EP0708725A1

EP0708725A1 - Drive unit for a ship

Info

Publication number: EP0708725A1
Application number: EP94925640A
Authority: EP
Inventors: Martinus Hubertus Cuypers; Jacob Lourens Gelling
Original assignee: TWIN TECHNOLOGY DEVELOPMENT BV
Current assignee: TWIN TECHNOLOGY DEVELOPMENT BV
Priority date: 1993-07-22
Filing date: 1994-07-21
Publication date: 1996-05-01
Also published as: WO1995003211A1; KR960704752A; NL9301292A; JPH09504244A

Abstract

The invention relates to a drive unit for a ship, comprising a gearbox (5) placed on a bedplate in the ship and two screw propellers (76, 77) rotating in opposite directions about a common axis. The screw propellers are driven by two concentrically rotating propeller shafts (2, 6), the inner shaft (2) being supported in at least two bearings (14, 17) provided with a lubricant supply. The bearings (14, 17) of the inner shaft each consist of stationary bearings which are connected by means of stationary bushes (15, 21) to the gearbox (5) in a fixed manner in the direction of rotation of the shafts. The inner shaft bearings (14, 17) are clad with bearing material, at the position of the bearing gap, on both walls facing the propeller shafts (2, 6), while lubricants are supplied to these bearings through one or more ducts disposed in the wall of the stationary bushes and running from the gearbox to the bearing gap. The bearings and the stationary bushes between the bearings and the gearbox are interconnected in such a way that the axial directions of two adjacent parts can form a small angle with each other. The bearings are in the form of hydrodynamically lubricated bearings, to which the oil is supplied uniformly over the bearing width of each bearing by means of an oil pump placed near the gearbox through ducts drilled in the stationary bushes.

Description

Drive unit for a ship.

The invention relates to a drive unit for a ship, in accordance with the preamble of claim 1.

Such a drive unit is known from EP-A-0221536. In the case of this known drive unit the oil is supplied to the inner bearing through bores disposed in the inner shaft. The inner bearing is a hydrostatically lubricated bearing.

This known drive unit has the disadvantage that the bearing arrangement is highly susceptible to disruptions in the oil supply, because oil has to be supplied at high pressure from stationary to rotating parts, with the result that there is a considerable chance of leakages. Another disadvantage is that the efficiency of the drive is adversely affected by the pumping power required for the hydrostatic bearing.

The object of the invention is to provide a drive unit by means of which a reliable bearing arrangement is achieved in the inner shaft in a simple manner, and in the case of which the parts which are subject to wear during normal operation, for example also when the ship is at sea, are easily accessible.

For that purpose, the drive unit is designed in accordance with the characterizing part of claim 1.

Due to the fact that the oil is supplied to the inner bearings through ducts incorporated in the stationary bushes, there are no rotating seals between the oil pump and the inner bearings, which increases the reliability of the oil supply. It is also simple to provide more than one duct, which increases operating reliability as well. In order to improve the operating reliability of the device by making the bearing arrangement less dependent on the oil pump, and in order to increase the tolerances which have to be maintained in the production of the various parts, and to reduce the power required for pumping the oil, the inner bearings are preferably in the form of hydrodynamically lubricated bearings.

Hydrodynamically lubricated bearings have good emergency running properties and can continue to function well if the oil supply decreases or fails. This is because in the case of well-dimensioned bearings the oil supply is necessary only for the dissipation of heat which has developed in the bearings. In order to provide a bearing arrangement in the case of which a locally high load on the bearings is prevented and the inner bearings can position themselves freely between the outside of the inner shaft and the inside of the outer shaft, the stationary bushes and the inner bearings are preferably connected to each other by coupling means which permit slight deviations in the axial directions relative to each other.

Due to the fact that at least one bush is placed between the gearbox and the bearings, the bearings can move slightly at right angles to the axis, and the directions of the bearing bushes can adjust in an optimum manner and within certain limits can follow the average shaft direction of the inner shaft and the outer shaft, with the result that the gap width between bearing and inner shaft and bearing and outer shaft can shift according to the load occurring, so that the bearing under load adjusts in such a way that the shaft load on the bearing is transmitted in full by the inner shaft to the outer shaft.

In order to prevent the oil in the gearbox from being contaminated with water and the gearbox from being filled up with oil, thus preventing additional loss of power in the gearbox, the drive unit is expediently designed with oil seals between the front stationary bush and the inner shaft and the hollow outer shaft. This makes it possible to fill the lubrication system of the propeller shaft bearing arrangement to above the top edge of the outer shaft with an excess pressure of oil, in order to prevent water from being able to penetrate from the outside into the lubricating system, while the oil level in the gearbox can remain low, so that no unnecessary churning losses occur there.

In order to ensure that even when the propeller shaft is running at very low speed of rotation there is sufficient lubrication at the rear inner bearing, a second duct is expediently provided, which duct runs through the stationary bushes and the inner bearings and connects a second oil pump to an oil supply duct at the position of the rear side of the rear inner bearing. This means that high-pressure oil can be introduced at that point between the bearing and the inner shaft and between the bearing and the outer shaft. High-pressure oil can be pumped by the second pump through the ducts when the pressure build-up in the hydrodynamic bearings is not working sufficiently during very slow rotation of the propeller shafts. Hydrostatic pressure then occurs between bearing and shafts, and corrosion between the bearing and the propeller shafts is prevented.

In order to reduce peak load on the rear bearing, arising through the deflection of the weaker inner shaft, the axial directions of the inner surface and the outer surface of the rear inner bearing are expediently differ¬ ent.

Since the stationary bearing does not rotate and the deflection of the rotating shafts relative to the stationary bearing does not change, it is possible to adapt the shape of the stationary bearing, so that the outer surface of the bearing runs parallel to the inside wall of the outer shaft, and the inner surface of the bearing runs parallel to the outside of the inner shaft.

The invention is explained in greater detail with reference to a drawing of an exemplary embodiment of the drive unit.

Figure 1 shows a drive unit according to the invention, mounted in the stern of a ship.

Figure 2 shows a longitudinal section of the gearbox of the drive unit of Figure 1.

Figure 3 shows a section of the gearbox of Figure 2, in a view along the line III-III. Figure 4 shows a longitudinal section of the rear inner bearing of the drive unit of Figure 1.

Figure 5 shows a section of the bearing of Figure 4, on an enlarged scale and in a section along the line V-V. Figure 6 shows the detail A in Figure 4, at the position of the oil supply duct.

Figure 7 shows the detail A in Figure 4, at the position of the oil discharge duct. Figure 8 shows the detail B in Figure 4.

Figure 9 shows an alternative gearbox, viewed in a section along the line IX-IX in Figure 10.

Figure 10 shows the gearbox of Figure 9, in a section along the line X-X in Figure 9. Figure 11 shows the diagrammatic section of the gravitationally loaded propeller shafts.

Figure 12 shows the rear inner bearing, in the case of which the bearing is fixed in a rigid position.

Figure 13 shows the rear inner bearing, in the case of which the bearing is fixed in a flexible position.

Figure 14 shows the rear inner bearing, in the case of which the bearing is adapted to the gap between the propeller shafts.

Figure 15 shows a simplified design of the bearing arrangement of the concentric propeller shafts.

The same parts are indicated by the same reference number in the figures.

In the description, it is assumed that known techniques require no further explanation, and that the parts and structures used in this field require no further description.

Figure 1 shows a stern 75 of a ship, which ship is driven by a front screw propeller 76 and a rear screw propeller 77, which propellers rotate in opposite direc- tions about an axis of rotation 78.

The front screw propeller 76 consists of a number of propeller blades 4 fixed on a hub 7. The hub 7 is fixed by means of a fixed connection to an outer shaft 6, for example by a conical clamping connection (not shown in any further detail) , possibly provided with a sunk key, the hub 7 being clamped by shaft nut 13 on the outer shaft 6. In order to make sufficient clamping force possible, the wall thickness of the outer shaft 6 at the position of the hub 7 is approximately equal to that of the hub 7. The outer shaft 6 is supported in a propeller shaft pipe 98 of the stern 75, in a rear outer bearing 16 and a front outer bearing 18. The oil seal of the outer shaft 6 and the propeller shaft pipe 98 is provided at the front, by means of an inner seal 19. A sliding bush 99 of the inner seal 19 is fixed in the usual manner on the outer shaft 6 by means of a clamping ring 20. At the water side the propeller shaft pipe 98 is sealed by the rear seal 8. Said rear seal 8 is protected from damage in the usual manner by a protective cap 9.

The outer bearings 16 and 18 are slide bearings, in the case of which bushes (not shown) are fitted in the propeller shaft pipe 98, in which bushes bearing material with oil supply grooves is disposed. At the bottom side of the rear bearing an additional oil supply is provided by means of additional ducts, in order to ensure that sufficient lubrication is present even at low speeds of rotation. There are also facilities for measuring the temperature in the rear bearing, in order to warn the user if undesirable situations occur.

The rear screw propeller 77 consists of propeller blades 4 which are fixed to a hub 3. The hub 3 is fixed by means of a fixed connection to an inner shaft 2, for example by means of a conical connection and a sunk key (not shown) , and the hub 3 is clamped on the inner shaft 2 by means of a shaft nut 12. The end of the propeller shaft 2, the shaft nut 12 and the hub 3 are covered by a cover cap 79.

The inner shaft 2 is mounted in a front inner bearing 17 and a rear inner bearing 14, each resting on an inner surface 100 of the outer shaft 6. The inner bearings 14 and 17 are hydrodynamically lubricated slide bearings, the design of which is comparable to the outer bearings 16 and 18. The space between the inner shaft 2 and the outer shaft 6 is sealed off at the water side by means of a shaft seal 10 mounted on hub 3 and hub 7. A protective cap 11 is fitted around the shaft seal 10.

In order to give the inner bearings 14 and 17 sufficient bearing power and in order to make the build-up of a stable lubricating oil film possible, these bearings are held fast in the direction of rotation by fixing them with a rear stationary bush 15 and a front stationary bush 21 to a centre part 31 of a gearbox 5. The centre part 31 is connected by means of a housing 25 and a support 96 to a bedplate 97 of the gearbox 5 (see also Figure 2) . The way in which the inner bearings 14 and 17 work will be discussed later.

The screw propellers 76 and 77 are rotated in opposite directions by transmitting the driving power of an engine (not shown) by means of a drive shaft 1 to the gearbox 5. The rotation supplied by way of the drive shaft 1 is reversed in direction in the gearbox 5, the output shaft of the gearbox having the same axis of rotation 78 as the input shaft.

The thrust generated by the rotation of the screw propellers 76 and 77 is taken up in the usual way in thrust bearings. The thrust generated by the rear propeller 77 is taken up in a thrust bearing (not shown) which is placed between the engine and the gearbox 5. The connection between the drive shaft 1 and the inner shaft 2 is therefore designed in such a way that said shafts are coupled both in the direction of rotation and in the axial direction. The thrust generated by the front propeller 76 is taken up in a thrust bearing which is immovably fixed to the outer shaft 6. For this purpose, a supporting ring 22 is immovably fixed by means of a clamping bush 23 to the outer shaft 6. The supporting ring 22 is bounded in the axial direction by thrust pads 24, which are fixed to a housing 38.

It can be seen from Figure 2 that the drive shaft 1 is coupled to the inner shaft 2 by a claw coupling 37, consisting of an outer toothing and an inner toothing, in the case of which the inner shaft 2 can be pushed into the drive shaft 1. The propeller shaft 2 is blocked by a ring 36 in the axial direction. For removal, the ring 36 is pressed inwards, following which the inner shaft 2 can be pulled out of the toothing of the claw coupling 37.

An intermediate shaft 34 is coupled to the drive shaft 1 at one side by means of a flexible coupling 35, and at the other side is coupled to a front crown wheel 28 by means of a claw coupling 33. The gearbox 5 also contains a rear crown wheel 27, one or more pinions 26 being fitted between the crown wheels 27 and 28. Selecting approximately the same number of teeth on the crown wheels 27 and 28 means that the speed of rotation of said wheels is approximately the same, but the rotation is in opposite directions. The rear crown wheel 27 is fixed by means of a claw coupling 32 to the outer shaft 6.

The crown wheels 27 and 28 can move freely in the axial direction over the claw couplings 32 and 33. The axial tooth forces are taken up by thrust pads 29 and 30, which are fixed to a cover 80 and to the housing 38 respectively.

Each pinion 26 rotates about a shaft 81 in a slide bearing 43 and a slide bearing 44, said bearings being of such dimensions that under the load of the tooth forces the compression in these bearings is such that the shaft 81 remains radially directed relative to the axis of rotation 78 of the crown wheels 27 and 28. The dimensions which determine the compression of the bearings in this case are the diameters of the slide bearings, the bearing width and the gap width, which are therefore adapted to the load situation and to the diameters of the crown wheels. The slide bearings 43 and 44 are fitted in the centre part 31, which is a part of a housing 39. In this case each pinion 26 can be removed in the axial direction by removing a cover 40. The front stationary bush 21 is retained by a support 47, which holds the bush in place and prevents its rotation. The support 47 is fixed to the centre part 31. The space inside the outer shaft 6 is completely filled up with a lubricant. This lubricant is separated from the lubricants which are used in the gearbox 5. For this purpose an oil seal 41 is fitted between the outer shaft 6 and the front stationary bush 21, and an oil seal 42 is fitted between the front stationary bush 21 and the inner shaft 2. An oil seal 45 and an oil seal 46 are fitted in order to keep the lubricants in the gearbox 5 and the thrust bearing.

Figure 3 shows a view and a partial section along line III-III of Figure 2, and it can be seen that three pinions 26 are used in this embodiment. Inter alia, the fastening of the front stationary bush 21 to the support 47 is shown here.

An oil circulation pump 101 is also shown, said pump supplying oil by way of a duct 49 to the inner bearings 14 and 17. Said oil discharge from the inner bearings passes by way of a duct 48 to a buffer tank 102 or the suction side of pump 101. The pump 101 is duplicated, one of the pumps serving as a reserve pump.

Figure 4 shows the rear inner bearing 14, as fitted between the inner shaft 2 and an inner surface 100 of the outer shaft 6. The rear stationary bush 15 is also shown here.

Figure 5 shows section V-V from Figure 4, in the case of which a bearing supporting ring 50 is provided on the outside with a bearing surface 51 and on the inside with a bearing surface 52. In the horizontal plane, both on the inside and on the outside of the bearing supporting ring 50, an oil groove 53 is provided in the inner surface 52 and an oil groove 54 is provided in the outer surface 51. The oil is supplied to said grooves through an oil supply duct 57, each pair of grooves having its own supply duct.

Figure 6 shows detail A from Figure 4, and shows the oil supply from the oil supply duct 57 to the oil grooves 53 and 54, the oil groove 54 being connected to the duct 57 by means of a bore 59, and the oil groove 53 by means of a bore 60. Several bores 59 and 60 of approximately the same cross-section, which is small compared with that of the supply duct, are provided along the length of the bearing, through which bores oil is fed uniformly along the length of the bearing. The supply duct 57 is shut off at the end of the bearing supporting ring 50. Oil grooves 53 and 54 are provided at each side of the bearing supporting ring 50, so there are also two supply ducts 57. The connection between the bearing supporting ring 50 on the rear stationary bush 15 and other bushes placed between the inner shaft 2 and outer shaft 6 is designed in such a way that only rotation between the two cylindrical parts is prevented, and the direction of the axes of the two cylindrical parts can vary by a small angle. The rear inner bearing 14 can consequently assume the most advantageous position, with the result that peak load on the bearing surface is avoided. This flexibility in the direction of the axes is also present at the connections between the inner bearings, the cylindrical bushes and possibly the support 47 in the gearbox 5. Due to the fact that the connections are slightly flexible, it is also possible for the stationary bushes to move in directions at right angles to the axis. This means that the inner bearings can position themselves in such a way between the two propeller shafts that the load is transmitted fully to the outer shaft 6 by the inner shaft of the inner bearings 14 and 17. The rotation between the two cylindrical parts is prevented by a coupling pin 56, which in this case is designed as a pipe with a bore 103 and also serves as the onward connection of an oil supply duct 55 in the sta¬ tionary bush 15 to the oil supply duct 57 in the bearing supporting ring 50. Since the leakage from the oil supply ducts 55 and 57 has to be minimized, the pins 56 are provided with sealing rings 58.

The stationary bush 15 and the bearing supporting ring 50 are coupled in the axial direction with some play by means of a spring washer 61, which is placed in a spring washer groove 62. By forcing the spring washer 61 with a screw 63 into the groove 62 through a hole in the outer shaft 6, it is possible to disconnect and remove the cylindrical parts 15 and 50. This is necessary for, for example, inspection and maintenance. If the spring washer 62 is made broader than the spring washer 61, a slight axial play is produced, and with sufficient play between the other parts which are in engagement with one another, the axial directions of the two parts to be connected can form a small angle with each other.

In Figure 7 detail A from Figure 4 is also shown at the position of an oil discharge duct 64. The oil is discharged through the oil discharge duct 64 in the bearing supporting ring 50 and an oil discharge duct 65 and a bore 66 in the stationary bush 15. Since the oil can be discharged without pressure, it can flow both on the inside and on the outside of the stationary bush 15.

The oil pressure in the supply lines is not high, and serves in particular to produce a uniform distribution of the oil supply over the length of the oil grooves. High pressure is not necessary, because the bearing power is produced through rotation of the propeller shafts 2 and 6 relative to the bearing surfaces 52 and 51, respectively. Since heat develops in this double bearing under the influence of the rotation and the load, it must be ensured that there is adequate circulation of the oil. A limited oil pressure is required for this, for example approximately 5 to 10 bar, which is supplied by the oil circulation pump 101 discussed in Figure 3.

The oil discharge shown can also be designed in an alternative way. It is, for example, conceivable for oil ducts to be disposed in such a way that the oil is supplied only through the bores in the lengthwise direction of the bearing supporting ring 50 and is discharged through bores through the outer shaft 6 or the stationary bush 15 at right angles to the axis 78, so that the oil can flow out of the bearings and the heat can be dissipated to the propeller shaft pipe 98, and from there, for example, to the ship's hull.

The design described above for the rear inner bearing 14 is used in a comparable way for the front inner bearing 17, the latter being coupled to both the rear and the front stationary bush. The connection between the various cylindrical parts can also be used for coupling parts of the cylindrical bushes 15 or 21 if the latter are split up in the axial direction into a number of shorter bushes.

Figure 8 shows detail B in Figure 4 at the position of the underside at the rear side of the rear inner bearing 14. Additional provisions are made here in order to achieve adequate lubrication in the event of the speed of rotation of the shafts 2 and 6 being too low to achieve hydrodynamic lubrication. Such situations can occur in the course of, for example, turning of the propeller shafts, for example during the cooling down of drive units. Then, and after a lengthy stationary period, there is a risk of the bearing material being under too great a load and corrosion occurring.

The additional provision is formed by an oil supply duct 82 which is coupled in the manner described earlier to a high-pressure oil pump. This pump (not shown) supplies oil at high pressure, for example at 60 bar, for a short period. This oil is supplied only during poor lubricating conditions. The oil supply duct 82 is coupled by means of a transverse bore 83, which runs approximately horizontally and at right angles to the axis of rotation 78, to a number of supply ducts 84 which supply oil both to the inner shaft 2 and to the outer shaft 6. The oil supply duct 82 is sealed by means of a sealing cap 85.

In order to be able to check whether there is sufficient lubrication, sensors (not shown) are provided in the region of the ducts 84, the cable work of which sensors is passed through a duct comparable to the oil supply duct 82 to a control panel (not shown) .

Figure 9 shows an alternative embodiment of the gearbox 5 at the position of the connection of the oil ducts to the gearbox 5. The centre part, which corresponds to centre part 31 in Figure 1, is made up of a front centre part 67 and a rear centre part 68, which makes fitting of the pinions simpler. These centre parts 67 and 68 are fitted in the housing 69. The front stationary bush 21 is worked to a cylindrical surface 70 at the front side. An O- ring 71 is fitted in this cylindrical surface 70, in order to prevent oil from running out of the propeller shaft pipe 98 and into the gearbox 5. The abovementioned oil seals 41 and 42 are also fitted for this purpose.

The oil discharge is achieved here by having the space between the outside of the stationary bush and the inside of the propeller shaft 6 communicate with the dis¬ charge duct 48. In this case a groove 72 is provided locally in the outside wall of the stationary bush 21. The oil supply duct 49 consists of a pipe 73 which is placed in an opening 74 of the front stationary bush 21. The opening 74 connects to the oil supply duct 55 in the stationary bush 21. The pipe 73 is fitted in an oiltight manner in the opening 74 and serves at the same time as a block against rotation of the stationary bush 21. The stationary bush 21 is easily removed for maintenance by removing the pipes 73 and pulling the bush out of the gearbox.

Figure 10 shows the section X-X in Figure 8, and here it can be seen clearly that there are several supply and discharge ducts.

In the exemplary embodiment described above, the drive is shown with a gearbox in the case of which the rotation in opposite directions of the two shafts is achieved by a gearbox in which a double right-angled transmission with crown wheels is fitted. It is also possible to design the unit according to the invention with other types of gearboxes, for example a single or a double planetary gearbox, without thereby affecting the essence of the invention.

Figure 11 shows the deformations in the propeller shafts 2 and 6, inter alia under the influence of gravity Gl on the rear screw and G2 on the front screw and a supporting force Fl and a supporting force F2. These deformations can be calculated with great accuracy on the basis of the loads, the rigidities and the dimensions of the various parts. Between the shaft direction of the inner shaft 2 and the outer shaft 6, there is an angle αl at the position of bearing 14 and there is an angle α2 at the position of bearing 17. These angles αl and α2 depend on the dimensions of the shafts and bearing arrangement. The order of magnitude adhered to for these angles can vary from 0.06 to 0.4 degrees. The shape of the inner bearings 14 and 17 is adapted thereto, and the axis of the bearing surface 51 forms an angle αl or α2 with the axis of the bearing surface 52. This makes it possible to provide the small gap widths necessary for the hydrodynamic lubrication between the rotating propeller shafts and the stationary bearings over the entire length of the bearings.

For hydrodynamic lubrication, this gap width is 0.001 x d to 0.002 x d, d being the shaft diameter. For a 500-mm shaft this means that the gap may be 0.5 mm to 1.0 mm. The space between the inner shaft 2 and the outer shaft 6 varies through the abovementioned angle by approximately 1 mm, as a result of the bend over the 1,000 mm length. For good bearing power, it is thus necessary to make the bearing of varying thickness in the lengthwise direction. Figures 12 - 14 show the bearing play under the influence of the various phenomena. In Figure 12 a bearing bush 88 is placed between an outer shaft 86 and an inner shaft 87, the bearing bush 88 being immovably fixed to the bedplate, so that the axial direction of the bearing bush 88 does not change and does not follow the direction of inner shaft 87 and outer shaft 86. As can be seen from

Figure 12, at the ends of the bush peak loads occur between the bearing bush 88 and the propeller shafts, with the result that a risk of corrosion between bearing and shaft occurs there. In Figure 13 the axial direction of the bearing bush 88 can change slightly, so that the peak load clearly lessens and the risk of corrosion is reduced.

In Figure 14 the axial direction of the inner surface and the outer surface of the bearing bush 88 is adapted to the direction of the inner wall and outer wall of the propeller shafts. It is found here that optimum use is now made of the bearing power of the bearing, and the risk of corrosion of the shafts in the bearing material is minimized. Figure 15 shows the bearing arrangement of two propeller shafts rotating in opposite directions about a common axis in a simplified design. This design could be used in particular in situations in which the dimensions of the parts are smaller and the deformations as the result of the loads are small compared with the usual tolerances and gap sizes in the bearings. A stationary bush 92 is fitted between an outer shaft 89 and an inner shaft 90. When the space inside the outer shaft is filled with oil, a lubricating film is built up between the stationary bush and the propeller shafts, under the influence of the rotation of both shafts. However, it is found in practice that without additional measures the bearing power of this hydrodynamic lubricating film soon decreases and becomes too low, since great heat is generated under the influence of the rotation, the viscosity of the oil reduces greatly and the bearing power of the bearings consequently decreases greatly.

For that reason at least one duct 91 is drilled in the axial direction in the stationary bush 92, by means of which duct oil is supplied to each bearing of the outer shaft 90, the oil being supplied under pressure by means of a circulation pump, which is preferably connected at the end of the stationary bush 92 to the ducts 91 in the bush. In this case it is possible to opt for separate ducts to each bearing, as indicated by a duct 93 and a duct 94, with the result that the oil flow during use can be regulated and monitored from the control room (not shown) . It is also possible to work with fewer supply ducts, in which case a restriction is placed in the ducts 93 and 94 at the position of the outflow, which restriction ensures that the same amount of oil reaches all points in the bearing. The oil flow is indicated by arrows in Figure 15.

For the discharge of oil, a duct 95 can be drilled in the stationary bush 92, in which case the oil flowing back is discharged at the end of the stationary bush 92. During this discharge the temperature of the oil can be measured, which is an indication that the oil flow should be increased or reduced. The abovementioned design can be simplified further through the use of grease instead of oil if the circumstances of use, for example low load on the bearings, make it possible. Particularly when grease is used, the use of one or more grease ducts to each bearing, preferably two ducts to each inner bearing and two ducts to each outer bearing, is a simple way of achieving good lubrication of the propeller shafts. The grease could also be discharged in the manner described above, but if environmentally harmless grease is used, it is also possible to reverse the direction of the grease discharge and direct it to the outside.

Claims

1. Drive unit for a ship, comprising a gearbox (5), a front screw propeller (76) and a rear screw propeller (77) , which propellers rotate in opposite directions about an axis of rotation (78) , the front screw propeller (76) being fixed to a hollow outer shaft (6; 86; 89) which is supported in a front outer bearing (18) and a rear outer bearing (16), which bearings (16, 18) are fixed in a stern (75) , and the rear screw propeller (77) being fixed to an inner shaft (2; 87; 90) which is supported in a cylindrical inner surface (52) of a front inner bearing (17) and a rear inner bearing (14), which inner bearings (14, 17) are provided with a cylindrical outer surface (51) which rests against an inner surface (100) of the outer shaft (6; 86; 89), characterized in that the inner bearings (14, 17) are connected to each other by means of a rear stationary bush (15) and to the gearbox (5) by means of a front stationary bush (21), and in that a first duct (55, 91) running through the stationary bushes (15, 21) and the inner bearings (14, 17) connects a first oil circulation pump (101) to the inner surface (52) and/or the outer surface (51) of the inner bearings (14, 17).

2. Drive unit for a ship according to claim 1, characterized in that the inner bearings (14, 17) are in the form of hydrostatically lubricated bearings.

3. Drive unit for a ship according to claim 1, characterized in that the stationary bushes (15, 21) and the inner bearings (14, 17) are connected to each other by means of coupling means (56) which allow small deviations in the axial directions relative to each other-

4. Drive unit for a ship according to claim 3, characterized in that the first duct (55, 91) at the position of the coupling means (56) is partly formed by a bore (103) in the length of a cylindrical pin (56) fixed in a sealing manner in the stationary bush (15, 21) and/or the inner bearing (14, 17).

5. Drive unit for a ship according to claim 4, characterized in that the coupling means (56) consist of the cylindrical pin (56) .

6. Drive unit for a ship according to one of the preceding claims, characterized in that there is a first oil seal (41) between the front stationary bush (21) and the hollow outer shaft (6; 86; 89) and a second oil seal (42) between the front stationary bush (21) and the inner shaft (2; .87; 90) .

7. Drive unit for a ship according to one of the preceding claims, characterized in that a second duct (82) is provided, which duct runs through the stationary bushes (15, 21) and the inner bearings (14, 17), and which connects a second oil pump to an oil supply duct (83, 84) at the position of the rear side of the rear inner bearing (14).

8. Drive unit for a ship according to claim 1, characterized in that the cylindrical outer surface (51) and the cylindrical inner surface (52) of the rear inner bearing (14) run in different axial directions.