Friction and Lubrication of Large Tilting-Pad Thrust Bearings
- Minimum thickness of the lubricant layer separating rotating collar from a pad surface is approximately 20–50 μm, which is less than the thickness of a human hair;
- Relative speed is equal to 40–45 m/s; this is about 150 km/h;
- The amount of heat generated in the bearing in some cases reaches 1 MW—heat flux generated in the film is equal to approximately 400 kW/m2;
- Axial load can reach several meganewtons (MN), which is over one thousand tons;
- Specific load may reach 5–6 MPa, but in the large bearings is usually limited to 2–3 MPa;
- Outer diameters of the thrust bearings exceed 5 metres—Three Gorges (China), Itaipu (Brazil)—5.2–5.3 m.
- Excessive pad deformations affecting generation of hydrodynamic pressure,
- Uneven load sharing among bearing pads,
- Excessive temperatures affecting the bearing alloy,
- Inadequate load carrying capacity at transient states.
2. Lubrication Systems of Large Tilting Pad Thrust Bearings
2.1. Bath Lubrication with Internal or External Cooling Systems
- The area of the heat exchanger is not limited by the size of the bearing housing.
- The heat transfer coefficient depends on the velocity of both fluids inside the cooler and when turbulent flow of both fluids is possible, which intensifies heat transfer—usually the heat transfer coefficient is 110–180 W/m2K for a built-in water oil cooler , and 600–680 W/m2K in an external plate heat exchanger (PHE) .
- The counterflow arrangement helps to fully utilize the natural differences of water and oil temperatures, which is not possible in internal cooling because of the complex flow pattern of oil inside the oil bath.
- The cooler output can easily be adjusted by the changing number of plates (in PHE).
- High velocity of fluids and turbulent flow prevents sedimentation of contaminants on the plates, and the output is not affected by the contamination (in PHE).
- The use of standard elements in the cooling systems increases quality and reliability
- Due to external coolers, water is not pumped into the bearing housing, so the danger of water leakages inside the housing is decreased—special design of the seals in PHE further increases the reliability.
- Oil circulation helps to provide continuous filtration of oil by a fine filter built in the system.
2.2. Directed Oil Supply
|Qlubr (L/min)||Lubrication System||Tmax (°C)||Tinlet (°C)|
3. Attempts to Reduce Bearing Losses
3.1. No Bath Lubrication
3.2. Decrease of Viscosity—Standard Oils, High Viscosity Index (VI) Oils
|Parameter||TurbWay SE||ISO VG 68|
|Viscosity at 40 °C (mm2/s)||46||69|
|Viscosity at 100 °C (mm2/s)||8.1||8.8|
3.3. Contribution of Polymer Lined Bearings
3.4. Hydrostatic Bearings
|Case||Number of Pads||Oil Viscosity||Pump Output||Film Thickness||Pocket Pressure||Pumping Power||Film Friction Loss|
|HD||16||Hydrodynamic bearing with measured power loss of 250 kW, given for reference|
3.5. Water Lubrication
4. Discussion and Conclusions
|Method of Friction Loss Reduction||Effects||Remarks|
|Decrease of viscosity—standard oils||Confirmed experimentally in large bearings, even a three-fold decrease of power loss, but at the cost of decreased film thickness||Decrease of the margin of safety|
|No-bath lubrication||Substantial decrease of power loss confirmed experimentally in tests in small bearings
Smaller effects in large bearings (approx. 10%) checked only by calculations
|New ideas required to provide safety in case of failures|
|Decrease of viscosity—high VI oils||In large journal bearings, decrease of power loss by almost 20%. A reduction by 13% in thrust bearings checked by calculations only||Oil price is the only barrier to extend the application, no other changes in bearings necessary|
|Polymer lining||20%–30% savings possible just by increase of oil bath temperature, 40%–50% savings possible in modified bearings of higher specific loads and decreased dimensions||Increase of oil temperature may affect its durability|
|Special hydrostatic bearings||In field tests, considerable decrease of temperature and increase in film thickness observed. Benefits of up to 50% were checked by calculations but require special design hydrostatic bearings||Not often used. New bearing designs and hydraulic systems necessary.
Methods of providing safety in case of failures
|Water lubrication||6–7 times reduction of losses is theoretically possible.||hydrodynamic or hydrostatic bearings—substantial development required, but the benefits may be remarkable|
Conflicts of Interest
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Wasilczuk, M. Friction and Lubrication of Large Tilting-Pad Thrust Bearings. Lubricants 2015, 3, 164-180. https://doi.org/10.3390/lubricants3020164
Wasilczuk M. Friction and Lubrication of Large Tilting-Pad Thrust Bearings. Lubricants. 2015; 3(2):164-180. https://doi.org/10.3390/lubricants3020164Chicago/Turabian Style
Wasilczuk, Michał. 2015. "Friction and Lubrication of Large Tilting-Pad Thrust Bearings" Lubricants 3, no. 2: 164-180. https://doi.org/10.3390/lubricants3020164