Patent Publication Number: US-10759510-B2

Title: Method of arranging the lubrication of a controllable pitch propeller arrangement of a marine vessel and a lubrication arrangement therefor

Description:
TECHNICAL FIELD 
     The present invention relates to a novel method of arranging the lubrication of a controllable pitch propeller of a marine vessel and a lubrication arrangement therefor. More closely the present invention relates to the method in accordance with the preamble of claim  1  and to the lubrication arrangement in accordance with the preamble of claim  14 . 
     BACKGROUND ART 
     Controllable pitch propellers are becoming more and more popular in marine vessels. The pitch control gives the operator a chance to alter the speed of the marine vessel by changing the blade angle or pitch of the propeller, and more importantly, to change the direction of movement of the marine vessel by turning the propeller blades from ahead direction to astern direction, whereby there is no need to provide the vessel with such a gearbox that is capable of changing the rotational direction of the propeller, or need to reverse the rotational direction of the engine. Such controllable pitch propellers are arranged in a so-called hub. The marine vessel propulsion arrangement comprises an engine, a drive means, a drive shaft and the hub with the propeller blades. The drive means is normally a reduction gear or an electric drive motor, which is used to drive the drive shaft. The pitch of the propeller is controlled by means for turning the propeller blades. The propeller blade turning means comprise actual mechanical turning arrangement arranged in the hub, and means for actuating the mechanical turning arrangement. The mechanical turning arrangement comprises a crank ring for each propeller blade. The propeller blade is rotatably coupled and sealed to the body or casing of the hub by means of the crank ring. The crank ring has a non-central or non-concentric pin extending towards the inside of the hub. The pin is fitted in a groove provided on a member arranged concentrically in the hub body and moving in the direction of the axis of the propeller. The groove extends, preferably, in a direction perpendicular to the axis of the propeller. Now that the moving member is shifted in axial direction by means of the actuating means, the crank ring/s and the propeller blade/s are forced to turn, whereby the pitch of the propeller blades is controlled. 
     There are basically two types of actuating means, i.e. a mechanical one and a hydraulic one. The mechanical one comprises a rod extending along a central bore in the drive shaft from the drive means to the inside of the hub such that the rod is coupled inside the hub to the movable member, and at the drive means end of the drive shaft to means for moving the rod axially. With regard to the hydraulic actuating means, of which U.S. Pat. No. 4,028,004 may be presented as an example, the movable member inside the hub is arranged to work as a piston in a hydraulic cylinder. In other words, on both axial sides of the movable member there are chambers, so called astern chamber and ahead chamber, into either one of which, when pitch control is desired, pressurized oil is delivered depending on the direction the propeller blades are to be turned. If the pressurized oil is taken to the ahead chamber, the piston, i.e. the movable member, turns the propeller blades such that the propeller blades move the marine vessel in ahead direction. And if the pressurized oil is taken to the astern chamber, the piston, i.e. the movable member, turns the propeller blades such that the propeller blades move the marine vessel in astern direction. Naturally, in the intermediate positions of the piston, the propulsive force of the propeller is reduced due to reduced pitch of the propeller blades. 
     The pressurized oil is taken to the astern and ahead oil chambers via a central bore in the drive shaft and by providing the central bore with a concentric tube such that two separate oil passages are formed. The oil is provided in the oil passages by means of an oil distribution box arranged in connection with the drive means, i.e. the purpose of the oil distribution box is to deliver oil from stationary oil pipes to rotary oil passages in the dive shaft. The oil distribution box receives pressurized oil from a so called hydraulic powerpack such that the operator of the marine vessel controls, by means of a pilot-operated main control valve arrangement, into which oil passage the pressurized oil is directed, whereby the other oil passage acts as a return passage returning the oil to the hydraulic powerpack. In other words, the hydraulic powerpack receives oil from an oil tank, pressurizes the oil to a required pressure, and by means of the above mentioned pilot-operated main control valve arrangement delivers the pressurized oil to the desired application. 
     The propeller pitch control works normally such that when starting to move the marine vessel in ahead direction the operator moves the pilot-operated main control valve in the ahead position whereby pressurized oil is allowed to enter the ahead oil chamber in the hub and the oil in the astern oil chamber is allowed to escape the astern oil chamber so that the movable member may turn the propeller blades in ahead direction against the hydraulic pressure water subjects to the rotary propeller blades. When the propeller blades are in their desired position the pilot-operated main control valve is moved in a position blocking the flow in both oil passages, whereby the blades are maintained in their position by keeping the ahead oil path blocked and the blades creating a certain pressure in the ahead oil path. If no leakage would occur in any part of the piping, the propeller blades would be locked in their current position. 
     However, keeping the ahead flow path closed by means of the pilot-operated main control valve is not, in practice, possible, as, in the oil distribution box the oil flows from stationary oil pipes to rotary oil pipes, causing a significant leakage due to the nature of this type of seal (annular pressure seal). An option to solve this leakage-related problem would be to move the pilot-operated main control valve to compensate for this leakage, but this would be very energy inefficient and difficult to control. Therefore a blocking valve (pilot-operated non-return valve or similar), not discussed in the above-mentioned U.S. Pat. No. 4,028,004, is placed in the ahead rotary oil pipe. In the ahead rotary pipe because propellers are normally designed such that the blades turn automatically to their astern direction even if no pressure is introduced in the astern oil chamber. Oil flow from the ahead oil chamber to the pilot-operated main control valve is blocked (leakage of the blocking valve is minimal and can be considered insignificant with respect to keeping the propeller blades at their current position) and, in fact, to the oil distribution box, whereby the oil is pressurized between the piston and the blocking valve in the ahead oil path, effectively locking the propeller blades in their current position without any need of supplying additional pressurized oil from the hydraulic powerpack. 
     The blocking valve functions such that pressurized oil flowing from the pilot-operated main control valve towards the ahead oil chamber may pass the blocking valve, so as to be able to change the blade pitch towards ahead. The blocking valve is connected by means of a pilot line to the astern flow path such that pressurized oil flowing from the pilot-operated main control valve towards the astern oil chamber, when exceeding a certain pressure (determined among others by the pilot ratio of the blocking valve), is able to open the blocking valve thereby allowing oil to escape from the ahead oil chamber and therefore to allow the pitch of the blades to be changed to astern. Allowing pressurized oil to flow towards the astern oil chamber will cause the movable member to eventually be stopped by a mechanical stopper. In other words, when the propeller blades are in their “full astern” position the mechanical stopper will lock the propeller blades in their current position, not the blocked oil in either one of the oil chambers. 
     In addition to the hydraulic actuating means the hub needs oil for the lubrication of the mechanical turning or control arrangement. There are in fact two options for arranging the lubrication. A first one would be to supply oil to and from the hub through the stern tube. But, however, it is not a preferred option, as it would connect the hub lubrication system with the stern tube lubrication system, whereby a problem, for instance water ingress, in one system would result in the same problem in the other system, too. A second, preferred, option, discussed in the above-mentioned U.S. Pat. No. 4,028,004, too, is to use the central bore of the shaft as the safe route for the lubricant delivery into the hub. Therefore, the central bore of the drive shaft is provided with another concentric tube so that three separate flow passages are arranged within the drive shaft. The two innermost flow passages communicate with the astern and ahead oil chambers whereas the outermost flow passage between the moving member and the body or housing of the hub communicates the lubricant chamber. 
     The lubricant chamber is provided with oil from the oil tank by gravity. The oil tank is arranged above the waterline of a fully loaded marine vessel for safety purposes such that the hydrostatic pressure inside the hub remains always higher than the water pressure outside the hub, whereby in case of sealing failure in the hub water is not able to enter the lubricant chamber, but the lubricant leaks to the surrounding water. 
     However, practice has shown that water may enter the inside of the hub, i.e. to the lubricant chamber, by condensation, by leakage, or for some other reason. Thus, it was, at some point of time, considered that some kind of lubricant circulation is needed in place of the earlier practice, discussed in the above-mentioned U.S. Pat. No. 4,028,004, where the same lubricant remained in the lubricant chamber and collected water and any other impurities until specific measures were taken to replace the used lubricant with fresh one. 
     Naturally, the first suggestion was to arrange a third concentric tube in the central bore of the drive shaft, which would have required also changes in the oil distribution box. As such a construction was considered quite complicated and risky, it was, for the second suggestion, realized that the hub already has pressurized oil in their ahead and astern oil chambers at least while the pitch of the propeller was changed. To take this suggestion into use only meant that an oil flow channel should be arranged from one of the ahead and astern oil chambers to the lubricant chamber. Since such an oil flow channel is a static element without any need for repair or maintenance it was taken into use between the astern oil chamber and the lubricant chamber. The arranging of the lubricant channel to start from the ahead oil chamber is not possible as the ahead oil chamber has to be kept blocked and pressurized when sailing ahead. Naturally, the oil flow channel, as well as the lubricant circulation, was in use only when moving the propeller blades to their astern position, i.e. the oil circulation took place only occasionally for a substantially short period of time. However, at that time it was considered sufficient as the mineral oil that was used for both the hydraulics and lubricating the mechanical turning arrangement was capable of dealing with certain amount of water without damage to either the oil itself or the surfaces it was supposed to lubricate. 
     However, now the environmental requirements are about to change such that in all positions where there is even a minor risk of leakage of oil to the surrounding water such oil has to be used that degrades easily when in contact with water. This is, naturally, a good property for oil in view of environment, but not so desirable in view of lubrication, as such an environmentally acceptable lubricant has to be watched continuously, as it may lose its lubrication capability in days or weeks depending on the amount of water that has got into contact with the oil. Therefore, modern controllable pitch propellers have to be provided with an oil circulation, at least when sailing ahead, to ensure that water is not able to collect into the hub. 
     BRIEF SUMMARY OF THE INVENTION 
     A first object of the present invention is to offer a solution to one or more of the above discussed problems. 
     A second object of the present invention is to suggest an almost continuous circulation of lubricant for the lubrication of a controllable pitch propeller. 
     A third object of the present invention is to offer a simple and reliable arrangement for circulating the lubricant in the hub of a controllable pitch propeller. 
     A fourth object of the present invention is to offer a simple solution for making it possible to use environmentally acceptable lubricants in connection with controllable pitch propellers. 
     At least one of the above and other objects of the invention are met by a method of arranging the lubrication of a controllable pitch propeller arrangement of a marine vessel, the controllable pitch propeller arrangement comprising a hub, a drive shaft, a drive means, an oil distribution box, a hydraulic powerpack and an oil tank; the hub having a number of propeller blades and mechanical and hydraulic means for controlling the pitch of the propeller blades; the mechanical control means being arranged in a lubricant chamber; the hydraulic control means comprising an astern oil chamber and an ahead oil chamber; the hub being attached to a first end of a drive shaft; the second end of the drive shaft being coupled to the drive means; the drive shaft having three oil paths, an astern flow path for connecting the astern oil chamber to the hydraulic powerpack via the oil distribution box and an oil pipe, an ahead flow path for connecting the ahead oil chamber to the hydraulic powerpack via the oil distribution box and an oil pipe and a lubricant path for connecting the lubricant chamber to the oil tank via the oil distribution box and an oil pipe, the method comprising the steps of connecting the astern oil chamber by means of at least one oil circulation channel to the lubricant chamber and the lubricant chamber being connected by means of at least one first lubricant passage to the lubricant path, and arranging, at least when the propeller blades are locked in a desired position, a pressure difference between the astern oil chamber and the lubricant chamber for circulating lubricant from the astern oil chamber via the lubricant chamber to the oil tank. 
     At least one of the above and other objects of the invention are met by a lubrication arrangement for a controllable pitch propeller arrangement of a marine vessel, the controllable pitch propeller arrangement comprising a hub, a drive shaft, a drive means, an oil distribution box, a hydraulic powerpack and an oil tank; the hub having a number of propeller blades and mechanical and hydraulic means for controlling the pitch of the propeller blades; the mechanical control means being arranged in a lubricant chamber; the hydraulic control means comprising an astern oil chamber and an ahead oil chamber; the hub being attached to a first end of a drive shaft; the second end of the drive shaft being coupled to the drive means; the drive shaft having three oil paths, an astern flow path for connecting the astern oil chamber to the hydraulic powerpack via the oil distribution box and an oil pipe, an ahead flow path for connecting the ahead oil chamber to the hydraulic powerpack via the oil distribution box and an oil pipe and a lubricant path for connecting the lubricant chamber to the oil tank via the oil distribution box and an oil pipe; the astern oil chamber being connected by means of at least one oil circulation channel to the lubricant chamber and the lubricant chamber being connected by means of at least one first lubricant passage to the lubricant path, the arrangement further comprising the astern oil chamber connected by means of at least one oil circulation channel to the lubricant chamber and the lubricant chamber being connected by means of at least one first lubricant passage to the lubricant path, and a source of pressurized oil arranged, at least when the propeller blades are locked in a desired position, in flow communication with the astern oil chamber. 
     Other characteristic features of the present method of arranging the lubrication of a controllable pitch propeller of a marine vessel and a lubrication arrangement therefor become apparent from the description herein. 
     The present invention, when solving at least one of the above-mentioned problems, provides at least the following advantages
         continuous circulation of lubricant when sailing ahead with the propeller blades locked in a desired position,   reliable lubrication of the controllable pitch propeller,   cost-effective means for arranging the circulation of the lubricant,   minimal changes in the present controllable pitch propellers,   easy to update existing controllable pitch propellers to include the novel lubricant circulation, and   use of environmentally acceptable, easily degradable lubricants in controllable pitch propellers.       

    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the following, the novel method of arranging the lubrication of a controllable pitch propeller of a marine vessel and a lubrication arrangement therefor is explained in more detail with reference to the accompanying Figures, of which 
         FIG. 1  illustrates schematically an exemplary lubrication and control arrangement of a prior art controllable pitch propeller, 
         FIG. 2  illustrates schematically a hub of a prior art controllable pitch propeller, 
         FIGS. 3 a -3 c    illustrate schematically a few blocking valve options to be arranged in connection with the hub of a prior art controllable pitch propeller, 
         FIG. 4  illustrates schematically a hub of a controllable pitch propeller in accordance with a preferred embodiment of the present invention, 
         FIG. 5  illustrates schematically a way of arranging lubricant circulation in the lubricant chamber in accordance with a first preferred embodiment of the present invention, 
         FIG. 6  illustrates schematically another way of arranging lubricant circulation in the lubricant chamber in accordance with a second preferred embodiment of the present invention, and 
         FIG. 7  illustrates schematically yet another way of arranging lubricant circulation in the lubricant chamber in accordance with a third preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF DRAWINGS 
       FIG. 1  illustrates a mechanically driven (though also electric drives may be used in connection with the invention) exemplary prior art controllable pitch propeller mainly in view of its control and lubrication arrangement. The arrangement of  FIG. 1  comprises a controllable pitch propeller  10  with its hub  12 , a shafting  14 , a drive means  16  (which is normally a reduction gear or an electric drive motor), an oil distribution box  18 , a hydraulic powerpack  20  and an oil tank  22  with appropriate oil pipes connecting the various components to one another as will be explained later on. The hub  12  comprises, among other components, both mechanical and hydraulic means for changing or controlling the pitch of the propeller blades, i.e. for changing the blade angle position between an ahead and an astern positions, as will be explained in more detail in connection with  FIG. 2 . The shafting  14  comprises a sterntube and a drive shaft. The sterntube is a non-rotary tube arranged to surround the drive shaft, to lead the drive shaft from the drive means through the marine vessel hull to the outside of the vessel and to protect such from any solid objects in the water. The drive shaft is, as will be shown in more detail in  FIG. 2 , provided with a central hollow interior or bore provided with two tubes inside one another. The tubes extend from the oil distribution box  18  to the hub  12 , form three flow paths from the oil distribution box  18  to the hub  12  for providing the hub with both lubricant and oil for controlling the blade angle of the propeller  10  with the hydraulic means for changing the pitch of the propeller blades. 
     The oil distribution box  18  is arranged, preferably but not necessarily, at the end of the drive shaft at a side of the drive means such that the required connections between the rotary shaft and the rotary tubes therein to stationary oil pipes may be made easily. The oil distribution box  18  connects the above mentioned three flow paths, which rotate with the drive shaft, by means of an oil pipe  24  to the oil tank  22  and two oil pipes  26  and  28  to the hydraulic powerpack  20 . The hydraulic powerpack  20  is connected by means of an oil pipe  30  to the oil tank  22  so that enough oil is always at the disposal of the hydraulic powerpack. The hydraulic powerpack  20  includes means for handling the oil used for operating the means for changing the blade angle of the propeller. In other words, the hydraulic powerpack comprises, among other components, hydraulic pump/s for pressurizing the oil, filter/s for keeping the oil clean etc. The hydraulic powerpack is used by a pilot-operated main control valve for operating the hydraulic means in the hub for changing the pitch or the blade angle position between an ahead and an astern position. 
       FIG. 2  illustrates schematically a hub of a prior art controllable pitch propeller. The hub comprises a hub body  40 , which is fastened to a flange  42  arranged at an end of the drive shaft  44 . To the hub  12  a plurality of propeller blades  46  has been rotatably arranged by means of attaching each blade  46  via its blade foot  48  to a rotatable crank ring  50  of its own, preferably by means of bolts. The blade foot  48  and/or the crank ring  50  has/have been sealed in relation to the hub body  40  such that oil used for lubricating the interior of the hub, i.e. the mechanical pitch control means, does not leak to the water surrounding the hub, when in operation. 
     The interior of the hub is provided with the hydraulic means for controlling the pitch or for changing the blade angle of the propeller, i.e. for changing the blade angle position between an ahead and an astern position as was already mentioned in connection with  FIG. 1 . At an end opposite to the drive shaft  44 , the hub  40  is provided with a cap  52  fastened to the hub body  40 . The interior of the cap  52  is provided with a piston  54  dividing the cylindrical interior volume of the cap  52  to two chambers, i.e. an astern oil chamber  56  and an ahead oil chamber  58 . The piston  54  is fastened to an end of a cylinder yoke  60 , which has a few functions. Firstly, it acts as a rod for the piston  54  by being supported and sealed within an opening in the wall of the hub body  40  and on the extension  62  of the drive shaft  44 . Secondly, the cylinder yoke  60  includes another part of the ahead oil chamber  58  to which pressurized oil is delivered from the hydraulic powerpack  20  when the propeller blades  46  are turned to an ahead position. And, thirdly the cylinder yoke  60  includes a part of the mechanical means used for controlling the pitch, i.e. for turning the propeller blades  46  between their ahead and astern positions. The mechanical means for controlling the pitch, i.e. the propeller blade turning means, comprises, for each propeller blade  46 , a groove arranged in the outer circumference of the cylinder yoke  60 , the groove being, preferably, but not necessarily, in a direction perpendicular to the axis of the cylinder yoke  60 . The groove is provided with a sliding block  64  having a round hole for housing a pin  66  (shown by broken lines) provided non-concentrically on the interior surface of the crank ring  50 . Thus, by moving the piston/cylinder yoke unit in axial direction the sliding block  64  in the groove in the cylinder yoke  60  forces the pin  66  to move and turn the crank ring  50 , whereby the blade  46  is turned, as the pin  66  is not located at the center of the crank ring  50  but at a side thereof. Simultaneously, the sliding block  64  also slides in the groove. 
     The actual operation of the hydraulic means for changing the blade angle of the propeller is based on the pressurized oil provided either in the ahead or in the astern oil chambers via the flow paths arranged in the drive shaft  44 . In other words, the drive shaft  44  has a hollow interior provided with two tubes arranged one inside another within the hollow interior of the drive shaft  44 . The inner one ( 68 ) of the two tubes extends from the oil distribution box  18  to and through the piston  54  to which it has been fastened such that it moves with the piston  54 . The interior of the tube  68  forms an astern flow path  70  for the pressurized oil from the hydraulic powerpack  20  via an astern oil pipe  26  and the oil distribution box  18  to the astern oil chamber  56 . The outer one  72  of the two tubes within the drive shaft interior forms together with the inner tube  68  an ahead flow path  74  for the pressurized oil from the hydraulic powerpack  20  via an ahead oil pipe  28  and the oil distribution box  18  to the ahead oil chamber  58 , i.e. first in the cavity inside the cylinder yoke  60 , i.e. inner part of the ahead oil chamber  58 , and from there along at least one hole  76  in the cylinder yoke  60  to the outer part of the ahead oil chamber  58 . The outer tube  72  extends from the oil distribution box  18  to the opposite end of the drive shaft  44 , i.e. to the end of the drive shaft extension  62  within the hub and is attached and sealed thereto. The outer tube  72  forms with the surface of the hollow interior of the drive shaft  44  a lubricant path  78  for the lubricant from the oil tank  22  along oil pipe  24 , the oil distribution box  18  and at least one first lubricant passage  80  arranged in the shaft extension  62  to the lubricant chamber  82  where the mechanical means for controlling the pitch, i.e. turning the propeller blades, are located. The first lubricant passage  80  opens in the lubricant chamber  82  in its innermost location so that any gas present (for any reason, including the assembly) in the interior of the hub is able to escape when the hub is rotated. In other words, when the hub is rotating any gas in the hub collects against the shaft extension at the innermost open part of the hub, as the oil being heavier occupies the rest of the hub interior cavity. In order for the lubrication to work properly the oil tank  22  is located well above the waterline of the fully loaded marine vessel (shown in  FIG. 1 ). With this arrangement the hydrostatic pressure within the lubricant chamber  82  is maintained higher than outside the hub  12 , whereby, in case of any leakage, the risk of water entering the lubricant chamber  82  is significantly reduced. 
     Prior art  FIGS. 3 a -3 c    illustrate schematically a few alternative valve arrangements used for blocking the oil flow from the ahead oil chamber to the hydraulic powerpack, i.e. for holding the pitch of the propelled constant. In the introductory part of this specification it was already mentioned that, as connections between fixed and rotating oil paths in the oil distribution box are, by nature, leaking, the oil path between the ahead oil chamber and the oil distribution box has to be provided with a blocking valve. In other words, the blocking valve has to be arranged in the rotary shaft in connection with the central tubes therein. The blocking valve by preventing oil flow from the ahead oil chamber to the oil distribution box locks the blades of the propeller at their desired pitch.  FIG. 3 a    comprises a valve means  88  (here a certain type of a pilot-operated non-return valve) arranged in the ahead flow path  74 , the valve means  88  allowing oil flow towards the hub, i.e. to the ahead oil chamber  58  of the hub, and controlling oil flow in the opposite direction from the ahead oil chamber  58  back in the direction of the hydraulic powerpack  20 , in practice blocking the oil flow, as long as the pressure in the astern flow path  70  is below a predetermined value (for example between about 3 to 80 bar). In other words, pilot line  90  introduces the pressure of the astern flow path  70  to the valve means  88  such that when the pressure in the astern flow path  70  is increased to the predetermined value, the valve means  88  open, and oil from the ahead oil chamber may flow to the hydraulic powerpack. Naturally, this also means that the pitch of the propeller starts changing to astern direction. As to the predetermined pressure it is a function of a number of variables, whereby the applicable range thereof is quite wide. The variables include, for instance, the spring used in the blocking valve (in principle fixed, but may be decided when choosing or designing the valve), the pilot ratio of the valve (area on to which the astern pressure acts divided by the area on to which the ahead chamber pressure acts) (in principle fixed, but may be decided when choosing or designing the valve), and the ahead chamber pressure (determined by the force the propeller blades exert on the moving cylinder yoke and determined by the propeller blade design and the actual operating conditions). 
     The described arrangement works such that when the marine vessel is moving or sailing ahead both the pilot-operated main control valve in connection with the hydraulic powerpack and the valve means  88  between the ahead oil chamber  58  and the oil distribution box have closed the ahead flow path  74 , i.e. preventing the ahead oil chamber  58  from emptying and locking the propeller blades at their desired position or pitch. When it is desired that the propeller pitch is adjusted to astern direction, the pilot-operated main control valve is moved to astern position, whereby the hydraulic powerpack pressure enters the astern flow path  70  and acts in the pilot line  90  of the valve means  88 . The hydraulic powerpack pressure exceeds the opening pressure of the valve means  88  thereby opening the valve means  88  such that oil may flow from the ahead oil chamber to the oil distribution box and further to the hydraulic powerpack, whereby the blades of the propeller are capable of turning in astern direction. 
       FIG. 3 b    illustrates another alternative for the valve means working in the same manner as that of  FIG. 3 a   . Here the valve means  92  (another type of a pilot-operated non-return valve) allow free flow along the ahead flow path  74  towards the ahead oil chamber to the left, whereas the flow in the opposite direction is blocked, until the pressure in the astern flow path  74  exceeds the predetermined value opening the valve means  92 . 
       FIG. 3 c    illustrates yet another alternative for the valve means working in the same manner as that of  FIG. 3 a   . Here the valve means  94  (a type of a counterbalance valve) allow free flow along the ahead flow path  74  towards the ahead oil chamber to the left, whereas the flow in the opposite direction is blocked, until the pressure in the astern flow path  74  exceeds the predetermined value opening the valve means  94 . 
     The astern flow path  70  may also be provided with blocking valve means similar in both construction and operation to those discussed in  FIGS. 3 a   - 3   c.    
     The above description contains the basic features and properties of the control and lubrication arrangement of a controlled pitch propeller. However, in order to work in a reliable manner the hub, and especially its lubricant chamber  82  is provided with means for circulating the lubricant. The oil circulation is needed as a significant amount of gas is always collected in the lubricant chamber  82  when assembling and installing the hub. For arranging the gas removal by means of the oil circulation, the piston  54  and the cylinder yoke  60  are provided with at least one oil circulation channel  84  (see  FIG. 2 ) from the astern oil chamber  56 , or from the flow path  70  leading thereto, to the lubricant chamber  82 . The lubricant circulation functions in the controlled pitch propeller hubs of prior art only when the astern oil chamber is pressurized in natural manner, i.e. when the travel direction of the marine vessel is changed from the ahead direction to astern direction or the speed of the marine vessel is decelerated by the propeller. In such a case the pressurized oil flows along oil circulation channel/s  84  to the lubricant chamber  82  and via the first lubricant passage  80  to the lubricant flow path  78  taking the air out of the lubricant chamber  82  in the manner discussed earlier. The operation of the prior art lubrication arrangement is discussed in more detail in the following. 
     When the operator of the marine vessel wishes to adjust the blade angle of the propeller to ahead direction he/she moves the pilot-operated main control valve in connection with the hydraulic powerpack to open the astern flow path  70  and to direct full oil pressure (for example 10-30 bar depending on the pressure in the ahead oil chamber caused by the external load on the propeller blades) to the ahead flow path  74 . Then, the pressure in the ahead flow path  74  moves the cylinder yoke  60  to the left forcing oil from the astern oil chamber  56  to the hydraulic powerpack  20 , and turning the propeller blades. As to the oil circulation from the astern oil chamber  56  to the lubricant chamber  82  and further to the tank  22 , it depends on the possible counter-pressure in the hydraulic powerpack. 
     When the operator of the marine vessel wishes to adjust the blade angle of the propeller to astern direction he/she moves the pilot-operated main control valve in connection with the hydraulic powerpack to open the ahead flow path  74  and to direct full oil pressure to the astern flow path  70 . Then, the pressure in the astern flow path  70  exceeds the opening pressure of the valve means  88 , the valve means  88  open allowing oil flow from the ahead oil chamber  58  to the hydraulic powerpack  20 , and the cylinder yoke  60  to move and turn the propeller blades. As to the oil circulation from the astern oil chamber  56  to the lubricant chamber  82  and further to the tank  22 , it continues without interruption. In other words, the only time period when the lubricant circulation is, in fact, really operating is when the propeller blades are turning in astern direction, i.e. the pilot-operated main control valve is moved to astern position. Thus, as soon as the desired propeller blade position is reached, and the pilot-operated main control valve is moved to neutral position, the lubricant circulation is ceased. 
     When, in prior art, the pilot-operated main control valve is in neutral position, it blocks all connections whereby there is no additional pressure in the astern oil chamber, and as a consequence, there is no forced lubricant circulation via the lubricant chamber. Thus, in practice, the lubricant circulates only a small portion of the time the propeller is running, as the sailing in ahead or astern direction, i.e. when the pilot-operated valve is in neutral position and the propeller blades locked in desired position, takes normally more than 90% of the time the propeller is running. Thus the lubricant circulation functions, at most, less than 10% of the time the propeller is running. 
     However, lately the environmental regulations have changed such that the oil used in the controlled pitch propeller hubs has to be environmentally acceptable. This far the oils have been mineral oils, but as they cause severe environmental problems when entering seawater in case of leakage, the environmental requirements dictate that less harmful oils have to be used. Environmentally acceptable lubrication oils, i.e. lubricants, are available, and their lubrication properties are as good as the mineral oils. However, they have a severe drawback, as they degrade very easily when getting into contact with water. Such degradation of lubricant may lead to wear and damage of the components requiring lubrication in the hub. This means, in practice, that the prior art occasional lubricant circulation described above cannot be considered sufficient, but the oil has to be circulated, in practice, nearly continuously such that any water in the lubricant may be detected and removed. Therefore, the lubricant circulation system has to be reconsidered. 
     A first improvement in the lubricant circulation is the provision of the lubricant chamber  82  with not only one route, but two different routes for the circulating oil. A first route, i.e. the first lubricant passage/s  80 , known from prior art, taking oil from the surface of the drive shaft extension  62  to the lubricant path  78 , and a second, novel route from the nearhood of the inner surface of the hub body  40  along at least one second lubricant passage  86  in the hub body  40  and in the drive shaft flange  42  to the lubricant path  78  as shown in  FIG. 4 . Now, when the hub is in use, i.e. rotating, the oil in the lubrication chamber is divided such that any light gas-containing oil collects at the surface of the drive shaft extension and escapes via the first lubricant passage/s  80 , whereas the heavy, possibly water-containing, oil collects, due to centrifugal force, against the inner surface of the hub body, i.e. to the outer circumference of the lubricant chamber  82 , and escapes via the second lubricant passage/s  86 . Both circulated oil fractions are taken along flow path  78  to the oil tank  22  for further treatment. As to dimensioning (length and diameter) the first and the second lubricant passages  80  and  86 , they may be, preferably but not necessarily, balanced such that the flow resistances or restrictions of the passages are substantially equal, whereby both gas-containing and water-containing oil may be removed from the hub. 
     A second improvement in the lubricant circulation is, unlike in prior art arrangements, in accordance with the present invention, the maintenance of a pressure difference between the astern oil chamber  56  and the lubricant chamber  82  at least when sailing ahead, and preferably when sailing astern, too, with the propeller blades locked in desired position. As a marine vessel is sailing ahead most (normally more than 90%) of the time of its operation, it is crucial for the quality of the oil that the lubricant is circulated when sailing ahead. However, the pressure is reduced when compared to the pressure used for controlling the pitch of the propeller. In other words, the pressure of the oil in the astern oil chamber  56  is adjusted to a predetermined value (for instance between about 1 and 7 bar) above the hydrostatic pressure in the lubricant chamber  82  and below the predetermined pressure needed for opening the valve means  88 , whereby the oil flows from the astern oil chamber  56  to the lubricant chamber  82 , and therefrom along lubricant flow path  78  to the oil tank  22 . 
     There are several optional ways of arranging the pressure difference and resulting oil flow through the astern flow chamber to the lubricant chamber for ensuring lubricant circulation. A way of arranging the lubricant circulation in accordance with a first preferred embodiment of the present invention is discussed in more detail in  FIG. 5 . It has been taught in  FIG. 5  that the return oil flow path  96  between the pilot-operated main control valve  98  and the hydraulic powerpack  20  is provided with a non-return valve  100 , which opens by means of a desired pressure in the return oil flow path  96  to the hydraulic powerpack  20 . Additionally, a flow connection  102  has been arranged between the pressure oil path  104  and the return oil path  96 , the flow connection  102  being provided with a control valve  106 , which is arranged to allow or to block pressurized oil flow to the return oil path  96 . The described arrangement functions such that in the neutral position of the pilot-operated main control valve  98  the flow connection  102  between the pressure oil path  104  and the return oil path  96  via control valve  106  is opened, for instance such that the pilot-operated main control valve  98  instructs the control valve  106  to open, and the pilot-operated main control valve  98  connects at least the astern oil pipe  26 , and possibly also the ahead oil pipe  28 , to the return oil path  96 . Now that the non-return valve  100  is arranged to open by a certain pressure, for instance 2 bar or 4 bar, oil at said pressure affects also in the astern oil chamber in the hub  12  ensuring lubricant circulation into the lubricant chamber and further into the oil tank (not shown). The non-return valve  100  is chosen such that an adequate oil pressure resulting in oil circulation, of the order of 2-20 l/min, in the lubricant chamber is accomplished. Here, the return path for the lubricant to the oil tank has not been shown. 
     When the pilot-operated main control valve  98  is moved to its ahead position, the flow connection between the pressure oil path  104  and the return oil path  96  is closed, for instance such that the pilot-operated main control valve  98  instructs the control valve  106  to close, the pressure oil path  104  is connected to the ahead oil pipe  28 , and the astern oil pipe  26  to the return oil path  96 . Now that the return oil path  96  is provided with the non-return valve  100 , a certain increased pressure is maintained in the astern oil chamber ensuring lubricant circulation into the lubricant chamber and further into the oil tank. 
     When the pilot-operated main control valve  98  is moved to its astern position, the flow connection between the pressure oil path  104  and the return oil path  96  is closed, for instance such that the pilot-operated main control valve  98  instructs the control valve  106  to close, the pressure oil path  104  is connected to the astern oil pipe  26 , and the ahead oil pipe  28  to the return oil path  96 . Now, in the astern oil pipe  26  and in the astern oil chamber there is, naturally, an increased pressure ensuring lubricant circulation into the lubricant chamber and further into the oil tank. 
     Another way of arranging the lubricant circulation in accordance with a second preferred embodiment of the present invention is discussed in more detail in  FIG. 6 . It has been taught in  FIG. 6  that the return oil path  96  between the pilot-operated main control valve  98  and the hydraulic powerpack  20  is provided with a pilot-operated control valve  108  and two non-return valves  110  and  112 , which open by means of desired pressures in the return oil flow path  96 . For instance, the non-return valve  110  may be arranged to open with a pressure of 2 bar and the non-return valve  112  with a pressure of 4 bar. Additionally, like in the embodiment of  FIG. 5 , a flow connection  102  has been arranged between the pressure oil path  104  and the return oil path  96 , the flow connection  102  being provided with a control valve  106 . The described arrangement functions such that in the neutral position of the pilot-operated main control valve  98  (shown in  FIG. 6 ) the flow connection  102  between the pressure oil path  104  and the return oil path  96  is opened, for instance such that the pilot-operated main control valve  98  instructs the control valve  106  to open, and the pilot-operated main control valve  98  connects at least the astern oil pipe  26 , possibly also the ahead oil pipe  28 , to the return oil path  96 . The pilot-operated control valve  108  is in a position that connects the return oil path  96  to the non-return valve  112 , i.e. to the valve opening at a higher pressure than the other counter-pressure valve  110 . Now that the non-return valve  112  is arranged to open by a certain pressure, for instance 4 bar, the oil pressure affects also in the astern oil chamber in the hub  12  ensuring lubricant circulation into the lubricant chamber and further into the oil tank. The non-return valve  112  is chosen such that an adequate oil pressure resulting in oil circulation, of the order of 2-20 l/min, in the lubricant chamber is accomplished. Here, the return path for the lubricant to the oil tank has not been shown. 
     When the pilot-operated main control valve  98  is moved to its ahead position, the flow connection between the pressure oil path  104  and the return oil path  96  via control valve  106  is closed, for instance such that the pilot-operated main control valve  98  instructs the control valve  106  to close, the pressure oil path  104  is connected to the ahead oil pipe  28 , and the astern oil pipe  26  to the return oil path  96 . Here, the pilot-operated control valve  108  may be chosen, as desired, to connect the return oil path  96  to either the non-return valve  112 , i.e. the one having a higher opening pressure, or the non-return valve  110 , i.e. the one having a lower opening pressure. Now that the return oil path  96  is provided with the non-return valves  110  or  112  a certain increased pressure is maintained in the astern oil chamber ensuring lubricant circulation into the lubricant chamber and further into the oil tank. 
     When the pilot-operated main control valve  98  is moved to its astern position, the flow connection between the pressure oil path  104  and the return oil path  96  via the control valve  106  is closed, for instance such that the pilot-operated main control valve  98  instructs the control valve  106  to close, the pressure oil path  104  is connected to the astern oil pipe  26 , and the ahead oil pipe  28  to the return oil path  96 . Now, as in the astern oil pipe  26  and in the astern oil chamber there is, naturally, an increased pressure ensuring lubricant circulation into the lubricant chamber and further into the oil tank, the pilot-operated control valve  108  may be moved to the other position connecting the return oil path  96  to the non-return valve  110  having a lower opening pressure, whereby the energy needed for returning the oil from the ahead oil chamber is reduced compared to the embodiment of  FIG. 5 . 
     Yet another way of arranging the lubricant circulation in accordance with a third preferred embodiment of the present invention is discussed in more detail in  FIG. 7 . In the third embodiment, a separate hydraulic pump  114  is arranged in or in connection with the hydraulic powerpack  20 . The hydraulic pump  114  is connected by means of an oil path  116  for pressurized oil to the astern oil pipe  26  such that the oil pressure is limited by means of some kind of a pressure-reduction valve  118  arranged into a return flow path between the oil path  116  and the hydraulic powerpack. In the illustrated arrangement the hydraulic pump  114  delivers oil continuously to the astern oil pipe  26  such that the pressure of the oil entering the astern oil pipe  26  is regulated by the pressure reduction valve  118 . The delivery of oil to the astern oil pipe  26  may take place irrespective of the position of the pilot-operated main control valve  98 . However, a more preferred option would be to arrange some kind of a control valve  120  in the oil path  16  between the hydraulic pump  114  and the astern oil pipe  26 , for instance the control valve  120  receiving its instructions from the position of the pilot-operated main control valve  98 . Naturally, when the pilot-operated main control valve  98  is in astern position a higher pressure is taken to the astern oil chamber from the pressure oil path  104 , whereby no flow is needed from the oil path  116 . Also, in the ahead position of the pilot-operated main control valve  98 , when the astern oil pipe  26  is returning oil from the astern oil chamber there is no need for the oil delivery by means of the hydraulic pump  114 . In the latter case, however, if lubricant circulation in the lubricant chamber is desired, it would be preferable to arrange a non-return valve in the return oil path as discussed in connection with the embodiments of  FIGS. 5 and 6 . Thus, it would appear advantageous if the pilot-operated main control valve  98  was made to instruct the optional control valve  120  to close or the hydraulic pump  114  to stop working whenever the pilot-operated main control valve  98  is shifted from its neutral position. 
     It has to be understood that the discussion of certain pressure values is only exemplary, as the pressure values cannot be decided alone but always together with the flow resistances in the various flow paths containing possible throttling orifices, the oil viscosity and the desired volume flow. Also the positioning of the hydraulic powerpack and the oil tank in relation to the vertical position of the waterline and the hub have an effect on the required pressure values. For instance, the pressure difference between the astern oil chamber  56  and the lubricant chamber  82  together with the flow resistance in the oil circulation channel/s  84  and the oil viscosity define the amount of circulating oil flow. If desired (each) oil circulation channel  84  may be provided with an orifice (of its own) of desired size to control the circulation of oil. In such a case the rest of (each) oil circulation channel  84  may be made wider, whereby its flow resistance may be neglected. 
     It should be understood, too, that the present invention, i.e. arranging, at least when the propeller blades are locked in a desired position, a pressure difference between the astern oil chamber and the lubricant chamber for circulating lubricant from the astern oil chamber via the lubricant chamber to the oil tank, and thereby a continuous flow from the astern oil chamber to the lubricant chamber, may also be applied intermittently. In other words such that, for instance, the oil circulation is coupled ‘on’ for a certain period of time (for instance 1 minute, 2 minutes or 5 minutes) and thereafter ‘off’ for another period of time (for instance 30 seconds, 1 minute, 2 minutes or 4 minutes), and then again ‘on’. By doing this, on the one hand some pumping energy is saved, but, on the other hand, some additional instrumentation is needed. 
     It should also be understood that the lubrication arrangement discussed above is not only applicable in new constructions, but also existing hub installations may be easily updated to include the novel lubrication arrangement. The only thing that needs to be done is the addition of the second lubricant passage/s in the hub and the installation of required valve/s, preferably but not necessarily, in connection with the oil distribution box or the hydraulic powerpack. In case the existing hub has no oil circulation, the required oil passage/s need to be drilled. 
     It should further be understood that the above is only an exemplary description of a novel and inventive method of lubricating a controllable pitch propeller arrangement of a marine vessel and a lubrication arrangement therefor. It should be understood that the above description discusses only a few preferred embodiments of the present invention without any purpose of limiting the invention to the discussed embodiments and their details only. In other words, it is obvious that there are numerous alternatives for arranging the hydraulic elements of the arrangement, whereby it is clear that the invention is not limited to the named elements, like for instance the valve types, discussed in the specification, but each and every element or group of elements performing the claimed function are covered by the claims. For instance the various flow paths in the drive shaft may not only be arranged by means of the two concentric tubes as discussed above, but the hollow interior of the drive shaft may be provided with three pipes leading from the oil distribution box to the appropriate position within the hub to be connected to the various oil chambers or the shaft may be provided with (at least) three bores extending from the oil distribution box to the appropriate position within the hub to be connected to the various oil chambers. Thus the above specification should not be understood as limiting the invention by any means but the entire scope of the invention is defined by the appended claims only. From the above description it should be understood that separate features of the invention may be used in connection with other separate features even if such a combination has not been specifically discussed in the description or shown in the drawings.