Abstract:
A preferred embodiment of an automotive vehicle roof system includes a detachable convertible roof that covers a rear storage compartment and at least a portion of a passenger compartment of the vehicle. In another aspect of the present invention, the retracted convertible roof may be removed entirely from the vehicle by disengaging quick release attachment devices. Yet another aspect of the present invention provides the ability to operate the convertible roof independently of the vehicle&#39;s rear backlite. In a further aspect of the present invention, an adjustable tensioning device is employed.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION  
         [0001]    The present invention relates in general to automotive convertible roof systems, and more particularly, to an automotive vehicle roof system having a detachable convertible roof.  
           [0002]    Automotive vehicles, such as sport utility vehicles (SUVs), have experienced a substantial increase in popularity in recent years. With the increase in popularity has come an increased demand for many of the amenities that were generally only found on other types automotive vehicles, such as convertible roof systems. Convertible roof systems designed for other automotive vehicles, such as passenger cars, are not, however, readily adaptable for use with SUVs because of the significant design differences between the two types of vehicles. For example, many SUVs have a rear storage compartment that is an extension of the passenger compartment, whereas in passenger cars the storage compartment is typically separate from the passenger compartment. Since it is generally desirable to be able to access the rear storage compartment from the rear of the vehicle, it would be advantageous that the convertible roof system be designed to allow for easy access to the rear storage compartment. SUVs also present a challenge as to how and where to store the convertible top when the top is fully retracted without unnecessarily using up valuable storage space.  
           [0003]    Various convertible roof systems for use with SUVs have been recently developed that attempt to address these and other problems associated with adapting a convertible roof system for use with an SUV. Examples of previously designed SUV convertible roof systems are disclosed in U.S. Pat. No. 6,068,326, entitled “Support Structure For A Canvas Top Bow On A Canvas Top Automobile,” which issued to Shiromura on Sep. 9, 1998; U.S. Pat. No. 5,775,767 entitled “Convertible Top For A Vehicle Having A Rear Egress,” which issued to Harrison et al. on Aug. 15, 1996; and U.S. Pat. No. 5,299,850 entitled “Automobile With Canvas,” which issued to Kaneko et al. on Aug. 27, 1992; the foregoing patents are incorporated by reference herein. While some of the patents have improved the art, a demand still exists to further simply the operation and improve the functionality of convertible roof systems for SUVs.  
           [0004]    In accordance with the present invention, a preferred embodiment of an automotive vehicle roof system includes a detachable convertible roof that covers a rear storage compartment and at least a portion of a passenger compartment of the vehicle. A further aspect of the present invention provides a linkage mechanism that is movable from an extended position to a retracted position in order to provide unobstructed access to the vehicle&#39;s storage compartment from the rear of the vehicle. In another aspect of the present invention, the retracted convertible roof may be removed entirely from the vehicle by disengaging quick release attachment devices. Yet another aspect of the present invention provides the ability to operate the convertible roof independently of the vehicle&#39;s rear backlite. In a further aspect of the present invention, an adjustable tensioning device is employed.  
           [0005]    The automotive vehicle roof system of the present invention is advantageous over previous designs in that the present invention provides a mechanism for retracting the convertible roof independent of the rear backlite. This allows for relatively unobstructed access to the rear storage compartment of the vehicle. Furthermore, not attaching the rear backlite to the convertible roof eliminates the need to retract or manually unsnap the convertible roof in order to access the vehicle&#39;s storage compartment from the rear of the vehicle. The present invention also provides a unique linkage mechanism that allows the convertible roof to be easily moved between its retracted and extended positions. If desired, the retracted convertible roof can be entirely removed from the vehicle with minimal effort by simply releasing the quick releasing latch and pin devices with common attachment points for both removable soft-top and removable hard-top convertible roofs; this attachment scheme also eliminates the typical loose fasteners that often get lost when the roof is detached. Moreover, an adjustable tensioning assembly causes roof bow-to-roof bow or roof bow-to-backlite frame spacing modifications which improve the fit and function of the pliable roof cover. Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a fragmentary perspective view showing the preferred embodiment of an automotive vehicle roof system with the convertible roof in a fully raised and attached position;  
         [0007]    [0007]FIG. 2 is a perspective view, from an opposite side of the vehicle from FIG. 1, showing a convertible roof linkage mechanism employed in the preferred embodiment automotive vehicle roof system, disposed in the fully raised position;  
         [0008]    [0008]FIG. 3 is a side elevational view, from the same side of the vehicle as with FIG. 1, showing the convertible roof linkage mechanism employed in the preferred embodiment automotive roof system, disposed in the fully raised position;  
         [0009]    [0009]FIG. 4 is an enlarged, perspective view showing a latch employed in the preferred embodiment automotive roof system for attaching the convertible roof to a trailing edge of a stationary roof panel;  
         [0010]    [0010]FIG. 5 is an enlarged, perspective view, similar to FIG. 2, showing a drive link and a latching mechanism employed in the preferred embodiment automotive roof system;  
         [0011]    [0011]FIG. 6 is a side elevational view, similar to FIG. 3, showing the convertible roof linkage mechanism employed in the preferred embodiment automotive roof system, disposed in a partially open position;  
         [0012]    [0012]FIG. 7 is a side elevational view, similar to FIG. 3, showing the convertible roof linkage mechanism employed in the preferred embodiment automotive roof system, disposed in a fully open position;  
         [0013]    [0013]FIG. 8 is a perspective view, similar to FIG. 2, showing the convertible roof linkage mechanism employed in the preferred embodiment automotive roof system, disposed in the fully open position;  
         [0014]    [0014]FIG. 9 is a side elevational view, similar to FIG. 3, showing the convertible roof linkage mechanism employed in the preferred embodiment automotive roof system, collapsed and removed from the vehicle;  
         [0015]    [0015]FIG. 10 is a side elevational view, similar to FIG. 3, showing an alternate embodiment automotive roof system, disposed in a fully raised position;  
         [0016]    [0016]FIG. 11 is a perspective view, similar to FIG. 2, showing the alternate embodiment automotive vehicle roof system, disposed in the fully raised position;  
         [0017]    [0017]FIG. 12 is a side elevational view, similar to FIG. 3, showing the alternate embodiment automotive roof system, disposed in a partially open position; and  
         [0018]    [0018]FIG. 13 is a side elevational view, similar to FIG. 3, showing the alternate embodiment automotive roof system, disposed in a fully open position.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]    FIGS.  1 - 3  show the preferred embodiment of a convertible roof system  20  employed in an automotive vehicle  22 , such as a notch-back sport utility vehicle, or alternatively, a station wagon or van. Roof system  20  consists of a stationary, rigid roof panel  24  and a soft-top convertible roof  26 . Stationary roof panel  24  is made from rigid sheet steel which extends from a front windshield header  28  back to a position approximately above a rear passenger seat  30  (see FIG. 3). Convertible roof  26  covers a rearmost portion of vehicle  22  and extends from a trailing edge  32  of stationary roof panel  24  back to a rear tailgate  34  of the vehicle. Tailgate  34  can be manually, downwardly pivoted to a horizontal open position about a bottom cross vehicle pivot axis or may be rotated open about a vertical, side pivot axis. Convertible roof  26  includes a pliable cover  36  that is supported by a underlying top stack linkage mechanism  38 . Cover  36  is preferably made from any of a variety of pliable or flexible materials, such as canvas or a multi-layer twill fabric.  
         [0020]    Convertible roof  26  has a U-shaped rear opening  40  that is adapted for receiving a rear backlite or back window  42 . Backlite  42  is movable between an extended position, as shown in FIG. 1, and a retracted position, as shown in FIG. 8, wherein the backlite is concealed within rear tailgate  34 . Backlite  42  may optionally include a frame, seals and attachment brackets. Convertible roof  26  further includes a pair of side windows  44  that are suitably affixed to openings in cover  36 . Side windows  44  are preferably constructed from a generally transparent pliable material, such as vinyl or, alternately, a transparent rigid material such as glass. Side window  44  may be permanently attached to cover  36 , but are preferably detachable using zippers, snaps or clips. If a rigid material is used for the side windows, however, it will be necessary for the windows to be removable in order to allow the convertible roof to be retracted.  
         [0021]    Cover  36  is supported by a top stack or linkage mechanism  38 , which is manually operable for moving convertible roof  26  between an extended and a retracted position. Linkage mechanism  38  employs three generally U-shaped roof bows, which include a forwardmost first roof bow  46 , a second roof bow  48 , and a rearmost third roof bow  50 . Each of the three roof bows has a cylindrical cross-sectional shape, although other geometries may alternately be used with equally satisfactory results. The three roof bows may have either solid or hollow cores, but will preferably utilize hollow cores to maximize the strength to weight ratio of the roof bows. Roof bows  46 ,  48  and  50  each have a pair of ends,  52 ,  54 , and  56 , respectively. Ends  52 ,  54 , and  56  have a generally rectangular cross-sectional shape, which facilitate attachment of the roof bows while also providing a more stable connection. Roof bows  46 ,  48  and  50  are preferably constructed of a metal material, such as aluminum or steel, but may also be made from other materials, including engineering grade polymers and composite materials containing fiberglass or carbon fibers. Roof bows constructed of polymeric or composite materials may also incorporate metallic ends that are suitably attached to the non-metallic portion of the roof bow.  
         [0022]    Referring to FIGS. 2 and 3, end  52  of first roof bow  46  is pivotally attached to end  56  of third roof bow  50  using a pivot attachment  58 , which may consist of a bolt, rivet, screw, or other suitable fixture device. Second roof bow  48  is pivotally attached to third roof bow  50  by connecting end  54  to a standoff  60  that is fixedly attached to third roof bow  50  using a pivot attachment  58 .  
         [0023]    First roof bow  46  has an intermediate section  62  that is oriented in a generally horizontal and cross vehicle direction when convertible roof  26  is attached to vehicle  22  in its raised position. Intermediate section  62  is positioned adjacent to a trailing edge  64  of stationary roof panel  24 . First roof bow  46  is secured to trailing edge  64  by a pair of latches  66 , which are shown in more detail in FIG. 4.  
         [0024]    Each latch  66  includes a cylindrically shaped body  68  that encircles an adjacent peripheral majority of intermediate section  62  of first roof bow  46 . Latch body  68  has an interior diameter that is preferably slightly larger than an outside diameter of intermediate section  62  of the first roof bow, which will permit latch  66  to freely rotate about its connection to the first roof bow. Latch body  68  contains an elongated opening  70  that runs parallel to a longitudinal axis of latch body  68  to facilitate assembly of the latch to first roof bow  46 . Latch body  68  is preferably made from a resilient material, such as an engineering grade polymer, that will permit flexing of the latch body during assembly. Latch  66  is assembled to first roof bow  46  by flexing latch body  68  so that the distance between edges  72  and  74  is greater than the diameter of intermediate section  62  of first roof bow  46 . This allows latch body  68  to be snapped over the first roof bow. Once attached to the first roof bow, latch body  68  is allowed to return to its non-flexed condition, thereby securing latch  66  to first roof bow  46 .  
         [0025]    Axial movement of latch  66  along a longitudinal length of intermediate section  62  of first roof bow  46  is prevented by a locating pin  76  that is affixed to an outer surface of first roof bow  46 . Locating pin  76  protrudes through an elongated slot  78  located in latch body  68 . Slot  78  has a width that is only slightly larger than an outside diameter of locating pin  76  in order to minimize axial movement of latch  66  along the length of first roof bow  46 . Slot  78  has a circumferentially elongated length that allows intermediate section  62  of first roof bow  46  to rotate within latch body  68 , in a lost motion manner, along a fore-aft plane of the vehicle when convertible roof  26  is moved between its extended and retracted positions.  
         [0026]    Each latch  66  further employs a stepped positioning pin  80  that engages a recess (not shown) located in trailing edge  64  of stationary roof panel  24 . A reduced diameter lead-in segment  82  projects from an enlarged segment  84  of positioning pin  80 . The smaller diameter of lead-in segment  82  assists in orienting and aligning enlarged segment  84  within the recess in stationary roof panel  24  when attaching convertible roof  26  to the vehicle. Moreover, positioning pin  80  has a tapered distal end. A tapered outside diameter of enlarged segment  84  produces a slight press fit between positioning pin  80  and the recess in trailing edge  64  when convertible roof  26  is attached to the vehicle. Additionally, a handle  88  is pivotally attached to a boss  90  that extends from latch body  68  by a pivot rod.  
         [0027]    Latch  66  employs a connecting member  94  which couples a hook  96  to handle  88 . Hook  96  engages a second recess (not shown) or striker pin located on an underside surface of stationary roof panel  24 . Latch  66  is engaged by manually pivoting handle  88  up toward the underside of the stationary roof panel  24 . This causes hook  96  to retract toward the handle, thereby pulling intermediate section  62  of first roof bow  46  toward trailing edge  64  of stationary roof panel  24 . Disengagement or unlatching is achieved by reverse handle rotation. Alternately, other latches may be employed such as that disclosed in U.S. Pat. No. 5,299,850 entitled “Automobile With Canvas,” which issued to Kaneko et al. on Apr. 5, 1994; and U.S. Pat. No. 4,817,999 entitled “Convertible Header Latch Mechanism,” which issued to Drew on Apr. 4, 1989; the foregoing patents are incorporated by reference herein.  
         [0028]    Referring to FIGS. 1, 2, and  8 , backlite  42  is movable between the open position (as shown in FIG. 1) and the closed position (as shown in FIG. 8). When fully retracted or open, backlite  42  is hidden from view within tailgate  34 . Backlite  42  may be either manually or automatically actuated using a variety of electrically driven window lift mechanisms  97 , such as that disclosed within U.S. Pat. No. 5,337,579 entitled “Vehicle Window Regulating Device,” which issued to Bergesio on Aug. 16, 1994, and U.S. Pat. No. 4,170,847 entitled “Tailgate Window Regulator,” which issued to Pickles on Oct. 16, 1979; both of which are incorporated by reference herein. Convertible roof  26  includes a backlite frame  98  that surrounds outer peripheral top and side edges  100  of backlite  42  when the backlite is in its fully extended or closed position, as shown in FIG. 1. Backlite frame  98  contains an elastomeric pocket seal  102  that is suitably attached to an inside periphery of backlite frame  98 . With backlite  42  positioned in its fully extended position, outer peripheral edges  100  of backlite  42  contacts pocket seal  102  to produce a weatherproof seal between backlight frame  98  and backlite  42 . Backlite  42  is preferably a rigid glass panel.  
         [0029]    As shown in FIG. 5, backlite frame  98  is attached to a body  104  of vehicle  22  by a pair of base links  106 . An end  108  of each base link  106  is pivotally attached to an end  110  of backlite frame  98 . Another end  112  of base link  106  is pivotally attached to a bracket  114  using a quick release, cylindrical pivot pin  116  with a transversely oriented, spring loaded detent ball to maintain the pin&#39;s engaged position against minor forces and vehicle vibration. Pin  116  is tethered to the convertible roof by a cable or the like. Each bracket  114  is fixedly attached to a side panel of vehicle body  104 .  
         [0030]    Referring to FIG. 2, rear backlite frame  98  has an intermediate section  118  that is oriented in a generally horizontal and cross vehicle direction. Third roof bow  50  has a similarly orientated intermediate section  120 . A pair of upper backlite links  122  connect intermediate section  118  of backlite frame  98  to intermediate section  120  of third roof bow  50 . Upper backlite links  122  are pivotally connected to roof bow  50 . The connection between upper backlite links  122  and backlite frame  98  is preferably rigid, but may also be of a pivoting type. Upper backlite links are preferably constructed in accordance with co-pending U.S. Serial No. 60/409,321 entitled “Convertible Backlite Control System” which was invented by the present inventor, filed on Sep. 10, 2002, and is incorporated by reference herein.  
         [0031]    A pair of control links  124  connect base links  106  to third roof bow  50 . An end  126  of each control link  124  is pivotally connected to a standoff  128  on base link  106 . Standoff  128  is positioned between ends  108  and  112  of base link  106 . An opposite end  130  of control link  124  is pivotally attached to an elbow of third roof bow  50  adjacent offset angled end  56 . The pivot connection of end  130  of control link  124  is spaced from the pivot connection coupling first roof bow  46  to third roof bow  50 .  
         [0032]    A pair of quick connect, rear latching mechanisms  132  are used to secure convertible roof  26  to rear portions of the side body panels. A rear latching mechanism can be constructed like those disclosed in U.S. Pat. No. 6,364,379 entitled “Vehicle Compartment Latch,” which issued to Roberts et al. on Apr. 2, 2002; U.S. Pat. No. 6,247,732 entitled “Vehicle Compartment Latch,” which issued to Alton on Jun. 19, 2001; U.S. Pat. No. 4,979,384 entitled “Trunk Lid Lock with Remote Release” which issued to Malesko et al. on Dec. 25, 1990; and U.S. Pat. No. 4,971,370 entitled “Self-Releasing Deck Lid Latch” which issued to Detweiler et al. on Nov. 20, 1990; the foregoing patents are incorporated by reference herein. Referring to FIG. 5, each rear latching mechanism  132  includes a releasable catch  134  that engages a striker  136  that downwardly extends from base link  106 . Rear latching mechanism  132  includes a release button or remotely actuable cable  138  that is accessible from an interior of vehicle  22 . Depressing release button  138  causes catch  134  to disengage from hook  136 , which allows convertible roof  26  to be cycled to its upwardly retracted or removed positions.  
         [0033]    To upwardly retract or collapse convertible roof  26 , rear backlite  42  is automatically moved to its retracted position as shown in FIG. 8 using electric motor driven window lift mechanism  97  (see FIG. 11). Rear latching mechanisms  132  are then released by manually depressing latch button  138 , which is accessible from inside the vehicle storage compartment. An operator subsequently manually rotates base link  106  about its connection to bracket  114  upwardly and toward the front of the vehicle as shown in FIG. 6. Continued movement of base link  106  causes third roof bow  50  to rotate about its connection with first roof bow  46  toward the front of the vehicle, and second roof bow  48  to rotate about its connection with third roof bow  50  toward the rear of the vehicle. With convertible roof  26  in its fully upwardly retracted position, as shown in FIGS. 7 and 8, first roof bow  46 , second roof bow  48 , and third roof bow  50  are placed in a nested position with the second roof bow being positioned adjacent to and between the first and third roof bows. Backlite frame  98  is thereby positioned adjacent to third roof bow  50 . As can be observed in FIG. 8, moving convertible roof  26  to its upwardly retracted position permits relatively unobstructed and clear rear access to the rear cargo space  140  of the vehicle while also giving the vehicle a more open and sporty appearance. A strap with snaps or buckles can be employed to maintain the convertible roof in its upwardly retracted position.  
         [0034]    Once in its upwardly retracted position, convertible roof  26  is designed to be quickly and easily detached from the vehicle as shown in FIG. 9. This is accomplished by manually releasing latches  66  used to retain first roof bow  46  to trailing edge  64  of stationary roof panel  24 . Quick release pins  116 , used to attach base links  106  to brackets  114 , are then disengaged. Convertible roof  26  can now be completely and easily removed from the vehicle without any loose parts.  
         [0035]    Convertible roof  26  can be easily reattached to vehicle  22  by first aligning positioning pin  80  of each latch  66  with the corresponding recess located in trailing edge  64  of stationary roof panel  24 . Hook  96  of latch  66  is then engaged with the corresponding recess located in the underside surface of stationary roof panel  24 . Handle  88  is subsequently fully pivoted to its over-center locked position, thereby securing first roof bow  46  to stationary roof panel  24 . End  112  of each base link  106  is next aligned with bracket  114  and quick release pins  116  are engaged to re-establish the pivot connection. Convertible roof  26  is then manually moved to its fully extended position. Finally, convertible roof  26  is secured in its fully extended position by applying a downward force to each base link  106  so as to engage hook  136  with catch  134  of rear latch  132 . Backlite  42  may now be cycled to its fully extended position if desired.  
         [0036]    FIGS.  10 - 16  show an alternate configuration  38 A of the convertible roof top stack linkage mechanism of the present invention. Alternate convertible roof linkage mechanism  38 A includes a first roof bow  140 , a second roof bow  142  and a backlite frame  144 . First roof bow  140  has a pair of ends  146  (best shown in FIG. 12) that are pivotally attached to second roof bow  142 . A control link  148  has an end  150  pivotally attached to an end  152  of second roof bow  142 . Another end  154  of control link  148  is pivotally attached to an end  156  of a base link  158 . Base link  158  is pivotally attached to a bracket  160  by a quick release pin  116  and bracket  160  which is fixedly attached to vehicle body  104 . An opposite end  162  of base link  158  is pivotally attached to an end  164  of backlite frame  144 .  
         [0037]    Furthermore, referring to FIGS. 11 and 15, backlite frame  144  is connected to second roof bow  142  by two tension-adjusting links  166 . Each tension-adjusting link  166  includes an outer shaft  351  coupled to second roof bow  142 , within which slides an inner shaft  353  coupled to backlite frame  144 . A fore-and-after elongated slot  355  disposed in a bottom of outer shaft  351  is aligned with a threaded hole  357  disposed in inner shaft  353 . A thumbscrew  359  has a manually rotatable knob  361 , an enlarged diameter bearing member  363  and a threaded dowel  365 . Dowel  365  engages hole  357  and allows sliding movement within slot  355  when loose such that the shafts can be manually, linearly expanded or retracted relative to each other. Thumbscrew  359  is then fully tightened to secure the shafts when in their desired positions. Thus, tension-adjusting links  166  allow an operator to vary the tautness of cover  36  when convertible roof  26 A is fully extended by controlling a distance  168  between second roof bow  142  and backlite frame  144 . This overcomes water leaks or wind noise caused by some traditional loose fitting roof covers, especially when the pliable cover may loosen over time due to the weight of snow or the like. Alternately, one or more tension-adjusting links  166  can be employed between any adjacent pair of roof bows.  
         [0038]    First roof bow  140  is secured to trailing edge  64  of stationary roof panel  24  using front latch  66  (see FIG. 4). Convertible roof  26  employs rear latches  170  to retain the rear portion of convertible roof  26 A in its fully extended position. Referring now to FIGS. 11 and 14, each rear latch  170  incorporates a locking pin  172 , having a transversely oriented, detented and spring loaded ball  173 , that detachably connects an offset lower flange  174  of backlite frame  144  within an adjacent vehicle body panel  104 . Thus, rotation of a handle  175  about a vertical pivot axis causes rear latch  172  to release the rear portion of the top stack mechanism from the vehicle body.  
         [0039]    Operating linkage mechanism  38 A involves steps similar to those used to operate linkage mechanism  38 . First, backlite  42  is retracted to its fully open position. Rear latch  170  is then actuated to disengage latch pin  172  from the hole  176  or recess located in vehicle body  104 . End  162  of base link  158  is then rotated upwards and toward the front of the vehicle as shown in FIG. 12. FIG. 13 shows convertible roof  26  in its fully upwardly retracted position. Convertible roof  26  is subsequently removed from vehicle  22  by unlatching front latches  66  and disengaging quick release pins  116  that connect base link  158  to bracket  160 . Convertible roof  26 A may thereafter be removed from the vehicle.  
         [0040]    A removable hard-top convertible roof  381 , illustrated in FIG. 16, can thereafter be attached to the vehicle by employing identically common attachment devices. In other words, first pin  82  (see FIGS. 4 and 16) of latch  66  is inserted into the corresponding recesses in trailing edge  64  of stationary roof panel  24 . The front convertible roof structure is secured to stationary roof panel  24  by engaging latch hook  96  with the recess located in the underside surface of stationary roof panel  24  and pivoting latch handle  88  towards the inside surface of stationary roof panel  24 . Moreover, quick release pins  116  are reengaged to establish a connection between a middle of convertible roof  381  and bracket  160 . Rear latches  170  are then rotated to connect a lower rear portion of convertible roof  381  with the vehicle body. Rear backlite  42  may thereafter moved between its open and closed positions if desired.  
         [0041]    While various aspects of the convertible roof system have been disclosed, it will be appreciated that many other variations may be incorporated without departing from the scope of the present invention. For example, alternate latching mechanisms and/or pins may be used to secure the first roof bow to the trailing edge of the stationary roof panel. Likewise, alternate latches and/or pins may be employed to retain the middle and rear portions of the convertible roof. Additionally, although the top stack linkage mechanism is disclosed as being manually operable, the top stack may also be automatically actuated using hydraulic or electric motor driven actuation mechanisms. Furthermore, various materials and linkages have been disclosed in an exemplary fashion, but other materials and linkages may of course be employed, although some of the advantages of the present invention may not be realized. It is intended by the following claims to cover these and any other departures from the disclosed embodiments which fall within the true spirit of the invention.