Chapters 6 & 7 (Selected Topics) Actuators Workhorses of the System

Objectives Describe the construction and operation of basic hydraulic cylinders, limited-rotation actuators, and motors. Compare the design and operation of various types of hydraulic cylinders. Select appropriate cylinder design options available for mounting hydraulic cylinders and reducing hydraulic shock. Compare the design and operation of various types of hydraulic motors.

Objectives Contrast the operation of fixed- and variable-speed hydraulic motors. Describe the construction and operation of a basic hydrostatic transmission. Size hydraulic cylinders and motors to correctly meet system force and speed requirements. Interpret manufacturer specifications for hydraulic cylinders.

Hydraulic Cylinders Actuators are the components used in a hydraulic system to provide power to a required work location Cylinders are the hydraulic system components that convert fluid pressure and flow into linear mechanical force and movement

Hydraulic Cylinders A basic cylinder consists of: Piston Piston rod Barrel

Hydraulic Cylinders Parts of a typical cylinder

Hydraulic Cylinders The piston forms sealed, variable-volume chambers in the cylinder System fluid forced into the chambers drives the piston and rod assembly Linear movement is produced

Hydraulic Cylinders Seals prevent leakage between: Piston and cylinder barrel Piston rod and head Barrel and its endpieces Wiper seal, or scraper, prevents dirt and water from entering the cylinder during rod retraction

Hydraulic Cylinders Various seals are used in a cylinder

Hydraulic Cylinders Rod wipers prevent contamination from entering on rod retraction IMI Norgren, Inc.

Hydraulic Cylinders Cylinders are typically classified by operating principle or by construction type Single-acting or double-acting Tie rod, mill, threaded end, or one piece

Hydraulic Cylinders Single- and double-acting cylinders Single-acting

Hydraulic Cylinders Single-acting cylinders exert force either on extension or retraction They require an outside force to complete the second motion

Hydraulic Cylinders Double-acting cylinders generate force during both extension and retraction Directional control valve alternately directs fluid to opposite sides of the piston Force output varies between extension and retraction

Hydraulic Cylinders Effective piston area is reduced on retraction due to the rod cross section

Hydraulic Cylinders Volume is reduced on retraction

Hydraulic Cylinders One-piece cylinder has the cylinder barrel welded to the ends Produces a compact actuator Cost effective to manufacture Cannot be serviced (throwaway)

Hydraulic Cylinders Hydraulic ram is commonly used in hand-operated jacks Rod is basically the same diameter as the inside of the cylinder barrel Large-diameter rod is more rigid under load, but cylinder can generate force in only one direction

Hydraulic Cylinders The force generated by a cylinder is calculated by multiplying the effective area of the piston by the system pressure

Hydraulic Cylinders Effective cylinder piston area

Hydraulic Cylinders Force generated during the extension of a double-acting cylinder with a single-ended rod is calculated as: extension force = system pressure  piston area Force generated during the retraction of a double-acting cylinder with a single-ended rod is calculated as: retraction force = system pressure  (piston area – rod area) (Calculations require consistent units of measure in these formulas)

Hydraulic Cylinders Speed at which the cylinder extends or retracts is determined by: Physical volume per inch of cylinder piston travel Amount of fluid entering the cylinder Effective area of the piston is used to calculate the volume of the cylinder per inch of piston travel

Hydraulic Cylinders Typical manufacturer’s catalog page Bailey International Corporation

Hydraulic Motors Hydraulic motors are called rotary actuators They convert fluid pressure and flow into torque and rotational movement

Hydraulic Motors Typical hydraulic motor application

Hydraulic Motors All basic hydraulic motors consist of three component groups: Housing Rotating internal parts Power output shaft

Hydraulic Motors Parts of a typical hydraulic motor

Hydraulic Motors System fluid enters the housing and applies pressure to the rotating internal parts This, in turn, moves the power output shaft and applies torque to rotate a load

Hydraulic Motors Primary parts that produce the rotating motion in most hydraulic motors are either: Gears Vanes Pistons

Hydraulic Motors Four requirements of a motor

Hydraulic Motors Displacement of a hydraulic motor indicates the volume of fluid needed to turn the output shaft one revolution Fixed displacement Variable displacement

Hydraulic Motors In a fixed-displacement motor: Internal geometry cannot be changed Same volume needed per output shaft revolution

Hydraulic Motors In a variable-displacement motor: Internal geometry can be changed Displacement per shaft revolution can be adjusted Motor can operate at variable speeds with a constant input flow

Hydraulic Motors Hydraulic motors may be classified by the type of load applied to the bearings of the output shaft Unbalanced indicates the output shaft is loaded from one side, side loading the shaft bearings Balanced indicates the bearing load is balanced by use of two inlet ports arranged opposite of each other and two outlet ports similarly arranged

Hydraulic Motors The external gear hydraulic motor is the most common and simplest of the basic motor types Fixed displacement Unbalanced load on the bearings

Hydraulic Motors The most common internal gear motor has a gerotor design Courtesy of Eaton Fluid Power Training

Hydraulic Motors The specially shaped gear teeth of the gerotor form variable-volume chambers that allow system fluid flow and pressure to turn the motor output shaft Gerotor motors are fixed-displacement units operating with an unbalanced bearing load

Hydraulic Motors An orbiting gerotor motor is a variation of the basic gerotor design Uses a fixed outer gerotor gear with internal teeth and an inner gear with external teeth Center point of the inner gear orbits around the center point of the fixed gear with internal teeth Motor operates at a slower speed, but has a higher torque output

Hydraulic Motors Orbiting gerotor motor Courtesy of Eaton Fluid Power Training

Hydraulic Motors Basic vane motor has a slotted rotor located off center in a circular chamber and fitted with movable vanes Space between the vanes creates a number of variable-sized chambers Forcing fluid into the small-size chambers causes the volume of the chambers to increase, turning the motor shaft Basic vane motor is fixed displacement with an unbalanced bearing load

Hydraulic Motors Basic vane motor

Hydraulic Motors Balanced vane motors evenly distribute the load on the bearings Achieved by operating the rotor and vanes in a slightly oblong chamber Allows two inlet ports and two outlets ports to be used in the motor Placing ports opposite each other balances bearing loading

Hydraulic Motors A basic, balanced vane motor

Hydraulic Motors Vane motors are available as either fixed or variable displacement The variable-displacement feature allows an operator to change the speed of a motor without changing the system flow rate

Hydraulic Motors In variable-displacement designs, the chamber in which the rotor and vanes operate is contained in a moveable ring When the center point of the rotor and ring are concentric, the displacement is zero Moving the ring so the center points are not concentric increases the motor displacement and changes motor speed

Hydraulic Motors Piston motors are available having either fixed or variable displacements In variable-displacement designs, the length of the piston stroke is changed to vary the volume of fluid needed to rotate the motor one revolution

Hydraulic Motors Two basic classifications of piston motors are axial piston and radial piston An axial piston motor has pistons with centerlines parallel to the axis of the output shaft A radial piston motor has pistons with centerlines perpendicular to the axis of the output shaft

Hydraulic Motors Axial piston motor The Oilgear Company

Hydraulic Motors Axial piston motors are available in two configurations: Inline Bent axis

Hydraulic Motors In an inline piston motor: Centerline of the barrel is concentric with the centerline of the power output shaft A swash plate transmits force from the pistons to the shaft

Hydraulic Motors Inline piston motor The Oilgear Company

Hydraulic Motors In a bent-axis piston motor: Centerline of the barrel is at an angle to the centerline of the output shaft A universal joint and other fittings are used to transmit force between the barrel and the output shaft

Hydraulic Motors Bent-axis piston motor Courtesy of Eaton Fluid Power Training

Limited-Rotation Hydraulic Actuators Limited-rotation devices are actuators with an output shaft that typically applies torque through approximately 360° of rotation Models are available that are limited to less than one revolution, while others may produce several revolutions

Limited-Rotation Hydraulic Actuators Most common designs of limited-rotation actuators are: Rack-and-pinion Vane Helical piston and rod

Limited-Rotation Hydraulic Actuators Rack-and-pinion limited rotation actuator IMI Norgren, Inc.

Limited-Rotation Hydraulic Actuators Vane limited-rotation actuator

Limited-Rotation Hydraulic Actuators Helical piston and rod limited-rotation actuator

Limited-Rotation Hydraulic Actuators Limited-rotation actuators are used to perform a number of functions in a variety of industrial situations Indexing devices on machine tools Clamping of workpieces Operation of large valves

Limited-Rotation Hydraulic Actuators Limited-rotation actuators are used in this robotic arm IMI Norgren, Inc.

MDMA Equipment—Menomonie Hydrostatic Drives Hydrostatic drive systems consist of the basic components typically found in other hydraulic motor circuits MDMA Equipment—Menomonie

Hydrostatic Drives Hydrostatic drives provide effective transmission of power and allow easy adjustment and control of: Output shaft speed Torque Horsepower Direction of rotation

Hydrostatic Drives When compared to conventional transmissions, hydrostatic drives: Have a high power output–to–size ratio May be stalled under full load with no internal damage Accurately maintain speed under varying load conditions Provide an almost infinite number of input/output speed ratios

Hydrostatic Drives Hydrostatic drives may be open or closed circuits Open circuit has the layout of a basic hydraulic motor circuit Closed circuit has the outlet of the pump directly connected to the inlet of the motor and the outlet of the motor directly connected to the inlet of the pump

Hydrostatic Drives Open circuit design

Hydrostatic Drives Closed circuit design Sauer-Danfoss, Ames, IA

Formulae US customary system

Formulae Metric system

Additional slides for lecture

Hydraulic Cylinders Tie-rod cylinder

Hydraulic Cylinders External tie rod bolts are used to secure the ends on the tie-rod cylinder design Commonly found on heavy industrial machines External tie rods increase chance of damage and promote accumulation of dirt

Hydraulic Cylinders Mill cylinders Yates Industries, Inc.

Hydraulic Cylinders Threaded-end cylinder Bailey International Corporation

Hydraulic Cylinders Typical hand-operated jack

Hydraulic Cylinders Telescoping cylinders are available for applications requiring long extension distances Rod is made up of several tubes of varying size nested inside of the barrel Each tube extends, producing a rod longer than the cylinder barrel Typical example is the actuator that raises the box on a dump truck

Hydraulic Cylinders Telescoping cylinders Star Hydraulics, Inc.

Hydraulic Cylinders Cylinders often use hydraulic cushions Provide a controlled approach to the end of the stroke Reduces the shock of the impact as the piston contacts the cylinder head

Hydraulic Cylinders Cylinder cushioning device

Hydraulic Cylinders A variety of mounting configurations are used to attach the cylinder body and rod end to machinery Fixed centerline Fixed noncenterline Pivoting centerline Expected cylinder loading is the major factor in the selection of the mounting style

Hydraulic Cylinders Head-end flange mount

Hydraulic Cylinders Fixed-noncenterline mount

Hydraulic Cylinders Pivoting-centerline, clevis mount

Hydraulic Cylinders Pivoting-centerline, trunnion mount

Hydraulic Cylinders Hydraulic cylinder manufacturers provide detailed specifications concerning: Construction Physical size Load capacity

Hydraulic Cylinders This information includes basic factors such as: Bore Stroke Pressure rating Other details, such as service rating, rod end configurations, and dimensions

Hydraulic Motors A number of alternate motor designs are used in specialized hydraulic applications Screw motor designs for quiet, continuous operation Special piston-motor designs allowing the direct mounting and drive of wheels for off-road, heavy-transport vehicles

Hydraulic Motors Hydraulic motors may be incorporated into circuits using series or parallel connections Series circuits: total system pressure is determined by adding the loads placed on each unit Parallel circuits: each motor receives full system pressure; loads must be matched or equal flow supplied to each motor if constant speed is desired from each unit

Hydraulic Motors Motors in series

Hydraulic Motors Motors in parallel

Hydraulic Motors Motors in parallel with flow control

Hydraulic Motors Braking circuits are used to slow hydraulic motors to a stop Inertia of a heavy rotating load can continue to turn the motor shaft Braking occurs when fluid discharged from the motor outlet port is forced to pass through an adjustable pressure control valve before returning to the reservoir

Hydraulic Motors Braking circuit

Hydraulic Motors An open-loop hydraulic motor system uses a layout typical of a basic hydraulic system Pump moves fluid from a reservoir, through a directional control valve, to the motor Fluid is then returned from the motor to the reservoir through the same control valve

Hydraulic Motors Closed-loop hydraulic motor systems continuously circulate fluid between the pump and the motor without returning it to a system reservoir These systems use a replenishment circuit to replace fluid lost through leakage

Hydraulic Motors Replenishment circuit

Hydrostatic Drives Four combinations of pump/motor arrangements can be used Fixed-displacement pump and motor Fixed-displacement pump and variable-displacement motor Variable-displacement pump and fixed-displacement motor Variable-displacement pump and motor

Hydrostatic Drives Fixed-displacement pump and motor: Maximum horsepower, torque, and output shaft speed are fixed Pump and motor have fixed displacement, so these characteristics cannot be changed

Hydrostatic Drives Fixed-displacement pump and variable-displacement motor: Maximum horsepower is fixed Torque and speed are variable Due to use of a relief valve, efficiency is lowered Output shaft rotation may be reversed if the pump is reversible

Hydrostatic Drives Variable-displacement pump and fixed-displacement motor: Torque output is fixed Horsepower and output shaft speed are variable Output shaft rotation may be reversed if pump is reversible

Hydrostatic Drives Variable-displacement pump and motor: Horsepower, torque, output shaft speed are variable Output shaft direction is reversible Most versatile of the four pump/motor combinations

Hydrostatic Drives Hydrostatic drives are typically considered hydrostatic transmissions when both the pump and motor have variable displacement This combination allows manual or automatic control of torque, speed, and power output

Hydrostatic Drives Two different general techniques are used in the construction of hydrostatic transmissions Integral Nonintegral

Hydrostatic Drives Integral construction combines all of the transmission parts into a single housing Nonintegral construction involves separate pump, motor, and accessories connected by hoses or tube assemblies

Review Question A(n) _____ cylinder can exert force during both the extension and retraction strokes. double-acting

Review Question A(n) _____ is the system component that converts fluid pressure and flow into linear force and movement. hydraulic cylinder

Review Question List the three basic configurations used to mount cylinders to equipment. A. Fixed centerline, B. fixed non-centerline, and C. pivoting centerline.

Review Question The three conceptual component groups that make up any hydraulic motor are: A. Rotor, vanes, and eccentric. B. Housing, rotating internal parts, and power output shaft. C. Housing, reciprocating internal parts, and power input shaft. D. Rotating internal parts, power input shaft, and power output shaft. B. Housing, rotating internal parts, and power output shaft.

Review Question To vary the displacement of a vane motor, a movable _____ is used to change the size of the pumping chambers. cam ring

Review Question List the four possible pump/motor arrangements that may be used with a hydrostatic system. A. Both pump and motor have fixed displacements, B. pump has a fixed displacement and the motor a variable displacement, C. pump has a variable displacement and the motor a fixed displacement, and D. both pump and motor have variable displacement.

Review Question During retraction, what is the effective area of the piston of a double-acting cylinder? The cross-sectional area of the piston minus the cross-sectional area of the rod.

Review Question A cylinder that has externally mounted metal rods holding the ends on the barrel is called a(n) _____ cylinder. tie-rod

Controlling the System . Chapter 8 (Selected Topics) Controlling the System Pressure, Direction, and Flow

Objectives Explain the function of each of the three general types of control valves used in hydraulic systems. Compare the design and operation of direct-acting and pilot-operated pressure control valves. Describe the function of the various types of pressure control valves used in hydraulic systems.

Objectives Compare the design and operation of two-way, three-way, and four-way directional control valves. Describe the characteristics of the various spool configurations used in three-position directional control valves. List and compare the methods used to position control spools in valves.

Objectives Compare the design and operation of non-compensated and compensated flow control valves. Explain the effect fluid temperature and pressure variations have on the operation of flow control valves.

Primary Control Functions in a Hydraulic System Control valves allow hydraulic systems to produce the type of motion or level of force needed to complete the functions expected of a hydraulic circuit

Primary Control Functions in a Hydraulic System A variety of valves can control actuator direction, speed, and force output Used with permission of CNH America LLC

Primary Control Functions in a Hydraulic System The three basic types of control valves are: Pressure control Directional control Flow control

Primary Control Functions in a Hydraulic System Pressure control valves can: Protect the system from damage due to excessive pressure Sequence motion Limit pressure in selected sections of a circuit

Primary Control Functions in a Hydraulic System A system pressure control valve

Primary Control Functions in a Hydraulic System Directional control valves direct fluid flow to establish and control actuator movement Used with permission of CNH America LLC

Primary Control Functions in a Hydraulic System Flow control valves control the operating speed of actuators They provide a means to vary the rate of fluid flow

Primary Control Functions in a Hydraulic System A typical flow control valve

Basic Structure and Features of Control Valves A fixed orifice is a precision hole either: Machined into the valve body or a component Pressed as a separate part into a valve passageway Both designs are used to control fluid flow

Basic Structure and Features of Control Valves Fixed orifices

Basic Structure and Features of Control Valves A spool is a cylindrical metal piece fitted into the bore of a valve body The spool is used to block or direct fluid through a valve to produce a desired fluid flow characteristic

Basic Structure and Features of Control Valves A typical spool Used with permission of CNH America LLC

Basic Structure and Features of Control Valves Internal and external forces are used to position the various valve elements Springs and pilot pressure are typical internal forces used to operate valve elements Manual, pilot pressure, and electromagnetic force are common external forces used for operation

Basic Structure and Features of Control Valves Precision fit, rather than separate seals, is used to prevent excessive internal leakage in most hydraulic control valves Internal leakage must be drained from valve chambers Fluid buildup causes backpressure Backpressure prevents the proper operation of internal valve elements

Basic Structure and Features of Control Valves Draining internal leakage

Basic Structure and Features of Control Valves Internal and external drains are used to remove internal leakage Internal drains may be used when the outlet line is not subjected to system pressure External drains are connected to low-pressure return lines leading to the reservoir

Basic Structure and Features of Control Valves Normal valve position refers to the position the internal elements assume when a hydraulic system is shut down Normally open Normally closed

Basic Structure and Features of Control Valves Symbols for normally open and normally closed valves

Basic Structure and Features of Control Valves Directional control valves may be referred to by the number of distinct flow positions provided by the valve Two position Three position

Basic Structure and Features of Control Valves Symbols for directional control valves

Valve Operation and Springs, Fluid Pressure, and Fluid Flow Springs, fluid pressure, and fluid flow are very important in the operation of hydraulic system control valves Springs are used in control valves to: Move spools and other internal elements Establish the maximum operating pressure Serve as a biasing force

Valve Operation and Springs, Fluid Pressure, and Fluid Flow Common uses for springs

Valve Operation and Springs, Fluid Pressure, and Fluid Flow Fluid pressure is used in control valves to: Directly open or close valves Remotely operate a valve element Operate a compensating device to obtain desired fluid flow

Valve Operation and Springs, Fluid Pressure, and Fluid Flow Fluid flow through an orifice is used in control valves to establish differences in pressure These pressure differences combined with balancing pistons and biasing springs are commonly used in the operation of pressure and flow control valves

Pressure Control Devices Pressure control valves may be grouped into one of five types System maximum pressure control Actuator sequence control Restrained movement control Pump unloading control Reduced pressure control

Pressure Control Devices Control valves can be classified by internal modes of operation Direct operation Balanced-piston operation

Pressure Control Devices Direct-operated valves depend on heavy internal springs to establish valve operating pressure Balancing-piston valves (compound relief valves) use lighter springs and system pressure acting on internal valve mechanisms to establish the desired operation

Pressure Control Devices Typical direct-operated relief valve

Pressure Control Devices Typical compound relief valve

Directional Control Devices Directional control devices allow a system operator to control the direction of fluid flow in the system Starting and stopping of actuators Control of actuator movement direction

Directional Control Devices Directional control devices can be grouped in four general classifications Shut-off or two-way valves Check valves Three-way valves Four-way valves

Directional Control Devices Symbols for directional control valves

Directional Control Devices The primary purpose of shut-off valves is to block fluid flow through a hydraulic system line Globe valve Gate valve Ball valve Spool valve Needle valve

Directional Control Devices Typical globe valve

Directional Control Devices Typical gate valve

Directional Control Devices Typical ball valve

Directional Control Devices Typical spool valve

Directional Control Devices Typical needle valve

Directional Control Devices The primary purpose of check valves is to allow free flow in one direction while preventing reverse flow Other functions include: Bypassing components during the return cycle of the system Providing flow resistance to maintain a minimum system pressure required for pilot operations

Directional Control Devices Typical inline check valve

Directional Control Devices Typical right-angle check valve

Directional Control Devices Three-way directional control valves provide a means to extend rams and single-acting cylinders The actuator is returned to its original position by an external force System load Spring built into the actuator

Directional Control Devices Typical three-way directional control valve

Directional Control Devices During extension, the three-way valve connects the actuator inlet line to a system supply line, allowing fluid to enter and extend the unit During retraction, the valve blocks the supply line and connects the actuator line to a system return line, allowing external force to return the actuator to its original position while directing displaced fluid to the reservoir

Directional Control Devices Four-way directional control valves provide a means to power actuators in either direction Valve has four external ports for connection to system supply line, reservoir, and inlet and outlet of the actuator Internal structure of the valve allows the ports to be alternately connected when a change in actuator direction is necessary

Directional Control Devices Four-way valve powers double-acting cylinder during extension and retraction

Directional Control Devices Four-way directional control valves are typically manufactured as two- or three-position valves This provides several operating options when designing circuits

Directional Control Devices Typical two-position, four-way valve

Directional Control Devices In two-position valves, the first position operates the actuator in one direction, while the second position reverses the direction In three-position valves, a center position is added that provides additional circuit operating characteristics

Directional Control Devices Typical three-position, four-way valve

Directional Control Devices A number of center position configurations are available Closed Open Tandem Floating Regenerative

Directional Control Devices Symbols for four-way valve center position

Directional Control Devices

Directional Control Devices

Flow Control Devices Flow control devices produce the desired rate of actuator operating speed by controlling the volume of fluid allowed to reach the actuator Flow control devices can be divided into two general types: Restrictor Bypass

Flow Control Devices Restrictor-type flow control valves limit the volume of fluid through the valve Excess pump output is forced to return to the reservoir through the system relief valve

Flow Control Devices Circuit containing a restrictor-type flow control valve

Flow Control Devices Bypass type flow control valves use an integral control port to return excess pump output to the reservoir The returned fluid is at a pressure less than system relief valve pressure

Flow Control Devices Circuit containing a bypass-type (bleed off) flow control valve

Additional slides for lecture

Directional Control Devices A standard check valve consists of a valve body containing a one-way valve located between inlet and outlet ports The one-way valve allows fluid flow through the valve in only one direction Some designs contain a spring that seats the valve poppet or ball In other designs, the poppet is seated only by fluid flow

Directional Control Devices Restriction check valves allow free flow in one direction and restricted flow when flow direction is reversed This is accomplished via a metering orifice machined into the poppet

Directional Control Devices Typical restriction check valve

Directional Control Devices Pilot-operated check valves can allow reverse flow through the valve Typically, pilot pressure opens the valve In some designs, pilot pressure may also hold the valve shut to block flow in both directions

Directional Control Devices Pilot pressure to open check valve

Directional Control Devices Pilot pressure to block flow through valve

Directional Control Devices For a three-position, four-way valve, there are number of center position configurations available Closed Open Tandem Floating Regenerative

Directional Control Devices The center position affects directional control characteristics and overall system efficiency Each style provides distinct operating characteristics that allow hydraulic system designers to obtain maximum performance from a system

Directional Control Devices A number of activation methods are used to shift the internal components of directional control valves Five general categories: Flow actuation Manual operation Mechanical operation Pilot operation Electrical operation

Directional Control Devices Flow actuation uses internal fluid movement to actuate the valve No external mechanism or force is used

Directional Control Devices Manual operation methods include: Handwheels Levers Push buttons Foot pedals These devices require constant operator presence and are typically found in less-complex systems

Directional Control Devices Mechanical operation methods include: Rollers Cams Levers Rams Mechanical operation is often used when the opening and closing of the valve must occur at a specific position in actuator travel

Directional Control Devices Circuit containing a mechanically actuated directional control valve

Directional Control Devices Pilot operation uses system pressure to activate the valve, rather than physical labor This method is effective when: Larger forces are need to shift the valve Remote operation is required because of safety or tight physical factors

Directional Control Devices Pilot-operated directional control valve Courtesy of Eaton Fluid Power Training

Directional Control Devices Electrical control of hydraulic systems is common in many types of equipment Simple solenoid devices to shift basic valves Electronic controllers operate proportional solenoid valves to produce extreme accuracy and repeatability

Directional Control Devices Typical electrically controlled valve The Oilgear Company

Directional Control Devices Multiple-position directional control valve may be held in a desired position using springs or detents Springs are located on the ends of the valve spool to return the valve to its normal operating position

Directional Control Devices Symbols for spring-return valves

Directional Control Devices Detents are locking devices that hold the spool in a selected position The spool may be held until the operator manually shifts the valve Increased system pressure at the end of an operation may automatically shift detent valves back to the normal position

Directional Control Devices Typical detent operation

Flow Control Devices Conceptual operation of a flow control valve may be traced to a basic orifice

Flow Control Devices The flow rate through a simple, sharp-edged orifice depends on: Area of the orifice Pressure difference between the inlet and outlet sides of the orifice Viscosity of the fluid, which varies with fluid temperature

Flow Control Devices Discharge coefficients are typically used in fluid mechanics formulas to simplify mathematical calculations These coefficients are available in most technical references covering fluid mechanics

Flow Control Devices Formula using a discharge coefficient to calculate flow through an orifice: Qa = Ao × Cd × 2 × g × H Where: Qa = actual quantity of flow Ao = cross-sectional area of orifice Cd = coefficient of discharge g = gravity H = head

Flow Control Devices Flow control valves may be noncompensated or compensated The flow rate through noncompensated valves varies as the load or fluid viscosity changes Compensated valves automatically adjust for fluid pressure variations to produce a consistent flow rate under varying load and temperature conditions

Flow Control Devices Noncompensated and compensated flow control valves may have: Fixed flow rate Adjustable flow rate

Flow Control Devices The simplest restrictor-type flow control valve is a simple orifice Basically a calibrated hole Serves as a noncompensated, fixed-rate flow control device

Flow Control Devices A needle valve is the simplest restrictor-type, noncompensated adjustable flow control device Consists of an orifice fitted with a tapered needle machined on a threaded stem Turning the threaded stem changes the effective area of the orifice, which adjusts the flow rate through the valve

Flow Control Devices Basic adjustable flow control valve

Flow Control Devices When using a restrictor-type, noncompensated flow control valve, actuator speed varies when system loads change Caused by the change in pressure drop across the control valve, which varies the flow rate through the valve

Flow Control Devices A pressure compensator maintains a constant pressure difference across the metering orifice of a flow control valve Senses pressure on the inlet and outlet sides of the orifice These pressures generate forces that act on the end surfaces of a sliding spool that is preloaded by a biasing spring

Flow Control Devices Force generated by the biasing spring establishes the constant pressure difference across the orifice This constant pressure difference maintains constant fluid flow through the valve even when system loads change

Flow Control Devices A basic pressure-compensated flow control valve

Flow Control Devices Pressure compensator operation

Flow Control Devices Pressure compensator operation

Flow Control Devices Temperature compensation is necessary in flow control devices if an accurate, consistent flow rate through a valve is needed This is due to the fluid viscosity changes that occur as fluid temperature changes

Flow Control Devices Temperature compensation is typically accomplished in flow control devices by: Specially designed, sharp edged orifice Heat-sensitive metal rod that operates a needlelike control device in the metering orifice of the valve

Flow Control Devices Temperature compensation using a heat-sensitive metal rod

Flow Control Devices In a circuit using a restrictor-type, pressure-compensated flow control valve: Pressure drop across the internal flow-control device in the valve remains constant, which produces a constant flow rate through the valve Actuator speed will not vary when system loads change

Flow Control Devices In a circuit using a restrictor-type, temperature-compensated flow control valve: Valve internal flow-control device is adjusted for viscosity variations that occur during fluid temperature changes Flow remains constant as system operating temperatures change

Flow Control Devices Circuit containing restrictor-type, compensated flow control valve

Review Question Control valves allow a hydraulic system to produce the type of _____ and level of _____ needed for machine operation. motion; force

Review Question Name the two types of pressure control valves that are used to prevent damage to hydraulic systems in cases where the system relief valve fails, causing excessive system pressure. A. Safety valve and B. hydraulic fuse.

Review Question What is the primary purpose of pressure control valves in a hydraulic circuit? To protect a machine and the hydraulic system components from damage caused by excessive pressure.

Review Question A four-way directional control valve has four ports. Describe what each port is connected to in the system. A. Pump discharge, B. cylinder or motor inlet, C. cylinder or motor outlet, and D. reservoir inlet.

Review Question Which type of a three-position, four-way directional control valve has a middle position with the pump, reservoir, and actuator ports connected? Open-center.

Review Question The _____ method of directional control activation uses system pressure, which may be remotely sensed, to shift the valve. piloting

Review Question What are the two general operating designs of flow control valves? A. Restrictor and B. bypass.

Review Question In an operating fluid power system, the _____ of the fluid will change as the operating temperature of the system changes. viscosity