Abstract:
A combined linear and rotary actuator has a shaft with a screw thread, a nut engaging the screw thread and engaging means movable between a first location where rotation of the turning mechanism produces substantially linear displacement of the shaft, and a second location where rotation of the turning mechanism produces substantially rotary displacement of the shaft. The linear actuator finds particular use on retractable marine propulsor such as used on submarines and the like.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is entitled to the benefit of British Patent Application No. GB 0702791.5 filed on Feb. 14, 2007. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to linear actuators that convert rotary motion into either linear or rotary motion. The invention finds particular application in the field of retractable units for marine applications. 
       BACKGROUND OF THE INVENTION 
       [0003]    Retractable thrusters are known for marine applications. Conventional retractable thrusters either drop-down vertically or pivot downwards around a hinge. Vertically retractable units have one set of hydraulic rams for lifting/lowering the thrusters and another mechanism for turning the thrusters to their desired orientation. The turning mechanism can be one or more hydraulic rams operating via a lever system to give rotary motion over a limited angle, or a geared motor (electric or hydraulic) acting on a large diameter steering gear via small diameter pinions (either singly or in groups spaced around the steering gear wheel). Such arrangements are complex and require both axial and rotary actuators with their concomitant control systems, cabling and hull penetrations. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an object of the present invention to seek to provide an improved linear actuator. It is a further object of the invention to seek to provide improved retractable thrusters. 
         [0005]    According to the present invention, there is provided a linear actuator having: a shaft having a thread, a rotatable turning mechanism engaging the thread, engaging means movable between a first location where rotation of the turning mechanism produces substantially linear displacement of the shaft, and a second location where rotation of the turning mechanism produces substantially rotary displacement of the shaft. 
         [0006]    Preferably, the engaging means include a pin or roller mounted to the shaft. 
         [0007]    Preferably, the engaging means includes a sleeve extending around the shaft and having a slot that extends parallel with the shaft. 
         [0008]    In the first position, preferably, the pin engages the slot and preferably in the second position the pin does not engage the slot. 
         [0009]    Preferably, the sleeve is provided with a limit stop outside the slot against which, when the pin abuts against the limit stop, prevents rotary displacement of the shaft in a first direction and permits rotary displacement of the shaft in the opposite direction. 
         [0010]    The rotatable turning mechanism may be connected to the sleeve through a thrust bearing. 
         [0011]    Preferably, the rotatable turning mechanism comprises a nut rotatable by a motor. 
         [0012]    Preferably, the nut forms at least part of a rotor in a rim driven motor. 
         [0013]    Preferably, the nut has a plurality of circumferentially arranged magnets. 
         [0014]    The shaft may further comprises a travel limiter which limits the maximum linear displacement of the shaft. 
         [0015]    Preferably, propulsion means are mounted to the shaft. 
         [0016]    The propulsion means may be a rim driven azimuthing thruster. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  depicts a linear actuator in accordance with the invention. 
           [0018]      FIG. 2   a  is a cross-sectional view of the linear actuator of  FIG. 1  taken across line C-C. 
           [0019]      FIG. 2   b  is a similar view to that of  FIG. 2   a  with the propulsor rotated through 90°. 
           [0020]      FIG. 3  depicts a linear actuator of a second embodiment in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]      FIG. 1  depicts a linear actuator  1  in accordance with the invention and in particular the use of the linear actuator to mount a thruster. 
         [0022]    The thruster in this embodiment is a rim driven propulsion pod though any other appropriate form of propulsor may be used. 
         [0023]    The thruster is located on a marine vessel, typically a submarine, and can alternate between a deployed position and a stowed position. The thruster is contained within a protective case in its stowed position and when required is moved outside the case to its deployed position. The casing may be flooded with seawater before deployment, or may be permanently open to seawater. 
         [0024]    When the thruster is in its deployed position it can be rotated to provide thrust vectoring. The thruster must have a raise and lower function with an additional ability to train the thruster propulsive element by rotation about a vertical axis in order to achieve a steering action. In the invention, both functions may be achieved by a single actuator, which obviates the need for separate actuators controlling both the linear and rotational movement. 
         [0025]    Propulsor  2  is mounted onto a shaft  4 . For clarity, the propulsor rim alone is shown, and this will encase a propeller, which is not shown. 
         [0026]    Shaft  4  has an external screw thread and the description below assumes a reverse or ‘left hand’ thread on nut  6  and shaft  4 . A conventional ‘right hand’ thread could also be used which would reverse the rotational direction of the actuator to achieve the same end results. 
         [0027]    The shaft may be hollow to carry power cables to the propulsor motor. These cables need to withstand a twist over the cable length of up to 270° and preferably up to 360°. The electrical connection to the propulsor motor is made via integral sealed connectors built into flange  30  and the propulsor housing  2 . Beneficially, the propulsor is de-mountable by removing the bolts at flange  30  and with use of the appropriate type of connector, could be accomplished while underwater. 
         [0028]    The nut  6 , which operates on the externally threaded shaft, is rotatable by a motor  8 . The nut  6  is held captive by the action of the weight of the shaft  4  and the load it carries which is reacted by the nut  6  via a thrust bearing  10  which in turn operates against a single or multipart fixed load bearing structure  12  which transmits loads to the wider ship structure. 
         [0029]    Motor  8  is also mounted to structure  12  either integrally, or as an additional bolt-on component. In the preferred embodiment, the training and raise lower motor  8  is of rim driven construction and comprises a rotor, the centre of which forms the nut  6  on the thread of strut  4 . The nut  6  is connected to the magnet carrying rim of the rotor  32  by spokes  34 , but depending on the torque required, the magnets could be mounted directly on the nut  6 . 
         [0030]    The structure  12  has a sleeve portion that extends downwards over the threaded portion of shaft  4 . The main radial bearing for the motor is provided by the interaction of shaft  4  and the inner bore of the sleeve. Accordingly, for this embodiment, the form of the thread is chosen such that the outer periphery of the thread forms a reasonable radial bearing surface when it operates in conjunction with the sleeve with which it has a close sliding fit. For an application, where only occasional operation is envisaged, this simple bearing arrangement is satisfactory. For applications requiring more regular deployment a more robust bearing system may be required. 
         [0031]    The extension sleeve of structure  12  incorporates a slot  14  in which operates a pin or roller  16  which is solidly mounted to the shaft  4 . The pin is a simple structure to manufacture. The roller, which rotates about the axis of the pin, offers an improved life to the pin. In this description, it will be appreciated that it is a matter of choice which of the components is used. Rotation of the shaft is prevented whilst a portion of the pin lies within the slot, but rotation of the shaft is enabled where the pin  16  is free of the slot  14 . Whilst rotation is prevented, i.e. the pin lies within the slot, rotation of nut  6  results in linear or axial movement of the shaft to raise or lower it relative to the fixed structure  12 . 
         [0032]    The slot is open at the lower end of the sleeve and the shaft is designed to permit a linear movement that is sufficient to move the pin  16  out of the slot  14 . Once the pin has moved out of the slot, friction between the nut and the shaft means that further rotation of the nut  6  results in simultaneous rotation of both the nut and the shaft. 
         [0033]    A flange  18  is provided at the opposing end of the shaft to that of the propulsor and is positioned to reach the nut  6  as the pin  16  exits the slot  14 . The flange therefore prevents further downward travel of the shaft and increases the friction between the nut and the shaft. A rubber ‘buffer’  20 , which is a ring, can be provided between the bottom of the flange  18  and top of nut  6  to help to prevent binding or locking together of the nut  6  and flange  18 . The buffer  20  also provides a degree of shock absorbance and prevents over tightening of the nut on the thread. In an alternative embodiment, the buffer may be a low friction ring of low compression material. Whilst this embodiment will not provide a shock absorbance functionality, the low friction material ensures binding between the flange and nut is avoided. 
         [0034]    Since further rotation of the nut  6  rotates the shaft  4 , any device  2  attached thereto is similarly rotated. Where the device is a propulsor, the maximum required rotation is 360° to give full directional movement. Where the propulsor can work in both forward and reverse, the maximum required rotation is 180° to give full directional movement. 
         [0035]    The angular position of the shaft  4  is controlled by the motor drive system, perhaps by the addition of a rotary encoder but more preferably by counting the electrical cycles applied to the synchronous or stepper motor including those items identified as  6 ,  8 ,  32 ,  34  of  FIG. 1 . 
         [0036]    Complete rotation to bring the pin  16  back into alignment with the slot  14  is prevented by a raised limit stop  22  on the lower surface of the sleeve.  FIG. 2   a  is a top view of the propulsor  2  in position as it is moved in a linear direction. The pin  16  is engaged by the slot  14  and is prevented from rotating. In  FIG. 2   b,  the pin has moved below the slot opening and is consequently able to rotate. During normal operation the shaft is rotated till it is away from the limit stop  22 , which enables the propulsor to have a wide swing angle  24  permitting thrust vectoring in nearly all horizontal directions. 
         [0037]    In preferred operation, the propulsor would be rotated 180° clockwise (when viewed from the top) so that pin  16  would be on the other side of the unit from that of the slot and limit stop. This is the neutral position with training or steering swing angle of plus or minus about 90° from that position. With the shaft  4  lowered and with the pin rotated away from slot  14  in the sleeve any operation of the training motor will result in rotation of the nut  6  and shaft  4  together, giving the training or steering action, within the plus or minus 90° range specified above. Clockwise rotation will tend to tighten the threaded shaft  4  against the nut  6  via the ring  20  and counter clockwise operation which would tend to loosen that union will be resisted by the friction in the nut as it will be easier to rotate the whole assembly than to lift the strut. Raising the shaft is also prevented by the action of pin against the lower surface of sleeve  12 . 
         [0038]    Therefore, during normal operations the shaft will rotate a maximum of 270° and the limit stop  22  will not be reached by pin  16 . However, should the limit stop be reached the actuator or motor could incorporate a fail-safe cut-out device to turn itself off. Alternatively or additionally, the limit stop can provide a mechanical reference to allow the rotational position keeping system to be reset to ‘zero’ if position information is lost for any reason. This latter function could be enhanced by also rotating the shaft  4  until the pin  16  contacts the limit stop  22  from one and then the other side. 
         [0039]    In order to retract the shaft  4 , the shaft is rotated so as to bring the pin  16  to a position where it abuts the limit stop  22  and is in line with the slot  14  at the bottom of the sleeve. The limit stop  22  prevents further rotation of shaft  4  with the motor overcoming the thread friction between nut  6  and shaft  4  to start to lift shaft  4 . As the shaft is lifted, the pin runs up slot  14 , which continues to prevent rotation of shaft. Slot  14  may be right through the wall of the sleeve as shown for clarity, or may be machined only part way through the wall to confer additional strength to the housing. When the strut reaches its uppermost position, the slot  14  comes to an end and prevents further lifting, so continued counter clockwise rotation of the training motor will move the pin into a detent  26 . Detent  26  is a horizontal extension of the slot which provides a security feature which prevents lowering of the shaft in back-up to the nut  6 , which continues to be the main support for the weight of shaft  4  and any attachments thereon. The detent  26  can incorporate a limit switch to turn off the motor and/or confirm that the strut is in the raised position. 
         [0040]    Software in the motor converter can be used to slow the training motor as it approaches the detent. The software can sense the change in current requirements as the lift operation starts and can then increase motor speed to get an appropriate lifting cycle time. As the motor is synchronous, the converter can count the AC cycles applied to get a good approximation of the vertical position of the unit, and start to slow the motor as the top of slot  14  is approached. The final ‘stop’ signal would be provided by a sensor in detent  26  or by sensing the rise in current required by the motor as the pin moves against the limit of the detent  26 . With the unit raised, the training motor can be de-energised and the weight of the unit will continue to be carried by the nut  6  as the thread pitch would be chosen such as to prevent reverse running, and the detent  26  would provide back-up support via pin  16 . If additional security is required, a hole through the wall in the region of detent  26  could be provided (if the slot does not cut right through the wall). Pin  16  may be spring loaded so that it comes through the hole making any further movement of shaft  4  impossible. When the unit is to be lowered, a solenoid-actuated pushpin could push the pin  16  back into the hole to allow movement when lowering is activated, but this would require additional hull penetrations, etc. for electrical connection. 
         [0041]    To deploy the propulsor, the motor first rotates nut and the friction loads between the nut and the shaft cause simultaneous movement of the shaft with the nut. The movement of the shaft moves the pin  16  out of detent  26  and into slot  14 . The pin is secured in the slot and further rotation of the nut causes the shaft  4  to be lowered until the pin  16  exits the slot  14  after which the whole of shaft  4  is free to rotate to the desired steering angle. 
         [0042]    In an alternative embodiment, as shown in  FIG. 3 , the sleeve  12  extends away from the propeller housing  2  relative to the actuating mechanism that includes components identified by numbers  8 ,  32 ,  10 ,  6 ,  20 , etc., rather than towards the propeller housing, as in the first embodiment. The arrangement of the alternative embodiment operates in substantially the same manner as that of the first embodiment. 
         [0043]    It will be appreciated that this embodiment has a smaller overall length than that of the first embodiment. Additionally, steering torque is reduced. Although not limited to electrical actuation, the invention is inherently compatible with it and can be used to eliminate a requirement for hydraulic systems. A single actuator is required which reduces the mechanical complexity of the unit and also requires fewer control connections that may be significant in certain applications, e.g., where power and control must pass through a boundary such as the pressure hull of a submarine, or the hull of a ship. 
         [0044]    The arrangement offers greater angular steering range than some existing hydraulically actuated units. Combining 180 degree rotation with thrust reversal (for thrusters application) gives effective 360 degree thrust vectoring capability. 
         [0045]    Additionally, the lift/lower and steering action may be faster than in conventional arrangements. 
         [0046]    The invention could be used in an aerospace application in, for example a nose wheel retraction and steering system, or steerable main undercarriage application, with the addition of suitable safety features and uprated materials properties and a suspension system. 
         [0047]    The invention could also be used in remote lock and release mechanism, e.g., for vehicle docking systems. The principle is the same as the primary application, whereby a locking probe extends into a mating locking plate, and is then rotated and operates on a stationary cam surface in the locking plate so as to pull two parts together and hold them firmly. Furthermore, it is relevant to any application requiring a reversible remotely operated ‘push and then twist’ or ‘pull and then twist’ action.