Directional control valves

Pressure valves set how hard. The directional control valve (DCV) chooses where the oil goes — it starts, stops and reverses an actuator. Learn to read its symbol and you can read almost any circuit.

Source: Rabie, Fluid Power Engineering, Ch. 5.

Before you start

What you need first

The basic circuit & ISO symbols — pump, tank, lines, actuator (Topic 5).
A cylinder extends when oil is fed to one side and the other side drains (Topics 5, 33).

What you'll be able to do

Read the ways / positions name (e.g. 4/3) and the ports P, T, A, B.
Decode a DCV symbol box by box and say what the cylinder does.
Choose a centre type (closed / open / tandem / float).
Recognise how a valve is switched (spring, lever, solenoid, pilot).

Start here · the one big idea

The DCV steers the oil

A directional control valve starts, stops and changes the direction of the oil flow — so it decides which way a cylinder moves.

Send oil to the cylinder's cap side → it extends.
Send oil to the rod side → it retracts.
Block both → it stops.

A DCV does not set the pressure or the speed — only the path. Think of it as the points on a railway: it chooses the track, not how fast or how hard the train pushes.

The "ways / positions" name

A DCV is named by two numbers, ways / positions:

ways / positions → e.g. 4 / 3DCV naming

Ways = the number of ports (oil connections). Count the ports drawn on one working box.
Positions = the number of states the valve can be in. Count the boxes in the symbol.

So a 4/3 valve has 4 ports (P, T, A, B) and 3 boxes. The box it rests in when nothing pushes it — usually held by a spring — is its rest position; for a 4/3 that is the middle box, the "centre".

Type	Use
2/2	Simple on/off (stop–start)
3/2	Single-acting cylinder
4/2	Double-acting, two positions
4/3	Double-acting + a neutral centre (most common)

The four ports of a 4/3 valve

the ports:
Port	Connects to
P	Pump (pressure in)
T	Tank (return)
A	One side of the actuator
B	The other side of the actuator

Older drawings label the same ports with letters or numbers — P = 1, T = 3, A = 2, B = 4 — but the meaning is identical. P always comes from the pump and T always goes to tank.

The key skill

Reading a DCV symbol, box by box

The symbol has one box per position. The lines inside a box show how the ports are joined in that state:

an arrow = oil flows from one port to another;
a small ⊥ stub = that port is blocked (a dead end);
the ports are drawn on the working (centre) box; you slide the other boxes over them in your mind to read each state.

Left = crossed (P→B, A→T), centre = all blocked (closed centre), right = straight (P→A, B→T) (after Rabie Fig. 5.27).

Box	Ports joined	Cylinder does
Centre (rest)	All blocked	Held still
Straight	P→A, B→T	Extends
Crossed	P→B, A→T	Retracts

Centre (neutral) types

The middle box is where the valve rests, so it decides what happens when nothing is commanded — should the cylinder lock, and should the pump rest? Four common centres:

Four common 4/3 centres — closed, open, tandem, float. Ports: A, B on top; P, T below (after Rabie Figs. 5.27–5.28).

Centre	What it joins at rest	Cylinder	Pump
Closed	All four ports blocked	Locked	Loaded (relief must dump — wastes power)
Open	All ports connected	Drifts	Rests (low pressure)
Tandem	P→T; A, B blocked	Locked	Rests
Float	A, B→T; P blocked	Floats free	Loaded (relief must dump)

Tandem is the best of both for many machines: the cylinder is locked and the pump unloads to tank — no drift and little wasted power (recall \(N=pQ\), Topic 23).

How a DCV is switched

To move the valve from one position to another, an actuator pushes the internal spool. The symbol shows the actuator on each end box:

Spring — returns the spool to its rest position when released ("spring-offset" / "spring-centred").
Lever / push-button / pedal — a person switches it.
Solenoid — a coil becomes a magnet when current flows and pulls the spool, so a controller can switch it electrically (good up to about 100 L/min).
Pilot-operated — for big flows, a small pilot valve moves a large main spool.

Common actuators (after Rabie Table 5.2).

Spool vs poppet. Most DCVs use a sliding spool (many positions, small leak). Poppet valves seat tightly for zero leak at very high pressure, but allow fewer positions. Most solenoid valves also have a small manual override pin to switch them by hand for testing.

✏️ Try it yourself

In "4/3", what does each number mean? How many ports and boxes does the symbol have?
A box joins P→B and A→T. What does the cylinder do, and why?
You need the cylinder to lock at rest and the pump to rest. Which centre type, and what does it join?

1. 4 = ports/ways (P, T, A, B); 3 = positions/states. The symbol has 4 ports and 3 boxes. 2. It retracts — oil is pushed into side B while side A drains to tank. 3. A tandem centre: it joins P→T (so the pump unloads to tank) while A and B are blocked (so the cylinder is locked).

Common mistakes to avoid

Mistake	Fix
Swapping the two numbers	It is ways / positions: ports first, then states. 4/3 = 4 ports, 3 boxes.
Reading the rest box as the working state	The cylinder moves only in the end boxes; the centre is the rest state.
Thinking a closed centre "saves power"	Closed centre locks the cylinder but the pump is dead-ended — its relief must dump (wastes power). Open/tandem rest the pump.
Thinking the DCV sets speed or pressure	It only sets the path; speed comes from flow control (Topic 27), pressure from the load.

Recap — the whole topic on one screen

Idea	What you own now
Naming	ways / positions — ports (P,T,A,B) over boxes; e.g. 4/3
Reading	One box per state; arrow = flow, ⊥ = blocked; read each box's P/T/A/B joins
Cylinder	P→A,B→T extends · P→B,A→T retracts · all blocked = held
Centres	closed = lock · open = pump rests · tandem = both · float = free
Switching	spring · lever / button · solenoid · pilot (big flows)

Next topic

Check valves

The DCV chooses the path; a check valve guarantees a path is one-way only — the simplest valve of all, and the basis of cylinder locking.

→ Check valves