Oil Seal O18 - Rotary Shaft Seal

Design and construction of the O18 oil seal

Applications: where does the O18 perform best?

The O18 is mainly used in applications where two media must be kept strictly separated or where a medium must not leak to the outside in designs with a stationary shaft and a rotating housing, hub or drum. The seal is mounted on the shaft and seals towards the outside, functionally acting as the mirror image of an SC seal in a stationary housing. Typical examples include bearing chambers filled with oil next to a compartment containing coolant or process fluid, as well as special drives, gearboxes and pumps in relatively clean industrial environments. The O18 keeps the lubricant clean and prevents media from mixing, but because it has no dust lip it is less suitable for heavy contamination with mud, sand or water. In shielded housings, the single lip performs very efficiently and with low friction.

Materials: options and material selection in practice

The typical oil seal O18 is made of NBR 70, which has excellent resistance against mineral oils and greases and is a reliable option in most industrial applications up to approximately +100 °C. If the temperature is higher or there are synthetic oils, fuels or chemically loaded fluids present, then FKM is a logical next choice due to its heat and chemical resistance, while HNBR provides an intermediate option with a higher mechanical strength and higher ageing resistance than standard NBR. For specific applications, materials like Silicone, ACM, CR or EPDM are available on request. Silicone performs excellent at lower temperatures but is mechanically less robust, while EPDM is particularly suitable for hot water, steam and brake fluids without oil. In practice, you’ll always begin with identifying the medium and operating temperature and then choose an elastomer that offers the best chemical and thermal match.

Material Table

Comparison: O18 versus SC

Structurally, the O18 and the SC oil seal are very similar: both are single-acting radial shaft seals with a spring-loaded lip and an elastomer-sheathed outer diameter, without a dust lip. The SC is used for inside sealing in a stationary housing around a rotating shaft, while the O18 has been developed for outside sealing in designs with a stationary shaft and a rotating housing or hub. In practice, you can therefore see the O18 as the functional opposite of the SC, which you deliberately select as soon as you need an external seal around a stationary shaft .

How to choose the right O18 oil seal

Choosing the right O18 oil seal starts with the dimensions: the inner diameter must match the shaft diameter, the outer diameter the housing diameter and the width the available installation space. You then check fit and surface finish: for the shaft, ISO H11 is preferred with a minimum hardness of around 45 HRC and a fine, torsion-free ground surface (approximately Ra 0.2–0.8 µm), and for the housing ISO H8 with a roughness around Ra 1.6–6.3 µm. The rubber outer jacket makes the O18 slightly more forgiving than a seal with a metal outer diameter.

Once the geometry is correct, you select the material based on the operating medium and temperature, choosing from NBR, FKM, HNBR or one of the special materials listed above. Keep in mind that the O18 is designed for non-pressurised applications up to around 0.05 MPa and circumferential speeds up to about 10 m/s; above these limits, wear increases rapidly and you are more likely to end up with special high-speed or PTFE-based seals. In heavily contaminated outdoor environments, an O18 without dust lip is less suitable and a model with dust lip or an additional wiper is usually the better option, whereas in relatively clean, shielded housings it is a very efficient solution for outside sealing and media separation.

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