The Properties of Hydraulic Hoses

Hydraulic hoses, or high-pressure hoses (HPH), as they are most often called, are flexible pipelines that carry a supply of liquids such as: mineral oil, fuel, paint materials, lubricants, and water. Efficient production or engineering is unthinkable without a structural element such as a hydraulic hose. As hoses are able to withstand high pressure, and help to reduce the vibration impact on specific parts of structures, they are integral to function of a verity of systems; any modern hydraulic system will be equipped with high-pressure hoses.

By Jurij Artiukhov, General Manager, PipeTime, LLC

Types of Hydraulic Hose

There are two principle technical designs of HPHs: braid and winding.

Braided Hoses

Braid hoses are used in high-pressure hydraulic systems with high-temperature working fluids such as: mineral oil, petroleum-based media, glycol or water emulsion for hot petroleum products, oil, air, and water. Its structure typically consists of a braided metal cord covered with a layer of rubber, and has an operating temperature range from -40˚C to +100˚C, refer to Image 1.

Image 1: Braided hose.

Most of the world’s HPH manufacturers are guided by the European production standards-EN853 and EN857. According to the EN853 standard, HPHs have a small bending radius, which differentiates them from other braid hoses, and are mainly used for equipment, such as construction cranes. The HPHs used on this type of equipment, specifically, are called ‘compact high-pressure hoses’. These hoses are commonly designated 1SC and 2SC, where the numbers indicate the number of braid layers.

The most common diameters of braided hoses are 6 mm to 25 mm, with a working pressure that ranges from 225 bar to 80 bar.

The EN857 standard represents four other types of braid hoses; 1ST, 2ST, 1SN, and 2SN. The principal difference between type 1SN/2SN and 1ST/2ST is that 1SN/2SN have a thinner outer layer of rubber. Types 1ST and 2ST hoses have a wire, which is able to withstand a breaking force of up to 20 kg, and are characterized by a thick outer layer of rubber, making them resistant to abrasion.

Image 2: 3SK hose.

In general, all four types are similar in manufactured diameters and operating pressure, with a range from 5 mm to 51 mm and an operating pressure range from 250 bar to 40 bar. All these types of high-pressure hoses are made of oil-and-gas-resistant synthetic rubber that is resistant to abrasion.

A special subtype of hydraulic hose is the 3SK hose. It has three layers of rubber, and metal braid, and can be used at ambient temperatures down to -55˚C. It also has a diameter of 5 mm is able to withstand working pressure up to 420 bar, refer to Image 2.

Winded Hoses

A winding design hydraulic hose is able to withstand high pressures, is resistant to impulsive loads, and is resistant to high temperatures. As a rule, the winding structure is inferior in flexibility to the braid structure, as it has a smaller bending radius, refer to Image 3.

Image 3: Winding hose.

According to the world production standards, which for most plants is the European standard EN856, there are four types of winding design hydraulic hoses: 4SP, 4SH, R12, and R13.

4SP hoses are manufactured by winding four spirals into a power cord. They are used in conditions of medium pressures that range from 450 bar to 165 bar. The design of the 4SH hose is also a winding of four spirals, but the wire has a special strength reinforcement, which allows the use of the hose in high pressure conditions. 4SH hoses are available in diameters that range from 20 mm to 50 mm with a working pressure range from 430 bar to 280 bar. This working pressure is an order of magnitude higher than the 4SP type.

The R12 design has four windings and is designed for long-term work in high temperatures with medium pressures, while the R13 is multi-spiral with six-windings. The R13 HPH is used for the most severe working environments; it is typically used with the highest pressures, heavy loads, and it has an increased service life, refer to Image 4.

Image 4: The R13 hose is a multi-spiral hose with six windings.

It is worth noting that the 4SP and 4SH RVs can withstand about 400,000 cycles at 100°C, and the R12 and R13 types can withstand at least 500,000 double cycles at 120°C, refer to Image 5.

Hoses designed with a winding construction are actively used in mining equipment and mining fields. Many of the characteristics of HPH, such as UV resistance, are therefore confirmed by such authoritative bodies as the Mine Safety and Health Administration (MSHA).

Image 5: 4SP and 4SH RV hoses.

Braided and Winded Comparisons

In the course of a dialogue with several manufacturers of high-pressure hydraulic hoses from Russia and China, the following comparison can be made between the two types of hose structures.

While the coils of the winding hose structure are wound so that the plane of the coil is perpendicular to the axis of the hose, the turns of the reinforcing layer of steel wire are wound along the line in a braided hose structure. Assuming that the same number of wires and thickness of rubber are used, the winding structure will therefore be able to withstand higher pressures then that of a braided structure.

The advantage of the braided hose, as already mentioned, its flexibility. As the wire turns are more easily diverged when bending a winding hose, they typically have a larger permissible bending radius. The minimum bending radius is an extremely important technical characteristic of the hose, because the convenience of its operation and ease of installation depend on the flexibility of the hose. That is why the braided construction prevails in the manufacturing process and is used more frequently.

Hydraulic hoses of the EN865 standard are widely used in excavator equipment.

Hydraulic Fittings

For connection to hydraulic systems, road, and excavator equipment, high pressure hoses are equipped with special fittings. The type of fitting for a HPH varies greatly depending on the metric system of the country, the type of equipment and technology being used, and a number of other parameters.

The primary way to discern which type of fitting is most suitable to a specific application is by the type of thread and type of seal it uses. Below is a list of the common thread and seal types:

Thread types:

  • European type (DIN);
  • English (BSP);
  • American (SAE).

Seal types:

  • BSP, JIC, JIS – cone seal;
  • DKO-L, DKO-S, SFS, SFL, STECK – ring or cone sealing;
  • DK – sphere seal;
  • NPTF, BSPT, NKT – tapered thread seal;
  • ORFS – gasket seal;
  • Banjo – bolt seal.

For convenience, fittings are also divided according to the type of shanks and the method of installation:

  • Universal series – used in all high pressure hoses of the braided series.
  • Interlock series – used for crimping coiled hoses, it is necessary to remove the inner and outer layers of the rubber of the hose.
  • CS series – designed for crimping winding-type hoses, removing the inner and outer rubber layer of the high-pressure hose is not required.
Image 6: Hydraulic crimping press.

Unlike industrial hoses, the crimping of fittings on HPHs is not done manually, but mechanically. Standard industrial hose clamps are capable of withstanding pressures up to 25-32 bar; in the case of hydraulic hoses, however, the pressure is at least three times higher. To install the fitting on the high pressure hose, special hydraulic crimping presses are used, refer to Image 6.

As a rule, hoses with a winding structure require stripping of the inner and outer rubber layer before installing the connection; stripping is performed mechanically, refer to Image 7.

Using special lubricating mixtures, the fitting is stored inside the body of the hose. In cases of large volumes of production, pneumatic units are used to push the connection into the hose. After that, the fitting is attached to the hose with a special ferrule.

Image 7: Mechanical stripping machine.

Users performing the crimping operation must diligently control the crimping moment, to prevent the inner reinforcing layer of the hydraulic hose from pushing through; such hoses must be rejected in the future. Before commissioning, the high-pressure hose with installed connections is hydrostatically tested for tightness.

Mitigating Risks with HPHs

As high-pressure hoses typically perform under difficult operating conditions, such as: bending, friction on abrasive surfaces, high temperatures, and fire hazardous areas, special systems have been developed to protect hoses from the effects of these hazardous factors. For example:

Image 8: Durable plastic spiral hose protection.
  • The spiral protection of the high pressure hose is a durable plastic resistant to friction and bending, refer to Image 8;
  • The bright color of the spiral makes it easy to locate the high pressure hose on the vehicle;
  • Refractory protection for high pressure hoses is used in the steel industry, where it is necessary to protect against metal splashes. This is also used in other industrial areas where the production process is associated with high temperatures, refer to Image 9;
  • HPH textile protection is made from polyester or polypropylene, a material that is highly durable and can be recycled. Products made of polypropylene also have enhanced properties to the effects of aggressive media (alkalis, acids), refer to Image 10;
  • Textile protection is installed on high-pressure hoses and is designed to protect people near the working units from a possible rupture of the high pressure hose.
Image 9: Refractory protection for high pressure hoses.

Final Thoughts

High pressure hoses are an important component of any hydraulic system. The manufacture of a HPH, and the installation of fittings, requires the use of specialized equipment and places high demands on the professionalism of the service personnel. As high-pressure hoses pose a potential risk to life, the operating organization, and its employees, manufacturers have an obligation to carry out tests and strictly follow the production standards of the industry.

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.

Image 10: HPH textile protection for hoses.