There is no one-size-fits-all material when producing mechanical cable. The material chosen to produce wire rope can vary from one application to the next, because commonly, no two applications place the same demands on the stranded cable. Consider tungsten, for instance. Also known as Wolfram (W, atomic number 74), tungsten possesses the highest melting temperature of all metals. For this reason, this rare earth metal is commonly found in light bulb filaments, x-ray tubes, and arc-welding electrodes. Because of such unique mechanical properties, tungsten has become an integral component in the manufacturing of many of today’s cutting-edge medical devices, including surgical robots.
Due in large part to tungsten’s imperviousness to fatigue over time, as well as its excellent tensile strength, ultrafine tungsten wire has become the go-to material in the construction of miniature, mechanical cables used in modern surgical robots. These surgical innovations are made the marvels that they are because of tungsten’s outstanding flexibility and resistance to abrasion. By comparison, were such robotic medical devices to use stainless steel cable, their lifespan would be greatly diminished, and potential for surgical failures would increase.
Likewise, one can also find such tungsten cables actuating the movements that humans achieve using their arms, elbows, and wrists. When tungsten is facilitating the musculoskeletal movement historically performed by the surgeon’s own body, the taxation is shouldered by the robot, thus allowing the doctor to perform numerous procedures without fatigue and strain.
The Less Obvious Benefits of Tungsten Cable
An engineer tasked with developing these complex tungsten cables, which are ever-decreasing in size, requires a material that offers properties greater than those found in standard stainless steels.
In today’s medical device applications, miniature tungsten cables use hundreds of filaments to create cables as small as a half a millimeter in diameter. Within that diameter, there could be as many as 200 or even up to 700 or more individual tungsten wires comprising the total construction of the cable. A single filament in such a cable construction may be as small as 0.0005 in. in diameter, making such wire six times smaller than a single strand of a human hair. That’s really small.
Tungsten’s Savings Over Time. Directly speaking, tungsten’s initial costs, while high compared with equivalent stainless steel cables, will outperform the competition over time. This makes tungsten less expensive in the long run.
There are two chief reasons why tungsten is the more cost-effective material in constructing miniature mechanical cables for medical and even industrial applications: lifetime and flexibility.
Tungsten Lifetime. Tungsten’s unique characteristics allow the cables to operate for longer periods of time without requiring maintenance or replacement cables. This is due to tungsten’s strength, flexibility, and resistance to temperature. First off, tungsten is among the strongest materials known to man. While diamonds possess a Mohs scale hardness of 10, tungsten is rated at 9. By way of comparison, stainless steel garners a Mohs hardness rating of about 6.
Tungsten Flexibility. Tungsten’s flexibility is, over radii smaller than 1 mm, is unrivaled. Stainless steel cable, applied the same tight bending radii, would likely fail due to bending stresses over many cycles.
Where the mechanical cable application requires a high tolerance for temperatures as well as superior tensile strength, tungsten continues to shine. Where 316 stainless steel melts at between 2,500–2,550 °F, tungsten only begins to do so as it reaches 6,192 °F. This makes tungsten 2.5 times more tolerant of extreme temperatures.
Unpacking Tungsten for High-Temperature Applications
Unlike surgical robots, where heat is less of a variable, tungsten has also found as welcome a home amid the semiconductor and solar markets. In these particular applications, technicians attach a small piece of silicon, called a boule, to a pure tungsten-made, mechanical cable assembly. The boule is dipped in pure molten silicon and is slowly pulled up in a crystal growing furnace. This process can take as long as 24 hours, yielding a piece of silicon between 12 and 18 in. in diameter and up to 6 ft long. This hardened crystal is subsequently sliced into wafers and polished to form flat substrates used for manufacturing semiconductors. Sava is one of the few mechanical cable manufacturers that produce these sophisticated tungsten cable assemblies.
Sava has also worked with the manufacturers of the furnaces used in these complex, silicon-making applications. Stainless steel, contrary to tungsten, quickly anneals at 1,900 °F. A stainless steel cable, rated for 2,400 lbs, would be derated to 1,200 lbs after only a short time exposed to these extreme furnace temperatures.
Tungsten on the other hand, due to its high melting point, is not vulnerable to such intense temperatures and retains its original cable strength throughout its lifespan. There are trade-offs, however, to consider when choosing a cable material to use in such applications. In these cases, using a less-expensive stainless steel cable would not tolerate the extreme temperatures involved in the silicon ingot manufacturing process.
While stainless steel cable continues to be used in the growing of silicon ingots, such cable would have to be replaced far more frequently, making the initial savings represented in using stainless steel only momentarily beneficial. Tungsten cable, though a considerably more expensive material to work with, extends cycles and reduces maintenance, downtime, and replacement costs tenfold.
Tungsten Cable Constructions
The evolution of cable constructions continues to unfold as medical robotics and medical devices place increased burdens on them. Common 1 × 7, 7 × 7, 7 × 19 constructions are being replaced with more sophisticated stranding such as 7 × 37, 19 × 19, and 19 × 37 19. These more complex constructions provide improved tensile strength over their predecessors, while adding high modulus and superior flexibility for more demanding applications found in today’s surgical instrumentation. What’s more, a 19 × 37 construction, a half millimeter in diameter, requires wires as small as 0.0005 in., which are nearly invisible to the naked eye.
Such pioneering constructions put ever-increasing demands into the manufacturing of these cables. Knowing the investment in science, research, labor, and time, it is not hard to understand why makers of these elegant, new cable innovations would turn to tungsten to prolong the lifespan of their inventions.
Working with Tungsten
Manufacturing tungsten cables on such small scales requires greater technical abilities to strand the wires together. After all, it’s not enough to manufacture tungsten cables. Makers of these ultrafine cables must possess the expertise to influence its behaviors, such as lay length, stiffness, and preforming.
Lay Length. Lay length is the linear distance for a single wire to wrap one complete revolution around its core. Because the wire filaments used in these fine, tungsten cables are so small in diameter, the manufacturer must employ tighter and tighter lay lengths to ensure properly functioning cable assemblies. Without achieving the desired lay lengths, cables are prone to fraying or “bird-caging” in use.
Preforming. Coupled with the lay length, it’s imperative to preform tungsten cables to help prevent failures as well. Preforming is the act of adding a sine wave to the wire prior to its closing, whether on strand or cable level. Preforming influences stiffness and ensures that the individual wires marry with one another to form a seamless, bonded strand of cable. When preforming tungsten cables as small as half a millimeter in diameter, the process can be made extremely challenging due to the machining complexities. In lieu of preforming, another option to keeping the wires and strands in position to minimize fraying and other deleterious effects would be to swage the cable OD.
When manufacturers forego or simply do not consider the stiffness requirements of tungsten cables, the cable’s behavior may not possess the physical abilities to be routed or traverse the device properly. When this happens, cables may bunch or fall apart while being asked to operate in the device’s environment. Swaging or preforming tungsten cables, as a means of achieving a desired stiffness, ensures that the cables perform optimally over extended periods.
The Future of Tungsten Mechanical Cables
The ultrafine tungsten wire market is expected to double by 2025 globally. While tungsten wire remains an increasingly popular material in the making of medical devices, tungsten is likewise growing in popularity anywhere ultrafine wire is required to tolerate a variety of demanding conditions.
Look for tungsten cables to get smaller and smaller over the next decade and beyond, pushing the boundaries of existing manufacturing technologies and placing growing demand on engineering expertise worldwide. As tungsten mechanical cables proliferate across a wide swath of industries, engineers are duty bound to harness the complex material’s potential and usher in a new era of medical and industrial devices with limitless scale and capabilities.
This article was written by Greg Soja, Senior Vice President of Engineering, and Scott Dailey, Vice President of Sales and Marketing, Carl Stahl Sava Industries, Riverdale, NJ. For more information, visit here .