Hose parameters such as flexibility for better/more compact routing or less product weight are directly influencing overall machine mass in a positive way. Lower hose assembly weight can also help to optimise the size of hydraulic components such as pumps, while hoses with improved bend radius deliver increased design flexibility and the potential to minimise hose lengths, again reducing weight for the machine manufacturer.
Alongside environmental benefits, a more reliable hose assembly with increased impulse qualification can reduce maintenance times and minimise the risk of field failures. Mobile machines often use several dozen metres of hose, so each design improvement can have a major impact on factors such as productivity, maintenance requirements and energy efficiency.
Of course, selecting a hydraulic hose that offers increased lifetime (higher number of impulse cycles) also promotes sustainability through reduced product replacement and disposal. Responsibility here falls firmly on the shoulders of hose manufacturers. However, long-life hoses are now available thanks to break-through technology developments in rubber compounding and new braid/spiral architecture. Here, the primary design path is reducing the thickness of different materials while simultaneously delivering greater fatigue life (impulse) at higher pressure and tighter bend radii.
A standard six-spiral hose will have 15 separate layers: six made from high-tensile steel wire and the remaining nine from specially formulated polymer. Each polymer layer has unique requirements, demanding a high degree of engineering proficiency.
The reason for this high level of innovation is to help avoid hydraulic hose failures, which typically fall into two categories: outside in, and inside out.
Inside-out failures occur due to stress from internal media flow as a result of pressure, temperature or bending stress over time, while outside-in failures happen due to stress generated by the external environment, such as hose cover abrasion, mechanical damage or cracking from ozone or heat.
With regard to the nine polymeric layers, identifying the right rubber formulation is key to obtaining the desired properties and helping prevent failures. Rubber formulation typically consists of 10 to 18 different chemicals and is extremely vital to hose performance. A poorly formulated rubber may withstand only 1000 fatigue cycles, whereas a highly engineered formulated rubber can withstand 2000,000 cycles.
Rubber formulation is clearly one of the most critical factors in hydraulic hose design. Among the compound properties which ensure better hose performance and longevity are compression set resistance, stress and strain, elastic modulus, tube to wire reinforcement adhesion, tear strength, oil resistance, and high and low temperature resistance.
Compression set is a measure of the dimensional change that occurs due to an applied force. Lower compression set means better hose and fitting compatibility, which leads to extended hose lifetime. It is possible to achieve lower compression through certain adjustments to the rubber recipe. For instance, using a rubber polymer with a high molecular weight helps to achieve better crosslink density, which means improved compression set resistance.
Optimised stress and strain properties in rubber compound can improve the endurance of hydraulic hoses exposed to mechanical loads, dynamic movements and oil pulsation. The result is longer service life in applications such as oil and gas drilling, agriculture, construction, mining equipment and heavy machinery.
Elastic modulus (tensile modulus, Young’s modulus) is the ratio of stress-to-strain and is equal to the slope of the material’s stress-strain diagram. Rubbers have a low modulus of elasticity and are not highly affected by deformation.
Regarding adhesion between components, any lack of adhesion between tube to wire reinforcement, wire reinforcement to insulation layers and cover to wire reinforcement, could potentially result in hose failure. Each hydraulic hose component should react together as one system under working conditions.
Tear resistance is especially important for the hose compound. Hydraulic hoses that are subject to dynamic loads from the pulsation of hot oil often fail due to a fracturing of the rubber component. A fracture may initiate in an area where the stress and load concentration are at maximum levels, causing it to increase in size and progress into a tearing action.
To satisfy the demands of low and high temperature resistance, a mixture of two or more polymers is sometimes necessary. These might include polymers with a low glass transition temperature, plasticisers offering low pour points or polymeric plasticisers with higher evaporation temperatures.
Reducing the negative effects caused by using petroleum-based materials has also become an important requirement of a more sustainable planet. The aim across many parts of industry is to expand the use of polymer materials obtained from renewable resources. Today, several such materials are beginning to emerge, with polylactic acid perhaps the foremost example.
There is also growing awareness of industry’s responsibility in waste and recycling. Addressing the problem at its source, in the production of these materials, offers the most potential for progress. Here, ethylene-propylene-diene monomer (EPDM), which Arlanxeo started producing in 2011 under the Keltan ECO trade name, is one of the biggest steps taken commercially. The Keltan ECO elastomer is a product in which all of the ethylene derives from plant and renewable sources, making it a huge milestone for green transformation in the rubber industry.
The use of filler materials in rubber compounds is another area of high interest in the shift towards a more sustainable rubber sector. Materials such as end-of-life tyres and ground-up construction rubble can serve as filler in rubber compounds, as can lignin, which is a by-product of paper production.
A further important point is vulcanisation systems. Dithiocarbamates, sulphonamides and thiurams, all of which are commonplace in rubber manufacture, produce secondary amines during vulcanisation, with a number of medical studies indicating a potential impact on human health. Vulcanisation system accelerators also require attention.
Optimised hose manufacturing is vital. If production lacks stability or consistency it can limit design innovation because the designer will have to compensate accordingly for potential process variations.
Consistent wall thickness, for example, is associated with high-quality inner tube extrusion, where critical parameters include extruder section temperatures, compound feeding rate and mould type. When any of these parameters is out of acceptance criteria, it can impact rubber viscosity, leading to wall thickness fluctuation. This effect can cause leakage, reduced abrasion resistance to chemicals/liquids inside the hose, or fitting assembly defects, for example.
The outside diameter is also critical, particularly for sizing the hose to the appropriate fittings and system components, as well as ensuring consistent protection against abrasion or ozone.
Achieving optimum concentricity is a further important factor as it can increase hose efficiency, help parts to wear evenly and predictably, better handle high pressures, and help ensure a tight fit between the hose and its fittings.