Abstract:
A wrap spring clutch having a spring constructed of a shape memory alloy. This wrap spring clutch operates in the same manner as any other basic wrap spring clutch except that the spring expands and releases when the spring is heated to a predetermined temperature. The heat may be applied to the spring through external sources or by an electrical current being applied to the spring.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable.  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    The present invention relates to methods and apparatus for wrap spring clutches. More precisely, the present invention relates to the novel application of using a shape memory alloy for the spring component of a wrap spring clutch. Yet more specifically, the present invention relates to using a spring constructed of a shape memory alloy in a wrap spring clutch assembly so that the energization of the spring can be controlled by temperature; wherein the assembly operates in a non-energized, normal mode, wherein the relative rotational movement of two shafts is restricted, and in an energized, released mode, wherein the relative motion is not restricted.  
           [0004]    Shape memory alloys refer generally to a group of metallic materials that demonstrate the ability to return to some previously defined shape when subjected to the appropriate thermal excursion. Generally, these materials can be plastically deformed at some relatively low temperature, and upon exposure to some higher temperature will return to their shape prior to the deformation. Materials that exhibit shape memory effects only upon heating are referred to as having a one-way shape memory. Materials that also undergo a change in shape upon recooling are referred to as having a two-way shape memory. The most common of the shape memory alloys is Nitinol, which is an alloy comprising primarily nickel and titanium. Other elements can be added to adjust or enhance the material properties.  
           [0005]    One-way shape memory effect describes the process of restoring the original shape of a plastically deformed piece of material by heating it. When the piece is made, it is formed to a desired shape during the heat treatment process. While the piece is below its transformation temperature, the material is in a soft martensitic form and can easily be plastically deformed. Heating the piece to the transformation temperature converts the material to its high strength, austenitic form, which returns the sample to its original desired shape. The piece can be cooled and the deformation and restoration steps performed multiple times. The temperatures at which this transformation takes place can be closely controlled through manipulation of the alloy and heat treatment. The shape memory effect is repeatable and can typically result in up to 8% strain recovery.  
           [0006]    Two-way shape memory effect is similar to the one-way process described above but the material assumes one shape when heated and another shape when cooled. This behavior is accomplished through the same mechanisms as one-way deformation but involves greater difficulty in production and involves a more complex series of heat treatment and manufacturing processes. One disadvantage of a two-way memory effect material is that when transforming at a high temperature it produces less force than a comparable one-way material transforming at the same temperature and when transforming at a lower temperature, even less force is produced. Therefore, although a two-way effect material can have two predetermined shapes it produces substantially less force than a one-way material transforming at a comparable temperature.  
           [0007]    Wrap spring clutches are well known in a variety of forms and are used in a variety of applications. In its simplest embodiment, the basic operation of many wrap spring clutch designs involves utilizing a spring coil surrounding two shafts to transfer torque from one shaft to the other in one direction only. As shown in FIG. 1, the basic wrap spring clutch comprises an input hub  12 , an output hub  14 , and a spring  16 . The spring  16  has an inside diameter that is close to or slightly smaller than the outside diameter of the two hubs.  
           [0008]    When the input hub  12  rotates in the direction of the spring winding  18 , the spring  16  wraps tightly down on the two hubs  12 ,  14  and positively engages the hubs allowing transmission of torque. When the input hub  12  rotates in the direction opposite the spring winding  18 , the spring  16  loosens and allows the hubs  12 ,  14  to rotate freely. This free rotation of the hubs is known as free-wheeling or over-running. The spring  16  may also have a control tang  20  that when pushed in a direction opposite the spring winding  18 , releases the spring  16  and allows freewheeling. The basic wrap spring clutch is useful because it provides a simple and robust clutch/brake design that offers almost instantaneous engagement and disengagement.  
           [0009]    While the simplest embodiment of a wrap spring clutch allows the transmission of torque in only one direction, wrap spring clutches are available that permit transfer of torque in both directions and freewheeling in both directions. These bidirectional wrap spring clutches are considerably more complex than the basic embodiment described above.  
           [0010]    Wrap spring clutches are currently being used in rotary valve actuators to control the movement of the valve. Many of these type valves used in industry are fail-safe close valves meaning that the valve is biased to the closed position and must be kept open by fluid pressure. In one application, the wrap spring clutch holds a rotary actuator in the open position. An electric solenoid is connected to a control tang on the spring and arranged so that the solenoid will pull the tang and release the spring if electrical power is lost. Therefore, if electrical power is lost, the solenoid will pull the tang to release the spring, which allows the rotary actuator to return to the fail-safe, closed position.  
           [0011]    The present invention is directed to improved methods and apparatus for the design and use of wrap spring clutches.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention relates to methods and apparatus for an improvement to the design of wrap spring clutches by taking advantage of the unique properties of shape memory alloys. In one embodiment the spring of a wrap spring clutch is constructed from a shape memory alloy. This wrap spring clutch operates in the same manner as any other basic wrap spring clutch except that the spring expands and releases when an electrical current applied to the spring to produce resistance heating or released from the spring allowing it to cool. This embodiment finds utility in providing less complex methods and apparatus for releasing a wrap spring clutch by using an electrical signal.  
           [0013]    Another object of the present invention is to provide a simple, reliable, fail-safe mechanism that actuates in response to environmental heating. In another embodiment of the present invention he spring relies on an increase in ambient air temperature (as would be experienced in a fire) to release the spring and allow freewheeling operation. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    For a more detailed understanding of the preferred embodiments, reference is made to the accompanying Figures, wherein:  
         [0015]    [0015]FIG. 1 is an isometric view of a prior art wrap spring clutch;  
         [0016]    [0016]FIG. 2 is a cross-sectional, isometric view of a wrap spring clutch in accordance with the one embodiment of the present invention;  
         [0017]    [0017]FIG. 3 is a cross-sectional, isometric view of a wrap spring clutch in accordance with another embodiment of the present invention;  
         [0018]    [0018]FIG. 4 is a cross-sectional, isometric view of a wrap spring clutch in accordance with yet another embodiment of the present invention; and  
         [0019]    [0019]FIG. 5 is a cross-sectional view of a valve using a wrap spring clutch. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    Referring initially to FIG. 2, there is depicted a simple wrap spring clutch mechanism  22 . The wrap spring clutch  22  comprises an input hub  24 , an output hub  26 , a spring  28 , a control collar  30 , and an electrical circuit  32 . The input hub  24  and output hub  26  are arranged coaxially. Spring  28  is circumferentially around both hubs  24 ,  26  and inside control collar  30 . Electrical circuit  32  is attached to each end  34 ,  36  of spring  28 .  
         [0021]    Spring  28  is a cylindrical helical spring preferably having a rectangular cross-section and constructed of a shape memory alloy, preferably Nitinol. Hubs  24 ,  26  and control collar  30  are preferably constructed of a non-conductive material. Alternatively, as shown in FIG. 3, hubs  24 ,  26  and the control collar  30  can be constructed of a metallic, conductive material as long as a nonconductive material  31  is placed so as to electrically isolate the spring  28  from any conductive components. In FIG. 3, metallic hubs  24 ,  26 , are isolated from the spring  28  by non-conductive material  31 , and a control collar  30  of a non-conductive material.  
         [0022]    Referring again to FIG. 2, spring  28  preferably fits snugly around the outer diameter of the hubs  24 ,  26 . The ends  34 ,  36  of spring  28  are restrained by the output hub  26  and the control collar  30 , respectively. Control collar  30  fits around spring  28  and maintains the position of spring  28  with respect to the hubs  24 ,  26 , while allowing the spring  28  to expand sufficiently to allow over-running, or free-wheeling, in both directions. The spring  28  is formed of a shape memory alloy and constructed so that when heated to a certain temperature, the diameter of the spring  28  expands.  
         [0023]    When electrical circuit  32  is not energized, i.e. no electrical current is flowing, the assembly operates as a typical wrap spring clutch. When the input hub  24  is rotated in the direction of arrow  38 , the spring  28  constricts around the circumference of hubs  24 ,  26  locking the two hubs together so that torque may be transmitted between them. When input hub  24  is rotated in the opposite direction, the spring  28  expands slightly and allows the input hub  24  to turn independently of the output hub  26 .  
         [0024]    When electrical circuit  32  is energized, i.e. electrical current is flowing, the spring  28  increases in temperature because of the inherent resistance of the material. Once the temperature reaches a predetermined level, the spring  28  returns to its preformed, slightly expanded condition. Once the predetermined temperature is reached, the spring  28  will change shape rapidly and with great force. The force exerted by spring  28  when expanding, is sufficient to move the spring  28  even under maximum torsional load from the hubs  24 ,  26 . Once the spring  28  expands, both hubs  24 ,  26  are free to rotate independently of each other, also known as overrunning or freewheeling.  
         [0025]    When using a one-way shape memory alloy, the spring is returned to its non-energized position by the movement of the hubs. This occurs because the amount of return deformation allowed is very small and limited by the control collar  30 . The control collar  30  thus maintains the spring  28  in a position so that it is returned to the non-energized position when the electrical current is removed.  
         [0026]    Although the above described embodiment uses a one-way shape memory alloy, it is also contemplated that a two-way shape memory alloy may be used giving additional flexibility to the arrangement and operation of the clutch. Using a two-way alloy, no assistance is needed from the control collar  30  to retain the spring  28  or return it to its non-energized position. It is also possible to manufacture the spring  28  so that, in the non-energized mode, the clutch can free-wheel in both directions and when the spring  28  is heated and in the energized mode, the clutch operates normally.  
         [0027]    Another embodiment of the present invention is shown in FIG. 4. This embodiment of a wrap spring clutch  40  is similar to the embodiment shown in FIG. 3 and described above except that this embodiment does not include an electrical circuit. The clutch  40  of FIG. 4 comprises metallic hubs  24 ,  26 , spring  29 , and a control collar  30 . Because there is no electrical circuit, all of the components can be constructed from metallic, conductive materials. This embodiment operates as a simple wrap spring clutch and finds particular utility as a safety release. The wrap spring clutch  40  can be used to hold a fail-safe close valve in the open position. Because the spring  29  is constructed of a shape memory alloy, it will expand if heated to a sufficient temperature. Therefore, the wrap spring clutch  40  will maintain the valve in an open position and in the event of fire, the heat of the fire will cause the spring to expand, releasing the clutch  40  and allowing the valve to close. Alternatively, the clutch  40  could hold a valve closed, for example a valve supplying a sprinkler system, and open the valve in response to an increase in heat.  
         [0028]    [0028]FIG. 5 shows a schematic view of a valve  50  incorporating a wrap spring clutch  52  having a spring constructed of a shape memory alloy. Valve  50  also comprises a valve body  54 , seat  56 , gate  58 , actuator housing  60 , actuator  62 , bearings  64 , and seals  66 . Gate  58  comprises a sealing portion  68  and a ball-screw shaft portion  70 . Sealing portion  68  acts with seat  56  to seal flowbore  51  in a first position and allows flow through the flowbore in a second position (not shown). Ball-screw shaft portion  70  makes up the shaft of a ball-screw, wherein the ball-screw nut portion  72  is comprised within the actuator  62 . Actuator  62  further comprises a torque connection  74  and a hub portion  76 . Actuator housing  60  is attached to valve body  54 , maintains seal  66  in place and comprises a hub portion  78 .  
         [0029]    In the closed position, gate  58  and seat  56  create a seal that prohibits flow through the flowbore  51 . To open the valve  50 , actuator  62  is rotated in a clockwise direction causing gate  58  to move linearly and moving the sealing portion  68  of the gate into the open position. Wrap spring clutch  52  is arranged so as to allow clockwise rotation of the actuator  62 . Valve body  54  and seat  56  are arranged so that the pressure within the flowbore  51  and valve body  54  creates a force on the gate  58  that will bias the gate to the closed position.  
         [0030]    The ball-screw shaft and nut  70 ,  72  are designed so that torque is converted to linear force at very high efficiencies. The shaft and nut  70 ,  72  are threaded with ball bearing races. When the shaft and nut  70 ,  72  are assembled with ball bearings  80 , the connection between the shaft and nut has very little friction. This, combined with the use of bearings  64  allows the bias force created by the pressure within the valve body  54  to close the valve. The closing of the valve is resisted by the wrap spring clutch  52  that will not permit the actuator  62  to rotate in the counter-clockwise direction.  
         [0031]    Preferably, a valve  50  of this type is placed in the open position during normal operation. If a fire were to occur in the vicinity of the valve, the spring of the wrap spring clutch  52  will expand with increasing temperature and the valve would be allowed to close. Thus, there is provided a valve that will close if the environmental temperature increases to a predetermined level without the need for any outside actuation or complex control system.  
         [0032]    The use of memory shape alloy springs in wrap spring clutches provides a simple, robust design that has the advantages of a wrap spring clutch while providing a simple, effective method for engaging and/or disengaging the mechanism. Wrap spring clutches constructed in accordance with the present invention can be used in any application where wrap spring clutches are currently used and any application where control of a rotating member is required.  
         [0033]    The embodiments set forth herein are merely illustrative and do not limit the scope of the invention or the details therein. For example, while it is preferred that the spring be constructed of Nitinol, any material having shape memory alloy properties may be used. It will be appreciated that many other modifications and improvements to the disclosure herein may be made without departing from the scope of the invention or the inventive concepts herein disclosed. Because many varying and different embodiments may be made within the scope of the inventive concept herein taught, including equivalent structures or materials hereafter thought of, and because many modifications may be made in the embodiments herein detailed in accordance with the descriptive requirements of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.