GB2225394A

GB2225394A - Shaft sealing arrangement

Info

Publication number: GB2225394A
Application number: GB8926600A
Authority: GB
Inventors: Wolfgang Weiler
Original assignee: MTU Motoren und Turbinen Union Muenchen GmbH
Current assignee: MTU Aero Engines GmbH
Priority date: 1988-11-25
Filing date: 1989-11-24
Publication date: 1990-05-30
Anticipated expiration: 2009-11-24
Also published as: DE3839843A1; GB2225394B; FR2639674A1; GB8926600D0; FR2639674B1; DE3839843C2

Description

1 SUMT SEALING ARRANGEMENT 2225394 The invention relates to a seal

arrangement, in particular for sealing regions acted upon by fluid media between coaxial shaft sections of turbo engines, particularly gas turbine power plants.

It is difficult to provide a seal of this type which operates without contact and at the same time has a relatively small overall radial and axial length.

This particularly applies to sealing points between rotor components and shaft systems which rotate about a common axis at different speeds and/or directions of rotation.

Labyrinth seals, which are well enough known,are generally used for axial sealing tasks. Known types of seals such as brush seals and radial slip ring seals are susceptible, among other things, to centrifugal force deformations which occur in operation. In the case of the brush seal the centrifugal forces lead to deformation of the bristles which touch and seal an associated shaft or a shaft section with their particular free ends; in the case of a brush seal which has bristles pointing substantially radially outwards this leads to, abrasion of the relevant butting coatings, while if the bristles point substantially inwards they tend to be lifted off the butting coating and thus the sealing capability is lost. In sealing arrangements using sealing rings, out-of-balance forces occurring in the course of operation can lead to destruction of the rings, and this applies both to the radial slip ring seals and to proposed sealing systems in which a brush seal is designed in combination with an inner ring as a sliding sealing ring.

According to the present invention there is provided a shaft sealing arrangement comprising an inner shaft, a coaxial hollow shaft surrounding the inner shaft and a sealing device arranged between the shafts so as to define two axially spaced regions to be 1 2 sealed from each other against a predetermined fluid pressure difference, wherein the sealing device comprises a sealing ring arranged between the two shafts, forming an annular gap of narrow radial extent with the inner or outer shaft and held in a sliding fit on both axial sides, and a spring element acting between the ring and the outer or inner shaft respectively resiliently to centre the ring radially so that the said narrow gap acts, in use, as an air bearing.

Embodiments of the invention may provide intershaft seals which, while of a relatively small overall axial length and, in particular, a small radial installation height, produce no significant wear and abrasion of material either on the shaft side or the sealing ring side due to the effects of operation, particularly out-of- balance and centrifugal forces. The sliding fit for the sealing ring may be formed by an annular recess in the one shaft section in which the ring is centered and which also contains the spring element. The recess may be defined in the axial direction by two wall members, one of which may be arranged to slide in the axial direction under a spring retaining force to hold the ring. 25 Advantageously the spring element,as seen in section in an axial plane, has the shape of a V or U lying on its side, at least over part of its circumferential extent. Embodiments of the invention may have the following advantages: only a short overall axial length and in particular also a small radial installation height requirement in view of close shaft spacings; freedom from contact between the two relevant shaft systems; independence of the direction of rotation; ease of installation; a sealing effect like a radial slip ring seal with a static housing, i.e. all that is required is a through-flow corresponding to about 10% of that of an equivalent labyrinth seal; a spring 3 characteristic for the spring element which can be designed comparatively rigid; the spring element need comprise only a single spring body basically closed upon itself in the shape of a ring; extremely fine adaptability of the spring stiffness to the particular application, particularly, if the spring element has essentially the shape of a prone V or U, by dividing one or both legs into individual so-called "bending bars", the number and dimensions of the "bending bars" being a free choice within wide limits; the assembly of spring element, sealing ring and recess walls may be designed either purely as a friction locking or as a form fitting connection as well.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:

Fig. 1 shows a section through a sealing assembly in accordance with the invention; Fig. 2 shows a second embodiment of the invention, also in section; Fig. 3 shows a part-sectional view 6f a spring element; Fig. 4 shows a view on the spring element, in the direc tion of arrow X in Fig. 3; and Fig. 5 shows modification of the Fig. 2 arrangement.

Fig. 1 illustrates a seal between two shafts 1 and 2 with housing-like guide 8,9 and spring means 6 for a sealing ring 3, assembled on to the inside shaft section. The regions or chambers which are acted upon by media such as liquids and/or gases at different pressures are respectively denoted R1 and R2. This arrangement is a so-called "inter-shaft seal", i.e. a seal between coaxial shaft sections 1, 2 which may form part of a gas turbine power plant by way of example. In the device according to Fig. 1 the sealing ring 3 is radially resiliently supported on one, in this case the inner, shaft section 2, the ring being retained in the k,' 4 manner of a housing and acting with this housing 8,9 as a secondary seal; the ring forms a narrow radial gap 4 with respect to the other shaft section 1 as a primary seal. The other shaft section 1 in this case may therefore be the relevant outer shaft of a gas turbine power plant.

According to the arrangement in Fig. 1, the sealing ring 3 is seated in a radially-extending annular recess 5 of one shaft section 2; further, the sealing ring 3 is centred on and supported, radially resiliently, by an (in section) essentially V-shaped annular spring element 6 which is arranged axially in the recess 5, is open to one side and has a degree of. rotational symmetry. The V-section is in a prone position with respect to the axis, with one leg braced against the ring 3 and the other against the inner shaft 2. Depending on the application and the desired local spring bias, however, this V-shaped spring element could also be designed, roughly speaking, more or less in the shape of a U. The dimensions of the radial gap 4 should further be made such that with a prescribed pressure difference P1 > P2 between the two chambers R1, R2 a supporting gap in the manner of an air bearing is formed. As may further be discerned from Fig. 1, the sealing ring 3 forms the radial gap 4 along a circumferential widened portion 7 which is Tshaped in axial section.

The radial recess 5 for the sealing ring 3 is formed between radially projecting walls 8, 9 of one shaft section 2. Both wall sections 8 and 9 may conceivably be shaft- integral component parts; but in Fig. 1 the left-hand wall 8 is formed integrally with the inner shaft section 2, the other wall 9 being designed as a separate structural part joined securely to the shaft section 2 on assembly. The wall 9 may be secured against rotating via, for instance, tongue and groove connecting means or the like. A circular or annular securing element 12 secures the other wall 9 in W 11 the axial direction.

As may further be roughly discerned from Fig. 1, on the one side facing towards one shaft section 2 and also on the side facing towards the sealing ring 3 the spring element 6 may, by way of example, be divided up into side piece sections 10' in the form of bending bars, distributed evenly in the circumferential direction.

Fig. 3 and 4, in particular, illustrate a spring element 6 of this type. In Fig. 3 the section is taken through a bending- bar-like side piece section of a spring element which is V-ring shaped and open to one side. Fig. 4 shows two of the side piece sections 10' in the manner of bending bars or bending lugs spaced uniformly over the circumference.

In the embodiment of Fig. 2 the housing-like guide, centering and spring means for the appropriate sealing ring are associated with the outer shaft element. Also there is additional axial plate-like contact pressure on the sealing ring produced by spring force against a sliding wall-like supporting means on the (outer) shaft. The other wall 9', again a separate structural part, is designed as a pressure plate which is fixed with respect to rotation on one shaft section 2' but is movable axially thereon under spring action. The wall 9' may further be fitted to slide in axial grooves 10 in the outer shaft section 2'. A spring element 11 acting in the axial direction engages between the slidable wall 9' and an annular retaining element 12' secured against rotation relative to the shaft 2' and if necessary releasable therefrom. Also discernable from Fig. 2 is the fact that the particular sealing ring 3' shown in this instance is a carbon ring mounted on a surrounding metal ring 13. 35 With respect to the sealing devices according to Fig. 1 and Fig. 2 which have been discussed and commented on above, it should further be observed that the sealing rings 3; V-should be arranged so that a 6 gap 4; 4' is produced between one shaft and the sealing ring of a width which lies within the known order of magnitude for air bearings. It follows from that that the sealing ring, like an air bearing, follows static 5 and dynamic shaft axis displacements as long as the out-of- balance forces which result from its rotation are smaller than the air bearing force.

With seals in accordance with the invention, the purpose of the particular, e.g. V-shaped, spring element 6 or 6' is that particularly in the starting and running-up phase the sealing ring is centred in relation to the particular housing means and that outof-balance forces deriving from errors of geometry and inhomogeneities of the sealing ring are absorbed until the air bearing force, resulting from the particular relative movement between the shafts or between the relevant shaft and the sealing ring, takes over centring of the latter in relation to the associated shaft contributing to the formation of the sealing gap.

Sealing devices in accordance with the invention may even succeed in perfectly meeting sealing requirements when storage chambers in gas turbines, for instance, have to be sealed off from the power plant inlet or from the ambience or from the exhaust gas flow, i.e. storage chambers which usually have a lower pressure level than the storage chamber in front of the relevant seal.

Taking Fig. 2 as an example, the inner shaft 1' may be the relevant shaft of the low pressure system of a gas turbine power plant. In this case the low pressure turbine would therefore have to be connected aero-thermodynamically after a gas generation turbine, and this in such a way that the inner shaft 1' is driven, in a form of construction mechanically independent from the outer shaft 2' of Fig. 2 which encases the relevant inner shaft 1' coaxially and which is a component part of the shaft system of the gas generator of the power plant.

1 7 Fig. 5 is an advantageous modified development of the sealing device in accordance with the invention, and shows the sealing ring and the spring element being held on the one housing-like shaft section not only in a friction-locking but also a form-locking manner. Unlike the essentially friction-locking mounting of the sealing rings 3,3' with the spring elements 6,6' on the one shaft section 2' which Figs. 1 and 2 show, a friction- and form-fitting arrangement here is achieved by the spring element 6' engaging in this example jjy means of tongues 14 or 15 protruding from the ends of the bending-lug-like side piece sections 10' and from the opposite leg of the V into axial grooves 10 of the shaft section 2' and axial grooves 16 of the metal ring 13 respectively. Thus a securing device against mutual circumferential rotation (sealing ring V/spring element V) is also produced. The tongues 14 or 15 and the axial grooves 10 or 16 are preferably arranged so as to be distributed evenly over the circumference.

Unlike Fig. 2, in Fig. 5 the securing element 12' is further designed as a wall section which is joined to the wall section 2.

Although the invention has particular application to a coaxial shaft system it is applicable to any apparatus requiring a shaft seal.

I- k_

Claims

1. A shaft sealing arrangement comprising an inner shaft, a coaxial hollow shaft surrounding the inner shaft and a sealing device arranged between the shafts so as to define two axially spaced regions to be sealed from each other against a predetermined fluid pressure difference, wherein the sealing device comprises a sealing ring arranged between the two shafts, forming an annular gap of narrow radiar extent with the inner or outer shaft and held in a sliding fit on both axial sides, and a spring element acting between the ring and the outer or inner shaft respectively resiliently to centre the ring radially so that the said narrow gap acts, in use, as an air bearing.

2. A shaft sealing arrangement according to claim 1, wherein the sliding fit for the sealing ring is formed by an annular recess in the one shaft section in which the ring is centered and which also contains the spring element.

3. A shaft sealing arrangement according to claim 2, wherein the annular recess for the sealing ring is formed between two parallel walls projecting radially from one shaft.

4. A shaft sealing arrangement according to claim 3, wherein the first wall is formed integrally with the one shaft and the second wall is a separate structural part.

5. A shaft sealing arrangement according to claim 4, wherein the second wall is secured against rotation relative to the one shaft.

6. A shaft sealing arrangement according to claim 4, wherein the second wall is designed as a pressure plate secured against rotation relative to the one shaft and axially displacable thereon under spring ii 1 1 action.

7. A shaft sealing arrangement according to claim 6, wherein the second wall is arranged to slide in axial grooves in the one shaft section and the spring action thereon is provided by a spring element which acts in the axial direction between the second wall and an annual retaining element fixed to the one shaft in a manner secure against rotation and releasable.

8. A sh,ft sealing arrangement according'to any preceding claim, wherein the sealing ring widens in the region forming the annular gap so as to be generally Tshaped in axial section.

9. A shaft sealing arrangement according to any preceding claim, wherein the spring element, as seen in section in an axial plane, has the shape of a V or U lying on its side, at least over part of its circumferential extent.

10. A shaft sealing arrangement according to claim 9, wherein one of the legs of the V or U is divided into flexible bar-like sections uniformly spaced around the circumference.

11. A shaft sealing arrangement according to claim 2 or any claim appendant to claim 2, wherein the sealing ring is secured against rotation in the housing-like recess both in a friction locking and in a form locking manner via the spring element.

12. A shaft sealing arrangement according to claim 11, wherein the spring element has tongues which engage in axial grooves respectively in the one shaft section and in the sealing ring, the tongues being designed particularly in the form of flexible lugs and distributed evenly over the circumference.

13. A shaft sealing arrangement substantially as described herein with reference to Figures 1, 2 or 5.

Funlishea iggeatThe Patent 0Ince, State House, 66.71 High Holborn, LondonWO1R4TP. Further copies maybe obtalnedfrom The Patent Office.