Scrapers
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Radial shaft seals
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Plastic Components with Flame Retardancy
Radial Shaft Seal (Oil Seal)
Definition and Purpose
A radial shaft seal (also called an oil seal; in German “Simmerring”) is a contacting seal for a rotating shaft. Contacting means that an elastic sealing lip rests on the shaft surface with a defined force. The seal sits with its outer shell in the housing and seals radially — that is, transverse to the shaft axis, directly on the shaft circumference.
What is it used for? It keeps lubricants such as oil or grease in the system and at the same time reduces the ingress of dirt, dust, or splash water from the outside. Radial shaft seals are therefore frequently found in gearboxes, motors, pumps, and bearings — wherever a shaft leads out of a lubricated housing.
The term “Simmerring” is a widespread colloquial name in German usage. Technically correct is radial shaft seal, because this name precisely describes the function (radial sealing at the shaft).
Structure and Operating Principle
A radial shaft seal at its core consists of an outer shell (metal or rubberized) and an elastomeric sealing lip. Frequently, a garter spring is integrated, which preloads the lip radially. In many designs, a dust lip is additionally added, sitting on the air side and protecting against external contamination.
How does it seal despite rotation? The sealing lip forms a defined contact zone with the shaft. In operation, a very thin lubricating film arises there, which limits friction and wear. The geometry of the lip and microscopic effects at the contact face can also support a pumping back effect. In this, an unavoidable small leakage is tendentially pumped back toward the oil side, so that the sealing effect remains stable.
Components Briefly Explained: Sealing Lip, Garter Spring, Dust Lip
- Sealing lip (main lip): It seals the medium (oil/grease) and is the functional center of the component.
- Garter spring: It keeps the contact force comparatively constant across temperature changes and increasing wear. As a result, the sealing function remains more stable over the application time.
- Dust lip (protective lip): It is a second lip on the outer side. It reduces the ingress of dirt but is usually not designed for the actual media sealing.
Designs, Materials, and Application Limits (Selection Guide)
Dimensions and basic designs are frequently based on DIN 3760 or ISO 6194. In practice, basic types such as A/B/C are frequently encountered. Variants with an additional dust lip are often marked with an S (e.g., AS/BS/CS). The specific design influences installation, sealing behavior, and robustness against contamination.
A central selection criterion is the material of the sealing lip, because it determines temperature, medium, and friction noticeably. Equally important are the application limits: radial shaft seals are typically designed for low pressure differentials. Rising overpressure can promote leakage and, depending on the application, requires special designs or additional measures. Speed (circumferential velocity), temperature, media chemistry, and contamination also set practical limits.
| Aspect | What matters | Typical consequence |
|---|---|---|
| Pressure differential | Usually only low overpressures are suitable | Special design may be needed under pressure |
| Speed / circumferential velocity | Influences friction, heat, lubricating film | Material and lip must match |
| Temperature | Determines elastomer hardness and aging | Wrong material leads to embrittlement / swelling |
| Medium (oil, additives, chemistry) | Influences resistance | Material selection is decisive |
| Contamination | Increases abrasion and lip damage | Dust lip / protective measures sensible |
Material Overview: NBR, FKM, PTFE
NBR (nitrile rubber) is frequently the standard choice for many mineral oils and customary temperatures. FKM (fluoroelastomer) is often used when higher temperatures or more demanding media occur. PTFE is very low in friction and can offer advantages at higher speeds, but it is frequently implemented in design concepts that differ from those of classic elastomer lips, because it behaves differently.
Shaft, Installation, and Typical Sources of Error
The sealing function depends strongly on the shaft, because the lip runs directly on its surface. Decisive is not only the roughness, which is often described via parameters such as Ra or Rz, but also the surface structure. A surface that generates lead (helical machining marks) through processing can unfavorably affect the pumping behavior at the lip and promote leakage. Concentricity, eccentricity, and the hardness of the running surface also affect wear and tightness.
During installation, what counts above all is that the sealing lip remains undamaged and does not run dry. In practice, a few clear rules help:
- The shaft edge needs a chamfer or a radius, so that the lip does not tear during installation.
- Before start-up, the sealing edge is usually wetted with the future lubricant, so that no dry running occurs.
- The installation direction must be correct: the main lip points toward the side of the medium to be sealed.
- Tools and seating surfaces must be clean, because dirt particles can damage the lip immediately.
Common sources of error are a damaged lip due to sharp edges, an unsuitable shaft surface (in particular with lead), dry installation, and a wrong combination of material, temperature, and medium. When operating conditions are unclear, brief specialized design or consultation is sensible.
