Product category:
Hydraulic Components
News Release from: Sterling Hydraulics | Subject: Accumulator charging blocks
Edited by the Engineeringtalk Editorial
Team on 02 May 2005
Putting the brake on off-road equipment
A new range of packaged accumulator charging blocks aims to simplify braking systems for off-road machinery.
Even though they're not normally driven by Jeremy Clarkson, heavy earth moving plant, forestry, construction and similar off-road machines take quite a bit of stopping These vehicles are too large for conventional master cylinder braking systems and rely on full power braking from their hydraulic systems
This article was originally published on Engineeringtalk on 20 Aug 2002 at 8.00am (UK)
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Safety dictates that these full power braking systems have to be fitted with an accumulator, 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's needed.
Sterling Hydraulics, the UK's leading designer and manufacturer of hydraulic cartridge valves, has developed a new range of packaged accumulator charging blocks aimed at simplifying these systems.
The vehicle's hydraulic pump, operating up to 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.
Further reading
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Flow restrictor is a sensitive type
The Sterling Hydraulics J02A2 is a needle-type flow restrictor valve with full 420bar pressure rating providing high flow capacity from a small cavity.
Shuttle valves for transmission applications
The K3A125 is a high-flow shuttle valve that is mostly used in transmission systems as a purge valve where a hydraulic parking brake is necessary.
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 setpoint, the pump has to be brought on load to recharge the accumulator and then taken off load when the accumulator pressure reaches an upper setpoint.
The differential between these two pressure setpoints - the "scan" - together with the size of the accumulator and the rate at which 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 charge rate is too high flow will be taken away from other hydraulic services, 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 not only be expensive; it would make the system complicated and susceptible to more failure modes than the one it is there to protect.
The new accumulator charging blocks from Sterling Hydraulics incorporate a unique new concept unloader valve which detects low pressure in the accumulator and gives an hydraulic signal to select supply from the pump.
The valve is spring loaded allowing the reload setpoint 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 which allows the charge rate to be set to suit the system requirements.
The unloader valve, scan relief valve, and check valve are all cartridge style valves and are assembled in a standard manifold block.
Additionally, accumulators for this type of application are almost invariably qualifying vessels under the Pressure Equipment Directive, consequently, the block has to be fitted with a compliant relief valve.
Unlike most relief valves, the approved 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 be fitted the wrong way round.
Sterling Hydraulics manufactures the valve at their Crewkerne factory and it carries the requisite CE mark which attests documented design, traceable materials, full testing at works and externally audited QA.
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.
"Sterling Hydraulics has a long history of designing and manufacturing custom manifold blocks, using our own cartridge valves and complementary valves from other manufacturers", says David Lyes, Sales Manager.
"We wanted to combine this skill with our valve design expertise to produce a stand alone block to control accumulator charging".
"The unloader valve is a completely new development for us and the test results have been excellent".
Lyes is referring to a series of tests carried out by Sterling Hydraulics' development engineers.
In these tests the block was cycled between an accumulator load pressure of 100bar and an unload pressure of 140bar with 75 and 25bar loads on an auxiliary service.
"The results showed reliable and reproducible unload and reload performance", says Lyes.
"The operation was consistently smooth with no pressure spikes and there was no detectable change with varying operating temperature".
Because all the valves used in the accumulator charging blocks are cartridge type, 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 like fan motors, lifts, rams, stabilisers and so on.
In this way all the traditional advantages of cartridge valves in manifolds can be introduced to the system such as reduced leak paths and less pipe work, ability to fit tight space constraints and improved serviceability.
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