Each finger seal consists of a number of multiple thin metal laminates, each with a multitude of flexible projections that are referred to as “fingers.” To form a functional seal, multiple laminates are formed into a stack, with each laminate oriented so that the slots between its fingers cover the slots of the neighboring laminates. For protection and support of the laminates, forward and aft cover plates are placed on each side of the finger seal stack. A high pressure exists on one side of the seal, providing a driving force to push the air from one side of the seal to the other.Each finger of the seal remains in light contact with the rotating shaft, limiting leakage that can pass between the seal and shaft. The pressure acting on the face of the upstream laminate can cause axial loading of the fingers against the downstream plate. In some seals, this axial loading is high enough to result in a high amount of friction between the downstream laminate and aft plate, as well as between the laminates themselves. This friction restricts the free motion of the fingers, preventing them from remaining in contact with the shaft, and harming the seal’s performance. To reduce this pressure-induced friction, a design has been created that includes a pressure balance circuit.
As pressure is applied to the finger seal, the laminates are pressed back into the dam near the inner diameter of the aft (downstream) plate. This creates a sealed cavity between the aft plate and downstream-most laminate, where air from the high-pressure side can be routed, which reduces the axial loading that acts on the laminate stack.
While the inclusion of this pressure balance circuit has improved the finger seal, it has caused the seal to be functional in only a single direction. During pressure loading in the design direction, the laminates of the finger seal are pressed into the dam by the pressure-induced axial load, sealing the pressure balance cavity, except for a small flow that leaks down between the finger slots. The overall leakage of the seal is low, and the majority of air passing through the seal occurs between the shaft and laminates. For reverse pressure loading, the pressure-induced axial load pushes the laminates away from the dam on the aft plate, opening an area for leakage. This allows a large amount of leakage to flow into the pressure balance cavity, through the pressure balance circuit, and to the low-pressure side. The overall leakage for a reverse pressured seal will be high, with the majority of leakage passing through the pressure balance circuit. To prevent this high leakage when pressure is reversed, the pressure-balanced finger seal design can only be used in applications where the pressure will never reverse.
Subscribe today to receive the INSIDER, a FREE e-mail newsletter from Embedded Technology featuring exclusive previews of upcoming articles, late breaking NASA and industry news, hot products and design ideas, links to online resources, and much more.