What is an Unloading Valve? - Types, Principle

In certain specific applications, there is a need for completely unload pumps flow to the tank instead of relieving it over a relief valve. This can be done using unloading valves.

If you are looking for more details, kindly visit Huade Hydraulic.

Unloading Valve

Image Courtesy: Mechatronics Control

Let us imagine a case of a system, where there are two pumps. Both pumps are required to perform specific work.

After completion of the job, there is a need to maintain the required pressure in the system by operating only one pump and the delivery of the other pump must be sent to the tank at low pressure. This job is done by an unloading valve.

Principle

An unloading valve is a pressure control valve that works on the principle of the hydraulic force as opposed to a spring force.

When pressure builds to the point, where hydraulic force is greater than that of the spring force, then the valve spool is shifted.

Purpose

The unloading valves are used for relieving the extra pressure in a system, that is at low pressure and connecting it to the tank when the delivery of the pump is not used.

The unloading valve may be controlled by a special cock or a pilot valve.

Types of Unloading Valves

The unloading valves are classified into two types.

They are as follows.

• Direct operated pressure Unloading Valve
• Pilot operated pressure Unloading Valve

Direct Operated Unloading Valve

A direct-acting unloading valve consists of a spool held in the closed state by a spring.

The spool blocks flow from the inlet to the tank port under normal operating conditions.

High-pressure fluid from the pump exerts a force against the pilot as it enters from the external pilot port.

When the system pressure increases to the force of the spring setting the fluid bypasses the tank.

When the pressure goes above the spring setting, the spool opens fully to dump the surplus fluid into the tank at little or no pressure.

Pilot Operated Unloading Valve

Unloading spool is the addition in a pilot-operated unloading valve, it is not found in the pressure relief valve.

Without the unloading spool, this valve would function the same as any pilot-operated relief valve.

Pressure buildup within the pilot section would open a certain amount of fluid flow to the tank. It makes unbalances the poppet, allowing it to open and relieve excess pump flow to the tank.

Unloading spool receives a signal through the remote-pilot port when the pressure in the working circuit goes more than its setting.

Simultaneously, fluid pressure on the spring-loaded ball in the pilot section starts to open it.

Pressure drop on the front side of the unloading spool brings down back force and pilot pressure from the high-pressure circuit forces the spring-loaded ball completely off its seat

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Double Pumps with Unloading Valve

The primary use for an unloading valve is associated with a dual pump circuit. A high-pressure, low-flow pump along with a low-pressure, high-flow pump is used double pump operated circuit.

An un-loading valve is used with two pumps, create high discharge flow, and the other one able to create a high line pressure with low oil discharge.

Deliveries of both pump are discharged into the circuit until the pressure approaches the setting of the unloading valve.

At this stage, fluid from the high pressure-low flow pump is passed through CV1 to the cylinder through the directional control valve but not allowed by CV2.

In a particular application, sheet metal punch press in which the hydraulic cylinder must extend rapidly over a length with low-pressure but high-flow requirements. This occurs under no load.

However during the punching operation for short motion, the pressure requirements are very high, but the flow requirements are low as the cylinder travel is small. 

It eliminates the requirement of having a very expensive high-pressure, high-flow pump.

As and when the punching operation initiates, the increased pressure opens the unloading valve to unload the low-pressure pump.

The purpose of relief valve is to protect the high-pressure pump from high pressure buildup at the end of cylinder stoke and when the directional control valve (DCV) is in its spring-centered mode.

The check valve protects the low-pressure pump from high pressure due to the high-pressure pump, which occurs during punching operation, at the ends of the cylinder stroke, and when the directional control valve (DCV) is in its spring-centered mode.

Advantages

• The pressure of unloading valve is typically less of the relief valve.
• The oil getting hot can be avoided by re-circulation with second pump in circuit. 
• Life span of O-rings can be extended by avoiding oil becoming hot.
• Less energy consumption.

Reference: Fluid Power Control Systems by MD Faiyaz Ahmed.

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We all know distributors are a valuable resource for end-users of fluid power products, often providing local, on-site, customer support. Closer to end-users than manufacturers, their customers should be able to rely on that local support.

To help provide that support, knowledge about the products they sell helps to meet the support needs of their customers/end-users. Fluid power distributors can take what they learn about customers&#; specific pumps, motors, and valves, and put that knowledge to use for their clients. If you are a distributor and your end-users can benefit from knowing more about the products you offer, this article is for you.

As pumps and motors are responsible for supplying power and energy within a hydraulic system, valves are the mechanical devices used to control that power by regulating the flow of fluid.

They can be used to redirect pressurized fluid, control the level of flow to a certain area, or completely close a line.

Although there are dozens of varieties that all serve different purposes, in this article, we will focus on these three major types of hydraulic valves:

• Pressure Control Valves
• Flow Control Valves
• Directional Control Valves

Each type has its own functions within a fluid power system and applications in which they provide the ideal result. Each hydraulics brand provides various configuration choices based on valve type. Acquainting yourself with these differences can be useful when offering clients support on valve-related issues.

If you are looking to provide guidance when your customer has a question or a problem, or if they&#;re asking you to consult on purchase decisions more confidently, we recommend starting with basic knowledge of how pressure control valves, flow control valves, and directional flow valves operate.

Pressure Control Valves

Hydraulic pressure control valves are used to regulate the fluid pressure that is passing through hydraulic systems and prevent rises in pressure that may result in system problems. They are also used to maintain a set pressure in a part of a hydraulic circuit.

Different types of pressure control valves include:

Each type has its own functions within a fluid power system and applications in which they provide the ideal result. Each hydraulics brand provides various configuration choices based on valve type. Acquainting yourself with these differences can be useful when offering clients support on valve-related issues.If you are looking to provide guidance when your customer has a question or a problem, or if they&#;re asking you to consult on purchase decisions more confidently, we recommend starting with basic knowledge of how pressure control valves, flow control valves, and directional flow valves operate.Hydraulic pressure control valves are used to regulate the fluid pressure that is passing through hydraulic systems and prevent rises in pressure that may result in system problems. They are also used to maintain a set pressure in a part of a hydraulic circuit.Different types of pressure control valves include:

• Pressure relief valves
• Pressure Reducing valves
• Sequence valves
• Counterbalance valves

Hydraulic relief valves limit system pressure to protect components. The valves also limit the hydraulic system&#;s maximum output force and control the amount of heat generated by balancing the hydraulic force with an adjustable spring force.

Pressure reducing valves are used to balance hydraulic circuit pressure with pressure in the main circuit. When the outlet pressure reaches a preset level, the valve opens, a piston moves to release pressure, and it keeps the outlet pressure constant.

Sequence valves are used to control the sequential operation of actuators. If the inlet pressure exceeds a preset level, they add pressure to the outlet side. A check valve allows for this directional control.

Counterbalance valves are used to maintain hydraulic pressure or load backpressure on a cylinder by releasing flow levels to keep pressure constant. Like sequence valves, they use a check valve to provide a counterbalance of flow, lifting the cylinder up to freely pass.

Flow Control Valves

Hydraulic flow control valves do what they say: They control flow rates in hydraulic circuits, with an end goal of controlling speed elsewhere in a fluid power system.

In hydraulic systems, these valves are used to regulate flow rates to actuators, such as motors and cylinders, and with it the speed of those components.

Flow rate also determines rate of energy transfer at any given pressure. A common physics concept is used to determine this transfer:

Actuator force x distance traveled = work done on load

This equation shows that the actuator force, multiplied by the distance through which it moves (stroke), equals the work done on the load. The energy transferred must also equal the work done, and speed determines the energy transferred.

In other words, regulating speed is the primary function of flow control valves because:

Energy transfer must equal work done, and is determined by multiplying force times stroke
Speed determines the amount of energy transferred
Flow rates determine the speed of an actuator

There are a variety of flow control valve designs, each intended for specific applications. The most common types include ball, diaphragm, needle, and butterfly.

Directional Control Valves

Directional control valves have three main functions: to stop fluid flow, to allow fluid flow, or to change direction of fluid flow between hydraulic cylinders and motors.

They typically consist of a sliding spool inside a cylinder, which restricts or permits fluid flow to control the fluid path. They can also consist as a poppet to control flow, which works in the same way.

Directional control valves can stop the flow suddenly and are sometimes referred to as either &#;switching&#; or &#;bang-bang&#; valves for the sound they make when operating.

There are two other types of directional valves that can control fluid flow in more complicated hydraulics systems:

1. Proportional valves -Allow infinite positioning of spools and flow volumes for more complex circuits
2. Servo valves - Operate with high accuracy, repeatability, low hysteresis, and high frequency response for sophisticated loop systems (and are often more expensive than proportional valves)

More Product Knowledge to Steer Satisfied Customers

In summary, hydraulic valves provide the control needed to keep hydraulic systems working. They act as the steering wheel to guide the moving parts of a functional hydraulics system.

Hydraulic relief valves limit system pressure to protect components. The valves also limit the hydraulic system&#;s maximum output force and control the amount of heat generated by balancing the hydraulic force with an adjustable spring force.Pressure reducing valves are used to balance hydraulic circuit pressure with pressure in the main circuit. When the outlet pressure reaches a preset level, the valve opens, a piston moves to release pressure, and it keeps the outlet pressure constant.Sequence valves are used to control the sequential operation of actuators. If the inlet pressure exceeds a preset level, they add pressure to the outlet side. A check valve allows for this directional control.Counterbalance valves are used to maintain hydraulic pressure or load backpressure on a cylinder by releasing flow levels to keep pressure constant. Like sequence valves, they use a check valve to provide a counterbalance of flow, lifting the cylinder up to freely pass.Hydraulic flow control valves do what they say: They control flow rates in hydraulic circuits, with an end goal of controlling speed elsewhere in a fluid power system.In hydraulic systems, these valves are used to regulate flow rates to actuators, such as motors and cylinders, and with it the speed of those components.Flow rate also determines rate of energy transfer at any given pressure. A common physics concept is used to determine this transfer:This equation shows that the actuator force, multiplied by the distance through which it moves (stroke), equals the work done on the load. The energy transferred must also equal the work done, and speed determines the energy transferred.In other words, regulating speed is the primary function of flow control valves because:Energy transfer must equal work done, and is determined by multiplying force times strokeSpeed determines the amount of energy transferredFlow rates determine the speed of an actuatorThere are a variety of flow control valve designs, each intended for specific applications. The most common types include ball, diaphragm, needle, and butterfly.Directional control valves have three main functions: to stop fluid flow, to allow fluid flow, or to change direction of fluid flow between hydraulic cylinders and motors.They typically consist of a sliding spool inside a cylinder, which restricts or permits fluid flow to control the fluid path. They can also consist as a poppet to control flow, which works in the same way.Directional control valves can stop the flow suddenly and are sometimes referred to as either &#;switching&#; or &#;bang-bang&#; valves for the sound they make when operating.There are two other types of directional valves that can control fluid flow in more complicated hydraulics systems:In summary, hydraulic valves provide the control needed to keep hydraulic systems working. They act as the steering wheel to guide the moving parts of a functional hydraulics system.

And whether they are used to control flow by redirecting pressurized fluid (pressure control valves), controlling the level of flow and speed (flow control valves), or completely stopping the directional flow (directional control valves), knowing about different types of valves and their intended function can be a great tool to help you support your customer&#;s individual needs.

Knowing the basics of different components and how they work is the key to offering this guidance, and to provide a valuable resource to your customers / end users.

Knowing the basics of different components and how they work is the key to offering this guidance, and to provide a valuable resource to your customers / end users.

Need more help? Call the HPS Customer Service team at (888) 477- for assistance. We&#;re here to help with your fluid power questions 8 &#; 6 ET, Monday through Friday.