2005/2006 i. hydraulic and pneumatic systems1 directional valves classification schemes: 1.spool...

2005/2006 I. Hydraulic and Pneumatic Systems 1

Directional valves

Classification schemes:

1. Spool valves or poppet valves2. Switching or continuously adjustable valves3. Number of ports and positions4. The kind of governing and positioning unit


Directional valves1. Spool valves or poppet valves:

Spool valves Poppet valves

have always leakages can be totally sealed

need relatively large displacements because a positive overlap is needed for sealing

need smaller displacements to let fluid through

preferred in all other cases because of their flexibility and lower price

preferred for large flow rates

need radial and axial pressure relief,


Directional valves

2. Switching or continuously adjustable valves:

Directional valves work either in certain switching positions or they are continuously adjustable.

Latter by electrohydraulic or electric governing units.


Directional valves3. Number of ports and positions:

Two positions Three positions

Basic symbols

4/3 valve with ports, operation positions and governing elements


Directional valves4. Kind of governing and positioning unit:

By hand, electrically or electrohydraulically

Cross-section:

Symbol:


Electrically governed continuously adjustable valves

Servo valves:

Servo valves Proportional valves

They are able to govern a very large output power with a very small input power.

Large masses have to be moved in a short time against varying forces to precise positions.

These valves are very expensive because they have to be manufactured with very high accuracy.

Because of this, their operation is also expensive, because the fluid has to be very well filtered.

They have usually two (for very large flow rates three) amplifying stages.


Servo valvesDouble nozzle – impinging plate (bridge circuit):

2

0

2

0

0

11

1

11

1

ss

ss

pp00 sss

User

s0-s s0+s

s Q2Q1

QU QU

Q01

p1

Q01

p0

p2

0p

p

0s

s1

0,8

-1

-0,8

Linear part in the middle


Servo valvesOperation:

If there is no current, there is no displacement, the valve body is kept in the middle.

If there is current, the iron core is displaced and the impinging plate that is rigidly coupled with it is also displaced and a pressure drop is created.

Note that there is a small negative overlap, the manufacturing must be very good.

There must always be a flow.

Disadvantage: it is difficult to hold the zero position, because of dry friction or contamination.

Solution:

There are also three-stage servo valves.

They are used for very large powers.


Continuously adjustable valvesProportional valves:

Continuously adjustable proportional magnets are able to adjust continuously and precisely the position of a directional valve.

The magnetic force is proportional to the current and nearly independent of the position.

The core reaches out far from the coil.

They might be connected with a position control, where hysteresis effects are smaller.

They are often used for flow rate control

but all pressure valves can work with a proportional magnet instead of a spring.

They are more precise.


Accessories

Liquid reservoir:

Tasks: Storing of necessary liquid volume

Conducting heat away from the system

De-airing of the liquid (large surface, long stay of the liquid)

Separation of condensed water

Carrying element of other system components (pumps, valves, pipes, etc.)

Pipes:Steel of flexible hoses.

Key point: sealing


AccessoriesFilter:

• Surface filter

• Volume filter – an order of magnitude better but not cleanable

It can be before the pump, after the pump, before every sensitive elements or in the back flow line.


System conceptsDelivering of liquid by:

Flow sources

Pressure sources

In case of flow sources normally for each actuator a separate source.

One pressure source can deliver several actuators parallel.

Pressure source should not be controlled by restriction valves because the losses will be then too high. *

Better is to control by adjusting the displacement volume of the actuator.

It can happen with a hydromotor easily but for a hydraulic cylinder one needs a hydraulic transformer.

This is called secondary control.

* → exp. on next slide


System conceptsCont.

* Basically there are two main subgroups:

1. The pressure is always constant

In this case the main task is to find a control system to change the flow rate for the oscillating flow rate needs of the actuators.

Various control mechanisms can be used here

2. The pressure is not constant

- Secondary control (see previous page)

- Load sensing control (much more complicated)


System conceptsConstant pressure sources:

- Pump with pressure relief valve: not very effective – lot of lost energy.

- The displacement volume or the rotational speed of a pump can be controlled by measuring the pressure. In fact flow rate control but indirectly pressure control.

The constant valve is for safety: it limits the maximum possible flow rate through the system and prevents a breakdown of the pressure. These valves are all between the actuator and the reservoir. This has the advantage that the actuator is between two liquid columns that are both under pressure.

VDr = Restriction valve

VW = direction valve

- All three versions have a 4/3 valve

- All three versions have two restriction valves, one constant and one adjustable

Position drives:


System concepts

Drives


Constant pressure sourcesPosition drives:

With the adjustable resistance the velocity can be controlled:

a) Is a simple circuit. Problem is drifting in the holding position since the spool valve has a leakage.

b) Solves this problem by putting a 2/2 directional valve before the 4/3 valve. This seals perfectly. In addition, a double check valve prevents the cylinder from moving.

c) Is even better. It allows a very accurate positioning. The directional valve 1 has the job to switch between a creeping mode and a normal speed mode. From the creeping mode it is easy to stop accurately.

Hold: 1

Normal speed: 2

Creeping speed: 0


Constant pressure sourcesVelocity drives:

VW = directional valve

EV = fast forward

ER = fast backward

AV = working forward

AR = working backward

VSZ = flow control valve

VDr = restriction valve Simpler circuits can be derived. If for example no working backward mode is needed then VW4 and VSZ1 can be omitted.

If the working velocity is so small that the flow rate is around the leakage flow rate, large errors occur. In this case spool valves may not be applied, instead pilot operated check valves.

Directional valve positions


Constant pressure sourcesForce drives: Holding of work pieces, lifting of weights, forming of materials

a) Simple circuit without adjustment of force. Why is it a constant force drive? Because the pressure pushes the cylinder as long to the right as there is no equilibrium with the spring force.

b) Force adjustable with a pressure relief valve.

c) Large force. Force is adjustable in two stages.

d) Weight balancing, not adjustable.

e) Weight balancing with two-stage adjustability.

a) b) c)

d)

e)


Big pictures

End of normal presentationBeginning of big pictures


Directional valvesSpool valves – pressure relieves

1 – axial, 2 – axial


Directional valves

Spool valves - overlaps

O > 0 O = 0 O < 0


Directional valvesSpool valves

Centred by springs

Pneumatically operated

Hydraulically operated

Spool typesWith two switching positions and latch



Electro-hydraulicaly governed, centred by spring

Electro-hydraulicaly governed , centred by pressure

Back



-

spool types


Directional valvesPoppet valves

With one ball

1 - ball 2 - spring 3 - poppet 4 - housing 5 – lever ? 6 – operating pin ?7 - ball 8 - poppet

Electrically governed 3/2 poppet valves

With two ball

Ball Taper Plate

Back


Directional valvesNumber of ports and positions:

a) 2/2

b) 3/2

c) 4/3

d) 5/3

e) 5/3

f) 4/3

g) 6/3


Directional valves5/3 valves:


Directional valvesKind of governing and positioning unit:


Servo valvesConstruction

Torque motor

Permanent magnetsIron core with coils tube

Bending tube

Double nozzle – impinging plate

Return springs

Spool valve

Constant restrictions

User


Servo valvesPositioning with spring


Servo valvesPositioning with inductive device


Servo valvesThree stage servo valve


Proportional valvesProportional directional valve


Proportional valvesProportional flow rate valve


Proportional valvesProportional pressure valve


AccessoriesFilters


System conceptsPositioning drives

Against bumpers Drift-free holding of the cylinder

Positioning out of creeping motion


Velocity drives

Circuit plan


Velocity drivesDirectional valve positions


Velocity drivesModification for very low working speeds


Force drivesWithout adjustment of force


Force drivesForce adjustable with pressure relief valve


Force drivesForce is adjustable in two stages


Force drivesWeight balancing, not adjustable


Force drivesWeight balancing with two-stage adjustability

2005/2006 i. hydraulic and pneumatic systems1 directional valves classification schemes: 1.spool...

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