A flexible drive 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.
In the history of aerospace manufacturing, there have been several innovative technologies that have changed the shape of flight. One case in point is the flexible drive shaft.
What makes flexible drive 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 drive 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 drive shaft is a highly effective means of transmitting rotary motion and is more efficient than propeller shaft, 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 drive 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 drive 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.