A look on water-lubricated composite bearings
Water-lubricated composite bearings are an ever-evolving area of material development, as Riina Haaja discusses.
Water-lubricated bearing applications are increasing in popularity as the shipping industry future-proofs its operations in preparation for tightening environmental regulation. But as water is not oil, it requires different bearing materials and designs to achieve the optimal friction, swell and durability. As such, new advanced materials are continuously being developed and perfected to ensure the best possible performance.
Propeller shaft bearings are some of the most critical components on a vessel that can affect performance. Finding the optimal material for water-lubricated composite bearings means improving the performance of the bearing and even the entire propulsion system, providing a longer lifecycle due to reduced wear. It is, therefore, no surprise that material development is high on the agenda of bearing manufacturers. Wärtsilä Lead Design Engineer, Seals & Bearings, Ross Strickland says he has seen major improvements in bearing product design in recent years.
‘In recent years, the focus has shifted mainly towards water-lubricated applications,’ he said. ‘We have developed a water-lubricated composite bearing product that uses an epoxy resin system with synthetic fibre reinforcement. Traditionally, the company’s bearings used phenolic resin, but the switch to epoxy has allowed for an advancements in manufacturing output and material performance.’
The main material properties required from composite bearings are low friction, low water absorption and a good wear life index. The compressive strength of the material does not need to be high, as the operating pressures for most applications are relatively low – only 0.6MPa for stern tube bearings and 10MPa for rudder bearings. Having some compliance in the material is an advantage, as this can help reduce issues of shaft misalignment and edge loading, which can often affect high compressive modulus materials.
‘Composite materials are used for water-lubricated applications primarily because of their anti-corrosion properties,’ said Strickland. ‘Although alignment of the shaft is essential, composite bearings are better equipped to withstand vibration and shaft misalignment,’ he added, explaining that very low friction levels are a significant benefit when running in water, as the lubricity of water is not as good as that of oil.
Improving dimensional control
The main material property changes moving from phenolic resin systems to epoxy are the reduction in material swell through water absorption. The phenolic bearing products typically have 1% swell allowance, whereas the epoxy materials currently used have 0.2%. This reduction allows for much tighter dimensional control of the material in service.
The latest water-lubricated materials also have a significant reduction in friction for both dry and wet running. Low-friction material provides excellent hydrodynamic performance, which allows for slower shaft speed operation, as the fluid film lift on the shaft is maintained for longer. This improves the wear life of the bearing over long-term operation.
A perfectly aligned hydrodynamic bearing will only wear when shaft rotation is started or stopped, or the shaft speed drops below the critical hydrodynamic threshold. At all other times, a fluid film separates the surfaces of the shaft and the bearing, and no contact takes place. The reality is, however, not always as ideal. If the shaft guide bearing runs in mixed friction conditions, there is still some contact between shaft and bearing, reducing bearing life. In such a situation, friction properties come into play. Alignment is also essential, as misaligned or damaged shafts can increase localised bearing pressure, accelerating wear and causing premature failure.
Differing market needs
According to Strickland, there are two main market areas for water-lubricated bearings – marine propeller shaft bearings, primarily stern tube and rudder bearings, and hydro turbine guide bearings. The latter is a developing market which is starting to see significant growth. However, these two segments have different requirements.
‘The propeller shaft bearings require the material to operate against a variety of different shaft journal materials, such as the softer bronze, but also much harder materials such as Inconel,’ said Strickland. ‘A newly developed water-lubricated bearing has focused on achieving the best hydrodynamic performance and wear life for all the commonly used shaft journal materials. Historic phenolic-based bearing products suffered operational issues with harder shaft journal materials.’
For the company’s main market area in marine, stern tube bearing design is covered under classification society rules, which dictate the bearing lengths and maximum load carrying allowance. They will also stipulate requirements for shaft alignment, to which the build shipyard must adhere. Wärtsilä has its own design standards for composite bearings, some of which are specific to each material being used while others relate to the retention methods for the surrounding metal work.
The hydro turbine guide bearings require tight control over the operating clearance between the shaft and bearing to minimise radial movement. As such, a bearing product was developed to provide dimensional stability with an extremely low swell allowance when running in water, allowing for reduced running clearances on installation.
Design to development
Development of new composite bearing materials and needs begins with the business case and insight into the future. Changing regulation is a strong driver, but operational improvements may have more influence on the customers’ decision-making. After identifying a market and designing a product to meet those material needs, a defined testing programme can begin.
The testing of materials and designs is a thorough and meticulous process. New materials are normally evaluated by mechanical testing to known industrial standards to determine characteristics such as compressive strength, friction, wear, swell and thermal expansion. Endurance trials can also be carried out on a test rigs against different shaft journal materials, with clean and abrasive water, and with different inlet temperatures and flow rates. The results of these tests can then be compared with other materials tested to the same regime.
But it is not enough to only examine the material properties. Machining trials are also carried out on all new materials to confirm the best machining feeds and speeds, as well as the required tool profiles to achieve the best surface finish possible. Ideally, the composite materials should be easy to machine on standard vertical borer, lathe and milling machines.
One such material consideration is that of rubber. Traditionally, rubber bearings often require an operating length four times that of the shaft diameter. Most modern composite bearing designs are half this length, as they do not have the same number of lubrication slots in the material and are able to maintain full hydrodynamic shaft support with a greatly reduced bearing length.
‘When it comes to design,’ Strickland said, ‘the main differences between manufacturers are in the lubrication profiles machined into the bores of the bearing and the methods for retaining the bearings within the support housings. Wärtsilä has its own preferred designs while other companies might adopt something slightly different, but it is mainly the bearing material itself which sets us apart.’
Choosing water lubrication
In the marine industry, the majority of vessels operating today still run with oil-filled stern tubes. Although sealing systems are extremely reliable, the possibility exists of leakage from rope entanglements round the propeller shaft, causing damage to an aft seal. With water-lubricated bearing products, there is no risk of pollution as there is no oil.
Reliability is a key attribute to most marine operators, and water-lubricated systems provide that assurance. Looking ahead, environmental regulation is making water-lubricated solutions more and more attractive to marine operators interested in future-proofing their businesses.