hydraulics & pneumatics presented by: dr. abootorabi introduction to valves pressure control...

HYDRAULICS & PNEUMATICS

Presented by: Dr. Abootorabi

Introduction to ValvesPressure Control Valves

1

Primary Control Functions in a Hydraulic System

Control valves allow hydraulic systems to produce the

type of motion or level of force needed to complete the

functions expected of a hydraulic circuit.

A variety of valves can control actuator direction, speed,

and force output.

2


The three basic types of control valves are:

Pressure control

Directional control

Flow control

3


Pressure control valves can:

Protect the system from damage due to excessive

pressure

Sequence motion

Limit pressure in selected sections of a circuit

4


A system pressure control valve

5


Directional control valves

direct fluid flow to establish

and control actuator

movement .

6


Flow control valves control the operating speed of actuators

They provide a means to vary the rate of fluid flow

7

Actuating force

With some types of poppet

valves, the actuating

force, which is dependent

on pressure and area, may

be very high. In order to

avoid this, pressure

compensation may be

provided at the valves.

8

Poppet principle

Valves are based either on the

poppet principle or slide

principle. In poppet valves, a

ball, a cone or a disc is pressed

by a spring against the seat of a

passage. Valves of this kind

provide a very efficient seal. The

illustration shows a cone used as

a sealing element. 9

Slide principleThis illustration shows the principle

of a longitudinal slide valve. In

order to allow the piston to move, it

has a certain clearance and floats in

hydraulic fluids. Ring grooves

ensure an even film of oil and thus

pressure equilibrium. The piston

can thus be moved with minimal

frictional losses.

This type of valve cannot provide a perfect seal, which

means that there is always a certain oil leakage.

10

Piston overlapThe switching characteristics of a valve are governed by, among

other things, its piston overlap. A distinction is made between

positive, negative and zero overlap. In the case of positive

overlap, the port in question is completely covered by the

piston, while with negative overlap it is less than completely

covered. In the case of zero overlap, the distances between the

control edges of the piston and of the port are exactly the same.

11

Negative switching overlap In the case of negative overlap, flow

from A to T is not quite closed when

the inlet P is opened. This means that

the pressure at port A rises slowly and

the piston starts gently.

In manufacturers' data sheets, overlap

positions are shown within dotted lines

between the switching positions, or the

overlap positions are shown in color.

12

Positive switching overlap In the case of positive overlap, the

left-hand piston does not open the

passage from P to A until the tank

has been completely isolated by

the other piston. Pressure is

immediately fed to the load

device (cylinder or hydraulic

motor) with the result that this

starts abruptly.

13

Basic Structure and Features of Control Valves

A spool is a cylindrical

metal piece fitted into

the bore of a valve

body.

The spool is used to

block or direct fluid

through a valve to

produce a desired fluid

flow characteristic.14


Internal and external forces are used to position the various

valve elements:

Springs and pilot pressure are typical internal forces

used to operate valve elements

Manual, pilot pressure, and electromagnetic force are

common external forces used for operation

15


Normal valve position refers to the position the internal

elements assume when a hydraulic system is shut down:

Normally open

Normally closed

16

Valve Operation and Springs, Fluid Pressure, and Fluid Flow

Springs, fluid pressure, and fluid flow are very important in

the operation of hydraulic system control valves.

Springs are used in control valves to:

Move spools and other internal elements

Establish the maximum operating pressure

Serve as a biasing force

17


Common uses for springs

18


Fluid pressure is used in control valves to:

Directly open or close valves

Remotely operate a valve element

Operate a compensating device to obtain desired

fluid flow

19


Fluid flow through an orifice is used in control valves to

establish differences in pressure.

These pressure differences combined with balancing

pistons and biasing springs are commonly used in the

operation of pressure and flow control valves.

20

Pressure Control Devices

Pressure control valves may be grouped into one of five types:

System maximum pressure control

Actuator sequence control

Restrained movement control

Pump unloading control

Reduced pressure control

21


Maximum system pressure control devices are referred to as:

Relief valves

Safety valves

Hydraulic pressure fuses

22


Relief valves are normally closed valves.

They open when system pressure approaches the set

maximum operating pressure.

The operation of relief valves can be classified as:

Direct operated

Balancing piston (compound or pilot-opereted)

23

Pressure Control DevicesDirect-operated relief valves use system pressure to

generate force to compress a spring.

This opens a ball or poppet valve, allowing excess fluid to

return to the reservoir.

24


Direct-operated relief valve

25


Compound relief valves consist of pilot- and balancing-

piston sections:

Combined into a single valve

More efficient and quieter than direct-operated

relief valves

26

Pressure Control DevicesTypical compound relief valve

27


The pilot section of the compound relief valve contains a

small, direct-operated relief valve.

The pilot section indirectly establishes maximum system

pressure by controlling the pressure in the balancing-piston

section of the valve.

28


The balancing-piston section of the compound relief valve

uses a metering orifice and a balancing spring to create

pressure and force differences.

These differences correctly position the piston to produce a

desired maximum system operating pressure.

29


Compound pressure relief valves are designed to

accommodate higher pressures than direct acting relief

valves at the same flow rate capacity.

30


Compound relief valves can also have the outlet port.

31

Outlet port


Compound relief valves can also be operated remotely by

using the outlet port from the chamber above the piston.

32


This chamber in turn can be vented to the tank through a

solenoid-operated direction control valve.

33


Cracking pressure: The point at which the internal pressure

of a hydraulic system triggers or actuates a valve. Also

called the blow-off pressure.

Full-flow pressure: The point at which a relief valve is

diverting flow at its maximum rate.

34

Pressure Control DevicesPressure override: The full-flow pressure minus the cracking

pressure. The pressure override is a measure of the increase

in pressure over the cracking pressure when additional flow

passes through the valve after it cracks.

35


The application of two direct-operated relief valves for

Cushioning of Hydraulic Motor:

36


Safety valves are used to prevent damage to the hydraulic

system if the system relief valve should fail to open.

Typically, safety valves are direct-operated relief valves.

Safety valves are generally set 25% higher than the normal

system operating pressure.

37


Hydraulic pressure fuses function as a pressure-limiting

device by using a disk that ruptures at a predetermined

pressure.

They act as a positive-pressure-limiting device for

systems where system pressure limits are critical to safe

system operation.

38

Pressure Control DevicesA typical hydraulic pressure fuse

39


Sequence valves allow the automatic sequencing of two

or more actuators in a hydraulic circuit

Primary actuator moves as soon as fluid flow is

directed to the actuator section of the circuit

Sequence valve blocks flow to the secondary

actuator until a predetermined pressure is reached,

then allows fluid flow to the actuator

40


A sequence valve is typically fitted with an integral check

(non-return) valve.

This allows free flow of fluid around the valve when the

direction of the actuator is reversed.

41

Pressure Control DevicesA circuit containing a sequence valve

42


Restrained movement control valves are used in circuits

to prevent unexpected actuator movement. Often

called:

Counterbalance (back pressure) valves when used

with cylinders

Brake valves when used with motors

43


Counterbalance valves prevent unexpected lowering of the

boom.

44


Counterbalance and brake valves are normally closed valves.

A check valve is required to permit free reverse flow around to

valve.

Counterbalance valves allow the downward movement of loads

supported by a cylinder only when the system pump is operating.

Force generated by system pressure and the force created by the

weight of the load are needed to move the load downward.45


Circuit containing a counterbalance valve

46


Brake valves are used to prevent an overrunning load

from continuing to turn a hydraulic motor after the

directional control valve has been closed.

Overrunning loads can turn the motor into a pump,

allowing the motor to turn past selected point.

47

Pressure Control DevicesCircuit containing a brake valve

48


Pump unloading controls hold a desired operating

pressure while the pump operates at near-zero

pressure. This reduces energy consumption and

maintenance costs. Unloading valves are

normally closed valves with external pilots.

Pump unloading controls use an unloading valve

and a dual pump or accumulator to maintain

desired system pressure while dumping

unneeded pump output to the reservoir at very

low pressure.

49


Circuit containing an unloading relief valve (using accumulator)

50

Pressure Control DevicesHi-Lo System (using dual pump)

51

Pressure Control DevicesReduced pressure control allows a portion of a system to

operate at a pressure below the maximum pressure setting of

the system relief valve:

Allows a system to operate using two or more pressures

Maximum pressure is determined by the system relief

valve with the additional pressures lower

Pressure-reducing valves are normally open valves with

an internal pilot and external drain.

52

Pressure Control DevicesPressure reducing valve

53

1

2 3


Circuit containing pressure reducing valve

54

The end.

55

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hydraulics & pneumatics presented by: dr. abootorabi introduction to valves pressure control...

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