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Bearings for longer shaft lifeAUTHOR: Tim Biswell , Product Development Director, Seals and Bearings, Propulsion, Wärtsi lä in the United Kingdom

Fig. 2 – Railko’s 200 mm test rig.Fig. 1 – Sternsafe bearing installation.

Composite marine bearing materials are non-metallic and have been specially designed to cope with extremes of operating conditions: loads, speeds, temperature fl uctuations, dirty conditions, etc, and depending on the application and grade, these thermosetting resin laminates can operate dry, partially lubricated, or fully lubricated in oil or seawater.

The bearings offer signifi cant advantages in terms of reduced vessel through-life operating costs. These include reduced maintenance, improved reliability, and less wear to shaft materials. The key to their success, however, lies in the high technology composite material, the excellent R&D work, engineering design, and skilful manufacturing and

Following the acquisition of the Railko Marine business in July 2007, Wärtsilä now is the leading developer and manufacturer of high performance composite bearings.

assembly. They are applied as propeller shaft bearings, rudder bearings, steering gear and deck machinery bushes.

Composite bearings Composite rudder bearings have been developed over a number of years to take the pressures and stresses inherent in rudder assemblies, and to provide continued performance under the most arduous conditions. One of the most advanced materials for rudder bearings is the CY160LS material, a fi lament wound material with high compressive and impact strength and a proven ability to resist fatigue shock damage, thus ensuring long bearing life.

The use of composite oil-lubricated stern tube bearings was introduced more than thirty years ago with the WA80H material. Today, thousands of ships worldwide have demonstrated the product’s exceptional performance benefi ts, such as the ability to operate in a seawater/oil emulsion or pure

seawater under emergency conditions. The material is also “kind” to shafts and will not seize, as metallic bearings can. A low modulus of elasticity and reduced wear and tear, together with a reduced weight for ease of handling, are other advantages. The bearings themselves can be easily machined and are available either press fi tted or resin bonded to reduce shipyard costs. CY160LS, the fi rst-generation of non-asbestos oil-lubricated bearings, manufactured using an asbestos-free reinforcement impregnated with a phenolic resin, offers high compressive strength, impact strength and wear rate, to deliver a long bearing life on the 300 vessels to which it has been applied.

Sternsafe developmentThe latest technology for oil lubricated stern tube bearings, however, is SternsafeTM. Developed in conjunction with a major shipowner and Classifi cation Society to meet the latest industry demands, this technology is based on

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Fig 3. – Bearing wear after 100 hours in gritted seawater.

50mm Journal test rig – Testing in gritted seawaterBearing wear rate – DTI

NF22 (Railko) RG22 (Railko) Elastomeric

CY160LS (Railko) Rubber

1.201.101.000.900.800.700.600.500.400.300.200.100.00

mm

0 10 20 30 40 50 60 70 80 90 100 110

Table 1. – Test conditions.

Metric units Imperial units

Sleeve: Stainless steel EN ISO 316 AISI 316Sleeve diameter 50.8 mm 2 inchShaft rotation: 55 rpm 8.8 m/min 28.9 ft/minBearing load 2500 N 550 lbfBearing pressure 0.48 N/mm 69 psi 4.8 kg/cm2

PV rating 42 kg/cm2. m/min 2000 psi. ft/minLubricant fl ow rate 7.5 litres/min 1.65 lmp. gallons/min 120 US gallons/hourLubricant tank capacity 88 litres – agitated

Fig. 4 –Elastomeric bearing wiped.

a composite material with a newly developed, improved bearing surface, which incorporates all the benefi ts of the existing products but with increased strength and durability. Sternsafe can also operate with some environmentally friendly bio-degradable oils.

The fi rst vessel fi tted with Sternsafe was the container ship Maersk Antwerp.This retrofi t was completed in April 2002 and the vessel has now completed over fi ve years of uninterrupted service. Sternsafe has since been fi tted to almost 100 ships covering a wide range of different shiptypes, with every possible shaft size operating under a variety of operating conditions.

In addition to this, Wärtsilä has recently

supplied what is thought to be the world’s largest composite stern tube bearing. The bearing, which is designed to suit a shaft of over 1000 mm in diameter, is now operating successfully on fi ve vessels in a series of what is reported to be the world’s largest container ships.

Water-lubricated systemsAlongside its oil-lubricated sterntube seals and bearings, Railko water-lubricated systems have also formed part of the company’s product portfolio for over forty years, supplying more than 30 of the world’s navies. Extensive independent end-user testing of the shaft bearing materials has been undertaken by several navies, all of whom consequently

adopted the systems for use in surface and submarine fl eets. Wärtsilä’s acquisition of Railko also includes the company’s sophisticated laboratory that has been at the forefront of bearing technology for nearly half a century.

Advanced 50 mm and 200 mm propeller shaft bearing test rigs have been used to confi rm the performance of the non-asbestos NF21/22 material intrinsic to the water lubricated stern tube bearing system. The material allows the system to offer a signifi cant reduction in the shaft speed at which the point of hydrodynamic operation is achieved. As a result, bearing friction and wear are reduced dramatically. Service wear measurements predict that a bearing life approaching 90 years could be possible.

In order to compare materials in current use, an arduous test programme was devised to analyse the performance of various bearing materials, in highly abrasive conditions, against stainless steel counter face materials.

Substitute seawater was used with silica particles added. The grit used was equivalent in particle size and shape to that found in the UK Portland area at a concentration accepted by the UK Ministry of Defence to be representative of aggressive British coastal water.

Bearing up to testsTo accelerate the comparative test, the concentration was increased by a factor of 10. The grit was kept in suspension in the

WÄRTSILÄ TECHNICAL JOURNAL 02.2007

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Fig. 5 – Rubber bearing shaft after 800 hours. Fig. 6 – NF shaft after 2000 hours testing.

seawater by means of a stirrer agitating the solution in the supply tank.

A pump was used to deliver the gritted seawater to the bearing and re-circulate it back to the tank. The fl ow rate for each of the test bearings was set at 7.5 l/m. Interestingly, the pump did not survive the fi rst set of tests. Test conditions can be seen in Table 1. The initial testing comprised running each material under the stated conditions for a period of 100 hours, measuring the bearing wear rate at 20 hour intervals.

As can be seen from the results in Figure 3, even though all the materials were tested under the same conditions, there was a spread of wear results. Most materials performed well over this time period with the exception of the elastomeric material, where signifi cant bearing wear and smearing occurred, and scoring on the shaft liner was noted. For consistency, all bearings were tested with the multi-axial groove confi guration; this tested the performance of the bearing material and not the design.

The second phase took the two best performing materials, Railko NF and rubber, testing them over a period of 2000 hours. Initially the rubber material performed well in comparison to Railko NF, but over time the wear rate increased rapidly. The rate of wear was such that the test on the rubber material was stopped at around 850 hours.

The tests showed that that there was signifi cant scoring on the shaft on the

rubber bearing when compared to the original shaft condition. This was caused by the silica particles becoming embedded in the rubber material. These particles then scored the shaft resulting in the typical failure mechanism of this type of material. The “gramophone” effect created on the shaft cannot maintain a hydrodynamic water fi lm, resulting in shaft to bearing contact. This greatly accelerates the bearing wear, leading to extensive “run-away” wear down.

Railko NF materialBy contrast, the Railko NF material, with its standard bearing design, exhibited a more linear (steady state) wear result. The test in this case was continued to 2000 hours. The bearing was still capable of further operation. It is worth noting that the Railko NF bearing took almost twice as long to reach the same wear down level as the rubber bearing.

As can be seen from Figure 6, the shaft sleeve was not heavily worn. As Railko NF is relatively harder than rubber, the material does not allow abrasive particles to embed in its surface, therefore maintaining a hydrodynamic water fi lm and greatly increasing both bearing and shaft sleeve life.

The NF21/22 material working in conjunction with gunmetal journal has over 25 years of proven service as a water-lubricated propeller shaft bearing in both naval and commercial vessel applications. In addition, based upon the additional

recent test work completed, Wärtsilä can confi dently offer its standard water-lubricated bearing material with its standard design – for both deep water and dirty river water conditions.

The compatibility of NF and stainless steel liners provides a durable system with long bearing life and extended shaft life. Railko bearings can readily replace rubber or elastomeric bearing materials and can be supplied as fi nish machined bearings, for press-fi tting into the stern tube or bracket, or in tube form for fi nal machining in the yard.

The addition of the Railko bearing materials to the existing Wärtsilä seals and bearings portfolio enables Wärtsilä to offer fully cost-effective seal and bearing package solutions for both water and oil lubricated systems, placing the company in an excellent position to satisfy shipowner demands for a totally environmentally friendly propeller shaft system.