Load control valves - a more detailed view

Tony Raikes of Sterling Hydraulics discusses the applications for load control valves.

Load control valves, also known as over-centre valves, counterbalance valves, hose burst check valves or boom lock valves, are critical for the control and safety of hydraulically controlled machines like manlifts, telescopic handlers, backhoes or anywhere where you need to control the movement of a hydraulically supported load.

They are usually three-port metering elements that combine three valve functions in a single cartridge, ie a relief valve with a reverse flow check valve and a pilot assisted opening feature on the main relief poppet.

In operation they are connected to the loaded port of an actuator, firstly to allow unrestricted input flow via the check valve to raise the load, and secondly, to control the outlet flow via the pilot assisted relief valve to lower the load.

A cross line pilot supply from the actuator's other port is connected to the valve's pilot port to give a feedback signal that causes the valve to modulate the load lowering pressure and so prevent a runaway situation.

Sterling load control valves are suitable for use up to 350 or 420bar with flow capacities up to 300 litre/min.

They are available in various styles, each with their own benefits and suitability to certain applications as detailed below.

The applications the valves are used on tend to have different requirements, the main ones being: If the hose between a loaded cylinder and its directional control were to fail the load would be unsupported and could fall.

Fitting a hose burst check valve between the cylinder and the directional valve will prevent this by locking the cylinder in position and only allowing movement when it is commanded to do so, generally by pressurising the opposite side of the cylinder.

The valve must be fitted either directly into the end cap of the cylinder or as a welded manifold with steel pipes welded to the annulus and full bore ends.

The drillings within the manifold or cylinder must be such that the valve is cross piloted from the cylinder's other port.

Typically the types of machine this arrangement is used on are manlifts or telescopic handlers or anywhere a load is supported on hoses.

Telescopic handlers have hose burst protection fitted on the boom lower, boom retract and fork tilt functions which ensures the operator retains control of the load at all times.

The fork tilt function usually has a vented valve (E6) as the directional valve has a closed centre where trapped pressure would act back in the spring chamber preventing the valve from opening.

Another important application for hose burst protection is on a backhoe when used for craning, in which case the machine has to comply with ISO8643.

This specification effectively means that in the event of a hose failure, with a load suspended from the backhoe, movement must be limited and that when operated dynamically the speed of the service must not increase over the limits set by the specification Sterling has introduced a new generation of control valves specifically designed to meet the ISO8643 criteria.

These valves often require a large degree of damping, as they tend to be too responsive to the induced pressures caused by small movements of the boom.

This can be achieved by incorporating internal hydraulic poppet damping, adding poppet friction or by severely restricting pilot flow to the valve.

These valves are part of our very successful E6 range of valves.

Rated to 350bar and 180 litre/min they are available with 8:1 and 3:1 pilot ratios with a minimal leakage of less than 6 drops per minute and with a range of damping options.

Not all requirements are for cylinders though, and load control valves can be used with motors in motion control and lock circuits to provide dynamic braking, anticavitation and a positive lock on a transmission or winch circuits.

In this application it would be usual to use valves with a higher pilot ratio (minimum 8:1) for efficiency, as the valve needs to be wide open under normal drive conditions but with the valve set to give good dynamic braking.

When used with a closed centre directional valve, the load controls can be combined with other valves to provide anticavitation, make-up and hot oil purge.

A brake shuttle and pressure reducing valve can also be incorporated in a manifold block with the other valves for operating a spring-applied brake.

Ideally the manifold should be gasket mounted directly onto the motor.

For motor spool directional valves a simpler arrangement with dual cross piloted load controls can be employed also with a brake shuttle and pressure reducing valve if required and again directly mounted onto the motor.

Counterbalance (over-centre) valves are used when a load goes over-centre and tends to run away from the pump potentially causing cavitation or a loss of control.

The load control valve is positioned to provide a metering element to the outlet side of the cylinder again piloted from the opposite cylinder port.

If the pilot starts to cavitate then the valve will close allowing the pilot pressure to rise.

In effect the valve will assume a metering position where it generates enough back pressure to maintain its own pilot and the load will always remain under control.

Sterling Hydraulics has specialised in the design and manufacture of hydraulic valves and customised manifold blocks since the 1960s and produces a wide variety of load control valves to suit specific applications.

Pilot assisted poppet valves cover a wide range of flows and are available with various pilot ratios to suit different applications.

As a general guide the higher the pilot ratio the more efficient the load controlling system, however lower pilot ratio valves tend to offer a more stable operating system so it is sometimes necessary to optimise valve selection to suit these needs.

Pilot assisted poppet valves with surge relief valve incorporate an additional relief valve that exhausts to a fourth port from the normal service line port to provide relief protection when the directional control valve port is closed off, or to give additional relieving flow capacity to cope better with surge flows from the load.

Pressure balanced, pilot assisted poppet valves benefit from being independent of the return pressure in the directional control valve port ie they are effectively sequencing load control valves.

They have a special sealing arrangement and a chamber that is vented to atmosphere that enables the sum of the pilot and relief areas to be vented to atmosphere without the need for a fourth port and associated vent line.

They are particularly suited for use with a service line relief valve on the directional control valve, in that the relief setting is not additive to the load control valve setting.

Within this group are damped valves for the control of instability.

This is achieved by adding extra friction to the valve poppet, by adding hydraulic damping inside the valve or by severely restricting the pilot both internally or externally.

For this we have a separate adjustable restricting element.

This element will allow free pilot flow away from the valve to allow the valve to close rapidly but will only allow pilot pressure to build at a very controlled rate.

A relief element is also incorporated to ensure the load control valve operates in cold weather.

Relieving and nonrelieving, pressure balanced, large pilot area assisted poppet valves have a relatively large pilot area that allows the valve to be set to a low pilot opening pressure.

This makes them suitable for certain pilot controlled systems but it is important to note that they will not automatically sense an over-centre runaway condition with out additional circuitry for this purpose.

They have good response to operator control of the pilot signal pressure and so can be used to accurately control the motion of a load.

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