Understanding hydraulic hose technology

Steve Payne, Industrial Hose Territory Manager at Parker Hannifin, explains the importance of hydraulic hose construction and correct specification.

Hydraulics technology has been used throughout industry for many years as an efficient and reliable method of delivering motive power, often at high operating pressures.

Although frequently taken for granted, the hose is one of the essential components in hydraulics, regardless of the application or type of system.

As a result, hose manufacturers invest considerable time and resources in product development, testing and production, to ensure that each hose is constructed to meet the needs of a wide range of operating conditions in industrial, offshore, military, aerospace and mobile applications.

In each case, the key to correct specification is to understand how the hose is constructed and this can potentially affect its installation, use and possible need for maintenance or repair.

There are generally three main elements to both hydraulic hoses, which are used for the movement of hydraulic fluids in power systems, and industrial hoses, which are used for anything from fuel delivery to the continuous transport of bulk fluids.

Working from the interior outwards, there is firstly the core tube, which transports the fluid media; this is then surrounded by a reinforcement and support layer which, in turn, is protected by an outer covering to prevent physical or environmental damage to the hose.

As the only component of the hose that actually comes into direct contact with the media being conveyed, it is essential that the raw materials used for the construction of the core tube are precisely matched to both the media and the application.

Typically, the materials used in hose manufacture are various grades of synthetic rubber, combined with chemical ingredients and blended to produce a rubber compound with the best combination of physical and chemical properties to suit each specific application.

Rubbers compounds that are commonly used include styrene butadiene rubber (SBR), acryloniltrile-butadiene rubber (NBR), chloroprene (CR), ethylene-propylene copolymers (EPM) and ethylene-propylene-diene terpolymer (EPDM), with each offering specific characteristics.

As an example, the increasing tendency for companies to respond to environmental pressures by opting for biodegradable oils rather than mineral hydraulic oils has led to a review of the compounds used for hose construction.

Biodegradable oils contain additives used in their part mineral, part synthetic make-up that can attack conventional hose materials, often causing them to disintegrate in as little as three months, as well as causing similar damage to the rubber seals in valves and other components.

This can, however, be easily prevented by changing the hose material to an inexpensive yet highly durable nitrile rubber compound.

While selecting the right material for the core tube is largely a matter of ensuring the correct chemical compatibility between the hose and media, the reinforcement layer is designed to cope with the physical property of pressure.

Similarly to the core tube, the materials used for reinforcement depend entirely on the application in which they are intended for use.

From cotton and man-made fibres, such as nylon and polyester, through to glass-fibre and ultra-high tensile steel wire, the materials used for reinforcement can vary dramatically.

In some particularly specialised applications, Parker has even used Kevlar (Aramide) on thermo plastic hoses due to its exceptional strength and its ability to resist extremely high operating temperatures.

Steel wire is most commonly used in hydraulic and high-pressure industrial hoses, where its strength and high flex modulus offer distinct advantages over fibre or textile reinforcement, which can kink as the bend radius of the hose is reduced.

As a substitute, a helix wire reinforcement can be used when a hose has to withstand severe bends without flattening or kinking, or when it has to withstand collapse under vacuum conditions inside the hose.

To prevent the hose from such collapse, a combination of spirally wound wire and reinforcing material, often cotton, are commonly used.

In essence, the steel helix maintains the shape of the tube, while the reinforcing material prevents the hose blowing through the gaps in the helix.

A more widely adopted form of hydraulic hose reinforcement is the braided hose.

As an example of its widespread use, Parker produces around 22 million metres of braided hydraulic hose each year, compared with just 2.2 million metres of spiral hose.

Categorised by the number of layers of reinforcement rather than the actual wire itself, braided reinforcement is available as either "single wire" or "two wire".

Accordingly, a two-wire hose would have two layers of braided wires separated by a thin layer of insulation rubber to prevent the wire layers rubbing together.

A key aspect of the braiding process, whether one or two wire, is the actual angle at which the wire is wound onto the tube, with the angle being measured from the longitudinal line of the hose.

The ultimate goal is to achieve a braid angle known as the "neutral angle of twist", which in practice is 54.7 degrees.

This angle or position is where the hose will always try to return to a straight line and therefore allows the hose to straighten under pressure.

Achieving the neutral angle of twist prevents undesired effects but can also be modified to impart specific properties to the hose.

For example, a 53-degree braid angle produces a hose that gets longer under pressure, whereas a 56-degree braid results in a hose that swells and shortens under pressure.

The final and most visible element of the hose is the outer covering, which is designed fundamentally to protect the reinforcement layer from abrasion and corrosion.

Although hoses with stainless steel wire reinforcement often do not need covers, other types generally employ various synthetic rubber coverings, which are again carefully selected to match the operating environment: silicon-free materials, for example, are typically used in car plants.

Regardless of how well a hose is constructed it can only be as good as its connection to up and downstream equipment.

Choosing the correct fitting is just as important as selecting the right hose.

Parker, for instance, pioneered the development of the "No-Skive" hose, which has a cover layer of uniform thickness.

This type of hose can be used with crimped fittings without having to peel back, or 'skive', the cover to make sure the fitting bites into the reinforcing layer.

With companies placing increasing emphasis on improving uptime, productivity and profits, factory and plant managers might be forgiven for preferring the "fit and forget" approach when sourcing equipment components.

The continuous innovation in hose design and manufacture means that these reliable, well-engineered items of equipment will operate for lengthy periods without undue maintenance, ultimately leaving managers to concentrate on their core business of running the factory or plant as efficiently and economically as possible.

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