Selecting seals for hydraulic systems

Hydraulic systems operate under high pressure, changing temperatures and continuous movement. Every movement in the system generates friction, every pressure peak stresses the material and every contamination can impair function. It is precisely at this interface that the seal takes on a central role: it keeps the system tight, stable and efficient in operation.

Selecting a suitable seal goes far beyond mere sealing. It is about operational reliability, maintenance intervals and long-term cost control. An unsuitable seal quickly leads to leaks, pressure loss or consequential damage to adjacent components.

When selecting seals for hydraulic systems, several factors must be considered at the same time: operating conditions, material, design and installation situation. Only the interaction of these factors ensures that the seal works reliably in the long term.

Requirements for hydraulic seals

Hydraulic seals fulfil several tasks at the same time:

• Seal pressure between moving components
• Prevent leaks even during dynamic movement
• Minimise friction for efficient operation
• Reduce wear for long service lives

Depending on the application, the requirements vary greatly. A hydraulic cylinder in mechanical engineering places different demands than a mobile hydraulic system in the vehicle sector.

Types of seals in the hydraulic system

Hydraulic systems consist of several seal types, each of which takes on specific tasks. Only the coordinated interaction of these components ensures that pressure is maintained, movement is enabled and contamination is prevented. Each seal type fulfils a clearly defined function within the system.

Rod seals

Rod seals seal the piston rod to the outside and reliably prevent hydraulic oil from escaping. They are in direct contact with the moving surface and must therefore have high wear resistance as well as good sliding properties. At the same time, they must withstand pressure peaks and remain permanently tight, even under changing operating conditions.

Piston seals

Piston seals provide the sealing within the cylinder and separate the pressure chambers from one another. They are decisive for force transmission in the system, since even minor leaks lead to loss of performance. Depending on the design, they must work reliably during both slow and fast movements and ensure uniform sealing.

Wiper seals

Wiper seals remove dirt, dust and moisture from the piston rod before they enter the system. They act as the first protective barrier and contribute significantly to the service life of all downstream seals. Without functioning wiper seals, particles enter the system, which can lead to increased wear and failures.

Guide rings

Guide rings do not perform a direct sealing function, but they are essential for the stability of the system. They guide moving components and prevent direct contact of metal surfaces. As a result, they reduce wear, ensure uniform force distribution and support the function of the actual seals.

In practice, these components always work together. A single wrong component can impair the entire system.

Materials for hydraulic seals

The choice of the right material influences the entire performance capability of the seal. NBR is one of the most frequently used materials and impresses with good oil resistance as well as a balanced price-performance ratio. Typical areas of application are at temperatures of around −30 °C to +100 °C and medium pressure loads.

FKM is used when higher temperatures or aggressive media play a role. The material remains stable even at temperatures up to around +200 °C and is suitable for demanding industrial applications.

PTFE offers particularly low friction values and very high chemical resistance. It is preferably used at high sliding speeds and with demanding media. Temperature ranges frequently extend from −200 °C to +260 °C.

Polyurethane is characterised by high wear resistance and pressure resistance and is therefore frequently used in dynamic applications. PU shows its strengths particularly at high pressures of over 400 bar.

The material choice always depends on the medium, temperature and mechanical load and should be matched exactly to the respective application.

Material comparison at a glance

Material	Temperature	Pressure	Friction	Application
NBR	−30 to +100 °C	medium	medium	standard applications
FKM	up to +200 °C	medium	medium	high temperatures
PTFE	−200 to +260 °C	high	very low	high performance
PU	−40 to +120 °C	very high	low	dynamic systems

Important selection criteria

• Pressure load: Typical hydraulic systems operate between 100 and 400 bar, high-pressure systems significantly above that. The seal must be designed accordingly.
• Temperature range: Areas of application often range from −20 °C to +120 °C, special applications significantly above that.
• Media resistance: Hydraulic oils, additives or aggressive media directly influence the material choice.
• Movement: High speeds require low-friction materials such as PTFE, slow movements allow more robust solutions.
• Installation space and tolerances: Precise manufacturing decides the sealing effect.

Decision logic for practice

• High pressure → use PU or PTFE
• High temperature → choose FKM
• High speed → prefer PTFE
• Dirt exposure → combine wiper seal + robust materials
• Standard application → NBR sufficient
• Pressure load: Hydraulic systems often operate with very high pressures. The seal must be able to absorb these permanently without deforming.
• Temperature range: Extreme temperatures influence elasticity and service life. The material must match the area of application exactly.
• Media resistance: Hydraulic oils, additives or aggressive media attack materials to varying degrees.
• Movement: Dynamic applications such as piston movements require wear-resistant materials, while static applications place different demands.
• Installation space and tolerances: The seal must fit exactly to the groove and geometry. Even small deviations lead to leaks.

Typical mistakes in selection

Many problems arise from fundamental wrong decisions:

• Wrong material for temperature or medium
• Underestimating the pressure load
• Neglecting friction and wear
• Unsuitable design for the application
• Lack of coordination between seal and mating surface

These mistakes usually lead to premature failure and unnecessary maintenance costs.

Typical damage patterns and causes

In practice, problems often become apparent through clear damage patterns that allow conclusions to be drawn about the cause.

• Leakage: Frequently caused by the wrong material choice, insufficient tolerances or damaged sealing lips.
• Scoring: Arises from dirt particles or unsuitable mating surfaces.
• Material hardening: Occurs at unsuitable temperatures or with the wrong material.
• Abrasion: Indication of excessive friction or insufficient lubrication.

The analysis of this damage enables targeted optimisation of the seal selection.

Designing the seal correctly

In addition to selecting the right seal, the design plays a central role for function.

The surface roughness of the mating surface significantly influences the sealing effect and wear. Surfaces that are too rough lead to abrasion, surfaces that are too smooth can impair lubrication. The installation clearance must be defined exactly, since tolerances that are too large encourage leaks and tolerances that are too small increase friction.

Lubrication in the system also contributes significantly to service life. Insufficient lubrication increases friction and leads to faster wear. At the same time, the seal must be optimally matched to the mating surface in order to ensure uniform loading.

Selection process step by step

1. Define operating conditions Pressure, temperature, medium, movement
2. Determine seal type Piston, rod, wiper or combination
3. Select material Based on load and environment
4. Check geometry Groove, tolerances, installation space
5. Consider service life and maintenance Assess economic efficiency in the long term

Selection matrix for hydraulic seals

Application	Pressure	Temperature	Material	Seal type
Standard hydraulics	medium	normal	NBR	rod seal
High-pressure systems	high	normal	PU	piston seal
Chemically exposed	medium	high	PTFE	special seal
High temperatures	medium	high	FKM	rod or piston seal

This overview serves as initial guidance for typical areas of application.

Industry examples

In mechanical engineering, the focus is on high pressure loads and continuous movements, which means wear-resistant materials are preferably used. PU and PTFE in combination dominate here.

In agriculture, dirt, moisture and changing temperatures are added. Robust sealing systems with wiper seals are standard here.

In vehicle hydraulics, compact designs and fast load changes play a role. Low-friction materials ensure efficiency and a long service life.

Internal links for selection

For a well-founded decision, combining this with further knowledge on elastomers, sealing materials and engineering plastics is recommended. These connections help to make the selection more precisely and to avoid typical mistakes.

In mechanical engineering, the focus is on high pressure loads and continuous movements, which means wear-resistant materials are preferably used. In agriculture, changing environmental conditions are frequently added, which means seals must additionally be protected against dirt and moisture. In vehicle hydraulics, compact designs and changing load conditions play an important role.

Practical example

A hydraulic cylinder in mechanical engineering operates under high pressure and continuous movement. In this environment, mechanical loads act continuously on all components. The seal must both seal and work with low friction while at the same time being protected against wear.

In practice, a combination of different materials is frequently used in order to specifically exploit the respective advantages. A wear-resistant solution on the piston rod ensures stability in operation, while low-friction components inside the cylinder increase efficiency. In addition, a wiper seal protects the system from external influences.

Through this coordinated combination, the service life can be significantly increased and the maintenance effort reduced.

Glossary of important terms

• Shore hardness: Measure of the hardness of an elastomer.
• Friction value: Figure for the resistance between two moving surfaces.
• Extrusion: Deformation of the seal under pressure into gaps.
• Media resistance: Resistance to liquids and chemicals.

Support with selection

Selecting the right hydraulic seal requires experience and a precise understanding of the operating conditions. A precise coordination of all factors ensures reliable systems, lower wear and plannable maintenance intervals.

Individual advice helps to find the optimal solution for your application and to reduce costs in the long term.

Frequently asked questions

Which seal is suitable for high pressures in hydraulic systems?

Polyurethane and PTFE are frequently used for high pressures. PU impresses with high wear resistance and pressure resistance, while PTFE scores in demanding applications with low friction and high media resistance.

Which material is suitable for high temperatures?

FKM is particularly well suited for higher temperatures. This material remains stable even under strong thermal load and is therefore frequently used in demanding industrial applications.

When is PTFE a good choice?

PTFE is particularly suitable for high sliding speeds, low friction and demanding media. PTFE is also often a suitable solution at very high or very low temperatures.

Why are wiper seals so important in hydraulic systems?

Wiper seals keep dirt, dust and moisture away from the piston rod. As a result, they protect the downstream seals and contribute significantly to a longer service life of the entire system.

What role do tolerances play in seal selection?

Precise tolerances are very important for the sealing effect. Even small deviations in the groove, installation space or mating surface can lead to leaks, increased wear or uneven loading.

What causes typical damage to hydraulic seals?

Frequent causes are the wrong material choice, high friction, insufficient lubrication, dirt ingress or unsuitable mating surfaces. Typical consequences are leaks, abrasion, scoring or material hardening.

How can the service life of hydraulic seals be increased?

A long service life results from the right combination of material, design, clean engineering, suitable lubrication and effective protection against contamination. A coordinated selection of all sealing components also significantly improves the service life.

Which seal is suitable for a standard application?

For many standard applications, NBR is an economical and proven solution. The material offers good oil resistance and is suitable for medium pressure and temperature ranges.

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