A Case for Tungsten Cable: Motion Control in Surgical Robots

.019 Ø, 19 × 19 with plasma welded ends.

Among the most common tungsten cable constructions used in surgical robotics include 8 × 19, 7 × 37, and a 19 × 19 configuration. 8 × 19 tungsten mechanical cable comprises 201 tungsten filaments, while 7 × 37 includes 259, and lastly, 19 × 19 possesses 361 helically stranded wires. And although stainless steel serves a number of applications, including numerous medical and surgical devices, there is no substitute for tungsten cable in surgical robotics applications.

But why is stainless steel, such a celebrated mechanical cable material, less and less popular in actuating motion in surgical robots? After all, stainless steel cable, particularly in miniature diameters, is used ubiquitously across military, aerospace and most importantly to the topic, countless other surgical device applications.

Well, the reason tungsten cable is supplanting stainless steel in surgical robotic motion control is actually less mysterious than one might think: it’s about strength. But because the strength of such mechanical cable is not measured merely by its linear pull strength, we must test strength, as a performance metricby collecting data from many field-appropriate scenarios.

Take for instance 8 × 19 constructions. One of the most commonly used mechanical cable constructions in achieving pitch and yaw in a surgical robot, 8 × 19 dramatically outperforms its stainless steel counterparts precisely as load increase.

8 × 19 Tungsten Cable Vs. Stainless Steel Cable

Note that as load increased, the cycle count and tensile strength improved with the tungsten cable, while the stainless steel cable alternative, under identical load, dramatically lost pace with tungsten’s strength.

Stainless steel cable, loaded to 10 lbs, approximately 0.018 ”Ø only achieves 45.73% of the cycles Tungsten achieves at the same 8 × 19 construction, wire diameter.

As a matter of fact, what this particular study revealed rather instantly was that, even at 10 lb (44.5 N), the tungsten cable exhibited more than twice the cycle count of the stainless steel cable. Given that, like all components, the miniature mechanical cable residing inside a surgical robot must meet or exceed rigid regulatory requirements, the cable must therefore tolerate anything thrown at it, right? Thus, the analysis demonstrates that using the same diameter 8 × 19 tungsten cable over stainless steel cable, possesses both inherent strength advantages, but also ensures the robot is powered by a cable material that is the strongest and longest-lasting of these two options.

Going further, and again in the case of 8 × 19 construction, the tungsten cables yielded a minimum of 1.94 times the cycle count of stainless steel of the same diameter and load. What’s more, the study showed that even as the load applied was incrementally increased from 10 lb to 30 lb, the stainless steel cable never caught up to tungsten’s resilience. As a matter of fact, the margins became increasingly distant between the two cable materials. Reaching as high as 3.13 times the cycles at the same loads at 30 lbs. And even more punctuated a finding was that the margins never narrowed throughout the study (up to 30 lbs). Tungsten always achieved a higher cycle count, by an average of 39.54%.

While this study examined specific wire diameters and cable constructions within a strictly controlled environment, it was undertaken to prove out that tungsten is stronger and can achieve a greater cycle count, under precise stresses, tensile loads and pulley configurations.

.0145 Ø, 8 × 19 tungsten cable with a hypo tube swaged and sleeve fitting.

Consult Tungsten Minature Cable Experts

Essential to achieving the cycle counts your surgical robotics application must is collaborating with your tungsten mechanical cable engineers.

No matter if its stainless steel, tungsten or any other mechanical cable material, no two cable assemblies serve the same masters. Often miniature cable applications for instance do not require that the strand itself, nor the fittings applied to the cable achieve almost impossibly tight tolerances.

In many cases, the deviation in length and size of the cable itself, as well as the location and yes, size of the fitting (s) are somewhat flexible. Such measurements are what comprise the tolerance (s) of the cable assembly. Providing your mechanical cable manufacturer can achieve a cable assembly that falls within the applications tolerances, the components are only then ready to be used in their live environment.

In the case of surgical robotics, where lives are in the line, achieving the design’s tolerances is the only acceptable outcome. It can therefore be argued that the ultrafine mechanical cables impersonating the surgeon’s every movement makes these cables among the most sophisticated on Earth.

The mechanical cable assemblies going inside these surgical robots will occupy small, crowded and compact spaces too. As a matter of fact, it’s nothing short of amazing how seamlessly these tungsten cable assemblies are nested within the narrowest channels, over pulleys no larger than the tip of a child’s crayon and accomplish both while supporting motion across a predictable cycle count.

.019 Ø, 19 × 19 tungsten beach.

Stainless Steel Cable Supplanted by Tungsten

Also important to note is that your cable engineers can advise on cable material early enough to potentially save time, resources and perhaps even costs, all of which are key variables when planning a thoughtful go-to-market strategy for your robot.

With an explosive surgical robotics market, it is no longer acceptable to merely supply the mechanical cables used to help achieve motion. The pace and poise with which a surgical robotics maker takes their marvel to market is certainly influenced by the ease with which the product is made ready for mass consumption. This is why it’s important to note that your mechanical cable engineers are studying, perfecting and building these cable assemblies daily.

It isn’t uncommon for example to find that a surgical robotics project may begin with a certainty of stainless steel’s strength, malleability and cycle count capabilities, but still resort to tungsten later on in their robot’s development.

It’s true.

Surgical robotics makers often arrive at stainless steel early in the robot’s design, but later opt for tungsten due to its superior data. And while this might seem abrupt a change to make in motion control considerations, it only masquerades as such. The change in material is the result of the indispensable collaboration had between the robot’s makers and the mechanical cable engineers hired to produce the cables.

.019 Ø, 19 × 19 tungsten cable with a swaged sleeve fitting.

Conclusion

Tungsten mechanical cable for surgical robotics has earned its seat at the table.

Stainless steel cable continues to prove itself to be a staple of the surgical instrumentation markets, specifically in the endoscopic devices space. However, for all its power to support motion in endoscopic / laparoscopic surgical procedures, stainless steel just does not yield the same tensile strengths as does its more brittle, but far denser and consequently, stronger counterpart known as tungsten.

And as ideally suited as tungsten is to unseat stainless steel as the cable material of choice for surgical robotics, one cannot understand the importance of sound collaboration from your cable makers either. Working with experienced ultrafine mechanical cable engineers doesn’t just ensure you’re cables are produced by world-class advisors and makers. Choosing the right cable manufacturer is also a surefire way to promise you’re prioritizing both perfecting the science and cadence of your build plan, which helps you realize your motion control ambitions faster than competitors trying to achieve the same things.

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