Abstract:
A lifting sling having warning markings that indicate the extent of elongation of the sling under load, whereby an operator can visually determine if the maximum safe load weight of the sling has been exceeded. The markings are disposed on the surface of the sling cover and initially covered by a non-elongating body, but are exposed as the sling elongates under load.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to the field of lifting slings, and more particularly to the field of such slings having means to sense, measure, indicate or warn of excessive elongation, strain, tension or impending failure.  
         [0002]     Lifting slings are devices similar to ropes, cables or chains that are used to lift large, heavy objects, typically with a crane or similar piece of equipment, with the sling being connected to or encircling the object and connected to a hook or similar attachment means on the crane. The lifting slings typically comprise one or more elongated bundles of fiber, thread or yarn forming a load-bearing core that is encased within a cover, jacket, sleeve, skein or the like. The fibers, yarns or threads are usually composed of a synthetic material, such as for example polyester or Kevlar, formed as multi-filaments or monofilaments, and they may be twisted or braided. The slings are typically of one of three types, either round (having the ends of the sling joined to each other to form a circle), flat web (having an elongated main body, the ends of which are bent back and secured to the body to form eyelets on each end), or eye-and-eye (a round sling enclosed with an elongated sleeve such that only relatively short loops extend from each end of the cover). Lifting slings are well known in the art, and examples are shown in U.S. Pat. No. 4,210,089 to Lindahl, U.S. Pat. No. 4,850,629 to St. Germain, and U.S. Pat. No. 5,727,833 to Coe.  
         [0003]     Lifting slings are load rated so that the operator does not attempt to lift too great a weight for a given sling. It is typical, for example, for a sling to be load rated at one fifth of its failure strength, such that a sling that would fail under a load of 30,000 pounds would be load rated for safe operation for loads up to 6,000 pounds. It is quite common under real working conditions that the actual weight of objects being lifted is not known, and thus there may be many occasions where loads are lifted by a sling where unbeknownst to the operator the load exceeds the load rating of the sling. In addition, the tenacity or resistance-to-elongation of a sling is likely to increase over time, such that load weights significantly below the load rating may be unsafe and result in failure for slings that have been weakened by excessive use, undetected damage or environmental degradation.  
         [0004]     All lifting slings elongate under heavy load to some degree, with slings made of polyester having greater elongation under load than a similarly rated sling composed of Kevlar or Aramid fibers. For example, a fourteen foot polyester sling load rated at  6 , 000  pounds may elongate up to five inches for a load approaching 6,000 pounds. Because elongation occurs under load, certain means for measuring or sensing the amount of elongation or any defects in continuity of the fiber core of a sling have been developed. Examples of such are shown in U.S. Pat. No. 4,757,719 to Franke and U.S. Pat. No. 5,651,572 to St. Germain, which disclose means comprising electrical circuits or optical fibers. Such systems add significant costs to the slings and are subject to environmental degradation or operational damage.  
         [0005]     It is an object of this invention to provide an elongation measuring or sensing means that provides an indication or warning to an operator that a load is approaching or exceeding the maximum safe load weight for a given sling. It is a further object to provide such a sling wherein the excessive elongation warning means is an integral component of the sling. It is a further object to provide such a sling wherein the excessive elongation warning means is relatively low cost, easily read and not readily susceptible to damage or degradation from environment or use. It is a further object to provide such a sling wherein the excessive elongation warning means is compatible with round, flat web or eye-and-eye slings.  
       SUMMARY OF THE INVENTION  
       [0006]     The invention is a lifting sling of the type comprising one or more elongated bundles of synthetic fiber, threads, yarn or the like, provided in multi-filament or monofilament form, preferably twisted or braided, and encased within an elongated cover or jacket, the fiber bundles comprising the load bearing core of the sling. The lifting sling may be of any configuration, such as for example round, flat web or eye-on-eye.  
         [0007]     Excessive elongation warning indicator means are provided, the dynamic indicator means comprising warning markings, indicia or other visible members that are disposed on, incorporated in, imprinted on or attached to the cover of the sling, and a static or stationary non-elongating body, housing or member that comprises demarcation means, such that the demarcation means references the markings in a visible manner, such that an observer may readily determine the extent of elongation of the sling and whether the sling is approaching or exceeding the maximum safe load. Preferably, the dynamic warning markings are non-uniform, having variations in color, size or content, such that certain markings indicate a safe load, other markings indicate a load approaching the maximum safe load, and still other markings indicate that the safe load has been exceeded. The static non-elongating body is affixed to the sling at a single location using suitable fastener means, such that relative motion between the load-bearing components of the sling and the non-elongating body occurs when the sling elongates under load. The demarcation means may include, for example, the non-affixed end of the non-elongating body, a slot, a window, a pointer, or similar structures.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is an exposed partial view of a round lifting sling in a non-load condition, such that the dynamic warning markings are concealed by the static non-elongating body of the excessive elongation warning indicator means, the non-elongating body comprising an extended portion of the sling cover.  
         [0009]      FIG. 2  is an external partial view of the lifting sling of  FIG. 1  under a load condition that does not exceed the maximum safe load weight for the sling, showing exposure of the dynamic warning markings as the load-bearing components of the sling elongate under load.  
         [0010]      FIG. 3  is a partial view of a lifting sling of any type in a non-load condition showing the static non-elongating body as being an added member affixed to the sling cover, the demarcation means of the excessive elongation warning indictor means comprising a window or slot disposed in the body.  
         [0011]      FIG. 4  is a partial view of the lifting sling of  FIG. 3  under a load condition that exceeds the maximum load weight for the sling.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]     With reference to the drawings, the invention will now be described in detail with regard for the best mode and the preferred embodiment. In general, the invention is a lifting sling that comprises indicator means to provide a visible warning to the operator when the elongation of the sling due to heavy load weight approaches or exceeds the maximum safe load rate for the sling.  
         [0013]     As shown in  FIG. 1 , a representative lifting sling  10  comprises a load bearing core  11  formed of one or more extended fiber bundles  12  that are enclosed within an extended cover, jacket, skein, sleeve or the like  14 . The fiber bundles  12  comprise fibers, threads, yarn or the like  13  most preferably composed of synthetic material such as polyester, Kevlar, Aramid or the like. The fibers  13  may be multi-filament or monofilament, and may be twisted, braided, interwoven or the like. While a sling  10  having a single core  11  is depicted in the drawings, it is to be understood that the sling of the invention may also comprise multiple cores  1   1 . The round sling  10  depicted in FIG. I has a first end  31  disposed within a second end  32  in known manner and the cover  12  of the second end  32  is extended to receive the first end  31 . The fiber bundle  12  is secured to the cover  14  both the first end  31  and the second end  32  by suitable bundle joining means  15 , such as stitching, mechanical fasteners or the like. The load bearing core  11  and cover  14  are dynamic components of the sling  10 , in that they will elongate to some degree when under heavy load.  
         [0014]     In this embodiment as depicted in  FIGS. 1 and 2 , the excessive elongation warning indicator means  20  comprises a static non-elongating body  21  that is composed of the extended sleeve portion of cover  14  on the second end  32 , and one or more dynamic warning markings, indicia or similar visible members  22  disposed on, imprinted upon, attached to or joined in suitable manner to the cover  14  adjacent the first end  31 . The dynamic warning markings  22  may be of any shape or configuration, preferably being non-uniform for easier visual recognition, and may for example comprise similar shapes of changing dimensions, shapes of differing configurations, changes in color, wording such as “safe”, “caution” and “overload”, weight percents such as “20%”, “40%”, “60%”, “80%” and “100%”, etc., as long as the markings  22  provide suitable visible indication as to the extent of elongation of the sling  10  relative to its maximum safe load weight. The indicator means  20  further comprises static demarcation means  23  to reference a particular warning marking  22 , with the demarcation means  23  comprising an edge, end, line, pointer or similar means to designate the marking  22  corresponding to the extent of elongation of the sling  10 . In  FIGS. 1 and 2 , the demarcation means  23  is simply the end of the static non-elongating body  21 . The warning markings  22  are dynamic in the sense that they move relative to the static demarcation means  23 . The separation distance between the individual warning markings  22  may remain the same, such as when a non-elongating material is affixed to the cover  14 , or may increase due to elongation under load, such as when the warning markings  22  are imprinted directly on the cover  14 . Some, all or none of the warning markings  22  may be covered by the non-elongating body  21  and/or exposed by the demarcation means  23 . Preferably, the warning marking  22  indicating that the load rating has been exceeded remains covered by the non-elongating body  21  until that condition is reached.  
         [0015]     As shown in  FIG. 2 , which depicts a typical load condition wherein the sling  10  is elongated under the weight of the object being lifted, the static non-elongating body  21  remains of unchanged dimension even with the sling loaded, since the indicator means body  21  and the demarcation means  23 , here the free end of the non-elongating body  21 , are only fixed to the sling  10  by fastener means  24  at one location and are not load bearing components. In other words, relative motion occurs between the dynamic components, cover  14  containing the markings  22 , and the static components, non-elongating body  21  and demarcation means  23 . As the sling  10  elongates under load, the cover  14  elongates such that some or all of the warning markings  22  are moved into an exposed position beyond the demarcation means  23 . As depicted in  FIG. 2 , the sling  10  has elongated under load such that the maximum safe load weight is being approached but not exceeded, since the maximum load warning marking  22 , shown as the longest of the bars, is not exposed.  
         [0016]     An alternative embodiment for the invention is shown in  FIGS. 3 and 4 , which may comprise a round, flat web or eye-on-eye sling  10 . In this embodiment, the excessive elongation warning indicator means  20  comprises a static non-elongating body  21 , such as a tubular member, that is affixed by fastener means  24  to the dynamic cover  14  of the sling  10 . Such excessive elongation warning indicator means  20  could also be a post-manufacture addition to slings already in use. In this embodiment, the demarcation means  23  comprises a slot or window, such that the warning markings  22  are visible therethrough. When the sling  10  is under load, as shown in  FIG. 4 , the cover  14  elongates and the position of the warning markings  22  relative to the demarcation means  23  changes. In this depiction, 100% of the maximum safe load weight has been reached and is indicated by visible exposure of the “100” warning marking  22 , and the operator should either lighten the load or switch to a higher rated sling.  
         [0017]     The separation distances of the warning markings  22  on the dynamic load bearing components of the lifting sling  10  will vary depending on the material components of the sling  10  primarily that of the load bearing core  11 , since different materials will have different elongation amounts under the same load. More than one excessive elongation warning indicator means  20  may be provided on a single sling  10 .  
         [0018]     It is understood that equivalents and substitutions to certain elements set forth above may be obvious to those skilled in the art, and therefore the true scope and definition of the invention is to be as set forth in the following claims.