Wärtsilä Shaft Line Solutions technical bulletin Wärtsilä Sternguard E Seals

Introduction

For more than 25 years, Wärtsilä Shaft Line Solutions’ Sternguard E seals have provided robust and cost-effective sealing for stern shafts on several thousand medium-sized commercial vessels ranging from tugs to trawlers and coasters. The impressively broad range of applications for which the seals can be used stands as proof of the core design’s durability. Simple, hard-wearing and flexible, this innovative design underpins the widespread acceptance of the Wärtsilä Sternguard E seals among vessel owners in several sectors.

Wärtsilä Sternguard E seals have been approved by all leading classification societies as meeting their requirements for wear and corrosion protection. They can be used with oil-, grease- and water-lubricated bearings and are available in three variants: a non-split inboard seal (Wärtsilä Sternguard EJ), a partially split inboard seal (Wärtsilä Sternguard EL) and a non-split outboard seal (Wärtsilä Sternguard EK). The seals are suitable for use with all stern shaft bearing lubrication systems and can operate in silty or abrasive water conditions.

Wärtsilä Sternguard E seals

Wärtsilä Sternguard EJ - non-split inboard seal

In use, the Wärtsilä Sternguard EJ seal is clamped to, and rotates with, the shaft, eliminating shaft wear and – because there is no friction against the shaft or a liner – these components will not need costly maintenance or refurbishment. The clamp also maintains the compression in the rubber sealing material that was set during installation.

Because the Wärtsilä Sternguard EJ is a non-split design, replacing the seal during a drydock requires the shaft to be dismounted before the body assembly (including the face) is removed.

Wärtsilä Sternguard E seals are equally suited to rudder stocks. Apart from the stock’s vertical orientation and lower rotation speed, the sealing requirements are similar. For this application, the EJ version is generally used for both top and bottom seals, with a clamp ring in both locations. Grease lubrication is catered for, rather than oil or water.

Wärtsilä Sternguard EL - partially-split inboard seal

A Wärtsilä Sternguard EL seal is a semi-split seal. Its split faces are held within a phosphor bronze face carrier, with a rubber sealing strip to seal the face insert into the carrier and apply radial compressive load to the insert.

In choosing whether to use the Wärtsilä Sternguard EJ or the EL inboard seal, consideration should be given to the non-split version’s ease of installation and drive pin function (this enables the seal body and face to rotate together securely) versus the split version’s ease of maintenance, bearing in mind the slightly increased risk of leakage with split seals.

Wärtsilä Sternguard EK - non-split outboard seal

A single option is available for the outboard seal, the non-split Wärtsilä Sternguard EK model. Unlike the inboard seals, the EK does not need a clamp because it is compressed between the propeller and stern frame. This arrangement also ensures that there is no shaft or liner wear, reducing the need for later shaft removal and refurbishment.

Standard and abrasion-resistant models of the EK variant are available and both can be supplied with oil ports on request. These seals are ideal for use with small thrusters or where the propeller is removable; other Wärtsilä seals are available for use with large thrusters.

Wärtsilä Sternguard E seals design features

All Wärtsilä Sternguard E seals have rubber bodies, which removes any possibility of galvanic corrosion that might arise if metal springs were used. It also accommodates the shaft’s axial, radial and angular motions and forms the seal’s loading component, imparting sufficient axial force to create a seal against the seal’s metal face.

This face is an integral part of the seal body for the non-split Wärtsilä Sternguard EJ and EK variants, while the split faces in the Wärtsilä Sternguard EL type are held within a phosphor bronze face carrier, with a sealing strip to seal the face insert into the carrier and apply radial compressive load to it. The seal also has a drive pin, as described above which enables simultaneous rotation of the seal body and face. The seal’s seat is a simple flange which can be modified. For the Wärtsilä Sternguard EK seal, this involves a seat insert supported by two concentric O-rings.

The rubber has a service life of 10 years from the date when it is cured and this date is advised on delivery. A stock of these rubber seals is held at the Wärtsilä Spain factory in diameters from 50mm, (Wärtsilä Sternguard EK and EJ) 70mm (Wärtsilä Sternguard EL) to 330mm (for all variants), so obtaining spares and replacements is quick and straightforward. Stock is also available for non-machined bodies.

Stock levels and turnover are such that there is likely to be at least eight years of use left when a new seal is delivered, or the rubber component is supplied as a spare part.

A special tool is supplied to fit the inboard seals, which compresses their rubber sealing material to its correct length while a clamp is secured to maintain that dimension; no other specialist tools are required to fit the seals.

No clamp is needed for the outboard seal, which is compressed between the propeller and stern frame, and in water-lubricated systems, an outboard seal may not be needed if the stern bearing has an ‘open water’ arrangement. For applications in which the stern tube and the propeller hub are highly distanced, an outboard version of the Wärtsilä Sternguard EJ seal can be installed. This will ensure the drive clamp can secure the working distance and the rotation of the seal body together with the shaft.

All three seals have the same performance and design features, and all are supported with strong but flexible elastomer bodies with integral face inserts. This maintains seal integrity despite both radial and axial shaft movements. In use, inboard seals can be checked without dry docking if the seal has an additional inflatable seal option, which reduces maintenance downtime and provides operators with additional time to focus on other operational aspects of their vessel.

All three versions can be supplied with or without inflatable seals (only advisable for the Wärtsilä Sternguard EJ and EL seals), which are actuated by air or fluid pressure and compress onto the shaft.

The table below summarises the main features shared by the various models:

Face seal benefits

Unlike most seals used on medium-sized vessels, Wärtsilä Sternguard E seals are face seals, giving important operational benefits over the more usual lip seals. While lip seals create a seal against the shaft, face seals use compression between two surfaces perpendicular to the shaft to form a seal. This means there is minimal wear on the shaft and allows for greater radial movement, which could otherwise break the seal or damage the shaft if a lip seal were used.

Face seals are also more compact than lip seals, which need a certain amount of length along the shaft line to create chambers for oil, but the Wärtsilä Sternguard E seals are particularly compact due their simple construction. 

A face seal is also easier to cool when that is required for oil-lubricated or closed water-lubricated systems. Cooling fluid can be channelled to within a few millimetres of the points of friction, which are at the faces rather than the shaft liner, as would be the case for lip seals.

Face seals also perform better if the shaft bearing should wear, causing the shaft itself to move lower in the bearing. Face seals are unaffected by this movement, but lip seals would experience a change in their compression, which can lead to water ingress.

This is a result of how the two types of seal experience friction: for face seals, it is axial while lip seals contend with radial friction. Because of this, any change in the shaft’s position within the bearing affects a lip seal’s shaft contact point and thus its effectiveness, while for a face seal it simply changes the location of the contact point but makes no significant change to the seal’s performance.

Conclusion

This brief overview of Wärtsilä’s Sternguard E seals underlines their versatility and simplicity, which combine to make them the ideal choice for small- to mid-sized commercial applications. Whatever the bearing’s lubrication system and the ship’s operating environment, these seals offer a compact and reliable solution, both for stern shafts and rudder stocks.

Combined with excellent service support that is backed by rapid spare parts supply from stock, Wärtsilä Sternguard E face seals provide a compact, competitively priced, reliable and long-lasting companion technology for any bearing installation.