Abstract:
A squeeze type pump that transfers slurry via an elastic tube by squeezing the elastic tube with pairs of rollers to elastically deform the tube by moving each pair of squeezing rollers. The elastic tube has an outer diameter, an inner diameter, and a thickness. A ratio of the inner diameter to the outer diameter is set within a range of 0.56 to 0.72, and the thickness is set within a range of 23 to 35 mm.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a squeeze type pump, which transfers slurry such as freshly mixed concrete, and more particularly, to an elastic tube preferably used for a squeeze type pump having squeezing rollers, which squeeze the elastic tube to elastically deform the tube and transfer slurry via the elastic tube. 
     Prior art squeeze type pumps include an elastic tube, which is arranged in a U-shaped manner along the inner surface of a cylindrical drum. A pair of support arms are mounted on a drive shaft that is inserted through a center of the drum. The support arms are separated from each other by an angle of 180 degrees and rotated synchronously. A pair of squeezing rollers are supported at a distal portion of each support arm by means of a support shaft and a bearing. The rollers squeeze the elastic tube from each side of its outer surface to elastically deform the tube into a flat shape. 
     The pairs of squeezing rollers squeeze the elastic tube to move concrete that is in front of the rollers through the tube along the revolving direction of the rollers. Furthermore, the succeeding pair of rollers revolve and squeeze the elastic tube to move concrete sealed within the tube, between the preceding rollers and the succeeding rollers, in the revolving direction of the rollers. Concrete is thus pumped. 
     However, in the prior art squeeze type pumps, which have an elastic tube that has a certain dimension, the elastic tube  61  is pressed against the inner surface of a drum  63  when the squeezing rollers  62  start to squeeze the tube  61 , as shown in FIG. 14 by the solid line. This prevents the tube  61  from being located in a normal position, as shown in FIG. 14 by the broken line. In such cases, it is necessary to replace the elastic tube or adjust the attachment position of the squeezing rollers. This reduces operation efficiency. 
     Furthermore, if these problems frequently occur, the elastic tube becomes worn in some locations, and the durability of the tube is reduced. 
     In addition, experiments show that the above problems occur with elastic tubes that have specific dimensions. As shown in Table 2, which will be described later, such elastic tubes have outer diameters ranging from 160 to 165 mm, inner diameters ranging from 120 to 145 mm, and thickness ranging from 7.5 to 22.5 mm. In such cases, the ratio of the inner diameter of the tube to the outer diameter thereof ranges from 0.73 to 0.91. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide a squeeze type pump having an elastic tube that is always located in a normal position between squeezing rollers when the rollers start to squeeze the elastic tube. 
     Furthermore, it is another objective of the present invention to provide an elastic tube used for a squeeze type pump capable of preventing local wear of the tube and improving the durability thereof. 
     A squeeze type pump according to the present invention transfers slurry via an elastic tube by squeezing the elastic tube with pairs of rollers to elastically deform the tube by moving each pair of squeezing rollers. The elastic tube includes an outer diameter, an inner diameter, and a thickness. A ratio of the inner diameter to the outer diameter is set within a range of 0.56 to 0.72, and the thickness is set within a range of 23 to 35 mm. It is assured that the elastic tube according to the present invention is thus squeezed while the tube is in the normal position. The local wear of the elastic tube is then prevented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention that are believed to be novel are set forth with particularly in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1 is a partial cross-sectional view showing an elastic tube; 
     FIG. 2 is a partial side cross-sectional view showing the elastic tube as viewed in the same direction as FIG. 1; 
     FIG. 3 is a partial enlarged cross-sectional view showing the elastic tube; 
     FIG. 4 is a partial cross-sectional view of the contacting walls of a squeezed and flattened tube showing a foreign body caught in the elastic tube as seen in the same direction as FIG. 1; 
     FIG. 5 is a partial cross-sectional view showing the elastic tube in an initial squeezing state; 
     FIG. 6 is a side view showing the squeeze type pump; 
     FIG. 7 is a cross-sectional view of the squeeze type pump taken along line  7 — 7  in FIG. 6; 
     FIG. 8 is a partial cross-sectional view showing the elastic tube squeezed by the squeezing rollers; 
     FIG. 9 is schematic side view showing the elastic tube arranged along the inner surface of a drum; 
     FIG. 10 is a cross-sectional view of the elastic tube when accommodated in the drum; 
     FIG. 11 is a graph showing the relation between the inner diameter of the elastic tube and the bend radius thereof; 
     FIG. 12 is a graph showing the relation between the bend radius of the elastic tube and the compression thereof; 
     FIG. 13 is a partial cross-sectional side view showing another embodiment of the elastic tube; and 
     FIG. 14 is a partial cross-sectional view showing a prior art squeeze type pump. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of a squeeze type pump according to the present invention will now be described with reference to FIGS. 1 to  12 . 
     The entire structure of the squeeze type pump will now be described. As shown in FIGS. 6 and 7, a cylindrical drum  11  is fixed to a vehicle (not shown), which transports the squeeze type pump. As shown in FIG. 7, a side plate  12  is formed integrally with a left end portion of the drum  11 . A reinforcing rib  13  is welded to the outer surface of the side plate  12 . A cover plate  14  is secured to the right end portion of the drum  11  by bolts to cover an opening. An attachment plate  15  secures a hydraulic motor  16 , which is inserted in an opening defined at the center of the cover plate  14 . The motor  16  includes a drive shaft  17 , which extends through a center portion of the drum  11 . A distal portion of the drive shaft  17  is supported by a center portion of the side plate  12  by a radial bearing  18 . 
     As shown in FIG. 6, a pair of straight support arms  19  are coupled to a middle portion of the drive shaft  17 . The support arms  19  are separated from each other by an angle of 180 degrees. As shown in FIG. 7, a pair of support shafts  20 , which extends parallel with each other, are fastened to each side of a distal portion of each support arm  19  by bolts  21 . A squeezing roller  22  is rotatably supported by each support shaft  20  to squeeze an elastic tube  24 . 
     A substantially semicircular supporter  23  is fixed, for example, by means of welding, to the inner surface of the drum  11 . The elastic tube  24  is arranged along the inner surface of the supporter  23 . As shown in FIG. 6, the elastic tube  24  includes an inlet portion  241 , which extends horizontally from an upper part of the drum  11 . The inlet portion  241  is connected to a concrete hopper (not shown) by a suction piping. An outlet portion  242  of the elastic tube  24  extends horizontally from a lower part of the drum  11  and is connected to a discharge piping. Concrete is thus provided to a construction site. A guide member  25  guides the elastic tube  24 . 
     A pair of polygonal attachment plates  26  are mounted on the drive shaft  17 . The attachment plates  26 , which extend parallel to each other, are arranged in the axial direction of the drive shaft  17  with a predetermined interval therebetween. The attachment plates  26  are welded to the drive shaft  17 . Rollers  27  are rotatably supported by opposing corner portions of the attachment plates  16  to contact the inner side of the elastic tube  24  and restore the cylindrical shape of the flattened tube. 
     A plurality of opposing support arms  28  are attached to the outer surface of each attachment plate  26 . A restricting roller  29  is rotatably supported by each arm  28  for restricting the position of the outer surface of the elastic tube  24 . 
     In the squeeze type pump of this embodiment, as shown in FIG. 7, the drive shaft  17  of the motor  16  rotates to cause integral revolution of the support arms  19 , the squeezing rollers  22 , the restoring rollers  27 , and the position restricting rollers  29 . Each pair of squeezing rollers  22  compresses the elastic tube  24  into a flat shape and revolves about the shaft  17 . This moves concrete located in front of the rollers  22  from the inlet portion  241  toward the outlet portion  242 . The concrete is thus transferred from a supply source to a desired location. 
     The structure of the elastic tube  24  will now be described. As shown in FIGS. 1 and 2, the elastic tube  24  includes a cylindrical tube body  40 , which is formed from rubber, and first, second, third, and fourth reinforcing layers  41 ,  42 ,  43 ,  44 . The first to fourth reinforcing layers  41  to  44  are embedded concentrically in the body  40 . The tube body  40  is formed from wear resistant and weather resistant rubber, which has, for example, the composition shown in Table 1. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Element 
                 Content (Parts by weight) 
               
               
                   
                   
               
             
             
               
                   
                 Natural rubber 
                 50 
               
               
                   
                 Styrene-butadiene rubber 
                 50 
               
               
                   
                 Carbon black 
                 50 
               
               
                   
                 Zinc white 
                 5 
               
               
                   
                 Softener 
                 5 
               
               
                   
                 Processing aid 
                 3 
               
               
                   
                 Sulfur 
                 2 
               
               
                   
                 Vulcanization accelerator 
                 1 
               
               
                   
                 Stearic acid 
                 2 
               
               
                   
                 Antioxidant 
                 1 
               
               
                   
                   
               
             
          
         
       
     
     As shown in FIG. 3, the reinforcing layers  41  to  44  are constituted by elongated synthetic fiber belts  47 . Each synthetic fiber belt  47  includes a plurality of nylon threads  45  and rubber  46 , which encompasses the nylon threads  45 . The nylon threads  45  lie parallel in the plane of each belt with an interval between one another. The nylon threads  45  are formed from nylon 6 or nylon 66, while the rubber  46  is formed from natural rubber or styrene-butadiene rubber. 
     The thickness of each synthetic fiber belt  47  is set within a range of 0.6 to 1.2 mm, while its width is set within a range of 200 to 500 mm, preferably within a range of 300 to 400 mm. The synthetic fiber belts  47  of the first and the second reinforcing layers  41 ,  42  extend helically about the axis of the tube in a first direction and in a second, opposite direction, respectively. In the same manner, the synthetic fiber belts  47  of the third and the fourth reinforcing layers  43 ,  44  extend helically in opposite directions. 
     As shown in FIG. 1, the dimension ratio of the diameter of the outer surface  244  (hereinafter referred to as outer diameter Φ 1 ) and the diameter of the inner surface  243  (hereinafter referred to as inner diameter Φ 2 ) of the elastic tube  24  (Φ 2 /Φ 1 ) is set within a range of 0.56 to 0.72. The elastic tube  24  is thus squeezed in an optimal manner, as shown in FIG. 5, during an initial period of squeezing by the squeezing rollers  22 . The basis for selecting the dimension ratio will hereafter be described. 
     An experiment was performed using a first elastic tube and a second elastic tube to move concrete therethrough. The first elastic tube had an outer diameter Φ 1  set at 159.0 mm, and an inner diameter Φ 2  set at 101.6 mm. The second elastic tube had an outer diameter Φ 1  set at 165.0 mm, and an inner diameter Φ 2  set at 105.0 mm. In the experiment, each elastic tube was squeezed in an optimal manner by the squeezing rollers (see Table 2). Furthermore, in third to sixth elastic tubes, the outer diameter Φ 1  of the elastic tube was set at either 159.0 mm or 165.0 mm with the thickness η of the elastic tube 24 set within a range of 23.0 mm to 35.0 mm. In such cases, the elastic tube was also squeezed in an optimal manner. 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Outer 
                 Inner 
                   
                   
                   
               
               
                   
                 diameter 
                 diameter 
                 Thickness 
                 Dimension 
                   
               
               
                 Tube No. 
                 φ 1  mm 
                 φ 2  mm 
                 η mm 
                 ratio φ 2 /φ 1   
                 Feasibility 
               
               
                   
               
             
             
               
                 1 
                 159.0 
                 101.6 
                 28.7 
                 0.64 
                 Feasible 
               
               
                 2 
                 165.0 
                 105.0 
                 30.0 
                 0.64 
                 Feasible 
               
               
                 3 
                 159.0 
                 113.0 
                 23.0 
                 0.71 
                 Feasible 
               
               
                 4 
                 159.0 
                 89.0 
                 35.0 
                 0.56 
                 Feasible 
               
               
                 5 
                 165.0 
                 119.0 
                 23.0 
                 0.72 
                 Feasible 
               
               
                 6 
                 165.0 
                 95.0 
                 35.0 
                 0.58 
                 Feasible 
               
               
                 7 
                 165.0 
                 120.0 
                 22.5 
                 0.73 
                 Unfeasible 
               
               
                 (Prior art) 
               
               
                 8 
                 165.0 
                 145.0 
                 10.0 
                 0.88 
                 Unfeasible 
               
               
                 (Prior art) 
               
               
                 9 
                 160.0 
                 120.0 
                 20.0 
                 0.75 
                 Unfeasible 
               
               
                 (Prior art) 
               
               
                 10  
                 160.0 
                 145.0 
                 7.5 
                 0.91 
                 Unfeasible 
               
               
                 (Prior art) 
               
               
                   
               
             
          
         
       
     
     Therefore, the dimension ratio (Φ 2 /Φ 1 ) of the elastic tube is preferably set within a range of 0.56 to 0.72. More preferably, the dimension ratio (Φ 2 /Φ 1 ) is set within a range of 0.60 to 0.68. The thickness η of the elastic tube is preferably set within a range of 23 to 35 mm, and more preferably, within a range of 28.7 to 30.0 mm. 
     If the thickness η of the elastic tube  24  exceeds 35 mm, the outer surfaces of the reinforcing layers  41 ,  42 ,  43 ,  44  may easily separate from the rubber body  40 . If the thickness η is smaller than 23 mm, the force for restoring the original shape of the flattened elastic tube  24  may be reduced. Furthermore, in such cases, heat may cause the outer surfaces of the layers  41 ,  42  to separate from the body  40 . 
     As shown in FIG. 3, the thickness γ of a rubber layer, which is defined by the innermost reinforcing layer, or the first reinforcing layer  41  and the inner surface  243  of the tube  24 , is set within a range of 10 to 15 mm. As shown in FIG. 4, the rubber layer prevents a foreign body  48  from cutting the first reinforcing layer  41  of the elastic tube  24  and the tube  24  is flattened by squeezing, when the foreign body  48  is caught in the tube  24 . 
     As shown in FIGS. 6 and 9, the elastic tube  24  of this embodiment is arranged in a semicircular shape along the inner surface of the drum  11 . A bend radius R of the elastic tube  24 , which is the distance from the center O 1  of the drum  11  to the axis O 2  of the elastic tube  24 , is determined as follows. 
     The elastic tube  24  has a circular cross section when it extends straight. However, the elastic tube  24  is deformed when a portion thereof is accommodated in the drum  11 , as shown in FIG.  9 . Then, as shown in FIG. 10, the elastic tube  24  has an oval cross section. In this state, a major axis D 1  of the inner surface  241  is parellel to the inner surface of the drum  11 , and a minor axis D 2 , extends perpendicular to the inner surface of the drum  11 , as shown in FIG. 10. A ratio of the minor axis D 2  to the major axis D 1 , or [(D 2 /D 1 )×100] indicates a compression τ of the elastic tube. As the compression τ becomes smaller, the suction amount of the pump becomes smaller. 
     When the elastic tube  24  is curved as shown in FIG. 9, a tensile force acts on an outer side portion of the tube  24  that contacts the drum  11 , while a compressive force acts on an inner side portion that is separated from the drum  11 . The bend radius R then becomes smaller to reduce the compression τ. If the elastic tube  24  is bent beyond its yielding point (restoration limit), a force acting on the elastic tube  24  becomes larger than the buckling resistance force T of the tube. This buckles the inner side portion of the elastic tube  24  as shown in FIG. 9 by the broken line. 
     In this embodiment, the compression τ of the elastic tube  24  is thus determined by the following equation so that a suction decrease corresponding to a compression decrease of the elastic tube  24  will be maintained under 10%, and the buckling of the tube will be prevented: 
     
       
         τ=[(D 2 /D 1 )×100]≧90%  (1) 
       
     
     The bend radius R, the thickness η, the rigidity G, and the ratio of the inner diameter Φ 2  to the outer diameter Φ 1 (Φ 2 /Φ 1 ) of the elastic tube  24  should be chosen to meet requirements of the equation (1). The rigidity G of the elastic tube  24  depends on the number N of the first to fourth reinforcing layers  41  to  44  and the winding angle a thereof (inclined angle of the layers  41  to  44  with respect to the axis O 2 , as shown in FIG.  9 ), the thickness η of the elastic tube  24 , and hardness Hs of the rubber. 
     An experiment was performed to determine a relation between the inner diameter Φ 2  and the bend radius R of the elastic tube  24  in light of the equation (1). The results are shown in the graph of FIG.  11 . As shown in this graph, a ratio of the bend radius R to the inner diameter Φ 2 , or R/Φ 2  is approximately 4.0. However, R/Φ 2 ≈5.0 is preferred to assure safety. 
     With the elastic tube  24  being bent in accordance with the bend radius R, an external force W (kg) acts on the tube  24  in a normal direction with respect to the axis of the tube  24 . The circular cross section of the tube  24  is thus deformed into an oval shape. In this state, the elastic tube  24  applies force that resists the external force, or the buckling resistance force T (kg). When the external force W becomes larger than buckling resistance force T, the bend radius R corresponds to a buckling bend radius while the buckling force T corresponds to a limit buckling resistance force. 
     The buckling resistance force T is determined by the following equation (2), and the rigidity G of the elastic tube  24  is determined by the following equation (3): 
     
       
         T= k   1 ×(η n /Φ 2   m )×G r   (2) 
       
     
     
       
         G= k   2 ×N×E  (3), 
       
     
     where k 1 , k 2  are constants, indices n, m, r are values that are experimentally determined, N is the number of the reinforcing layers  41  to  44 , and E is a constant that is determined experimentally based on the material of the reinforcing layers  41  to, 44 , the thickness of fiber of the layers, and the fiber density thereof (the number of fibers contained in an inch (2.54 cm)). 
     Furthermore, the winding angle α of the reinforcing layers  41  to  44  affects the curvature characteristics of the tube  24 . If the winding angle α is zero, the tube is hard to bend and easy to buckle. However, the tube is not easily stretched axially by pressure acting in the tube. If the winding angle α is 90 degrees, the tube is easy to bend and hard to buckle. However, the tube is easily stretched axially by pressure acting in the tube. Therefore, the winding angle α is set normally within a range of 50 to 70 degrees, and preferably within a range of 50 to 60 degrees. In this embodiment, the winding angle α is set to be 54′55″. This structure enables a balance between the axial component and the radial component of the force acting on the tube. 
     A plurality of elastic tubes  24 , inner diameters Φ 2  of which are 38, 50, 75, and 100 mm, were produced to determine relations between the bend radius R and the compression τ of each tube  24 . The results are shown in FIG.  12 . The bend radius R of the elastic tube is obtained in accordance with the graph shown in FIG.  12  and is represented by the following equation (4): 
     
       
         R= k   3 ×(Φ 2 +η)×(Φ 2 /η)  (4), 
       
     
     
       
         where  k   3 ∝(1/G)  (5). 
       
     
     If the value of N, or the number of the reinforcing layers  41  to  44 , increases in the equation (3), the rigidity G represented in the equations (3), (5) becomes larger. This reduces the value of constant k 3  represented in the equations (4), (5). If the constant k 3  is smaller, the bend radius R determined by the equation (4) becomes smaller, even though the thickness η of the tube  24  and the compression τ thereof are constant. The hardness Hs of the rubber, which is related to the rigidity G, is set normally within a range of 50 to 70 degrees. Furthermore, the constant k 3  varies in accordance with the diameter of the drum  11 , and is set normally within a range of 0.8 to 1.2. 
     A plurality of elastic tubes, nominal diameters of which are 38, 50, 75, and 100 mm, were designed and produced to have a compression τ determined by the equation (1) and in accordance with the experimental equation (4). Table 2 shows calculated values and actual values of the bend radius R of the elastic tubes  24  and actual values of the compression τ of the elastic tubes  24 . The inner surface of the drum  11  has a radius that is determined by adding a half value of the outer diameter Φ 1  of the elastic tube to the actual value of the bend radius R. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Data of the tubes 
                 Bend radius R 
                   
               
             
          
           
               
                   
                   
                   
                 Number 
                   
                   
                   
               
               
                 Nominal 
                 Inner 
                 Thick- 
                 of rein- 
                 Calcu- 
                   
                 Compres- 
               
               
                 Diameter 
                 diameter 
                 ness 
                 forcing 
                 lated 
                 Actual 
                 sion 
               
               
                 φ 2  (mm) 
                 φ 2   
                 η 
                 layers 
                 value 
                 value 
                 τ (%) 
               
               
                   
               
               
                 38 
                 38.1 
                 12.7 
                 4 
                 152.4 
                 128.3 
                 92 
               
               
                   
                   
                   
                   
                 k 3   
               
               
                 50 
                 50.8 
                 16.6 
                 6 
                 208.2 
                 215.3 
                 95 
               
               
                   
                   
                   
                   
                 k 3   
               
               
                 75 
                 76.2 
                 19.0 
                 6 
                 381.8 
                 267.9 
                 96 
               
               
                   
                   
                   
                   
                 k 3   
               
               
                 100 
                 101.6 
                 28.5 
                 4 
                 463.8 
                 421.0 
                 93 
               
               
                   
                   
                   
                   
                 k 3   
               
               
                   
               
             
          
         
       
     
     The number of reinforcing layers is preferred to be set within a range of four to six or a range of two to eight. In Table 3, if the nominal diameter is 38 mm, the value of k 3  is determined by dividing the bend radius 128.3 by the calculated value 152.4 mm (≈0.84). If the nominal diameter is 50 mm, k 3  is (≈1.03). 
     As described above, particularly in the embodiment constructed as described above, the dimension ratio (Φ 2 /Φ 1 ) of the elastic tube  24  is set within a range of 0.56 to 0.72, and the thickness η of the elastic tube  24  is set within a range of 23 to 35 mm. Therefore, when the squeezing rollers  22  start to squeeze the elastic tube  24 , the elastic tube  24  is located in the normal squeezing position without being pressed against the inner surface of the drum  11 . This structure prevents the elastic tube  24  from being damaged by excessive stress that acts locally thereon. The durability of the tube is thus improved. 
     The dimension ratio (Φ 2 /Φ 1 ) may be set within a range of 0.60 to 0.68, which is smaller than the range of 0.56 to 0.72. This facilitates squeezing of the elastic tube  24  at a proper squeezing position. Therefore, the durability of the tube is further improved. 
     The elastic tube  24  is constituted by the rubber tube body  40  and the reinforcing layers  41  to  44  that are embedded in the body. This structure improves the durability of the elastic tube. Furthermore, the reinforcing layers  41  to  44  are arranged in the tube body  40  with a predetermined interval between one another in the radial direction. The reinforcing layers  41  to  44  extend helically in opposing directions. This further improves the durability of the elastic tube  24 . 
     The reinforcing layers  41  to  44  are formed from the synthetic fiber belts  47 . Each synthetic belts  47  includes the plurality of synthetic fibers  45 , which are formed from nylon, polyester, or the like. With the synthetic fibers  45  arranged in a row, the rubber  46  encompasses their outer surfaces. This structure also improves the durability of the elastic tube  24 . 
     The thickness γ, which is defined by the inner surface  243  of the elastic tube  24  and the innermost reinforcing layers, or the first reinforcing layer  41  of the rubber body  40 , is set within a range of 10 to 15 mm. This structure prevents the foreign body  48  from cutting the reinforcing layer  41  when the foreign body  48  is caught in the elastic tube. Thus, the durability of the elastic tube  24  is further improved. 
     The bend radius R is set to enable the compression of the elastic tube to be 90% or larger. The bend radius R is determined by the equation (4). This prevents the buckling of the elastic tube  24 , and thus the durability of the tube is improved. 
     The present invention is not restricted to this embodiment and may be embodied as follows. 
     As shown in FIG. 13, a fifth reinforcing layer  51  and a sixth reinforcing layer  52  may be formed in the elastic tube  24  in addition to the first to fourth reinforcing layers  41  to  44 . Alternatively, one, two, three, seven or more reinforcing layers may be formed in the elastic tube  24 . 
     The body  40  of the elastic tube  24  may be formed from nitrile rubber (acrylonitrile-butadiene copolymer), styrene rubber (styrene-butadiene copolymer), acrylic rubber (acrylonitrile-acrylic ester copolymer), polyethylene rubber (chlorosulfonated polyethylene), polyurethane rubber or the like. 
     The synthetic fibers  45  of the synthetic fiber cords  47  may be formed by twisting a plurality of fibers together. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention.