Scrapers
Rod seals
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O-ring measuring towers
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Large format 3D printing
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Plastic Components with Flame Retardancy
U-Cup / Lip Seal
Definition and Classification
A U-cup (also called a lip seal; in German “Nutring”) is a dynamic seal for hydraulics and pneumatics. Dynamic here means: it seals at a location where components move against each other. For this purpose, the U-cup sits in a groove (a circumferential, geometrically defined recess in the component) and rests with its sealing edges against the mating surface.
It is mainly used in cylinders. There, it typically seals either the piston against the cylinder tube (piston seal) or the piston rod against the environment (rod seal). The goal is that the pressure medium (e.g., hydraulic oil or compressed air) does not escape at the moving sealing point, or only in a controlled, minimal amount, so that the pressure chamber remains stable.
Operating Principle (Pressure-Energized Lip)
The U-cup frequently has a U-shaped cross-section. The two legs form sealing lips — that is, narrow, defined contact zones to the mating surface. Even without system pressure, the preload from the material elasticity ensures a baseline seal. This preload matters so that the lip seal also seals at low pressures and so that leakage does not occur immediately when motion starts.
As the pressure in the system rises, it acts into the U-space and supports the lips. The sealing lip is pressed more strongly against the running surface, with the result that the sealing effect rises with pressure. This behavior is particularly typical for translational motion — that is, the back-and-forth movement of the piston or piston rod. In practice, friction behavior is a central secondary condition here, because higher contact pressure often also raises the friction force and thus heat and wear.
Piston vs. Rod Seal
| Installation location | Where does the seal sit? | What is sealed? | Typical consequence |
|---|---|---|---|
| Piston seal | Groove on the piston | Pressure chambers in the cylinder against each other | frequently bilateral pressurization relevant |
| Rod seal | Groove in the cylinder head/cover | Pressure chamber against the environment | dirt ingress and surface protection become more important |
Designs and Materials (Practice-Oriented)
U-cups come in symmetrical and asymmetrical profiles. A symmetrical profile can often be used more flexibly, because it is less strictly tied to one main pressure side. An asymmetrical profile, by contrast, is frequently optimized for a preferred pressure direction — for example through a robust main lip or an additional lip that keeps dirt away.
Pressure application also plays a role in the selection. In many applications, pressure acts only from one direction (single-acting), while in cylinders alternating pressure directions often occur (double-acting). The profile and installation situation must then fit accordingly, so that the lips work sensibly in both load cases.
Among the materials, a few standard groups dominate. They differ primarily in wear behavior, friction, and media resistance:
| Material (example) | Strengths | Typical limits |
|---|---|---|
| NBR (nitrile rubber) | good standard for many hydraulic oils, cost-efficient | limited resistance with special media/temperatures |
| PU (polyurethane) | often very wear-resistant, mechanically robust | friction and media compatibility are more application-specific |
| PTFE (polytetrafluoroethylene) | very low friction, chemically resistant, low stick-slip tendency | frequently needs a preload element, sensitive to assembly errors |
Which combination fits depends on which medium is used, which temperature applies, and how fast the motion is. Especially at dynamic sealing points, friction and wear are worth treating as equally weighted selection criteria, because they directly influence service life.
Design and Typical Risks (Gap Size, Extrusion, Surface)
A central design topic is the gap size. This refers to the gap that is unavoidable by design between moving and fixed components — for example between piston and cylinder tube or between rod and guide. At high pressure, sealing material can be pushed into this gap. This process is called extrusion and often leads to material loss, breakouts at the lip, and premature leakage.
Alongside pressure, temperature, speed, and the pressure direction also influence the risk. Pressure peaks are particularly critical, because they generate very high loading for short periods. In many cases, a guide is also needed when transverse forces act. Here, guide rings come into play, which absorb side forces and thus keep the sealing gap stable.
When Are Back-Up Rings Sensible?
Back-up rings (anti-extrusion rings) are sensible when the combination of high pressure and larger gap size makes extrusion likely. They sit on the pressure side of the seal and support the material so that it cannot flow into the gap. In practice, this is particularly relevant when the system operates with pressure peaks or when, for manufacturing and tolerance reasons, a very small gap cannot be realized.
Surfaces and Installation Conditions
The sealing function depends strongly on the quality of the sliding surface. A surface that is too rough acts like sandpaper and increases wear. An unfavorable surface can also promote micro-leakage, because the lip does not hold contact stably. Equally important is a clean, dimensionally accurate groove geometry, so that the U-cup is correctly seated, sufficiently guided, and does not twist or get damaged.
Specific limit values — for example for the recommended roughness (Ra) — are generally data-sheet-dependent, because they depend on profile, material, and operating conditions. For design, the following parameters are therefore typically clarified: pressure level and pressure direction, medium, temperature range, speed, gap size/tolerances, and the required leakage class.
In the end, the interplay of geometry, material, and surface quality often decides on service life. Under demanding operating conditions, specialized consultation or manufacturer approval can be sensible.
