Modern mechanical carbon materials are being used in a wide variety of applications — a few examples include aircraft gearboxes, air turbine motor starters, and main shaft seals for both aircraft turbine engines and aircraft auxiliary power units (APUs).
These unique, self-lubricating materials are composed of fine-grained, electrographite substances that are impregnated with proprietary inorganic chemicals to improve lubricating qualities and oxidation resistance. These materials are ideal for 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.
Engine manufacturers and operators, who are constantly working to extend their time between overall (TBO), are using these modern seal materials in seal designs. Their goal is to ensure that carbon main shaft seals will no longer be a factor in limiting time between overhaul (TBO) on new aircraft turbine engines.
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.
High sliding speed in shaft seals
Aircraft gearboxes are used to reduce the main engine shaft’s rotational speed from as high as 26,000 rpm down to about 3,400 rpm, so the shaft can 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 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 are sealed 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, from Metallized Carbon Corp., is often used as the stationary ring. It 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.
Shaft seals must withstand extremely high shaft speed
Air turbine motor starters typically use the same carbon-graphite versus 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 APU 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. Metcar Grade M-45 is used in air motor starter seals because of its outstanding self-lubricating qualities at the required operating conditions.
Main shaft seals for aircraft turbine engines and APUs
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 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 rings are used.
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