Abstract:
A drive shaft including an inner shaft having a first end that is adapted to be connectable to a first connecting shaft that is rotated by torque applied by an external electrical machine, and a second end that is adapted to be connectable to a second connecting shaft. A hollow outer shaft, coaxial with the inner shaft, defines at least part of a rotor assembly of an associated electrical machine. The outer shaft is adapted to be releasably connected to the inner shaft so that the drive shaft is selectively configurable in a first arrangement for normal operating conditions where the outer shaft is connected to the inner shaft for rotation therewith, and a second arrangement where the outer shaft is not connected to the inner shaft.

Description:
FIELD OF THE INVENTION 
       [0001]    Embodiments of the present invention relate to drive shafts, and in particular to drive shafts for applications where two electrical machines (e.g., electric motors) are used to drive a single drive shaft. 
         [0002]    The drive shafts can be used in marine or ship propulsion assemblies. 
       BACKGROUND ART 
       [0003]    It is not unusual for two electrical machines (e.g., electric motors) to be arranged to apply torque to a single drive shaft. The electrical machines are typically arranged in a twin or tandem configuration. A fault in one of the electrical machines will typically mean that the drive shaft cannot be used until the fault is cleared. 
       SUMMARY OF THE INVENTION 
       [0004]    Embodiments of the present invention provide a drive shaft, with two driving electrical machines, where an electrical machine, such as the electrical machine that is closest to the load, can be disconnected easily, when faulty, so that the drive shaft can be returned to service and driven by the remaining electrical machine. 
         [0005]    In particular, embodiments of the present invention provide a drive shaft comprising: an inner shaft having: a first end that is adapted to be connectable to a first connecting shaft that is rotated by torque applied by an external electrical machine (e.g., an electric motor); a second end that is adapted to be connectable to a second connecting shaft; and a hollow outer shaft, coaxial with the inner shaft, the outer shaft defining at least part of, or being operatively coupled to, a rotor assembly of an associated electrical machine (e.g., an electric motor that is part of a drive assembly), and being adapted to be releasably connected to the inner shaft so that the drive shaft is selectively configurable in a first arrangement for normal operating conditions where the outer shaft is connected to the inner shaft for rotation therewith, and a second arrangement where the outer shaft is not connected to the inner shaft. 
         [0006]    It will be readily appreciated that the first and second connecting shafts do not form part of the drive shaft per se but that the ends of the drive shaft are connected to the first and second connecting shafts in use. The second connecting shaft can be directly or indirectly coupled to any suitable load, e.g., a propulsion means such as a propeller, impeller, water jet etc. in the case of a marine propulsion assembly. Similarly, the external electrical machine and the non-related parts of the associated electrical machine (e.g., the stator assembly, active parts of the rotor assembly etc.) do not form part of the drive shaft per se but can apply a torque to the drive shaft in use. The electrical machines can have any suitable construction and can be arranged in a twin or tandem configuration. The outer shaft of the drive shaft can carry the active parts of the rotor assembly of the associated electrical machine or can be directly or indirectly coupled to the rotor assembly. In general terms, the external electrical machine will be operated to apply torque to the inner shaft through the first connecting shaft to which it can be directly or indirectly coupled. The associated electrical machine will be operated to apply torque to the outer shaft. 
         [0007]    The inner shaft can be rated to take a higher torque than that delivered by the associated electrical machine. 
         [0008]    The first end of the inner shaft can include a connecting flange that is connectable to a corresponding connecting flange of the first connecting shaft. Similarly, the second end of the inner shaft can include a connecting flange that is connectable to a corresponding connecting flange of the second connecting shaft. The first and second connecting shafts are therefore connected to the first and second ends of the inner shaft in use for rotation therewith. The connecting flange at the second end of the inner shaft can be releasably connected to the inner shaft. This allows the connecting flange to be fitted to the inner shaft after it has been properly located within the outer shaft. The respective connecting flanges can be connected together using mechanical fixings such as bolts, pins, clamps etc. 
         [0009]    The inner shaft can include a first intermediate flange that is releasably connected to a flange provided at a first end of the outer shaft, e.g., using mechanical fixings such as bolts, pins, clamps etc. that can be removed to disconnect the inner and outer shafts at the first end and configure the drive shaft in the second arrangement. In one arrangement, the mechanical fixings are expanding hydraulic bolts. The drive shaft can further comprise a spacer between the first intermediate flange and the first end flange of the outer shaft. Any suitable spacer can be used. In one arrangement the spacer can be a segmented ring spacer that is divided into two or more segments, each segment receiving at least one mechanical fixing such as a bolt. Such a spacer can be removed in a radial direction while the inner shaft remains in place. 
         [0010]    To provide additional protection against high shock loading, the inner and outer shafts can be releasably connected at both ends of the outer shaft. In particular, the inner shaft can further include a second intermediate flange that is releasably connected to a flange provided at a second end of the outer shaft, e.g., using mechanical fixings such as bolts, pins, clamps etc. that can be removed to disconnect the inner and outer shafts at the second end and configure the drive shaft in the second arrangement. The drive shaft can further comprise a spacer between the second intermediate flange and the second end flange of the outer shaft. Once again, any suitable spacer can be used, e.g., a segmented ring spacer. Each spacer between an intermediate flange and the adjacent end flange of the outer shaft can be removed when the inner and outer shafts are disconnected to allow a clear gap to be created between the respective flanges. 
         [0011]    In general terms, it will be readily appreciated that any suitable means can be used to releasably connect the inner and outer shafts to enable the drive shaft to be selectively configurable in the first or second arrangement. Such means can be provided at one or both ends of the outer shaft. The means can be a coupling of any suitable type (e.g., flange, viscous, magnetic, flexible etc.) or a clutch assembly of any suitable type (e.g., dog, friction, magnetic etc.). It will be readily appreciated that such means do not necessarily need to use corresponding flanges on the inner and outer shafts. The inner and outer shafts can be physically adapted or constructed in other ways to facilitate their releasable connection. 
         [0012]    During normal operating conditions, torque from the external electrical machine can be transmitted between the first and second connecting shafts through the inner shaft. Torque from the associated electrical machine can also be transmitted between the outer shaft and the inner shaft through the intermediate flange(s). In the case of a fault condition which prevents the outer shaft from being rotated, the inner and outer shafts can be disconnected (e.g., by removing the bolts, pins, claims or other mechanical fixings, or by operating the clutch assembly) so that torque can still be transmitted between the first and second connecting shafts through the inner shaft. This means that the drive shaft can still be used even if there is a fault that requires the outer shaft to remain stationary. 
         [0013]    In an embodiment, the connecting flanges and intermediate flanges of the inner shaft are located axially outside the outer shaft. 
         [0014]    The outer shaft can further comprise one or more collars to prevent lateral movement of the drive shaft. Such collars are not normally intended to bear thrust load. 
         [0015]    An embodiment of the present invention further provides a drive assembly comprising: a drive shaft as herein described; and an associated electrical machine having a rotor assembly defined at least in part by, or operatively coupled to, the outer shaft. 
         [0016]    The drive assembly can further comprise one or more bearings for supporting the drive shaft. 
         [0017]    The drive assembly can further comprise locking means for selectively preventing rotation of the outer shaft when the drive shaft is in the second arrangement. 
         [0018]    The drive shaft (or the drive assembly) can be part of a marine propulsion assembly where the second connecting shaft can be used to drive propulsion means, e.g., a propeller, impeller, water jet etc. A marine propulsion assembly can also include components such as one or more plummer blocks, thrust block etc. 
         [0019]    An embodiment of the present invention further provides a method of operating a drive shaft comprising: an inner shaft having: a first end that is adapted to be connectable to a first connecting shaft that is rotated by torque applied by an external electrical machine; a second end that is adapted to be connectable to a second connecting shaft; and a hollow outer shaft, coaxial with the inner shaft, the outer shaft defining at least part of, or being operatively coupled to, a rotor assembly of an associated electrical machine, and being adapted to be releasably connected to the inner shaft; the method comprising the steps of: connecting the outer shaft to the inner shaft during normal operating conditions; and disconnecting the outer shaft from the inner shaft in response to a fault condition. 
         [0020]    If the inner shaft includes a first intermediate flange that is releasably connected to a flange provided at a first end of the outer shaft using mechanical fixings such as bolts, pins, clamps etc. the method can include the step of removing the mechanical fixings to disconnect the outer shaft from the inner shaft in response to a fault condition. If the drive shaft includes a clutch assembly, the method can include the step of operating the clutch assembly to disconnect the outer shaft from the inner shaft in response to a fault condition. 
         [0021]    The method can further include the step of preventing rotation of the outer shaft when disconnected from the inner shaft. 
     
    
     
       DRAWINGS 
         [0022]      FIG. 1  shows a drive assembly incorporating a first drive shaft according to an embodiment of the present invention; 
           [0023]      FIG. 2  shows a drive assembly incorporating a second drive shaft according to an embodiment of the present invention; 
           [0024]      FIG. 3  is a schematic view of a marine propulsion assembly incorporating a drive shaft according to an embodiment of the present invention; and 
           [0025]      FIG. 4  is a schematic view of the marine propulsion assembly of  FIG. 3  where the associated electric motor is out of service. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    With reference to  FIG. 1 , a drive assembly  1  for a marine propulsion assembly includes a first drive shaft  2  according to an embodiment of the present invention. It will be readily appreciated that the drive assembly  1  is not limited to marine applications and can be used for other purposes. 
         [0027]    The drive shaft  2  includes an inner shaft  4  and a hollow outer shaft  6  that is coaxially located with respect to the inner shaft and spaced apart by an axial gap  8 . 
         [0028]    The inner shaft  4  has a first end  4   a  and a second end  4   b.  The first end  4   a  includes a connecting flange  10  that is connected to a connecting flange  12  of a first connecting shaft  14  by means of a series of circumferentially spaced bolts  16 . The second end  4   b  includes a connecting flange  18  that is connected to a connecting flange  20  of a second connecting shaft  22  by means of a series of circumferentially spaced bolts  24 . The connecting flange  18  can be fitted to the second end  4   b  after the inner shaft  4  has been inserted through the outer shaft  6 . 
         [0029]    The outer shaft  6  includes a first end  6   a  and a second end  6   b.    
         [0030]    The inner shaft  4  includes an intermediate flange  26  that is releasably connected to an end flange  28  at the first end  6   a  of the outer shaft  6  by means of a series of circumferentially spaced bolts  30 , e.g., expanding hydraulic bolts. Although not shown, it will be readily appreciated that the inner and outer shafts  4 ,  6  can be releasably connected together by other types of mechanical fixing or by a clutch assembly. 
         [0031]    A segmented ring spacer  32  is located between the intermediate flange  26  and the end flange  28 . As described above, the spacer  32  is divided into two or more segments and each segment receives one or more of the bolts  32  so that they are retained in position between the flanges  26 ,  28 . 
         [0032]    The outer shaft  6  includes a pair of axially spaced collars  34  that prevent lateral movement. Each collar  34  is positioned adjacent a bearing  36  that supports the drive shaft  2 . 
         [0033]    An associated electric motor  38  (or ‘aft motor’) includes a rotor assembly  40  that is provided on the outer shaft  6 . 
         [0034]    During normal operation, the intermediate flange  26  and the end flange  28  are connected by the bolts  30 . 
         [0035]    Torque is provided by an external electric motor (not shown) to the first connecting shaft  14  and is transmitted to the second connecting shaft  22  by the inner shaft  4 . Torque provided by the associated electric motor  38  is transmitted to the second connecting shaft  22  by the outer shaft  6  and the inner shaft  4  through the intermediate flange  26  and the end flange  28 . 
         [0036]    In the event of a fault where the outer shaft  6  cannot rotate, the bolts  30  can be manually removed to disconnect the intermediate flange  26  and the end flange  28 , and hence disconnect the stationary inner and outer shafts  4 ,  6 . The segmented ring spacer  32  is also removed to provide a clear gap between the intermediate flange  26  and the end flange  28 . The outer shaft  6  can optionally be locked to prevent rotation by a locking means (not shown). 
         [0037]    The inner shaft  4  is still capable of transmitting torque from the external electric motor (not shown) to the second connecting shaft  22 . No torque is applied to the outer shaft  6  by the inner shaft  4  during a fault condition. 
         [0038]    When the fault condition has been cleared, the inner shaft can be held stationary while the segmented ring spacer  32  is repositioned between the intermediate flange  26  and the end flange  28  and the bolts  30  are manually reinserted to reconnect the intermediate flange  26  and the end flange  28 . 
         [0039]      FIG. 2  shows a drive assembly  1 ′ that includes a second drive shaft  2 ′ according to the present invention. The second drive shaft  2 ′ is similar to the first drive shaft shown in  FIG. 1  and like components have been given the same reference numbers. 
         [0040]    The second drive shaft  2 ′ provides additional protection in situations where the associated electric motor  38  is subject to high shock loading. The inner shaft  4 ′ includes a first intermediate flange  26   a  that is releasably connected to an end flange  28   a  at the first end  6   a  of the outer shaft  6 ′ by means of a series of circumferentially spaced bolts  30   a.  The inner shaft  4 ′ also includes a second intermediate flange  26   b  that is releasably connected to an end flange  28   b  at the second end  6   b  of the outer shaft  6 ′ by means of a series of circumferentially spaced bolts  30   b.  The second intermediate flange  26   b  can be fitted to the inner shaft  4 ′ after it has been inserted through the outer shaft  6 ′. A segmented ring spacer  32   a  is located between the intermediate flange  26   a  and the end flange  28   a.  Similarly, a segmented ring spacer  32   b  is located between the intermediate flange  26   b  and the end flange  28   b.  In the configuration shown in  FIG. 2 , i.e., during normal operation, the second drive shaft  2 ′ is capable of withstanding high shock loads. 
         [0041]    In the event of a fault where the outer shaft  6 ′ cannot rotate, the bolts  30   a  can be removed to disconnect the intermediate flange  26   a  and the end flange  28   a,  and the bolts  30   b  can be removed to disconnect the intermediate flange  26   b  and the end flange  28   b.  The segmented ring spacers  32   a,    32   b  are also removed to provide a clear gap between the respective intermediate flange and the end flange. 
         [0042]    When the fault condition has been cleared, the inner shaft can be held stationary while the segmented ring spacers  32   a,    32   b  are repositioned between the respective intermediate and end flanges  26   a,    28   a  and  26   b,    28   b.  The bolts  30   a  are then manually reinserted to reconnect the intermediate flange  26   a  and the end flange  28   a  and the bolts  30   b  are manually reinserted to reconnect the intermediate flange  26   b  and the end flange  28   b.    
         [0043]    With reference to  FIGS. 3 and 4 , a marine propulsion assembly  100  includes a drive assembly  1  as described above with reference to  FIG. 1 . 
         [0044]    The first connecting shaft  14  passes through a bulkhead seal  102  in a watertight bulkhead  104  and is connected to an electric motor  106  (or ‘forward motor’). 
         [0045]    The second connecting shaft  22  is connected to a propeller  108  by means of a first plummer block  110 , a thrust block  112  and a second plummer block  114 . A stern seal  116  is provided in the hull  118  of the marine vessel. 
         [0046]      FIG. 3  shows the drive assembly during normal operating conditions. 
         [0047]      FIG. 4  shows the drive assembly during a fault condition where the electric motor  38  is out of service. In particular, the bolts  30  and the segmented ring spacer  32  have been removed so the outer shaft  6  is disconnected from the inner shaft  4 . 
         [0048]    The inner shaft  4  is still capable of transmitting torque from the electric motor  106  to the second connecting shaft  22  to rotate the propeller  108 . 
         [0049]    The shaft that supports the rotor assembly of the electric motor  106  is connected to the first connecting shaft  14 . The first connecting shaft  14  can be formed as two intermediate shaft sections  14   a,    14   b  that can be disconnected from each other in a similar manner to that discussed above. If there is a fault condition where the electric motor  106  is out of service, it can be disconnected from the drive assembly  1 . In particular, the support shaft can be disconnected from the first shaft section  14   a  (which can also optionally be removed completely) and the second shaft section  14   b  can be disconnected from the first end  4   a  of the inner shaft. The second shaft section  14   b  passes through the bulkhead seal  102  and can be supported on a temporary cradle. 
         [0050]    The electric motor  38  can still be operated and torque is transmitted to the second connecting shaft  22  by means of the outer shaft  6  and the inner shaft  4  to rotate the propeller  108 . 
         [0051]    This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.