Rubber hoses for the supply of hydraulic fluids such as fuel, mineral oil, paints, and varnishes, are called hydraulic hoses and are designed for significant pressures that classic rubber hoses cannot withstand.
By Jurij Artiukhov, General Manager, PipeTime, LLC
Any production process or special equipment cannot exist without high-pressure hoses (as hydraulic hoses are often called), because in addition to ensuring an uninterrupted operation process (supply of process fluids), they also significantly reduce the effect of vibration on structural parts.
Technical Design of Braided Hoses
There are two technical designs of hydraulic hoses: braided and coiled. Braid construction is the design most often used in manufacturing processes. A braided type hydraulic hose is made with a particularly strong metal cord coated with a rubber layer. This type of hose operates at high pressure and is able to withstand operating temperatures from -40°C to +100°C. The design of the hose withstands significant vibration loads and interacts with fluids such as oil, glycol, water, air, etc.
In almost any country in the world, European standards for the production of hydraulic hoses are generally taken as a basis: they include the EN853 and EN857, quality standards for the manufacture of high-pressure hoses. A local quality standard can also be applied.
High pressure hoses manufactured according to EN853 (Figure1) are mainly used for construction equipment and have a small bend radius. In most cases, these are short sections of hoses up to 1.5-2 m. Such hoses are marked 1SC and 2SC, where
the numbers indicate the number of braided layers of steel cord. Depending on the production task, diameters vary from 6 to 25 mm, with working pressure from 225 to 80 bar.
Figure 1: High pressure hoses manufactured according to EN853.
The EN857 (Figure 3) standard includes 4 types of braided hoses; 1ST, 2ST, 1SN, 2SN.
Brass wire acts as a braided cord in 1ST and 2ST hoses, which are able to withstand a breaking force of up to 15-20 kg. In addition, these hoses are characterized by an increased layer of rubber coating to increase the wear resistance of the coating, which, in turn, reduces the bending radius of the hose by an order of magnitude.
Types 1SN, and 2SN have a thinner outer layer, respectively, are less resistant to abrasion, but have a high bending radius and are installed in the most inaccessible places.
High pressure hydraulic hose 3SK (Figure 4), is a hose that has a triple layer of metal cord and rubber braid, withstands negative temperatures up to -55°C, which is especially important for those regions where equipment works in cold climates. The operating pressure of the hose can be up to 420 bar with a nominal hose diameter of 5 mm.
Essentially, all listed types of hydraulic hoses are quite similar in their technical parameters, such as working pressure from 250 bar to 420 bar and diameters from 5 mm to 51 mm.
Figure 2: Coiled design hoses.
Technical Design of Coiled Hoses
In all cases, the hose is made of wear-resistant rubber, that is not impacted by oil products and chemical environments. Coiled design hoses are resist-ant to significant vibration loads, high positive temperatures, and are also used in systems where operating pressure does not allow the use of braided hoses (Figure 2).
Coiled design hoses have a small bending radius due to the fact that they have a large amount of metal cord and rubber layer. There are four main types of coiled hydraulic hoses; 4SP, 4SH, R12 and R13. 4SP hoses withstand pressure from 170 bar to 450 bar, the number “4” means that the hose has four wound metal spirals as a power cord.
Figure 3: The EN857 braided hoses.
Figure 4: High pressure hydraulic hose 3SK.
Figure 5: R13 is a six-spiral high-pressure hydraulic hose.
The construction of the 4SH hose is a four helix coil like the 4SP, but the wire is reinforced and the hose can withstand higher pressures. 4SH hoses have nominal diameters from 20 mm to 50 mm with operating pressures up to 430 bar.
R12 has four coils, a heavy hose, designed for continuous work at high temperatures and medium pressures. R13 is a six-spiral high-pressure hydraulic hose for the most severe operating conditions, withstands significant compensatory loads (Figure 5).
Coiled design hoses, like braided de-sign hoses, are made of oil and petrol resistant rubber that can withstand abrasion. Like any hose, hydraulic hoses are connected to each other and connected to the equipment through special hydraulic connections.
Given the high pressure, the fittings are installed on the hoses mechanized – special crimping machines are used. The main division occurs according to the type of seal or the type of fitting thread (Figure 6).
Figure 6: Types of hydraulic hose fittings.
Figure 7: Textile protection without insulation.
Testing and Safety
After installing the fitting on the hydraulic hose, it is imperative to carry out a hydraulic leak test. The use of high pressure hydraulic hoses is subject to harsh operating conditions, such as high and low temperatures, friction on the surfaces in contact with the hose, bends, kinks, and other factors that can affect the smooth and safe operation of the hose.
To eliminate all these factors as much as possible, various methods have been developed to protect the hydraulic hose:
• Spiral Hose Guard – A highly durable plastic that wraps around the outer surface of the hose and protects it from mechanical stress.
• Flame retardant protection for high pressure hose – Used where there is a risk of hot metal getting on the body of the hose, etc.
• Textile protection for the hose – For the protection of personnel who work near the hose and there are potential risks of its depressurization (damage). It can be a thermal cover to maintain the temperature of the liquid inside the hose (Figure 7).
Automotive hoses are special hoses and tubes for cooling systems of internal combustion engines and for use in heating systems. As a rule, these are straight or curved hoses designed to work both under pressure and without pressure in cooling systems using ethylene glycol-based coolants in internal combustion engines of vehicles (the mass of the vehicle, as a rule, is not more than 3.5 tons).
In most cases, automotive hoses are made from silicone, which has a standard hardness of 65 Shore A and a density of about 1.26 g/cm3. The outer layer is represented in blue, rarely red, to make it easier to identify the hose in the vehicle.
The outer and inner layers are always made of oil and petrol resistant silicone, able to withstand the influence of petrochemicals, combustion products, and more. Reinforcement can be represented by various types of fiber and, de-pending on the type of fiber – polyester or fiber glass, withstand temperatures from -50°C to +330°C. Diameter varies from 6 to 152 mm, operating pressure from 1.6 to 14 bar (safety factor 3:1).
The most common standard for this type of hose is SAE J20R1, which refers to the design of the hose without a steel helix inside, the steel helix version of the hose is regulated by the J20R2 standard and has significant resistance to kinking and vacuum operation.
It is also worth noting that the standard length for automotive hoses is a 1 m branch pipe, if necessary, the length of the hose can be increased up to 20 m. Hoses can also have special nozzles (Figure 8) that allow you to design a pipeline or a transmission system for liquids and gases, and can have al-most any configuration from a straight shape to a shape with numerous bends and turns. For example, nozzles can have an angle of 180°, 90°, 135°. The length of such a branch pipe varies from 400 mm to 1000 mm.
Figure 8: Silicon nozzle.
Figure 9: Reinforced break hose.
Branch pipes are attached to the aggregate units using clamps. It is also worth mentioning separately one of the most important automotive hoses – brake hoses, which are the artery of the car’s brake system. With the help of brake hoses, brake fluid is supplied to the working brake cylinder, which acts on the pads and, as a result, the braking process occurs.
Brake hose wear is a very insidious and often imperceptible breakdown. Depressurization of the hose leads to a rapid loss of brake fluid and, consequently, failure of the brake system. In addition, the hose is constantly under pressure, so hose integrity is critical.
In most cases, to replace rubber, special reinforced hoses are used (Figure 9), consisting of an inner layer – a reinforced plastic tube that can withstand high pressures and exposure to aggressive gases and liquids.
The outer layer is a special metal net-work that does not allow the hose to deform. More and more manufacturers cover this network with a layer of poly-vinyl chloride to prevent damage to other components of the mechanism by the reinforced surface of the hose.
The average pressure for which this hose is designed is 300 bar, which means that the hose is able to with-stand emergency braking at speeds of 150-170 kilometers per hour. The same fittings are used to install the hose as for high pressure hydraulic hoses.
High pressure hydraulic hoses, as well as automotive hoses, are indispensable elements of any mechanism or production process where there is high pressure and significant overloads. Accordingly, the combination of all these factors imposes additional responsibility on the maintenance technical personnel, compliance with the requirements of production standards, as well as constant checks for tightness and operability of the hose.
ABOUT THE AUTHOR
Jurij Artiukhov works as the General Manager at PipeTime, LLC. He previously worked as a Sales Manager for MTT Company and Project Manager for SOL SP Company. He holds a graduate degree from the University of Foreign Affairs, Economy and Law.