A Retrofittable Mechanical Pre-Failure Fuse
for Wire Rope Cable and Other Applications.
(Patent Pending)
 

  Wire rope cables are in wide use across many market segments generally to either support a structure or lift a load.  The market for wire rope and associated hardware is very large and diverse, but can be generally divided into two parts, standing rigging and lifting/pulling applications.  

It might be said of cables that, despite their importance (not just from the standpoint of commerce, but viewed from the perspective of safety as well), they are “dumb” tools.  They provide virtually no indication of when they are about to fail, or clues as to the loads they have experienced relative to those they are designed to withstand.  This is why mechanical fuses should be attached to these cables.  The fuse can provide proof that a cable has experienced loads of a magnitude that may call into question the integrity of the cable.  With such information, the user may more intelligently determine that the cable is suspect and should be replaced.  

Patents for cable fuses have existed for years, but the known patents all share the same flaws.  They are relatively complex devices in terms of their construction and tripping modes and, perhaps more importantly, they all require the device to be fitted to the cable prior to installing the cable’s end fittings and subsequently installing the cable at its destination.  They are also affixed to the cable by means of the swaging process, a process that is most practically performed at the plant site where the cable assembly originates.  Therefore, to get the benefits of these fuse devices one must essentially start from scratch by removing and replacing all the cables.  

The Retro-Fuse is different.  

There are two basic embodiments of the Retro-Fuse, symmetrical and asymmetrical.

Figure 1: Symmetrical embodiment of the Retro-Fuse.

  Fig. 1 shows the main body of the symmetrical fuse has a planar rectangular shape and is clamped onto the wire rope with malleable inserts (see Figure 2) pressed into the wire rope strands by rigid end caps bolted to the fuse main body. The main body of the fuse has a rectangular opening to accommodate the slack portion of the cable that supports no tensile load. The top and bottom horizontal members of the main body include center sections of reduced area. The fuse is designed to trip (separate) at these sections when the cable tensile load reaches a designated magnitude. The load that causes the fuse to trip is the fuse rated load and may be set to a percentage of the wire rope estimated breaking load by adjusting the type and dimensions of the material used, as well as the design and dimensions of the reduced areas. Details of the fuse assembly are shown in Figure 2.  

The exploded view of Figure 2 shows the details of the split sleeve inserts made of malleable metal.  They fit into grooves formed in the main body and the rigid end caps.  The inserts each have a male channel formed across their longitudinal axis on their outer surfaces.  Those channels fit within female grooves in the main body and rigid end caps.  The channels prevent the inserts from slipping along the axis of the cable and the inserts themselves provide a swage-like bonding of the fuse to the cable strands when the bolts and nuts are sufficiently tightened.  

The Retro-Fuse is easy to install using ordinary hand tools for all the most common cable sizes.  Using figures 1 and 2 as visual aids, the installation steps are as follows:  

1. Create slack in the cable by loosening the tensioning device.

2. Insert the split sleeves into the main body and bracket channels (if not already accomplished at the factory.)

3. Place the fuse main body onto the cable with the cable fitting into the grooves in the sleeve inserts (ideally at about eye level.)

4. Place the rigid end caps (with sleeve inserts in place) on the fuse main body and cable, aligning the holes.

5. Insert the bolts and nuts and tightened lightly (allowing movement of the cable.)

6. Using a “C” clamp, create a slack loop in the center of the span of cable within the center rectangular section of the fuse by drawing the cable toward the top or bottom horizontal member of the fuse main body.

7. Tighten the bolts to the torque specifications.

8. Re-tension the cable.

  Figure 3 represents the asymmetrical embodiment.  It is installed in the same manner as the symmetrical version.

 

    
Figure 3: Asymmetrical Retro-Fuse.

 

The asymmetrical design causes the horizontal member of the main body to bend under load.  The deformation will increase under increasing loads and, within limits, will do so without the fuse body completely separating at the center section of reduced/weakened area.  This embodiment can also be produced without a weakened area in the center of the span, so that deformation continues until the slack portion of the cable is fully loaded.  The amount of bending deformation can be measured as shown in Figure 4 using common measuring devices, and may be correlated to cable loads.  

Figure 4:  A finite element model of an asymmetrical Retro-Fuse deforming under load.  

For example, a field inspector can measure the bending deformation and record the cable load by referring to a look-up table, as illustrated in Figure 5. The cable load history can thus be documented and the fuse replaced if it has been damaged even prior to total failure.  A history of high loads on the cable can be used to determine when it should be replaced.

 

 
Figure 5: Example of Fuse Deformation vs. Load Characteristics.

Cable loads are not the only force measurable by the Retro-Fuse.  The load on a cable correlates to the load on the structure.   As it does with respect to cables (shown in Fig. 5), the Retro-Fuse provides an economical means of determining stresses on the structure connected to the cable, a tower for example.

  As depicted in Figs. 6 & 7, the tension forces recorded by the fuses on the guy lines can be correlated to the compression forces on the corresponding sections of a tower.  This data can be used as a key ingredient in determining whether damage to the tower may have resulted from a storm or earthquake, requiring closer inspection of the tower itself.  Conversely, following a storm or quake, a quick inspection of the fuses where no excessive loads are indicated can signal that the more costly inspection of the tower structure itself is not necessary.

 

Fig 6:  Correlation of cable tension forces to tower section compression forces.  

Fig 7: Correlation of cable tension forces to tower section compression forces on a guy-supported electric transmission tower

The geometry of the structure in fig. 7 is more complex than the tower in fig. 6.  Members within this type of tower will be subjected not only to compressive forces, but also to forces causing bending.  Even though the selected tower components in fig. 7 are not directly connected to a guy cable, the forces acting on individual members may be correlated to tension forces on the guy cables that are measurable with the asymmetrical Retro-Fuse.

 

 

 

 

A symmetrical Retro-Fuse with shock absorption and a load transfer means can be added to electrical transmission lines above the insulator (Fig. 8) for protection against damage to the towers caused by swaying from an earthquake or the forces of a storm.  Upon tripping of the fuse, the load will be immediately transferred to the shock absorption member, with the provision of a tertiary load path member for added security.  The event will also deploy an indicator readily visible from the ground or air.

Fig. 9 shows an embodiment of Fig. 8 in its normal state.  It is created using a symmetrical Retro-Fuse body surrounded by a coil spring mounted to the body at both ends.  A slack cable is contained within the body and is swaged to clevis fittings at both ends, which are retained by the fuse body.  Clevis fittings are offered only as examples of fittings suitable for this application.  

 

The simplicity of the Retro-Fuse invention lends itself to additional advantages.  Figure 12 (next page) shows a doubling of the main bodies in both the asymmetrical and symmetrical versions of the fuse.  Doubling the main bodies eliminates the need for the rigid end caps and aligns the stress forces acting on the axis of the cable with the axis of the fuse.

Fig. 10 shows the fuse in its “tripped” condition. The body of the fuse has separated at the location of its reduced area, transferring the load to the spring. The spring can be designed to elongate to a variety of ranges. In this example it will elongate to approximately 6 inches.

The tripping of the fuse also deploys the indicator. In this example, a long red ribbon.

The cable provides a tertiary load path for extra safety. Under normal flexing of the spring (in the tripped state of the fuse) the cable is not loaded and it is presumed the fuse would be replaced in a timely fashion following an event that caused
it to trip. However, if the spring should fail, the load is supported by the cable.

For purposes of visual clarification, Fig. 11 shows the fuse with the coil spring removed, revealing the fuse body, cable and
visual indicator.

The indicator is covered in a protective shell on the upper half of the fuse. When the fuse trips, causing the halves of the fuse body to separate, the ribbon (the end of which is attached to the lower half of the body) deploys with the aid of the wind force that caused the fuse to trip.


The simplicity of the Retro-Fuse invention lends itself to additional advantages. Figure 12 (next page) shows a doubling
of the main bodies in both the asymmetrical and symmetrical versions of the fuse. Doubling the main bodies eliminates the
need for the rigid end caps and aligns the stress forces acting on the axis of the cable with the axis of the fuse.

 

Figure 12:  Doubling of the main bodies of the asymmetrical and symmetrical fuse.

  More significantly, doubling the fuse main body doubles the load rating making a given main body dimension applicable to multiple cable thickness and/or maximum rated loads.  To accommodate the differences in cable dimension, the inner dimension (channel diameter) of the malleable inserts is changed.  Changes in the dimensions of the reduced area further enhances the versatility of the design, allowing a few basic components, used in combination, to create fuses for a wide variety of applications.

  In another example of versatility, figure 13 shows how two asymmetrical fuse bodies can be combined to form one symmetrical fuse, changing both the load rating and the trip dynamics.

 

          

Figure 13: A Symmetrical fuse formed by combining two asymmetrical bodies.

         

Benefits of the Retro-Fuse

 

1. The guy wire/rigging embodiments can be easily installed in the field without removal of the end fittings and without cutting the cable.  For most common cable sizes, this can be done using common hand tools and by operators with minimal training.

 

2. It deforms and/or trips at a pre-determined load (e.g. tensile force and/or bending force) less than the maximum rated tension of the cable.

 

3. Embodiments of the Retro-Fuse can provide a record of high loads experienced by the cable without tripping.  It can also provide stress load data for components not directly connected to it.

 

4. Yet another embodiment provides shock load absorption from storms or earthquakes, making it suitable for application to high tension electrical lines.

 

5. It is simple and economical to produce.

 

6. It is versatile.  It can be configured for a multitude of applications.