Liquid asset

Despite being a relatively mature technology, statistics show that the use of hydraulics is gaining in popularity for a wide variety of control applications where high torque or power is needed.

Hydraulic systems — where force is transmitted via liquid, usually water or oil, under pressure — comprise a number of elements. These include a drive, motor or pump at the front end, often leading to a cylinder, via an array of components including reservoirs, valves, filters and hoses.

Applications can be very large-scale. For example, in Las Vegas hydraulics are used as part of the show at the MGM Grand Hotel. In one performance, the Cirque du Soleil's 'KA', a massive 7.5 x 15m stage platform (known as the 'sand cliff deck') must be raised above 'the abyss' — the hotel basement several floors below.

The hydraulic system behind this 2m-thick moving platform provides 4,500kW of power, enabling it to be positioned quickly enough to integrate seamlessly into the show.

The system uses four 22m-long Parker Hannifin single-rod cylinders — believed to be the world's longest — with one as a spare. When the cylinders are lifting the stage deck they move more than 6,000 litres of oil a minute, at 0.6m/sec. In addition to this vertical lifting, the set-up also incorporates two hydraulic servo valves, which are used to control the tilt of the stage.

Fluid power, with its ability to deliver enormous amounts of energy, is also used in construction. In Brussels, for instance, a 140m-long railway bridge was placed in position using a hydraulic system. The bridge, which weighed more than 1,600 tonnes, could not be built in situ, so it was assembled in parts from its manufacturer (Victor Buyck Steel Construction) near to where it was required. For such a project, where huge structures are being moved unevenly, stresses could be large enough to cause structural damage.

In this case Enerpac built a system using hydraulics (with an associated stress measurement system and control computer) to apply support where required. The system contained 32 hydraulic cylinders fed from a central pump with a fluid pressure of up to 700 bar. As the bridge was moved into place on multi-axle platform wagons — again hydraulically controlled — the cylinders made sure the bridge was at the correct height and angle to minimise damage.

Another recent application was the installation of equipment to allow new ferries with an extra level to operate between Southampton and the Isle of Wight.

The Red Funnel Group, ferry operator and owner of the new ships, needed to upgrade its vessels to meet ever-increasing demand for the service. The ferries unload cars via a 30m linkspan bridge to land as part of the company's roll-on, roll-off (ro-ro) service. Due to the movements of ships caused by tides, the linkspans have to be able to cope with considerable vertical movement and uneven loads.

Bosch Rexroth supplied, installed and commissioned a system using hydraulic cylinders and variable displacement power units.

The upgraded system comprises two hydraulic cylinders either side of the linkspan, each with its own power unit. The technology is auto-levelling and also has a safety circuit to hold the linkspan in a 'failsafe position' in the event of the bridge losing contact with the ferry, or unexpected movements.

Statistics from the British Fluid Power Association (BFPA) show a positive growth rate over the past few years. The latest figures available show that growth in hydraulic shipments was more than five per cent year-on-year.

With their high pressures and large power transmission characteristics, there is an obvious need to ensure hydraulic systems are used safely. One area attracting particular attention is that of hose safety.

An estimated 80 per cent of hydraulic hose failures result from external damage caused by pulling, kinking, crushing or abrasion, or by dynamic fluctuations in the fluid carried in the hose.

If a hose breaks there are consequences beyond expensive downtime to consider. These include clean-up and replacement of fluids and damage to other components, as a result of loss of pressure or contamination. The costs can be considerable.

The importance of hose safety is acknowledged globally. NASA's Glenn Research Centre safety manual, for example, devotes a chapter to setting out regulations for high-pressure hose. Here in the UK, The British Fluid Power Distributors Association (BFPDA) has produced guidelines aimed at ensuring that the number of hose failures are minimised.

Director Ian Morris said: 'Reputable suppliers and users of fluid power equipment know the importance of hydraulic hose integrity and safety. They are aware of the havoc that hose failure can cause. This might be injury to staff, harm to valuable plant, or environmental damage, which is expensive in legal liability terms in today's tough regulatory climate. There are also fluid clean-up costs.

'The main players — including Sterling Hydraulics, Brevini, Rotary Power, Challenger Hydraulics, Linde Hydraulics and Eaton — already have rigorous quality procedures in place, ensuring that hydraulic hose assemblies are specified and made up correctly from the right components. Full audit trails will be in place to prove it.'

On the safety issue, Terry Light, director of Rotec Hydraulics, said: 'As a result of the general increase in working pressures, the drive for greater reliability and most importantly, the safety of personnel working in the vicinity of pressurised systems, the old-time practice of repairing used hoses has become totally unacceptable.

'Safety and cost implications of the failure of a repaired hose assembly are far outweighed by the reliability gained by fitting a replacement using component parts designed and matched together and rigorously tested.'