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Fluid Power Systems (ME353)
Fall 2012
Lecture 13
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Applying Hydraulic Power
Typical Circuits and Systems
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Hydraulic systems involve a group of components that closely worktogether to produce a desired performance
– Pressure control
– Flow control
– Motion control
– Miscellaneous functions
A hydraulic system typically includes several of these segments
Basic pressure-control circuits limit the maximum operating pressure of a
hydraulic system
Variations of this function include circuits that can:
– Control pressure from a remote location
– Provide for the selection of multiple pressures in a single circuit
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A basic pressure-control circuit
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Remote pressure control in a circuit
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Multiple and Reduced pressure in a circuit
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With open- and tandem-center
directional control valves, a
spring-loaded check valve may be placed in the return line to create
low back pressure
– Maintains low system
pressure necessary to operate
pilot-controlled functions – This variation still produces
an energy-efficient circuit that
is a feature of these valve
center designs
Back-pressure check valve
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Three basic flow control circuits are used in hydraulicsystems:
– Meter in
– Meter out
– Bleed off
These basic circuits meet the varying flow-control demands
for systems with positive and negative loads
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The meter-in flow control design places the flow control valve between the
pump and the inlet of the actuator
– Should only be used for positive loads
– Cannot provide accurate control under a negative load condition
– The prime mover is always operating against the maximum pressure
setting of the system relief valve
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The meter-out flow control design places the flow control valve between
the actuator outlet and the reservoir
– Can provide accurate control for positive and negative loads
– The prime mover is always operating against the maximum pressuresetting of the system relief valve
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The bleed-off flow control design places the flow control valve in a tee in
the working line between the directional control valve and the actuator inlet
– Outlet of the flow control is directly connected to the reservoir
– Measured flow is diverted to the reservoir while remaining flow operatesthe actuator
– The prime mover operates against a pressure only high enough to move
the load
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Meter-in and meter-out circuits provide the most accurate actuator
speeds
Both meter fluid flow delivered directly to or from the actuator
The meter-out circuit is the best method for negative loads that may pull the actuator
The bleed-off flow control circuit is less accurate than either the meter-in or
meter-out system
– Flow is metered back to the reservoir while the remaining pump outputestablishes actuator speed
– The remaining flow can vary because of pump efficiency and system
leakage
– Flow control valve metering accuracy under varying load conditions is
also a factor
The bleed-off circuit is the most energy-efficient design
This is due to the fact the prime mover operates at a pressure only high
enough to move the load
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Several other circuit designs can also be used
to control flow to a circuit actuator
– High-low pump circuit
– Regenerative-cylinder-advance circuit
– Rapid-advance-to-work circuit
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The high-low pump circuit allows a system to use two pumps in tandem
– Provides high-volume flow during low-pressure demand
– Provides low-volume flow during high-pressure demand
– Lowers equipment costs by reducing prime mover size and cuts ongoingsystem energy consumption
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A regenerative-cylinder-advance circuit provides a rapid advance to
the point where a load is encountered
– Fluid is recirculated from the rod end into the blind end and
coupled with pump output to increase flow into the blind end, thusincreasing cylinder extension speed
– Recirculation is possible due to the difference in area between the
cylinder piston ends
During the rapid-extension phase of a regenerative circuit, the effective
force is limited to the maximum pressure setting of the system multiplied by the cross-sectional area of the cylinder rod
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A mechanically activated deceleration valve is often used in rapid-advance-
to-work circuits
This controls the rate at which the advancing cylinder is slowed from the
rapid-advance speed to the slower feed rate
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Sequence circuits operate actuators in a specific order
Two common methods for actuator sequencing are:
– Spring-loaded check valves
– Specially designed sequence valves
Check valves are inexpensive
Valves designed for specific sequence control provide more accurate movement
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Specially constructed pressure control valves can be used to sequentially
block fluid flow to actuators
– When a preselected pressure is reached, these valves open to allow fluid
flow to the actuator – This provides the order of operation
– Check valves allow reverse flow around these pressure control valves
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Synchronizing the movement of cylinders requires accurately metering
fluid flow into the actuators
– Accurate flow control valves
–
Other metering devices, such as mechanically connected hydraulicmotors
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Synchronization with mechanically coupled
motors
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Two factors that are basic to motor
operation are:
– Fluid flow rate establishes motor
speed – Pressure establishes torque output
Other factors must also be considered:
– Motor freewheeling
– Motor braking
Motor control circuits with a three-
position directional control valve can
allow a hydraulic motor to:
– Rotate with full speed and torque
capability
– Freewheel using the inertia of the
load to sustain rotation
– Brake to a stop under controlled
conditions