Abstract:
A screw with triple regions of shaft provides an optimal drilling/securing power as well as debris-containing capacity. To encompass the above opposing functions in a screw, the middle region is configured in two alternative ways: (1) Both the number of spiral threads per unit length and the outer diameter of spiral threads in the middle region are reduced to create more capacity for debris; and (2) Replace voluminous spiral threads with an equivalent space-saving securing structure of rhombic threads.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a screw. In particular, it relates to a tri-region screw with an effective drilling ability and debris removal capacity. 
         [0003]    2. Description of Related Art 
         [0004]    In general, a wood product is made of a superficial or top level of composite lumber, and an inner level of wood. When a screw is operated, the lower region of a screw drills through the top level of composite lumber before screwing into an inner wood level. At the time the lower region is inside the inner wood level, the upper region of a screw is most likely to be at the top level of composite lumber. 
         [0005]    Given the above-described screwing process, two important factors are always considered in designing screws. They are:
       (a) the density of spiral threads (i.e. the number of spiral threads per unit length) contained; and   (b) the capacity for containing debris (which are generated during screwing).       
 
         [0008]    The rule of thumbs is that the more closely disposed spiral threads are, the faster screwing speed shall be, and also the more securing a screw will be inside the wood. Nevertheless, the more closely disposed spiral threads are, the less room drilled by a screw is left for debris. As a result, the accumulated or “yet-to-be-digested” debris adversely acts as a resistance to screwing, and thereby slowing down the operation. 
         [0009]    To solve the above dilemma between density of spiral threads and capacity for debris, a conventional screw  1  is disclosed in U.S. Pat. No. 6,966,737 and shown in  FIG. 1 . U.S. Pat. No. 6,966,737 offers a solution to the above dilemma in a divided-in-half approach: the upper region of a screw shaft serves more for securing a screw, while the lower region, more for containing debris, in addition to the default drilling and securing function associated with spiral threads in the two regions. 
         [0010]    This divided-in-half approach is realized by reducing the volume of that specific region of a screw shaft specialized more in containing debris, resulting in a non-uniform screw shaft with two regions of different radius and different density of spiral threads disposed thereon. The screw  1  in  FIG. 1  comprises more spiral threads per unit length in the upper region, and less spiral threads in the lower region made of smaller radius. In other words, the upper region with dense spiral threads takes more responsibility in locking a screw; while an extra capacity for debris is created by reducing the volume of the lower region as well as the number of spiral threads, as discussed in more details below. 
         [0011]    The screw  1  in  FIG. 1  includes a head  11  and a shaft  12  connecting to the head  11 . The shaft  12  comprises a distal end  121 , a 1 st  thread region  122  and a 2 nd  thread region  123  spirally disposed thereon, and a cross-sectional area  124  disposed between the 1 st  thread region  122  and the 2 nd  thread region  123 . The number of threads per unit length in the 2 nd  thread region  123  exceeds that of the 1 st  thread region  122 . 
         [0012]    Having described U.S. Pat. No. 6,966,737, we proceed to summarize some other prior US Patents such as U.S. Pat. No. 7,037,059, Pat. No. 6,666,638, Pat. No. 6,074,149, and Pat. No. 5,816,012. All these four Patents reveal a screw with a three-region shaft (like the present invention). Nevertheless, the middle region disclosed in these patents are merely a section of naked shaft free of spiral threads, serving a single function—a debris container, and hence cannot be counted as an optimal solution to the conventional dilemma between density of spiral threads and debris-containing capacity. 
         [0013]    The key to solve the dilemma lies in creating an extra debris capacity at the minimal cost of reducing the numbers of spiral threads which work to drill a hole and to secure the screw in position. It is therefore the purpose of this invention to provide a screw retaining conflicting dual functions, drilling and securing vs. debris-containing capacity, at an optimal level. The screw thus configured is able to drill effectively into the wood (with or without spiral threads), and at the same time, efficiently removes debris upwards by creating an extra space without reducing the number of spiral threads. 
       SUMMARY OF THE INVENTION 
       [0014]    The present invention strategically solves the above-stated conventional dilemma in designing screws by resorting to: 
         [0015]    (1) rhombic threads, a thread-equivalent structure that retains the drilling and locking function of spiral spreads with minimal volume needed; or 
         [0016]    (2) diminishing the outer diameter of spiral threads (and hence the volume occupyed by spiral threads) without reducing the number of threads per unit length. 
         [0017]    A middle region (or a 3 rd  region) of a screw shaft in the present invention serves as a balancing point between the conventional dilemma. The middle region in the present invention plays a very different role from prior arts revealing a middle region of a screw shaft free of spiral threads. The middle region in this invention fulfill two essentially opposing functions in one region: drilling or locking vs. debris-containing; while the middle region in the prior arts acts simply as a debris container. 
         [0018]    To be more specific, in this invention, the drilling ability of a screw is retained by having a middle region configured with spiral threads of smaller diameter, or alternatively, with an equivalent drilling structure (such as rhombic threads) requiring less volume. Due to the less voluminous rhombic threads or spiral threads of smaller diameter, the debris-containing capacity increases which effectively “digests” or reduces the resistance of debris to drilling, and thereby accelerates the drilling speed. In other words, the 3 rd  region disposed in between the shaft of a screw functions as a driller, a debris container, and a locker. 
         [0019]    Being mainly as a debris container, two restrictions are placed on the configuration of spiral threads in the 3 rd  region: 
         [0020]    (a) The outer diameter of the threads in the 3 rd  region does not exceed that of the 1 st  region and the 2 nd  in order to allow more debris to stay in the 3 rd  region. 
         [0021]    (b) The number of threads per unit length in the 3 rd  region does not exceed that of its two neighbor regions in order to increase the debris container space. 
         [0022]    The screw in accordance with the present invention therefore comprises a head and a shank engaging the head. The shank has a distal end, a 1 st  region disposed near the distal end, a 2 nd  region located near said head, and a 3 rd  region sandwiched between the 1 st  and the 2 nd  region. The 3 rd  region could be implemented either with spiral threads or with rhombic threads, with the latter (i.e. the 3 rd  region with rhombic threads) allowing for maximal storage room for debris. 
         [0023]    The threads of the 1 st , 2 nd , and 3 rd  region all spiral in the same direction to facilitate drilling. As the upfront contact point with a wood product, the number of threads per unit length in the 1 st  region is less than that in the 2 nd  region for the 1 st  region to optimally contain or “digest” the debris and reduce the resistance created by debris. 
         [0024]    The outer diameter of the 3 rd  region is smaller than that of the 1 st  and 2 nd  region, creating a spacious container for the waste product of debris moving upwards from the 1 st  region. As a result, the resistance to drilling is effectively reduced, and the screwing action is accelerated. 
         [0025]    The 2 nd  region, interfacing directly with the outside, is designed with the most number of threads per unit length to ensure securing of the screw inside the wood product. 
         [0026]    The advantages of the present invention over the known prior arts is capitulated as follows:
       (1) The present invention, a tri-region screw, differs from U.S. Pat. No. 6,966,737 (a dual-region screw) in that the present invention increases the debris container space without changing the size (or radius) of the screw shaft. Instead, it simply reduces the diameters of spiral threads in the middle region. Or alternatively, it replaces the voluminous spiral threads with volume-saving rhombic spreads.   (2) It differs from U.S. Pat. No. 7,037,059, Pat. No. 6,666,638, Pat. No. 6,074,149, Pat. No. 5,816,012 (all of which are tri-region screws) in that the 3 rd  region (or the middle region) of these prior arts are merely a section of the naked shaft without spiral threads, whose only function is to contain debris. Whereas by having spirals threads with diminished outer diameter, or alternatively, by having rhombic threads, the 3 rd  region thus configured possess triple functions—drilling, debris-containing, and securing, depending on different stages of screwing.       
 
         [0029]    The present invention will become even more apparent to those of ordinary skilled in the art upon reading the following descriptions in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  is a perspective view illustrating a screw of the U.S. Pat. No. 6,966,737; 
           [0031]      FIG. 2  is a perspective view illustrating a preferred embodiment of the present invention; 
           [0032]      FIG. 3  is a perspective view illustrating the preferred embodiment in  FIG. 2  in locking position; and 
           [0033]      FIG. 4  is a perspective view illustrating a second preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0034]    Referring to  FIG. 2 , the preferred embodiment of the present invention comprises a head  21  and a shank  22  engaging the head  21 . The head  21  has a top surface  211 , a bottom surface  212 , and a periphery  213  disposed between the top surface  211  and the bottom surface  212 . The bottom surface  212  is larger than the top surface  211 . The head  21  forms an undercut  214  on the bottom surface  212 ; the periphery of the head  21  is inclined. 
         [0035]    The ratio of the maximum outer diameter of the head  21  to that of the shank  22  is somewhere between 1.787:1 and 2.13:1. The shank  22  has a distal end  221 , a 1 st  region  222  disposed near the distal end  221 , a 2 nd  region  223  disposed near the head  21 , and a 3 rd  region  224  disposed between the 1 st  region  222  and the 2 nd  region  223 . The 3 rd  region  224  contains a plurality of threads spirally disposed on the shank  22 . The threads  2221  of the 1 st  region  222 , the threads  2231  of the 2 nd  region  223 , and the threads  2241  of the 3 rd  region  224  spiral in the same direction. 
         [0036]    The number of threads per unit length in the 2 nd  region  223  exceeds that in the 1 st  region  222 . The outer diameter of the 3 rd  region  224  is smaller than that of the 1 st  region  222  and the 2 nd  region  223 . The inner diameter of the 1 st  region  222  equals to that of the 2 nd  region  223 . The number of threads per unit length in the 3 rd  region  224  equals to that of threads per unit length in the 1 st  region  222 . Each thread  2221 ,  2231 , and  2241  in the 1 st  region  222 , the 2 nd  region  223 , and the 3 rd  region  224  forms a thread peak  2224 ,  2234 ,  2243 , respectively. 
         [0037]    Partial threads  2221  of the 1 st  region  222  form a downward-inclined face  2222 . The opposite partial threads  2221  form an upward-inclined face  2223 . The size of the angle d 1  (formed by the upward-inclined face  2223  and the thread peak  2224 ) is different from that of the angle d 2  (formed by the downward-incline face  2222  and the thread peak  2224 ). To be more specific, the angle d 1  formed (by the downward-inclined face  2222  and the thread peak  2224  is smaller than the angle d 2  (formed by the downward-inclined face  2223  and the thread peak  2224 ). 
         [0038]    Likewise, partial threads  2231  of the 2 nd  thread region  223  form a downward-inclined face  2232 ; the opposite partial threads  2231  form an upward-inclined face  2233 . The size of the angle d 3  between the upward-inclined face  2233  and the thread peaks  2234  is different from that of the angle d 4  (formed by the downward-inclined face  2232  and the thread peak  2234 ). To be more specific, the angle d 3  (formed by the upward-inclined face  2233  and the thread peaks  2234 ) is larger than the angle d 4  (formed by the downward-inclined face  2232  and the thread peak  2234 ). 
         [0039]    Referring to  FIG. 3 , while in operation, each thread region screws into a composite lumber  3  and a wood product  4  in order. Because the threads  2221  of the 1 st  region  222 , and the threads  2231  of the 2 nd  region  223  all spiral in the same direction, the 2 nd  region  223  will not extend the hole being screwed into. Due to the differences in size of the angle d 1 , d 2 , d 3 , and d 4 , the 2 nd  region  223  will also be tightly screwed into the composite lumber  3 , and the screw  2  will be rapidly screwed into the composite lumber  3  and the wood product  4 . 
         [0040]    With the outer diameter of the 3 rd  region  224  being smaller than that of the 1 st  region  222  and the 2 nd  region  223 , the room for containing debris in the 3 rd  region  224  is augmented. When in operation, most debris will be pushed out rapidly with some debris accumulated in the spacious 3 rd  region  224 . This greatly reduces the resistance to screwing and speeds up the process. 
         [0041]    Finally, with the head  21  being somewhat smaller and the periphery  213  of the head  21  being inclined, the screw  2  will sink into the composite lumber  3  in a fast and flat manner, keeping flatten the surface of the composite lumber  3 . 
         [0042]      FIG. 4  shows a 2 nd  preferred embodiment of the present invention. The differences between this embodiment and the first embodiment is that the 3 rd  region  224  now being a plurality of rhombic threads  2242  disposed on the shank  22 . This second embodiment achieves a very similar effect as the first embodiment. The spacious room formed by rhombic threads  2242  has a large capacity for containing debris. When in operation, the screw  2  fastens tightly with the object due to the rhombic threads  2242 , which are surrounded by debris in all directions. 
         [0043]    Having shown and described the embodiments in accordance with the present invention, it should be clear to those skilled in the art that further embodiments may be made without departing from the scope of the present invention.