Patent Publication Number: US-6712181-B2

Title: Safety rope grab device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/267,227, filed on Feb. 7, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a safety device which is carried by a lifeline or tether line and which will hold fast when weighted, but which is slidable along the lifeline or tether line when unweighted. 
     BACKGROUND OF THE INVENTION 
     FIG. 1 illustrates a form of Prusik knot  10 , also known as a triple sliding hitch or “monkey fist.” Such a knot can be used in fall arrest safety systems which include a lifeline or tether line  12  having one end portion  14  adapted for connection to an anchoring device. For example, one end of the lifeline  12  can be formed with a sturdy eye  16  for receiving a carabiner  18  which, in turn, is secured to the eye of a roof anchor. The lifeline hangs from the anchor. The Prusik knot, when unweighted, can be slid along the lifeline as desired. A termination knot  20  can be provided to limit the extent of sliding movement of the Prusik knot along the lifeline. 
     In general, the Prusik knot can be formed of a single stretch of rope having several wraps  22  around the lifeline  12 . The free end portions  24  of the rope are tucked through the longitudinally extending portion of a loop  26 . In the embodiment illustrated in FIG. 1, the free ends of the rope are threaded through rings  28  of a thimble  30  which forms a central eye  32 , and then are hand tied to form the end knot  33 . A carabiner  34  is carried in the eye  32  and can be connected by a lanyard to a worker&#39;s harness. In normal use, with little or no weight applied to the eye portion of the Prusik knot, the wraps  22  can be slid along the lifeline to a desired position. If substantial weight is applied to the eye, such as by way of the carabiner  34 , the knot holds fast on the lifeline. 
     There are a myriad of regulations and specifications for fall arrest safety equipment. For the design shown in FIG. 1, the Prusik knot should contain at least six wraps  22 . However, sometimes the knot will be unfastened by sliding the thimble  30  and rope ends  24  through the loop  26 , resulting in fewer wraps  22  on the lifeline  12 . The entire knot can be unfastened in this manner. Also, there can be concerns with respect to the type of connection from a user&#39;s harness and/or lanyard to the eye  32 . It is generally preferred that a large loop carabiner  34  be used, but sometimes a snap hook having a smaller opening will be connected to the thimble. In that case, a twisting force of the snap hook in the thimble eye may disengage the hook by damaging the swinging clasp of the snap hook. Also, in the construction of FIG. 1, some protection is formed for the inside portion of the rope loop around the thimble, but the exterior portion is exposed and may become frayed. Further, in fall arrest systems it is desirable for an integral component such as a the Prusik knot to be replaced if it has been subjected to the substantial force of a fall, but whether or not the knot has been subjected to such a force is not immediately ascertainable. Finally, it is possible for the hand tied knot  33  to become loosened over time, requiring frequent and careful inspection if the safety system utilizing the knot is to be reliable. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved rope grab device operating on the principles of a Prusik knot. In a preferred embodiment, a casing or cover is provided for the free end portions of the stretch of rope forming the knot. The cover can be formed of complimentary halves defining an internal cavity receiving the free end portions of the knot. One free end portion is much longer than the other and is looped in an internal channel to form a bight. This end portion can be secured to the other end portion, such as by a swage. The cover can completely encase the end portions of the knot so that they are not subjected to cutting or fraying, and can assist in reliably securing the free end portions of the knot without hand tying. In other aspects of the invention, the cover permits convenient and reliable interconnection with other components without compatibility problems such as can occur with certain snap hooks. The cover can be formed to provide a reliable indication of a strong force having been exerted on the safety device, and to lessen the likelihood of the knot unraveling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 (prior art) is a side elevation of components of a fall arrest safety system having a known rope grab device, namely, a Prusik knot; 
     FIG. 2 is a side elevation of a rope grab device in accordance with the present invention, using a terminal casing or cover; 
     FIG. 3 is a top perspective of a cover in accordance with the present invention; 
     FIG. 4 is a top plan of part of the cover of FIG. 3; 
     FIG. 5 is a section along line  5 — 5  of FIG. 4; and 
     FIG. 6 is a diagrammatic top plan of a cover in accordance with FIG. 4, showing the interconnection of the casing with the free ends of a stretch of rope extending from a Prusik knot. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The improved rope grab device in accordance with the present invention can be used for any system in which it is desired to have a strong, reliable device which will hold fast to a line when weighted but be slidable along the line when unweighted. For example, components of a fall arrest safety system in which the rope grab device of the present invention may be used are illustrated in FIG.  2 . These components correspond generally to components of the system shown in FIG. 1. A lifeline or tether line  12  has an end termination  14  adapted for connection to a safety anchor, such as by provision of an eye  16  to receive a carabiner  18 . The rope grab device  40  of the present invention can, when unweighted, be slid along the lifeline to a desired location. A termination knot  20  may be provided to prevent the rope grab device  40  from inadvertently being slid off the end of the line  12 . 
     The rope grab device  40  includes a Prusik knot  10  having several generally helical wraps  22  around the line  12 , and the longitudinal loop  26  through which the free end portions  24  of the knot rope are threaded or tucked. However, rather than terminating in a hand tied knot, the free end portions  24  of the Prusik knot are secured within a casing or cover  42 . Cover  42  has an eye  43  designed for receiving a connection device such as carabiner  34 . The carabiner can, in turn, be connected to a body harness, such as by way of a lanyard. 
     The size of the cover  42  prevents it from being slipped back through the loop  26 . Also, the cover completely encloses the end portions  24  of the stretch of rope forming the Prusik knot. 
     With reference to FIGS. 3-5, the cover  42  can be formed with upper and lower halves  44  and  46 . One such half is shown in FIGS. 4 and 5. Each cover half  44 ,  46  has a near or proximate end  48  with openings  50  contoured to receive the rope ends  24 . Openings  50  lead to a larger interior cavity  52  and a generally oval channel  54  of approximately semicircular cross section. The channel encircles the eye  43 . The two halves of the cover are nearly identical, the only difference being that one half has holes  58  for externally threaded fasteners and the other half preferably has internally threaded inserts in the form of nuts for receiving the threaded fasteners. 
     The two halves will be secured together by the nuts and fasteners to the condition shown in FIG. 3, but only after securing and routing of the rope ends  24  as shown in FIG.  6 . In FIG. 6, one rope end  24 A is shown textured while the other end  24 B is drawn white (not textured) for ease of illustration. End  24 A is at least twice as long as end  24 B. Each end extends through a swage  60  which initially is cylindrical but which is crimped in a die for tightly grasping the free ends  24 . End  24 B, drawn white in FIG. 6, extends through the central portion of the swage with a small part projecting from the other end. The other rope end  24 A, drawn textured, extends through the swage, in the same direction as the first end  24 B, then forms a bight or loop  62  in the channel  54  and extends back through the swage. The three rope sections extend alongside each other and are secured together by crimping the swage. The internal cavity  52  of the combined cover  44 ,  46  is sized to snugly receive the swage therein. The size of the bight  62  is selected to match the length of the internal channel  54  of the cover, and the openings  50  of the cover are sized to snugly receive the rope ends  24 A,  24 B. 
     With the rope ends swaged together as shown in FIG. 6 (the swage  60  being positioned in the cavity  52 ), and with the bight  62  routed in the channel  54 , the two halves are secured together by the fasteners. At that point, the cover and Prusik knot will appear as shown in FIG.  2 . The cover has a convenient handle portion  64  which is sized and contoured for convenient manipulation of the Prusik knot along the lifeline  12 . The opposite end portion forms the eye  43 , which deliberately is quite small and set inward from the outer periphery of the cover. This prevents inappropriate snap hooks from being secured to the cover, which could be dangerous. The overall size and length of the cover prevent it from being passed back under the loop  26 , so that the improved safety device cannot be removed from the lifeline without either sliding it off an end or severing the Prusik knot. 
     Another advantage of the cover is that it can be formed of a tough, abrasion resistant material, but with its strength selected so as to bend or break in the event of a large force being applied (in excess of a predetermined force), as would occur when arresting a fall. Structural integrity of the device for halting a fall is based on the rope characteristics, rather than the cover. The cover forms a convenient indicator or telltale for excessive force having been applied at one time or another to the device, which indicates that it should be replaced in accordance with current regulations. 
     While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.