Created By : Majid Iqbal

INSTITUTE : Construction Technology Training institute Islamabad

TECHNOLOGY: Mechanical with specialization in Construction Machinery

CONTACT : +923005524644

In the name of ALLAH, the Beneficent and the Merciful

Hydraulic System

Chapter : 04

CONTROL VALVES

Question must be asked at the Question must be asked at the end of presentationend of presentation

Control valve

Pressure control valve

Flow control valve

Directional control valve

Pressure Regulating ValvePressure Regulating Valve

Pressure Regulating valve serves to adjust the pressure in the circuit.

SAFETY VALVESAFETY VALVE

Safety is actuated only when pressure in circuit increases excessively high unexpectedly.

SYMBOL

Types of pressure control valve

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

Poppet type valve

PARTS

OPERATIONOPERATIONValve will close when Pressure is low.

Valve is open as soon as the pressure

become high.

PISTON TYPE VALVE

OPERATIONOPERATIONPressure is low to

overcome the spring thrust.

When pressure increases piston will push the valve to open exhaust port

Pilot operated valve

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.

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

A relief valve is actuated only when the Pressure in any part of circuit increases unexpectedly to an excessively high Value

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.

Troubles and corrective measures

weak spring

Loosening of adjusting screw

Troubles

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.

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

HOLD LOWER RAISE

Control Valve:-The directional control valve determines the path that fluid takes through the system.

Direction control valve

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.

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

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.

SpoolsGrooves Metering Notches

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

FLOW CONTROL VALVE

Flow control valve serves to restrict the flow of fluid.

SYMBOL :

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.

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.

APPLICATIONS

USED IN FORKLIFT TRUCKS

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.

When raising forks

When lowering forks

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.

SYMBOLS

APPLICATION

BUILT IN A VALVE SPOOL OF HYDRAULIC CONTROL VALVE

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.

TENDEM CONNECTION

Construction simpler and compact. Smaller size comparatively.Simultaneous operation is impossible.

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.

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.

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.

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.

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.

VALVE CHATTERINGVALVE CHATTERING

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