seals & bearings
of the bearing and a resultant increase in contact area.
The new bearing segments can be pre- machined at the Orkot factory and no additional machining is required at the shipyard. Segments are easily fitted using simple tools and may be removed and replaced, in an emergency, without docking the vessel and provided split composite rope guards are used. The bearing systems can be retrofitted into any existing plain bearing housing without modification. When sterntube and bracket bearings are not required to be removed with the shaft in place, semi-finished tubes can also be supplied for shipyard machining. These can be fitted into the bronze or composite housings by removing the existing rubber bearing from the housing, usually by machining. The new bearings can then be refitted by press or by shrink fitting. Trelleborg
Sealing fully seal Solutions has
developed composite shaft coupling protection covers for both hydraulic and flange couplings, to
these from seawater ingress.
Designed for the life of the vessel, these can be removed and replaced with no need for epoxy bandaging or paint protection. Originally produced for the Royal Navy, with the trial cover fitted to HMS Cornwall Type 22 frigate 11 years ago, these covers have now been retrofitted to all Type 22, 23 and 42 vessels and are fitted new on Type 45 Queen Elizabeth class and Royal Navy of Oman’s Khareef corvettes. The system has proved successful in operation
12.0 maximum allowable clearance* 10.0 8.0
THORDON COMPAC SEAWATER LUBRICATED PROPELLER SHAFT BEARING CLEARANCES (for Grand Princess)
current clearance to date (Apr 2011) average of part and standard
6.0
4.0
2.0 also 0.0 0 2 4 6 8 10 12 14 16 18
based on classification societies maximum clearance recommendation for 642 mm (25.25”) shafts years
Based on recent inspection, the Compac bearing on M/V Grand Princess will achieve a service life of close to 20 years
with a recent inspection showing no corrosion or water ingress when a cover was removed. Manufactured
composite material, covers either the shaft liners or on split location
from Orkot machineable the
locate on
sleeves bonded to the shaft and sealed with O-rings and gaskets. To further protect the shaft coupling, the internal voids are filled with Orkot Envirogrease, which is specially formulated and biodegradable. MP
Briefing: water-lubricated bearings
Water-lubricated bearings were first used around 150 years ago to provide underwater support for propellers on the earliest generations of steam-powered vessels. The earliest water-lubricated bearings consisted of a tubular housing with longitudinal slots along the interior, into which were placed dense wooden staves, which reduced the running bore of the bearing. To control the lubricating water flow, Victorian design engineers optimised the gap between the staves and shaft surface, as well as the ratio of stave width and the ‘flute’, or gap, between the staves. Today’s water-lubricated bearing designs have come a long way since these early examples but still apply the same principles of optimisation. The limitations of early products were mostly due to the nature of the materials available at the time. Wooden staves, for example, were prone to rapid wear in service and needed regular replacement. This issue was not overcome until early in the twentieth century, when it was found that the wear
48 I Marine Propulsion I February/March 2012
characteristics of the rubber were far better, thus marking the end of the use of wooden staves. Nitrile rubber, installed in a stave- and-flute pattern inside metal tubes, then became the standard material in the design of water-lubricated bearings. The material was a logical choice as it exhibits minimum swell in water and is very hard wearing. It is still widely used in modern water-lubricated bearing products. Water-lubricated bearings are now seen in a range of industries and, within the marine sector, they are applied in a variety of applications. This is particularly so where low noise signatures are required and, for at least the last half century, rubber-lined water-lubricated shaft bearings have been commonplace in many types of surface ship and submarine used by navies worldwide. “It was thought for many years that shaft
rotation in a water-lubricated bearing gave lift to raise the shaft above the bearing surface,” said Mr Neave. “It was not until 2003 that sponsored work conducted by Liverpool
John Moores University proved this to be incorrect.” Practical in-service microscopic measurements
of operating bearings,
backed up with computational fluid dynamics analyses, demonstrated that, under the rotating shaft load, the flexibility of the rubber surface causes it to deflect. Although the effect is slight, this leaves a pocket a few microns in depth where water can support the shaft, although still just rubbing tangentially on either side of the pocket. This research explained why the use of non-elastomeric materials such as wood, or composites, creates much greater wear and noise than rubber, as their lack of flexibility does not enable them to deflect under shaft loading. Results also emphasised the importance of understanding how rubber technology enhances the service life of the bearing, by ensuring the rubber is optimally cured and possesses optimal wear and compression set properties.
Acknowledgement: Icon Polymer Group
www.mpropulsion.com 20
clearances (mm)
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