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.
OIL LUBRICATED PROPELLER SHAFT BEARINGS
Traditional oil lubricated tailshaft bearings are ‘white’ metal lined – failures occur due to wiping of the liner due to: high loads, low speeds (where a hydro-dynamic oil film is difficult to generate), displacement of the tailshaft whilst manoeuvring and fatigue of the bearing metal caused by repeated shock loading. These types of white metal failures can be catastrophic, with the shaft becoming welded to the bearing.
ContainerShip(AK) ‘Maj Bernard F Fisher'
(US Navy photo)
Railko’s latest material development is SternsafeTM. Together with the well-proven CY160LS grade, these materials have been designed for oil lubricated tailshaft bearings and are applied to all sizes of merchant ships and cruise liners. SternsafeTM and CY160LS have proven not to seize or damage the shaft in the event of oil loss due to worn or wire-damaged seals. The benefit to the Owner/Operator is that the vessel can be converted to water lubrication at sea and get to its destination at normal speed.
There are no towing charges
- There is no need for machining down or replacing the shaft (assuming the wire has not done irreparable damage to the shaft)
- There is no need to run to the cost of keeping a spare shaft in stores and the additional cost of freighting it to the repair yard
- There need be no knee-jerk reaction to get the vessel into the nearest drydock – this can be planned – again resulting in minimal disruption to the chartering programme and enabling the most cost effective dry-docking location to be selected.
Railko ‘SternsafeTM’ is the first composite propeller shaft bearing material that holds all major Classification Society approvals for both oil and water lubricated systems.
For those Owners/Operators sailing through environmentally sensitive areas, Railko stern tube bearing materials are compatible with Vickers Hydrox Bio68.This biodegradable oil is compatible with most seal types and is approved by many seal manufacturers.
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 bearings are also used on ferries and supply ships.
Here, the benefits to the Owners are again the fact that Railko composites cause much less/no damage to the shaft than the ‘traditional’ rubber or elastomeric materials used. Sand particles embed in the softer materials and, acting like a piece of ‘sandpaper’, scratch the shaft. This scoring leads to the inability of the system to develop and maintain a full hydrodynamic water film; causing the shaft to touch the bearing and wear it away, resulting in the frequently observed undulating ‘cylindrical gramophone’ effect on the shaft. Owners complain about this scoring of the shaft but it has become second nature to locally weld up the shaft or replace it. Railko material, being relatively harder than rubber or elastomeric bearing materials, does not allow sand particles to embed in its surface and consequently maintains a hydrodynamic fluid film, causes less/no damage to the shaft and gives longer life.
Spanish Navy F101 Frigate
(Photo property of Navantia)
To summarise the benefits of using Railko NF material/design as water lubricated propeller shaft bearings; it is used because of its:
- low wear rate
- extremely low/nil noise signature
- ability to run hydrodynamically at very low RPM's - with low/no stick-slip
- higher thermal capability than rubber or elastomeric material
- proven shock resistance
- higher load carrying capacity than rubber or elastomers – therefore shorter bearing lengths are possible, leading to…..
- weight savings
- low thermal expansion - better stability (less reduction of running clearance) if there is a reduction of seawater (cooling) flow, causing over-heating of the system
- low swell characteristic
- creep resistance - does not allow the shaft to 'sink-in' to the bearing (squeezing the water out from the bearing/shaft interface), leading to a 'dry' (usually noisy) start
- kindness to shafts - does not tend to allow abrasive particles to embed in its surface (like rubber and elastomeric materials can), therefore causes less scoring of the shaft
- consistency of quality (not variable like Lignum Vitae)
- Railko design and supply complete bearing/housing assemblies
- ease of replacing existing rubber or other bearing materials/systems
- ease of machining/fitting and.............
Railko’s knowledge/technical support/track record of supplying materials to the industry - In conjunction with the UK Ministry of Defence, Railko have developed the 3rd generation of shell bearing design from the original stave (lignum vitae, thence to rubber) concept; this gives much better hydro-dynamic performance of the propeller shaft bearing system and better control of the shaft. The length of a Railko bearing is much shorter (half) than the traditional design of rubber bearing, resulting in less shaft:bearing misalignment issues.
Norwegian Navy F310 Frigate
(Photo property of Navantia)
Service time - the UK Navy has furnished data of a series of measurements from several Frigates which concludes that the life of a set of Railko NF propeller shaft bearings can be 97 years - obviously, they are changed earlier than that, at key docking periods - but they have the confidence to know that NF will last to the next planned docking. Independent tests, completed some years ago, show the performance of NF over an increasing/decreasing speed range in seawater with added abrasive particulate. The tests concluded that “NF had 1/5th the wear of rubber bearings, with less deterioration to the surface of the shaft sleeve and friction levels generally lower than rubber”.
RUDDER BEARINGS
Rudder, rudder head carrier and steering gear bearings are subject to some of the most arduous operating conditions encountered in marine engineering. Whilst metallic bearings may be suitable when loads are moderate and speeds are high enough (or arc of shaft movement is great enough) to create a continuous lubricant film, they are not the ideal solution for the rigorous demands of rudders and steering systems, where impact loading and lack of lubrication can severely limit the operational life of the bearing.
Railko have developed RG22 to meet the latest specifications set by the Classification Societies. Containing ‘built-in’ lubricants, the product not only meets the stringent requirements of pressure capability (RG22 is approved for pressures up to 10 N/mm2), low friction (very low stick-slip) and low swell in water, but it also has approvals for totally dry operation for upper pintle bearings.
OTHER BEARING APPLICATIONS
In addition to the SternsafeTM, CY160LS, NF and RG22 materials mentioned previously, Railko supply other materials from their range; be-it the general purpose JL material or the more specialised high temperature material RG15. With zero, or negligible lubrication requirements, tolerance to abrasive conditions and non-corrodibility, applications such as deck machinery bushes, pipe-work and tank expansion pads, seawater lift pump bearings, stabiliser bushes and many applications in submarines can all be serviced from the comprehensive range of Railko products.
