In the history of aerospace manufacturing, there have been several innovative technologies that have changed the shape of flight. Some of these technologies aren’t well known or influential on their own, like the propeller or jet engine, but are instead innovations that occurred behind the scenes, quietly enabling steady improvements to aerospace technologies in various ways. One case in point is the flexible shaft.
A flexible shaft is a precisely defined and configured nested group of springs, tightly wound, so that it has torsional or rotational strength, as opposed to the tensile strength found in standard wire rope or cable, to which flexible shafts bear a resemblance. What makes flexible shafts particularly useful is that flexible shafts can bend, but also still rotate; a flexible shaft transmits rotary motion much like a solid shaft, but it can be routed over, under, and around obstacles that would make using a solid shaft impractical.
A flexible shaft assembly consists of a rotating shaft (sometimes called a core) with metal end fittings for attachment to mating parts. A protective outer casing is used when necessary. This casing has fittings (called ferrules) that keep it stationary during use and can serve as an environmental protection.
A flexible shaft is a highly effective means of transmitting rotary motion and is more efficient than universal joints, gears, sprockets and chains, and belts and pulleys. It is typically lower in cost than these other devices and offers the added benefit of compensating for misalignments. Flexible shaft assemblies are successfully used in everything from 787 airplanes to children’s toys. Their long lives are not affected by continuous operation at speeds up to 50,000 RPM, and they can withstand temperatures ranging from -300 to 1000° F.
Flexible shafts are used in many aerospace applications. Some common applications include thrust reverser actuation systems (TRAS), flap actuation, valve override systems, afterburner nozzle actuation, and more.
Flexible shafts are a vital part of TRAS systems on many large commercial aircraft. TRAS systems are essentially an engine brake for these aircraft. Located at the back of the engine, the thrust reverser of a turbofan/jet engine essentially closes the by-pass and diverts the thrust forward to slow the plane. In forward motion, the thrust comes in the intake and out the exhaust and by-pass. In the stop-motion, the thrust comes in the intake and, since the by-pass is closed, the thrust reversers divert the by-pass flow through forward facing matrices, causing the plane to stop. There are four flexible shafts used in this system, which synchronize and connect the actuators that open both halves of the thrust reverser. There are two other flexible shafts used in the trans cowl lock, which keep the TRAS from deploying while in flight.
Why flexible shafts?
While some applications in which flexible shafts are deployed appear extremely complex or demanding, other applications seem straightforward. However, flexible shafts are a constant in all these applications. Flexible shafts are often the preferred choice in certain applications for rotary motion transmission over gearboxes, universal joints, and belts-and-pulleys.
One company, S.S. White Technologies, designs custom flexible shafts for each application to maximize effectiveness. This is part of the beauty of the flexible shaft. Each flexible shaft assembly can be carefully designed and created to meet the needs of a specific application. If one application needs a precise amount of torsional strength or power, the flexible shaft can be made to match that exactly. This is not easy to do. Flexible Shafts are a surprisingly complex product and is not easily modeled by simple calculations.
For example, one of the most challenging aspects of designing a flexible shaft is balancing the opposing properties of bending flexibility and torsional deflection. S.S. White engineers solved this problem with a program called Perflexion, a computer modeling software. This software allows design engineers to optimize the performance characteristics of a flexible shaft. With Perflexion, S.S. White fully models the behavior of all the wires within a flexible shaft and arrive at a design that provides maximum bending flexibility while allowing minimal torsional deflection with 20 to 30% improvement above accepted industry standards.
When transmitting rotary motion, especially in a tight space, the flexible shaft is efficient, versatile, and durable. The flexible shaft is one of the backbones of the aerospace industry. From windshield wipers to TRAS systems to jet afterburners, flexible shafts are there, doing their job quietly and efficiently, unbeknownst to many, but indispensable to all.