Abstract:
A pre-failure warning indicator is provided for use with a sling. The pre-failure warning indicator triggers at a point that is predictable within a relatively narrow range, thereby increasing the possibility that a damaged sling is removed from use. The pre-failure warning indicator includes a dedicated strand of material that is placed in close proximity to the load-bearing core yarns of the sling but remains separate and independent from the core yarns; the ends of the dedicated strand are connected via a sacrificial “ring.” A warning fiber having an end that is visible to operators/riggers works in conjunction with the sacrificial strand and the ring. The ring is designed to fail when the sling is subjected to a specifically chosen condition (e.g., excessive weight). The failure of the ring causes the warning fiber to withdraw from the rigger&#39;s view thereby warning the rigger that the sling was subjected to the specifically chosen condition and may be damaged.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The present application claims the benefit under any applicable U.S. statute, including 35 U.S.C. § 119(e), to U.S. Provisional Application No. 60/683,987 filed May 23, 2005, in the name of Dennis St. Germain, titled Sling Having Predictable Pre-Failure Warning Indicator and Associated Method.  
         [0002]     This application incorporates by reference U.S. Provisional Application No. 60/683,987 as if fully set forth herein. 
     
    
     FIELD OF THE INVENTION  
       [0003]     This invention relates generally to industrial slings used to lift, move and transport heavy loads and, more particularly, an apparatus for notifying operators/riggers who use synthetic slings of an overload or damage situation that may lead to sling failure.  
       BACKGROUND OF THE INVENTION  
       [0004]     Wire rope slings made of a plurality of metal strands twisted together and secured by large metal sleeves or collars are common in the industry. During the past thirty years, industrial metal slings have seen improvements in flexibility and strength. However, compared to non-metal or synthetic fiber slings, metal slings are relatively stiff and inflexible.  
         [0005]     Synthetic fiber slings have gained popularity over the last fifteen years and are replacing metal slings in many circumstances. Synthetic slings are usually comprised of a lifting core made of twisted strands of synthetic fiber and an outer cover that protects the core. The most popular design of synthetic slings is a roundsling in which the lifting core forms a continuous loop and the sling has a circular or oval-shaped appearance.  
         [0006]     An advantage of synthetic slings is that they have a very high load-lifting performance strength-to-weight ratio which provides for a lighter, more flexible and even stronger slings than their heavier and bulkier metal counterparts. Even with such advances in the art of sling making, the riggers who use these improved synthetic slings still suffer and endure some of the age old problems of sudden failure and loss of a load caused by a sling breaking without warning because it was fatigued (or overly stretched) from being subjected previously to overload conditions. After a sling has been fatigued, it does not usually provide any physical indication that it was damaged—even to the trained eye. (One of the few advantages of a metal sling over a non-metal sling is that there is equipment available that can be used to conduct a non-destructive test of the metal. For example, similar equipment is routinely used to determine whether the wings of an airplane have become fatigued.)  
         [0007]     Standard break tests have been established for determining how large of a load a sling can endure. Slings are attached to a testing machine that applies a steady but increasing force on the sling until it is unable to withstand the stress of the force being applied to it and the sling ultimately breaks. Such break tests have enabled manufacturers of industrial slings to rate the load-bearing capacity of the sling. The load capacity is determined to be a point well below the load used to break the sling and also below the point where the sling is fatigued or damaged. Most sling manufacturers will affix some type of tag notice on the sling which states the load capacity (rated capacity) of the particular sling. This rated capacity gives the maximum amount of load to which the sling may be subjected and still be considered a safe use of the sling.  
         [0008]     Unfortunately, even conscientious operators/riggers who do not take unsafe shortcuts and who operate in a safe responsible manner sometimes are surprised by a sling breaking in use even when they believed it was being used within the load limits of its rated capacity. For example, when industrial slings are in continuous heavy use over three shifts around the clock, the operators on a later shift may not be aware that someone on an earlier shift had subjected the sling to a substantial overload which may have caused serious damage to the lifting core strands of the sling. When a synthetic fiber sling is overloaded beyond its tensile strength or weight-lifting capacity at maximum stretch, it is considered to be fatigued and may never return to its normal strength and load bearing capacity.  
         [0009]     When subjected to an overload condition above its rated capacity, a roundsling can be permanently damaged/deformed if the load stretches the fibers of the load bearing core material beyond their yield point. An over-loaded sling may be susceptible to fracture at a stress point. This condition is similar to the stretching of a rubber band beyond its point of normal elasticity so that when the load or tension is removed or relieved, the rubber band will never regain its normal configuration and its strand dimensions may be permanently stretched which will cause it to fail under a load which is less than its tensile strength load. As stated previously, it is nearly impossible to determine, upon a cursory visual inspection, that a sling has been damaged because of the large size of such slings (on the order of  6  feet or more) and because the load-bearing core is hidden inside the outer cover.  
         [0010]     Once the load-lifting core of the synthetic sling is stretched beyond its yield point, it can actually change in its physical structure and be restricted at a stress point. To date, there has been no precise method or apparatus available to an operator or rigger to determine if a sling with a protective cover was subjected to an overload or damage-causing condition. If a roundsling has been fatigued or structurally changed, the sling may no longer lift a load according to its maximum rated load capacity and, most importantly, becomes a serious threat to the operators and riggers using the sling.  
         [0011]     Thousands of roundslings are being used on a daily basis in a broad variety of heavy load lifting applications which range from ordinary construction (e.g., skyscrapers and bridges), plant and equipment operations, to ship building (e.g., oil rigs), nuclear power plants and the like. The lifting core fibers of such roundslings may be derived from natural or synthetic materials, such as polyester, polyethylene, nylon, and the like. Although the outer covers of synthetic slings are designed to reduce damage, the core fibers are still susceptible to damage from abrasion, cutting by sharp edges, or degradation from exposure to heat, cold, ultraviolet rays, corrosive chemicals or gaseous materials, or other environmental pollutants.  
         [0012]     In certain instances, the core yarn of a synthetic sling could weaken, melt or disintegrate when subjected to elevated temperatures, or to prolonged exposure to either ultraviolet light or chemicals. Still another safety concern flows from abuse by the user when the core yarn is damaged from abrasive wear when the slings are not rotated and the same wear points are permitted to stay in contact for extended periods of time with a device used for lifting (such as hooks on a crane), or on the edges of the load itself. Such abrasion is accelerated for certain types of synthetic fiber material and especially if the load contact section is under compression or is bunched. Riggers in the field are concerned that the inner lifting core yarn of their roundslings may be damaged on the inside without a means for them to detect such defects through the sling cover. Even if the cover is removed it may be impossible to tell if the lifting core has been damaged to the point where it cannot lift its rated load. Since there is no reasonable non-destructive testing techniques for synthetic fiber slings, a synthetic sling that is only suspected of being damaged must be removed from service for safety reasons.  
         [0013]     The structural integrity of the roundsling lifting core material is difficult to determine when it is hidden inside a protective cover of opaque material which renders the lifting core yarn inaccessible for inspection. A stretched or fatigued roundsling could experience a sudden catastrophic failure without warning to the rigger, which may result in the loss of lives and property. Many in the industry have sought to provide safe slings to its riggers to avoid bodily injury, property damage and product liability claims.  
         [0014]     Several roundsling constructions are known which have a failure indicator. For example, it is known in the art to incorporate a failure indicator synthetic strand as an integral member of the lifting or load-bearing core. The failure indicator strand in prior art constructions was always an extension of the core yarns.  
         [0015]     A popular design of prior art roundslings was to twist a plurality of yarns together to form a single strand; the strand is then rolled into an endless parallel loops of strands that form the core, which is then encased in a protective cover material. If the sling was designed with a prior art failure indicator, an indicator strand would be incorporated into and twisted with the core yarns. The two ends of the indicator strand (sometimes referred to as tell-tails), extend freely through an opening in the cover material. When the sling is subjected to an overload condition, the tell-tail would partially withdraw within the cover and the freely extending tell-tail ends would be visibly shorter than the tell-tails of an undamaged sling; if the overload condition exceeded the maximum rated load of the sling, one or both tell-tails would usually withdraw completely within the cover. In either event, the rigger is warned of the occurrence of a potentially damaged sling by either the absence of one or both tell-tails, or a “significant” withdraw of at least one tell-tail inside the cover. However, there usually was no consistency on how the tell-tails would react when triggered, even when the slings were manufactured under identical conditions.  
         [0016]     A drawback of prior art failure indicators based on an indicator strand is that there is no predictable way of determining when the failure indicator will be triggered. Synthetic slings have a safety factor designed into their construction. For example, if the sling is rated at 6,000 pounds, it typically will not be damaged unless the sling is subjected to a force five times greater (i.e., around 30,000 pounds, a 5-to-1 design factor) than the rated capacity; the tell-tail may be triggered and indicate an overload condition when the sling is subject to a force of between four to five times the rated capacity (i.e., about 24,000 lbs) by retracting into the sling&#39;s cover. Therefore, the tell-tail will provide a visual indication that the sling may have been damaged or subjected to a situation that may have been detrimental to the overall condition of the sling before the sling actually is subjected to such a condition. Unfortunately, there was no way of ensuring that the tell-tails would consistently withdraw within the cover at about 24,000 pounds.  
         [0017]     In other words, two slings having prior art failure indicator strands contemporaneously made under the same conditions would have two different trigger points (for example, one sling may trigger at about 22,050 pounds and the other sling may trigger at about 26,000 pounds). In addition, one sling may react to a trigger event by completely withdrawing one of the tell-tails, while the other sling may react to a trigger event by partially withdrawing both tell-tails.  
         [0018]     If the tell-tail is not withdrawn completely within the cover, one rigger&#39;s opinion of a “significant withdrawal” towards the opening in the cover may differ from another rigger&#39;s opinion. Therefore, a “small” movement of one or both of the tell-tails, which may result from the constant use and handling of the sling, may appear to one rigger as an indication that an overload condition was reached when, in fact, the sling was not subjected to an overload condition. Therefore, the visual inspection of the tell-tails in prior art failure indicators and the eventual determination of a trigger event becomes a subjective test.  
         [0019]     Another prior art roundsling construction utilizes an optical fiber strand that enables the operator/rigger to test it by shining a light on one end of the optical fiber to determine if the light can be seen at the other end of the optical fiber. In U.S. Pat. No. 5,651,572 to Dennis St. Germain, it is taught to incorporate a flexible fiber optic “signal” cable into the lifting core strands of the roundsling.  
         [0020]     As indicated previously, in a roundsling, the lifting core is configured in endless parallel loops of strands which are then encased within a protective cover material. The cover will have openings or orifice slits out of which the two ends of the fiber optic signal strand emerge. The aforesaid ends of the fiber optic cable are designed to extend freely through a slit in the sling&#39;s cover so that they are easily accessible by the rigger.  
         [0021]     The optical signal strand member conducts light from a light source at one end to an observer looking at the opposite end for testing the integrity and the continuity of the core strands. The inclusion of the fiber optic cable in the lifting core yarn of the roundsling converts the inaccessible inner core area into an observable test check area by means of the passage of light through the fiber optic component of the lifting core.  
         [0022]     Fiber optic materials are capable of transmitting light into endless parallel relationship with the fibers of the lifting core yarn. This fiber optic signal strand comprises fiber or rod material which permits the propagation of light that enters the fiber material at one end and is totally reflected back inward repeatedly from the fiber wall through the entire length of the fiber optic strand which enables the light being transmitted within the fiber optic cable to pass from one end of the fiber optic cable to the other end. If the light emerges at the other end of the fiber optic cable, it indicates that the integrity of the fiber optic cable throughout the path of the roundsling lifting core bundle is intact and, by reasoning, the integrity of the lifting core yarns are also intact.  
         [0023]     Since the fiber optic cable member is incorporated into the lifting core of the roundsling disclosed in U.S. Pat. No. 5,651,572, it tends to develop somewhat similar breaking or snapping characteristics as the lifting core fiber materials. If the fibers of lifting core yarn break or fracture, then the fiber optic cable will also be damaged which will prevent the transmission of light from one end to the other end of the emerging fiber optic cable. If the light fails to pass from one end of the signal fiber optic cable to the other end, then the rigger is warned that the lifting core strands may be damaged, and to remove the protective cover from the roundsling for further inspection. If, upon inspection, it is determined that the roundsling was damaged, it will be immediately removed from service, and replaced with a new sling.  
         [0024]     Although the apparatus disclosed in U.S. Pat. No. 5,651,572 is currently the leading product for determining whether the lifting core yarns of a synthetic sling have snapped or been damaged, in the stages where the sling has been subjected to an overload condition, the fiber optic signal strand still does not have the identical stretching properties of the load-bearing core yarns. Accordingly, unless the fiber optic cable breaks completely, some light may still be able to traverse the entire length of the fiber optic cable such that the degradation in the intensity of the light may be imperceptible to the naked eye.  
         [0025]     Alternatively, the fiber optic cable, being more brittle than the synthetic core material, may be damaged by normal handling (and dropping) of the sling, or at a force less than the rated capacity of the sling. In such cases, the light transmission through the fiber optic cable may be disrupted causing the fiber optic cable to indicate an overload condition when, in fact, no overload condition was reached.  
         [0026]     Finally, under other excessive or damage-causing situations (e.g., excessive heat, acidic or chemical exposure, and ultraviolet exposure) it can be expected that the fiber optic cable will be affected differently than the synthetic strands of the lifting core. If, for example, a sling with the fiber optic signal cable is exposed to certain chemicals, the fiber optic signal cable may be relatively unaffected (or only its exterior surface is affected leaving the light path through the center of the cable unscathed), while the lifting core has been degraded to the point where it no longer meets its load rating. Therefore, as stated previously, the need to precisely determine whether the load bearing core of a synthetic sling was subjected to an excessive or damage-causing situation still exists.  
       SUMMARY OF THE INVENTION  
       [0027]     The present invention discloses a pre-failure warning indicator for use with a sling that is more accurate and predictable than prior art indicators. In the present invention, the failure indicator strand is separate and independent from the load-bearing core yarns.  
         [0028]     One of the most popular designs of a roundsling is to twist a plurality of yarns together to form a single strand; the strand is then rolled into endless parallel loops of strands that form the core. In accordance with the present invention, a pre-failure warning indicator includes a separate dedicated strand of material, a ring made of a specially chosen material, and a separate warning fiber having an elongated indicator whip end.  
         [0029]     The dedicated strand is placed proximate and substantially parallel to the loops of core strands of the sling; the ends of the dedicated strand are brought within close proximity (in a preferred embodiment several inches) to each other and are terminated with eyes or another configuration that can secure the ring. The ring is inserted through or secured to both eye terminations, thereby bridging the gap between the ends of the dedicated strand, and usually forms an oval-shaped loop. One end of the warning fiber is attached to one of the eyes of the dedicated strand, and the free end of the warning fiber is placed along the ring and threaded through the opposite eye; the free end of the warning fiber is then double-backed along the length of the ring. A tubular cover material encases the lifting core and the pre-failure warning indicator. The free end of the warning fiber extends through an opening in the cover material and is referred to as the indicator whip.  
         [0030]     In a specific embodiment, a tag is attached to the strand (and preferably one of the terminating eyes) and is also drawn through the slot so that it extends freely outside the cover. The tag is designed to provide an indicator that the sling has been tampered with or sabotaged.  
         [0031]     The ring is designed to fail when the sling is subjected to an excessive or damage-causing situation. A common damage-causing situation is when the sling is over-loaded. The ring will break when the sling is placed in an overload situation, thereby causing the termination eyes to separate, resulting in the complete withdrawal of the whip inside of the cover.  
         [0032]     By choosing the ring carefully, relatively accurate predictions of the force needed to trigger the warning fiber can be made. In addition, the ring may be chosen to fail and thereby convey a damage situation when the sling is being used under unusual environmental conditions (e.g., excessively hot, acidic, or ultraviolet rays from, for example, sunlight).  
         [0033]     Previous indicators either of the fiber optic nature or of the tell-tail type could give false indications of an overload or other internal damage. In the case of fiber optics, the ability to transmit light can be impeded by dirt, grease, and other debris that can retard the transmission of light through the fiber optic cable by jamming the ends. In the case of tell-tails, the movement of the sling&#39;s outer cover from friction with a load can give a false implication that the tell-tails were pulling under the cover when it was really the cover moving over the tell-tails. In the current invention, these areas of confusion are eliminated by a simple visual identification of the external warning indicator. Also, the dedicated strand can be locked into place by permanent attachment to the cover. If the cover shifts, the entire assembly of this invention moves with it in concert so a false indication of overload is eliminated.  
         [0034]     Additional objects and advantages will be evident to one skilled in the art after a reading of the detailed description that follows. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and, together with the following description, serve to explain the principles of the invention. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred, it being understood, however, that the invention is not limited to the specific instrumentality or the precise arrangement of elements or process steps disclosed.  
         [0036]     In the drawings:  
         [0037]      FIG. 1  is a perspective view of a single-path roundsling which incorporates a predictable pre-failure warning indicator in accordance with the present invention;  
         [0038]      FIG. 2  is an enlarged cross-sectional view of the roundsling illustrated in  FIG. 1  taken along line  2 - 2 ;  
         [0039]      FIG. 3  is a side view of a pre-failure warning indicator in accordance with the present invention;  
         [0040]      FIG. 4  is a side view of another embodiment of a pre-failure warning indicator in accordance with the present invention, utilizing multiple rings linked together;  
         [0041]      FIG. 5  is a side view of another embodiment of a pre-failure warning indicator in accordance with the present invention for use with a two-path sling;  
         [0042]      FIG. 6  is a perspective view of a two-path sling incorporating the pre-failure indicator of  FIG. 5 ;  
         [0043]      FIG. 7  is a side view of a pre-failure warning indicator in accordance with the present invention which also incorporates a sabotage indicator means; and  
         [0044]      FIG. 8  is a perspective view of a single-path roundsling incorporating the predictable pre-failure warning indicator of  FIG. 3  and the sabotage indicator of  FIG. 7 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0045]     In describing a preferred embodiment of the invention, specific terminology will be selected for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents that operate in a similar manner to accomplish a similar purpose.  
         [0046]     The subject invention is an apparatus and method for determining whether a synthetic fiber sling has been damaged (because of an overload or other condition that could weaken the sling&#39;s load-bearing core) to a point where the sling should be removed from service and returned to the manufacturer for internal inspection and, if necessary, repair or disposal. Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings in which a roundsling having a pre-warning failure indicator in accordance with the present invention is generally indicated at  10 . The various preferred embodiments will be described with reference to the drawing figures that form a part of this description where like numerals represent like elements throughout.  
         [0047]      FIG. 1  illustrates a perspective view of a roundsling in accordance with the present invention.  FIG. 1  specifically shows a single-path roundsling, but the principles disclosed herein may be applied to other slings including multiple-path slings.  FIG. 2  is a cross-sectional view of the roundsling shown in  FIG. 1  taken along line  2 - 2 , and illustrates the primary interior components of a typical roundsling.  
         [0048]     Referring to  FIGS. 1 and 2 , the roundsling  10  comprises an inner core  12  encased within an outer protective cover  25 . The outer cover  25  shown in  FIG. 2  is meant to convey that the cover  25  is larger than the load-bearing core  12  and moves relatively freely with respect to the load-bearing core  12  and not necessarily that the cover  25  has a cross-sectional shape of an oval. The core  12  is designed to bear the entire weight of the load to be lifted. The primary purpose of the outer cover  25  is to prevent physical damage to the core from abrasion, sharp edges on the load, etc.; the cover  25  will also help to reduce damage to the sling when it is used in an environment that will subject it to harsh elements such as heat, ultraviolet light, corrosive chemicals, gaseous materials, or other environmental pollutants. As will be explained hereinafter, the cover  25  can also be designed to notify a user when physical damage has occurred to the cover.  
         [0049]     The lifting core  12  is preferably made of a single or multiple strands  17  configured in a plurality of endless parallel loops of strands to form a single core or multiple cores, all of which are contained inside the protective cover material  25 . The use of a single strand or multiple strands in this configuration is typical in the construction of roundslings.  
         [0050]     The lifting core  12  of such roundslings may be derived from one or more natural or synthetic materials, such as polyester, polyethylene, nylon, K-Spec® (a proprietary blend of fibers), HMPE, LCP, para-aramid or other types of synthetics. The material chosen for the core primarily depends on the maximum weight the sling is designed to lift and environment in which the sling  10  will be used. Such sling constructions have a high lifting and break strength, lighter weight, high temperature resistance and high durability, compared to wire rope or metal chain slings.  
         [0051]     Referring now to  FIG. 3 , the pre-failure warning indicator  11  in accordance with the present invention is illustrated in side view and is shown without the cover  25  and without core  12 . In a preferred embodiment, the sling  10  may be manufactured with only a pre-failure warning indicator  11 , or with both a pre-failure warning indicator  11  and a tamper-evident means  35 . Initially, the operation of the pre-failure warning indicator  11  will be disclosed; the tamper-evident means  35  will be described later with respect to  FIG. 7 .  
         [0052]     A separate (preferably single) strand  20  of yarn is dedicated to the pre-failure warning indicator  11 . The dedicated warning strand  20  is located within cover  25 ; it is preferably placed proximate the core  12  and may either be twisted around the load-bearing strands of the core  12  or it may just lay next to the core  12 , as illustrated in  FIG. 2 .  
         [0053]     In a different embodiment, it may be desired to permanently affix the dedicated strand  20  to the inside of the cover  25 . When a sling is used over a period of time, the cover will develop wear points at specific locations, for example, where the sling hangs from a crane&#39;s hook. Accordingly, it is usually advisable to rotate the cover with respect to the load-bearing core  12 . By securing the dedicated strand  20  to the inner cover, movement of the cover (either intentionally or non-intentionally) will not affect the operation of the pre-failure warning indicator  11 .  
         [0054]     First end  22  and second end  24  of the dedicated strand  20  are terminated in eyes  32 ,  34 , respectively. The dedicated strand  20  and eyes  32 ,  34  are preferably made of the same material as the core strands  17 .  
         [0055]     The eyes  32 ,  34  are connected by a ring  26 , thereby forming an endless loop with the dedicated strand  20 . The shape of the separate dedicated strand  20  generally matches the shape of the endless parallel loops formed by the core strand  17  (i.e., generally circular or oval). Although the term “ring” implies a circularly-shaped object, as used herein “ring” is defined as any closed link or band that will connect the ends of a dedicated strand.  
         [0056]     In one preferred embodiment, the ring  26  is chosen to have a lower tensile strength than the core  12 . The sling manufacturer may choose to do this any number of ways, e.g., by making the ring  26  out of a different material than the dedicated strand  20 , cutting a notch or notches in the ring to physically weaken it, or by making the ring  26  out of the same material as, but of a smaller diameter than, the core strands  17 . When ring  26  is chosen to have a lower tensile strength, the pre-failure warning indicator  11  is designed to trigger and thereby notify the rigger or other users of the sling that the sling  10  has been subjected to an overload condition (i.e., the sling was subjected to a force that was pre-determined to compromise the integrity of the sling, and is sometimes determined to be about four times greater than the sling&#39;s rated capacity).  
         [0057]     Attached to first termination eye  32  is a warning indicator fiber  29 . Warning indicator fiber  29  is an elongated strand that is placed substantially parallel to the ring, is threaded through the second termination eye  34 , is then double-backed along the ring  26  towards the first eye  32 , and directed out an opening in the sling cover  25 . (The external end  40  of the warning indicator fiber  29  that extends through the sling cover  25  is sometimes referred to as a “whip.”) Although the sling cover  25  is not shown in  FIG. 3 , the preferred orientation of the warning indicator fiber  29  is illustrated, i.e., it forms a substantially “J” shape within the sling cover  25 .  
         [0058]     Referring again to  FIG. 1 , the whip  40  of the warning indicator  29  extends freely through cover  25 . Although not necessary, cover patch  30  may be attached (preferably by sewing), to the cover to protect the opening through which the whip end  40  of the warning indicator  29  extends.  
         [0059]     The dedicated strand  20  is preferably made of similar material as the strands  17  of the load-bearing core  12 ; this promotes the relatively equal stretching of all components of the sling  10 . In a preferred embodiment, the ring  26  has a pre-selected lower tensile strength than the material used to make the core strands; in this embodiment, the ring  26  will fail before the lifting core  12  is stretched or fatigued. Alternatively—or in addition—the ring  26  may be designed to have a lower resistance to abrasion, heat, cold, and/or chemical exposure. By carefully choosing the properties of ring  26 , a sling manufacturer can control the condition(s) under which the subject pre-failure warning indicator  11  will trigger.  
         [0060]     In one example, the sling manufacturer may design the ring  26  to fail at 70% of the tensile strength of the inner core. Accordingly, the material from which ring  26  is made and/or its cross-sectional thickness may be chosen to meet the pre-selected tensile strength.  
         [0061]     When the sling  10  is placed under a load that exceeds its recommended rating, ring  26  will fail before damage can occur to either the load bearing core strands  17  that form the core  12  or the dedicated strand  20 . When ring  26  fails, the termination eyes  32 ,  34  begin moving in opposite directions away from each other, and the physical distance between the eyes  32 ,  34  and/or ends  22 ,  24  of the dedicated strand  20  increases.  
         [0062]     As the eyes  32 ,  34  move apart, the whip portion  40  of warning indicator fiber  29  (i.e., the end that extends freely outside the cover  25 ) is drawn back inside the cover  25  until it no longer extends through the cover. If the whip end  40  of the warning indicator  29  is not visible, an inspector or rigger will immediately be able to determine that the sling  10  may have been subjected to a condition that would prevent the lifting core  12  from lifting its maximum rated load and will therefore remove the sling  10  from service for further inspection. The double-back configuration of the indicator fiber  29  ensures that the whip end  40  moves twice the distance compared to the distance the eyes  32 ,  34  move apart, ensuring that every time a trigger event occurs, the whip end  40  will completely disappear. (It should be noted that the whip end  40  of the warning indicator  29  may be shaded in a high visibility color or otherwise marked, so that its visibility or lack thereof will be more noticeable.)  
         [0063]     An important feature is that the ring  26  is designed to fail before damage occurs to the lifting core, thereby warning the riggers that they must either stop using the sling  10  in the manner in which they are using it or, if they continue, the sling  10  will be permanently damaged. If the rigger stops using the sling, the integrity of the lifting core  12  may remain intact. In this case, the sling  10  can be returned to the manufacturer and the pre-failure warning indicator  11  can be replaced or repaired; usually only the ring  26  will have to be replaced.  
         [0064]     A primary advantage of the pre-failure warning indicator  11  in accordance with this invention is that the ring  26  may be designed to more precisely fail at a controlled point (regardless of whether it is at a specific strength, abrasion, temperature, etc.). The ring  26  can be used as an indicator of an overload condition by making it weaker than the individual core strands  17 . In a second embodiment, the ring  26  can be made from a material that would fail from yarn-on-yarn abrasion damage. In a third embodiment, the ring  26  can be made to fail from excessive temperatures (either heat or cold, or both). In a fourth embodiment, the ring  26  could be made from a material that would deteriorate in the presence of chemicals at a concentration lower than would damage the strands  17  of the load-bearing core. In still another embodiment, the ring  26  can be made of a material or combination of materials that would fail when subjected to more than one of the pre-determined conditions (e.g., overload and excessive heat).  
         [0065]     In all of the above conditions, the ring  26  is preferably designed to fail at the pre-determined or desired condition at a relatively precise point. For example, if the sling is rated to lift 6,000 pounds (with a five-to-one design factor), the ring  26  can be designed to break relatively close to 24,000 pounds every time. Therefore, the ring  26  can be made to fail before the built-in safety factor of 30,000 pounds and well before any damage occurs to the sling  10 . The use of the predictable pre-failure warning indicator  11  as disclosed herein, gives a sling manufacturer a more predictable and accurate way of incorporating a failure notification means into any sling it designs or makes. In other words, the present invention introduces a degree of predictability into the manufacturing of roundslings since the failure point of the ring  26  can be selected and consistently reproduced. In prior art tell-tail indicators, the failure point was unpredictable and was not consistently reproducible.  
         [0066]     A prototype was made in order to meet the following requirements: 
        Tensile strength of 30,000 lbs.;     Vertical Rated Capacity=6,000 lbs. at a 5 to 1 design factor;     Overload Warning Indicator triggers at 20,000-25,000 lbs. with a Design Factor between 3 &amp; 4 to 1;     Lightweight: 6′ prototype weighs 1.7 lbs;     Double contrasting color cover: Outer Green and inner Red for easy cut inspection;     Low stretch;     Impervious to salt water and most chemicals including oil, diluted acids and bases;        
 
         [0074]     Made with K-Spec® proprietary blend of high performance core yarn.  
         [0075]     The above prototype was tested and it was determined that the whip  40  of the pre-failure warning indicator  11  consistently disappeared (meaning that ring  26  consistently broke) at between 23,000 and 24,000 lbs and the final tensile strength of the sling  10  was 32,860 lbs.  
         [0076]     When the whip  40  of the warning indicator  29  is no longer visible, the sling  10  should be returned to the sling manufacturer for inspection and/or repair. The ring  26  consistently broke before damage occurred to either the dedicated strand  20  or the load-bearing core  12 . In many cases, the sling manufacturer will only have to replace the ring  26  in order to refurbish the sling and return it service. (In the above example, the ring  26  failed around 24,000 pounds and the sling  10  did not approach its maximum tensile strength of 30,000 pounds.) Under certain conditions, even though the ring  26  may have been designed to fail first, the sling  10  may have degraded to a point where it must be discarded entirely. For example, if the sling  10  was exposed to an acidic environment for an extended period of time, especially after the ring  26  failed, the sling  10  (and, specifically, the strands  17  that make up the load-bearing core) may have been damaged to such an extent that it can no longer meet its rated capacity. (The selection of the material for the core is the primary factor in determining whether the subject sling is impervious to sea water, oil, acids and other chemicals. Also, the cover  25  plays an important factor in protecting the core especially from abrasion or from sharp edges.)  
         [0077]     It should be noted that a person skilled in the art, after reading the present disclosure could produce equivalent embodiments. For example, even though virtually all synthetic slings have a load-bearing core protected by an outer cover, a sling manufacturer can eliminate the outer cover (or shorten the outer cover) so that the ring  26  is visible. In this embodiment, a dedicated strand is not required and an operator can determine that a sling overload condition (or other failure condition) was met by observing the integrity of the ring  26 .  
         [0078]     Referring now to  FIG. 4 , another preferred embodiment is disclosed. In this embodiment, pre-failure warning indicator Ha incorporates a plurality of rings  26   a ,  26   b ,  26   c , etc. connected together (i.e., as links in a chain) between termination eye  32  and termination eye  34 . In this manner, a sling  10   a  can be designed to indicate whether it has been subjected to multiple excessive conditions—any one of which could cause the controlled destruction of one of the linked rings  26   a ,  26   b ,  26   c , etc. and which would then trigger the warning indicator  11   a  in a similar manner as when there is only one ring  26 . (Although this example uses three rings  26   a ,  26   b , and  26   c , two rings, four rings or more rings may be used depending on the number of failure conditions the sling manufacturer wishes to incorporate into the sling.)  
         [0079]     The warning indicator fiber  29  has a secured end and a whip end. The secured end is attached to one termination eye  32 ; the remainder of the indicator fiber  29  is placed along all of the rings  26   a ,  26   b ,  26   c ; the indicator fiber is then threaded through the other termination eye  34 , is double-backed along all the rings, and is finally directed through the slit in the cover  25  where the whip is visible to an operator.  
         [0080]     For example, as shown in  FIG. 4 , ring  26   a  could be designed to fail when the sling is subjected to an overload (excessive weight) condition, ring  26   b  could be designed to fail under an excessive heat condition, and ring  26   c  could be designed to fail when exposed to a specific concentration of a particular chemical. Therefore, if the sling is subjected to any of the pre-determined failure conditions, one of the rings  26   a ,  26   b ,  26   c  will fail, causing the termination eyes  32 ,  34  to pull away from one another, thereby causing the whip portion  40  of the warning indicator whip  29  to completely retract inside the cover  25 . In this manner, a single predictable pre-failure warning indicator  11   c  can be used to signal one of a multiple possible failure conditions. By marking the individual rings before assembly of the sling, one can determine the exact condition which the sling was subjected to that caused the pre-failure warning indicator to trigger. So, for example, if ring  26   b  failed (and ring  26   a  and ring  26   c  remained intact), the sling manufacturer would know that the sling was subjected to a high temperature for an extended period of time.  
         [0081]     An improved synthetic roundsling having multiple cores is manufactured by Slingmax, Inc. and is disclosed in U.S. Pat. No. 4,850,629 to Dennis St. Germain. An embodiment disclosed in U.S. Pat. No. 4,850,629 is a two-core roundsling (sold under the brand name TWIN-PATH®) which has two-load lifting cores inside a single cover. The cover is also divided into two separate paths. U.S. Pat. No. 4,850,629 is incorporated by reference as if fully set forth herein.  
         [0082]     Similar to a sling having a single core (and a single pre-failure warning indicator), in a multiple-core or multiple-path roundsling  50 , each core incorporates a predictable pre-failure warning indicator  11   a ,  11   b , as taught herein. Referring now to  FIG. 5 , a first dedicated strand  20   a  is associated with the first core  12   a  of a two-path sling  50  and a second dedicated strand  20   b  is associated with the second core of the two-path sling. The dedicated strand  20   a  is terminated by termination eyes  32   a ,  34   a , and dedicated strand  20   b  is terminated by termination eyes  32   b ,  34   b , respectively. A ring  26   d ,  26   e , as disclosed previously in a one-path sling  10 , is incorporated into each path of the two-path sling  50 .  
         [0083]     Referring now to  FIG. 6 , whip  40   a  is associated with the predictable pre-warning indicator  11   a  in the first path of the sling  50 , and whip  40   b  is associated with the predictable pre-warning indicator  11   b  in the second path. (It should be noted that the warning indicator fiber  29   a  is attached to one termination eye  32   a , threaded through the other termination eye  34   a , and the whip end  40   a  is passed through the cover  25   a , and operates in a similar manner as the “basic” single-path sling  10  illustrated in  FIGS. 1 through 3  using only one ring  26 . Similarly, warning indicator strand  29   b  is attached to one termination eye  32   b , threaded through the other termination eye  34   b , and the respective whip end  40   b  is passed through the cover, and operates in a similar manner as when there is only one ring  26 .)  
         [0084]     Sling  50  is comprised of a two-path core; as illustrated in  FIG. 6  the warning indicator whips  40   a  and  40   b  are passed through the cover  25   a  and emerge in free extension apart from the cover  25   a . This embodiment provides a pre-failure indicator for each path that can convey sling damage or overload when either core of the TWIN-PATH® sling is subjected to a load which exceeds its tensile strength or rated capacity. When this happens, one or both of the extended warning indicator whips,  40   a  and/or  40   b , which emerge outside of the cover material  25   a  will retract completely within the cover thereby alerting the operator or rigger to a sling overload condition.  
         [0085]     In a Twin-Path® sling having exactly two cores, each core is identical to the other. Referring again to  FIG. 5 , an interesting variation for a two-core sling is the ability to design into the sling two distinct and separate damage-indicating parameters into a single sling. For example, in the first path, the ring  26   d  could be designed to fail only at a lower tensile strength than the core  12 ; while in the second path, the ring  26   e  could be designed to fail only when the sling is exposed to a certain chemical in the environment. The whips  40   a ,  40   b  of warning indicators  29   a  and  29   b  can be marked or coded in order to indicate which whip is associated with which ring so that if a ring breaks, the rigger will know the condition that was exceeded (i.e., if ring  26   d  breaks it was because the TWIN-PATH® sling was subjected to a load approaching it&#39;s maximum load rating; alternatively, if ring  26   e  breaks if was because the TWIN-PATH® sling was exposed to the chemical for a period of time such that it deteriorated the integrity of the sling). Therefore, if a three-core sling is made, three separate conditions may be simultaneously and independently tested using the predictable pre-failure indicator  11  taught herein; a four-core sling can be used to simultaneously test for four separate conditions, etc.  
         [0086]     In this manner, if the two-path sling  50  is subjected to either one of the pre-selected conditions to a point that causes either ring  26   d  or ring  26   e  to fail, the rigger will be alerted and will have more information than would otherwise be available to him. Designing the rings  26   d ,  26   e  to fail under different situations may also assist the sling manufacturer in analyzing the sling or further improving the sling, if the sling is ever returned for inspection or repair. However, there are situations in which it will be necessary to design the rings  26   d  and  26   e  to fail under the same condition (e.g., an overload condition).  
         [0087]     The pre-failure warning indicator  11  in accordance with the present invention is designed with a trigger mechanism that will generate a magnified force on the whip end  40  of the external warning indicator  29  in order to move the whip end  40  out-of-sight almost instantaneously, if any of the pre-engineered conditions are met and the ring fails. The reason why the force on the whip end  40  of the warning indicator fiber  29  is magnified is because of the double-back design of the warning indicator fiber  29  through the termination eyes  32 ,  34 . After the ring  26  breaks, the termination eyes  32  and  34  separate at a certain speed; however, since the warning indicator fiber  29  is tied to one eye  32 , threaded through the opposite eye  34 , and doubles-back along the ring before emerging through the cover  25 , the whip end  40  of the warning indicator is moving twice as fast (and twice the distance) as the speed (and distance) at which the eyes  32 ,  34  are moving away from each other. Accordingly, the whip end  40  withdraws inside the cover entirely so that there is no question as to whether a trigger event occurred.  
         [0088]     Another feature to note, is that because the whip  40  of the warning indicator  29  is moving so fast, it creates a sound that is audible to the operator. Therefore, the present invention not only gives a visual indication that a sling has reached a critical damage point, but also gives an audible warning. The audible warning is especially important when the sling is positioned so that the operator cannot see the whip  40  (e.g., when the sling is hanging thirty feet in the air).  
         [0089]     Another notable feature of the subject pre-failure warning indicator  11  is the ability to warn the rigger of an overload and other dangerous situations without affecting the overall strength of the roundsling  10 . If the rigger stops lifting the load promptly after the pre-failure warning indicator  11  is triggered, the sling  10  retains 100% of its residual strength.  
         [0090]     The color code safety feature of this invention may be achieved by encasing the load-bearing core in two separate covers, each cover having a different color. For example, the outer cover could be green or blue, and the inner cover could be orange or red; since the inner cover is a different color from the outer cover, it will show through whenever the outer cover is cut or worn through. This double-cover feature provides a visible safety warning for any user of the sling that abrasion or other damage not normally detectable, has occurred.  
         [0091]     In another embodiment of the present invention, a pre-failure warning indicator  11  can be adapted with a sabotage or tamper-evident means. Referring now to  FIG. 7 , a tamper-evident tag  35  is attached to either the dedicated indicator strand  20  or, preferably, to one of the eyes  32  or  34 . The free end of the tamper-evident tag  35  is passed through the cover via a slit. The slit can be the same one through which the whip  40  passes through.  
         [0092]     If the pre-failure warning indicator  11  is triggered (by, for example, an overload condition), this means that ring  26  has been broken, the ends  22 ,  24  of the dedicated strand  20  are free, causing whip  40  to withdraw completely within the cover. Upon inspection, the tamper-evident tag  35  can be easily pulled out from inside the cover  25  along with a portion of the dedicated strand  20 , as illustrated in  FIG. 8 , when the pre-failure warning indicator  11  has been triggered. If the whip end  40  of the warning indicator is not visible because of an intentional intervention by a user, the tamper-evident tag  35  will remain secure and cannot be pulled from the cover  25 . In this manner, sabotage of the sling  10  can be evidenced by the supervisor on the work site. (In order to avoid work, some users will cut off the whip end  40  of the warning indicator  29  in an attempt to make it appear that the sling was subjected to a damage situation and, therefore, work must be temporarily stopped so that the sling can be removed for inspection and, if necessary, replaced with a new sling.)  
         [0093]     As part of the inspection process, the inspector may yank on the tamper-evident tag  35 . If the tag is secure, the sling  10  is useable; but, if the tamper-evident tag  35  can be pulled out from inside the cover, the sling  10  must be removed from use because the pre-failure warning indicator  11  has been triggered. Of course, if a saboteur cuts both the whip end  40  and the visible portion of the tamper-evident tag  35 , the inspector will immediately know that the sling  10  has been tampered with, and should remove the sling from service.  
         [0094]     It is important to note that no other prior warning indicators have the ability to quickly inspect the condition of a roundsling. Also, prior warning indicators are not as accurate as the subject warning indicator  11 . If the whip end  40  of the warning indicator is visible and the cover  25  is intact, the roundsling can be used for the next lift; if the whip end  40  of the warning indicator is not visible, the sling should be removed from service and inspected. The subject pre-failure warning indicator is the first completely pass/fail inspection system—it is a completely objective test and not subjective.  
         [0095]     It should also be noted that one skilled in the art, after reading this disclosure, may develop variations that are contemplated as being equivalent in scope to the various embodiments specifically set forth in the present disclosure. For example, the termination loops  32 ,  34  may be eliminated and the ends of the dedicated strand  20  may be tied directly to the ring  26 . (Alternatively, slip-knots or other means may be used to secure the ends of the strand  20  to the ring  26 .) Although this invention has been described and illustrated by reference to specific embodiments, it will be apparent to those skilled in the art that various changes, modifications and equivalents may be made which clearly fall within the scope of this invention. The present invention is intended to be protected broadly within the spirit and scope of the appended claims.