Tungsten mechanical cables are the most popular for surgical robot makers.
Connor Chiuchiolo and Scott Dailey, Carl Stahl Sava Industries
The mechanical cables that reside within the motion control systems of the most modern surgical robots can be so small that the individual filaments themselves are nearly impossible to see with the naked eye.
So tiny are the wires comprising tungsten cables for surgical robotics that they contain individual strand diameters as small as one-fourth the thickness of a single piece of paper.
When a surgical robotics designer comes to a cable manufacturer with a tungsten cable application, there is a specific set of criteria that the cable must satisfy. The cable must tolerate a precise load, for example, while maintaining a miniature diameter, as well as a critical factor of safety. After all, a tungsten cable must shoulder its load requirements without getting too large to fit within the tight form factors of today’s surgical robots. Therefore, manufacturing tungsten cable for surgical robotics involves the delicate process of understanding how tungsten wire behaves at incredibly small diameters once it’s been stranded into helical cable constructions.
While engineers must design manufacturing processes that prevent mating components from damaging the ultrafine tungsten cable, they must also ensure that mating components and the live environment coalesce to deliver world-class motion actuation when a human life is on the line.
Plainly said, miniature tungsten cable behaviors get more complex as the engineer introduces additional parts, stresses and other external conditions to the design process.
Like the evolving miniaturization of surgical products in general, making tungsten cable for surgical robotics is an incredibly sophisticated engineering process, made more challenging by its increasingly small footprint and sophisticated behavioral characteristics.
Why tungsten mechanical cable?
Surgical robotics designers use tungsten cables in more and more of their products because of the material’s unique ability to both bear high load and traverse tight radii.
The pulleys inside these surgical marvels may be as small as the tip of a crayon. Mechanical cable that needs to round such sharp turns must therefore possess qualities that permit the product to withstand the beating the cable will take over thousands of cycles, without fear of premature degradation.
Tungsten (also known as wolfram) is ideal for these applications, as it is has the highest tensile strength of any natural metal on Earth.But in addition to its inherent might, tungsten cable also possesses a “deadness” to it. As a matter of fact, when stranded, tungsten miniature cable behaves similarly to a shoelace: held in the center of its length, tungsten cable falls limp. For contrast, stainless steel of the same size and strength would possess a spring-like quality that forces it to return to its original form. And although stainless steel is used to support motion in surgical robots, its natural rigidity causes the material to want to defy the tight turns in tomorrow’s surgical robots.
Introducing mating components
When an engineer combines a sleeve, ball fitting or other cable fitting to the tungsten cable, the natural behavior of the tungsten strand will change in and around these fittings. The changes these mating parts produce often range from merely unwanted in nature to entirely unacceptable. In a nutshell, this is the engineer’s challenge when designing miniature tungsten cable for surgical robotics.For example, take a tungsten mechanical cable assembly that includes a 3-in. length of 19×19 cable (comprising 361 wires) with a ball fitting attached to the center of the assembly. In this case, it is not uncommon for the tungsten filaments to slightly separate or “bird cage” immediately to the left and right of the ball fitting when an industrial press places pressure on the fitting itself. Cable design engineers are prepared for this and make adjustments to the fitting and/or the press applying pressure to the mating components.
The designers of tomorrow’s surgical robots are concerned with the ball’s diameter, placement and performance, along with the tungsten cable assembly itself. However, cable design experts must concern themselves with the aforementioned requirements as well as tooling, machining and manufacturing processes that create the perfect product every time.
And remember that the cable design engineer needs to not only solve temporaneous and predictable complications that emerge from handling tungsten cable, but do so at a mass scale so robotics makers can compete within fiercely aggressive and rapidly moving global markets.
This article was written by Carl Stahl Sava Industries Associate Design Engineer Connor Chiuchiolo and VP of Marketing Scott Dailey. For more information about Riverdale, New Jersey-based Carl Stahl Sava Industries, visit www.savacable.com.
The opinions expressed in this blog post are the author’s only and do not necessarily reflect those of MedicalDesignandOutsourcing.com or its employees.