Turning in the Sky

Simple motion transmission component has been flying for decades

by Richard Mandel

Even before two brothers succeeded in making an internal combustion engine airborne for 120 feet under its own power, certain design principles for flight were pretty clear. Ideally, components were to be lightweight, strong, and occupy minimal space. In those days before specialty alloys and cheap aluminum, aviators were kept aloft with sticks and fabric, materials that were used on powered aircraft into World War 2.

Locations where flex shafts are used

Most new technologies were build on existing technologies and aviation was no exception. The engine that powered the Wright craft at Kitty Hawk was not invented by the Wright Brothers--it was taken from another application. Likewise, so were many of the components that were eventually used on airplanes.

War-time technology

By the time of WW2, aviation had mostly advanced beyond simple cable-and-pulley control systems that were suited for the simpler one- or two-man craft. Mazes of hydraulic lines snaked through and around rigid steel bulkheads, but there was still a need for mechanical links from one point of rotation to another. Designers called on S.S. White, Piscataway, NJ, to develop various drive assemblies using flexible shafts that the company first created as dental drill drive shafts in 1844. Similar shafts had found use in early automobiles as links to the mechanical speedometers, from a gear positioned at either a wheel or in the transmission. The aircraft assemblies were successfully incorporated into flap, rudder, aileron and trim controls, as well as part of the handcranked backup systems if the hydraulics failed.

Thrust reverser on a Pratt & Whitney jet engine

Current applications for flex shafts in aviation still include many areas of commercial and military aircraft. Smaller private planes and business jets are using flex shafts for flap and slat actuation, and for trim controls on most 6-passenger twin-turboprop platforms. Because of the limited area around front windows, windshield wipers are driven by a remote-mounted motor. Similarly, commercial aircraft, some of which use up to 38 flex shafts, operate power latches on several doors concurrently with just a single motor.

On engines, shafts made from high-temperature materials like Inconel and Monel operate cowling air bypass systems and afterburner spray actuation and control. Thrust reversers, the big clamshell doors that slip over the exhaust of a jet engine to aid braking on landing, are driven by flex shafts spinning at speeds up to 100,000 rpm. The inherent spring action of the cable absorbs the impact of the reversers as they hit their stops. Not only do flex shafts carry out primary actuation, they also perform as a secondary mechanical link for synchronization. F4 Phantom jets have these shafts driving their fuel pumps, and even the rotation of radome on the AWACS is helped along by a flex shaft connecting a fuselage-installed motor/gearbox assembly to an upper gearbox on the radome support structure.

Why select a flexible shaft

As indicated, flex shafts can handle tough situations. They can supply rotary motion close to high-temperature sources such as engines, where electric motors would fail. Flex shafts have an almost unlimited reach, allowing the designer to place motors and actuators where they best fit, while the shafts can be routed around obstacles without requiring redesign of drive components or structures. The flexibility of the shafts require less precession, and they can absorb manufacturing differences in aircraft assembly. And the ability to operate several remote devices from a single motor allows simple synchronization control while lowering parts count and cost.

Since they are nearly a direct-drive component, flex shafts are 90 to 95% efficient, compared to gears, u-joints, belts and pulleys which have a lower efficiency due to frictional losses. The shafts can also maintain that efficiency even with offsets as high as 180 degrees--flex couplings allow only 5 degrees of offset, and u-joints 30 degrees, but with a 40 - 50% decline in efficiency. The flexibility also frees up design from the high precision required for many other power transmission systems.

Designing with flex shafts

Once a decision has been made to use a flex shaft in a system, there are a few factors to consider besides length and material.

* Is the application uni-directional or bi-directional? A uni-directional shaft are designed for continuous operation in one direction of rotation, and are made to maximize torque carrying ability and torsional stiffness for that particular direction. Bi-directional shafts are for shorter duty cycle applications requiring rotation in both directions.

* What are the torque requirements? Maximum continuous torque through the shaft is a determining factor in specifying shaft diameter. Flex shafts typically can handle start-up torques 20% higher than their maximum continuous torque.

* What is the minimum bend radius? There is a direct relationship between operating torque and operating radius--as the radius gets smaller, the torque capacity reduces due to increased friction within the shaft itself.

Page from 1940's brochure

* Bending stiffness or torsional stiffness? It is preferable to select a shaft with the lowest bending stiffness, should there be a choice that bears the same torsional stiffness. If shaft twisting under load is a concern, then select a shaft with higher torsional stiffness.

* What is the continuous operating speed? As speed increases, the level of power transmitted can increase as the torque load is reduced proportionally to the increase in shaft speed. Faster speeds, therefore, reduce torque load in flex shafts. Excessive speed, especially in tight bend radii, can cause heat buildup and premature failure.

* Does the application require a casing? When lengths exceed 8 in. or the operating environment is dirty, dusty or corrosive, a casing is usually recommended. A casing supports the shaft and prevents it from flexing and losing torque capacity at lower-than-expected loads. Casings also minimize injury or damage where shafts are run at high speeds.

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