After more than half a century, hydraulics continues to hold an important role in commercial aircraft control functions. Pilots deploy thrust reversers just after a plane touches down. Thrust reversers make up the rear shroud of engines and help decelerate a plane by moving to a position that deflects thrust from the engine forward. This action relieves much of the load on an aircraft’s hydraulic brakes.

Thrust reversers normally are locked into a stowed position mechanically, and when the pilot activates a switch to unlock and deploy them, solenoid operated hydraulic valves route pressurized fluid to feed sets of hydraulic actuators that position segments of the thrust reverser. Once the aircraft has slowed, the pilot commands the thrust reversers back to their stowed position, and the valves reverse to retract the cylinders.

Getting in synch

Many methods exist for synchronizing position of multiple cylinders. (Refer to our January 2011 issue for an article outlining eight other ways of synchronizing cylinder motion.) However, a mechanical connection often proves the most reliable because it will perform without electricity or regardless of fluid pressure.

The synchronization is accomplished by integrating a ball screw assembly with each cylinder and driving the ball screws in series using flexible rotary shafts. The ball screws follow the position of the hydraulic actuator, and the rotary shaft connecting each pair ensures that the rotational position of each ball screw matches the other.

Brian Parlato, vice president at S. S. White, Piscataway, N. J., says ensuring that ball screws move in precise synchronization is essential for these functions. Parlato explains that in its simplest form, a flexible rotary shaft can be thought of as a speedometer cable. However, a speedometer cable is designed to transmit only rotational motion at the extremely low torque needed to drive a speedometer. It also is designed for only one direction of rotation.

Photo shows example of how flexible shafts transmit rotational power between components — many times the only solution for such cramped quarters.

Flexible rotary shafts from S. S. White, however, are designed to transmit bidirectional rotation and torque. Rotational power is transmitted through layers of wires made of stainless steel or other high-strength material and helically wound in opposite directions. Reaction forces from the layers helically wound in opposite directions tend to cancel each other out, so the layers don’t unravel regardless of which direction they turn. The shafts also transmit high torque within a small OD — in fact, the shafts are often located within the hydraulic tubing routed to each actuator.

Here’s how it works. As an actuator extends, the ball nut follows the motion. The ball nut, therefore, rotates the ball screw according to its pitch. The flexible shaft transmits the rotational motion of the ball screw to the ball screw of the next assembly in succession. This ensures that all ball screws, ball nuts, and actuator pistons all move in concert.

Flexible benefits

Construction of a flexible shaft begins with a core, around which high-strength wire is helically wound either clockwise or counter-clockwise. A second layer of wire is then helically wound in the opposite direction over the first layer. This alternating clockwise and counter-clockwise continues until the desired diameter is reached. Lubrication, an outer casing, and end fittings are then added to complete the assembly. To reduce maintenance, and eliminate potential contamination in aerospace applications, S.S. White also offers Flexcellent, a permanently lubricated system.

Flexible rotary shafts offer many benefits over alternative technologies, such as gearing and universal joints. Flexible shafts allow generous design freedom because component alignment (both angular and parallel offset) is almost a non-issue. They have a higher power density and are quieter and more efficient than gearing or universal joints. They are also easier and less costly to install, and can be designed in at the later stages of a project.

For more information on flexible rotary shafts from S.S. White Technologies, visit, call (732) 474-1700, or e-mail Click here for a brief tutorial on flexible shafts.