# Hydraulic Valves - Classification, Functions, Selection

> Types of hydraulic valves in construction machinery: pressure, directional, check and flow control valves. How they work, failure symptoms, repair or replace.

![Hydraulic pump with valves and hoses in the compartment of a JCB machine](https://wibako.com/media/articles/article-100/slot1-e-4c74a2ab.webp)

**Hydraulic valves** control three parameters of the oil in a hydraulic power system: flow direction, pressure and flow rate. They determine whether a cylinder extends with the right force and speed, whether slewing stops smoothly and whether the pump is protected from overload.

In this guide we cover the types of hydraulic valves used in excavators, loaders and cranes and explain how they work. We also describe typical failure symptoms and help you decide when a valve is worth repairing and when it should be replaced.

## Hydraulic valves in construction machinery

A machine's hydraulic system is made up of the pump, the control valve, the actuators (cylinders, hydraulic motors) and a whole network of valves that make sure the oil goes where it is needed at the right pressure. Construction machines use oil hydraulic valves. They are designed for mineral or biodegradable oil, pressures often exceeding 300 bar and continuous duty. Valves for water or heating systems are an entirely different category and cannot be used as substitutes.

A valve can be a separate unit bolted to a subplate (subplate mounting) or a cartridge valve screwed into a manifold block. It is also often part of a larger assembly: a main control valve, a pump regulator or the valve block of a travel motor.

## Types of hydraulic valves by function

![Yellow hydraulic valve with plugged ports](https://wibako.com/media/articles/article-100/slot2-u-2adf2f5f.webp)

Hydraulic valves are primarily classified by the function they perform in the system. There are three groups:

- **Directional valves**: directional control valves, shut-off valves, check valves and pilot-operated check valves.
- **Pressure control valves**: pressure relief valves (safety and overflow valves), pressure reducing valves, sequence valves and counterbalance valves.
- **Flow control valves**: throttle valves, flow regulators and flow dividers.

The second classification is based on the operating mode. Switching valves have defined positions, e.g. open or closed. Continuously acting valves, i.e. proportional valves and servo valves, adjust flow or pressure smoothly according to the control signal.

## Pressure control valves – relief, reducing, sequence

![Hydraulic valve in an aluminium body with electrical connector](https://wibako.com/media/articles/article-100/slot3-e-6569a422.webp)

Pressure relief valves are divided into **safety valves** and **overflow valves**. A safety valve limits the pressure in the system and protects the pump, hoses and actuators from damage, which is why every system must have at least one. An overflow valve controls the pressure in the supply line: when there is too much oil, it passes the excess back to the tank. Such valves are also referred to as dump, unloading or bypass valves.

The key component is a spring that presses a poppet or ball against the seat. Its preload, set with an adjusting screw, determines the cracking pressure. For high flow rates, pilot-operated valves are used, as they work more stably.

An excavator usually has a main relief valve at the control valve and port relief (overload) valves on the individual sections, often combined with anti-cavitation valves. They protect the cylinder, e.g. when the bucket strikes an obstacle. Other pressure control valves include:

- **Pressure reducing valve** – lowers the pressure in a selected branch, e.g. in the pilot control circuit of the joysticks.
- **Sequence valve** – opens flow to the next actuator only once a set pressure has been reached.
- **Counterbalance valve** – holds the load, prevents the boom from dropping uncontrollably and stops the travel motor from overrunning on a downhill slope.

## Check valves and pilot-operated check valves

A one-way check valve lets oil flow in one direction and blocks reverse flow. By design, there are poppet, ball, diaphragm and flap check valves. They are also divided into spring-loaded and non-spring-loaded valves.

A pilot-operated check valve, single or double, normally works like an ordinary check valve. When pilot pressure is applied, it opens and allows return flow. It is used to lock outrigger, dozer blade or boom cylinders in a set position. A leaking seat shows up as the cylinder slowly creeping down under load.

## Flow control valves

These valves set the amount of oil flowing to the actuator and therefore its speed of movement. A throttle valve changes the flow cross-section. It is simple, but the actuator speed varies with the load. A flow regulator has a built-in pressure compensator and keeps the speed constant regardless of disturbances. Flow regulators are divided into **2-way** (in-line) and **3-way** (bypass) types.

A flow divider splits the oil into two branches at a fixed ratio, e.g. so that two cylinders move in sync.

## The control valve – a directional valve

![Mechanically operated hydraulic control valve with two levers](https://wibako.com/media/articles/article-100/slot4-e-13ea333a.webp)

The directional control valve routes oil to a specific actuator and, in the neutral position, can unload the pump by returning oil to the tank. There is no hydraulic power system without one. Because it works continuously, it is one of the most heavily loaded components. The most common faults are a worn or damaged spool, a spool jammed by contamination, leaking seals and broken centring springs.

By design, directional control valves are divided into:

- **spool valves** – the most common type in construction machinery,
- **poppet (seat) valves** – very tight in the closed position,
- **rotary valves**.

A designation such as 4/3 or 2/2 gives the number of ways (ports) and the number of positions. Valve symbols on hydraulic schematics are read the same way.

## Valve actuation and how a solenoid valve works

![Bucher Hydraulics valve with solenoid coil and electrical connector](https://wibako.com/media/articles/article-100/slot5-e-5aa9bdd3.webp)

Valves can be operated mechanically (by lever, as on older machines), hydraulically (by pilot pressure from the joystick) or electrically. In a solenoid valve, the energised coil creates a magnetic field that moves the armature and, with it, the spool or poppet. When the current is switched off, a spring returns the valve to its initial position.

In proportional valves, the machine controller regulates the coil current, and the coil force, i.e. the valve opening, increases in proportion to this current. This gives smooth movements. Newer designs also feature valves with on-board electronics that communicate with the controller via a bus system and report faults.

## Symptoms of a faulty hydraulic valve

- **Loss of power** on one function or in the whole system – the main relief or port relief valve is leaking or out of adjustment.
- **Oil overheating** – a valve is constantly dumping oil to the tank and turning energy into heat.
- **Boom or outriggers drifting down** under load – a leaking counterbalance valve, pilot-operated check valve or control valve spool.
- **Jerking, vibration, squealing** – a chattering poppet, worn spring, air in the system.
- **Function does not work at all** – a faulty coil, no power supply or a jammed spool.

A worn pump can cause the same symptoms, so it is worth checking its condition before replacing a valve. We cover this in the article [How often to service pumps in excavators](https://wibako.com/about-us/news/817-how-often-should-excavator-pumps-be-serviced-rebuilding-a-dpvo-165-from-a-liebherr-a316).

## Diagnostics – how to test a valve

1. Measure the pressure with a gauge at the test ports and compare the reading with the machine documentation.
2. Carry out a drift test: raise a load, measure the cylinder movement over a set time and rule out components one by one.
3. Check the temperature of the valve bodies. A noticeably warmer valve indicates that oil is flowing through it.
4. On a solenoid valve, check the voltage at the connector and the coil resistance.
5. After removal, inspect the spool and seat: look for scoring, wear and metal particles.

Do not raise the relief valve setting "to see what happens". Higher pressure overloads the pump, hoses and cylinders and does not fix the original fault. An example of complete system diagnostics is described in our project on the [Menzi Muck A91 walking excavator](https://wibako.com/about-us/news/835-the-customer-said-it-has-no-power-we-stripped-the-entire-hydraulics-of-a-menzi-muck-a91-walking-excavator).

## Repair or replace the valve?

Repair makes sense when the valve is contaminated, has damaged seals, a broken spring or a slightly worn poppet seat that can be lapped. Replacement is necessary with a worn spool and bore (excessive clearance), a cracked body or a destroyed seat. Cartridge valves are usually replaced as a complete unit. In the WIBAKO shop you will find new and used components, e.g. the [Vickers CVU25UB29W25011 hydraulic valve](https://wibako.com/shop/hydraulics/valves/6768X7921-hydraulic-valve-vickers-cvu25ub29w25011) or the [Hydromatik A6VE107 hydraulic motor](https://wibako.com/shop/hydraulics/hydraulic-motors/40748X4641-hydraulic-motor-hydromatik-a6ve107hz3-63w-vzl020b-r909604870) with valve block.

After repair, a pressure control valve must be set using a pressure gauge. The cause of the failure must also be eliminated: most often it is contaminated oil, so the filters and oil should be changed and, if metal particles are present, the system flushed.

WIBAKO provides hydraulic system diagnostics and service as well as [hydraulic pump and motor overhauls](https://wibako.com/about-us/service/431-repairs-of-liebherr-hydraulic-pumps-and-motors). On-site diagnostics are charged per man-hour (PLN 230 net per mechanic) plus travel (PLN 3 net per km, both ways). Contact us and describe the symptoms – we will help you find out whether the valve is to blame.

## Frequently asked questions

### What are the types of hydraulic valves?

By function, they are divided into three groups. The first is directional valves: directional control valves, shut-off valves and check valves. The second is pressure control valves: relief, reducing, sequence and counterbalance valves. The third is flow control valves: throttle valves, flow regulators and flow dividers.

### How does a relief valve work in a hydraulic system?

A spring presses a poppet or ball against the seat. When the system pressure exceeds the value set by the spring preload, the valve opens and returns the excess oil to the tank. This protects the pump, hoses and actuators from overload.

### Why does the excavator boom drop on its own after the engine is switched off?

The most common causes are a leaking counterbalance valve or pilot-operated check valve, a worn control valve spool or worn cylinder piston seals. This is determined with a drift test under load, ruling out components one by one.

### How do you test a hydraulic solenoid valve?

First check whether there is voltage at the connector with the function activated. Then measure the coil resistance with a multimeter and compare it with the manufacturer's data. If the coil is fine but the valve does not switch, the cause is usually a jammed spool, most often due to contamination in the oil.

### Can a hydraulic valve be repaired, or does it have to be replaced?

A valve with contamination, damaged seals, a broken spring or a slightly worn seat can usually be repaired. Replacement is necessary when the spool and bore are worn, the body is cracked or the seat is destroyed. After any repair, a pressure control valve must be set using a pressure gauge.

Published: 2024-02-29

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Source: https://wibako.com/about-us/blog/100-hydraulic-valves-classification-functions-selection
Contact: +48338704200, biuro@wibako.pl