Really two pieces in one, Qualiseal Controlled Gap/Bushing seals are the best solution when pressure is an issue.
Controlled gap or bushing seals are comprised of a carbon insert that is shrink-fitted into a separate retaining band. This floating circumferential ring assembly grows at a rate similar to that of the shaft when exposed to elevated temperatures.
Under normal operating conditions, the pressure on the airside is higher than the pressure on the oil side. This creates some air leakage through the controlled gap between the carbon ring and shaft. The differential air pressure, along with the spring load, provide the closing force necessary to maintain contact between the carbon and housing secondary sealing face. Meanwhile, the shaft turns freely within the carbon ring.
The seal design also features a unique stabilizer ring with fins which minimize stress on the carbon ring, thereby maintaining the roundness of the circumferential ring assembly. The dimensional tolerance of the critical inside diameter of the bushing ring is tightly controlled to minimize leakage variations.