Fluid Power - (ME353)- Lec13

8/17/2019 Fluid Power - (ME353)- Lec13

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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

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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

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Fluid Power - (ME353)- Lec13