Abstract:
A length of flexible helically corrugated rubber tubing is provided with certain characteristics that enable it to be axially compressed to less than half of its normal relaxed length using minimal force while still providing a high resistance to crushing. The tubing has a wall with a uniform thickness, the wall defining external and internal threads with a specially defined pitch and thread height related to the wall thickness.

Description:
BACKGROUND OF INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to crush-resistant flexible tubing for conveying gases. The tubing is formed of elastomeric material that initially is in an uncured condition and, when formed and cured, provides a strong but flexible length of tubing. More particularly, the invention relates to tubing that has helical corrugations along its length and also, relates to a method for making the tubing. 
         [0003]    2. Description of Related Art 
         [0004]    Corrugated tubing of the general type to which the invention relates is both flexible and strong, yet still retains its tubular form in a semi-rigid condition. This is accomplished by providing the tubing with helical corrugations. This type of tubing is used in a variety of home and industrial applications, such as for vacuum cleaning systems, engine exhaust systems for automotive service facilities and for numerous other purposes. 
         [0005]    In accordance with prior practice, helically corrugated tubing may be made by first sliding an extruded sleeve of uncured rubber axially over a rotatable mandrel, the mandrel having a continuous thread formed on its outer surface. When in place on the mandrel, the sleeve is forced into the helical groove or root of the thread by a length of cord wrapped around the sleeve as the sleeve rotates with the mandrel. This serves to impart a desired corrugated shape to the uncured rubber sleeve. 
         [0006]    The resulting assembly is then removed from its rotary support preparatory to curing. In this condition the rubber sleeve is cured in an oven or autoclave to set the helical corrugations. The result is a cured length of rubber tubing with helical convolutions. 
         [0007]    The cord is removed from the corrugated tubing by placing the assembly back on a rotary support and rotating the mandrel in the reverse direction while unwinding the cord from the cured tube. After the removal of the cord, the corrugated tube length may be removed from the mandrel by introducing air under pressure between the outer surface of the mandrel and the inner surface of the corrugated tube. The overall method described above is generally known as the “cording” process and is shown and described in more detail in U.S. Pat. No. 2,832,086. The process of removing the cured tube from the mandrel using pressurized air is shown and described in U.S. Pat. No. 2,888,712. 
         [0008]    The resulting tubular product with helical convolutions is both flexible and resilient but rigid enough to resist crushing such as when the tube is stepped on. Also, the tube has wall with a generally uniform thickness, the wall defining continuous internal and external threads with alternating crests and roots along the tube length. In other words, the wall portion that defines a crest portion of the external thread also defines on its opposite side, the root portion of the internal thread. 
         [0009]    As viewed in longitudinal or axial section, the wall has a sinusoidal form with alternating crests and roots as to both the internal and external surface portions. Both the internal and external crests are rounded or semi-circular as viewed in longitudinal section and have about the same shape (i.e. about the same radius). Likewise, both the internal and external roots are rounded or semi-circular as viewed in longitudinal section and have about the same shape (i.e. about the same radius). However, the radii of the external and internal crests are greater than the radii of the respective roots, the difference being about equal to the thickness of the tubular wall. 
         [0010]    Also, in past practice, the sinusoidal shape of the wall as viewed in axial section is such that the center of the semi-circular crest shape for the outer surface has about the same radial distance from the axis of the tube as the center of the semi-circular crest shape for the inner surface. In order to achieve this characteristic for the tubing, the thread has a very small pitch that is achieved by having a minimal spacing between the convolutions of the cord when the tube is formed on the rotating mandrel. These related characteristics result in a high resistance to axial compression of the tube. At the same time, these characteristics produce a high resistance to crushing. 
         [0011]    While resistance to axial compression is desirable for many applications, there are some circumstances where it would be desirable to be able to axially compress crush-resistant flexible tubing for conveying gases with minimal force and retain it in a collapsed condition (e.g., for storage and transport). One application where axial shortening or compression of the corrugated tubing would be beneficial, if not essential, is in the case of mobile military field equipment. In modern combat operations, it is necessary to be prepared for chemical and/or biological attack. With certain current types of biochemical weapons, gas masks alone would not protect against casualties. In the case of some biochemical agents, any contact with the body could disable the soldier. 
         [0012]    Accordingly, to protect against biochemical warfare, military vehicles must have air purification equipment to provide safe air to the vehicle&#39;s occupants. Also, sealed enclosures (e.g. tents) with air purification capability are needed. To achieve this, it is necessary to have means to connect either a vehicle&#39;s own air purification system or a separate mobile air purification unit to an adjacent sealed enclosure. Flexible corrugated tubing of the general type described above is ideally suited for this purpose, provided it may be axially compressed with minimal force to reduce its size for storage and transport. This is particularly important in the case of combat vehicles and other mobile combat equipment because space is at a premium. 
         [0013]    Accordingly creative modifications to the design and production of helically corrugated tubing using the “cording” process have been developed to meet the requirements described above. The tubing and process for making it as embodied in the present invention achieve these results and afford other features and advantages heretofore not obtainable. 
       BRIEF SUMMARY OF THE INVENTION 
       [0014]    In accordance with the invention, crush resistant helically corrugated tubing is provided with certain dimensional characteristics to enable the tubing to be axially compressed while still retaining a high resistance to crushing. More particularly, the tubing so formed, is axially compressible from its normal relaxed length to a collapsed condition that is from about 20% to about 80%, and in some embodiments from about 50% to about 75%, shorter than its normal length upon application of a load of about 25 lbs. Thus, the ratio of a length of tubing in a normal relaxed state compared to a length of the same tubing in an axially compressed state will range from about 5:1 to about 1.25:1, and in some embodiments, from about 4:1 to about 2:1. The force needed to compress the tubing is relatively small and can easily be applied by an average person. 
         [0015]    The tubing has a wall with a uniform thickness, the wall defining a continuous external thread with an external crest and root and a corresponding continuous internal thread with an internal crest and root. The size of the convolution must be larger than the size of convolutions utilized in prior art tubing. In one embodiment, the thread has a pitch of about six to ten times the wall thickness and a thread height, from root to crest, of about three to five times the wall thickness. The utilization of larger convolutions provide the desired axial compressibility while still maintaining resistance to crushing. 
         [0016]    In accordance with the method of the invention, helically corrugated tubing having the above described characteristics is made by first sliding an extruded sleeve of uncured rubber axially over a rotatable mandrel. The mandrel has continuous thread with a crest and root, formed on its outer surface sufficient to produce a convolution of sufficient size to allow for axial compressibility. In one embodiment, the thread has a pitch of about six to ten times the wall thickness of the extruded sleeve and a thread height from root to crest of about three to five times the wall thickness. When in place on the mandrel, the sleeve is forced into the helical root of the thread by a length of cord wrapped around the sleeve as the sleeve rotates with the mandrel. 
         [0017]    The resulting assembly is placed in an oven to cure the rubber sleeve and set helical corrugations. The assembly is then removed from the curing oven and the cord is removed from the cured rubber tubing by reverse rotation of the mandrel. Finally, the cured rubber tubing is removed from the mandrel using techniques well known in the art and as described above. 
         [0018]    The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a fragmentary side elevation of a length of helically corrugated rubber tubing of prior art design, produced in accordance with the prior art “cording” method, with a portion broken away and shown in section for the purpose of illustration. 
           [0020]      FIG. 2  is a fragmentary side elevation of a length of helically corrugated rubber tubing embodying the invention and showing the tubing in its normal relaxed condition and with one portion broken away and shown in section for the purpose of illustration. 
           [0021]      FIG. 3  is a fragmentary side elevation of the length of tubing of  FIG. 2 , showing the tubing axially compressed to a substantially shortened length and with a portion broken away and shown in section for the purpose of illustration. 
           [0022]      FIG. 4  is a fragmentary side elevation showing a sleeve of uncured rubber positioned on a forming mandrel with an outer surface defining a thread with helical convolutions formed in accordance with the invention, and further showing a cord wrapped around the sleeve to form helical corrugations, portions being broken away for the purpose of illustration. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Referring more particularly to the drawings,  FIG. 1  shows a portion of a length of helically corrugated, crush resistant rubber tubing  10  of the type incorporating a prior art design. The tubing  10  may be formed of many different types of elastomers. This design represents the type of tubing produced by the prior art “cording” method described above. The tubing typically has an inner diameter of from 3 to 6 inches but the other dimensions of the helical convolutions would be about the same for a variety of sizes. The tubing  10  is both flexible and resilient but is rigid enough to resist crushing such as when the tubing is stepped on. 
         [0024]    The tubing  10  has a wall with a uniform thickness, the wall being shaped to define a continuous external thread  11  and a corresponding continuous internal thread  12 . The external thread  11  has a crest  13  and a root  14  while the internal thread  12  has a crest  15  and root  16 . The portion of the tubular wall that defines the crest  13  for the external thread  11  also defines, on its opposite side, the root  16  of the internal thread  12 . Likewise, the portion of the tubular wall that defines the crest  15  for the internal thread  12  also defines on its opposite side, the root  14  of the external thread  11 . 
         [0025]    As shown in  FIG. 1 , the wall of the tubing as viewed in a longitudinal (or axial) section has a sinusoidal form, with alternating crests and roots. Both the external and internal crests are rounded, as are the external and internal roots. However, the radii of the external and internal crests  13  and  15  are greater than the radii of the respective roots  14  and  16 . 
         [0026]    The pitch (i.e., distance from one helical convolution to the next) as designated in  FIG. 1  by the letter “x”, is relatively small. In some applications, the pitch of the threads for helically corrugated tubing of the prior art is about four times the wall thickness or about the pitch shown in  FIG. 1 . Also, the typical height of the external and internal threads (i.e., the distance from root to crest) as designated in  FIG. 1  by the letter “y”, is relatively small. In some applications, the height of the external and internal threads is about twice the wall thickness. These parameters are used to provide optimal resistance to crushing. However, these parameters also result in corrugated tubing with a high resistance to axial compression. Normally, such tubing can be axially compressed from its normal relaxed length to a collapsed condition by only a small amount (e.g. &lt;15% under a 25 lb. load). 
         [0027]      FIGS. 2 and 3  show a portion of a length of helically corrugated rubber tubing  20  embodying the invention, the tubing being so formed as to permit substantial axial compression unlike the prior art tubing of  FIG. 1 , while still maintaining an acceptably high resistance to crushing.  FIG. 2  shows the tubing  20  in its normal relaxed condition, while  FIG. 3  shows the tubing of  FIG. 2  in its axially compressed condition, a condition that may be achieved in accordance with the unique characteristics provided by the invention. The tubing  20  may be formed from a variety of different elastomers. Butyl rubber is particularly suitable for military applications since it is impermeable to the types of gases used in chemical warfare. 
         [0028]    The tubing  20  has a wall with a uniform thickness, the wall being shaped to define a continuous external thread  21  and a corresponding continuous internal thread  22 . The external thread  21  has a crest  23  and a root  24 , while the internal thread  22  has a crest  25  and a root  26 . The portion of the tubular wall that defines the crest  23  for the external thread  21  also defines, on its opposite side, the root  26  for the internal thread  22 . Likewise, the portion of the tubular wall that defines the crest  25  of the internal thread  22  also defines, on its opposite side, the root  26  of the external thread. 
         [0029]    As shown in  FIG. 2 , the wall of the tubing, as viewed in longitudinal (or axial) section, has a sinusoidal form similar to that shown in  FIG. 1 , however, the convolutions are significantly larger in size. In the embodiment of the invention illustrated in  FIG. 2 , the pitch of the convolutions is designated by the letter “x”, and is about eight times the wall thickness and the thread height is designated by the letter “y”, and is about four times the wall thickness. In some embodiments of the invention, the threads have a pitch of about six to ten times the wall thickness and a thread height of about three to five times the wall thickness. There does not appear to be a linear mathematical relationship between wall thickness, pitch and thread height. It is critical, however, that the convolutions be larger than the convolutions used in conventional helical tubing. The convolutions are preferably at least 25% larger in size for comparable diameter prior art tubing, and more preferably about 32% larger in size for comparable diameter prior art tubing tubing. 
         [0030]    Within the parameters described above, a combination of pitch and thread height may be determined experimentally to achieve the desired axial compressibility. The preferred combination may vary depending on the type of elastomer used, the diameter of the tubing and the wall thickness. In some embodiments (e.g., 4″ diameter butyl tubing), excellent results can be obtained when the product of the ratio of pitch to wall thickness (typically within the range of 6 to 10) and the ratio of thread height to wall thickness (typically within the range of 3-5) is 30±2. 
         [0031]      FIG. 3  shows a portion of the length of tubing of  FIG. 2 , axially compressed to about 40% of its normal relaxed length (i.e., a tubing in which the relaxed length to compressed length ratio is about 2.5:1). At the same time, the tubing is flexible and resilient and has a high resistance to crushing. The amount of compressibility that may be obtained using the parameters described above may vary from about 20% to about 80%, and in some embodiments from about 50% to about 75%, shorter than its normal length upon application of a load of about 25 lbs. Thus, the ratio of a length of tubing in a normal relaxed state compared to a length of the same tubing in an axially compressed state will range from about 5:1 to about 1.25:1, and in some embodiments, from about 4:1 to about 2:1. 
         [0032]    A typical embodiment of the invention is defined in the specifications listed in Tables I and II below: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
             
             
               
                   
                 Type of Elastomer: 
                 butyl rubber 
               
               
                   
                 Normal Relaxed Length: 
                 65.00″ 
               
               
                   
                 Internal Diameter: 
                  3.75″ 
               
               
                   
                 Wall Thickness: 
                  0.11″ 
               
               
                   
                 Pitch: 
                  1.11″ 
               
               
                   
                 Thread Height: 
                 0.422″ 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
             
             
               
                   
                 Axially Compressed Length: 
                 17.5″ 
               
               
                   
                 Compression Ratio: 
                 3.171 to 1 (27%) 
               
               
                   
                 Force Required for Axial Compression: 
                 25 lbs. 
               
               
                   
                 Force Required to Maintain Compression: 
                 10 lbs. 
               
               
                   
                   
               
             
          
         
       
     
         [0033]    The method of the invention incorporates a particular adaptation of the “cording” method described above. This particular adaptation is illustrated in  FIG. 4  that shows a portion of an assembly  30  that includes a rotatable mandrel  31  with an uncured rubber sleeve  32  positioned thereon and formed into a helical corrugated shape using a length of cord  33  wrapped around the sleeve. The process is begun by sliding the uncured rubber sleeve  32  over the rotatable mandrel  31 . The mandrel has a continuous thread  34  with a crest  35  and root  36 , formed on its outer surface. The thread has a pitch of about six to ten times the wall thickness of the extruded sleeve and a thread height (from root to crest) of about three to five times the wall thickness. The preferred combination of pitch and thread height may be determined experimentally for a particular embodiment. 
         [0034]    The sleeve  32  is forced into the helical root  36  of the mandrel  31  by wrapping the cord  33  around the sleeve as the sleeve rotates with the mandrel. This process forms helical convolutions in the uncured rubber sleeve  32 . 
         [0035]    The resulting assembly  30 , a portion of which is shown in  FIG. 4  is then removed from its rotary support and placed in a curing oven or autoclave where it is heated to cure the sleeve and set the helical corrugations. When curing is complete, the assembly  30  is removed from the oven and placed so that the mandrel is supported for rotation. Then the cord is removed from the cured rubber tubing by reverse rotation of the mandrel. Finally, the cured rubber tubing is removed from the mandrel such as by introducing air under pressure between the outer surface of the mandrel and the inner surface of the corrugated rubber tubing. 
         [0036]    The resulting helically corrugated rubber tubing has a high resistance to crushing but also may be axially compressed using a small amount of force that may be easily applied by an average person. A typical amount of force required would be around 25 lbs. About 10 lbs of force is typically required to hold the tubing in its axially compressed condition. 
         [0037]    While the invention has been shown and described with respect to specific embodiments thereof, this is intended for the purpose of illustration rather than limitation and other variations and modifications of the specific embodiments herein shown and described will be apparent to those skilled in the art, all within the spirit and scope of the invention. Accordingly, the patent is not to be limited in scope and effect to the specific embodiments shown and described nor in any other way that is inconsistent with the extent to which the progress in the art has been advanced by the invention. 
         [0038]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and illustrative examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.