Thrust Bearing - an overview (2023)

12.3.3 Seal thrust bearing

II.D.7 Thrust Bearings

Oftentimes the pad is either deliberately crowned or it deforms under the combined action of load and frictional heat. Performance of such pads is still calculated in the manner discussed previously, except that the film thickness distribution now depends not only on the tilt parameters but also on pad deformation. Furthermore, cavitation of the lubricant film within its diverging geometry must be considered.

Condition Monitoring

Compressors and their associated auxiliary systems are instrumented to ensure safety and reliability. Standard alarms, shutdowns, and control systems are discussed in detail within API 614 and API 670 standards. Typical parameters that are monitored online are listed below. Many of these measurements are fed into a machinery protection system. Trips related to overspeed and bearing oil pressure are generally required. Trips related to vibration level and axial position are often specified as well.

Manual inspections are also of importance. The compressor and the auxiliary equipment should be inspected for leaks at piping connections. Lubricating oil should be periodically checked for the presence of water and for degradation.

Thrust Management

It is extremely rare to use a double helical gear in an IG turbomachine because the net pinion thrust (i.e., sum of axial impeller gas forces) would act to load one face of the helical gear more than the other. Therefore, almost all IG turbomachinery use a single helical gear. Furthermore, thrust bearings are almost exclusively fluid film-type configurations with the oil and gas industry. To control the axial position of the large bull gear, a double-acting tapered-land thrust bearing is most commonly applied. The relatively low speed of the bull gear means that minimal fluid shear losses are present in the bull gear shaft thrust bearing.

VI Dynamic Interaction between Engine and Hull

We have observed that development of marine engines has reached a point where the design of the engine no longer dominates the design of the ship; usually a choice can be made among several engine types, and none will demand a large share of hull volume and lifting capacity. Nonetheless, a ship designer should not design hull and select machinery independently. There are design linkages, such as the effect of engine weight and volume on payload, that have been mentioned earlier.

Possible dynamic interactions are perhaps the most important of all, since poor design can seriously impair the operation of a ship in a way that a minor loss in payload, say, never would. The dynamic interactions arise from the almost inevitable tendency of a propeller to generate torsional, longitudinal, and transverse excitation as its blades rotate through regions of differing water velocity, and from the several sources of excitation within the engine.

If the engine is a diesel, the torque applied to the crankshaft by each piston varies periodically as the cylinder gas pressure varies, and as the inertia forces from piston acceleration vary. If any of the many harmonics of these periodic torques resonates with a natural frequency of the engine-shaft-propeller system, severe torsional vibration may occur. The torsional vibration may be destructive only of the rotating machinery, but by its nature, the propeller is a converter of torque to thrust, and so it is that a strong longitudinal vibration may be caused also.

Longitudinal vibratory forces are transmitted to the hull by the thrust bearing that transmits the propulsive thrust, so that one of the natural frequencies of hull vibration may be excited. A low-speed diesel, in particular, may vibrate longitudinally, acting in the manner of a vertical cantilever beam. If one of its natural frequencies resonates with the frequency of the longitudinal shaft forces, the engine may vibrate excessively, and in turn excite surrounding ship structure.

A reciprocating engine transmits to its shaft bearings the periodic forces that must accompany the periodic accelerations of its pistons and associated moving parts. Usually, however, the forces are canceled through the agency of counterweights on the crankshaft, but it may not be possible to cancel the moments that these forces produce. The degree of moment cancellation depends largely on the number of cylinders, but most typically some significant degree of second-order (meaning frequency equal to twice the rotational frequency of the engine) moment exists. This moment tends to bend the engine vertically about a transverse axis (i.e., bend its ends up and down). Since the engine structure is not infinitely stiff, this bending is transmitted to the engine foundation, and may thereby excite hull vibration.

The phenomena that give rise to these dynamic interactions are largely unavoidable—the pulsing nature of diesel engine torque, for example, is an inevitable characteristic of a reciprocating engine. The principal remedy is to design the moving parts of the engine and shafting system so that resonances between its vibratory modes and the excitations do not occur. In some instances this may include stiffening the engine structure by adding sway braces between the hull and the upper level of the engine (particularly so for the tall low speed diesels). The consequences of unbalanced torques within the engine may be minimized by selecting the number of cylinders for a low moment value, building extra stiffness into the foundation, and by mounting the engine near a node of the expected hull vibration mode.

The smooth torque of turbine engines reduces their dynamic hazard, but unacceptable torsional and longitudinal vibrations can be excited by the propeller. There have also been instances in steam turbines of severe vibration excited by the periodic passage of turbine blades through the steam jets. As with the reciprocating engines, the principal remedy is knowledge of excitations, and of frequencies of vibration, so that resonances can be avoided.

2.2 Bearings used in RDTs

RDTs work in underwater environments, so water-lubricated bearings are best suited to carrying the radial and axial forces of the rotor because of their advantages of environment-friendly, simple structure and no need of sealing device and good compatibility in deformation. Brunvoll, Voith and some other commercial RDTs have successfully used water-lubricated bearings. For hub type RDTs, radial bearings and thrust bearings (e.g., TSL products) can be installed inside and at both ends of the hub. However, for hub-less type RDTs, both kinds of bearings (e.g., Schottel products) can be installed in the duct.

The duct thickness and hub diameter must be as small as possible to ensure favorable hydrodynamic performance of the RDT, which makes the bearing design very complicated. Small size, high load carrying capacity, and wear-resistant water-lubricated bearings have yet to be effectively designed. Especially when the bearing is installed in the duct, the friction arm of the rotor is much larger than when it is installed in the hub, which results in a larger starting resistance torque of hub-less type RDT than that of a same size hub type one. Even in the course of normal operation, the friction loss caused by bearings of hub-less type RDT is greater than that of hub type.

The load carrying capacity and service life of water-lubricated bearings are lower than that of oil-lubricated bearings. Voith (2017) has developed a non-water-lubricated roller bearing system including bio-oil compatibility for its hub type RDTs. The system features a well-proven design and material combination, with bearings equipped with a leakage-free and redundant sealing system. Rolls-Royce also builds oil-lubricated bearings into its newly developed AZ-PM thrusters, which have now already run more than 1500h trouble-free (Rolls-Royce, 2017a,b). The disadvantages of oil lubricated bearings include their structure is complex (especially for thrust bearing), the machining accuracy and installation requirements are high, and reliable sealing devices are needed to prevent the leakage of lubricating oil. According to 2013 Vessel General Permit (VGP) Requirements issued by US Environmental Protection Agency (EPA) and some other regulations enacted by European Union, vessels installed of RDTs using oil lubricated bearings may be prohibited from entering in certain protected waters. These environmental protection laws and regulations may limit the application of oil lubricated bearings on RDTs.

4.1.4 Olistostrome-type OPS

This third type of OPS occurs in southwest Lleyn, is about 20m thick, and consists of many weakly or unmetamorphosed lenses of white quartzite, red chert, dolomitic limestone, and basaltic greenschist embedded in a matrix of black mafic mudstone. Most lenses are 5–30cm across, some reach 6m, and one is 25m long (Maruyama et al., 2010). Kawai et al. (2007) interpreted these rocks as an olistostrome-type mélange (Suppl. Fig.6), which consists of already-formed accretionary material that had slumped down the inner hanging wall of a trench to be incorporated into a gravitational debris flow. The olistostrome is overlain by a phyllonite-bearing thrust and underlain by unbroken ridge–trench OPS.

This third type of OPS occurs in southwest Lleyn, is about 20m thick, and consists of many weakly or unmetamorphosed lenses of white quartzite, red chert, dolomitic limestone, and basaltic greenschist embedded in a matrix of black mafic mudstone. Most lenses are 5–30cm across, some reach 6m, and one is 25m long (Maruyama et al., 2010). Kawai et al. (2007) interpreted these rocks as an olistostrome-type mélange (Suppl. Fig. 6), which consists of already-formed accretionary material that had slumped down the inner hanging wall of a trench to be incorporated into a gravitational debris flow. The olistostrome is overlain by a phyllonite-bearing thrust and underlain by unbroken ridge–trench OPS.

Finally, we point out that understanding OPS can provide important, indeed unique, information on the world's glaciations. Recognition of glacial stratigraphy and structures, necessary for reconstructing for example the Neoproterozoic Snowball Earth, has largely come from tillites that were deposited on-land and on continental margins, and rarely from dropstones deposited in continental slope basins (e.g. Frimmel et al., 2002). However, if there was anything like a global glaciation at the time of the Snowball Earth, then ice or icebergs would have covered many of the deep oceans, enabling ice-rafted dropstones to be deposited in deep-water mid-oceanic sediments, but such oceans have all been destroyed, and there are no reports of dropstones in mid-oceanic sediments. However, theoretically, dropstones should have been deposited in pelagic cherts and mudstones. If we can recognize such pelagic sediments in accreted OPS, then we have a chance to find such glacial debris. Kawai et al. (2008a,b) reported matrix-supported exotic dropstones of sandstone, chert and basalt (arc or continental material from Avalonia) in a 1m-thick hemi-pelagic, mafic mudstone on top of red chert on Llanddwyn Island. The depositional age was estimated to be 595–550Ma, which is coincident with the ca. 580Ma Gaskiers glaciation (Trindada and Macouin, 2007), which from palaeomagnetic data occurred in high- to low-latitudes. The Llanddwyn dropstones are close in age to ca. 590–570Ma diamictites and dropstone beds in Ireland and Scotland, but these occur in continental-based turbidites hundreds of meters thick (Condon and Prave, 2000).

4.6 How do crevasse squeeze ridges and concertina eskers form?

The relationships between crevasse squeeze ridges (CSR) and flutes (Sharp, 1985a) suggest there might be more than one mode of CSR formation. Most studies suggest that CSR form as a consequence of sediment infilling basal surge crevasses from the bed upwards and subsequent melting out during quiescence. The mechanics of this process are, however, not entirely clear, mainly because of the lack of detailed sedimentological and geomorphological investigations. Rea and Evans (2011) assessed the potential for the formation of crevasses and their infilling with sediments, using linear elastic fracture mechanics approach and empirical data derived from the literature, for seven surge-type glaciers from Svalbard, Iceland, Greenland and Alaska. They concluded that CSR most likely resulted from the infilling of basal crevasses, driven for the most part, bottom-up, by high basal water pressures. Studies from Svalbard surge-type glaciers have invoked englacial thrusting mechanisms for producing the CSR, suggesting they being derived from debris-bearing thrust faults (Hambrey and Huddart, 1995; Bennett et al., 1996; Glasser et al., 1998). Lovell et al. (2015) found that debris-rich englacial structures observed at surge-type tidewater glaciers in Svalbard display a variety of characteristics and morphologies, which they interpreted to represent the incorporation and elevation of subglacial till via squeezing into basal crevasses and hydrofracture exploitation of thrust faults, reoriented crevasse squeezes, and pre-existing fractures. These structures were observed to melt-out and form embryonic geometrical ridge networks during quiescent phase ice stagnation and ice-margin recession. Cross cutting relationships between flutes and CSR have neither been widely studied nor well explained, and the processes operating between infilling of basal fractures by till and CSR becoming exposed on the foreland during the quiescent phase melting out of dead-ice are not clear. Thorough sedimentological studies, linked to detailed observations of processes operating during surges (Kristensen and Benn, 2012; Lovell et al., 2015), could further highlight the processes explaining the CSR-flute relationships and thus increase our understanding of subglacial ice-flow mechanisms at work during surging.

Although concertina eskers are uniquely associated with surge-type glaciers there are still some aspects as to their formations that are poorly understood. The original explanation of Knudsen (1995) of concertina eskers being formed by compression and deformation of pre-surge englacial eskers has largely been abandoned in favour of them being the result of supraglacial or englacial meltwater deposition in linked cavities or crevasses during or immediately after the surge (Evans and Rea, 2003; Benn and Evans, 2010; Ólafsdóttir, 2011). Along with observations and monitoring of glacier surges, further sedimentological and structural studies of concertina eskers could clarify the processes of their formation in the course of a surge as well as better explaining their spatial pattern.