Scrapers
Rod seals
Piston Seals
Guide rings
Back-Up rings
O-Rings / X-Rings
Flat seals
Spring-loaded seals
Radial shaft seals
Rotary seals
High pressure seals
Sealing sets
O-ring measuring towers
Turned parts
Milled parts
Turned and milled parts
Bushes or bearings
Large format 3D printing
Post-processing of 3D printed parts
Heat-resistant 3D printed components
Plastic Components with Flame Retardancy
Water Hydraulics
Definition and Distinction from Oil Hydraulics
Water hydraulics is the transfer of force and motion in hydraulic systems with water or water-based pressure fluids as working medium. The medium is pressurized by pumps and moves actuators such as cylinders or motors via valves and pipes. For sealing technology, what is decisive is how the pressure medium behaves at the sealing edge, mating surface, and in the sealing gap.
Compared with oil hydraulics, water differs primarily through its strong polarity (water molecules attract each other electrically). This polarity changes the interaction with elastomers and plastics and can cause swelling or hardness changes. Water also has a low lubrication effect, which is why friction and wear at dynamic sealing points (moving seals, e.g., piston and rod seals) more frequently become the dominant design risk. At the same time, water promotes corrosion on metallic components; corrosion products can act as hard particles and damage sealing edges.
Why is water hydraulics used? Frequently when fire protection, cleanliness (e.g., no oil carryover), or environmental aspects (leakages) are in focus. These advantages are usually paid for with higher demands on materials, surfaces, and cleanliness.
Water-Based Hydraulic Fluids: Classes and Significance for Material Selection
In practice, water hydraulics is not limited to pure water. Many systems use water-based, fire-resistant hydraulic fluids whose composition and additive packages strongly influence seal compatibility. Decisive is therefore first the question: which fluid is actually being used and in which class does it lie?
The common classification distinguishes among others HFA, HFB, and HFC. It serves as orientation for water content, polarity, and additives — and thereby for the risk of swelling, embrittlement, or hydrolytic breakdown.
| Fluid class | Base type | Orientation on water content | Important for sealing technology because … |
|---|---|---|---|
| HFA | Oil-in-water emulsion | Often ≥ 80 % | Very water-like: low lubrication, corrosion and compatibility questions become prominent |
| HFB | Water-in-oil emulsion | Typically > 40 % | Combines oil and water portions: material reactions can lie between oil hydraulics and water operation |
| HFC | Water-glycol | Frequently approx. 35–65 % | Polar, additivized: friction, swelling, and aging depend strongly on recipe and temperature |
The classification is not pure formalism. It practically answers the question of whether a sealing material is loaded rather oil-specifically or rather water/glycol-typically. As a result, it becomes clear early whether a material proven in oil hydraulics will likely fail in water hydraulics or at least has to be tested.
HFA, HFB, HFC: Brief Characteristics and Typical Consequences
With HFA, the water content dominates. This reduces lubricating film formation at the sealing edge and increases the importance of surface quality and guidance. With HFB, oil and water phase act together; seals can benefit from a certain lubrication component but still react to water and additives. HFC (water-glycol) is a frequent compromise in many systems but brings its own interactions with elastomers through glycol and additives.
For sealing technology, this means: the same material designation (e.g., PU or FKM) can deliver very different results depending on recipe and fluid class. Without media specification, resistance remains an assumption.
Sealing-Related Challenges in Water Hydraulics
In water hydraulics, it is often not a single effect that decides but the combination of friction, corrosion, and mechanical loading. Because water lubricates worse, friction work at dynamic seals rises. This increases wear at sealing lips and can promote stick-slip (jerky sliding) when guidance and lubrication are not suitable.
Corrosion acts as an amplifier. When metallic components rust, particles arise. These particles are frequently hard and act abrasively — that is, like grinding media. They damage mating surfaces and sealing edges, whereby leakage and further wear increase. Cleanliness, filtration, and low-corrosion materials are therefore particularly relevant in water-based systems.
Mechanically, water hydraulics is often operated at high pressures and dynamic load cycles. Pressure peaks and high sliding speeds increase the risk of extrusion. Extrusion means that sealing material is pressed into a gap between components and shears or tears there. Temperature cycles act in addition because they can change material stiffness, swelling, and friction coefficients.
Typical Damage Patterns and Test Parameters
When evaluating a seal for water hydraulics, the question “does it swell?” is not sufficient. In many cases, damage only shows over time through aging and friction wear. Frequent damage patterns are:
- Swelling or shrinkage: dimensional changes through media absorption or release.
- Hardness change: material becomes softer or harder; both can worsen the sealing function.
- Hydrolytic breakdown: chemical degradation through water, particularly relevant with certain PU/TPU types.
- Extrusion and edge tear-off: under pressure, gap, and insufficient support concept.
As practical test and evaluation parameters, the following are frequently used: compression set (permanent deformation after compression), friction/wear in dynamic operation, and aging under temperature in the real fluid. These parameters answer the question of whether a seal maintains contact pressure, geometry, and surface condition after weeks or months.
Material Selection for Seals: Orientation by Elastomers and Plastics
Material selection sensibly starts with the question of which medium (HFA/HFB/HFC or water) is present at which temperature and with which additives. Then it becomes clear whether chemical compatibility or rather wear and extrusion resistance plays the main role. In sealing technology, this decision is frequently supplemented by design measures — for example through back-up rings against extrusion or through optimized mating surfaces.
The following overview provides orientation but does not replace a media-related release, because recipes and additives can shift behavior noticeably:
| Material family | Tendency in water / water-glycol | Typical limits from a sealing perspective |
|---|---|---|
| EPDM | Frequently well suited | Usually unsuitable for mineral oils; specific release depends on additives and temperature |
| NBR | Often critical in strongly water-based media | Can react unfavorably; preferred for oil hydraulics |
| FKM | Partly suitable depending on recipe, often requires testing | Water/glycol suitability is strongly fluid- and recipe-dependent |
| PU/TPU | Mechanically strong (abrasion/extrusion) but selective | Risk of hydrolytic breakdown under moisture/heat; type selection and verification are central |
| PTFE | Very good chemical resistance | Not elastic; frequently requires preload elements and suitable design |
At high pressures, extrusion protection is frequently decisive. Back-up rings reduce the free gap and thereby the extrusion tendency. At the same time, surfaces and guidance remain important, because otherwise low lubrication leads to early sealing edge wear.
Short Checklist for Design (Fluid, Operation, Construction)
- Clarify fluid class and specific recipe: HFA/HFB/HFC or water, additives, water quality.
- Define operating conditions: temperature, pressure, pressure peaks, sliding speed, cycle rate.
- Evaluate cleanliness and corrosion: filtration, materials, possible particle sources.
- Pre-select material and safeguard the design: e.g., check candidates like EPDM for many water / water-glycol cases; plan back-up rings at high pressures.
- Validate: media storage, friction/wear tests, and practical experience in the specific fluid are often decisive.
In the end, what counts is whether seal, mating surface, and medium run stably as a system. When application limits are tight or the fluid mix changes, specialized consultation and test-based release are usually sensible.
