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

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