Abstract:
Apparatus and methods for fitting mechanical parts to minimize prevent free play are described. Additionally, a design for an apparatus having a piston, which restrains the piston before actuation and which allows the piston to gain momentum before striking an object, is detailed. The described apparatus are applicable to devices involving shearable elements. An embodiment of the invention as a pyrotechnically activated valve incorporating such features to minimize free play in conjunction with such features to restrain the piston and also allow the piston to gain momentum is described.

Description:
CLAIM OF PRIORITY  
       [0001]    This application makes reference to, incorporates the same herein by reference, and claims all benefits accruing under 35 U.S.C.§119(e) from my application PISTON-A CTIVA TED VALVE filed as a provisional application in the U.S. Patent and Trademark Office as U.S. application Serial No. 60/261,199, filed on Jan. 16, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to mechanical fittings and tolerancing, in particular to fittings found in shearable devices, and more particularly to pyrotechnically activated devices such as valves.  
           [0004]    2. Background of the Invention  
           [0005]    Pyrotechnic valves are commonly used in rockets for valves which must be opened reliably one time. Typically, these valves contain a seal tube which holds a pressurized gas, and this seal tube is pyrotechnically sheared to open the sealed end of the seal and start the flow of gas. The seal tube typically needs to be very strong in order to support the high gas pressure, such as by helium gas, dictated by the design. Also, initiator redundancy is typically needed to promote reliability. Moreover, for reliability, for example, these devices are typically built with two pyrotechnic initiators for redundancy. Also, the valve typically must be able to function using only one initiator loaded to 75% or with two initiators loaded to 125%. Such valve requirements create a range of initial ballistic gas pressures with a ratio of approximately 4:1, for example. Further, peak pressures are limited by the ability of O-rings and other gas-tight fittings to contain such pressures.  
           [0006]    One potential method of addressing the above described needs in a pyrotechnic valve is to have the piston to supply a much larger force at the beginning of stroke. This could be accomplished, for example, by either increasing the initial ballistic gas pressure or by increasing the area of the piston that is acted on by the gas. However, such pressure increase in this first potential method can be unacceptable because of a need for initiation redundancy in the device.  
           [0007]    A second potential method of addressing the above desired needs in a pyrotechnic valve, is to increase the piston area, which can be undesirable for two reasons. First, increasing the piston area can make the piston physically large and heavy. Second, more propellant charge or possibly even an additional booster charge would be required in the initiators. These characteristics can be undesirable from a design standpoint and can generally raise the cost of the pyrotechnic valve device.  
           [0008]    Moreover, the construction of pyrotechnic valve devices must be done to exacting tolerances. Pyrotechnic valve devices, such as those used in rocketry, for example, are often subject to considerable vibration. If there is “play” between the parts, vibrational damage can result.  
         SUMMARY OF THE INVENTION  
         [0009]    It is therefore an object of the present invention to provide an improved shearable device.  
           [0010]    A yet further object of the invention is to provide an improved shearable device for use with pyrotechnic initiators.  
           [0011]    It is further an object of the invention to provide a device which is less sensitive to vibration.  
           [0012]    A still further object of the invention is to provide an improved pyrovalve.  
           [0013]    These and other objects are met by the present invention. In one embodiment, the present invention provides method and apparatus for preventing free play in a device, such as a pyrotechnic valve device. An apparatus of this embodiment includes an eccentric sleeve, that is, a sleeve with a cylindrical outer surface and a bore which is not coaxial with the outer surface. Rotation of the eccentric sleeve adjusts the position of the bore in the eccentric sleeve, which is mated with a pin, such as a shear pin.  
           [0014]    In another embodiment of the present invention, the present invention provides methods and apparatus for a device, such as a pyrotechnic valve device, the apparatus of this embodiment including a piston which is driven to strike an object. The piston is mounted to a mount in the device by a shear pin, and upon application of force, the shear pin shears and the piston moves through a gap before striking the object. An apparatus of this embodiment includes: amount; a piston adjacent to the mount; the piston having a shape defining the movement direction of the piston; a shear pin having an end partially inserted in a hole in the mount and another and of the shear pin connected to said piston, for restraining the piston relative to the mount a hammer region formed on an end of the piston located in the direction of motion of the piston, and a strikable part mounted in the direction of motion of the piston from the hammer region and separated from the hammer region by a gap. The other end of the shear pin is, in one alternative embodiment, connected to the shear pin through a sleeve, such as an eccentric sleeve. In another alternative embodiment, the shear pin is positioned in spaced relation from the strikable part in the direction of motion of the piston. In a further embodiment an end of the shear pin is press fit into the mount.  
           [0015]    Also, embodiments of the invention as a pyrovalve are also described. In an embodiment the pyrovalve, includes: a housing having a cylindrical bore; a pyrotechnic initiator mounted in an upper portion of the housing; a seal tube mounted in an extending out from the housing, the seal tube having a generally cylindrical configuration and the axis of the seal tube being positioned perpendicular to the axis of the bore of said housing, the seal tube further including a shearable cap on the end of said seal tube inside the housing; a piston located inside in the bore of the housing so as to define a direction of motion, the piston including: a flowpath formed perpendicular to the direction of motion defined by the piston; a hollow formed in a side of the piston, further from the pyrotechnic initiator than the flowpath, and shaped to enclose the shearable cap, the hollow being larger in cross-section than the shearable cap so as to define a gap between an overhang of the piston and the shearable cap; and a shear pin connecting the shearable cap to the piston through a sleeve, for restraining play in said piston. Desirably, the sleeve is an eccentric sleeve. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    A more complete appreciation of the invention, and many of the attendant advantages, thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:  
         [0017]    [0017]FIG. 1A is an embodiment of the present invention in the form of a pyrotechnically activated valve, seen in cross-section before activation of the valve;  
         [0018]    [0018]FIG. 1B is a view of the pyrotechnically activated valve of FIG. 1A shown after activation of the valve;  
         [0019]    [0019]FIG. 2 is a cross-section of the valve shown in FIG. 1A, prior to activation of the valve, taken across the line labeled “2” of FIG. 1A and the line “2” of FIG. 6B;  
         [0020]    [0020]FIG. 3A is a cross-sectional view of a device, such as can be used in a valve device, illustrating a general embodiment of the invention incorporating an eccentric sleeve;  
         [0021]    [0021]FIG. 3B is a plan view taken from the bottom of the device of FIG. 3;  
         [0022]    [0022]FIGS. 4A and 4B are a cross-sectional views of a device illustrating another general embodiment of a device of the invention, respectively before and after actuation of the device, such as a valve device;  
         [0023]    [0023]FIG. 5 is a close up of a portion to illustrate the shear pin area of the valve device shown in FIG. 1A;  
         [0024]    FIGS.  6 A- 6 F are a series of views of the valve device shown in FIGS. 1A and 1B, with FIG. 6A illustrating an external side view of the valve shown in sectional view in FIGS. 1A and 1B along the section line “1A/1B”, with FIG. 6B illustrating an external front view of the valve of FIGS. 1A and 1B with the sectional view of FIG. 2 taken along line “2” and with the sectional view of FIG. 6F taken along the line “6F”, with FIG. 6C illustrating a right side view of the valve of FIGS. 1A and 1B, with FIG. 6D illustrating a top external view of the valve of FIGS. 1A and 1B, with FIG. 6E illustrating a bottom external view of the valve of FIGS. 1A and 1B, and with FIG. 6F illustrating a partial sectional view of the valve of FIGS. 1A and 1B of FIG. 6B along the sectional line “6F” of FIG. 6B;  
         [0025]    FIGS.  7 A- 7 C are a series of views of the eccentric sleeve of the valve device shown in FIGS. 1A and 1B, with FIG. 7A illustrating a left side external view of the sleeve of FIGS. 1A and 1B, with FIG. 7B illustrating a sectional view of the sleeve of FIG. 7A taken along the section line “7B” of FIG. 7A, and with FIG. 7C illustrating a right side external view of the sleeve of FIGS. 1A and 1B; and  
         [0026]    FIGS.  8 A- 8 I are a series of views of the piston of the valve device shown in FIGS. 1A and 1B, with FIG. 8A illustrating a front external view of the piston of FIGS. 1A and 1B, with FIG. 8B illustrating a right side external view of the piston of FIGS. 1A and 1B, with FIG. 8C or illustrating a left side external view of the piston of FIGS. 1A and 1B, with FIG. 8D illustrating a cross-sectional view of the piston of FIGS. 1A and 1B illustrated in FIG. 8C taken along line “8D” of FIG. 8C, with FIG. 8E illustrating a sectional view of the piston of FIGS. 1A and 1B illustrated in FIG. 8D taken along line “8E” of FIG. 8D, with FIG. 8F illustrating a top view of the piston of FIGS. 1A and 1B, with FIG. 8G illustrating a detail view of an O-ring groove on the piston of FIGS. 1A and 1B as defined by the view illustrated in FIG. 8C, with FIG. 8H illustrating a sectional view of the piston of FIGS. 1A and 1B illustrated in FIG. 8D taken along the line “8H” of FIG. 8D, and with FIG. 8I illustrating a bottom view of the piston of FIGS. 1A and 1B. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    An embodiment of a valve device of the present invention is a pyrovalve, that is, a pyrotechnically activated valve. Pyrovalves can be used, for example, in rockets for valves which are activated only once. For example, a pyrovalve can be a normally closed valve mounted in proximity to a main rocket motor. The valve can be installed in a flow path between a pressurized helium tank and the interior of the main motor, for example. During the first phase of flight, the main motor burns to provide propulsion. During this time, the valve remains in the normally closed position, preventing the flow of helium to the motor. When the main rocket motor burns out, the valve is actuated to provide a helium purge to the motor. This brings the motor rapidly to a zero thrust permitting stage separation shortly thereafter.  
         [0028]    An embodiment of a valve device of the present invention as a pyrovalve is shown in FIGS. 1A, 1B,  2 ,  5 ,  6 A- 6 F,  7 A- 7 C and  8 A- 8 I, with FIG. 1A illustrating the pyrovalve in the pyrovalve&#39;s normally closed position prior to activation. Referring to FIGS. 1A, 1B,  2 ,  5 ,  6 A- 6 F,  7 A- 7 C and  8 A- 8 I, in this embodiment, the pyrovalve is designed to be installed connected to a gas source at the right side in the FIG. 1A, at “inlet fitting” I. The inlet fitting I leads to seal tube  2 , which is generally machined from a solid material, such as a precipitation hardened stainless steel, with no outlet, to block the flow of gas through the pyrovalve. At an end of seal tube  2  is a shearable cap  30 , which is an integral part of seal tube  2 , the shearable cap being formed of the material of the seal tube  2 , such as a precipitation hardened stainless steel. Groove  40  in seal tube  2  “necks down” the seal tube  2 , and is generally formed by machining. This necking down allows the shearable cap  30  to be mechanically sheared when the pyrovalve is actuated, but to withstand the gas pressure of the gas source, such as a helium gas source, prior to shearing. For example, in an actual embodiment of a pyrovalve, the seal tube  2  has been designed to withstand internal gas pressures in excess of 12,000 pound per square inch, but shearable cap  30  shears off at a ballistic pressure in the vicinity of 4,000 pounds per square inch to open the flow path F when required. After actuation, the gas, such as helium, will flow from right to left toward the “Outlet Fitting” O in the pyrovalve.  
         [0029]    Continuing with reference to FIGS. 1A, 1B,  2 ,  5 ,  6 A- 6 F,  7 A- 7 C and  8 A- 8 I, within housing  1  of the pyrovalve is, piston  3  being formed of a precipitation hardened stainless steel, for example. Piston  3  is also shown in greater detail in FIGS.  8 A- 8 I. The piston  3  can be cylindrical in shape. Piston  3  can be seen in FIGS. 1A and 2 to have an upper flowpath  45 , which becomes the gas flowpath F after activation of the pyrovalve. Piston  3  also has lower portion  3   a  which has a hollow region surrounding shearable cap  30 . The hollow region is shaped to provide a gap  37  above shearable cap  30 , and a portion of the piston  3  between flowpath  45  and the hollow region forming as overhang or hammer region  35 , since overhang  35  appears to overhang the shearable cap  30  as seen in FIGS. 1A and 2.  
         [0030]    Referring again to FIGS. 1A, 1B,  2 ,  5 ,  6 A- 6 F,  7 A- 7 C and  8 A- 8 I, mounted in the end of seal tube  2  and extending away from the seal tube is a shear pin  5 , shear pin  5  being formed of any suitable fracturable material, such as aluminum, for example. Shear pin  5  is installed in the bore  18   a  of sleeve  18 . The outside of sleeve  18  is cylindrical in shape, and sleeve  18  has a cylindrical bore  18   a  which is parallel to but not coaxial with the outside of the sleeve  18 . Thus, sleeve  18  has an eccentric bore  18   a  . Sleeve  18  is shown in greater detail in FIGS.  7 A- 7 C. The outside of sleeve  18  is mounted in a bore  3   b  in piston  3 . Set screws  20  pass through a portion of the piston  3  and contact the outside of sleeve  18 . Also, the axis Z 1  of the seal tube  2  is desirably positioned perpendicular to the axis Z 2  of the bore  1   a  of the housing  1 , as illustrated in FIG. 1A, for example.  
         [0031]    Also shown in FIGS. 1A and 2 are O-rings  7 , seal  19  and ball bearing or ball member  21  and lock plug  4 . Also shown is Faraday cap  15 , present before installation to prevent accidental discharge of the initiators  16 .  
         [0032]    In order to activate the pyrovalve, a sufficient amount of electric current is applied to one or both of electrical initiators  16 . These pyrotechnic initiators produce hot pressurized gas, which are the products of combustion of the pyrotechnic materials contained within them, in the cavity C over piston  3 . Gas within this cavity C is sealed by O-rings  7 . The pressure, acting over the area of the piston  3 , produces a force to drive the piston  3  downward as shown in the Figures, such as FIGS. 1A and 1B. The motion of the piston  3  is resisted by the shear pin  5 , which is held in place by seal tube  2 , a portion of the shear pin  5  being held snugly in the seal tube  2 , such as by being press fit into the seal tube  2 . The shear pin  5  is designed to shear across the plane where shearable cap  30  at the left side of seal tube  2  mates with piston  3 . In this particular design, the force to shear the shear pin  5  is nominally around 1,600 pounds. The shear pin  5  is designed to shear off at a lower force than is necessary for the shearing of shearable cap  30  from seal tube  2 . For example, in this particular design, the shearable cap  30  requires a shear force of approximately 4,000 pounds to shear.  
         [0033]    When the force produced by the gas is sufficient, shear pin  5  shears and piston  3  begins to travel downward in the direction of arrow D of FIG. 1B. Since the shear pin  5  is designed to shear more easily than the shearable cap  30 , the shear pin  5  shears off before shearable cap  30  at the end of the seal tube  2 . That is, piston  3  begins to move before the flow path F opens. After shear pin  5  shears, the piston  3  accelerates downward in the direction of arrow D, building up kinetic energy until overhang or hammer region  35  in the piston  3  strikes the shearable cap  30 , the shearable cap  30  designed to be sheared off from the seal tube  2 . For example, in the specific example shown, the piston  3  travels approximately ¼ inch before striking the shearable cap  30 . At this point, the kinetic energy of the moving piston  3  is well in excess of the energy required to shear off the end of the seal tube  2 .  
         [0034]    During the portion of the stroke that the piston  3  is shearing off the shearable cap  30 , the piston  3  will generally temporarily slow down. Piston kinetic energy is partially depleted and used as shear energy. The piston  3  then accelerates as a result of the expanding ballistic gas still acting on the piston  3 . At the end of the piston stroke a skirt  50  which is machined into the piston  3  flares out into a conical cavity  51  formed by the space between the lock plug  4  and housing  1 , the lock plug  4  and housing  1  being made of precipitation hardened stainless steel, for example. The lock provided by the skirt  50  flaring out into the conical cavity  51  both holds the piston  3  permanently in the final position P and gently slows the piston  3 , minimizing shock. In the final position P. illustrated in FIG. 1B, the final lock is engaged and the skirt  50  of piston  3  is in a flared position in over the lock plug  4 . Also, in the final position P, the shearable cap  30  of the seal tube  2  and the portion of shear pin  5  in the shearable cap  30  are restrained in a portion of the cavity  51  of FIG. 1A at or near position S illustrated in FIG. 1B.  
         [0035]    When actuated, as shown in FIG. 1B, shear pin  5  can be seen to be broken into pieces  5   a  and  5   b  as illustrated in FIG. 1B. Piece  5   b  is retained in shearable cap  30 , which has been sheared from seal tube  2 .  
         [0036]    A valve such as that shown in FIGS. 1A and 1B is typically required to survive a very high-level level shock and random vibration environments, such as during the period of time between launch and the actuation of the valve. These environments can be particularly brutal on internal components that have any “free play”. This free play can allow impact loads on internal components that rapidly cause damage. The valve of the present invention, such as the valve device of FIGS. 1A and 1B, incorporates a design to minimize such free play, which is described more generally in FIGS. 3A and 3B.  
         [0037]    Referring now to FIGS. 3A and 3B. In FIG. 3A, an embodiment of a device of the present invention such as can be used in a valve device, to minimize free play is shown having part  303  and part  310 , where parts  303  and  310  are to be connected so as to avoid free play in the directions shown in double headed arrow  320 . Part  303  has a cylindrical bore  303   a  in which sleeve  318  is fitted. Sleeve  318  has a cylindrical outer surface which fits snugly into the bore  303   a  in part  303 . Sleeve  318  also has a cylindrical bore  318   a  which is parallel to, but not coaxial with, the outer cylindrical axis of sleeve  318 . That is, sleeve  318  has an eccentric bore  318   a  . Pin  305 , such as a shear pin or any shearable feature of part  310   310 , extends from part  310 . Pin  305  is cylindrical and fits snugly into the bore of sleeve  318 . In using the device of the present invention of FIGS. 3A and 3B, sleeve  318  is rotated within the bore  303   a  in part  303 . As seen in FIG. 3B, since sleeve  318  is eccentric, rotation of sleeve  318  would cause the bore  318   a  in sleeve  318  to rotate around the axis A of the bore  303   a  in part  303 , and this allows for negligible variation in the position of pin  305  in the direction of arrow  320 , but sufficient variation in the lateral direction L, as indicated by the double-headed arrow L in FIG. 3B, to allow assembly of parts manufactured to reasonably large tolerances.  
         [0038]    The device of FIGS. 3A and 3B can also include a securing means, such as a set screw  320 . After pin  305  is inserted into sleeve  318 , set screw  320  would be tightened to lock and possibly deform sleeve  318 , thus locking the position of pin  305  with no or minimal free play. This general design of a device for minimizing free play of FIGS. 3A and 3B is incorporated into the pyrotechnic valve shown in FIGS. 1A and 1B. In the pyrotechnic valve of FIGS. 1A and 1B, the piston  3  has to be contained due to the vibration of the environment during use. Free play of 0.010 to 0.020 inches, for example, would lead to impact loads which would destroy the shear pin. Thus, in the design of a device for minimizing free play incorporated and shown in FIG. 1A, one end of shear pin  5  is press fit into shearable cap  30  of seal tube  2 . Also, the pin  305  can be integrally formed as a part of part  310 , as illustrated in FIG. 3A, for example. Sleeve  18  is incorporated to slip fit into bore  3   a  of piston  3 . The eccentric design of sleeve  18  allows alignment of shear pin  5  into sleeve  18  by rotation of the sleeve  18 , thereby avoiding or minimizing free play in the connection of shear pin  5  to piston  3 , thereby reducing any free play such as to only approximately 0.001 to 0.002 inches, for example. Set screws  20  in conjunction with ball bearing  21  or ball member as the securing means further lock and deform sleeve  18  and further prevent or further minimize any free play.  
         [0039]    The valve shown in FIGS. 1A and 1B also incorporates a design which allows the piston  3  to gather momentum before striking the shearable cap  30 , as can be used in conjunction with the sleeve  318  of FIGS. 3A and 3B, as described above. A type of this design is described more generally with reference to a general embodiment of the present invention shown in FIGS. 4A and 4B.  
         [0040]    Referring to FIGS. 4A and 4B, in FIG. 4A, a device is shown including mount  410  and piston  403 . Mount  410  can be a cylinder in which piston  403  rides, or can more generally be any part providing a guide surface for the downward motion of the piston  403 . Likewise, piston  403  may be a cylindrical piston, or can be of any of a variety of shapes which can travel downward in the direction of the arrows indicating a propelling force  460  as shown in the FIGS. 4A and 4B. Piston  403  is mounted to mount  410  by shear pin  405 , which is inserted in holes or apertures  410   a  and  403   a  respectively in mount  410  and piston  403 .  
         [0041]    Continuing with reference to FIGS. 4A and 4B, the lower part of piston  403  has hammer surface or hammer region  435  which is designed to deliver a blow to strikable part or shearable element  430  when the piston  403  moves. Here, part  430  can be any part to be struck. There is a gap  470  between the hammer surface  435  of the piston  403  and the shearable element  435 .  
         [0042]    In the particular embodiment illustrated in FIGS. 4A and 4B, strikable part  430  or shearable element  430  is attached to a stationary part  440 . As illustrated in FIGS. 4A and 4B, stationary part  440  is mounted to mount  410 , but stationary part  440  could be mounted to anything stationary relative to the piston  403 . Also, as shown in FIG. 4A, piston  403  can have a ledge  445  which is not aligned with hammer surface  435 .  
         [0043]    In operation, a propelling force  460  indicated by the arrows in FIGS. 4A and 4B is applied to the top of piston  403 . This propelling force can be gas pressure, pneumatic pressure, an electric or mechanical force, etc. When the force reaches a certain value, shear pin  405  which can be spaced from the shearable element  430  in the direction of travel or motion of the piston  403 , as illustrated in FIG. 4A, shears and the piston  403  moves downward in the direction of the arrows  460  indicating the propelling force in FIGS. 4A and 4B. Alternatively, when the mount corresponds to a seal tube, such as seal tube  2  of FIGS. 1A and 1B, the pin  405  can be press fit into the shearable cap  30  of the seal tube  2 , as described previously with respect to FIGS. 1A and 1B. Piston  403  moves through gap  470  before hitting the strikable part of shearable element  430 ; and thus, piston  403  is able to gather momentum before the impact with the strikable part or shearable element  430 . When used as a valve device, such as illustrated in FIGS. 1A and 1B, the shearing of the shearable element  430  would typically create a flow path G formed integrally as a passage in the piston  403 , the location of the flow path G being indicated in FIG. 4B.  
         [0044]    In the particular embodiment shown of FIGS. 4A and 4B, the strikable part or shearable element  430  shears off of stationary part  440  upon impact at a shearable link  430 a connecting the strikable part or shearable element  430  and the stationary part  440 . Due to the ledge  445 , the stationary part  440  is not hit by the piston  403  when shearable element  430  is impacted by the piston  403 . Stationary part  440  can thus serve as a detent for stopping the downward motion of the piston  403 . In such a case, the total stroke of the piston  403  is given by gap  450  illustrated in FIG. 4A. In general, there will be some sort of a detent for stopping the piston after it has hit the object to be worked.  
         [0045]    Although shown in FIGS. 4A and 4B for a shearable object, the device of the present invention illustrated in FIGS. 4A and 4B is applicable to any object to be struck. For example, hammer surface or hammer region  435  could be striking a rivet or nail, punching a hole, making an impression, etc., for example.  
         [0046]    In the pyrovalve embodiment shown in FIGS. 1A and 1B, however, piston  3  is mounted through shear pin  5  to shearable cap  30  which is part of seal tube  2 , which is held in place by the housing  1  and the shear pin  5  is not initially spaced from the shearable cap  30  in the direction of travel or motion of the piston  3 . Upon activation, the piston  3  moves downward, and overhang  35  moves through gap  37  before striking shearable cap  30 . In the invention as shown in FIGS. 1A and 1B, the interaction of skirt  50  with the conical cavity formed by the space between the lock plug  4  and housing  1  serves as the detent.  
         [0047]    In the pyrovalve of the invention, the piston design, incorporating the sleeve design to minimize free play, allows the piston to gain momentum before striking the shearable cap. One result of this design is that less of the pyrotechnic explosive is required to shear the cap than in a comparable design with no shear pin in which the piston initially contacts the shearable cap.  
         [0048]    While there have been illustrated and described what are considered to be preferred embodiments of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. In addition, many modifications may be made to adapt a particular situation to the teaching of the present invention without departing from the scope thereof. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the present invention, but that the present invention includes all embodiments falling within the scope of the appended claims.