Pressure control valves (PCVs)

Directional control valves (DCVs; direct- and pilot-operated)

Flow control valves (FCVs)

Check valves

To control and regulate pressure in hydraulic system

Types ◦ Pressure relief valve

◦ Pressure regulator

connected with high-pressure and return low pressure lines.

used to limit the maximum operating pressure in the high-pressure lines. The relief valve consists mainly of a poppet, loaded by a spring.

P

T

The poppet is pushed by the spring to rest against its seat in the valve housing. The spring pre-compression force is adjusted by a spring seat screw or by inserting distance rings.

The following formula is used to calculate the cracking pressure for this valve

PCV

Normal position

Pressure control valve practically is

used in every hydraulic system.

It is normally a closed valve whose

function is to limit the pressure to a

specified maximum value by diverting

oil from pump flow back to the tank.

Refer to the circuit : When the motor is

on and the control valve is un-actuated,

the fluid pressure in the pipe line will

increase. At the maximum pressure, the

pressure relief valve will open and

allow the fluid to return back to the

tank Without the pressure relief valve

the pipe line may explode or the pump

damage.

If the valve is actuated, the piston will

extend. When the piston fully extend

the pressure will also increase and

again the pressure relief valve will

open.

Reduce the output pressure to safe/usable pressure.

Automatically shuts off hydraulic flow at a certain pressure.

A

P T

DCVs are used to start, stop, or change the direction of fluid flow.

The valves are specified by the number of connected lines (ways) and the number of control positions.

The control positions determine the way in which the lines are interconnected, and consequently the directions of fluid flow.

E.g. A 4/3 directional control valve is connected to

the pressure line (P), return line (T), and cylinder lines (A and B). In its neutral position, the valve closes all of the four lines and the cylinder is stopped. By switching the valve to any of the other positions, the cylinder moves in the corresponding direction.

Types:

1. 2 ports/2 way

2. 3 ports/2 way

3. 4 ports/2 way

4. 4 ports/3 way

◦ 1 working port and 1 pressure port

◦ Control delivery by closing or opening the passage

Type of 2/2 Way Valve. (Normally Open and Normally Closed)

2/2 Way Valve (Normally Close) type

2/2 Way Valve (Normally Open) type

◦ 1 working port,1 pressure port and 1 tank connection

◦ Control delivery

Normal position – P is closed and A to T is open

Actuated position – T is closed, flow from P to A

Type of 3/2 Way Valve. (Normally Open and Normally Closed)

3/2 Way Valve (Normally Open) type

3/2 Way Valve (Normally Closed) type

3/2 Valve (Normally Closed)

Use for single acting cylinder

NORMAL POSITION

ACTUATED

◦ 2 working port (A, B), 1 pressure port and 1 tank connection

◦ Control delivery

Normal position – flow from P to B and A to T

Actuated position – flow from P to A and B to T

4/2 Valve (PB Type )

4/2 Valve (Lever type) NORMAL POSITION

Use for double acting cylinder

4/2 Valve (Lever type ) ACTUATED

The 4/2 way DCV used in previous circuit are sometime impractical since they continuously sending flow and pressure into hydraulic actuator. The should give a relief to the actuator, a NEUTRAL position should be used, where the actuator is not subjected to pump pressure.

NEUTRAL POSITION

4/2 Valve

4/3 Valve

HYDRAULIC AND PNEUMATIC

4/3 Valve With Closed Neutral Position

Closed Neutral Position:

(1) The Pump is blocked.

(2) The flow goes to

Pressure Relief Valve.

(3) The maximum allowed

pressure of the system is

set by the pressure relief

valve.

HYDRAULIC AND PNEUMATIC

4/3 Valve With Tandem Neutral Position

Tandem Neutral Position:

(1) A desirable condition

where the only pressure

involve line resistance.

(2) The power consumption

is reduced. Is said to be

UNLOADED.

(3) Less energy wasted.

HYDRAULIC AND PNEUMATIC

4/3 Valve With Open Neutral Position

Open Neutral Position:

(1) A combination of a

tandem and float neutral

position.

(2) A flow will return to the

tank from the pump.

(3) At the same time, A bit

of flow might still be

flowing (floating) at the

actuator.

HYDRAULIC AND PNEUMATIC

4/3 Valve With Float Neutral Position

Float Neutral Position:

(1) A desirable condition for a

circuit with motor, since it

allow the motor to spin until

fully stop.

(2) This condition is said to allow

the motor to float or spin

freely without influence of

pump.

(3) Not using it may caused

pressure increase at the motor,

since spinning motor might

give a resistance when the

positioned is changed.

HYDRAULIC AND PNEUMATIC

4/3 Valve With Regenerative Neutral Position

Regenerative Neutral

Position:

(1) A term used where the

flow returning from

actuator is feed back

into the pressure line to

give supplemental

power.

Also known as non-return valve

generally used to allow for free flow in one direction

prevent (obstruct) the fluid flow in the opposite direction

Type: ◦ Spring loaded

◦ Without spring

Check Valves in Circuit

FCV is used to regulate speed

It limit the amount of flow in a main or branch hydraulic circuit

Types: ◦ Throttle valve

◦ One-way FCV

Used to restrict the flow from either direction

AKA as needle valve.

A throttle valve control the

flow rate in the circuit.

By controlling the flow rate,

the speed of the actuator can

be controlled.

FCV: Throttle valve

Restrict the flow in one direction while allowing a bypass in the opposite direction

Used to reduce the speed of cylinder movement

100%

A one way FCV give a controlled flow direction

from left to right (A) and a free flow in reverse

(B)

END OF LECTURE