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Category: Hydraulic Valve

Hydraulic Valves: Directional, Proportional, and Servo

For  decades, hydraulic valves have operated in more or less the same simple way: they flipped back and forth, opening first one line, and then another. Normally, one pipe was the ‘do nothing’ line, and one was the ‘do something’ line. But as our understanding of hydraulic systems increased, our desire to manipulate them in more precise manners piqued — and in response, different kinds of valves developed. Here’s a brief summary of the three major types of hydraulic valve.

 

Directional Valves

Also known as ‘switching valves,’ or more colloquially, ‘bang-bang valves’ because of the noise they made when switching, these are the valves described above. Directional valves have evolved over the past decades, however, from a simple left-right decision to a single valve that can contain several different outputs. Many also moderate the speed of the hydraulic fluid by altering the aperture through which it can flow.

 

Directional valves are useful, but simple: every change in direction, flow, or pressure requires its own directional valve, making even modestly complex hydraulic circuits enormous in size and expensive to produce.

 

Proportional Valves

Proportional hydraulic valves use solenoids to allow the valve to take any desired position between and including ‘closed,’ ‘left output,’ and ‘right output.’  This means they can adjust the flow to any proportion between the two outputs. This gives the ability to put speed, flow, and directional controls all on a single valve, dramatically reducing the space taken up by a complex circuit.

 

In addition, the ability of a proportional valve to adjust the speed of a circuit anywhere between ‘stop’ and ‘full power’ means that a single source and a single hydraulic pump can be used to power a wide variety of hydraulic devices, even if they require entirely different flow speeds and/or pressures to operate. That meant not only were circuits smaller, but there were fewer circuits necessary for any given set of jobs.

Servo Valves

Servo valves aren’t new — they’ve been around since the 1940s — but they’re rare, because they’re expensive. They operate using a combination of input pressure from the hydraulic line and electronic controls to create a valve that doesn’t ‘bang-bang’ — it moves smoothly and accurately. The net effect is that a servo valve lasts a very long time, responds quickly to controls, and has low hysteresis compared to the other two valve types.

 

Three Points on Understanding Hydraulic Valves

Hydraulic valves are an important part of machines and certain equipments. If you do not work with these valves on a daily basis, you may not understand the importance of the hydraulic valve, the way it operates, and the different parts involved in the process of a working valve.

Point 1: The Functions of the Hydraulic Valve

Hydraulic valves are responsible for directing the flow of fluids beginning from the input port of the valve and coming through the output port. The commonly used fluid is oil. The spool position inside of the valve determines the direction that the fluids flow through the valve. The force of the fluids flowing through the valve is driven by a force motor.

Point 2: The Specific Parts of the Hydraulic Valve

Hydraulic valves have three specific parts that are included along with the valve in order to create the right amount of pressure to allow the fluids to flow quickly, powerfully, and easily. Below are the parts and their primary function:

  • Power Conversion: this part gives isolation between the power valve, external field bus, and the auxiliary supplies. It is also the source of voltages to regulate functional blocks that may occur while the valve is at work.
  • Field Bus Interface and Control: this part gives isolation between the control system and the signals that come from the field bus. The control system turns all information coming from the field bus into instructions for the DSP.
  • Valve Control: this part takes a role in positioning the spool and measuring temperature and pressure. This part is also responsible for indicating certain alarm conditions.

Point 3: The Primary Uses for Hydraulic Valves

The hydraulic valve is a necessity for carrying out certain hydraulic mechanisms. Most typically, hydraulic valves are used in machinery and equipment that lifts heavy objects. But hydraulic valves are also present in heavy machinery, like vehicles, military machinery, and aircraft. If you work on a construction site, in a manufacturing facility, or do everyday normal activities like drive a car, you are most likely using some type of hydraulic valve.

Hydraulic valves are an essential part of major tools, machines, and equipment. The valves are responsible for transporting fluids from one end of the valve to the other. It gives off high pressures and power which makes it powerful enough to manage its role in big machinery and equipment.

 

Two Kinds of Hydraulic Manifolds, Part I: Single-Piece

Hydraulic manifolds come in two basic types: the single-piece design that contains all of the valves and passages needed in a single metal chunk, and the modular design where each block contains exactly one valve and the various passages needed for that one valve to work. Each has their own advantages; in this article, we’ll discuss the benefits of choosing a single-piece hydraulic manifold.

Single Piece Manifolds come in two basic kinds: ‘lamniar’ and ‘drilled-block.’

Laminar Type Manifolds are composed of multiple layers of metal with holes drilled in them such that when the layers are brazed together, they form the necessary passages. As each layer is formed, the necessary mechanics of each valve are put in place. There is no limit to the number of valves — or the size of valves — that can be mounted in a laminar type manifold.

Laminar manifolds can withstand pressures of up to ten thousand psi, and can be custom-designed for any task. Because of the brazed construction and permanently shaped passages, however, it cannot be modified if changes become necessary; it must be replaced entirely. Laminar manifolds are generally more expensive than drilled-block manifolds as well.

Drilled-Block Manifolds are similarly custom-made for specific applications, and also cannot be altered after creation. Most often made from a single slab of iron, steel, or aluminum, a number of straight passages are drilled to create the flow passages and to provide the space necessary to insert the valves and other moving parts. Some such manifolds use threaded passages to screw the machinery into place; others lock the parts into place using plates attached to the manifold’s surface.

Drilled-block manifolds, being single metal pieces, can with stand more pressure than the valves within them are able to — meaning they’re effectively unbreakable, as in almost any case, a hydraulic valve will give out and the fluid will be ejected before the manifold itself cracks. They’re also the least expensive kind of manifold to produce, in general.

Single-piece manifolds in general have the advantages of withstanding greater pressures and being less expensive than modular manifolds. To learn the advantages of modular manifolds, come back next time for Part II.

Understanding Hydraulic Pump Types and Differences

There are many types of machinery that are driven by or actuated by a hydraulic pump. There are a variety of different systems that are used to generate the flow and pressure required and they all have a hydraulic fluid and a system that controls the fluid and pressure with hydraulic valves. The pump needs to driven and this can be done by any force generating device such as an electric motor, an internal combustion engine, wind power or even a person operating a lever or crank.

How It Works
A hydraulic fluid is put under pressure by the hydraulic pump and the pressure can then be used to drive a piston or drive unit via hydraulic lines. A hydraulic valve is used to switch the force on and off to give control of the device. The control can be mechanical or electrical and may be actuated manually through a lever or a button or automatically through control system.

Volume and Pressure
There are many different hydraulic systems and they all used a combination of volume displacement and pressure to work. The higher the pressure the more robust a system needs to be because of the tremendous forces involved. In general higher pressure systems are more efficient and the higher the pressure the less flow is required for the same application of force. There are two general types of pumps fixed displacement types that displace the same amount of fluid every cycle and adjustable displacement types that can vary the displacement for increased or decreased pressure.

Pump Types
There are many different types of hydraulic pumps that have different applications. Screw type pumps are good for high volumes at relatively low pressure. They are simple and effective but not particularly efficient. A gear pump has a more balanced pressure and flow and is very simple but is not very efficient particularly as pressure increases.

The vane pump is widely used in system of medium pressure up to 150 bar and beyond. While the axial piston pump is used in applications that require the highest efficiency. Where high pressure above 300 bars is needed the radial piston pump combine high pressure and low flow rates needed in these applications.

The Hydraulic Pump Decoded

With a name like ‘hydraulic’, it is kind of obvious that a hydraulic pump is somehow related to fluids. We can even deduce that it’s somehow used to pump fluids. But, what exactly does its role with fluids? What does it do? Well, here’s a short overview:

How it’s built?
So, we know from basic science that energy (or any substance, for that matter) has the tendency to flow from where there is a lot of it to where there is little. The pump, though, reverses this process by its mechanical action (therefore, mechanical energy). In a hydraulic pump, any mechanical energy is transferred to fluids. The energy generated by the pump is transferred to the fluid within the pump, therefore energizing the fluid to move from an area of low density to high density. Further in the chain of things, this energy held by the fluid is eventually transferred to other parts of the pump (or machine) completing the action. The amount of energy needed, and therefore produced, can be controlled with the use of hydraulic valves.

Types of Pumps

  1. Hydrostatic Pumps: In this type of pump, there is a fixed amount of fluid that is constantly used to generate energy. The amount of energy generated cannot be adjusted.
  2. Hydrodynamic Pumps: These pumps generally use hydraulic valves to allow the amount of energy produced to be adjusted, basically by adjusting the volume of fluid that is being displaced within the pump.

What Does It Do?
This is such a basic design that there can literally be thousands upon thousands of applications of the pump in residential as well as industrial settings. Here are just a few popular applications:

  • Water pumps: The most obvious application of this kind of pump would be to use it to pump water to buildings.
  • Construction: These pumps used in series can be used to build heavy machinery like an excavator.
  • Lifts: Not elevators, but small lifts like in your dump truck and such can be powered by hydraulics.

While these are only a few popular applications of hydraulic pumps, there are tons of applications of it in our day to day lives.

Keep Your Hydraulic Manifolds Working Longer By Reducing Cavitation

Cavitation is a deeply interesting physical phenomenon that occurs when liquids are subjects to sudden massive changes in speed. For example, if fluid moving under 40 megapascals (MPa) of pressure suddenly hits a hydraulic valve and the pressure drops to 20MPa, the sudden increase in speed (described by Bernoulli’s Theroem) would lead us to the conclusion that a small bubble of nearly empty space — a hard vacuum — would open up within the fluid at the point of pressure drop.

This does in fact happen, and the bubble both forms and then collapses at supersonic speeds, creating a tiny sonic boom within the hydraulic fluid itself. The shockwave of the bubble’s collapse generates enough energy that it can literally tear apart nearby molecules of hydraulic fluid — or, if the bubble collapses near the inner walls of the manifold, it can rend the molecules of the manifold itself, causing “erosion”.

Reducing Cavitation
That’s perhaps a misnomer — there’s no realistic way to reduce cavitation within a given system short of reworking it to avoid sudden pressure changes. But the critical phrase above is “if the bubble collapses near the inner walls of the manifold.” It may not be possible to realistically prevent cavitation bubbles, but it is possible to more carefully control where those bubbles collapse.

The simple rule of thumb is this: increase backpressure by 5% of the total drop in pressure across the valve or metering edge (or whatever else is causing the change in pressure.) In the above example, where pressure drops by 20 MPa, an increase in backpressure of just 1 MPa should force the cavitation bubbles toward the middle of the fluid stream, thus causing them to collapse where they will do as little damage as possible to the hydraulic manifold.

If that doesn’t work, switching from an aluminum manifold to a ductile iron manifold will reduce the rate of “erosion” by 90%. Alternately, if possible, increasing backpressure to 10% of the total pressure drop will have a similar effect, though often switching manifold materials is the easier solution.

How do you know if it worked? Listen carefully at the point of pressure change. White noise coming from within the system is the sound of cavitation bubbles collapsing; if the noise is reduced or gone, your attempt was successful.

Anatomy and Types of Hydraulic Valve, Part II

In Anatomy and Types of Hydraulic Valve Part I, we went over two of the five basic kinds of hydraulic valve: relief valves and reducing valves. Today, we’ll take a brief look at sequence, counterbalancing, and unloading valves.

Sequence valves
Whenever you have a hydraulic circuit wherein two actuators that are in sequence along a single circuit must move in succession (rather than all moving simultaneously), a sequence — or, more accurately, a sequence of sequence valves — is the tool for the job. Each sequence valve looks much like a relief valve (see last entry), except instead of draining into a reservoir, when the sequence valve cracks, the hydraulic fluid flows down the circuit and continues working — and, a sequence valve shuts it’s own input if the valve is over-pressured. Sequence valves do also have a drain placed above the point at which the circuit continues, so if they are forced open by over-pressure, excess fluid will drain out until the valve can open again and resume normal function.

To illustrate: you have a pipe along which fluid is being pumped. The first juncture splits two ways: to an actuator and to a sequence valve. The sequence valve is set to crack at a pressure above the amount needed for the actuator to do it’s work, so the actuator works first, then as pressure builds, the sequence valve cracks. Next in line is another actuator/valve juncture, and once again the sequence valve won’t crack until the actuator has done it’s work, and so forth, ensuring that each actuator goes off in sequence.

Counterbalancing valves
A counterbalancing valve looks and acts much like a sequence valve, but it has no drain to allow excess fluid to be removed from the system if it is over-pressured. Instead, a line feeds fluid from the over-pressure escape back into the valve’s output line, creating a feedback effect that limits the pressure that an actuator is able to force back into the system. This is vitally important, for example, when an actuator is working under a heavy load — the weight of the load could potentially cause extraordinary strain on the hydraulic circuit, but a counterbalancing valve alleviates that danger.

Unloading valves
Unloading valves are basically user-operated doorways that open into a tank and allow hydraulic fluid to flow out of the system into the tank. They’re used in several circumstances, but the most common is in a system where you have two hydraulic pumps and an actuator that is designed to move in two ‘modes’ — under low fluid volume, both pumps activate and move the actuator in a fast (high flow rate) but weak (low pressure) fashion. Or, under high volume, a single pump moves the actuator in a slow but powerful fashion. An unloading valve is the key to switching between those two modes.

Anatomy and Types of Hydraulic Valve, Part I

Hydraulic valves, or pressure-control valves, are found in virtually every hydraulic system. They perform or assist in a variety of functions, from keeping pressure below safety limits to maintaining a constant pressure in areas of the hydraulic circuit. There are many kinds of hydraulic valve, including (but not limited to)

  • Relief
  • Reducing
  • Sequence
  • Counterbalance
  • and Unloading

All of these valves are normally open, but close under specific circumstances — with the exception of the Reducing valve, which is normally open but closes when systemic pressure gets too high. Of the remainder, all of them normally close under circumstances internal to the system, with the exception of the Unloading valve, which is manually operated and closes when the system user directs it to.

Relief Valves
Hydraulic systems are designed to operate within a pre-set pressure range based on the amount of force the actuators at the ‘business end’ of the system are designed to generate. If the pressure inside the system gets too high, the actuators or other machinery can be damaged. Relief valves are designed to ‘relieve’ the system of extra pressure.

A relief valve is essentially a plug held in place by a spring, next to which which is a reservoir. When systemic pressure gets too large (called a relief valve’s ‘cracking pressure’) the spring is compressed and the plug retracts slightly, allowing hydraulic fluid to escape into the reservoir. Once the excess pressure is released, the spring forces the plug back into place. Most relief valves have screws attached to the springs so that the pressure at which they crack can be changed by the user.

Reducing Valves
Reducing valves also operate to modulate pressure within a system, but they act without allowing any fluid to leave the hydraulic system. Instead, if pressure gets too high on one end of the reducing valve, the valve simply shuts. The pressure on one side of the valve remains high, but on the other side, it stabilizes.

Reducing valves are a common component of hydraulic manifolds, wherein the manifold itself (usually a single piece of shaped metal) can endure significantly higher pressures than the pipes, junctures, and valves that are not part of the manifold.

Come back next time for more on Sequence, Counterbalance, and Unloading valves.