. In position 1, the cylinder does not move, because the pressure is ventedto the tank.In position 2thecylinder moves to theright, since pressureis applied tothe left side of the piston. In position 3, the cylinder moves to the left, because pres-sureisappliedtotherightsideofthepiston.Commontypesoffixedpositionvalvesarecheckvalves,poppetvalves,spoolvalves, and rotary valves. Figure 10.41 illustrates check and poppet valves. Thecheck valve allows flow in one direction only.The poppet valve is a check valvethat canbeforced open to allowreverseflow.As illustrated in Figure 10.42,a spool valve consists of a cylindrical spool withmultiplelobesmoving within a cylindrical casing containing multiple ports.The spoolcan be moved back and forth to align spaces between the spool lobes with input andoutputportsinthehousingtodirecthigh-pressureflowtodifferentconduits inthesys-tem.Thespool isbalanced ineachpositionbecausethe staticpressureisthesame onopposing internalfaces ofthe lobes.Therefore,noforce is required tohold a position.In theleftposition,portAispressurized, and portBisvented tothetank;and intherightposition,portBispressurized andportAisvented.Tomovethespool betweenpositions,an axial force is required,from an actuatorormanual control lever,to overcomethehydrodynamicforcesassociated with changingthemomentum oftheflow.In the design of a spool valve where large hydrodynamic forces occur, a pilotvalve is added, as shown in Figure 10.43. The pilot valve operates at a lower pres-sure, called pilot pressure, and at much lowerflow rates and thereforerequires lessforce to actuate.Thepilot valve directs pilot pressure to one side of the main spool,and theforce generated by the pressure acting over the main spool lobeface is largeenoughto actuatethemainvalve.Theeffect ofthepilotvalveis toamplifyforceprovided by the solenoid or mechanical lever acting on the pilot spool. In the figure,the pilot spool is in the full left position, causing pilot pressure to be applied to thepistonP4/3valveBFigure10.40Double-actinghydrauliccylinderup-down plunglight springballseano flowunlessplunger isAKVflowflovdown(b) poppet valve(a), check valveFigure 10.41 Check and poppet valves
Confirming Pages to opposite ends of the cylinder, applying or venting pressure on opposite sides of the piston. In position 1, the cylinder does not move, because the pressure is vented to the tank. In position 2, the cylinder moves to the right, since pressure is applied to the left side of the piston. In position 3, the cylinder moves to the left, because pressure is applied to the right side of the piston. Common types of fixed position valves are check valves, poppet valves, spool valves, and rotary valves. Figure 10.41 illustrates check and poppet valves. The check valve allows flow in one direction only. The poppet valve is a check valve that can be forced open to allow reverse flow. As illustrated in Figure 10.42 , a spool valve consists of a cylindrical spool with multiple lobes moving within a cylindrical casing containing multiple ports. The spool can be moved back and forth to align spaces between the spool lobes with input and output ports in the housing to direct high-pressure flow to different conduits in the system. The spool is balanced in each position because the static pressure is the same on opposing internal faces of the lobes. Therefore, no force is required to hold a position. In the left position, port A is pressurized, and port B is vented to the tank; and in the right position, port B is pressurized and port A is vented. To move the spool between positions, an axial force is required, from an actuator or manual control lever, to overcome the hydrodynamic forces associated with changing the momentum of the flow. In the design of a spool valve where large hydrodynamic forces occur, a pilot valve is added, as shown in Figure 10.43 . The pilot valve operates at a lower pressure, called pilot pressure, and at much lower flow rates and therefore requires less force to actuate. The pilot valve directs pilot pressure to one side of the main spool, and the force generated by the pressure acting over the main spool lobe face is large enough to actuate the main valve. The effect of the pilot valve is to amplify force provided by the solenoid or mechanical lever acting on the pilot spool. In the figure, the pilot spool is in the full left position, causing pilot pressure to be applied to the Figure 10.40 Double-acting hydraulic cylinder. P T A B piston 4/3 valve Figure 10.41 Check and poppet valves. (a) check valve (b) poppet valve flow no flow ball seat light spring up-down plunger no flow unless plunger is down flow 10.8 Hydraulics 471 alc80237_ch10_431-477_sss.indd 471 lc80237_ch10_431-477_sss.indd 471 10/01/11 10:24 PM 0/01/11 10:24 PM Introduction to Mechatronics and Measurement Systems, Fourth Edition 127 McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution
McGraw-Hill CreateTM ReviewCopyforInstructorNicolescu.Notfordistribution128Measurement Systems472CHAPTER10ActuatorsspoollobeforoTAPBTITATTPBITleft position (P-A, B-T)right position (A-T, P-B)(a) schematicSPOOLUGROOVESImproves contamination toleranceofspools over conventional V grooves.SEALED WET ARMATURE SOLENOIDSSignificantly reduces spool hang uPMaximum protection againstmoisture,resulting in loss of production. corrosion and dirt.OPERATORPROTECTIONHigh temperature elements arejisolated from direct contact.STANDARDMOUNTINGSConforms to NFPA andANSI/ISO standards.2WIRELEAD50/60 Hz coils standard forincreased application flexibility.INTERCHANGEABLE SPOOLSProvide easy field maintenance.LOW-PRESSURE DROPSNo matching of parts.Reduces heat load andincreases efficiency,(b) actual(CourtesyofContinental Hydraulics, Savage,MN)Figure10.42Spoolvalve.pilot valvesolenoid-operatedpilot spoolmain spool pilotNBAFigure10.43Pilot-operatedspoolvalveleft side of the main spool and venting fluidfrom theright side of the main spool to1tank,thusdrivingthemainspooltothefull rightposition.Thisappliesmainpressureto port B and vents port A.VideoDemo10.24 illustrates how a pilot valve can beVideo Demousedtocreateanamplifiedhydraulicforce.10.24PilotvalveThediscussionof spoolvalvessofarhasbeenlimitedtooperationbetweenhydraulic amplifiertwo positions only: on and off. Continuous operation can be achieved by using acut-awayproportionalvalve,onewhosespool movesadistanceproportional toamechanical
Confirming Pages Figure 10.42 Spool valve. (a) schematic T A P T A P left position (P-A, B-T) right position (A-T, P-B) force spool lobe casing B T T B SEALED WET ARMATURE SOLENOIDS Maximum protection against moisture, corrosion and dirt. OPERATOR PROTECTION High temperature elements are isolated from direct contact. 2 WIRE LEAD 50/60 Hz coils standard for increased application flexibility. INTERCHANGEABLE SPOOLS Provide easy field maintenance. No matching of parts. LOW-PRESSURE DROPS Reduces heat load and increases efficiency. SPOOL U GROOVES Improves contamination tolerance of spools over conventional V grooves. Significantly reduces spool hang up resulting in loss of production. STANDARD MOUNTINGS Conforms to NFPA and ANSI/ISO standards. (b) actual (Courtesy of Continental Hydraulics, Savage, MN) Figure 10.43 Pilot-operated spool valve. T T A P B solenoidoperated T T pilot spool pilot press. pilot valve main spool 472 C H A P T E R 10 Actuators left side of the main spool and venting fluid from the right side of the main spool to tank, thus driving the main spool to the full right position. This applies main pressure to port B and vents port A. Video Demo 10.24 illustrates how a pilot valve can be used to create an amplified hydraulic force. The discussion of spool valves so far has been limited to operation between two positions only: on and off. Continuous operation can be achieved by using a proportional valve, one whose spool moves a distance proportional to a mechanical Video Demo 10.24 Pilot valve hydraulic amplifier cut-away alc80237_ch10_431-477_sss.indd 472 lc80237_ch10_431-477_sss.indd 472 10/01/11 10:24 PM 0/01/11 10:24 PM 128 Measurement Systems McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution