SubscribeRailko Limited is a leading developer and manufacturer of engineered high performance plastics and specialises in composite bearings for marine and industrial use.
Railko's bearing materials are non-metallic and have been specially designed to cope with extremes of operating conditions: loads, speeds, temperature fluctuations, dirty conditions, etc. Depending on the application and grade, these thermosetting resin laminates can operate dry, partially lubricated, or fully lubricated in oil or seawater.
Railko's marine bearings offer significant advantages in terms of reduced vessel through-life operating costs: reduced maintenance, improved reliability and 'kindness' to shaft materials. They are applied as propeller shaft bearings, rudder bearings, steering gear and deck machinery bushes.
WATER LUBRICATED PROPELLER SHAFT BEARINGS
Railko NF material was developed in the early 1980's for water lubricated (and dry) bearings applications - it is used by over 30 Navies around the World for water lubricated bracket and stern tube bearings in surface vessels and submarines. Railko water lubricated bearings are also used on ferries and supply ships.
Spanish KG Navy F101 Frigate
(Photo property of Navantia)
Railko was asked "Do you have a better material for propeller shaft bearings in dirty river conditions?" - to replace the rubber and elastomeric materials being used.
In order to compare materials in current use Railko embarked upon an arduous test programme to analyse the performance of various bearing materials, in highly abrasive conditions, against stainless steel counter face material.
'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.
To accelerate the comparative test, the concentration was increased by a factor of 10. The grit was kept in suspension in the seawater by means of a stirrer agitating the solution in the supply tank.
Test Rig
A pump was used to deliver the gritted seawater to the bearing and re-circulate back to the tank. The flow rate for each of the test bearings was set at 7.5 litres per minute. Interestingly the pump did not survive the first set of tests, it was noted not to be fitted with Railko bearings.
The applied load to the bearing was via a levered arm providing an applied load of 2500 N.
| Metric Units | Imperial Units |
|---|---|---|
Sleeve: Stainless Steel | EN ISO 316 | AISI 316 |
Sleeve diameter | 50.8 mm | 2 inch |
Shaft rotation 55 rpm | 8.8 m/min | 28.9 ft/min |
Bearing load | 2500 N | 550 lbf |
Bearing pressure | 0.48 N/mm | 69 psi |
PV rating | 42 kg/cm2. m/min | 2000 psi. ft/min |
Lubricant Flow rate | 7.5 litres/min. | 1.65 Imp. gallons / min |
Lubricant Tank Capacity | 88 litres - agitated | 88 litres - agitated |
The initial testing comprised of running each material under the stated conditions for a period of 100 hours, measuring the bearing wear rate at 20 hour intervals.
Graph 1
As can be seen from the results in Graph 1 above, 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 significant bearing wear and smearing occurred and scoring on the shaft liner was noted. For consistency, all bearings were tested with the multi-axial groove configuration; this tested the performance of the bearing material and not the design.
Elastomeric bearing wiped
The 2nd phase took the two best performing materials, Railko NF and rubber, testing them over a period of 2000 hours. As can be seen from the results shown in Graph 2, initially the rubber material performed well in comparison to Railko NF however, 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.
Scoring of shaft
Graph 2
Rubber bearing and Shaft
Looking to the pictures above of the rubber bearing and shaft sleeve after testing it can be seen that that there is significant scoring on the shaft in comparison to the original shaft condition shown right. This is caused by the silica particles embedding in the rubber material - these particles then score 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 film, resulting in shaft to bearing contact. This greatly accelerates the bearing wear, leading to the 'run-away' wear down shown in Graph 2 .
By 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.
NF bearing and Shaft
As can be seen from the photograph, the condition of 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 film and greatly increasing both bearing and shaft sleeve life.
"Do you have a better material for propeller shaft bearings in dirty river water?" - Based upon the test work completed, Railko has satisfied itself that it can confidently offer its standard water lubricated bearing material 'NF' - with its standard design - for dirty river water conditions. The compatibility of NF and stainless steel 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 finish machined bearings, for press-fitting into the stern tube or bracket, or in tube form for final machining in the yard.
Railko NF is approved by all major Classification Societies and can be used at 2:1 L:D ratio for the aft bearing, typically half that of some other water lubricated bearing systems.
'UN' Ro-Ro Photo Courtesy of Flensburger
Schiffbau Gesellschaft mbH & Co KG