For circumferential main shaft seal rings, carbon-graphite segments that fit with close end clearance in slots in the stationary housing are used. 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 are used to create lift so that these seals run on an aerodynamic or hydrodynamic film. Rotating speeds can be as high as 26,000 rpm, and temperatures in the seal rings can reach as high as 800 F.
Metcar Grade M-595 type materials are used for aircraft main shaft seals, because the chemical additive in these grades prevents oxidation at high temperature and provides improved lubricating qualities at the required operating conditions.
Commercial aircraft turbine engines have an established TBO. During the overhaul, the turbine engine is disassembled so that internal parts can be cleaned and inspected, and all parts that are out of specification can be replaced. Smaller commercial aircraft turbine engines typically have TBOs on the order of 3,000 to 5,000 hours, and the cost of the overhaul can be as much as $250,000.
Often during the engine overhaul, the carbon main shaft seals need to be replaced. With the use of modern carbon seal materials and aerodynamic lift off seal designs, the hope is that carbon main shaft seals will no longer be one of the factors that limit TBO on new aircraft turbine engines.
APUs are small gas turbine engines that are 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.
Oil-free, self-lubricating mechanical carbon materials possess a unique combination of characteristics that make them ideal for use in both commercial and military aircraft seal applications.
The materials are self-lubricating, self-polishing, and dimensionally stable, which ensures 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 they can be supplied lapped and polished to a flatness specification of one helium light band.
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