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Pre-stretching and Proof Loading

Pre-stretching and Proof Loading

Friday, April 29, 2021

Applications that utilize mechanical cables and cable assemblies cannot afford to fail when deployed in the field. Take a surgical robot, for instance. The intricate and miniature tungsten cable assemblies that actuate the grippers and other robotic appendages require tremendous testing to ensure they do not fail during delicate surgical procedures. If mechanical cables used in these sophisticated surgical robots were to fail, the surgeon would not be able to perform the procedure or worse, put the patient at risk.

But even outside of surgical applications, from aircraft to spacecraft, and from military and defense equipment to an array of industrial applications, wire rope and mechanical cable assemblies must perform as designed and as planned. For example, Sava has proof loaded cable assemblies that have been used throughout the aerospace industry on projects such as the NEA Scout, and other rockets that require cable assemblies must remain taut when engaged.  

With life and death scenarios surrounding the mechanical engineers testing these mission-critical cables, it’s no wonder cable manufacturers conduct proof loading and pre-stretching operations to add functionality to cable assemblies. 

What is a Proof Load Mechanical Cable?

Proof loading is the act of stretching mechanical cable assemblies usually to 60 percent of their breaking stretch. So, for example, a stainless steel cable assembly rated to break at 100 lbs, is proof loaded at 60 lbs. Proof loading benefits the cable assembly’s design because this critical engineering operation removes constructional stretch that, if present, may leave cable filaments not being uniformly loaded. In plainest terms, proof loading allows the core of the cable construction, and helically-configured strand to make closer contact with one another. Proof loading mechanical cable results in a slightly smaller diameter profile, as the filaments move toward each other.     

Proof loading ensures that the cable is going to perform as expected under its planned load. Without proof loading, the cable possesses an area of uncertainty, inasmuch as the filaments have not yet made optimal contact with one another. This absence of ideal contact leaves a measurable nonlinear region, much like a rubber band lying limp when the application requires it to be taut.   

Pre-stretching Mechanical Cable

You may hear the terms "proof load" and "pre-stretching" used interchangeably. This would be slightly incorrect in that pre-stretching wire rope is the act of proof loading cable without fittings, or said another way, the cable alone, not custom mechanical cable assemblies. It is not uncommon to pre-stretch spools of wire rope, as they do not contain mating components such as eyelets, or threaded plug fittings. 

Although the high-quality cables manufactured by Carl Sava Stahl Industries are made to tight tolerances, and in accordance with military specifications (mil spec), there is an element of uncertainty that can only be eliminated once the cable is placed under load. To stretch wire rope is to simulate an actual working environment, which seeks to apply requisite load to the cable until the nonlinear region of the cable’s performance is eliminated. 

Proof Loading Milspec Cable

Engineers at Sava proof load mechanical cable assemblies used in milspec applications to prevent stretch when the assembly is introduced to its application environment.   

Milspec, or military specification, cables are mechanical cable components that meet the military's rigorous quality standards. The majority of military applications require pre-stretching and proof loading wire rope or custom mechanical cable assemblies prior to service. However, mil spec cable is not just for the military. There are many non-military uses, such as aircraft and defense applications, which honor mil spec regulations, or even more stringent governance for quality and performance.  

Proof loading cable assemblies are a vital engineering operation that promises the constructional stretch of the cable is eliminated, leaving only the optimal load characteristics remaining. Thus, mechanical cable assemblies cannot perform if nonlinear areas are present. Remembering the rubberband example, a cable assembly elongating, when it must remain static, introduces points of failure that could be catastrophic to either or both the motion system entirely, or critical nearby components.   

Takeaways

  • The proof loading process is necessary when the cable assembly must remain taut.

  • Pre-stretching involves removing constructional stretch from mechanical cable only.

  • Applications such as pull-based cable assemblies benefit from proof loading because it removes the potential for dynamic regions to be present within the cable.

Our engineers are experts in pre-stretching and proof loading cable. To speak with one of our pre-stretching and proof loading professionals, or to view our entire product catalog, visit Sava today.

April 29, 2021