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Thinking beyond the simple black rubber pipe

13 November 2015

The humble hydraulic hose has seen many improvements in recent years, and a significant amount of advanced engineering has gone into today’s high performance products. Luca Pozzi reports.

The key influencing factors that have driven the development of hydraulic hoses fit for the 21st century are the increasingly challenging and demanding environments in which they are required to work – and work effectively and maintenance-free - for long periods at high pressures and temperatures. Pressures of 400bar and temperatures ranging from -57°C to +125°C are not uncommon.

Equipment manufacturers and end-users quite rightly do not expect hydraulic hoses to be the weak link in their systems. When choosing a hose, there are some essential features to look out for if reliability is to be maintained and downtime minimised. Users need to select the right hose for their specific application: does it need to be heat, ozone or weather resistant? Does it need to be resistant to chemicals, oil, fire, abrasion or ultraviolet (UV) radiation? Does it need to fit into tight spaces and/or be capable of withstanding some substantial bending?

If abrasion is a problem, hoses are available with a choice of covers that eliminate the need for additional protective sleeves. Some proprietary products offer significant abrasion protection (as much as 450 times that of a standard rubber-covered hose), as well as ozone resistance. Where high pressure performance is needed, then ‘no-skive’ spiral hoses should be considered. Skiving is what happens when part of the hose cover and/or inner tube is removed or shaved before hose ends are attached. A no-skive design simply means that the hose and fittings are designed to work without any skiving being required, so the hose core is not exposed to possible damage and hose assembly is much faster.

Spiral hoses were initially created for use in large, off-road speciality equipment where there are extremely high impulse pressures. Hydrostatic drives using multiple wire spiral hoses are now widely used in earthmovers and lawn tractors. Medium pressure hoses usually have a single or double wire braided construction, whereas most low pressure hoses are made of a much more flexible braided textile for use with ethylene glycol anti-freeze, hot lubricating oil, petroleum-based fluids and diesel fuel, as well as air and water.

However, it is the inner part of the hose that makes all the difference. The inner tube of a hose can be constructed from a range of rubber compounds, each of which reacts differently to the duty fluid. Among the compounds used are PKR rubber, which is resistant to petroleum-based fluids as well as ozone and weathering. Synthetic rubber offers excellent resistance to petroleum-based and environmentally friendly fluids, as does EPDM rubber, which is also highly resistant to phosphate ester fluids and dry air. Other compounds commonly used include Nitrile (NBR) and chlorine-free NBR.

The layline
An important element of any hose is the layline, which is the printing and illustration that can be seen along the length of the hose. Depending on the hose, laylines can be applied in various ways – using ink, a mylar tape or by embossing. Making it easy to identify and re-order a hose, information on the layline will include a unique part number and details of the manufacturer as well as the inside diameter of the hose, its maximum working pressure, the industry standards it meets plus the date and place of manufacture.

The layline will also include industry performance specifications, special performance icons and the hose's pressure rating in Mega Pascals and psi. For example, one of the icons is a half-bend icon indicating that the hose is rated to one half of the SAE bend ratings, signifying that it can be installed and perform at a tighter radius than a standard SAE hose. A hose showing the low temperature icon will have a lower working temperature range than a standard hose, meaning it is made with special compounds that enable it to work well at extremely low temperatures.

To make life easier for an engineer fitting or replacing a hydraulic hose, laylines are now getting shorter so that the data they contain is repeated more often on the hose. This means that no matter where the hose is cut to make an assembly, there is a much better chance that all the key data will still be visible and not be chopped off.

A particularly important piece of information on the layline is the date code, which will provide an indication of when a hose is due to be replaced (the SAE specifications say that the shelf life of bulk rubber hose is ten years from the date of manufacture). The layline can also provide a simple visual guide when routing and tightening the assembly to avoid any chance of a hose becoming twisted or kinked.

My own company has developed a novel tracking system (the PTS), which can aid the management of hose repair and replacement. Using a Web-based application, PTS generates a unique identification code for each hose assembly, which is printed on a durable barcode or RFID label. It provides fast and accurate product identification to speed replacement regardless of where the original assembly was made.

In a remote environment, a burst or leaking hose could pose severe problems in terms of downtime, environmental pollution and personal injury, depending on what media the hose is carrying, and robust construction is essential for such duties. So, whatever use you might have in mind for the seemingly humble hydraulic hose, there is a great deal more to it than just getting hold of a black rubber pipe and fitting it to your equipment. The significant growth over the years in demanding and challenging environments has meant that hydraulic hoses have had to become much more complex in their construction and design.

Luca Pozzi is product manager, hydraulic hoses, Parker Hannifin

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