hydraulics

Chapter 8 

Hydraulic valves

1

Introduction

There are three types of valves Directional control valves Determinesthe path through which a fluid traverses a given circuit. Pressure control valves Protectsagainst overpressure which may occur due to excessive actuator loads or due to the  closing of a valve  (pressure relief, pressure reducing, sequence unloading, and counterbalance valves) Noncompensated flow control valves are used where precise speed control is not required   flow rate varies with pressure drop across a flow control valve Pressure‐compensated flow control valves automatically adjust to changes in pressure drop to  produce a constant flow rate 2 The most important considerations in any fluid power system is control. The controlling elements  are called valves

3

Directional Control Valves (DCV)

A valve is a device that receives an external signal (mechanical, fluid pilot signal, electrical or electronics) to release, stop or redirect the fluid that flows through it.

DCV is to control the direction of fluid flow in any hydraulic system

List the DCV

Directional Control Valves (DCV)

Directional Control Valves (DCV)

DCV may be actuated by a variety of methods

When an electric coil or a solenoid is energized. It creates a magnetic force that pulls the armature into the coil. It causes the armature to push the spool

It can be shifted by applying a pilot signal (hydraulic or pneumatic) against a piston at either end of the valve spool.

In manual, the spool is shifted manually by moving a handle pushing.

In mechanical, the spool is shifted by mechanical linkages such as cam and rollers.

Directional Control Valves (DCV)

Check Valve allows flow in one direction but blocks the flow in the opposite direction

Pilot operated check vlave: always permits free flow in one direction but permits flow in

normally blocked opposite direction if the pilot pressure is applied at the pilot pressure point of the valve.

Check Valve

In the free flow direction, the fluid pressure overcomes the  spring force at about 5 psi In the opposite direction, the fluid pressure pushes the poppet  in the closed position and no flow is permitted Graphic symbol of a check valve along with its no‐flow and  free‐flow directions

Shuttle Valve allows two alternative flow sources to be connected in a one branch circuit.

Directional Control Valves (DCV)

It has two inlets and one outlet

Outlet receives flow from an inlet that is at a higher pressure.

One application for a shuttle valve is to have a primary pump inlet P1 and a secondary pump The secondary acts as a backup supplying flow to the system if the primary pump loses pressure

2 way 2 position DCV (Normally closed)

Directional Control Valves (DCV)

The valve in its normal state. The valve is held in this position by the force of the spring

The valve in its actuated state. The valve is shifted into this position by applying a force to overcome the resistance of the spring.

The flow is allowed to go to the outlet port.

2 way 2 position DCV (Normally Open)

The spring holds the valve in a position in which ports P and A are connected

When the valve is actuated, the flow is blocked from going to A

Application 2/2 DCV

Directional Control Valves (DCV)

A pair of two-way valves is used to fill and drain a vessel

Valve 1 is shifted to the open position while valve 2 is remains closed

Valve 2 is shifted to open position while valve 1 is remains closed. This drains the vessel.

3 way 2 position DCV (Normally closed), with push button actuation and spring return

Directional Control Valves (DCV)

The valve in its normal state. The pressure port P is blocked and outlet A is connected to the tank

In the actuated position, the pressure port is connected to the tank and the tank port is blocked.

3 way 2 position DCV (Normally Opened)

The valce sends pressure to the outlet and blocks the tank port in the normal position.

In the actuated postion, the pressure port is blocked and the outlet is vented to the tank

Application 3/2 and 3/3 DCV

Directional Control Valves (DCV)

3/3 DCV may be used to fill and drain vessel

The closed neutral (tandem) position is required to hold the vessel at some constant fluid level

Hold Fill Drain

Application 4/2 and 4/3 DCV

Directional Control Valves (DCV)

4/2 DCV is used to control a double acting cylinder

The valve is in the normal position, the pump line is connected to the end of the cylinder and the blind end is connected to the tank

The cylinder retracts

Fully rectracted, the pump flow goes over to the pressure relief valve and back to the tank The pump line is connected to the blind end of the cylinder and the rod end to the tank

The cylinder extends

Solenoid Actuated valve

Directional Control Valves (DCV)

It can be actuated using a solenoid

When the electric coil is energized, it creates a magnetic force that pulls the armature into the coil This causes the armature to push on the push pin to move the spool of the valve.

2 types of solenoid designs used to dissipate the heat developes in electric current flowing in the coil

Air gap solenoid: dissipates the heat into surrounding air

Wet pin solenoid: the push pin contains an internal passage way that allows the tan port oil to

communicate between housing of the valve and the housing of the solenoid

Pilot operated DCV

Directional Control Valves (DCV)

They are used in a hydraulic system operating at a high pressure

The force required to actuate the DCV is high

The operation at a high pressure uses a small DCV that is actuated by either a solenoid or manually

Application of Pilot operated DCV to control the table of a surface Grinder

Directional Control Valves (DCV)

The table is fitted with adjustable stops

The pilot valve is DCV that is actuated by a push button

When stop S₁hits push button B₁, the pilot valve Sends a pilot signal to the main valve to shift the Configuration in the right envelope of the main valve The cylinder will extends

At the end of the extension, S₂hits B₂.

The pilot signal directions to be reversed.

The main valve moves to the configuration in the left enveloppe of the main valve and the double cylinder retracts.

Example

A cylinder with a bore diameter of 7 cm and a rod diameter of 3.125 cm is to be used in a system with a 45 LPM pump. Use the graph to determine the pressure drops across the DCV when the cylinder is rectracting. (P>A, B>T)

The flow from P to B is the pump flow into the rod end

3.2bar

p

 

The flow from A to T is the return flow out the blind end. The flow rate is greater than the pump flow and must be determined



₂



 

2 ₂

7

38.5cm

4

4

piston piston

A





D







 

₂





2 ₂

3.125

7.7cm

4

4

rod rod

A





D







38.5

45

56.26 LPM

38.5 7.7

Piston return pump Piston rod

A

Q

Q

A

A















6.2bar

p

 

Pressure control Valves

Pressure Reducing valve

It is used to maintain reduced pressures in specified locations of hydraulic systems It is actuated by downstream pressure and tends to close as this

pressure reaches the valve setting

It uses a spring loaded spool to control the downstream pressure If the downstream pressure is below the valve setting, the fluid flows freely

When the outlet pressure increases to the valve setting, the spool moves to partially block the outlet port.

If the valve closes completely, leakage past the spool causes downstream pressure to build up above the valve setting

This is prevented from occuring because a continuous bleed to the tank is permitted via a seperate drain line to the tank.

Pressure control Valves

Pressure Reducing valve

Two cylinder are connected in parallel

The circuit is designed to operate at a maximum pressure

P1, which is determined by the PRV

It is the maximum pressure at which cylinder 1 operates By the function of this machine, cylinder 2 is limited to

pressure P₂(P₂< P₁). This can accomplished by placing a pressure reducing valve

If the pressure in the cylinder 2 circuit rises above P2, the pressure reducing valve closes partially to create a pressure drop across the valve.

The disadvantage is that the pressure drop represents the lost energy that is being converted into heat.

Example: the primary part of the circuit is operating at 180 bar. A secondary circuit supplied from the primary via a pressure reducing valve requires a constant flow of 30 LPM at 100 bar. Find the power loss over PRV. _180 100_ 30

4kW 600   _

Pressure control Valves

UnloadingValve

It is used to dump excess fluid to tank at littlet or no pressure

A common application is in high low pump circuits where two pumps move an actuator at a high speed and low pressure.

The circuit the shifts to a single pump providing a high pressure to perform work

Pressure control Valves

CounterbalanceValve

It is normally closed valves. Used to maintain a back pressure on a vertical cylinder to prevent it from failling due to gravity

During the downward movement of the cylinder, the counterbalance velveis set to open at slightly above the pressure required to hold the piston up

Pressure control Valves

SequenceValve

It is used to force two actuators to operate in sequence.

Without the sequence valve, the cylinder extends together

Instead of sending flow back to the tank, a sequence valve allows flow to branch circuit, when a preset pressure is reached

The clamp cylinder extends completely and then the bend

The pressure rises and open the sequence valve 1.

During the retraction the sequence valve has no effect on the circuit

Orifice as a flow meter or flow control device

Such a device can be used as a flowmeter by measuring the pressure drop (p)across the orifice

SG

p

CA

Q

 0851

0

.



SG

p

CA

Q



38

.

1



Needle valves

It is used to regulate the speed of hydraulic cylinders and motors by controlling the flow rate to these actuators

It is designed to give fine control of flow in small diameter piping Their name is derived from their sharp, pointed conical disk and

matching seat

Easy read & adjust flow control valve. A locknut prevents unwanted changes in flow

G v

S

P

C

Q





C_v=Valve Capacity Coefficient

25

Example 8-5

Servo Valves

It is a directional control valve that has infinitely variable positioning capability

It can control not only the direction of fluid flow but also the amount

It is coupled with feedback sensing devices which allow for the very accurate control of position, velocity, and acceleration of an actuator

27

Mechanical type servo valves

1- Force amplifier used for positioning control

2- Small input force shift the spool to right makes the oil flows to P1 retracting hydraulic cylinder to right.

3- Feedback link shifts the sliding sleeve to right until it blocks off flow to hyd. Cylinder.

4- A given input motion produced a specific and controlled amount of system 5- Applied f output motion.

6- System with output is feedback to modify the input is a closed –loop in hydraulic power steering system of automobiles, hydropower plants

Electrohydraulic servo valves

Electrically controlled- Proportional metering valve used in mobile vehicles & Industry

Torque motor includes : coils, pole pieces, magnets, and an armature

Armature is supported for limited movement by a flexure tube

Flexure tube provides a fluid seal between hydraulic and electromagnetic portions of the valve

Flapper attaches to the center of the armature and extends down, inside the flexure tube

29

Electrohydraulic servo valves

Torque motor includes : coils, pole pieces, magnets, and an armature

Armature is supported for limited movement by a flexure tube

Flexure tube provides a fluid seal between hydraulic and electromagnetic portions of the valve

Flapper attaches to the center of the armature and extends down, inside the flexure tube

Nozzle is located on each side of the flapperflapper

motion varies the nozzle openings

Inlet pressurized hydraulic fluid is filtered and the supplied

to each nozzle through one of the two inlet orifices located at the ends of the fliter

The variation of the differential pressure between the ends of the spool is made by the flapper motion between the nozzles

Four way valve spool driects flow from supply (pressure port) to the two outlet-to-load ports in an amount proportional to spool displacement

Electrical current causes either clockwise or counterclockwise torque in the armaturedisplaces the flapper between the two nozzles

Differential nozzle flow moves the spool to either right or leftuntil the feedback torque couteracts the electromgnetic torque

Armature/flapper is returned to centerspool stops and remains displaced until the electrical input

Electrohydraulic servo valves

Armature/flapper is returned to centerspool stops and

remains displaced until the electrical input changes to a new level

Valve spool position is proportional to the electric signal

The actual outlet flow from the valve to the external load will depend on the load pressure

31

Proportional control valves

It is called electrohydraulic proportional valves, similar to electrohydraulic servo valves (electrically controled)

Servo valves Proportional valves

Used in closed loop systems Used in open loop systems Electric current in a torque motor

coil to control the movement of  the valve spool

Uses a solenoid produces a force  proportional to the current in its

coil

Controlling the current in the solenoid coil  the position of the spring loaded spool can be controlled A standard solenoid, a proportional valve can provide both directional and flow control capability in

a single valve The graphic symbol contains two horizontal lines( one @ top & bottom) to indicate the infinite

Proportional control valves

The spool is designed to provide precise oil metering for good speed control for cylinder or motor

To accomplish this precise control, clearances between the spool and mating valve bore are very small (0.0005 in)

33

Cartridge valves

Integrated hydraulic circuits are compact hydraulic systems formed to make the hydraulic systems more efficient and economical

It is formed by integrating various cartridge valves and other components into a single, machined, ported manifold block

The cartridge valve is assembled into the manifold block either by screw threads

Advantages & Functions of Valve manifold Blocks

1.

Reduced no. of fittings to connect hydraulic Lines between various

components in a system.

2.

Reduce oil leakage & contamination due to fewer fittings .

3.

Lower installation time & cost.

4.

Reduced service time by changing faulty cartridge.

5.

Required small space for the system.

A variety of Different valve functions can be provided using cartridge valves

35

Integrated hydraulics technology

It can provide easier installation and servicing, greater reliability, reduced leackage, expanded design flexibility, and lighter, neater hydraulic packages for a variety of hydraulic applications

Hydraulic fuses

It is analogue to an electric fuse because they both are one shot devices

It prevents hydraulic pressure from exceeding an allowable value in order to protect circuit components from damage

Exceeds a design valuethe thin metaldisk ruptures to relieve the pressure as oil is drained back to the oil tank

Partial circuit : pressure compensated pump hydraulic fuse 37

Problem 8-43

in

x

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lb

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2000

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65

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0

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psi

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crackingpressure

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300 

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Problem 8-47

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38

Problem 8-51

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C= 0.8 (sharp edge) 2 2

14

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4

in

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gpm

9

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

Problem 8-57

9

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0

687

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2



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SG

kPa

p

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Lpm

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p

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60

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39

Problem 8-58

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Psi

P

rod

piston

A

P

W

A

P

1848

1

2

4

2000

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4

750

)

(

2 2 2 2 2

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A

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1000

1

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2

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

Problem 8-59

s

in

gpm

Q

87

.

23

/

60

231

65

.

22

9

.

0

1848

5

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0

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in

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2 2

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s

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2 2

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