Abstract:
A lifting sling and a method for making the same are disclosed. The lifting sling comprises an elongated, flexible member having a working surface, a lifting end portion for attaching to a lifting mechanism and a choker end portion. A plurality of longitudinally spaced apart gripping members are attached to the working surface of the flexible member for frictionally engaging the outer surface of a load being lifted. To make the lifting sling, the flexible member is laid on the face of a mold having cavities for forming the gripping members. Liquid casting material is introduced into the mold cavities and allowed to penetrate the surface of the flexible member for permanent bonding thereto.

Description:
FIELD  
       [0001]     The present invention concerns lifting slings for lifting, holding or otherwise manipulating heavy articles.  
       BACKGROUND  
       [0002]     Previous inventions have been designed for the carrying or lifting gas cylinders. One such device is disclosed in U.S. Pat. No. 5,556,146 issued to Kirk, which describes a metal hoop having a handle. The metal hoop, which is sized slightly larger than the diameter of a cylinder to be lifted, is slipped around the cylinder so that when the cylinder is lifted by the handle, the hoop retains the cylinder through frictional contact on the bottom and top of the device. This device, however, is disadvantageous in that it is suitable for lifting cylinders of only one size, the metal hoop tends to damage the surface finish of cylinders, and it is susceptible to slipping.  
         [0003]     Conventional nylon web slings configured to have a choker mechanism at one end also have been used to lift gas cylinders. In such a device, one end of the sling is threaded through the choker to form an adjustable loop which is then placed around the cylinder to be lifted. The loop is tightened around the cylinder and the free end of the sling is connected to a lifting mechanism, such as a load hook or a forklift tine. As the lifting mechanism is raised, the sling is pulled taught, causing the loop to frictionally engage the outer surface of the cylinder and lift the cylinder. Nylon slings are desirable because they can be adjusted to fit around a cylinder of any size and do not damage the surface finish of the cylinder. The use of nylon slings, however, is limited because the coefficient of friction provided by the nylon may not be sufficient to adequately grip heavy or wet cylinders.  
         [0004]     One device attempts to improve upon the inadequate frictional properties of nylon by encasing a nylon strap in an elastomeric sleeve. Although the sleeve increases gripping capability, this type of sling proves to be difficult to use when handling metal cylinders. When a loop is formed and tightened around a cylinder, the elasticity of the sleeve causes the loop to lose the shape of the cylinder unless tension is maintained on the sling. As a result, an operator must maintain the loop with one hand while simultaneously operating the lifting mechanism with his other hand until there is sufficient tension in the sling to grip the cylinder. This practice is cumbersome and may lead to operator injury. The elastomeric-sleeved lifting sling is also undesirable in that the sleeve produces unwanted surface friction against the choker mechanism as slack is removed from the loop.  
         [0005]     Therefore, a need exists for a new and improved lifting sling that overcomes the foregoing and other disadvantages of the prior art.  
       SUMMARY  
       [0006]     The disclosed embodiments of the present invention seek to overcome the foregoing problems of the prior art by providing an improved lifting sling for lifting and moving heavy loads, such as cylinders containing pressurized gas. According to one embodiment of the present invention, a lifting sling comprises an elongated, flexible member having a working surface, a lifting end portion for attaching to a lifting mechanism and a choker end portion. A plurality of longitudinally spaced apart gripping members are attached to the working surface of the member for frictionally engaging the outer surface of the load to be lifted. The sling preferably includes a choker ring fastened to the choker end portion through which the lifting end portion may be inserted to form an adjustable loop with a variable diameter to fit around the outer surface of a load of any size or configuration.  
         [0007]     When lifting or otherwise moving a load with the sling, the sling should be configured to form an adjustable loop, as previously described, which is placed about the load. The size of the loop is adjusted to contact a surface of the load by manually tensioning the sling in a direction away from the load until the loop fits snugly around the outer surface of the load. The lifting end portion, which is secured to a lifting mechanism, is moved in a direction away from the load to remove the slack from the sling. As the sling is pulled, tension in the sling is converted to a circumferential gripping force about the load and the load is lifted  
         [0008]     The foregoing features and advantages of the present invention are described further in the following detailed description, which proceeds with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a perspective view of an operator lifting a gas cylinder with a working embodiment of a lifting sling according to the present invention.  
         [0010]      FIG. 2  is an enlarged perspective view showing the adjustable loop that is formed with the lifting sling of  FIG. 1 .  
         [0011]      FIG. 3  is a cross sectional side view showing the lifting sling of  FIG. 1  before it is configured to form an adjustable loop.  
         [0012]      FIG. 4  is a top plan view showing the lifting sling of  FIG. 1  before it is configured to form an adjustable loop.  
         [0013]      FIG. 5  is a top plan view of a mold used to make the lifting sling illustrated in  FIGS. 1-4 .  
         [0014]      FIG. 6  is a cross sectional view of the mold of  FIG. 5  taken along line  6 - 6 .  
         [0015]      FIG. 7  is a cross sectional view of the mold of  FIG. 5  taken along line  7 - 7 .  
         [0016]      FIG. 8  is a perspective view of the mold of  FIG. 5 .  
     
    
     DETAILED DESCRIPTION  
       [0017]     Referring to  FIGS. 1-4 , a working embodiment of a lifting sling  12  is illustrated. As shown in  FIG. 1 , the lifting sling  12  may be used by an operator  10  to lift and move a load, such as the illustrated gas cylinder  14 . The manner in which the lifting sling  12  is used to move a load is described in greater detail below.  
         [0018]     Lifting sling  12  comprises an elongated, flexible member  16  having a working surface  18 , a choker end portion  20  and a lifting end portion  22 . Member  16  may be constructed of any flexible material having a strength sufficient to avoid failure under the weight of a load to be lifted. For reasons which will become apparent, it is preferred that member  16  comprise a material having a permeable surface, such as a fabric or fabric-like material, to permit fabrication of the sling  12  according to the manufacturing method described herein. Conventional two-inch wide nylon webbing or nylon strapping was found to be a suitable material for member  16 , although wider nylon webbing also may be used.  
         [0019]     As best shown in  FIG. 3 , a loop  24  is integrally formed in the lifting end portion  22  by folding the terminal end  22   a  of the lifting end portion back and onto an adjacent part  22   b  of the lifting end portion, and then fastening together the terminal end  22   a  and the adjacent part  22   b  of the lifting end portion, such as by stitching. The loop  24  is sized so as to fit around a conventional lifting mechanism (not shown), such as a load hook or a forklift tine.  
         [0020]     Secured to the choker end  20  of member  16  is a choker ring  25 . A disclosed embodiment comprised a first link  26  and second link  28  mounted to the first link  26  in any conventional manner, such as by welding. In the illustrated embodiment, the first link  26  is generally elliptical in shape with oblate side portions  27  and the second link  28  is welded to one of the side portions  27  inside the first link so as to form first and second apertures  30  and  32 . As best illustrated by  FIG. 2 , member  16  is fastened to the choker ring  25  in a working embodiment by extending the terminal end  20   a  of choker end portion  20  through the second aperture  32 , looping the terminal end  20   a  back unto an adjacent part  20   b  of the choker end portion and coupling together the terminal end  20   a  and the adjacent part  20   b  in a conventional manner, such as by stitching or rivets. The second aperture  32  generally is dimensioned to be only slightly larger than the width of the choker end portion  20 , the significance of which will be explained below in the description of the sling&#39;s operation. As further shown in  FIG. 2 , the first aperture  30  of the choker ring  25  is of sufficient size such that the lifting end portion  22  of member  16  may be slidably inserted therethrough to create an adjustable loop  34  of variable diameter that is adjustable to fit around, and configure to, the outer surface of a load of any size or configuration.  
         [0021]     A plurality of longitudinally spaced, elastomeric gripping members  36  are secured to the working surface  18  of member  16  to provide the necessary gripping force when the adjustable loop  34  is placed around a load for lifting. By spacing the gripping members  36  along the length of the sling  12 , the flexibility of member  16  is maintained to effect proper setting of the loop around the cylinder without maintaining tension on the sling. It has been found that the proper dimensions and spacing for the gripping members  36  are those which maximize surface area of the gripping members and provide for adequate flexibility of the sling for the particular load being lifted. For example, where less flexibility is required, such as when lifting a load having a relatively large radius of curvature, the widths of the gripping members may be increased and spacing between gripping members may be decreased so as to increase the overall surface area of the gripping members  36 . Conversely, when lifting a load having a relatively small radius of curvature, the widths of the gripping members should be decreased and spacing between gripping members should be increased to provide a sling that is more flexible. In a working embodiment sized for lifting commercial gas cylinders, each gripping member  36  is preferably about 0.4 inches in width, about 1.7 inches in length, and is spaced approximately 0.35 inches from an adjacent gripping member. These dimensions were found to work best for safely lifting gas cylinders of about 7 to 15 inches in diameter and weighing up to 500 pounds. Of course, larger diameter and/or heavier cylinders may be lifted by increasing the size of the gripping members  36 .  
         [0022]     The gripping members  36  are made of a material capable of providing sufficient frictional contact with the surface of a load to grip and lift the load when the sling  12  is pulled taut. In addition, the gripping members  36  should be constructed of a material that is sufficiently elastic so as to conform to any irregularities on the load surface and not damage the surface finish of the load. For example, many of the commercially available thermoplastic and thermosetting elastomers would be suitable for use in the present invention. In particular, molded polyurethane, such as product number PMC-746, manufactured by Smooth-On, Incorporated of Easton, Pa., was found to be an exemplary material for the gripping members in a working embodiment. Tests have shown that molded polyurethane provides a gripping force for lifting loads of up to 2,000 pounds without slipping.  
         [0023]     When lifting or otherwise moving a load with the sling  12 , the sling must be first configured to form an adjustable loop  34 , as previously described, which is then placed around the load, such as around cylinder  14 . The adjustable loop  34  is then set in place by manually tensioning the sling in a direction away from the load until the loop fits snugly around the outer surface of the load. At this point, tension may be released because the gripping members  36  provide sufficient traction to prevent the loop  34  from slipping once the loop has been set. Thus, an operator is permitted to operate the lifting mechanism without having to simultaneously maintain tension in the loop  34  until enough slack has been removed by the lifting mechanism to prevent premature slippage of the loop  34 . It therefore should be appreciated that handling of a load with the present invention may be easily accomplished by one operator. Further, the illustrated embodiment of the invention reduces the possibility of serious injury, as the operator is able to operate the lifting mechanism at a safe distance from the load being lifted.  
         [0024]     After the adjustable loop  34  has been set and the lifting loop  24  has been placed around a lifting mechanism, the lifting mechanism is moved in a direction away from the load to remove slack from the sling  12 . As the sling  12  is pulled, the tension in the sling is converted to a circumferential gripping force around the surface of the load. This is referred to as the “acquisition stage” of the lifting process. Once the sling  12  is pulled taut under the weight of the cylinder, the load is lifted off the ground due to the frictional contact between the gripping members  36  and the surface of the load. Because the second aperture  32  is sized to be only slightly larger than the width of the choker end portion  20 , the choker ring  25  is prevented from rotating relative to the choker end portion  20  as tension is applied in the upward direction (as shown in  FIG. 1 ). Therefore, the choker end portion  20  and the portion of the sling  12  in the first aperture  30  maintain contact with the oblate side portions  27  to provide a smooth upward transition from the acquisition stage to the actual lifting of the load.  
         [0025]     Referring now to  FIGS. 5-8 , a mold  38  is illustrated for casting the gripping members  36 . The mold  38 , the length of which generally is equal to the length of the sling  12 , has a plurality of longitudinally spaced mold cavities  40 , each of which forms an individual gripping member  36 . In the alternative, a series of individual molds having a cavity similar to cavities  40  wherein each individual mold corresponds to a gripping member  36  may be employed rather than using a single mold having multiple cavities.  
         [0026]     The molding process has been accomplished as follows. Member  16  made of conventional two inch nylon webbing material is laid on the face  44  of the mold  38  and held in place by a clamping mechanism (not shown). Liquid casting material, which is poured into a longitudinally extending reservoir  46 , flows into each of the mold cavities  40  through gates  42 . As shown in  FIGS. 6 and 7 , the gates  42  taper outwardly from the mold cavities  40  to the reservoir  46  to facilitate removal of the mold  38  once the casting material has cured. After the cavities  40  have been filled with casting material, the casting material is allowed to cure. The casting material penetrates the surface of the nylon webbing material so that the gripping members become permanently secured to the webbing material when cured.  
         [0027]     The proper temperature and time required for adequate curing will vary depending on the casting material and the dimensions of each gripping member. When using the type of polyurethane available from Smooth-On, Incorporated, as mentioned above, it has been found that the mold should be cured for about 1 hour at approximately 130° Fahrenheit to form gripping members that are 1.7 in length, 0.4 in width and 0.09 in thickness. Once the casting material has cured, the clamping mechanism is removed and the mold is separated from the sling.  
         [0028]     In addition to the molding technique described above, it should be appreciated that other conventional methods of molding, such as injection molding, may be employed to form the gripping members.  
         [0029]     Whereas the invention has been described in connection with a representative embodiment, it will be apparent that the invention is not limited to that embodiment. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.