Engineering Plastic Solves Design Challenge for Aircraft Tire Pressure Monitor

By Mark Shortt
Editorial Director, Design-2-Part Magazine

Landing Gear Hubcaps Are Injection Molded of Strong, High-temperature Plastic

Crane Aerospace & Electronics knew exactly what it needed when it was developing a new tire pressure monitoring system for the landing gear of the Boeing 777 aircraft: a material with high temperature resistance, mechanical strength, and chemical resistance to the caustic elements found on the runway. The company, a major supplier of systems and components for critical aerospace applications, specified VICTREX® PEEK™ polymer for the hubcaps containing the system.

“Maintaining proper tire pressure is critical to aircraft safety,” says Jeff Tonn, design engineer at Crane, based in Lynwood, Washington. “The tire monitoring system on the Boeing 777 is designed to assure proper tire air pressure for take-off and landing. The hubcaps include a conduit which allows a rim-mounted pressure sensor to communicate with an onboard system, providing real time data to the cockpit.”

Aircraft landing gear must be able to survive some of the most extreme environments and loading conditions. “For example,” explains Tonn, “temperatures at altitude can be as low as -54°C (-65°F), whereas temperatures on braking can reach more than 200°C (392°F). Landing gear must be able to withstand repeated hard landings, as well as impacts from flying debris. In addition, landing gear is exposed to both ozone and to chemicals in jet fuel, hydraulic fluid, and de-icing solutions.”

The landing gear hubcaps are able to withstand these extreme conditions because they incorporate Victrex PEEK polymer. John Walling, regional business manager for Victrex, explains that “Victrex PEEK polymer maintains its mechanical properties in hot and wet environments. With a continuous-use temperature of 260°C (500°F), the polymer has proven to be the most stable when compared to other high heat thermoplastics. It exhibits no loss in tensile strength after aging at 200°C (392°F) for one year.” 

In addition to outstanding thermal performance, the polymer provides excellent fatigue performance, wear resistance, and chemical resistance. “This unique combination of properties has allowed Victrex PEEK polymer to successfully displace metals, traditional composites, and other plastics in a growing number of aerospace applications,” adds Walling.

Weight also played a major role in Crane’s selection of Victrex PEEK polymer to displace aluminum for the hubcaps. As a result of specifying Victrex PEEK polymer, each hubcap now weighs less than 7 kg (1.5 lbs), according to Tonn. “With 14 wheels per plane, the weight reduction is considerable,” he says.

Another advantage of using the polymer is that it can be easily fabricated into tight tolerance parts. “The hubcaps are injection molded and painted to provide an added layer of UV protection,” says Walling. “A nominal wall thickness of 3.75 mm is thick enough to protect the hubcap from high impact events. Molds are filled evenly on conventional equipment with standard viscosity Victrex PEEK polymer.” 

Altek (www.altek-inc.com), a custom manufacturer in Liberty Lake, Washington, was given the nod to produce the finished product. The company provides a full range of services that include collaborative engineering design, precision machining, mold making, injection molding, and electromechanical assembly. Its custom finishing capabilities include aerospace-certified painting.

“Through a concerted effort with Victrex and its technical team, we were able to develop a stable and automated molding process,” said Mark Adams, program manager at Altek. “Victrex was able to offer support during all phases of our operation. Our extensive machining and aerospace painting experience made the secondary finishing go like clockwork. The design and processing guides were very helpful, and we were all very pleased at how easy it is to work with this material.”

In addition to the Boeing 777 aircraft, the Crane Tire and Brake Monitoring System may be selected for use on other commercial aircraft.

Victrex USA Inc., a division of Victrex plc, is the sole manufacturer and supplier of Victrex PEEK polymer worldwide. For more information, visit the Victrex website at www.victrex.com.

For more on Crane Aerospace & Electronics, visit www.craneae.com.

For more on Altek Inc., visit www.altek-inc.com.

VICTREX® is a registered trademark of Victrex Manufacturing Limited. PEEK™ is a trademark of Victrex plc.