Patent Publication Number: US-9849959-B1

Title: Marine pod drive system

Description:
TECHNICAL FIELD 
     This patent disclosure relates generally to power transmission in a marine vessel and, more particularly, to transmission of power from an inboard engine to a marine pod drive unit that vertically extends through the hull of the vessel. 
     BACKGROUND 
     Powerboats and other marine vessels are operatively powered by various types or configurations of marine drive systems. For example, in an inboard drive system, an inboard engine is completely located inside the hull of the vessel and rotates a driveshaft that extends through the hull and to which is attached the prop or propeller. In an outboard drive system, the engine and any shafts or gearing that connect the engine to the propeller are typically suspended over the transom or stern of the vessel such that the drive system is substantially disposed outboard of the hull. Recently, some drive systems of marine vessels have been configured to utilize one or more marine pod drive units, sometimes referred to azimuth thrusters. A marine pod drive unit typically includes an upper part or section that is fixedly disposed inboard of the vessel hull and a lower part or section that protrudes vertically through the bottom of the hull into the water and that supports the propeller in a generally horizontal orientation with respect to the hull. Further, the lower pod section is rotatably connected to the upper pod section so that it and the propeller can turn with respect to the upper pod section and the hull to steer the vessel. The motor and gearing to rotate the lower pod section with respect to the upper pod section may be disposed on the upper pod section along with clutches, transmission components, and the like to adjust the power output of the drive system. 
     To provide power to the marine pod drive unit, an internal combustion engine can be separately disposed inside the vessel hull and can be operatively connected to the upper pod section through a driveshaft. Possible advantages of physically separating the marine pod drive unit and the engine in this manner include that physical separation of components enables customized selection of different pod and engine combinations and that it protects the engine if the pod were to strike the seabed floor or underwater object. An example of this arrangement of a marine pod drive unit and an inboard engine is illustrated in FIG. 1 of U.S. Pat. No. 9,187,164 (“the &#39;164 application”), which shows the inboard section of a marine pod drive unit receiving rotational power from a horizontally oriented driveshaft extending from the vessel&#39;s engine. The present disclosure is directed to a similar arrangement for transmitting rotational power from an inboard engine to a marine pod drive unit on a marine vessel. 
     SUMMARY 
     The disclosure describes, in one aspect, a marine drive system for a marine vessel including an inboard engine and a marine pod drive unit extending through the hull of the marine vessel. The inboard engine has an output shaft and the marine pod drive unit includes an input shaft. A driveshaft operatively connects the output shaft and input shaft. To protectively enclose the driveshaft, a guard sleeve having a tubular configuration is disposed around the driveshaft. The guard sleeve include a first sleeve end coupled to the marine pod drive unit using a first annular packing to isolate vibrations and enable angular displacement of the guard sleeve and marine pod drive unit. The guard sleeve is further coupled to the inboard engine at a second sleeve end using a second annular packing to isolate vibrations and enable angular displacement between the guard sleeve and the inboard engine. 
     In another aspect, the disclosure describes a method of operatively connecting a marine pod drive unit and an inboard engine in a marine vessel. The method connects the input shaft of the marine pod drive unit and the output shaft of the inboard engine with a driveshaft. To protect the driveshaft, a guard sleeve having a tubular configuration is disposed around the driveshaft. A first sleeve end of the guard sleeve is coupled to a first coupling collar mounted to the marine pod drive unit with a first annular packing to isolate vibration. A second sleeve end of the guard sleeve is coupled to a second coupling collar mounted on the inboard engine with a second annular packing also included to isolate vibration. 
     In yet another aspect of the disclosure, there is disclosed a guard sleeve for protectively enclosing a driveshaft on marine vessel. The guard sleeve includes a tubular body extending between a first sleeve end and a second sleeve end and that has a sleeve diameter to accommodate a driveshaft. The first sleeve end is configured to receive a first coupling collar and has a first annular groove directed radially outward. The first annular groove can accommodate a first annular packing so that the first annular packing is sandwiched between the first sleeve end and a second annular groove disposed in the first coupling collar. The second sleeve end is configured to receive a second coupling collar and has a third annular groove that is directed radially outward. The third annular groove can partially accommodating a second annular packing disposed between the second sleeve end and the second coupling collar. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a marine vessel including a horizontally oriented driveshaft enclosed in a guard sleeve that extends between and operatively connects an inboard engine and a marine pod drive unit that extends through the bottom hull of the vessel. 
         FIG. 2  is a cross-sectional image of the driveshaft interconnected to the input shaft of the marine pod drive unit and protected by the guard sleeve that is coupled to a coupling collar on the marine pod drive unit with an annular packing in accordance with the disclosure. 
         FIG. 3  is a cross-sectional image of the driveshaft interconnected to the inboard engine and similarly protected by the guard sleeve coupled to a coupling collar disposed on the engine with an annular packing. 
         FIG. 4  is a perspective view of an embodiment of the guard sleeve including a first semi-cylindrical half and a second semi-cylindrical half that extend adjacent to each other around the driveshaft and that are joined together by vibration isolator mounts. 
         FIG. 5  is a perspective assembly view of the area indicated in  FIG. 4  of an embodiment of the vibration isolator mount shown with respect to the first and second semi-cylindrical halves. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure relates to the powertrain in a marine vessel and to ways of transmitting rotational power and torque between an inboard engine and a propeller supported in a marine pod drive unit. Referring to  FIG. 1 , there is illustrated a representative illustration of a marine vessel  100  in the particular form of a powered pleasure boat; however, the present disclosure may be implemented on any type of marine vessel including commercial vessels, military vessels, and the like. The marine vessel  100  includes a hull  102  that is the body of the vessel that is disposed in and interacts with the water. The hull  102  may have any particular shape and configuration known among marine designs, including V-hulled or flat-hulled, and can define the areas or components that are inboard  104  as opposed to outboard  106  of the marine vessel  100 . To propel the marine vessel  100  in the water, the vessel can include a marine pod drive unit  110  that extends through the hull  102  and that is configured to receive and redirect kinetic power from a separately located inboard engine  112  to one or more propellers  114  that are typically positioned underwater to drive the vessel. To provide forward and/or aft thrust, the propellers  114  are typically oriented horizontally and parallel with respect to the longitudinal length of the hull  102 . 
     The marine pod drive unit  110  can include an upper pod section  120  that is disposed inboard  104  and a lower pod section  122  that is disposed outboard  106  in the water and which supports the propellers  114  in a torpedo-shaped housing. The upper pod section  120  and the lower pod section  122  can interconnect with each other through an opening  124  disposed in the bottom wall  126  of the hull  102  using a stuffing box, packings, clamps, gland seals, and the like to ensure a water-tight seal is provided in the opening. The upper pod section  120  can include an externally protruding input shaft  130  to receive rotational power input and can include various clutches, gear sets, and transmission components to variably adjust the power input that drives the vessel. For example, to redirect rotational power from the input shaft  130 , which may be oriented horizontally and parallel with the lengthwise extension of the hull  102 , to the propellers  114  in the lower pod section  122 , the marine pod drive unit  110  can include a vertical pod shaft  132  that engages with the input shaft  130  through a bevel gear set  134  or, in other embodiments, a differential gear set. In an embodiment, to provide cooling water to the inboard engine  112 , the upper pod section  120  can include an intake water valve  136  that communicates with an intake grate or the like in the lower pod section  122  to receive water from outboard  106  through the hull  102 . 
     To steer the marine vessel  100 , in an embodiment, the lower pod section  122  can be configured to rotate or turn with respect to the upper pod section  120  and the hull  102 . For example, the upper pod section  120  can be attached in a fixed manner inboard to the hull  102  and can interconnect with the lower pod section  122  through bearing assemblies that enable relative horizontal rotation between the upper and lower pod sections. The marine pod drive unit  110  can further include a servomotor and a gear arrangement that causes the lower pod section  122  to turn and redirect thrust from the propellers  114  in the water. In various embodiments, the marine vessel  100  can include a plurality of marine pod drive units  110  disposed on opposite sides of the hull  102  that function together in parallel or non-parallel operation to steer and maneuver the marine vessel  100 . 
     To generate power for the marine pod drive unit  110 , the inboard engine  112  can be an internal combustion engine of the type that combusts hydrocarbon-based fuels to convert the chemical energy therein to kinetic energy in the form of rotational torque delivered by an output shaft  138  protruding from the engine, which may be in the form of a crankshaft, engine flywheel, or any other type of engine coupling. In various embodiments, the inboard engine  112  can be a diesel burning, compression ignition engine or can be a gasoline burning, spark ignited engine. The inboard engine  112  can be fixedly attached to the hull  102  in, for example, an engine compartment or similar designated area. The inboard engine  112  can also be separate from and located remotely from the marine pod drive unit  110 . Separating the inboard engine  112  from the marine pod drive unit  110  allows for positioning of the engine to better balance the vessel and can reduce potential damage to the engine in the event the marine pod drive unit strikes an underwater object. The marine pod drive unit  110  and the inboard engine  112  may be separated by any distance and, depending upon the size of the marine vessel  100 , may be separated by a considerable distance on the order of several feet or meters. 
     To physically transmit torque and rotational power to the marine pod drive unit  110  from the inboard engine  112 , a driveshaft  140  can disposed between the components and can be operatively coupled to the input shaft  130  and the output shaft  138 . The driveshaft  140  can be a cylindrical, elongated structure and can extend between a first driveshaft end  142  and an oppositely oriented, second driveshaft end  144 . The driveshaft  140  can be composed of a single integral part or multiple conjoined parts and typically is a solid structure of material having sufficient strength to withstand the torsion and sheer stresses it may encounter. The first driveshaft end  142  can be intended for connection to the input shaft  130  of the marine pod drive unit  110 , for example, by a universal joint that allows for angular displacement of the shafts in multiple degrees of freedom, as is known in the art. In other embodiments, an elastomeric spider coupling or similar flexible coupling can be used to connect the driveshaft  140  and input shaft  130  that also allows for relative angular displacement between the shafts. The second driveshaft end  144  can be intended for similar connection to the output shaft  138  of the inboard engine  112  via a similar universal joint or spider to allow relative angular displacement of the shafts. The elongated driveshaft  140  may delineate a driveshaft axis line  146  extending between the first driveshaft end  142  and the second driveshaft end  144  that can be oriented generally horizontally with respect to the lengthwise extension of the hull  102 . With respect to the embodiment of  FIG. 1 , in which the driveshaft  140  is illustrated at a slight downward angle extending from the inboard engine  112  to the marine pod drive unit  110 , it should be appreciated the term “generally horizontally” allows for a scope of alignment different from perfectly horizontal in keeping with the nature of disclosed driveshaft as used in a marine vessel. 
     During operation, to prevent contact or interference with the rotating driveshaft  140  that may be spinning at several thousand RPM, a tubular guard sleeve  150  can be disposed radially around and extend axially along the driveshaft  140 . In various embodiments, the guard sleeve  150  can be an extruded structure or, as explained below, can include multiple parts to facilitate assembly about the driveshaft. To accommodate the driveshaft  140 , the guard sleeve  150  can be a hollow tubular casing having a sleeve diameter  158  larger than the driveshaft diameter  148  of the driveshaft  140  and can be generally coextensive in length with the driveshaft. When assembled, the guard sleeve  150  can extend between and be supported by the marine pod drive unit  110  and the inboard engine  112 . Accordingly, the guard sleeve  150  provides a stationary, protective enclosure in which the driveshaft  140  rotates. To support the elongated guard sleeve  150  around the driveshaft  140 , the guard sleeve  150  can include a first sleeve end  152  that is operatively coupled to the marine pod drive unit  110  and a second sleeve end  154  that is operatively coupled to the inboard engine  112  in manner that accommodates misalignment and reduces or isolates vibration from the active components. 
     For example, referring to  FIG. 2 , there is illustrated an embodiment of the connection between the first driveshaft end  142  of the driveshaft  140  and the input shaft  130  protruding from the marine pod drive unit  110 . The connection between the driveshaft  140  and the input shaft  130  is at least partially protectively enclosed by the first sleeve end  152  of the guard sleeve  150 . To operatively couple with and support the tubular guard sleeve  150 , the marine pod drive unit  110  can include a first coupling collar  160  having a similar tubular configuration that is fixedly attached to the upper pod section of the pod drive unit. The tubular first coupling collar  160  can be a thin-walled structure made from wrapped or extruded material and can be radially disposed around the exposed length of the input shaft  130  protruding from the upper pod section. To ensure the input shaft  130  is covered, the first coupling collar  160  may have an axial length greater than the exposed length of the input shaft  130  projecting from the marine pod drive unit. 
     In an embodiment, to join the first sleeve end  152  with the first coupling collar  160 , in an embodiment, the first coupling collar can have a collar diameter  162  that is slightly smaller than the sleeve diameter  158 . The difference in dimension between the collar diameter  162  and the sleeve diameter  158  enables the first coupling collar  160  to be inserted or received into the first sleeve end  152  of the guard sleeve  150 . To further facilitate insertion, the first coupling collar  160  can be formed with a tapered distal end  164 . The difference between the sleeve diameter  158  and the collar diameter  162 , along with the tapered distal end  164 , provides a gap or radial clearance  166  between the first sleeve end  152  and the first coupling collar  160 . However, while the present embodiment illustrates the first coupling collar  160  dimensioned to be inserted into the guard sleeve  150 , it should be appreciated the arrangement in other embodiments may be reversed with the first sleeve end  152  inserted into the tubular first coupling collar  160 . The combination of the first sleeve end  152  and the first coupling collar  160  prevents unintentional access to the connection between the input shaft  130  and the driveshaft  140  and further can protect the intake water valve in the event the shaft connection should fail. 
     To dampen or isolate vibrations that may originate from the marine pod drive unit  110 , the connection between the first sleeve end  152  of the guard sleeve  150  and the first coupling collar  160  can include a first annular packing  170  disposed at the interface between the first sleeve end  152  and first coupling collar  160 . In an embodiment, the first annular packing  170  can be in the form of an o-ring made from a suitable elastomeric or resilient material such as natural or synthetic rubber, fluoroelastomers, nitrile rubber, silicone rubber, and the like, and/or blends thereof. Such resilient materials can volumetrically compress and recover under applied loads. The first annular packing  170  in the form of an o-ring may have a cross-sectional thickness  171  that is generally circular as illustrated, although in other embodiments, the first annular packing may have other profiles such as oval or elliptical, square or boxed, x-shaped, lipped, etc. Moreover, the annular shape of the first annular packing  170  enables it to be disposed radially around the first coupling collar  160  as illustrated. More specifically, the annular diameter  172  of the first annular packing  170  can be dimensioned so it can be disposed between and sandwiched by the first coupling collar  160  that is received and accommodated in the first sleeve end  152  of the guard sleeve  150 . Accordingly, the annular diameter  172  can be slightly larger than the collar diameter  162  and slightly smaller than the sleeve diameter  158 . 
     To further accommodate the first annular packing  170 , the first sleeve end  152  of the guard sleeve  150  and the first coupling collar  160  can include complementary first and second annular grooves  174 ,  176  or indentations that circumferentially extend around the first sleeve end  152  and first coupling collar  160  respectively. The first and second annular grooves  174 ,  176  may be disposed in opposing orientations with the first annular groove  174  directed radially outward and the second annular groove  176  directed radially inward. Further, the first and second annular grooves  174 ,  176  can be curved or arched in shape as determined by the curvature of the grooves and indicated in  FIG. 2  by as radius of curvature  178  associated with the grooves. In an embodiment, the degree or dimension of curvature of the radius of curvature  178  defining the curved shape of the first and second annular grooves  174 ,  176  can be equal to or larger than the curvature corresponding to the cross-sectional thickness  171  of the first annular packing  170 . Accordingly, the width of the first annular packing  170  can fit within the first and second annular grooves  174 ,  176  as shown. 
     When assembled, the first annular groove  174  and the second annular groove  176  can axially align coextensively with each other to accommodate the first annular packing  170 . Further, the oppositely directed radial orientation of the first and second annular grooves  174 ,  176  and the radius of curvature  178  causes the radial clearance  166  between the first sleeve end  152  and the first coupling collar  160  to increase. The cross-sectional thickness  171  of the first annular packing  170  may be dimensioned to cause a slight compressive fit between the first annular packing  170  and the first and second annular grooves  174 ,  176  in which it is accommodated. When retentively positioned in the first and second annular grooves  174 ,  176  between the first sleeve end  152  and the first coupling collar  160 , the elastic characteristic of the first annular packing  170  can absorb and isolate vibrations transmitted between the components. Furthermore, as described below, the first annular packing  170  allows a limited range of relative angular motion between the guard sleeve  150  and the first coupling collar  160  to accommodate angular misalignment between them. 
     Referring to  FIG. 3 , there is illustrated an embodiment of the connection at the oppositely located second driveshaft end  144  of the driveshaft  140  as coupled to the output shaft  138  of the inboard engine  112 . To restrict access to the interconnection between the rotating output shaft  138  and the driveshaft  140 , the connection can be protectively enclosed by the second sleeve end  154  of the guard sleeve  150 . To join with and support the second sleeve end  154 , the inboard engine  112  can have mounted thereon a second coupling collar  180 , having a similar thin-walled, tubular configuration as the first coupling collar to radially extend about and to project over the exposed length of the output shaft  138 . The second coupling collar  180  can have a collar diameter  182  that is again dimensioned to enable insertion of the second coupling collar into the larger sleeve diameter  158  associated with the second sleeve end  154  of the guard sleeve  150 . The difference between the sleeve diameter  158  and the collar diameter  182  can produce a radial clearance  186  between the second sleeve end  154  and the second coupling collar  180 . 
     To isolate or absorb vibrations originating from the inboard engine  112 , a second annular packing  190  can be made from elastic or resilient material and can be disposed at the interface between the second sleeve end  154  and the second coupling collar  180 . The second annular packing  190  can be an elastomeric o-ring similar to or the same as the first annular packing or can have a different size, profile, or configuration. For example, the second annular packing can have a cross-sectional thickness  191  that corresponds to a circular cross-section. Additionally, the second annular packing  190  can have a second annular diameter  192  sized to enable radial positioning around the second coupling collar  180  when received in the second sleeve end  154 . 
     In a variation, the second annular packing  190  can be accommodated by a single, third annular groove  194  disposed into either the second sleeve end  154  as illustrated or, alternatively, in the second coupling collar  180 . The third annular groove  194  can have a curved configuration designated by a curvature or a radius of curvature  198  that corresponds to or can be larger than the cross-sectional thickness  191  of the second annular packing  190  and, in the illustrated embodiment, can project radially outwards. In addition to dampening induced vibrations, the second annular packing  190  can accommodate various degrees of angular and axial misalignment between the second coupling collar  180  and the guard sleeve  150 , for example, due to angular tilting of the driveshaft  140  away from the true horizontal plane. 
     Referring to  FIG. 4 , there is illustrated an embodiment of the guard sleeve  150  that is specifically configured to facilitate assembly about the driveshaft extending between the marine pod drive unit  110  and an inboard engine (not shown). The guard sleeve  150  again extends axially along the driveshaft axis line  146  between the first sleeve end  152  and the second sleeve end  154  to provide a tubular enclosure for the driveshaft. To facilitate assembly, the guard sleeve  150  can be constructed from multiple parts including a first semi-cylindrical half  202  and a corresponding and complementary second semi-cylindrical half  204 . As illustrated, both of the first and second semi-cylindrical halves  202 ,  204  can be axially elongated and are shaped as a partial cylinder. When the first and second semi-cylindrical halves  202 ,  204  are joined lengthwise about the driveshaft axis line  146 , they provide a tubular bore  206  that can accommodate and encase the driveshaft. The first and second semi-cylindrical halves  202 ,  204  thereby simplify installation of the guard sleeve  150  about the driveshaft  140 . 
     To join the first and second semi-cylindrical halves  202 ,  204  together, each semi-cylindrical half can be configured to abut each other lengthwise along their semi-cylindrical edges. Specifically, in the illustrated embodiment, the semi-cylindrical shaped curve or arc of the first and second semi-cylindrical halves  202 ,  204  can terminate at a first semi-cylindrical edge  210  and an opposite second semi-cylindrical edge  212 . On the first semi-cylindrical half  202 , a flat, planer first abutment flange  220  can project from the first semi-cylindrical edge  210  and a similar second abutment flange  222  can likewise project from the second semi-cylindrical edge  212 . Both the first and second abutments flanges  220 ,  222  can extend along the axial length of the first semi-cylindrical half  202 . Likewise, the second semi-cylindrical half  204  can include a similar third abutment flange  224  projecting from the first semi-cylindrical edge  210  and can include a fourth abutment flange  226  projecting from the second semi-cylindrical edge  212 . When the first and second semi-cylindrical halves  202 ,  204  are assembled together, the abutment flanges  220 ,  222 ,  224 ,  226  extend radially outward from and parallel with respect to the driveshaft axis line  146  with the first and third abutment flanges  220 ,  224  and the second and fourth abutment flanges  222 ,  226  respectively abutting each other. 
     To couple the first and second semi-cylindrical halves  202 ,  204  of the guard sleeve  150  to the marine pod drive unit  110  in a manner that isolates vibration between the components, the first and second semi-cylindrical halves  202 ,  204  can be joined at the first sleeve end  152  using one or more vibration isolator mounts  230 . Vibration isolator mounts  230  are a class of passive devices that can be rigidly mounted to interconnect two more structures and which include a flexible or vibration dampening portion to absorb or dissipate vibrations between the structures. An example of one embodiment of a vibration isolator mount  230  is illustrated in  FIG. 5 , which includes a cylindrical body  232  with threaded male studs  234  projecting from opposite ends. The cylindrical body  232  can be made from an elastomeric material to dampen or dissipate energy associated with the vibrations. However, other embodiments of the vibration isolator mounts can utilize gels, fluids, springs, meshes, or the like to absorb and dissipate vibratory forces. Moreover, the vibration isolator mounts can be joined to the structures using any suitable techniques such as clips, welding, adhesives, etc. 
     In the embodiment illustrated in  FIGS. 4 and 5 , to utilize the vibration isolator mounts  236 , the portions of the abutment flanges proximate the first sleeve end  152  can extend radially outward and form flat, planar sleeve tabs  240  that extend adjacent to each other. The first sleeve end  152  of the guard sleeve  150  may also have one or more sleeve apertures  242  disposed through it that are proximate to the planar sleeve tabs  240 . The sleeve apertures  242  may function as slots or openings to accommodate one or more collar tabs  244  projecting generally radially outward from the first coupling collar  160  to extend through the sleeve apertures. The sleeve tabs  240  and collar tabs  244  can be arranged in a spaced-apart manner to receive a vibration isolator mount  230  there between. Hence, the first sleeve end  152  of the guard sleeve  150  is further coupled to the first coupling collar  160  on the marine pod drive unit  110  in a vibration-isolated manner through use of the vibration isolator mounts  230 . While  FIGS. 4 and 5  illustrate the vibration isolator mounts  236 , sleeve tabs  240 , and collar tabs  244  on only one lateral side of the guard sleeve  150 , it can be appreciated the same features may be present on the other lateral side of the guard sleeve. Further, although the illustrated embodiment shows two vibration isolator mounts  230  in a stacked arrangement, other embodiments can have different numbers of mounts in different arrangements. Further, different spacing arrangements for the sleeve and collar tabs  240 ,  244  can be used, while in other embodiments, the collar tabs  244  can be eliminated and the vibration isolator mounts  230  can be used only to securely clamp the sleeve tabs  240  together. 
     INDUSTRIAL APPLICABILITY 
     Referring to  FIG. 1 , the disclosed guard sleeve  150  and coupling arrangements provide vibration-isolating effects that support the guard sleeve when coupled to and suspended between a marine pod drive unit  110  and an inboard engine  112  to protectively enclose a driveshaft  140  operatively interconnecting those components. Vibrating forces may arise from operation of the inboard engine  112  due to the inherent reciprocal motion of the pistons in the combustion chambers during the combustion cycles and vibrations or shock loads may be generated from operation of the marine pod drive unit  110  that is subjected to various moments and applied forces through action of the propellers  114  and the water. Referring to  FIGS. 2 and 3 , inclusion of the first and second annular packings  170 ,  190  at the interfaces where the tubular first and second sleeve ends  152 ,  154  receive the respective first and second coupling collars  160 ,  180  provides the vibration isolated characteristics. 
     At the marine pod drive unit  110 , the circular cross-section of the first annular packing  170  and its accommodation in the first and second annular grooves  174 ,  176  enables the guard sleeve  150  (and the enclosed driveshaft  140 ) to tilt out of angular alignment with the input shaft from the marine pod drive unit  110 . The radial clearance  166  between the first sleeve end  152  and the first coupling collar  160  provides spacing that further enables those parts angularly articulate with respect to each other without locking up. Accordingly, in this aspect, sliding or rolling motion between the first annular packing  170  and the first and second annular grooves  174 ,  176  in which it is disposed functions as a pivotal joint or linkage enabling the guard sleeve  150  to articulate angularly with respect to the output shaft  138 , as indicated by the angular offset of the driveshaft axis line  146 . Furthermore, the cross-sectional thickness  171  of the first annular packing  170  and the radial clearance  166  provided between first and second annular grooves  174 ,  176  can cause a compression fit compressing the first annular packing  170 . The compressed first annular packing  170  can constrain axial motion between the first sleeve end  152  of the guard sleeve  150  and the first coupling collar  160 , preventing the parts from decoupling. Specifically, it can be appreciated if the coextensive first and second annular grooves  174 ,  176  where to move axially with respect to each other, the radial clearance  166  defined between the grooves would shrink. Hence, first annular packing  170  as accommodated in the space provided between the first and second annular grooves  174 ,  176  would be further compressed in the shrinking radial clearance  166  to such an extent any further compression is not possible. The first annular packing  170  would thereafter functions as a lock or obstacle to relative axial movement of the first sleeve end  152  and the first coupling collar  160 . 
     At the inboard engine  112 , rolling or sliding motion between the circular cross-section of the second annular packing  190  and the radius of curvature  198  of the third annular groove  194  also enables the guard sleeve  150  to tilt angularly with respect to the second coupling collar  180 . The radial clearance  186  between the second sleeve end  154  and the second coupling collar  180  also provides spacing that enables those parts to angularly articulate with respect to each other without locking up. However, because the second annular packing  190  is received only in the single third annular groove  194 , the second sleeve end  154  can experience a degree of axial movement with respect to the second coupling collar  180 . More specifically, because the radial clearance  186  between the second sleeve end  154  and the second coupling collar  180  is substantially fixed, no further compressive forces are induced on the second annular packing  190  if the second sleeve end  154  and the second coupling collar  180  axially move relative to each other. The second annular packing  190  can therefore slide or roll partially along the axial extension of the second coupling collar  180 . This enables the coupling connection between the second sleeve end  154  of the guard sleeve  150  and the second coupling collar  180  to accommodate relative axial motion due to, for example, thermal expansion of the parts. 
     It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 
     The use of the terms “a” and “an” and “the” and “at least one” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The use of the term “at least one” followed by a list of one or more items (for example, “at least one of A and B”) is to be construed to mean one item selected from the listed items (A or B) or any combination of two or more of the listed items (A and B), unless otherwise indicated herein or clearly contradicted by context. 
     Accordingly, this disclosure includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.