Abstract:
A towline fuse device for use in a towing system with a towing vehicle and a towline is provided. The towline fuse device comprises a fuse mechanism for failing at a predetermined level of tensile force, the fuse mechanism mounted between the towing vehicle and the towline wherein the predetermined level of failure of the fuse mechanism is less than the towing ratings of the towing system. The strain energy in the towline is then harmlessly dissipated in the towline fuse device.

Description:
The present application is a continuation-in-part of pending provisional patent application Ser. No. 60/582,315, filed on Jun. 23, 2004, entitled “Towing Fuse for Cable/Rope/Chain Towing Devices”. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to a towing system for towing equipment with a cable, rope, or chain towing device and, more particularly, the invention relates to towline fuse and strain energy dissipation device included in the towing system that fails at a predetermined level of tensile force that is lower than the towing ratings of the rest of the towing system components, thereby acting as a “fuse” for the towing system and harmlessly dissipates the strain energy stored in the stretched towline. 
   2. Description of the Prior Art 
   When using a cable, rope, or chain, hereafter referred to as the towline, to tow a load with a mobile vehicle, the line or the towing attachments can be easily overloaded, causing failure of some portion of the towing system. When the towline separates, strain energy stored in the line can cause the parts of the system to fly off in unpredictable directions, endangering life, and/or property. Such failures have been known to cause injury and death. 
   SUMMARY 
   The present invention is a towline fuse device for use in a towing system with a towing vehicle and a towline. The towline fuse device comprises fuse means for failing at a predetermined level of tensile force, the fuse means mounted between the towing vehicle and the towline, wherein the predetermined level of failure of the fuse means is less than the towing ratings of the towing system. In addition, the towline fuse device comprises means for dissipating strain energy stored in the elastically stretched towline through friction, eddy currents, aerodynamic or hydrodynamic turbulence, or other dissipative means. 
   The present invention further includes a method for towing with a towline and a towing vehicle. The method comprises mounting a fuse device between the towing vehicle and the towline, failing the fuse device at a predetermined level of tensile force, and dissipating towline strain energy immediately after the failure of the fuse device, the fuse device failing at a predetermined level of tensile force wherein the predetermined level of failure of the fuse device is less than the towing ratings of the towing system and the strain energy stored in the stretched towline is harmlessly dissipated. 
   Furthermore, the present invention includes a towline device for use in a towing system with a towing vehicle and a towline. The towline device comprises towline strain energy dispersion means for dissipating towline strain energy of the towline upon failure of the towline. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view illustrating a towline fuse device embodying calibrated fracture of a tension member and frictional dissipation of towline strain energy, constructed in accordance with the present invention; 
       FIG. 2  is a sectional view illustrating a towline fuse device embodying controlled frictional release to initiate separation of the towing fuse and frictional dissipation of towline strain energy, constructed in accordance with the present invention; 
       FIG. 3  is a sectional view illustrating a towline fuse device embodying controlled fracture of composite material to initiate separation of the towing fuse and continued progressive fracture of the composite material to dissipate towline strain energy, constructed in accordance with the present invention; 
       FIG. 4  is a sectional view illustrating a towline fuse device embodying calibrated fracture of a tension member and eddy current heating to dissipate towline strain energy, constructed in accordance with the present invention; 
       FIG. 5  is a graph illustrating the hook velocity of a 0.25 kilogram hook as a function of time from the release event with and without aerodynamic dissipation (drag chute) of strain energy from a 15-foot long tow strap; 
       FIG. 6  is a graph illustrating the hook velocity of a 0.25 kilogram hook as a function of distance from the release point with and without aerodynamic dissipation (drag chute) of strain energy from a 15-foot long tow strap; 
       FIG. 7  is a graph illustrating the hook velocity of a 0.25 kilogram hook as a function of time from the release event with and without eddy current dissipation of strain energy from a 15-foot long tow strap; 
       FIG. 8  is a graph illustrating the hook velocity of a 0.25 kilogram hook as a function of distance from the release point with and without eddy current dissipation of strain energy from a 15-foot long tow strap; 
       FIG. 9  is a graph illustrating the kinetic energy of a 0.25 kilogram hook as a function of distance from the release point with and without eddy current dissipation of strain energy for a 15-foot long tow strap; and 
       FIG. 10  is an elevational side view illustrating the towline fuse device, constructed in accordance with the present invention, secured between a vehicle and towable equipment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As illustrated in  FIGS. 1–4  and  10 , the present invention is a towline fuse device, indicated generally at  10 , positioned between towing equipment  12 , i.e., a vehicle, and a cable, rope, or chain  14  for use in a towing system. In particular, the towline fuse device  10  of the present invention is a “link” included in the towing system that fails at a predetermined level of tensile force that is lower than the towing ratings of the rest of the towing system components, thereby acting as a “fuse” for the towing system. The towline fuse device  10  is designed in such a manner that the towline fuse device  10  absorbs any strain energy during the failure process, preventing towing system components from becoming deadly missiles which can injury any persons nearby.  FIG. 10  illustrates the towline fuse device  10  as part of a towing system. 
   The means for dissipating the strain energy with the towline fuse device  10  of the present invention can take a number of forms, including, but not limited to:
         Mechanical friction as the components of the towline fuse device  10  separate;   A given negative slope to the stress strain curve following failure of the towline fuse device  10 . This would most likely require composite material construction of the towline fuse device  10 , as described in further detail below;   Eddy current dissipation of strain energy from magnetic induction;   Aerodynamic dissipation of strain energy through turbulent mixing;   Hydraulic dissipation of the strain energy following failure of the towline fuse device  10 ; and   Tethering of the towline fuse device  10  to prevent parts from becoming missiles. This is the least preferred embodiment, since the tether can also store significant strain energy and may break.       

   An embodiment of the towline fuse device  10  of the present invention is illustrated in  FIG. 1 .  FIG. 1  illustrates a towline fuse device  10  embodying frictional dissipation of towline strain energy. The towline fuse device  10  includes a calibrated fracture pin  16  designed to fail at a predetermined tensile load, whereupon a drawbar  18  being pulled from a socket  20 , the towline strain energy dissipates as heat through the interaction of the drawbar  18  with friction material  22 . The friction material can be solid or reinforced rubber material or metallic, ceramic, or synthetic fibers such as Kevlar™ embedded in a resin matrix. This type of towline fuse device  10  is reusable by simply replacing the fracture pin  16  after use. The fracture pin  16  could be held in place by screw threads, retaining rings, or pins, or the like. 
   A shear pin is a second method (not shown) of obtaining a specific release loading and could readily replace the fracture pin  16 . A third method of obtaining a specific release loading is to utilize a spring loaded latch mechanism (not shown). A spring loaded latch mechanism would allow adjustment of the release loading. 
   The strain energy dissipation could also be adjustable by including an adjustable spring loading mechanism for the friction material (not shown). Eye attachments are shown for the towline fuse device  10 , but any standard towline attachment means, such as hooks, clevises, swaged fittings, or the like, could be used to incorporate the towline fuse device  10  into the towline  14 . The other components would necessarily need to have higher load ratings than the towline fuse device  10  for the towline fuse device  10  to operate correctly. These attachment considerations apply to all embodiments of the towline fuse devices  10 , as described herein. 
   The friction fuse embodiments of the towline fuse device  10  do not necessarily need to have a calibrated fracture pin  16  or a shear pin. Since the coefficient of static friction is greater than the coefficient of dynamic friction, a towline fuse device  10  can be designed to release at a particular static friction loading. The draw bar  18  will then slide from the socket  20 , dissipating strain energy in the same manner as the friction towline fuse device  10  discussed above. This type of towline fuse device  10  could also be made adjustable and reusable. 
     FIG. 2  illustrates the construction of an embodiment of the present invention, a friction towline fuse device  10  without a calibrated fracture pin  16  in accordance with the present invention.  FIG. 2  illustrates a towline fuse device  10  embodying controlled frictional release and frictional dissipation of towline strain energy. This method of load control would be less precise than a fracture method, but would be simpler and less expensive. 
   As illustrated in  FIG. 3 , in still another embodiment of the present invention, the towline fuse device  10  has a controlled failure strain curve towline fuse.  FIG. 3  illustrates a towline fuse device  10  embodying controlled fracture of composite material to dissipate towline strain energy. The composite material sleeve  24  connecting the socket  20  with the draw bar  16  would be engineered to fail in a progressive, controlled manner. This is accomplished by selecting various ply materials and matrix materials, and then manipulating ply orientation and fiber volume to achieve progressive, controlled failure. 
   Composite material fuse elements  24  would be non-adjustable and non-reusable. The towline fuse device  10  could be designed to accept a replaceable composite material cartridge, similar to replacing an electrical fuse. 
   As illustrated in  FIG. 4 , in yet another embodiment of the present invention, the towline fuse device  10  dissipates the strain energy through eddy currents.  FIG. 4  illustrates a towline fuse device  10  that includes a calibrated fracture pin  16  designed to fail at a predetermined tensile load, and the towline strain energy is dissipated through eddy current heating. The draw bar  18  would be attached to the socket  20  with a fracture pin  16 , shear pin, or spring loaded latch, similar to the friction embodiment shown in  FIG. 1 . When the draw bar  18  releases, a permanent magnet  26  is drawn through a copper sleeve  28  and the eddy currents produced by the moving magnetic fields dissipate the strain energy as heat in the copper sleeve  28 . This embodiment could be adjusted by changing the magnet to one with high or lower magnet energy product to achieve higher or lower energy dissipation, respectively, and could be reusable with the replacement of the fractured element or in the case of the spring loaded latch, by replacing the draw bar  18 . 
   Hydraulic means of dissipating the strain energy are within the scope of the present invention. In general, these will be more complex and more expensive to implement. Hydraulic means would, however, allow precise control of the dissipation of strain energy throughout the release cycle. One means of doing this would be to have a variable area groove in a hydraulic cylinder wall, so that the resistance of the fluid bypassing the piston would decrease to match the decrease in the strain energy as the draw bar was withdrawn. A greater level of engineering design would be required to make such a device. 
   Aerodynamic means of dissipating the strain energy are also within the scope of the present invention. In such an embodiment, a small drag chute would be attached to the towline and packed into a receiving socket. Upon failure or release of a calibrated tensile fracture pin, a shear pin, or a spring loaded latch, the drag chute would be pulled from the socket by the towline and deployed by aerodynamic forces. However, aerodynamic forces vary as the square of the relative velocity between the drag chute and the surrounding atmosphere and vary with the density of the atmosphere, so the performance of such an embodiment would vary with such factors as the ambient temperature, the elevation, and the strength and direction of the local wind. Significant velocity of the released portion of the towline would have to be achieved for aerodynamic dissipation of strain energy to be operable. 
   The inventor of the present application has completed theoretical modeling of energy dissipation from a stretched, elastic tow rope, or strap using two different techniques, aerodynamic friction, and eddy current friction. Some assumptions were made to assure a conservative analysis:
         the tow rope or strap has no mass,   the tow rope or strap has no internal damping,   the tow rope or strap is modeled as a linear spring with a spring constant of 15725 N/m/m,   the tow rope or strap is 15 feet (4.6 meters) long,   a hook of mass 0.25 kilograms is accelerated by the strain in the tow rope or strap,   without any dissipation mechanisms, the strain energy is entirely converted into kinetic energy of the hook, and   the towing fuse is located near the towing vehicle, but the acceleration of the vehicle away from the towed vehicle upon the activation of the towing fuse is negligible.       

   Using these assumptions, the velocity of the hook as a function of time and as a function of distance of the hook from the release point was calculated using a time step of 1μ second. The velocity as a function of time and distance for aerodynamic dissipation, i.e. a drag chute, are shown in  FIGS. 5 and 6 , respectively. 
   The velocity as a function of time and distance for eddy current dissipation, using a magnet of 1-inch diameter and 2-inches length, are shown in  FIGS. 7 and 8 , respectively. 
   The magnet parameters used in the analysis were: 
   1-inch diameter, 
   2-inches length, 
   Neodymium-Iron-Boron (NeFeB) magnet, 
   A magnet energy product of 17.5 MGOe, and 
   0.03125-inch air gap. 
   The initial assumptions for this analysis yielded hook velocities in the absence of dissipative methods that are significantly higher than actual velocities would be, since all sources of parasitic losses are ignored. Therefore, the dissipative mechanisms in this analysis should be able to perform at least as well as predicted. The eddy current dissipation looks particularly promising in keeping the kinetic energy of the hook low.  FIG. 9  illustrates kinetic energy of the hook as a function of distance from the release point for eddy current dissipation. The maximum hook kinetic energy in this case is only 1/100 that of the freely accelerated hook. Other dissipative mechanisms, such as air friction, ground friction, and internal fiber friction in the rope or strap, should then bring the hook to a halt not far from the release point. The main drawback of the eddy current dissipation device is that it would have to be about as long as or longer than the stretch in the tow rope or strap. In the example calculations the stretch is about ⅓ of a meter or about 1 foot. 
   Aerodynamic dissipation is also possible, but the physical parameters are not as favorable. The results of the calculations that are shown in  FIGS. 1 and 2  are for a drag chute 1 meter in diameter. Larger drag chutes will, of course, provide better performance, but even a 1-meter diameter chute is probably unreasonably large for a 4.6 meter long tow strap. 
   This analysis clearly shows that dissipation of the strain energy in a stretched tow rope or strap is possible using different mechanisms. The eddy current dissipation mechanism could be built into a practical device, essentially a tube one and one-half (1½″) inches in diameter and about one (1′) foot long. The tube would consist of an iron exterior tube lined with a copper tube about 1/32 inch thick. 
   The potential market for the present invention include:
         Automotive towing, potentially applicable to those individuals who carry towing equipment in their personal vehicles   Industrial towing applications. This idea specifically stems from the problem of towing large equipment in a surface coal mine where safety considerations are stringent and the forces and strain energies involved are extremely large.   Winch operation, both automotive and industrial. The automotive market may be better than the industrial market, since torque limiting equipment can be incorporated in larger, expensive winches.       

   The foregoing exemplary descriptions and the illustrative preferred embodiments of the present invention have been explained in the drawings and described in detail, with varying modifications and alternative embodiments being taught. While the invention has been so shown, described and illustrated, it should be understood by those skilled in the art that equivalent changes in form and detail may be made therein without departing from the true spirit and scope of the invention, and that the scope of the present invention is to be limited only to the claims except as precluded by the prior art. Moreover, the invention as disclosed herein, may be suitably practiced in the absence of the specific elements which are disclosed herein.