Ball bearing Roller bearing Preformed sleeve bearing Slipper bearing Figure 1-76. Types of main bearings used for gas turbine rotor support. stop oil leakage. These two types of seals are very similar,  differing only in thread size and the fact that the labyrinth  seal is pressurized. Another type of oil seal used on some of the later engines is  the carbon seal. These seals are usually spring loaded and  are similar in material and application to the carbon brushes  used in electrical motors. Carbon seals rest against a surface  provided to create a sealed bearing cavity or void; thus, the  oil is prevented from leaking out along the shaft into the  compressor airflow or the turbine section.  [Figure 1-77] The ball or roller bearing is fitted into the bearing housing and  may have a self-aligning feature. If a bearing is self-aligning,  it is usually seated in a spherical ring. This allows the shaft  a certain amount of radial movement without transmitting  stress to the bearing inner race. The bearing surface is usually provided by a machined journal  on the appropriate shaft. The bearing is usually locked in  position by a steel snap ring or other suitable locking device.  The rotor shaft also provides the matching surface for the oil  seals in the bearing housing. These machined surfaces are  called lands and fit in rather close to the oil seal. Figure 1-77. Carbon oil seal.  Spacers Seal housing Seals Bearing inner race 1-55