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Product category: Hydraulic Components
News Release from: Sterling Hydraulics | Subject: Custom hydraulics
Edited by the Engineeringtalk Editorial Team on 07 March 2005

Hydraulic solutions meet on- and
off-road demands

The challenge of engineering is to find innovative solutions to new problems, and Sterling Hydraulics specialises in the design of novel hydraulic solutions.

The challenge of engineering is to find innovative solutions to new problems Sometimes the solution involves the development of a new product but, more often, it uses existing products in a novel way

Sterling Hydraulics has been working closely with customers to find the best solutions for their specific hydraulic problems since the 1960s and, with operations in Europe and the USA, is now recognised as a world leader in the design and manufacture of screw-in hydraulic cartridge valves and manifold systems.

Heavy earth moving plant, forestry equipment, construction vehicles and similar off-road machines are too large for conventional master cylinder braking systems, and rely on full power braking from their hydraulic systems.

Safety dictates that these full power braking systems have to be fitted with an accumulator for braking, and many are fitted with two, to give independent braking to the front and rear wheels.

As a result, the associated hydraulic control systems can be complex, incorporating a number of individual valves to ensure that hydraulic pressure is available at the right place when it is needed.

Sterling Hydraulics' brief was to develop a new range of packaged accumulator charging blocks aimed at simplifying these systems.

The vehicle's hydraulic pump, operating at typically 200bar, provides all the power necessary for braking, but only as long as the engine is running.

Loss of engine power would lead to complete brake failure with disastrous results.

An hydraulic accumulator - essentially a pressure vessel charged with nitrogen - that is pressurised by the hydraulic pump can retain sufficient hydraulic fluid to execute several braking operations, even after the engine has been stopped.

Accumulators are an essential safety feature, but ensuring that they are always charged to the right pressure is a challenge for hydraulic engineers.

When the pressure in the accumulator falls below a lower set point, the pump has to be brought on load to reload the accumulator and then taken off load when the pressure reaches an upper set point.

The differential between these two pressure set points - the "scan" - together with the size of the accumulator and the rate at which the pressure discharges, determines the number of loading cycles for the pump.

The accumulator charge rate is also important because the higher the charge rate the more impact there is on the rest of the hydraulic system.

If the charging rate is too high then the accumulator charging can starve the rest of the system, causing a loss of hydraulic supply to cylinders and motors.

The situation is further complicated if there are two accumulators driving dual braking systems.

The objective of independent braking for front and rear wheels is simple: in the event of a catastrophic hose failure on either system, the remaining system will still provide braking.

But this means that the two accumulators have to be pressurised individually otherwise the accumulator on the intact system would simply discharge to the failed one.

Providing individual pressure control valves and pump loading controls for each accumulator would be not only expensive but would make the system complicated and susceptible to more failure modes than the one it is there to protect.

Sterling Hydraulics developed a new accumulator charging block that incorporates a unique unloader valve which detects low pressure in the accumulator and gives an hydraulic signal to load the pump.

The valve is spring loaded allowing the reload set point to be adjusted manually.

The unloader valve vents through an adjustable scan relief valve, which allows the unload pressure to be set manually and independently of the reload pressure.

Unlike most conventional valves, this means that the system scan can be adjusted independently of any other system settings, so that the operation of the accumulator can be optimised in terms of the frequency of pump loading cycles and valve operations.

In the case of normal production valves, the two control pressures can be factory set to prevent tampering but, where system operating conditions may change, for example in development projects, they can be left accessible for the user to adjust them.

Further flexibility is provided by an interchangeable flow control orifice that allows the charge rate to be set to suit the system requirements.

The unloader valve, scan relief valve, check valve and orifice are all cartridge elements and are assembled in a standard manifold block.

Because accumulators for this type of application are almost invariably larger than one litre and have a pressure-volume ratio greater than 50bar/litre, they are qualifying vessels under the Pressure Equipment Directive PED 97/23/EC.

Consequently, the block has to be fitted with a compliant relief valve.

Unlike most relief valves, the Sterling Hydraulics cross line cartridge valve is bidirectional, so it can be retrofitted into any common cavity manifold block, irrespective of direction of flow.

In short it can't be fitted the wrong way round.

Sterling Hydraulics manufactures the valve at its UK factory and it carries the requisite CE mark which attests documented design, traceable materials, full testing at works and is to externally audited QA procedures.

The charge control block is available in two versions: one generating a pressure signal that controls an integral diverter valve for fixed displacement pump systems, and one generating an analogue pressure signal that can be fed directly to a variable displacement load sensing pump.

Users can specify not only the scan and charge rate but also port sizes and threads of the accumulator-charging block to suit their own application.

Both versions of the accumulator-charging block can be fitted with Sterling Hydraulics' new inverse shuttle valve for double accumulator applications.

This is a precision three port shuttle check valve which automatically opens the lowest pressure accumulator to the charging circuit and, in doing so, simultaneously isolates the higher pressure accumulator.

In the event of a catastrophic failure, like a ruptured hose, to either braking system, the inverse shuttle valve automatically opens the failed system side to the charging circuit.

Although this means a potential loss of hydraulic fluid from the failed accumulator, the rate of loss of fluid from the system is low because it is controlled by the orifice to a value that has little impact on the system pressure.

More importantly, the inverse shuttle valve closes the intact circuit, retaining its accumulator pressure and, therefore, its braking capability.

Because all the valves used in the accumulator charging block are common cavity cartridge versions, it is quite an easy task to incorporate the accumulator charging circuit into a custom designed manifold block.

In this way it can be an integral part of an overall control system including other machine functions for lift-rams, stabilisers and so on.

The custom manifold block is a good example of how standard products can be used to create a cost effective solution to an hydraulic engineering problem.

The Sterling Hydraulics standard range of screw-in cartridge valves includes two and three position solenoid valves, pressure and flow control valves, check valves, diverter valves and shuttle valves as well as manual valves.

In fact all the components necessary to create complex control systems.

Connecting individual components together by hydraulic hoses is untidy, space consuming and prone to leaks because of the many joints involved.

The custom manifold block brings all the components together in a single block of steel or aluminium in which flow channels or galleries are machined.

The individual valves simply screw into precision engineered cavities and control the flow of hydraulic fluid in the galleries.

The block has ports for connection to the hydraulic supply pump and to hydraulic motors, actuators, cylinders, and so on that have to be controlled.

The ports are machined to meet the size and thread requirements of individual customers.

It is the hydraulic equivalent of a printed circuit board.

side from the benefits of compact size, this monolithic approach means that individual valves can be easily removed for servicing or replacement with spares, which, because the valves are standard products, are readily available from stock.

So the custom manifold block is a solution that Sterling Hydraulics offers on a routine basis to solve unique problems - problems like the one that confronted Gibbs Technologies when it was developing its highly desirable Aquada amphibious car.

One of the main problems with amphibious cars in the past has been the drag created by the road wheels when the vehicle is on water resulting in reduced speed and manoeuvrability and increased fuel consumption.

The challenge that Gibbs presented to Sterling Hydraulics was to provide components to meet the requirements of the Gibbs patented system that would allow the wheels to retract when in water, but to be capable of providing a conventional standard of ride and handling on the road.

The result is a unique retracting suspension system that, at the touch of a button on the dashboard, transforms the Aquada from a sleek but fairly conventional-looking road car to a high performance boat.

Once the button has been pressed to select "water mode" the Aquada's sensors check to see if the car is afloat.

Only then does the logic circuit allow hydraulic pressure to transfer the drive from the road wheels to the water jet pump that pushes the vehicle through water.

At the same time, pressure on the hydraulic struts swing the wheels in an upwards 90-degree arc to retract them into the wheel housing above the water line, but clever design leaves just enough of the bottom of the tyres protruding from the sides of the hull to act as mooring fenders.

The suspension provides conventional spring dampening with variable ride height for dry-land driving.

Together with the retraction mechanism, this requires seventeen hydraulic control valves.

Sterling Hydraulics incorporated them into five precision-engineered manifold blocks, one on each wheel arch and one central master, machined from aluminium to minimise weight and maximise corrosion resistance.

The bidirectional poppet valves, that switch the hydraulic flows from gallery to gallery within the manifold blocks, are standard products from Sterling Hydraulics' range of cartridge valves, which means ease of maintenance and rapid spares availability.

But Sterling Hydraulics did have to produce a special solenoid valve to handle the unusually high flow rate of hydraulic fluid and this was designed and manufactured in its Chicago facility.

So off road or on, Sterling Hydraulics can offer practical solutions to a wide variety of engineering problems.

Using standard components from their extensive product range, the solutions are reliable and cost effective with safety built in as part of the design.

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