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Table of Contents

         1.0 Abstract 
         
2.0 Background of the Invention 
         3.0 Brief Description of the Present Invention
         4.0 The Primary Function of the Fuse 
         5.0 The Secondary Function of the Fuse
         6.0 Benefits/Summary of the Invention 
         7.0 Proto-types and Test Results  
         8.0 Discussion of Prior Art 

1.0 ABSTRACT

A device, referred to as a “Mechanical Fuse”, is installed at any location along a wire rope that provides a pre-failure warning that the wire rope has been loaded beyond a specified magnitude. The device also provides a pre-failure warning that swaged fittings are in jeopardy of slipping. The device is swaged onto the wire rope in the same manner as eye and clevis end fittings are swaged onto the wire rope. When a specified tensile load in the wire rope is exceeded, the fuse fails, or trips, providing a visible indicator that an overload condition has occurred. Unlike the electrical fuse counterpart that interrupts the current to protect the circuit, this “Mechanical Fuse” transfers the load from the fuse body to the wire rope when the overload condition occurs so as to maintain the integrity of the load path. The “Mechanical Fuse” may be locally work hardened to provide specific material failure characteristics and “Gauge Marks” may be applied so as to record a more detailed load history.

2.0 BACKGROUND OF THE INVENTION

A pre-failure indicator for wire rope is the subject of this invention.

Flexible wire rope, for example, 7x19 of various sizes are commonly used in cranes and lifting equipment. Such flexible wire rope lends itself to this type of application since it can be threaded through various pulleys and sheave configurations as well as being wrapped and unwrapped from storage drums. Such wire rope is commonly used in aircraft, marine halyards and running gear, automotive, and other types of mechanical control systems. Less flexible wire rope, for example, 1x19 of various sizes is more often used in more static applications such as stays to support constructions elements, crane booms, towers, telephone poles, construction masts, and marine rigging.

All these applications of wire rope have one thing in common, how to install the wire rope into the load path.  Two techniques are commonly used. The first wraps the wire rope around a thimble clapping the free end of the wire rope to itself with either a mechanical clamp or a lead lug that is crimped onto the wire rope with a crimping tool. The second method swages a designed eye or clevis end fitting onto the wire rope by extruding a section of the fitting into the twisted stands of the wire rope using a hydraulic die. This reduces the diameter of the fitting within this section securing the fitting onto the wire rope.

The need to protect such systems from catastrophic failure due to the failure of a wire rope component within the system is the subject of this invention.

3.0 BRIEF DESCRIPTION OF THE PRESENT INVENTION

The wire rope “Mechanical Fuse” is a cylindrical machined device tapered at each end. A concentric longitudinal hole is machined through the entire length of the device. The diameter of this longitudinal hole is set to fit a specific wire rope size. A lateral slot slightly wider than the diameter of the longitudinal concentric hole is machined in the center one third of the device from side to side through the entire width. This section of the fuse is referred to as the tension-free-slot. This tension-free-slot is then locally work hardened to remove the ductility of the material, and then a circular groove is machined at the midpoint of the slot length and around the circumference of the center barrel of the fuse. The device so described is a wire rope  “Mechanical Fuse” and will be referred to throughout the remainder of this presentation simply as a “fuse”. The details of this description are shown in Figure 1.


Figure 1:  Isometric view of the armed fuse

 A wire rope is threaded through the entire length of the fuse and then the fuse is positioned along the wire rope at the desired location. Once the fuse is positioned, one end of the fuse is swaged onto the wire rope, and then the wire rope is pushed toward the center forming a small arc or slack in the wire rope. The remaining end of the fuse is then swaged onto the wire rope securing the small arc or slack in the wire rope within the tension-free-slot. The wire rope within this slotted center portion of the fuse will support no tensile load, which is the origin of the name tension-free-slot. The fuse is now armed. The details of this description are shown in Figure 2.


Figure 2: A side view and sectional view of the armed fuse

To understand why the fuse is armed, consider the tensile load path in the wire rope when a load is applied. As the wire rope tensile force approaches the fuse, the tensile force is transferred from the wire rope in the swaged section of the fuse and then to the outer center section or outer barrel of the fuse. The tensile force in the outer section of the fuse is then transferred back into the wire rope as it continues through the swaged section at the opposite end of the fuse. That portion of the wire rope with the slight circular arc or slack in the center of the lateral slot supports no load.

The section perpendicular to the longitudinal axis of the fuse at the groove location is the critical cross section of the fuse due to the stress concentration created by the groove and the reduced cross-sectional area at the groove location. The fuse barrel will fail at this groove cross section at the mid-point of the tension-free slot if tensile load become excessive.

4.0 THE PRIMARY FUNCTION OF THE FUSE.

The fuse is designed to fail at a specified tensile load magnitude for the size of the wire rope. For example, the fuse may be designed to fail at 50% of the estimated breaking load of the wire rope. When this load is exceeded, the outer barrel of the fuse fails at the critical cross section within the reduced area section thus transferring the load to the wire rope in the previous tension-free-slot. Figure 3 shows the failed fuse.


Figure 3: Isometric view of the failed fuse

The elastic properties of the fuse and wire rope are considered so that shock loading of the wire rope is minimized during this failure. The failure characteristics of the material within the tension-free-slot might also need to be adjusted by local work hardening before the reduced area is machined in the fuse.  Figure 4 shows a side view and section view of the failed fuse.


Figure 4: Side view and sectional view of the failed fuse

In conclusion:

If the fuse is observed to have failed in the reduced area section, then the wire rope system has been overloaded.

For a wire rope system that is repeatedly loaded and unloaded and is a component of a designated critical safety item, “Gauge Marks” may be added to the “Mechanical Fuse” on either side of the reduced area. Figure 5 illustrates the placement of such “Gauge Marks”.


Figure 5: Wire rope armed fuse showing “Gauge Marks” and “Gauge Length”

The distance between the “Gauge Marks”, designated as “Gauge Length”, may be any convenient distance. Measuring the “Gauge Length” while the wire rope is unloaded provides a more definitive approach to tracking the load history. If the “Gauge Length” changes, then the wire rope has been loaded to a magnitude close to the “not to be exceeded load” even though the fuse has not completely failed. With this “Gauge Mark” embodiment added to the fuse, critical safety items that contain a wire rope component can be inspected more quantitatively for safety.

5.0 THE SECONDARY FUNCTION OF THE FUSE

If the swaged section of either of the wire rope’s end fittings were to fail, for example by permitting the wire rope to slip within the swaged section, then the wire rope would fail to support load. Granted, the swaging process, if properly applied is stronger than the parent wire rope. However, extended service time, repeated loading, fatigue, corrosion, misuse, and any number of additional unknowns, require close inspection of the swaged sections during required periodic safety inspections. The secondary function of the proposed fuse addresses this issue.

Assume that the swaged section at each end of the fuse is swaged onto the wire rope in the same manner as the end fittings are swaged onto the wire rope. Assume that the swaging is performed by the same person using the same swaging tool and using the same technique. Further, in the design of the fuse, the swaged sections at both ends of the fuse are individually of less length than the swaged length of any of the end fittings. The length and location of the swaged sections of the fuse are shown in Figure 6.


Figure 6: Isometric view of disabled fuse

If a swaged section were to fail, it is logical to suggest that one of the fuse’s swaged sections would fail first since they are identical and of less length. If this should occur, the slack in the wire rope within the tension-free-slot would straighten due to the wire rope now supporting tensile load. Figure 7 shows a side view of the fuse along with a sectional view clearly showing the geometry of the wire rope in the tension-free-slot.


Figure 7: Side view and sectional view of the disabled fuse

Notice that the fuse in Figure 7 has not failed yet the wire rope has straightened. The only way this could occur is if the wire rope slipped within the swaged section at either end of the fuse.

 If the wire rope in the tension-free-slot is observed as being straight while the fuse barrel is intact, then the swaged section of the fuse has slipped and the fuse is disabled. All other swages within the system should also be considered in jeopardy of failing.

While several other embodiments of this wire rope fuse are described in the patent application, only two additional embodiments are presented here.Figure 8 shows the wire rope fuse incorporated as an integral part of a typical eye or clevis end fitting.


Figure 8: The wire rope armed fuse as an integral part of typical end fittings

The functionality of the fuse in these configurations is the same as previously described.

6.0 Benefits / Summary of the Invention

(1) The mechanical fuse may be located at any desired location along the wire rope.  The fuse may be placed at a location convenient for viewing so that the status of the fuse may be frequently checked.

(2) The mechanical fuse may be incorporated within the structure of standard end fittings.

(3) Visual evaluation of the fuse status does not require special training. The fuse is either broken or intact. If intact, the rope is either straight or curved.

(4) The mechanical fuse is disabled if the wire rope becomes straight within the tension-free slot without the fuse breaking. This is a signal that all other swaged sections in the wire rope system are also in jeopardy of slipping. This visual evaluation of the fuse status does not require special training.

(5) If more definitive load history is required, the “Gauge Mark” concept may be used.

(6) The current mechanical fuse is approximately the same size as an eye or clevis end fitting.

(7) The cost of manufacturing would be comparable to the standard end fittings.

(8) Installing the fuse, i.e. swaging the end sections of the mechanical fuse onto the cable, is the same technique as attaching the standard end fittings.

(9) The skill level, type and quality of equipment required to install the mechanical fuse is comparable to the skill and equipment required to install the standard end fittings.

(10) The mechanical fuse has the same appearance and general shape as that of the standard end fittings. Its presence would appear to be a normal part of the system.

(11) The mechanical fuse can be designed to trip/fail at various percentages of the wire rope breaking strength.

7.0 Proto-Type and Test Results

As proof of concept, a schedule of mechanical fuses for a 1/4” 9x19 wire rope were manufactured and tested for a variation of critical sections. These proto-types were installed and tensile tested to failure to validate the geometry of the critical section for a 65% failure based upon the reported breaking load for a ¼” wire rope cable.

Figure 9 shows the installed fuse while Figure 10 is a close-up of the fuse’s critical section showing the cable slack as illustrated in Figure 2.



Figure 8: The ¼”wire rope fuse ready for tensile testing.

Figure 10: Close-up view of ¼” wire rope fuse critical section.

Figure 11 shows the armed fuse before testing while Figure 12 shows the failed fuse after testing.

 


Figure 11: The ¼”wire rope armed fuse ready for tensile testing.
Figure 12: The ¼”wire rope failed fuse at 65% of the rated breaking load of the cable.

Granted this procedure must be repeated for each cable size, but proof of concept however has been established.