control valve use in hydraulic system of heavyt equipment
TRANSCRIPT
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Created By : Majid Iqbal
INSTITUTE : Construction Technology Training institute Islamabad
TECHNOLOGY: Mechanical with specialization in Construction Machinery
CONTACT : +923005524644
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In the name of ALLAH, the Beneficent and the Merciful
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Hydraulic System
Chapter : 04
CONTROL VALVES
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Question must be asked at the Question must be asked at the end of presentationend of presentation
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Control valve
Pressure control valve
Flow control valve
Directional control valve
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Pressure Regulating ValvePressure Regulating Valve
Pressure Regulating valve serves to adjust the pressure in the circuit.
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SAFETY VALVESAFETY VALVE
Safety is actuated only when pressure in circuit increases excessively high unexpectedly.
SYMBOL
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Types of pressure control valve
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Kinds of relief valveKinds of relief valve
Relief valve
Pilot type
Ball type
Spool type
Poppet type
Piston type
Differential pressure type
Single stage
Two stage
Direct acting type
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Poppet type valve
PARTS
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OPERATIONOPERATIONValve will close when Pressure is low.
Valve is open as soon as the pressure
become high.
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PISTON TYPE VALVE
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OPERATIONOPERATIONPressure is low to
overcome the spring thrust.
When pressure increases piston will push the valve to open exhaust port
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Pilot operated valve
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Operation At Low PressureOperation At Low Pressure
Pilot valve will be close at low pressure.
It will block the oil flow to balancing port.
Main valve will not actuate because of equal pressure on both sides.
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Operation At High PressureOperation At High Pressure Pilot valve will be open
at high pressure.
Oil will flow to balancing port and creates a pressure difference at both side of main valve
In this way Main Valve will open the main port
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A relief valve is actuated only when the Pressure in any part of circuit increases unexpectedly to an excessively high Value
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Adjustment of set pressure
The spring tension can be reduced by increasing the number of shims for lower set pressure.
To increase the spring tension, number of shims used should be reduced to increase set pressure.
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Troubles and corrective measures
weak spring
Loosening of adjusting screw
Troubles
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Corrective measures
Set pressure can be restored by removing the shims.
If spring is too weak to give desired set pressure, spring should replace.
If the adjusting screw is loosened, it should retightened to restore desire set pressure.
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Directional control valves are used to direct oil into separate circuits of a hydraulic system.
Directional Control Valves provide links between various parts of the hydraulic system, by connecting of disconnecting and or changing the oil flow direction.
Directional control valve
INTRODUCTION
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HOLD LOWER RAISE
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Control Valve:-The directional control valve determines the path that fluid takes through the system.
Direction control valve
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FUNCTION OF CONTROL VALVE Move the spool to left-hand position and this closes the right side outlet port by spool land. now inlet port is connected to left side outlet port. Move the spool to right hand position, this closes left side outlet port by spool land. this connects inlet port to right side outlet port.
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Designation:
The number of ports and the number of valve positions that a DCV has will be the basic to describe the type of valve designation.
DCV DESIGNATION
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Hold a small amount of oil for lubrication between body and spool.
Assists with sealing.
Retains small particles of contamination.
What is the function of Grooves?
What is the function of Metering Notches?
Minimizes the possibility of hydraulic lock up.
Better control of flow rates with fine operation and cushioning.
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SpoolsGrooves Metering Notches
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Graphic Symbols:Graphic Symbols:
5. Control Valve, hand actuated
This is an OLSS valve.
Control valve has three positions:
Hold.
Tilt.
Dump.
The check valve is to stop work equipment from dropping when the spool is just opening.
5
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FLOW CONTROL VALVE
Flow control valve serves to restrict the flow of fluid.
SYMBOL :
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Operation at Forward Flow Operation at Forward Flow
When the oil is flowing in Direction indicated by arrow, valve opens allowing the oil to flow through center port and surrounding.
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Operation at Reverse Flow Operation at Reverse Flow
When the oil is flowing indicated by blue arrow, Valve closes allowing oil to flow through center port only.
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APPLICATIONS
USED IN FORKLIFT TRUCKS
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Oil should be fed into cylinder to lift the forks. The more oil is fed, the more the forks lifting speed.When forks lower under the own weight, causing oil in cylinder to flow back to oil tank very rapidly.To prevent the danger of rapidly lowering forks, oil must be restricted by letting it go through a smaller port to slow the lowering forks.
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When raising forks
When lowering forks
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CHECK AND MAKEUP VALVESCheck valve is used in a hydraulic circuit to constitute the one-way passage of oil and block the opposite direction passage.The makeup valve is used to make up the lack of oil so as to prevent the vacuum from being produced in the cylinder..it also prevents reverse flow of oil from cylinder to tank.Check valve and makeup valve has same construction but has different functions.
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SYMBOLS
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APPLICATION
BUILT IN A VALVE SPOOL OF HYDRAULIC CONTROL VALVE
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Oil is fed from the pump, the check valve opens, allowing the oil to flow into cylinder to raise the bucket. When stopping the oil feed or abnormal loading on bucket will give a possibility of oil from cylinder to flow back under bucket load. then check valve closes to prevent reversal of flow and preventing bucket from lowering.
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TENDEM CONNECTION
Construction simpler and compact. Smaller size comparatively.Simultaneous operation is impossible.
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SERIES CONNECTION
Simultaneous operation is possible. however, its becomes impossible when first cylinder piston reaches the end stroke. Two spools equally divide oil pressure, it reduces output force of each actuator to half of what it can display when operated independently.
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PARALLEL CONNECTION
All spools can be operated simultaneously and independently. During simultaneous operation, the cylinder with lighter load will operate earlier and at a faster speed. it is necessary to keep loads roughly equal.
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RESTRICTION
It is a device for reducing flow cross-section and provides resistance in fluid passage. It can be provided by chock and orifice restrictions.
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ORIFICE It is a small hole in oil passage. The orifice length is shorter than its cross-sectional area. The pressure after orifice is lower than before it. Pressure difference is increased when flow increases and when orifice becomes smaller.
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CHOKE
It is a throttle device which is long as compared with its internal cross-sectional area.It can act both as a orifice and as a choke.If area increases, it will behave as a orifice and if area decreases, it will behave as a choke.
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VALVE CHATTERINGVALVE CHATTERING
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