17. A ring seal (fig. 9-7) comprises a metal ring which is housed in a close fitting groove in the static housing. The normal running clearance between the ring and rotating shaft is smaller than that which can be obtained with the labyrinth seal. This is because the ring is allowed to move in its housing whenever the shaft comes into contact with it.
18. Ring seals are used for bearing chamber sealing, except in the hot areas where oil degradation due to heat would lead to ring seizure within its housing.
19. This method of sealing is often used between two rotating members to sea a bearing chamber.
Unlike the labyrinth or ring seal, it does not allow a controlled flow of air to traverse across the seal,
20. Hydraulic seals (fig. 9-7) are formed by a seal fin immersed in an annulus of oil which has been created by centrifugal forces. Any difference in air pressure inside and outside of the bearing chamber is compensated by a difference in oil level either side of the fin.
21. Carbon seals (fig. 9-7) consist of a static ring of carbon which constantly rubs against a collar on a rotating shaft. Several springs are used to maintain contact between the carbon and the collar. This type of seal relies upon a high degree of contact and does not allow oil or air leakage across it. The heat caused by friction is dissipated by the oil system.
22. Brush seals (fig. 9-7) comprise a static ring of fine wire bristles. They are in continuous contact with a rotating shaft, rubbing against a hard ceramic coating. This type of seal has the advantage of with-standing radial rubs without increasing leakage.
Hot gas ingestion
23. It is important to prevent the ingestion of hot mainstream gas into the turbine disc cavities as this would cause overheating and result in unwanted thermal expansion and fatigue. The pressure in the turbine annulus forces the hot gas, between the rotating discs and the adjacent static parts, into the turbine disc rim spaces. In addition, air near the face of the rotating discs is accelerated by friction causing it to be pumped outwards. This induces a comple-mentary inward flow of hot gas.
24. Prevention of hot gas ingestion is achieved by continuously supplying the required quantity of cooling and sealing air into the disc cavities to oppose the inward flow of hot gas. The flow and pressure of the cooling and sealing air is controlled by interstage seals (fig. 9-5),
CONTROL OF BEARING LOADS
25. Engine shafts experience varying axial gas loads (Part 20) which act in a forward direction on the compressor and in a rearward direction on the turbine. The shaft between them is therefore always under tension and the difference between the loads is carried by the location bearing which is fixed in a static casing (fig. 9-8). The internal air pressure acts Internal air system
upon a fixed diameter pressure balance seal to ensure the location bearing is adequately loaded throughout the engine thrust range.
26. To provide cabin pressurization, airframe anti-icing and cabin heat, substantial quantities of air arebled from the compressor. It is desirable to bleed the air as early as possible from the compressor to minimize the effect on engine performance. However, during some phases of the flight cycle it may be necessary to switch the bleed source to a later compressor stage to maintain adequate pressure and temperature.