Frequently asked questions about hydraulics

A hydraulic hose is a flexible, tubular hose with a braided or spiral-shaped pressure carrier, suitable for high pressures. The hose can be made up of one or more layers.

Hose lines transport suitable pressurised fluids in pressure, return, suction and leakage lines. They should safely transport the pressurised fluids from the producer (pump) to the consumer (e.g. cylinder) and function as reliable equipment (component) in the hydraulic system. 

In order for the hose to be used as intended, it must be ensured that the hose material is suitable for the intended use. A number of parameters must be taken into account:

• Ageing
• Effect of the pressurised fluid on the inner and outer layer 
• Fire resistance 
• Fire behaviour
• Sensitivity of the material to pressure loads, pressure fluctuations, pressure peaks, pressure pulses
• Is the material silicone-free? 
• What is the permeability? 
• Is the hose material resistant to ozone? 
• What is the electrical conductivity? 

In general, the manufacturer's specifications must be observed and hose lines must be used in accordance with the specifications of the machine/system manufacturer.

In most cases, hose lines are laid ‘open’, also to ensure good access. Safety is always the focus in order to provide the best possible protection for personnel and the surrounding area. In the event of visible damage, it is therefore essential to take measures to ensure that the machine can continue to be used safely. In most cases, it is advisable to replace damaged hoses immediately. To increase safety, a special protective hose can also be used. In order to minimise the risks to life and limb, it is advisable to carry out a hazard analysis of all hose lines on an ongoing basis.

Qualified persons to carry out tests on hydraulic hose lines in accordance with the Ordinance on Industrial Safety and Health. To do so, they must have completed technical vocational training or have sufficient technical qualifications. Inspection practice should be guaranteed on the basis of several inspections per year.

There are many different reasons for replacing a hydraulic hose line. All hose lines should therefore be visually inspected and reassessed on a regular basis. If there is even the slightest doubt about the proper condition of a hydraulic hose line, it is better to replace the hose line with a new one. Reasons for replacing the hose line include

• Poor routing
• Kinking
• Stress due to twisting (torsion)
• Inadequate integration
• Falling below the minimum bending radius
• Ageing
• Period of use exceeded
• Damage from outside and much more 

In accordance with DGUV Rule 113-020, once a year under normal load and ½ a year under high load (e.g. in multi-shift operation).

The outside temperature has a significant influence on hose lines, as a hose line is always designed for its intended use. This includes not only the operating pressure or the nominal diameter, but of course also the temperature of the medium and the ambient temperature. The heat generated by the pressurised fluid and the heat coming from outside - for example from solar radiation - influence the service life of the hose line. If the materials are not matched to the temperature level as intended, there is a risk of damage and failure.

In principle, yes! The special thing about hydraulic hose lines made of elastomers is the elasticity of the material. Temperature fluctuations cause the material to stretch and relax, so if a rubber hose is initially very warm and suddenly cools down considerably due to the operating environment, the elastomers can lose their elasticity and freeze as hard as glass. The elastomers regain their resilience and elasticity when they are carefully warmed up again. If these temperature fluctuations are very large and frequent, the hose lines are likely to fail at an early stage.

The inner layer of a hose must be resistant to the pressurised fluid. This is a prerequisite for proper and safe operation. It goes without saying that the pressurised fluid used must be known and it must also be ensured that the materials are compatible with each other and that no swelling of the inner or outer layer occurs. It must always be ensured that the hydraulic fluid is handled properly and that the hose materials and hydraulic fluids are mutually compatible.

No, because hydraulic hose lines are only ever designed for certain pressures. There are hose standards that define the technical and safety features. For example, the maximum permissible pressure should always be printed on the hose. Of course, many other parameters are described in these hose standards according to certain aspects. They should generally be a binding decision-making aid and usually represent the current state of the art. For example: EN 853-2SN, EN 856-4SP, EN 856-4SH etc.

With regard to the service life of hose lines, strong pressure fluctuations / pressure pulses play a particularly important role. If, for example, the number of pressure peaks is very high because the line is to be used in hammer operation, this is detrimental to the hose line. If the load is known more precisely, hammer fittings and / or a more impulse-resistant hose location can be used if necessary. Tip: Include possible pressure peaks in the maximum permissible operating pressure of the hose lines.

Escaping pressurised fluid usually means danger! In most cases for people and machines, as a jet of pressurised oil can penetrate the skin and cause extremely serious injuries or inflammation. In the case of machines, it must be checked on a case-by-case basis whether escaping hydraulic fluid could be damaging to the machine parts. In many cases, however, it simply means a risk of injury, for example from slipping on the fluid.

The task of hydraulic hose lines is to safely transport pressurised fluids from the producer to the consumer. Under normal circumstances, they are a reliable working tool. However, if visible damage, faulty routing, incorrect or leaking connection fittings, missing protective hoses or a number of other defects are recognisable on the hose line, it must be replaced. It is therefore advisable to carry out regular visual inspections of the pipework. Qualified persons’ are the right people to contact for this. As a rule, they are characterised by the appropriate specialist knowledge required to inspect the hose lines.

Failure of a hose line cannot be 100% avoided, as there is always a risk of pressurised medium escaping under high pressure, whipping of the hose line and a risk of fire. Additional measures must therefore be taken to protect the area around hose lines that are laid freely, e.g. by using additional whipping protection or shielding. Special protective hoses can also help to minimise the risks - e.g. from escaping pressurised fluid.

Due to ageing, which also occurs during storage, safe use after the storage period has expired is not possible without first having a thorough inspection carried out by a competent person. This person is familiar with the current regulations and legal requirements and is able to make an appropriate recommendation, also after consultation with the manufacturers of the hose lines and the machine manufacturer.

Torsion of the hose line reduces the service life enormously. It is better to consult an expert. A hose line that is not in proper condition can pose a life-threatening hazard.

There are various accessories and methods for protecting hose lines against kinking. The ideal situation is always to prevent kinking behind the fitting in the first place. The fitter who installs the hose lines is responsible for this.

In general, we talk about maintenance for machines and systems with hydraulic equipment. This naturally also includes hydraulic hose lines. All spare parts must comply with the machine manufacturer's specifications. Regular visual inspection is an essential part of the maintenance of hose lines. To this end, hose lines must be visually inspected for external defects at least once a year by authorised persons.

One option may be to use special protective hoses. In general, make sure that the temperature (pressurised medium) in the hydraulic system is not too high.