Ships are propelled by propellers, most ships being provided with propeller shafts between a propeller outside the hull and the motor inside the hull. Thrusters are a particular embodiment, wherein the propeller including the propeller shaft can rotate in the horizontal plane. The propeller is driven by a pair of right-angled gear transmissions, step 1 from the horizontal propeller shaft to a vertical drive shaft and step 2 from this vertical shaft to a horizontal shaft which is connected to the drive motor inside the hull. This embodiment is also referred to as a 'Z' drive. A particular embodiment is an 'L' drive, wherein the drive motor, often an electric motor, is arranged vertically and drives the vertical drive shaft directly. The term thrusters is in herein to be understood to refer to both the 'Z' and 'L' drive systems. In order to achieve a good streamline of the water to the propeller, it is important to limit the external dimensions of the gondola housing around the bottom gear transmission, which results in gears which are subjected to relatively heavy loads. With increasing power, these loads increase and the loads on the teeth become increasingly critical. The present invention is therefore primarily concerned with relatively large thrusters, from 250 kW, more particularly thrusters from 1 MW.

Thrusters are known, inter alia, from US 3,896,757 and WO 2014/175746 Al . Thrusters are used to direct the thrust in all desired directions in the horizontal plane. As soon as the ship sets sail, the water flows into the propeller at a speed and is accelerated there. However, during steering, this inflow direction is not at right angles to the direction of rotation of the propeller and greatly varying loads occur. At increasing angle and sailing speed, these loads increase and result in a greatly varying load, not only in terms of magnitude, but also in terms of direction. An additional problem with propellers is cavitation: as soon as the negative pressure on the rear side of the propeller blade becomes too low, cavitation bubbles are generated which then implode and cause significant vibrations and additional power peaks. As a result of these varying loads, the propeller is not only subjected to a purely axial thrust and drive torque, but also to a complicated interplay of forces in terms of magnitude and direction. This results in the propeller transmitting these forces onto the propeller shaft and elastically deforming the latter on account of these forces. A large gear wheel of the bottom right-angled gearbox is attached to this propeller shaft and is driven by a small pinion gear wheel on the vertical drive shaft. Usually, the toothing consists of a bevel gear with spiral toothing (according to EN 5276), in which the large wheel is referred to as wheel and the small gear wheel as pinion. In order to prevent possible confusion, the small and the large gear wheel will be referred to as pinion and gear wheel, respectively.

Due to the varying deformation of the propeller shaft, the gear wheel which is rigidly mounted on the propeller shaft will also move concomitantly, as a result of which the mutual position / adjustment of both gear wheels will vary, causing additional wear and damage. In order to minimize this deformation, the propeller shaft is often made relatively thick and rigid. The rigidity is increased further by the small horizontal distance between the propeller and the gear wheel. This rigidity causes the greatly varying loads on the propeller to be transferred directly as greatly varying loads on the gear wheels. In extreme cases, this may lead to fatigue cracks in the tooth root on the one hand or to the gear wheels losing the mutual contact force and starting to rattle, on the other hand, with disastrous consequences for the lubricating film and wear.

In addition, a thruster also undergoes complex deformations due to thermal expansion, these complications being caused on the one hand by the fact that the surrounding gondola housing is cooled by the water flowing around it and slightly increases in temperature, and on the other hand, by the fact that the internal parts of the propeller shaft, gear wheel and bearings do increase in temperature. This opposite effect results in undesired mutual deformations and may lead to a significant variation between both gear wheels.

So as to prevent damage to the teeth of the pinion and the gear wheel, it is very important that the lubricating film between the running surfaces remains intact. Such a lubricating film between the running surfaces of the teeth of the pinion and the gear wheel often has a thickness of 0.1 mm or less. With known thrusters, the distance between the pinion and the gear wheel may quickly vary by 1 mm or more as a result of mechanical deformations, which may lead to interruptions in the lubricating film and damage to the teeth. It is the object of the present invention to prevent these problems by configuring the propeller shaft as an insert shaft in a hollow gear shaft, wherein the coupling between the propeller shaft and the hollow gear shaft is situated at the free end of the propeller shaft. As the propeller shaft no longer has to support the gear wheel in a rigid manner, a thinner and more flexible shaft may be used. The propeller shaft is mounted, preferably between one bearing near the propeller and one at the free end. The coupling is fitted to this free end, between the propeller shaft inner shaft and a separately mounted hollow shaft gear wheel construction. It should be understood that it is also possible to pass the propeller shaft through the hollow gear shaft and to mount it at the free end in the direction of the gondola housing. In this embodiment, the coupling is then situated on the propeller side of this end bearing.

According to a further embodiment of the present invention, the coupling is embodied as being flexible. This coupling may comprise any constructions conceivable in the prior art. The coupling may comprise flexibility with regards to bending and thus prevent the bending of the propeller shaft from being passed on to the hollow gear shaft. This coupling may then, for example, be embodied as a splined coupling. The coupling may also comprise flexibility with regards to torsion and thus prevent torsion peaks from being passed from the propeller to the gear wheel.

According to a further embodiment of the present invention, the gear wheel is rigidly connected to the hollow gear shaft. As, in case of bending and/or twisting of the propeller shaft, the gear wheel no longer bends or twists along with it, wear of the gear wheel is prevented.

According to a further embodiment of the present invention, the propeller shaft is mounted by means of a propeller shaft bearing near the propeller and by means of the splined coupling in the hollow gear shaft on the opposite side. The propeller shaft is thus mounted in at least two points which are spaced apart along the longitudinal direction of the propeller shaft. The propeller shaft may project from the hollow gear shaft beyond the splined coupling and may be mounted, for example, via a bearing spaced apart from the gear shaft with respect to the gondola housing. However, it is preferred if the propeller shaft is only mounted in the hollow gear shaft at its end which faces away from the propeller.

According to a further embodiment of the present invention, the diameter of the propeller shaft gradually decreases by at least 15% from the propeller shaft bearing towards the free end of the propeller shaft remote from the propeller. As a result thereof, a saving on material of the propeller shaft is made and the propeller shaft can bend and/or twist in order to absorb the bending forces and/or torsional forces exerted on the propeller. According to a further embodiment of the present invention, the bearing is adapted to substantially absorb the thrust of the propeller shaft. This substantially prevents the thrust from being transferred onto the gear wheel in the longitudinal direction of the propeller shaft and via the propeller shaft.

According to a further embodiment of the present invention, the propeller shaft and the propeller shaft bearing are configured, near the propeller, to substantially block translation of the propeller shaft in the longitudinal direction of the propeller shaft with respect to the propeller shaft bearing. This prevents the propeller shaft from exerting a push force on the gear shaft or on the gear wheel along the longitudinal direction which could result in damage to the gear wheel. According to a further embodiment of the present invention, the coupling is such that both parts can move freely in the axial direction. As a result thereof, neither deformations of the propeller shaft caused by forces or thermal effects are passed on in the direction of the hollow gear shaft. For example, in the case of the propeller shaft being bent, this prevents the gear shaft from translating with respect to the housing along the longitudinal axis of the gear shaft. According to a further embodiment of the present invention, the thruster comprises a vertical pinion shaft, a pinion and pinion bearings, wherein the pinion, the vertical pinion shaft and the pinion bearings may be fitted in a hollow pipe housing which can be demounted in its entirety from the gondola housing. When the pipe housing has been demounted from the gondola housing, the pinion and the gear wheel of the hollow gear shaft can easily be inspected.

According to a further embodiment of the present invention, the bearing assembly of the hollow gear shaft is positioned directly next to the gear wheel and the bearing assembly comprises juxtaposed mirror-symmetrical cone bearings. Preferably, the bearing assembly comprises two directly juxtaposed bearings which are placed around the gear shaft in the longitudinal direction of the hollow gear shaft. Bearings which are placed close to each other in this way are subjected to similar changes in temperature. As a result thereof, the bearings of the bearing assembly will expand in a virtually identical way when the temperature changes, which leads to an improved mounting of the hollow gear shaft.

According to a further embodiment of the present invention, the hollow gear shaft is mounted in the gondola housing by means of a first cone bearing which is positioned directly next to the gear wheel and by means of a second cone bearing which is situated at the end of the hollow gear shaft. The first and second cone bearing are preferably positioned mirror-symmetrically with respect to each other, with the wide cone ends of the first and second cone bearings facing away from each other. The cone bearings keep the hollow gear shaft, and in this case the gear wheel as well, correctly aligned with respect to the pinion.

According to a further embodiment of the present invention, the pinion is preferably supported on both the top and bottom side by a radial bearing, wherein both bearings are mounted in a separate demountable carrier (housing). This carrier may be mounted in or demounted from the surrounding gondola housing in its entirety. This makes it possible to carry out accurate mounting and adjustment of the bearings already beforehand. In addition, axial adjustment of the pinion and the gear wheel with respect to each other is simpler if the carrier can be placed in an axial direction in its entirety.

According to a further embodiment of the present invention, the gear wheel and the hollow shaft construction are supported by several bearings in the demountable carrier (housing), which may be mounted in or demounted from the surrounding gondola housing in its entirety. As is the case with the pinion carrier, this makes it possible to carry out accurate fitting and adjusting of the bearings already beforehand. This demountable carrier is situated around the free end of the propeller shaft and therefore on the opposite side of the propeller. The gondola housing of a large number of thrusters consists of a large moulded piece and a cylindrical opening at the location of the end cover. Often, a bearing flange is fitted in this cylindrical opening first, followed by a separate streamline cover. This carrier may consist both of one integrated flange cover construction and of separate parts.

According to a further embodiment of the present invention, the gear wheel is supported on the hollow shaft by a mirror-symmetrical arrangement of two cone or thrust bearings, directly next to the gear wheel. This arrangement results in minimal thermal deformations of the short shaft between the bearings and the wheel and thus in minimal variation in tooth play of the gear wheel in the direction of the pinion.

According to a second aspect, the invention provides a method for assembling a thruster, preferably a thruster as described herein, wherein the thruster comprises: a gondola housing which is provided with a propeller; a propeller shaft which runs from the propeller to the opposite side of the gondola housing, wherein the propeller shaft can be connected to a hollow gear shaft which is provided with a gear wheel via a flexible coupling, wherein the flexible coupling is configured as a splined coupling and is adapted for preventing a bending of the propeller shaft from being passed on to the hollow gear shaft, wherein the method comprises the following steps:

assembling a first carrier part for the gear wheel, wherein the first carrier part comprises a demountable cover of the gondola housing which has bearings mounted therein for mounting the hollow gear shaft in the first carrier part, and comprises the hollow gear shaft,

assembling a second carrier part, wherein the second carrier part comprises a hollow pipe housing which has a vertical pinion shaft fitted therein, with a pinion and bearings for the vertical pinion shaft,

fastening the assembled first carrier part and the second assembled carrier part to the gondola housing, and

subsequently pushing the propeller shaft, which is mounted in a propeller shaft bearing placed in a flange, into the gondola housing, in such a way that the propeller shaft is partly inserted in the hollow gear shaft, the spline coupling is formed between the propeller shaft and the hollow gear shaft, and fastening the flange to the gondola housing.

The method makes it possible to mount the propeller shaft and the associated propeller shaft bearing to the gondola housing in a very simple manner.

Depending on requirements, the present invention may be carried out using a so- called pusher propeller or tractor propeller, that is to say, in the first case, water is moved past the housing by the propeller, whereas in the second case, the water is pushed away from the housing by the propeller. It is also possible to fit a sleeve-shaped jet nozzle around the propeller in order to increase the thrust of the propeller at relatively low speeds. The thruster may be configured as a main drive or as an auxiliary drive, wherein, in the latter case, it may also be arranged in the hull of a vessel in a direction at right angles to the sailing direction of a vessel. Obviously, the thruster can also be moved in its entirety in a vertical direction using a separate mechanism so that it can be moved both inside and outside the hull.

The invention will be explained in more detail by means of illustrative embodiments, in which: Fig. 1 diagrammatically shows a cross section of a standard thruster;

Fig. 2 diagrammatically shows an example of the effect of bending in the propeller shaft;

Fig. 3 diagrammatically shows an example of the effect of torsion in the propeller shaft;

Fig. 4 diagrammatically shows a cross section of the bottom part of the thruster according to the present invention;

Fig 5 diagrammatically shows both carrier parts, wherein the gondola housing is indicated by means of dashed lines;

Fig 6 diagrammatically shows a sectional view of a propeller shaft having a diameter which gradually decreases towards the end facing away from the propeller.

In Fig. 1, a standard thruster is denoted by reference numeral 1. The thruster comprises a gondola housing 2 provided with a propeller 3, a jet nozzle 4 and a propeller shaft 5. The propeller shaft is mounted by means of a bearing 10 near the propeller and a bearing (assembly) 11 near the gear wheel 8. Gear wheel 8 is driven by the pinion 9 via a vertical pinion shaft 6. The vertical pinion shaft is driven in turn by the right-angled gear wheels 14 in the upper box and by the horizontal shaft 7 which is connected to the drive motor (not shown).

Fig. 2 diagrammatically shows the bending of the propeller shaft resulting from a non-uniform load on the propeller blades. If a higher load is applied to the top blade 15 than to the bottom blade 16, a bending moment is produced, as a result of which the propeller shaft will assume a concave shape between bearing 10 and bearing assembly 11. Due to this bending, the gear wheel 8 will also assume a slightly oblique position and push against the pinion gear (not shown) in the axial direction, resulting in an undesired tooth play. The solid lines indicate the initial position, the dashed lines indicate the deformed position. With an opposite distribution of the load between the top blade and bottom blade, the propeller shaft will assume a convex position. As a result thereof, gear wheel 8 will assume a slightly oblique position in the opposite direction, again causing an undesired tooth play. It will be clear that small deviations in the tolerance of the gear wheel (both a relatively large and a relatively small distance) will have a significant effect on the wear behaviour and the service life. Fig. 3 diagrammatically shows the torsion in the propeller shaft 5 resulting from varying the load on the entire propeller 3. The propeller shaft is supported by bearings 10 and 11. Due to the short rigid propeller shaft 5 between the propeller 3 and the gear wheel 8, these peaks are passed on directly. The solid lines of the propeller 3 indicate the initial position, the dashed lines indicate the deformed position.

From the above, it can clearly be seen that, on the one hand, a thick propeller shaft is desirable in order to prevent bending, as is illustrated in Fig. 2, and, on the other hand, a thick propeller shaft is undesirable in order to provide elasticity between propeller and gear wheel, as is illustrated in Fig 3.

Fig. 4 shows the bottom part of the thruster according to the present invention.

This thruster contains gondola housing 20, provided with propeller 21 and propeller shaft 22. This propeller shaft is mounted near the propeller by means of a bearing 23 and, on the opposite side, by means of a splined coupling 24 in the hollow gear shaft 25. This hollow gear shaft 25 is provided with gear wheel 28 and is mounted on the spline side and by bearing assembly 27 next to the gear wheel 28. This gear wheel is driven by pinion 29 and by the vertical pinion shaft 30. Further driving is performed in a similar way to that of a standard thruster. In this embodiment, the torsional rigidity of the connection between the propeller and the gear wheel is an order of 2-3 lower than with the standard thruster. Due to additional flexibility in the coupling, this torsional rigidity may be reduced by a factor of 3-5 compared to a standard thruster or, if desired, an even higher factor. In addition, it is also possible to increase the length of the cover, as a result of which the length of both the propeller shaft and the hollow gear shaft increases, providing advantages for torsional flexibility. This makes it clear that the length of the gear shaft preferably has to be at least 50% of the length between the propeller shaft bearing 23 and splined coupling 24, if possible 60% or more.

Fig 5 shows the two both carrier parts, carrier part 31 for the gear wheel and carrier part 33 for the pinion. Carrier part 31 consists of the removable cover 32 of the gondola housing in which the bearings 26 and 27, the hollow gear shaft 25 and the gear wheel 28 are fitted. This entire carrier can be assembled before it is fitted into the gondola housing 20 (indicated by a dashed line). As a result thereof, the assembly can be fitted accurately, including the adjustment of the bearings. The limited dimensions make it possible to manoeuvre this carrier in the workshop. Carrier part 33 consists of a hollow pipe housing 34 in which the bearings for the vertical pinion shaft 30 and the pinion 29 are fitted. In this case, there are advantages with regard to fitting and adjustment as well.

When assembling the thruster, both carrier parts may be fitted in the gondola housing 20 first and, during this procedure, the tooth play can be inspected from the inside in the case of relatively powerful motors, as the propeller shaft 22, bearing 23 and associated flange have not yet been fitted. Then, the propeller shaft 22, including bearing 23 and associated flange 35, may be pushed into the gondola housing. It is also simple to remove the propeller shaft again for inspection and maintenance, without having to remove both carriers with the associated tooth play. In case of damage to the propeller shaft, it suffices to replace this simple steel part, instead of the complicated shaft provided with gear wheel.

Fig. 6 diagrammatically shows a sectional view of a propeller shaft 62 such as may be used in a thruster according to the invention. The propeller shaft 62 is partly inserted in a hollow gear shaft 65. Although not shown, the gear shaft 65 is provided with a gear wheel similar to the gear wheel 28 from Fig. 5. The diameter of the propeller shaft 62 gradually decreases from bearings 63 in the direction of the free end of the propeller shaft remote from the propeller, so that the diameter d2 of the propeller shaft just before the splined coupling 64 is at least 15 % smaller than the diameter dl of the propeller shaft at the bearings 63. In this way, the propeller shaft is securely mounted in the bearings 63, while the thinner part of the sole propeller shaft allows a greater degree of bending and/or twisting than the part having the relatively large diameter dl . Due to the flexible spline coupling 64 between the hollow gear shaft 65 and the propeller shaft 62, it is possible to absorb bending and/or twisting of the propeller shaft without this leading to a corresponding change in orientation of the gear shaft 65 or the gear wheel thereof.

After reading the above, it will be clear to those skilled in the art that the invention has many variants. Such variants are obvious in view of the above and are covered by the attached claims.