A second-generation design for a finger seal has been proposed to reduce a hysteretic effect that gives rise to increased leakage in a first-generation finger seal. As explained below, the second-generation design provides for balancing of pressure drops along the flow paths within the seal in such a way as to reduce a friction force believed to cause the hysteresis.

Figure 1. A First-Generation Finger Seal works well at low or monotonically increasing speed, but not during deceleration from high speed.
Like a labyrinth or brush seal, a finger seal is used (typically in a gas turbine) to minimize a leakage flow along a rotating shaft. A finger seal partly resembles a brush seal in appearance and function, but costs only about one-fifth as much.

Instead of the random array of fine wires found in a brush seal, a finger seal contains a stack of precisely photoetched sheet elements. The photoetched details of the sheet elements are series of intricate geometrical features repeated at small intervals along the circumference at the inner diameter (see Figure 1.) The key features are slender, curved beams (the fingers) with contact pads at their tips. The fingers spring-load the contact pads into gentle contact with the shaft.

Another key feature is a series of pairs of holes, into which rivets are inserted during assembly of the seal. The holes are spaced such that when successive elements are alternately indexed to the holes, the empty spaces between the finger/ pad subelements of each sheet element are covered by finger/pad subelements of the adjacent sheet element. Usually, a first-generation finger seal comprises four sheet elements, a spacer on the forward (high-pressure) side, and forward and aft cover plates.

Figure 2. A Second-Generation Finger Seal would differ from the first-generation seal in several details, such that the friction force believed to cause hysteresis would be greatly reduced.
The seal is fitted over a sealing land on the shaft with a small amount of interference and thus gentle spring loading. Flow through the seal is impeded by the staggered finger/pad features as well as by the radial contact between the land and the pads. The flexible fingers can give radially to accommodate shaft excursions and relative growth of the seal and rotor resulting from rotational forces and thermal expansion.

In tests at constant temperature and pressure, first-generation finger seals exhibited the following hysteretic phenomenon: Leakage remained close to an initial low level as the rotation of the shaft was ramped up from zero to a maximum speed of about 40,000 rpm, but then the leakage increased significantly as the speed was ramped from maximum down to zero. It has been conjectured that (1) as the speed increases, the fingers move out in response to a combination of centrifugal growth of the rotor, thermal mismatch, rotor runout, and other causes; and (2) as the speed decreases, the seal fingers do not spring back and instead become stuck in their radially outermost positions, so that the seal/shaft gaps become wider, allowing more leakage. It has been further conjectured that the reason the fingers become stuck in their outermost positions is that the force of friction between the aft cover plate and the fingers is greater than restoring spring force of the fingers.

The proposed second-generation design would reduce the pressure-drop force between the aft cover plate and the adjacent finger/pad subelements, thereby reducing the friction force believed to cause the hysteresis. A second-generation finger seal (see Figure 2) would comprise forward and aft cover plates, forward and aft spacers, and four sheet elements. By means of a series of radial and axial passages, the gap between the forward plate and the foremost sheet element would be connected to the gap between the aft cover plate and the aftmost sheet element. This connection would cause the intermediate pressure between the aft cover plate and the aftmost sheet element to track very closely the high pressure at the forward end. The aft cover plate would include a narrow dam in contact with the finger subelements of the aftmost sheet element. This dam would provide the seal between the intermediate pressure and the low pressure.

This work was done by Gulshan K. Arora and Donald L. Glick of Allied-Signal Aerospace Co. for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Mechanics category.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-16840.