Bosch Rexroth Hydraulic Valves, New Zealand

In 4WE6D6X/EG24N9K4 the letter after the size is the spool symbol, and it decides what happens to the cylinder when the solenoids are de-energised. Hold, drift, float or vent. Change that letter and you change what the machine does on an emergency stop.

Directional valves are the highest volume hydraulic enquiry we get, and the spool is the field people copy least carefully.

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Reading the type code

Field	Means
4	Number of main ports
WE	Directional spool valve, solenoid operated
6	NG size
D	Spool symbol
6X	Series
G24	Solenoid voltage
N9K4	Connector, seals and further options

Related families read the same way. WEH is pilot operated for larger flows, WRE and WRZ are proportional, and the size, spool and voltage fields sit in the same places.

The spool symbol is a machine behaviour decision

The most important content on this page.

The spool determines how the ports connect in each position, including the position the valve sits in with no power.

Centre condition	With the solenoids off, the cylinder
Closed centre	Is held, oil trapped both sides
Open or float centre	Is free. It can be pushed or it can drift
Tandem centre	Cylinder held, pump unloaded to tank
Regenerative and special spools	Application specific behaviour

Two valves that are identical in size, voltage and connection, and different in one letter, will behave completely differently the moment the machine stops.

Three consequences worth being blunt about:

On a vertical or suspended load, the centre condition is part of how the load is held. Fitting an open centre spool where a closed centre one was means the load can settle.

On a machine with an emergency stop, the centre condition is part of what happens on the stop.

And a tandem centre unloads the pump. Substituting a closed centre spool for it can leave the pump on relief, running the system hot and wasting energy, which presents weeks later as an overheating problem.

So copy the spool letter exactly. Whether a different spool is acceptable belongs with whoever holds the machine's safety and hydraulic design. We supply what you specify, and we will flag it if you ask for a spool that differs from what you described.

A closed centre spool is not a load holding device

Worth saying, because it is a common misunderstanding.

Spool valves leak internally by design. A closed centre spool holds a load for a while and not indefinitely.

A cylinder that creeps down slowly is usually normal spool leakage, not a faulty valve. Genuine load holding uses a pilot operated check valve or a counterbalance valve, which is a separate component.

So if a machine has started creeping, look for that valve rather than replacing the directional valve.

NG sizes and interchange

Rexroth	Also called
NG6	CETOP 3, D03
NG10	CETOP 5, D05
NG16	CETOP 7, D07
NG25	CETOP 8, D08

The mounting face is standardised, so valves from different manufacturers bolt onto the same manifold. That is genuinely useful when a machine is down and the original has a lead time.

Standardised	Not standardised
Bolt pattern and port positions	Spool availability and symbols
Overall mounting dimensions	Flow rating and pressure drop
	Response time and switching characteristics
	Coil, connector and seal specification

So an interchange valve is a real option and not automatically a like-for-like. We will offer one where it genuinely suits, tell you what differs, and recommend staying with the original on proportional, servo or safety related duties.

The coil is a separate part

The same money-saving fact as pneumatics, and it applies here just as strongly.

The solenoid coil slides off the valve and is orderable on its own.

Symptom	Often
Valve will not shift, no signal at the coil	Wiring or the control system
Signal present, valve does not shift	Coil, or a stuck spool
Coil hot or discoloured	Coil, or the wrong voltage fitted
Works cold, fails when hot	Coil breaking down at temperature

Use the manual override first

Most solenoid valves have a manual override pin on the end of the solenoid. Pushing it shifts the spool without electrical power.

Result	Conclusion
The cylinder moves normally	Hydraulics fine. The fault is electrical, and probably the coil
Nothing happens	Spool stuck, no pressure, or something downstream
Moves slowly or weakly	Pressure, flow or the actuator

That splits the fault in half in seconds, exactly as it does on a pneumatic valve. Stand clear of the machine before doing it, because the actuator will move.

Coils are not generic

Voltage and connector type both have to match. 24 V DC and 230 V AC coils are both common on the same site, and fitting the wrong voltage burns the coil out quickly.

The connector matters too, since the standard rectangular plug, moulded connectors and various circular styles all appear. Tell us the voltage and send a photo of the plug.

Proportional valves and amplifier cards

Where the largest wasted orders happen on this brand.

A proportional valve is driven by a variable current, not a simple on-off signal. Something has to generate that current, and there are two arrangements.

	External amplifier card	Onboard electronics
Where the electronics live	A card in the panel	Built into the valve
Control signal from the PLC	Analogue reference into the card	Analogue reference straight to the valve
Interchangeable	No. They are different valve variants

A valve with onboard electronics will not work driven from an amplifier card, and a plain proportional valve will do nothing connected to an analogue output. That is a wasted order and a wasted shutdown.

So look in the panel before ordering. If there is a card with adjustment potentiometers wired to the valve, you have the external arrangement. If the valve has a multi-pin connector and no card exists, it has electronics on board.

Cards are tuned to the machine

Adjustment	What it sets
Gain	How much valve movement per unit of signal
Ramp	How fast the valve is allowed to change
Deadband compensation	Takes up the spool overlap
Offset and zero	Where the valve sits at zero command

A replacement card arrives at factory settings. So the axis will move and it will not move like it did, which reads as a faulty valve or a mechanical problem.

Photograph the potentiometer positions before removing the old card. On a card with screwdriver adjusters that photograph is the tuning record, and it is often the only one that exists.

Contamination kills proportional valves first

Proportional and servo valves have tighter internal clearances than on-off valves and need cleaner oil to survive.

Symptom	Usually
Axis drifts or hunts	Contamination or a sticking spool
Sticky at low command, fine at high	Contamination in the spool clearance
Same valve fails repeatedly	Oil cleanliness. Not the valve
Fails soon after a system repair	Debris introduced during the repair

If a machine has eaten more than one proportional valve, stop buying valves and get an oil sample tested. A cleanliness result costs a fraction of the valve, and without it the next one fails the same way.

Worth checking at the same time: the filter element and its indicator, the tank breather, and whether the return line filter was ever fitted.

Seals and fluid

The seal specification is a field in the type code, and it has to suit the fluid.

Standard seals suit ordinary mineral oil. Fire resistant and some biodegradable fluids need a different specification, and a valve with the wrong seals fails weeks or months later as a leak or a swollen seal rather than immediately.

So if the system runs anything other than mineral oil, say so in the enquiry. One line, and it prevents a repeat job.

Before ordering a replacement

Symptom	Check first
Valve will not shift	Manual override, then the coil and its signal
Cylinder creeps down	The load holding valve, not the directional valve
New card fitted, axis moves badly	Card tuning. It arrives at factory settings
Proportional valve sticky or drifting	Oil cleanliness
Second valve failed the same way	Get an oil sample tested
Coil burning out repeatedly	Voltage, or a spool that is sticking and holding the coil in
System hot	Spool centre condition, relief valve, or cooler
Leak weeks after a change	Seal specification against the fluid

What to send us

The complete type code. Especially the spool letter and the voltage.

A photo of the valve on the manifold, showing the coils and the connectors.

The solenoid voltage and connector type.

What fluid the system runs.

For a proportional valve, whether there is an amplifier card in the panel.

Whether the load is vertical or suspended.

The symptom, and what the manual override did.

What we can supply

Directional valves. 4WE and 4WEH across the NG sizes, spools and voltages.

Pressure and flow valves. Relief, reducing, sequence, throttle and flow control.

Check valves. Including pilot operated check and load holding valves.

Proportional valves. With external amplifier or onboard electronics, plus the amplifier cards.

Coils and connectors. In the common voltages and plug styles.

Manifolds, subplates and seal kits.

Interchange. Equivalent valves from other manufacturers on the same NG mounting, where genuinely suitable, with the differences stated.

A sensible hydraulic valve spares holding

Item	Why
Solenoid coils in your voltages	The part that actually fails, and cheap
Filter elements	The single best protection for everything else
One directional valve per spool and size you run	Usually only a few combinations across a machine
Seal kits and subplate seals	Cheap, and a leak stops a machine
Photographs of every amplifier card's settings	Free, and it is the tuning record

Common questions

Can you get a part that is not listed on your site?
Usually. The site shows what we hold, not what we can source.

What does the letter after the size mean?
The spool symbol. It decides how the ports connect, including with no power.

Can I use a different spool?
Not without changing what the machine does when it stops. On a vertical load or an emergency stop that is a design decision.

My cylinder creeps down.
Spool valves leak internally by design. Look for a pilot operated check or counterbalance valve.

Can I fit another brand's valve?
Mechanically often yes, because the mounting is standardised. The spool, flow rating and connector still have to match.

Do I need the whole valve?
Often not. The coil is a separate part and a fraction of the price.

Why does my proportional valve keep failing?
Oil cleanliness, in most cases. Get a sample tested before buying another.

What is the lead time?
Ex stock items ship the next working day within New Zealand. We give you a firm lead time with the quote.

Do you quote in NZD?
Yes, in NZD including GST for New Zealand customers.

Send us your type codes

All of it, and the spool letter in particular. And tell us what the manual override did, because that answer decides whether you need a coil or a valve. Quotes in NZD including GST.

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Industech is an independent supplier of new, surplus and used industrial automation equipment. We are not an authorised distributor of, approved by, sanctioned by, or affiliated with Bosch Rexroth. Bosch Rexroth and Rexroth are trademarks of Bosch Rexroth AG. CETOP is a trademark of the European Oil Hydraulic and Pneumatic Committee. All trademarks, brand names and type codes are the property of their respective owners and are used here for identification purposes only.