Abstract:
A portable towing apparatus for towing a disabled golf cart. The towing apparatus is preferably mounted onto the rear portion of a golf cart that is to be used in towing the disabled golf cart. The towing apparatus comprising, in part, a substantially inverted T-shaped member that has attached thereto a strut assembly and a gimbal cross member. A winch is provided and is mounted onto the inverted T-shaped member. The gimbal cross member engages the disabled golf cart and the winch in concert with the strut assembly disposes the disabled golf cart for towing. The gimbal movement of the cross member enables the golf cart under tow to traverse all types of terrain.

Description:
FIELD OF THE INVENTION 
     The invention generally relates to an apparatus and method for towing. In particular the invention relates to a portable apparatus and method for towing a golf cart. More particularly the invention relates to a portable, golf cart mountable, apparatus for towing a disabled golf cart from a golf course and the method of use thereof. 
     BACKGROUND OF THE INVENTION 
     A golf course and clubhouse represent a considerable investment for the owners. The meticulously kept grounds that surround the golf course and clubhouse are one of the reasons why golfers frequent a particular golf course. Of course, the principle reason is to engage in or play a game of golf but the grounds and clubhouse serve to facilitate the golfer&#39;s interest. 
     The golf course itself is designed with the golfers&#39; best interest in mind. The golf course is generally designed as a challenge for golfers and the aesthetics of the course are generally indicative of the cost of membership. Generally, the higher the cost of membership, the higher the aesthetics of the golf course and consequently, the higher the maintenance of the course. The maintenance of the golf course generally involves a vehicle of some type. Most golf courses are designed for a minimum of vehicle traffic. The primary vehicles found on a golf course are tractors for mowing the grass and golf carts for transporting golfers about the course. 
     If for any given reason, a golf cart becomes disabled while out on the golf course, it must be returned to a maintenance facility for repair. Removing the disabled golf cart from the interior of a golf course may very well be a challenge. If a standard truck-like wrecker is employed to tow the disabled golf cart back to the maintenance facility, the grounds of the course may suffer from the weight of the wrecker. The roads in the interior of the golf course are designed for golf cart traffic and do not easily accommodate truck traffic. If the grounds keeping tractor is used, it must first be transformed into a vehicle that can tow a golf cart. Simply attaching one end of a rope or chain to the tractor and the other end to the disabled golf cart is not feasible. Towing a disabled golf cart with a chain or rope up and down hills or inclines is inherently unsafe and may be destructive to both the tractor and the golf cart. 
     It would be desirable to have a towing system that could be easily assembled and portable. The towing system would also be of convenient size to enable mounting onto a standard golf cart. The golf cart would become a golf cart wrecker for towing disabled golf carts off the golf course to a maintenance facility. 
     SUMMARY OF THE INVENTION 
     The golfing industry is well aware of the importance of the aesthetics of a golf course. They are also keenly aware of the cost of maintaining the grounds of the golf course and the equipment such as golf carts. Golf carts that become disabled for a variety of reasons need to be quickly removed from the golf course. Once the golf cart is removed from the course, it may be repaired in a maintenance facility. The economies of quickly removing a golf cart and repairing it are self-evident. When a golf cart is disabled, it cannot be rented to golfers who desire to be transported about the golf course. 
     The present invention provides a portable towing system that is delineated into three distinct subassemblies. The towing system when assembled forms an apparatus that converts an ordinary golf cart into a golf cart wrecker. The towing system may also be easily disassembled and stored when not in use. 
     In the preferred embodiment of the present invention, the first subassembly comprising at least one elongated vertical member is connected to at least one elongated horizontal member. The union of these two members forms an inverted T-shaped structure or member. The inverted T-shaped member may be positioned in the rear of the golf cart in the area normally provided for golf clubs. Straps may be employed to secure the inverted T-shaped member to at least one canopy support rod of the golf cart. An L-shape member is connected to the inverted T-shaped member and extends outward from the rear of the golf cart. 
     The second subassembly is a strut assembly that is pivotally connected along the aforementioned L-shaped member. The strut assembly extends outward from the L-shaped member thereby providing mechanical advantage to the inverted T-shaped member. A winch is mounted along the vertical member and is connected to the strut assembly by a cable and pulley arrangement. 
     The third subassembly of the towing system is a cross member assembly that is gimbally connected to the strut assembly. The gimbal cross member engages the disabled golf cart and provides stability and security to the cart while in route to a maintenance facility for repair. When the winch is engaged the strut assembly pivots the cross member arcuately. The arcuate motion of the cross member has the effect of raising or lowering the disabled golf cart. The gimbal motion of the cross member enables the golf cart under tow to be easily maneuvered over all types of terrain. 
     In the second embodiment of the present invention, the first subassembly comprising at least one elongated vertical member is connected to at least one elongated horizontal member. The union of these two members forms an inverted T-shaped structure or member. The inverted T-shaped member may be positioned in the rear of the golf cart in the area normally provided for golf clubs. At least one strap may be employed to secure the inverted T-shaped member to at least one canopy support rod of the cart. The inverted T-shaped member has an extension arm connected along its vertical member. The extension arm projects outwardly from the inverted T-shaped member. This outward projection by the extension arm provides the towing system with an extended mechanical advantage. An L-shape member is connected to the inverted T-shaped member below the extension arm and extends outward from the rear of the golf cart. 
     The second subassembly of the second embodiment is a strut assembly. The strut is pivotally connected to the L-shaped member of the inverted T-shaped member. The strut assembly extends outward from the L-shaped member thereby providing mechanical advantage to the inverted T-shaped member. A winch is mounted along the extension arm and is connect to the strut assembly by a cable and pulley arrangement. 
     The third subassembly of the towing system is a cross member assembly that is gimbally connected to the strut assembly. The gimbal cross member engages the disabled golf cart and provides stability and security to the cart while in route to a maintenance facility for repair. When the winch is engaged the strut assembly pivots the cross member arcuately. The arcuate motion of the cross member has the effect of raising and lowering the disabled golf cart. The gimbal motion of the cross member enables the golf cart under tow to be easily maneuvered over all types of terrain. 
     When taken in conjunction with the accompanying drawings and the appended claims and reading the following detailed description of the embodiments of the invention, other features and advantages of the present invention will become apparent. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is illustrated in the drawings in which like reference characters designate the same or similar parts throughout the figures of which: 
     FIG. 1 illustrates a perspective view of the preferred embodiment of the present invention, 
     FIG. 2 illustrates a side view of the preferred embodiment of FIG. 1 engaging a vehicle to be towed, 
     FIG. 3 illustrates a partially exploded side view of the preferred embodiment of FIG. 2, 
     FIG. 4 illustrates a partially exploded side view of the second embodiment of the present invention, 
     FIG. 5 illustrates a side view of the preferred embodiment of FIG. 2, 
     FIG. 6 illustrates a side view of the second embodiment, 
     FIG. 7 illustrates a front view of the preferred embodiment of the present invention, 
     FIG. 8 a  illustrates a front view of the cross member assembly of FIG. 7, with the cross member pivoted to one side, 
     FIG. 8 b  illustrates a front view of the cross member assembly of FIG. 8 a,  with the cross member pivoted to the other side, 
     FIG. 8 c  illustrates a front view of the cross member assembly of FIG. 8 b,  with the cross member pivoted to center. 
     FIG. 8 d  illustrates a front view of the cross member assembly of FIG. 8 b,    
     FIG. 8 e  illustrates a front view of the cross member assembly of FIG. 8 b,    
     FIG. 9 a  illustrates a side view of the method of use of the preferred embodiment of the present invention, 
     FIG. 9 b  illustrates a side view of the present invention of FIG. 9 a,    
     FIG. 9 c  Illustrates a side view of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment of the present invention is a towing system for small to medium size vehicles, generally illustrated at  10 , FIG.  1 . The vehicle  12  may be any type of vehicle that allows the towing system  10  to be mounted thereon. Vehicle  12  may, if desired, be a golf cart that is normally used to transport golfers about a golf course. Golf cart  12  requires no special modifications to receive the towing system  10 . 
     The towing system  10  may, if desired, be mounted anywhere on the frame or body of the golf cart  12  that allows the operation of the towing system  10 . The towing system  10  may, if desired, be mounted at the rear of vehicle  12  to easily engage a vehicle  14 , FIG. 2, that is in need of towing. The attaching of the towing system  10 , FIG. 1, to the golf cart  12  transforms golf cart  12  into a golf cart wrecker  12 . Preferably, the towing system  10  is mounted to the rear of the golf cart wrecker  12 . The golf cart wrecker  12  is used primarily on golf courses to tow disabled golf carts to a maintenance facility for repair. 
     The towing system  10 , FIG. 1, (discussed in detail herein) is positioned in a space normally occupied by golf club bags. The towing system  10  may, if desired, be releasably secured to the body or frame of the golf cart wrecker  12 . Once the towing system  10  is positioned in the cart, a portion of the system extends outward from the rear of the golf cart wrecker  12 . This portion is releasably secured to the rear bumper or frame of the golf cart wrecker  12 . The towing system  10  is now secured to the golf cart wrecker  12  and is ready for towing the disabled golf cart. 
     The second embodiment of the present invention is a towing system and is generally illustrated at  98 , FIG.  6 . The towing system  98  enables the user of the system to vertically transfer the tongue weight of the vehicle being towed from the base of the towing system to the top most portion of the towing system. The towing system  98  like the towing system  10  is mounted onto a golf cart, transforming the golf cart into a golf cart wrecker. 
     The towing system  10  comprises three distinct, portions, subassemblies or subsystems that may, if desired, be conveniently assembled or dissembled for transporting and for attaching to the golf cart wrecker  12 . The first subsystem  20 , FIG. 3, comprises an inverted T-shaped member  18  and winch assembly  19 . The inverted T-shaped member  18  may, if desired, be fabricated from a horizontal member  22  and a vertical member  24  or, if desired, be cast into a single inverted T-shaped member. The vertical member  24  may, if desired, be connected anywhere along the horizontal member  22  that allows the towing system  10  to operate. Preferably, the vertical member  24  is medially connected to the horizontal member  22  to form the inverted T-shaped member  18 . The material used to fabricate the inverted T-shaped member  18  may, if desired, be any material that allows the towing system  10  to operate. The vertical member  24  and the horizontal member  22  are fabricated from stock steel tubing having a diameter of 1 to 3 inches. Preferably, the stock steel tubing is 1½ inches in diameter. The vertical member  24  and the horizontal member  22  are then welded together using welding techniques known in the art. 
     The inverted T-shaped member  18  has a winch assembly  19  mounted thereto. The winch assembly  19  may, if desired, be mounted in any convenient place on the inverted T-shaped member  18 . Preferably, the winch assembly  19  is mounted at the end of the vertical member  24  opposite from the horizontal member  22 . The winch assembly  19  may, if desired, be motorized for the convenience of the user or it may have a hand crank  26  for raising and lowering the disabled golf cart. 
     The inverted T-shaped member  18  has a concave polyhedral member or a D-shaped ring  28  connected thereto for attaching straps  30  and  32 , FIG.  1 . The straps  30  and  32  loop around the D-ring  28  and secure the inverted T-shaped member to the golf cart wrecker  12 . A pair of stop block members  34  and  36  are connected to the inverted T-shaped member  18  in any convenient place. The stop block members  34  and  36  may, if desired, be connected to the inverted T-shaped member  18  to provide support and in concert with the straps  30  and  32  secure the inverted T-shaped member to the golf cart wrecker  12 . The stop block members  34  and  36  may, if desired, be fabricated from the same or different material as the inverted T-shaped member  18 . The inverted T-shaped member  18  further comprises two pairs of safety chains  38 ,  40  and  42 ,  44  connected thereto for added retention or safety during the towing of the disabled golf cart. 
     A substantially L-shaped member  46 , FIG. 3, is connected to the inverted T-shaped member  18  in any convenient position that allows the towing system  10  to operate. Preferably, the L-shaped member  46  is connected along the vertical member  24  and is oppositely spaced from the winch assembly  19 . The L-shaped member  46  has one end  48  that extends over the bumper  50  of the golf cart wrecker  12 . The L-shaped member  46 , by extending over the bumper  50 , provides support for the inverted T-shaped member  18 . The L-shaped member  46  also has a moveable or adjustable locking mechanism  52 . The locking mechanism  52  may adjustably slide along the L-shaped member  46  providing various securing positions according to the type of golf cart wrecker  12  that is used. Once a desirable position is found, a retaining screw  54  is tightened thereby securing the L-shaped member  46  and the inverted T-shaped member  18  to the golf cart wrecker  12 . 
     The second subsystem  56 , FIG. 3, is a pivotal strut member. The pivotal strut member  56  comprises a vertical member  60  connected along a horizontal member  62 . The vertical member  60  is spaced from the fulcrum point connecting the L-shaped member  46  and the pivotal strut member  56  in such a way as to impart the maximum mechanical advantage. The pivotal strut member  56  is positioned 10 to 24 inches from the aforementioned fulcrum point. Preferably, The pivotal strut member  56  is positioned 15 inches from the aforementioned fulcrum point. The vertical member  60  may, if desired, be supported by brace  64  or  66 . Both braces  64  and  66  may, if desired, support the vertical member  60 . The pivotal strut member  56  may be fabricated from the same or different material as the inverted T-shaped member  18 . 
     The pivotal strut member  56  is pivotally connected to the L-shaped member  46  by any convenient method known in the art. Preferably, pin  72  traverses one end of the pivotal strut member  56  and the L-shaped member  46  thereby pivotally connecting them together. A securing nut  73  may, if desired, connect to pin  72  for securing the two members together. The pivotal strut member  56  is free to rotate about the fulcrum point delineated by the insertion of the pin  72  connecting the two members together. 
     The third subsystem  74 , FIG. 3, is a gimbal cross member. The gimbal cross member  74  is gimbally connected to the pivotal strut member  56  in any convenient arrangement that allows the towing system  10  to operate. The gimbal cross member may, if desired, be gimbally connected in the same plane as the strut member  56 . The gimbal cross member  74 , FIG. 1, comprises an elongated member  75  that has two oppositely spaced end members  76  and  86  connected along its horizontal lengths. The end members may, if desired, be connected to the ends of the elongated member  75 . At least one supporting stop block  78  is connected about the end portion of the union of elongated member  75  and end member  76 . If desired a second support end block  88  may be connected to the union of elongated member  75  and end member  86 . A pair of stop blocks  136  and  138  may be added to members  86  and  76  respectively for grasping the cross member of the disabled vehicle being towed. A U-shaped bracket  80  is connected along the elongated member  75  in any convenient place that allows operable connection to the towing system  10 . Preferably, U-shaped bracket  80  is medially connected between the stop blocks  78  and  88 . The pin  84  traverses the U-shaped bracket  80  and the pivotal strut member  56 . The U-shaped bracket  80  in concert with pin  84  provides the connection mechanism for the gimbal cross member  74  and the pivotal strut member  56 . The pivotal cross member  74  may, if desired, be fabricated from the same or different material as the inverted T-shaped member  18 . 
     The gimbal cross member  74 , FIG. 7, is free to rotate about the connecting pin  84 , FIGS. 8 a  and  8   b.  The angle of rotation about the axis formed in the horizontal plane by the intersection of the pivotal strut member  56 , pin  84 , and the gimbal cross member  74  may, if desired, be any angle between 0° and 180°. The angle of rotation about the axis formed in the vertical plane by the intersection of the U-shaped bracket  80 , pin  82 , and the gimbal cross member  74  may, if desired, be any angle that allows cross member  74  to freely rotate. Preferably, the angle of rotation is in the range of 5° and 25° (degrees), FIG. 8 c.  The intersection of the pivotal strut member  56 , the U-shaped bracket  80  and the gimbal cross member  74  in concert with pins  84  and  82  form two mutually perpendicular and intersecting axes of rotation, thus providing free angular movement or gimbal movement in two directions, FIGS. 8 d  and  8   e.  The two degrees of freedom of movement produced by the gimbally mounted gimbal cross member  74  allow for the rolling and yawing correction of movement of the golf cart  14  under tow. In normal operation, the cross member  74  grasps the axle of the disabled golf cart  14  to be towed. The size and relative position of the axle to the disabled car and the unison of movement between the golf cart wrecker  12  and the disabled golf cart  14  determine the relative or radius of rotation of the gimbal movement of the gimbal cross member  74 . 
     A cable  92 , FIG. 3, has one end affixed to the winch assembly  19  and the other end to a detachable retaining device  94 . The cable  92  passes through a plurality of pulleys each exhibiting varying degrees of mechanical advantage before connecting to a hook  146 . Preferably, the cable  92  passes through pulley  96  and a pulley  70  before connecting to a hook  146 . Preferably, the pulleys  96  and  70  are strategically positioned on the inverted T-shaped member  18  and the pivotal strut member  56  respectively to provide maximum mechanical advantage to the towing of the disabled golf cart. A typical position for pulley  96  on the inverted T-shaped member  18  is 35 inches above the horizontal member  22 . A typical position for pulley  70  on the pivotal strut member  56  is 40 inches above the horizontal member  62 . The detachable retaining device  94  may, if desired, be a hook with a safety clasp connected thereto. 
     The Second Embodiment of the Towing System 
     The second embodiment of the present invention is a towing system, generally illustrated at  98 , FIG.  4 . The towing system  98  generally has three distinct portions, subassemblies or subsystems that may, if desired, be conveniently assembled or disassembled for transporting and for attaching to the golf cart wrecker  12 . The subsystem  100 , FIG. 4, comprises a inverted T-shaped member  102  with an extension arm  114 . The inverted T-shaped member  100  may, if desired, be fabricated from a horizontal member  106 , a vertical member  102 , and an angularly displaced extension arm  114 . Alternatively, if desired, the subsystem  100  may be cast into a single inverted T-shaped with an extension arm  114 . 
     The vertical member  102  may, if desired, be connected anywhere along the horizontal member  106  that allows the towing system  98  to operate. Preferably, the vertical member  102  is medially connected to the horizontal member  106  to form the inverted T-shaped member  18 . The extension arm  114  is connected at one end of the vertical member  102 . The extension arm  114  is angled from the vertical member  102  in such a way as to provide the maximum mechanical advantage to the towing system  98 . A typical angle for the extension arm  114 , measured from the vertical member  102 , is in the range of 45° to 135° (degrees). The material used to fabricate the inverted T-shaped member  100  may, if desired, be any material that allows the towing system  98  to operate. The vertical member  102 , the horizontal member  106 , and the extension arm  114  are fabricated from stock steel tubing having a diameter of 1 to 3 inches. Preferably, the stock steel tubing is 1½ inches in diameter. The vertical member  102 , the horizontal member  106 , and the extension arm  114  are then welded together using welding techniques known in the art. 
     The extension arm  114  has a winch assembly  104  mounted thereto. The winch assembly  104  may, if desired, be mounted in any convenient place along the extension arm  114 . Preferably, the winch assembly  104  is mounted at the end of the extension arm  114  adjoining the vertical member  102 . The winch assembly  104  may, if desired, be motorized for the convenience of the user or it may have a hand crank (not shown) for raising and lowering the disabled golf cart. 
     The inverted T-shaped member  100  has a D-shaped ring  108  connected thereto for attaching straps  30  and  32 , FIG.  1 . The straps  30  and  32  loop around the D-ring  108  and secure the inverted T-shaped member  100  to the golf cart wrecker  12 . A pair of stop block members  110  and  112  are connected to the inverted T-shaped member  100  in any convenient place. Preferably, the stop block members  110  and  112  are connected to the inverted T-shaped member  100  to provide support and in concert with the straps  30  and  32  secure the inverted T-shaped member  100  to the golf cart wrecker  12 . The stop block members  110  and  112  may, if desired, be fabricated from the same or different material as the inverted T-shaped member  100 . 
     A substantially L-shaped member  116 , FIG. 4, is connected to the inverted T-shaped member  100  in any convenient position that allows the towing system  98  to operate. Preferably, the L-shaped member  116  is connected along the vertical member  102  and is oppositely spaced from the winch assembly  104  of the inverted T-shaped member  100 . The L-shaped member  116  has one end  118  that extends over the bumper  50  of the golf cart wrecker  12 . The L-shaped member  116  provides support for the inverted T-shaped member  100 . The L-shaped member  116  also has a moveable or adjustable locking mechanism  120 . The locking mechanism  120  may adjustably slide along the L-shaped member  116  providing various securing positions according to the type of golf cart wrecker  12  that is used. Once a desirable position is found, a retaining screw  122  is tightened thereby securing the L-shaped member  116  and the inverted T-shaped member  100  to the golf cart wrecker  12 . 
     The second subsystem  124 , FIG. 4, is a pivotal strut member. The pivotal strut member  124  is pivotally connected to the L-shaped member  116  by any convenient method known in the art. Preferably, pin  126  traverses one end of the pivotal strut member  124  and the L-shaped member  116  thereby pivotally connecting them together. A securing nut  127  may, if desired, connect to pin  126  for securing the two members together. The pivotal strut member  124  is free to rotate about the fulcrum point delineated by the insertion of the pin  126  connecting the two members together. 
     The third subsystem  74 , FIG. 4, is a gimbal cross member. The gimbal cross member  74  in this embodiment is identical to the gimbal cross member  74  discussed above. In this embodiment the gimbal cross member  74  operatively connects to the strut assembly  124  as discussed above. 
     A cable  130 , FIG. 4, has one end affixed to the winch assembly  104  and the other end to a detachable retaining device  94 . The cable  130  passes through a plurality of pulleys each exhibiting varying degrees of mechanical advantage before connecting to a hook  146 . Preferably, the cable  130  passes over pulley  132  and around a pulley  134  before connecting to a hook  146 . The pulleys  132  and  134  are strategically positioned on the inverted T-shaped member  100  and the pivotal strut member  124  to provide maximum mechanical advantage to the towing of the disabled golf cart. A typical position for pulley  132  is on the end of the extension arm  114 . The typical position for pulley  134  is on the pivotal strut member  124  and is 10 to 20 inches from the gimbal cross member  74 . Preferably, the pulley  134  is 13 inches from the gimbal cross member  74 . The detachable retaining device  94  may, if desired, be a hook with a safety clasp connected thereto. 
     Operation of the Present Invention 
     The present invention is a towing system for disabled golf carts. Both embodiments of the invention, FIGS. 5 &amp; 6 and delineated herein are operated in relatively the same manner. They both transport disabled golf carts found on the golf course back to the maintenance facility. For illustrative purposes, only the operation of the first embodiment  10  is discussed. 
     The towing system  10 , FIG. 9 a,  may, if desired, be permanently installed on a golf cart or it may remain portable. If the towing system  10  is desired to be portable it may be transported or mounted onto any golf cart. 
     The towing system  10 , FIG. 9 a,  is mounted to the rear of cart  12 . Normally, golf club bags occupy the space or cavity  140  now occupied by the towing system  10 . The straps  30  and  32  traverse the D-shaped ring  28  and canopy poles  142  and  144  respectively and securing the towing system  10  to the cart  12 . The locking mechanism  52  is properly positioned against the rear bumper  50 . The towing system  10  is now securely installed onto the cart  12  and is ready to engage a disabled golf cart and tow it to the maintenance facility. 
     The user of the towing system  10  positions the system to the front or to the rear of the disabled golf cart. The towing system  10  functions equally well in towing disabled carts from the rear or from the front. The user grasps handle  26  of the winch  19  and begins to angularly lower the gimbal cross member  74 . The gimbal cross member  74  in concert with the strut assembly  56  pivots about pin  72 . The pivotal action and the unwinding of cable  92  causes the leading edge of the gimbal cross member  74  to approach the surface or plane containing the disabled golf cart. 
     A typical golf cart has a front suspension, crossbar, or other framework attached to the cart. A typical example is crossbar  145  positioned to the front of disabled golf cart  14 . The gimbal cross member  74  is positioned under the crossbar  145 , FIG. 9 b.  The user reverses the direction of the winch  19  causing the gimbal cross member  74  to engage the crossbar  145 , lifting the disabled golf cart  14  upward. The actual distance the front end of the disabled golf cart is lifted above the ground is a matter of convenience for the operator of the towing system  10  and the ability of the cart to be towed. If desired, safety chains  150  and  148 , FIG. 9 c,  may be used to secure, via hooks  90  and  91 , FIG. 7 the towing system  10  to the golf cart  14 . The disabled golf cart  14  is now ready for transport to the maintenance facility. 
     Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims, means-plus-function clause is intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.