Sealing between coaxial shafts rotating at high and differing speeds has long been a serious problem. A case in point pertains to multi-shaft aircraft turbine engines wherein the shaft carrying the low pressure compressor and the low pressure turbine is coaxial with and inside the tubular shaft carrying the high pressure compressor and the high pressure turbine. The shafts may be corotating or counterrotating and the mean diameter of the annulus between them can be of the order of two inches to eight inches. Shaft speeds of 10,000 to 20,000 RPM are common.
Typically, sealing this annulus has involved use of ring type carbon seals with gaps so that centrifugal force could dilate the rings and cause them to seat tightly against the bore of the outer shaft. Such a seal is shown in FIG. 1 where the annulus 2 between an outer shaft 4 and an inner shaft 6 is sealed by a plurality of segmented carbon seal rings 8 each of which is adapted to bear against the inner surface 10 of outer shaft 4 and against a mating ring 12 secured to inner shaft 6 by virtue of shoulder 14, spacing rings 16, annular ring 18 and sleeve clamp 20.
The principal problem presented by this type of seal stems from the magnitude of the centrifugal force on the ring. For example, if a carbon seal ring of 5 inch mean diameter and of 1/4 inch by 1/4 inch cross-section should rotate at 12,000 RPM the radial loading could be calculated as follows:
The weight of the carbon ring, using a specific weight of 0.065 lb per cubic inch would be about 0.004 lb per inch of circumference. PA1 The centrifugal field would be equivalent to: ##EQU1## Thus the loading on the ring would be: 0.004 lb/inch.times.10,227=40.9 lb/inch.
Therefore, the ring would be practically fixed by the centrifugal force in the bore and would require a force equivalent to 8 or 10 lb per inch to move it axially. Since the shafts would have different rotational speeds, even if corotational, the ring would need to rub either on its outer arcuate surface or on its transverse face. Obviously, the loadings on both these surfaces are too high for desirable wear life.
The present invention partially relieves the high centrifugal loading due to the rotation of the seal ring. The invention also can cause the seal ring to rotate at a speed intermediate the speed of the two shafts, thus reducing the maximum rubbing velocity. The invention relieves the centrifugal loading with relatively simple means without resorting to a multiplicity of hinged or articulated counterweights or similar complex mechanisms. The loadings at the rubbing surfaces are markedly reduced.
The use of hydrodynamic pockets to produce negative and positive lift in shaft seals is known to the art as is seen in U.S. Pat. No. 4,082,296, the disclosure of which is incorporated herein by reference. As pointed out in said patent, positive lift provided by hydrodynamic pockets has been used to reduce rubbing loads in seals associated with single shafts. In accordance with this invention pressure developed in hydrodynamic pockets cooperating with one shaft is communicated to closed pockets cooperating with the other shaft to control relative rotation speeds of the seal ring and the shafts.