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8/17/2019 Fluid Power - (ME353)- Lec10
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Fluid Power Systems (ME353)
Fall 2012
Lecture 10
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Directional Control
Devices (Cont.)
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Three-way directional control valves provide a means to extendrams and single-acting cylinders
The actuator is returned to its original position by an external force
– System load
– Spring built into the actuator
Typical three-way directional control valve
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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 toits original position while directing displaced fluid to the reservoir
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Four-way directional control valves provide a means to poweractuators 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
Four-way valve powers double-acting cylinder during extension and
retraction
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Four-way directional control valves are typically manufactured
as two- or three-position valves
This provides several operating options when designing circuits
In two-position valves, the first position operates the actuator in
one direction, while the second position reverses the direction
Typical two-position, four-way valve
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In three-position valves, a center position is added that provides
additional circuit operating characteristics
Typical three-position, four-way valve
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A number of center position configurations are available
– Closed
– Open
– Tandem
– Floating
– Regenerative
Symbols for four-way valve center position
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
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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
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Flow actuation uses internal fluid movement to actuate the valve - Noexternal mechanism or force is used
Manual operation methods include:
– Handwheels
– Levers
– Push buttons
– Foot pedals
These devices require constant operator presence and are typically found inless-complex systems
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
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Circuit containing a mechanically actuated directional control
valve
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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
Pilot-operated directional control valve
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Electrical control of hydraulic systems is common in many types ofequipment
– Simple solenoid devices to shift basic valves
– Electronic controllers operate proportional solenoid valves to produceextreme accuracy and repeatability
Typical electrically controlled valve
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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
Symbols for spring-return valves
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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 automaticallyshift detent valves back to the normal position
Typical detent operation
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Flow Control Devices
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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
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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
Circuit containing a restrictor-type flow control valve
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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
Circuit containing a bypass-type flow control valve
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Conceptual operation of a flow control valve may be traced to a
basic orifice
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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
Discharge coefficients are typically used in fluid mechanics formulas to
simplify mathematical calculations
These coefficients are available in most technical references covering fluid
mechanics
Formula using a discharge coefficient to calculate flow through an orifice:
Qa = Cd × Ao × 2 × g × HWhere:
Qa = actual quantity of flow
Ao = cross-sectional area of orifice
Cd = coefficient of discharge
g = gravity
H = head
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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
Noncompensated and compensated flow control valves mayhave:
– Fixed flow rate
– Adjustable flow rate
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The simplest restrictor-type flow control valve is a simple orifice
– Basically a calibrated hole
– Serves as a noncompensated, fixed-rate flow control device
A needle valve is the simplestrestrictor-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 theorifice, which adjusts the flow
rate through the valve
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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
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
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
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A basic pressure-compensated flow control valve
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Pressure compensator operation
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Temperature compensation is necessary in flow control devices if anaccurate, consistent flow rate through a valve is needed
This is due to the fluid viscosity changes that occur as fluid temperature
changes
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
Temperature compensation using a heat-
sensitive metal rod
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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
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
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Bypass-type flow control
valves:
–
Provide accurate flow to actuators – Direct any excess flow from the
pump directly to the reservoir
through an integral port
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Operation of a bypass flow control valve
during increasing or decreasing load
The operating pressure of a system using a bypass-type flow control valve is
determined by the load on the actuator plus the pressure needed to overcome
the force of the biasing spring
The relief valve functions only when actuator loads are great enough toincrease system pressure above the cracking pressure of the relief valve
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Operation of a bypass flow control valve during steady load
Operation of a bypass flow control valve with stalled actuator
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The bypass flow control design provides an efficient operating
flow control circuit
– Pressure in the system is only as high as needed to move the
load and operate the valve compensator
– This reduces system heat generation and energy
consumption
– Care must be taken to accurately determine actuator loads
and the cracking pressure of the system relief valve
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Priority and proportional divider valves are designed to divideone fluid supply between two circuit subsystems
Many of these valves can also be used to combine the flow from twodifferent circuits
Priority divider valves provide flow to one port before providing flowto a second port
Often used in mobile equipment where pump output is controlled by engine
speed
Typical priority valve
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Circuit containing a priority divider valve
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Proportional divider valve splits input port flow into two
proportional output flows
Ratio between the output flows may be fixed or variable
Ratio of 50-50 is most common