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Mechanical Carbon Seals
MATERIALS USED MOSTLY IN AIRCRAFT COULD PROVE BENEFICIAL IN OTHER TURBOMACHINERY APPLICATIONS
Mechanical carbon materials are being used in such applications as aircraft gear boxes, air turbine motor starters, and main shaft seals for both aircraft turbine engines and aircraft auxiliary power units. These self-lubricating materials are composed of fine-grained, electrographite substances that are impregnated with proprietary inorganic chemicals to improve lubrication qualities and oxidation resistance.
They are favored in aircraft applications because of a low coefficient of friction, low wear rate at high sliding speed, high thermal conductivity and resistance to oxidation in high temperature air. These properties may also make the materials of interest to designers of other high-speed rotating equipment, such as high-speed rotary gas compressors and steam turbines.
Take the case of aircraft gearboxes, which are used to reduce the main engine shaft’s rotational speed from as high as 26,000 rpm down to about 3,400 rpm. This enables the shaft to drive such system components as hydraulic pumps, generators and air conditioning compressors.
To seal the oil lubricant within the gearbox and protect it from leaking at the point where the shaft enters the gearbox, most aircraft gearboxes use face seals. These usually contain a carbon-graphite stationary ring and a silicon carbide or tungsten carbide rotating ring. These rings that make the dynamic face seal are lapped flat and held together with springs or magnets so liquids cannot flow between the ring faces even though they are spinning against each other at high rpm.
The two rings in relative motion are sealed to the shaft or the gear box housing with static seal rings such as polymeric O-rings. 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 reduces the friction and wear of the seal faces (Figure 1).
Figure 1: Metcar Grade M-45 is used in air motor starter seals because of its self-lubricating qualities at the required operating conditions[/caption]
Air turbine motor starters typically use the same carbon-graphite vs. silicon carbide or tungsten carbide dynamic face seal materials that are 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 180,000 rpm, or a sliding speed of about 1,000 feet/second, which is nearly the speed of sound. The seals are designed with wedges and gas flow passages to produce aerodynamic or hydrodynamic lift-off.
Speed and temperature
Face seal rings, with carbon-graphite primary rings, and carbon-graphite circumferential seal rings are used in aircraft engine main shaft seals to control the air flow 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 that fit with close-end clearance in slots in the stationary housing are used. The carbongraphite 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.
Auxiliary power units are another area of use for carbon-graphite seals. APUs are essentially small gas turbine engines that create electric power, air conditioning, or cabin heat, when the main engines are turned off at the gate to save fuel. These seals are similar to, but smaller than, the main engine seals.
This carbon-based material’s oil-free, self-lubricating, self-polishing and dimensionally stable nature ensures a good sealing mate. As it is heat resistant and has high thermal conductivity, this helps conduct frictional heat away from the sliding surface. In addition, carbongraphite is machinable for exacting aerospace dimensional tolerances.
