Modern Mechanical Carbon Materials for Aircraft Seal Applications

Modern mechanical carbon materials are found in a wide variety of applications – a few being aircraft gearboxes, air turbine motor starters, and main shaft seals for both aircraft turbine engines and aircraft auxiliary power units (APUs).

The self-lubricating materials are composed of fine-grained, electrographite substances impregnated with proprietary inorganic chemicals to improve their lubricating qualities and their oxidation resistance. These modern carbon-based materials are finding use in aircraft applications because of their low coefficient of friction, low wear rate at high sliding speed, high thermal conductivity, and resistance to oxidation in high-temperature air.

These properties also make the materials of interest to designers of other high-speed, rotating equipment; for example, high speed rotary gas compressors and steam turbines.

Withstanding High Speed
Aircraft gearboxes reduce the main engine shaft’s rotational speed from as high as 26,000rpm down to about 3,400rpm, so the shaft can drive such system components as hydraulic pumps, generators, and compressors.

To seal the oil lubricant within the gearbox and protect it from leaking out at the point where the shaft enters and exits the gearbox, most aircraft gearboxes use face seals. The face seals usually contain a carbon-graphite stationary ring and a silicon-carbide or tungsten-carbide rotating ring. The rings that make the dynamic face seal are both lapped flat and are held together with springs or magnets so that liquids cannot flow between the ring faces even though they are spinning against each other at high rpm.

The two rings in relative motion that make the dynamic seal attach to the shaft or the gearbox housing with static seal rings such as polymeric O-rings. Seal designers use spiral grooves, straight grooves, and wedges to channel or pump a thin film of air or oil between the two sliding sealing faces. This creates aerodynamic or hydrodynamic lift, which greatly reduces the friction and wear of the seal faces.

For example, Metcar Grade M-45 finds success used as the stationary ring. This material is especially suited for these shaft seals because it is impermeable and thus able to support an aerodynamic film. It also has the ability to run at high speed with low friction and low wear.

Air turbine motor starters typically use the same carbon-graphite versus silicon-carbide or tungsten-carbide dynamic face seal materials used in gearbox seals, but the sliding speed is much higher. These air turbine motor starters are actually small turbines that use the exhaust gas from the auxiliary power unit to create the power necessary to start the main engines. The shaft speed on air motor starters can be as high as 180,000rpm, or a sliding speed of about 1,000ft/second, which is nearly the speed of sound. The seals, designed by aircraft seal manufactures, feature wedges and gas flow passages to produce aerodynamic or hydrodynamic lift-off. Metcar Grade M-45 is used for air motor starter seals because of its self-lubricating qualities at the required operating conditions.

Handle Speed, High Temps
Face seal rings, with carbon-graphite primary rings, and carbon-graphite circumferential seal rings, find use in aircraft engine main shaft seals to control the airflow and combustion gas flow inside the engine. They also seal the oil lubricant in the main engine bearings that allow the compressor shaft and the combustion gas turbine shaft to rotate freely. Both circumferential and face type seal ring are used.

For circumferential main shaft seal rings, carbon-graphite segments with close end clearance are used in slots in the stationary housing. The carbon-graphite segments are tensioned against a ceramic or hard metal coating on the rotating shaft using a “garter” spring.

Lifting wedges and machined configurations work to create lift so that these seals run on an aerodynamic or hydrodynamic film. Rotating speeds can be as high as 26,000rpm, and temperatures in the seal rings can reach as high as 800°F.

Use of Metcar Grade M-595 type materials is recommended for aircraft main shaft seals, because the chemical additive in these grades prevents oxidation at high temperatures and provides improved lubricating qualities at the required operating conditions.

Auxiliary power units (APUs) are small gas turbine engines used to create electric power, air conditioning, or cabin heat, when the main engines are turned off at the gate to save fuel. APUs contain carbon-graphite seals that are similar to, but smaller than, main engine seals.

Unique Characteristics
Oil-free, self-lubricating mechanical carbon materials possess a unique combination of characteristics that makes them ideal for use in both commercial and military aircraft seal applications.

The materials are self-lubricating, self-polishing, and dimensionally stable, which insures a good sealing mate. The materials are heat resistant and have a high thermal conductivity, which helps conduct frictional heat away from the sliding surface. In addition, these materials are readily machinable to exacting aerospace dimensional tolerances, and are available lapped and polished to a flatness specification of one helium light band.

Metallized Carbon Corp.
Ossining, N.Y.
www.metcar.com

About the author: Glenn H. Phelps, is technical director at Metallized Carbon Corp., and can be reached at ghphelps@metcar.com.

LEARN MORE: Download the materials properties brochure – http://bit.ly/14YZuEJ – for more detailed information.