Unlocking the Mysteries of Flexible Shaft Technology

The modern airplane is a marvel of engineering. Designs cost billions of dollars and take years to develop, with experts in all manner of mechanism working together to build a craft that works in harmony. Of course, in the field, all that harmony is subject to defeat by circumstance. When one or several of the many valves controlling the movement of fluid fails, a system built to be controlled from the cockpit falls apart. These fluid systems are responsible for vital functions such as anti-icing and even engine ignition. So, the mechanism responsible for quickly bringing inaccessible valves out to a point where the mechanic or flight crew can manually operate the valves is vitally important. Without it, the flight might not be able to depart. Flexible shafts are the components given this critical task. However, until recently, the design of these shafts was completely reliant on one method: simple trial and error.

That strategy had worked—somewhat—since 1874 when an employee of a dental drill company founded by dentist Samuel Stockton White invented the flexible shaft. Solid shafts can only transmit rotation in a straight line; flexible shafts transmit the same rotary motion, but can also bend—making it applicable in a huge variety of applications where a solid shaft could not work. Still, for the first three decades of the flexible shaft, the technology was only used in dental applications. Then, Ford Motor Company installed flexible shafts for speedometer cables, and shortly thereafter the shafts were used to improve World War One airplanes, and flexible shafts spread beyond the dentist’s drill while S.S. White continued production. “That is what we are,” chuckles Rahul Shukla, Chief Executive Officer and President of S.S. White Technologies Inc., “we are the flexible shaft division of the dental company.” Working with another engineer, he created PERFLEXION, the first software to predict the characteristics of flexible shaft designs.

By the time Shukla joined S.S. White, the preeminent flexible shaft manufacturers, as a Quality Control Inspector in 1973, the technology about to turn one hundred years old remained largely a mystery to those who built it. “I went to some senior engineers at the company and asked them how we designed flexible shaft,” he recounts, and they shrugged. “How can you not know?” he asked. “Our company has been making it for 100 years.” Their reply was always the same: we do it by trial and error. That failed to satisfy the young man, who promptly posed the question to the manager of the chief engineers. “[I] said to him, ‘We design flexible shaft by trial and error,’” Shukla recalls, and “he said, ‘I know that. We don’t know how to design flexible shafts, and neither do our competitors, so there is no problem.’”

The Mysteries of Flexible Shaft

Each flexible shaft is made of layers of carbon steel wire. Often these wires number forty or fifty around a core or mandrel. Each layer consists of a handful of wires wrapped in the same direction relative to the center, and layers alternate in direction: left, right, left, etc. Depending on how the wires are wound, the top layer will tighten when the shaft is twisted in one or another direction, closing down on the layers underneath. The layer it closes on, in turn, opens up to resist it. This interaction enables the transmission of torque. A good flexible shaft has to have good bending flexibility, but it should not have torsional flexibility – when you rotate it, it should not wind up.

It is a relatively simple design, but there are a number of variables that can affect the performance of a shaft. The thickness of wires in each layer, the number of wires in each layer, the number of layers, and so on can all change how a shaft acts. Generally, the two outermost layers define the shaft’s torque, but each layer is subject to influence from other layers. In a four-layer shaft, for example, layers three and four define the torque characteristics with influence from layer two.

To the engineers Shukla challenged, these subtly influential variables seemed to be unpredictable. Additionally, the simplicity of the machine allowed for simple tweaks. By adding a wire or by removing one, by changing a wire’s thickness (diameter), a shaft could be tweaked closer and closer toward the required specifications. It was certainly a simple process, but the costs to time and material were avoidable.

Now PERFLEXION analyzes all these factors and predicts performance on a number of key characteristics, like torsional stiffness, bending stiffness, and torque carrying capacity. With the breakthrough software, flexible shaft design takes about a week, start to finish. “You can imagine if you had to experiment with actual shaft designs, it would take 100 days,” says Shukla.

To Shukla, dethroning trial and error was about cost efficiency, but it was also about discovering the unknown. It seemed that the question had neither been seriously asked or seriously answered: How exactly do flexible shafts work? “I told my wife, ‘I don’t want to die before I unlock the mysteries of flexible shaft.’”

PERFLEXION and the End of Trial and Error

For a time, the mysteries refused to reveal themselves. Shukla toiled over successive formulas for eight years without developing a reliable solution. “My degree of confidence was only 50%,” he recalls, “When the coefficient of correlation is only 50 %, the relationship is not strong at all. The coefficient of correlation should be 80% or .80 for you to have confidence in your mission.”

Then, an S.S. White engineer bumped the level of confidence to 65%, but it was not Shukla. Adam Black was a newly hired mechanical engineer at the company when he took the formulas to the next level. He had developed a spring formula which described a flexible shaft as a series of springs wound around the core. Shukla took notice, and encouraged Black to pursue the project as a doctoral study. After a few years, Dr. Black presented his dissertation, replete with page-and-a-half long formulas describing the mechanics of the flexible shaft.

Promptly, Shukla translated the academic findings into a computer program. The result: PERFLEXION, the first proprietary software that can accurately predict flexible shaft characteristics. With it, Shukla surpasses his goal of 80% accuracy with prediction, and as a computer program, PERFLEXION does in minutes what would have taken weeks. Before, modeling was comparative. Now, PERFLEXION makes it absolute.

What PEFLEXION Enables

With PERFLEXION cutting time costs of the flexible shaft design process down in a profound way, flexible shafts are poised to continue their integration into ever more applications. While Ford in the days of the Model T could wait for a trial and error design, modern engineers gain by getting their flexible shaft designs quickly. This not only moves the greater design process along but also gives engineers the chance to realize the power of the flexible shaft to solve more design quandaries on a single project. “What we have found is that – take an aerospace company, for example – once an engineer works with us to design an application, that person will find three more uses for the flexible shaft,” says Shukla. The flexible shaft, after all, offers a lot to engineers. By design, a flexible shaft takes up misalignments, so it can save engineers time that would otherwise be spent ensuring tight drive system tolerances. Its freedom of movement makes it ideal for hand tools and other applications which require a lot of movement. It also tolerates vibration, works bidirectionally, and ensures reliability with very few moving parts.

Shukla also points out that engineers often first unlock the exciting technology for their practice by interacting with the S.S. White design team. “No universities teach flexible shaft, no textbook has a chapter on a flexible shaft,” so S.S. White’s expertise is invaluable to partner engineers. Over the years, S.S. White has added to PERFLEXION with supplementary calculations and tools to predictively calculate fatigue life, determine limit torque parameters, and predict a host of other characteristics.

Popular aerospace applications range from TRAS (Thrust Reverser Actuation Systems) that drive multiple actuators from a single point or synchronize multiple actuators, to flap actuation systems that extend flaps from aircraft wings on take-off and landing, to manual override for valves, but the list is not exhaustive. Applications range from thermostat control on gas boilers or furnaces to docking systems on spacecraft, from abattoirs to ophthalmic surgery—and S.S. White specializes in developing unique, customized solutions for any use. That expertise stems largely from a sense of mission on the part of the company’s engineers. “My passion for flexible shaft cannot be explained,” Shukla admits, but the mechanics of the flexible shaft can be explained and understood using S.S. White’s PERFLEXION.