Fluid power has a critical role to play in the operation of many types of plant and equipment in industries ranging from manufacturing and offshore to construction and agriculture. Jerry Hughes reports
One of the most important parts of any hydraulic system is the hydraulic hose, having to operate at often very high pressures, shock loads and temperatures.
There are various types of hoses available, and it is important to choose one that is best suited for your particular application. One of the first things to ensure is that you source the right size of hose for the task at hand. A trained system designer should know what the required flow rate and pressure requirement will be, as well as what type of fluid will be used, and should therefore be able to determine an appropriate type of hose. However, if there is any doubt as to the best specification of hose, I would recommend that you seek assistance from a reputable hose manufacturer.
Hose liner compatibility
Depending on the specific uses and pressures involved, different types of hose material might be more suitable than others. For example, in the case of the hose liner, this should primarily be chosen to suit the media – ie, the fluid – that is to be carried. Most applications in hydraulics tend to use a standard mineral oil. However, in the case of certain more specialist applications, other oils will be required. In aircraft testing, for example, Skydrol fluid is commonly used. This requires a particular type of hose liner that is recognised as being resilient enough to transfer this type of oil safely and efficiently over a long life cycle.
Correct hose material
Also, it is important to consider the external environment where the hose will operate. If exposed to sunlight, salt water and ozone – for example, on a ship in the Gulf – hydraulic hoses will naturally degrade quicker than if they were installed in a system used within an enclosed factory environment where there would be limited exposure to such elements. The temperature within most factories is likely to be much more consistent and controllable than on a ship; where the temperature range could change from -40 to +40c very easily, depending on where the vessel is operating.
Matching hoses and end fittings
A hose assembly comprises a piece of hose and two end fittings, and it is important that this is made up of what is often referred to as matched components – that is to say, the hose end should be tested by the manufacturer in conjunction with the particular hose it is to be fitted to; not just tested with any hose meeting the relevant ISO-standards. This is because the tolerances outlined in ISO-standards cover a wide spectrum. If a hose assembly is designed and assembled to cope with the full range of tolerances within the hose standards it would be very difficult to determine a crimp dimension that would cover all tolerance eventualities. Even if they do not actually manufacture hoses, most hose suppliers will be familiar with the process of testing various combinations of hoses and hose ends.
Testing and standards compliance
Indeed, this is a mandatory requirement when building systems in accordance with the Machinery Directive. The requirement to test both hose and hose end is also noted in the BS EN ISO 4413 standard ‘Hydraulic fluid power – general rules and safety requirements for systems and their components’ published in 2010. Nevertheless, some companies continue to supply hose assemblies from non-matched combinations, and this is something to be aware of.
BS EN ISO 4413 also states that hose assemblies shall (‘shall’ being defined as a mandatory requirement) be constructed from hoses that have not previously been used in operation as part of another hose assembly, and that fulfil all performance and marking requirements given in appropriate standards. Essentially, this means, first, do not re-end a hose and, secondly, the assembly when deriving the crimp dimensions for the hose should be impulse tested in accordance with the standard. I therefore advise any purchasers of hydraulic hoses to quote BS EN ISO 4413 to their supplier. The International Standards Organisation (ISO) has also introduced a variety of standards concerning a range of hose types; two-wire braid, multi-spiral, etc.
Choosing the right end couplings
It is also important to choose the right end couplings made from the most suitable materials for their intended purpose. In the offshore and marine environment, for example, it will be necessary to choose end fittings made from stainless steel, which is more resilient to corrosion from salt water. These stainless end fittings require to be separately tested with the hose in the same way as a mild steel to determine correct crimp dimensions.
Cleanliness of hose and hose assemblies is also important. If you are looking to fit a hose assembly onto a brand-new system you should flush the system as a matter of course and ensure the hose assembly is part of the flushing loop. However, if you are looking to replace a hose on an existing system and you are not able to re-flush the system, ensure that the new hose is supplied clean of black rubber debris and swarf from the metal braiding by flushing separately.
Never re-end a hose
Never re-end a hose, or accept a re-ended hose from a supplier. This can never be emphasised enough and there is a good reason for taking this stance. A hose is constructed from a number of layers; it has a hose liner, one or more layers of wire – either braiding or spiral wire depending on the type of construction – and an outer cover. When dimensions are derived for crimping the hose they will be based on a virgin hose; i.e. one that has never been pressurised. When the hose is pressurised everything moves; including the rubber and wires. As soon as this occurs it can no longer be categorised as new.
This is why BS EN ISO4413 states that a hose assembly should be constructed from hoses that have never previously been used in operation as part of a hose assembly. If previously used hoses are used, or if these hoses have been re-crimped and re-ended, there is a far greater risk of failure, which could not only have serious consequences concerning productivity, but also with regard to the health & safety of operators, maintenance personnel and passers-by. Any risk of a fluid injection injury should be taken very seriously due to the potentially life-changing or fatal consequences involved.
With this in mind, I would recommend that end-customers and designers always ask for hose assemblies to be supplied with a certificate to prove all hoses have been pressure tested and conform to the relevant standards. I would also advise that all hoses be sourced from a certified member of the BFPA’s (British Fluid Power Association’s) and BFPDA’s (British Fluid Power Distributors Association’s) Hose Accreditation Scheme for added confidence and security.
With the correct sourcing, testing, installation and maintenance regime firmly in place, your plant and equipment should be in the best position to operate efficiently and safely. Extra time and attention invested in getting things right in the first instance can pay major dividends for the months and years ahead.
Jerry Hughes is director of independent management consultancy Jemlyn Limited, and is a consultant member of the British Fluid Power Association. He is chairman of the BFPA committees MCE 18 (BFPA/T&SPC) and MCE 18/4 (BFPA/TC 4 Connectors).