Hydraulic Hose Line

A hydraulic hose is a flexible component in hydraulic systems that transports hydraulic fluid under high pressure between components. It consists of a multilayer hose with fittings crimped onto both ends. Its flexibility allows for movement, vibration, and compensation for length changes between rigid components, which fundamentally distinguishes it from piping.

Construction of a Hydraulic Hose Assembly

Every hydraulic hose follows a multi-layer design principle that withstands various mechanical and chemical stresses. The structure is divided into three functional layers, each of which performs a specific task.

Inner Layer

The inner layer is in direct contact with the hydraulic fluid, usually mineral oil, but also special fluids such as HFC or HFD fluids. It consists of oil-resistant synthetic rubbers such as NBR (nitrile rubber) or, for higher temperature requirements, FKM (fluorinated rubber). The inner layer must be chemically resistant to the pumped medium and, at the same time, have a smooth surface to minimize flow resistance.

Reinforcement Layer (Pressure-Bearing Layer)

The reinforcement layer absorbs the actual pressure load. It consists of steel wire that is wrapped around the inner layer either as a braid or as a spiral. The number of plies determines the pressure rating of the hydraulic hose. Rubber interlayers are placed between the individual wire layers to reduce friction between the wires and provide support between the layers. Alternatively, textile or aramid fibers are used for lower pressure ratings.

Outer Layer (Cover)

The outer layer protects the pressure-bearing components from environmental influences such as ozone, UV radiation, mechanical abrasion, and chemicals. It consists of weather-resistant synthetic rubber, often CR (chloroprene rubber). In particularly abrasive environments, additional protective hoses or spiral protective springs can be fitted over the hose.

Design Types and Standards

Hydraulic hoses are classified according to the type of reinforcement and the corresponding product standards. These standards define the requirements for pressure ratings, temperature ranges, and pulse tests.

Wire-braided hoses according to DIN EN 853

Hoses with one or two layers of braided steel wire fall under DIN EN 853. Types 1SN and 2SN cover pressure ratings up to approximately 225 bar, depending on the nominal diameter range. They are characterized by good flexibility and are used in applications with moderate operating pressures, such as in agricultural machinery or in return lines of industrial plants.

Spiral-wound steel wire hoses according to DIN EN 856

For high operating pressures up to 420 bar and above, hoses with four or six spiral-wound steel wire layers are used, as specified by DIN EN 856. Types 4SP, 4SH, and 6SP are designed for systems subject to high pressure pulses, such as those found in presses, injection molding machines, and construction equipment. The spiral construction offers higher pressure resistance than braided hoses, but is associated with a larger minimum bend radius and lower flexibility.

Compact hoses according to DIN EN 857

DIN EN 857 specifies hoses with highly flexible wire braids (Types 1SC and 2SC). They achieve pressure ratings of up to 400 bar while maintaining a compact design and a small bend radius. This characteristic makes them particularly attractive for mobile hydraulic applications where tight installation spaces and frequent swivel movements are required.

Performance Classification According to ISO 18752

ISO 18752 classifies hydraulic hoses by performance classes (Class A through D) rather than by design. This performance-based approach takes pressure, pulsation, and temperature into account within a single classification system and is increasingly replacing design-based classification. Design engineers select the appropriate class based on actual operating conditions, not on the wire arrangement.

Hydraulic Hose Assemblies versus Piping

In practice, hose lines and piping complement each other within a hydraulic system. The choice varies depending on the requirements for flexibility, durability, and pressure stability.

Piping is typically used for main pressure and return lines in stationary systems where there is no relative movement between the connection points. Hydraulic hose lines, on the other hand, connect moving components such as cylinders, swivel actuators, or booms that swivel on machines. In many systems, both types of lines form a single integrated system.

Assembly and Fittings

The term “assembly” describes the process of manufacturing a ready-to-use hydraulic hose assembly from the individual components—hose and fittings. The hose is cut to the required length, and the fittings are crimped onto both ends.

Crimping as a Joining Process

Proper crimping is the most critical step in assembly. During this process, the fitting is deformed using a hydraulic press so that it forms a positive-lock, leak-tight connection with the hose. The crimping pressure, crimping depth, and tool selection must strictly adhere to the fittings manufacturer’s specifications. Improper crimping can result in leaks or the fitting being torn out under pressure.

Selecting Fittings

Fittings must be compatible with the hose and the connection type. The pressure rating of the fitting must be at least equal to that of the hose. The applicable operating pressure of the finished hydraulic hose assembly is determined by the weakest component—that is, by the lower rated pressure of either the hose or the fitting. This is expressly stipulated in DGUV Rule 113-020.

Minimum Bend Radius and Installation

The minimum bend radius is one of the most important design parameters for a hydraulic hose assembly. If this radius is not maintained, the inner layer on the outside of the bend will be overstretched, while the pressure-bearing layers on the inside will be compressed. This results in cracks in the core, reduced pressure resistance, and premature failure.

The minimum bend radius depends on the hose type and the nominal diameter range. Spiral hoses have larger minimum bend radii than braided hoses of the same nominal diameter. When installing the hose, the following applies: The actual bend radius should significantly exceed the minimum value to maintain a safety margin. In addition, no tensile stress may act on the connection area between the hose and the fitting.

Causes of Failure and Maintenance

Hydraulic hose lines are subject to natural aging, as the elastomers change their properties over time. External factors also accelerate wear. The most common causes of failure can be divided into several categories.

Mechanical Causes

External abrasion caused by contact with adjacent components or other hoses is one of the most common causes of failure. Failing to maintain the minimum bend radius and kinks also lead to premature failure. Tensile stresses at the connection to the fitting weaken the crimped joint.

Thermal Causes

Operating temperatures above the permissible limit significantly accelerate elastomer aging. Even exceeding the continuous service temperature limit by just a few degrees can significantly reduce service life. Conversely, cold temperatures cause the rubber to become brittle.

Chemical Causes

An incompatible hydraulic fluid can attack the inner layer, causing it to swell or shrink and thereby reducing its compressive strength. Aggressive environmental media such as oils, solvents, or acids can also damage the outer layer.

Aging

Even under optimal operating conditions, elastomers age. Ozone and UV radiation attack the outer layer, even when the hydraulic hose assembly is not in use. For this reason, manufacturers specify a maximum storage age that must not be exceeded at the time of commissioning.

Inspection Requirements According to DGUV Rule 113-020

DGUV Rule 113-020 governs the safe use of hydraulic hoses and hydraulic fluids in Germany. It replaces the former BGR 237 and BGR 137 and is based on the Industrial Safety Regulation (BetrSichV).

Hazard Assessment

Operators must conduct a risk assessment that accounts for all stresses on the hydraulic hose assembly: operating pressure, flow rate, fluid, temperature, environmental influences, and dynamic loads. The selection of the appropriate hose type and pressure rating is based on this assessment.

Regular Inspections

DGUV Rule 113-020 requires regular visual and functional inspections. Visible damage such as cracks, bulges, abrasion, or leaks at the fittings requires immediate replacement. The inspection intervals are determined by the risk assessment. As a general guideline, a visual inspection should be conducted every six to twelve months; more frequent inspections are required for particularly critical applications.

Replacement Intervals

The regulation recommends replacing hydraulic hose lines after six years at the latest, unless prior damage necessitates an earlier replacement. The date of manufacture is printed on the hose and must be taken into account during commissioning. Hoses that are already more than four years old at the time of installation should no longer be used.

Conclusion

Hydraulic hoses are an indispensable component of any hydraulic system, compensating for movement and vibration between components. Their reliable operation depends on correct selection based on pressure rating and standards, professional assembly, adherence to the minimum bend radius, and regular inspection. Adhering to these principles minimizes the risk of failure and ensures safe and efficient system operation.