Controlling power

Lighter and longer extendable booms means they become increasingly difficult to control. Mark Venables
reports on the radical approach to technology in the hydraulic systems and load control valve design.

Increased pressure in the European mobile engineering sector over the past 20 years, plus the incorporation of electronics, are two of the trends that have permeated the latest round of hydraulic product releases.

Hydraulics experts point out that in the 80s the trend went from 210 bar up to 350 bar, but since the late 90s this has increased to 420 bar — and in some applications 450 bar.

Sterling Hydraulics’ sales and marketing director David Symes explained that this allows OEM design engineers to reduce the size and weight of the hydraulic actuators and therefore permits more payload to the vehicle, or extended booms on manlifts, cranes, concrete pumps or telescopic handlers.

Because the extendable booms on this type of equipment are now lighter and longer, the control of their movement becomes increasingly difficult.

‘This has required the development of new technology in the hydraulic systems — and especially in load control valve design,’ said Symes.

‘Specific valves are required for these applications with additional damping control and features to create smooth movement under difficult load situations.’

Sterling has taken a  new approach to this problem with the introduction of a range of load control valves. These are damped to control instability by adding extra friction to the valve poppet, using hydraulic damping inside the valve or by severely restricting the pilot, both internally and externally.

‘For this the valves have a separate adjustable restricting element to allow free pilot flow away from the valve. This means the valve can close rapidly, but will only allow pilot pressure to build at a very controlled rate,’ said Symes.

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

Accumulators are in common use in heavy mobile plant and are typically used in the braking systems of haul trucks and other wheeled vehicles. The standard design requires that accumulators are recharged with pressurised oil after vehicle braking, and valves providing this function have been around for many years.

After taking a fresh look at these design requirements and following extensive trials, Sterling has introduced the Accumulator Charge Valve System comprising two cartridges, which can be built in to a dedicated Accumulator Charge valve block or be part of a larger composite valve block incorporating other vehicle functions such as steering systems, cooling fan speed control and tipping control.

Many existing valves provide the basic functions, but often have undesirable characteristics inherent to the design. These include pressure spikes when switching that shocks the circuit. This in turn can be uncomfortable for the vehicle driver. Some designs can also be unreliable — especially with wide variations in oil temperatures.

An extreme high-pressure example — 700 bar — was utilised to deliver safe, reliable and precise power in challenging environments in the laying of an 80km string of sub-sea pipeline anchor off Australia’s north-west coast.

The hydraulic system was custombuilt by Enerpac for two anchor feed beds used to lift, position and lower the 32- tonne anchors by shipboard crane to the ocean floor. Reliability and accuracy was integral to the roll-out of the string of anchors — weighing a total of 28,800 tonnes— because their correct positioning ensures stability of the new 42in second trunkline from the Woodsideoperated Goodwyn and Rankin gas condensate fields.

Four RR308 double-acting cylinders were selected to provide a total of 120 tonnes of lift capacity for each of two anchor feed beds (each cylinder had a 30- tonne capacity on push function, five tonnes on retraction and a 209mm stroke). The cylinders, designed for high-cycle industrial uses, were powered by an Enerpac PAM-9808N air pump with manifold incorporating valving to ensure fine control of advance and retract functions, as well as safety shutdown capacity.

At mobile solutions manufacturer Bonfiglioli it is the integration of electronics into hydraulic devices that is driving its innovation.

Cheshirebased Bonfiglioli UK’s John Adair said: ‘There are increasing trends towards more electronic componentry that corresponds with Bonfiglioli’s global mechatronic policy. Items such as load-sensing pumps are leading to more efficient use of power.

‘In the excavator market, for example, more customers are responding to the needs of the Kyoto agreement and are specifying tier 2 and tier 3 engines to give lower emissions, reduced noise and greater efficiency. Despite this, pressure levels are unlikely to increase due to the attendant higher costs of build and maintenance associated with the material strengths of machine components.’

Hydraulic lifting cylinders, supplied by Bosch Rexroth, form part of an installation at a pair of high-level linkspan bridges at Southampton and East Cowes.

Isle of Wight ferry operator Red Funnel has extended and refurbished two vessels, necessitating an additional roll-on/roll-off linkspan level at both ports, as well as the refurbishing of existing hydraulics lifting equipment.

At East Cowes, the original linkspan was operated by two hydraulic cylinders mounted on an overhead gantry with a purpose-built machinery room and operator kiosk located alongside. The additional linkspan, designed to connect with the new upper car decks, is now mounted over the original, with a concrete bankseat constructed as an abutment for the linkspan hinge and a fixed cradle on the original linkspan to support the front end.

This revised design increased the load on the cylinders, so Rexroth was required to increase the capacity of the hydraulic system and install a new control panel, without disrupting ferry operations.

Four cylinders, two each side, now support the two-tier linkspan, which are operated by two power units housed in a bigger machinery room. In addition to refurbishing and refitting the existing equipment Rexroth supplied two 280mm bore, 125mm rod, 5.1m stroke hydraulic cylinders, installed a new hydraulic power pack and connected all four cylinders to the tandem power units in a revised configuration.

The company also incorporated an existing diesel-powered generator into the system to provide emergency standby power, and linked together three hydraulic fluid reservoirs with a large balance line, complete with separate isolators.