STERN TUBE BEARING SYSTEM OF CONTRA-ROTATING PROPELLER

Abstract: A stern tube bearing system of contra-rotating propeller includes an inner shaft having a rear propeller and an outer shaft having a front propeller, both shafts being rotated in opposite directions. A shaft center portion of the inner shaft is made hollow, a plurality of oil supply holes are bored radially in the inner shaft so as to pass therethrough perpendicularly to the outer peripheral surface thereof, and a plurality of rows of these oil supply holes are arranged in the longitudinal direction of the inner shaft, whereby seizing of the bearing due to offset touch as its rear end caused because of the propeller being overhung can be prevented.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a stern tube bearing system of contra-rotating propeller for large-size marine vessels. 2. Description of the Prior Art Figs. 14 through 18 show conventional stern tube bearing systems of contra-rotating propeller. Referring to these drawings, marine vessels employing conventional contra-rotating propeller systems use the well-known vessel-propulsion method in which an inner shaft 02 having at its point a rear propeller 01 and an outer shaft 04 having at its point a front propeller 03 are rotated in opposite directions via a reversing device by a prime mover not shown ( such as diesel engine, steam turbine, and gas turbine ). To drive propeller shafts smoothly, generally, a front bearing 06 and a rear bearing 07 are disposed between the outer shaft 04 and a hull 05, and a reversing front bearing 08 and a reversing rear bearing 09 are disposed between the inner shaft 02 and the outer shaft 04. Further, between the hull and the outer shaft and between the outer shaft and the inner shaft are disposed stern shaft seals 010 and 011, respec­ tively, in a stern section. The bearing disposed between the outer shaft 04 and the hull 05 is of the ordinary type of stern tube bearing and has no peculiar technical difficulty. However, because the inner shaft 02 and the outer shaft 04 rotate in opposite directions, the bearing incorporated there­ between is confronted with some technical difficulties. The reasons are as follows. Fig. 15 is a schematic cross-sectional view showing the mode of counter-rotation as viewed in the axial direction. Because the hull remains stationary, a normal fluid lubrication plain bearing is formed between the outer shaft 04 and the hull 05. However, because the inner shaft 02 and the outer shaft 04 are rotating in opposite directions as indicated by the arrows, fluid lubrications can hardly be achieved therebetween. Further, when the above two are rotating in opposite directions substantially at the same velocity, fluid lubrication ( separation of the shafts by means of oil film ) cannot be effected. To cope with the aforementioned difficulty, such bearing systems as shown in Figs. 16 and 17 were proposed in the prior art. The system of Fig. 16 is characterized in that a floating bush 012 is disposed between the inner shaft 02 and the outer shaft 04 and held substantially stationary to form fluid lubrication oil films between the inner shaft and the floating bush and between the floating bush and the outer shaft. Fig. 17 shows an outer shaft 013 having taper/land portions on its inner surface, that is, section A indicates a taper portion, B a land portion parallel to the surface of the inner shaft, and C a dynamic pressure generating taper portion effective when the whole system reverses ( goes astern ). The conventional bearing system creates easily the fluid lubrication oil film ( the dynamic pressure ). However, if the marine vessel is running at a low speed, that is one feature arising during the service of the vessel, the oil film becomes thin, as a result, metal-to-­ metal contact tends to occur on the bearing surface, and a rear end portion of the bearing 09 tends to cause offset touch as shown in Fig. 18 because of the propeller being overhung, thus, there is a defect that the bearing will be seized. SUMMARY OF THE INVENTION It is an object of the present invention to overcome the aforementioned defect, thus to provide a bearing system which permits operation at a further low speed in view of a demand of making bulky a propeller and of enhancing the propulsion efficiency thereof and prevents seizing of a rear end portion of the bearing even under offset touch that will be caused because of the propeller being overhung. To achieve the foregoing object, the present invention provides a stern tube bearing system of contra-­ rotating propeller of the type wherein an inner shaft having a rear propeller and an outer shaft having a front propeller are rotated in opposite directions, which is characterized in that a shaft center portion of the inner shaft is made hollow, a plurality of oil supply holes are bored radially in the inner shaft so as to pass there­ through perpendicularly to the outer peripheral surface thereof, and a plurality of rows of these oil supply holes are arranged in the longitudinal direction of the inner shaft. According to the foregoing structure, by letting h denote the thickness of an oil film at an oil supply hole, P o the outlet pressure of the oil supply hole, and P s the oil pressure inside the hollow portion of the inner shaft; a run-out rate Q toward the bearing is represented by Q ∝ h s P o , and a flow rate Q passing through the oil supply hole ( equivalent to the run-out flow rate from the bearing ) is represented by Q ∝P s -P o . As will be apparent from the aforementioned relationships, the larger the h , the smaller the P o . Conversely, the smaller the h , the larger the P o . Accordingly, by determining the distribution of static pressure in the longitudinal direction of the bearing section at the oil supply holes, it is possible to bring out a static pressure, i.e. a load carrying capacity, acting so as to lift the weight of the inner shaft. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view showing a first embodiment of a stern tube bearing system of contra-­ rotating propeller according to the present invention; Fig. 2(A) is a diagram explanatory of the operation principle of the system shown in Fig. 1; Fig. 2(B) is a diagram illustrative of the distribu­ tion of static pressure corresponding to Fig. 2(A); Fig. 3 is a cross-sectional view showing a second embodiment of the present invention; Fig. 4 is a side view of an inner shaft as shown in Fig. 1 or Fig. 3; Fig. 5 is a diagram explanatory of the effect obtained by the inner shaft of Fig. 4; Fig. 6 is a longitudinal sectional view showing a stern shaft system in a third embodiment of the present invention; Fig. 7 is a fragmentary enlarged sectional view corresponding to Fig. 6; Fig. 8 is a fragmentary sectional view of an inner shaft taken in the direction of the arrows along line VIII-VIII in Fig. 7; Figs. 9 through 12 are cross-sectional views showing other embodiments of the bearing system according to the present invention; Fig. 13 is a graph of experimental results which is illustrative of the influence of setting depth of an orifice on abrasion depth of a bearing metal; Figs. 14 through 18 show conventional stern tube bearing systems of contra-rotating propeller, in which Fig. 14 is a sectional side view of a stern bearing section; Fig. 15 is a cross-sectional view of a contra-­ rotating bearing system; Figs. 16 and 17 are cross-sectional views of other contra-rotating bearing systems; and Fig. 18 is a sectional side view corresponding to Fig. 14, showing the state in which both inner and outer shaft bearings are causing offset touch because of the propeller being overhung. DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will now be described with reference to Figs. 1 through 13. In Fig. 1 showing a first embodiment, an outer shaft 4 and an outer shaft bearing 5 are identical to those of the conventional system. But, the system of Fig. 1 differs in that an inner shaft 2 has a concentric hole 19 concentrical with the outer periphery thereof for leading an oil pressure supplied from a hydraulic power source not shown, which hole communicates with three or more radial holes 20 bored radially in the inner shaft. An outer peripheral portion of each radial hole 20 has a screw 25 tightened therein which has a pinhole having the orifice-throttling function, and each outer peripheral portion is finished so as to assume the same circumference as of the outer periphery of the inner shaft. In Fig. 3 showing a second embodiment, an outer shaft 4 and an outer shaft bearing 5 are identical to those of the first embodiment. But, the system of Fig. 3 differs in that an inner shaft 2 has, as three of more radial holes 20 communicating with the oil pressure leading concentric hole 19, holes having the capillary-throttling function. The operation principle of the foregoing first and second embodiments will now be described with reference to Figs. 2(A) and (B). Referring to Fig. 2(A), consider a peripheral spot 23 of one, where the oil film has a thickness of h , among a number of radial holes 20 of the inner shaft. By letting P o denote the outlet pressure of the radial hole 20, and P s the supply pressure of the concentric hole 19, a run-out flow rate Q toward the bearing is represented by Q ∝ h s P o . Further, a flow rate Q passing through the hole or orifice ( equivalent to the run-out flow rate from the bearing ) is represented by Q ∝P s -P o . As will be apparent from the aforementioned relation­ ships, the larger the h , the smaller the P o . Conversely, the smaller the h , the larger the P o . Accordingly, the distribution of static pressure around the inner shaft in case a number of radial holes 20 are provided is not uniform about the shaft center as shown in Fig. 2(B). Thus, there is produced a static pressure, i.e. a load carrying capacity, acting so as to lift the weight 21 of the inner shaft. Of course, a similar action to that of the orifice is also realized in case of capillary throttling. In Fig. 2(A), h₁ and h₂ indicate the minimum and the maximum thickness of the oil film, respectively. Fig. 4 shows in side view the inner shaft as shown in Fig. 1 or Fig. 3. The radial holes 20 are arranged in the longitudinal direction of the inner shaft 2 at irregular intervals, and the spacing between adjacent holes becomes small as approaching the stern. Of course, the area where the holes are arranged is limited to the extent of of the width ( the length ) of the bearing. Describing the function and effect of the foregoing arrangement with reference to Fig. 5, if the static pressure holes are arranged at regular intervals, a rear end section of the shaft involves a minimal oil film, whereas a front section leaves a large space, as a result, a great amount of wasteful oil is consumed through the static pressure holes of the front section. Thus, if the holes are provided at irregular and mutually-closed intervals in an area of the rear end section where offset touch is heavy, the extent of rising of the shaft in the rear end section can be made large with remarkably reducing the necessary amount of oil in comparison with the arrangement of regular interval. Of course, in case an inclination of the shaft is small of a consumption of oil does not matter, the holes may be provided at regular intervals. As described above, the following effects are produced by application of the stern tube bearing system of contra-rotating propeller according to the present invention: (1) Low-speed operation is permitted, whereby the propeller can be enlarged and the propulsion efficiency can be enhanced; and (2) There occurs no seizing due to offset touch of a rear end portion of the bearing that would be caused because the propeller is overhung. Figs. 6 through 12 show a third embodiment of the present invention and modifications. In these drawings, reference numeral 101 designates an inner shaft; 102 an outer shaft; 103 a rear propeller; 104 a front propeller; 105, 106, 107 and 108 seal members; 109 a two-piece hollow shaft; 110 a coupling of the outer shaft; 111 an inner shaft bearing on the stem side; 112 an outer shaft bearing on the stem side; 113 an outer shaft bearing on the stern side; 114 an inner shaft bearing on the stern side; 115 a lobe guard; 116 a hull; 117 a rear bulkhead of an engine room; 18 an oil feeding hole for passing therethrough an oil a of high pressure; 119 an oil supply hole formed radially in the inner shaft 101; 120 an oil drain hole formed in the outer shaft; 121 a lubricating oil outlet pipe; 114-1 a bearing metal applied to the inner shaft bearing 114 on the stern side; and 114-2, 114-3, 114-4 and 114-5 oil drain holes formed in either or both of the outer peripheral surface of the inner shaft bearing 114 and the inner peripheral surface of the outer shaft 102 for making communication between both ends of the inner shaft bearing. Of course, the inner shaft bearing 111 on the stem side also has a bearing metal and oil drain holes. The inner shaft 101 is linked to a propelling prime mover not shown and drives the rear propeller 103. The outer shaft 102 is rotated by a reversing device or a second propelling prime mover not shown in the opposite direction to the rotation of the inner shaft 101 to drive the front propeller 104. To lubricate the inner and outer shaft bearings 111, 112, 113 and 114, a lubricating oil is high pressure is supplied from a hydraulic power source not shown to the oil feeding hole 118 which is formed in a shaft center portion and extends in the axial direction of the inner shaft 101 for driving the rear propeller 103. This lubricating oil coming through the oil feeding hole 118 passes through the oil supply holes 119 formed radially in the inner shaft 101 to lubricate the inner shaft bearings 111 and 114, after lubrication it ( the drain oil ) goes directly, or after passing through the oil drain holes 111-2 and 114-2 formed in the outer peripheral sections of the inner shaft bearings 111 and 114, to the oil drain hole 120 bored in the outer shaft 102, lubricates the outer shaft bearings 112 and 113, and returns through the lubricating oil outlet pipe 121 to the hydraulic power source. Examples of the oil drain hole are shown in Figs. 9 through 12, in which Fig. 12 shows those passing through the insides of the inner shaft bearings 111 and 114. To adjust the supply amount of oil to the inner shaft bearings 111 and 114, each oil supply hole 119 has an orifice 130 or a fluid throttling element ( hereinafter referred to simply as an orifice ) having the same function as that of the orifice as shown in Fig. 8. The manner of determining the setting depth of the orifice will be described below. The setting depth of the orifice is determined on the basis of the results of collision erosion test that was performed by causing the lubricating oil flowing out from the orifice 130 to collide with the bearing metal 114-1 applied to the inner peripheral surfaces of the inner shaft bearings 111 and 114 and the like. As shown in Fig. 13, if the ratio, ℓ/d ( see Fig. 8 ), of setting depth to diameter of the orifice 130 is small, abrasion due to erosion cannot be avoided even in case of purified oil. In case solid particles are mingled, the extent of abrasion increases abruptly as shown in Fig. 13. However, if the ratio ℓ/d is increased the amount of abrasion decreases remarkably, and in the range of ℓ/d> 25, substantially zero abrasion is exhibit­ ed. In this way, by providing the orifice 130 at the position of the oil supply hole 119 as proposed by the present invention, irrespective of amount of solid particles mingled in the lubricating oil, the bearing metal provided on the inner peripheral surface of the inner shaft bearing 111, 114 cannot be damaged by erosion. As described above, the third embodiment is charac­ terized in that the inner shaft bearing is attached to the inner peripheral surface of the outer shaft, which has the bearing metal on its inner peripheral surface, a plurality of oil drain holes are formed in the outer peripheral surface of the inner shaft bearing being in contact with the outer shaft or in the inner peripheral surface of the outer shaft for making communication in the axial direction between both ends of the inner shaft bearing, and the orifice for adjusting the supply amount of oil or fluid throttling element having the same function as that of the orifice, having a diameter d , is provided in each oil supply hole with a setting depth ℓ under the condition that ℓ/d> 25. Thus, the following effects are produced: (1) After performing lubrication of the bearings, the lubricating oil passes through the oil drain holes formed in either or both of the outer peripheral surface of the inner shaft bearing and the inner peripheral surface of the outer shaft, and is drained smoothly, thus, even when the slide surfaces of the inner and outer shafts cause metal-to-metal contact by any chance, the bearing metal can surely be prevented from proceeding to destructive damage; and (2) Since the setting depth of the orifice and the like provided in the radial oil supply hole of the inner shaft is made larger than is necessary, there is no generation of erosion in the bearing metal that would otherwise be caused by a jet flow of the lubricating oil of high pressure. While the preferred embodiments have been described, it is not intended to have the present invention limited to the specific embodiments thereof, and many changes and modifications may be made without departing from the spirit of the present invention.

Claims: 1. A stern tube bearing system of contra-rotating propeller of the type in which an inner surface having a rear propeller and an outer shaft having a front propeller are rotated in opposite directions, characterized in that a shaft center portion of the inner shaft is made hollow, a plurality of oil supply holes are bored radially in the inner shaft so as to pass therethrough perpendicularly to the outer peripheral surface thereof, and a plurality of rows of these oil supply holes are arranged in the longitudinal direction of the inner shaft. 2. A stern tube bearing system of contra-rotating propeller according to claim 1, further including an inner shaft bearing having a bearing metal applied to its inner peripheral surface which is attached to a portion of the inner peripheral surface of the outer shaft corresponding to the area where the oil supply holes are opened, a plurality of oil drain holes formed in either or both of the outer peripheral surface of the inner shaft bearing and the inner peripheral surface of the outer shaft, and a plurality of fluid throttling elements having the function of an oil supply adjusting orifice, each being provided in a respective oil supply hole under the condition that ℓ/d>25, where ℓ is the distance from the setting position of each fluid throttling element to the outer peripheral surface of the inner shaft, and d is the diameter of a flow path formed in each fluid throttling element.

Inventors: ASANABE, SADAO NAGASAKI TECHNICAL INSTITUTE OF, KODA, MASATOSHI NAGASAKI SHIPYARD & ENGINE WORKS, MATSUMOTO, SUSUMU NAGASAKI TECHNICAL INSTITUTE OF, MOROHOSHI, SHOZO NAGASAKI TECHNICAL INSTITUTE OF, SAKI, KUNIO NAGASAKI TECHNICAL INSTITUTE OF, TANIGUCHI, SUSUMU NAGASAKI TECHNICAL INSTITUTE OF, Koda, Masatoshi Nagasaki Shipyard &amp; Engine Works
Applicants: MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1986
Main classification: B63H23
IPC codes: B63H23, B63H1