SPECIFICATIONSeal Between Relatively Rotatable MembersThis invention relates to a seal between relatively rotatable members.
It has been difficult in the past to devise an effective and long-lasting seal of this nature in conditions where the pressure differential across the seal is large and the ambient temperature is high. These conditions apply in various instances, one of which occurs in the turbine region of the gas turbine engine.
A recent development in the design of such seals has been the introduction of the brush seal.
These seals use annular arrays of bristles which are retained to one of the members and extend across the gap between the members to touch or to closely approach the surface of the other member. In high temperature applications the bristles must be of high temperature resistant material, and where a high pressure is to be borne, at least one side plate is necessary to support the flexible fibres against the pressure load.
One possible problem with these seals is that of short life. At least in the normal operating condition of the seal it is normal to arrange for the bristles to touch the surface with which they coact; even in this condition there is possibly considerable wear of the bristles and as the members expand differentially due to thermal and centrifugal effects the wear can be considerably accelerated.
The present invention provides a seal which has provision for the clearance or degree of interference between the bristles and the member with which they coact to be continually adjusted as the seal operates.
According to the present invention a seal between relatively rotatable members comprises an annular array of bristles mounted coaxial with the axis of said relative rotation on one said member and adapted to cooperate with a continuous annular surface portion of the other said member to provide a seal, said continuous annular surface portion being provided with a raised portion adapted to displace said bristles by its passage and thereby to modify the clearance or degree of interference between the bristles and the remainder of the annular surface.
Preferably the height of the raised portion above the remainder of said annular surface is calculated to maintain a substantially zero clearance between the bristles and the remainder of the surface; thus the height may be approximately one seventh of the degree of interference between the bristles and surface at the normal running condition of the seal.
We prefer to make the elevated portion as a pad of ceramic which may be flame sprayed or otherwise deposited on the base surface, and then blended smoothiy into the base surface.
Normally the annular array of bristles will be supported by a side plate lying against that face of the array subject in use to the lower pressure.
The invention will now be particularly described with reference to the accompanying drawings in which:Figure 1 is a sectional view of a seal in accordance with the invention, andFigure 2 is a view on the arrow 2 of Figure 1.
In Figure 1 the seal of the invention extends between an outer static casing member 10 from which the sealing member itself is carried, and an inner shaft 11 adapted to rotate within the casing 1 The space between the shaft and casing with high pressure gas to its left hand portion 1 2 as shown in the drawings, while its right hand portion 13 is vented to a lower pressure area.
Clearly it is necessary to provide a barrierbetween the high pressure and low pressureareas 12 and 13 and to this end a brush seal generally indicated at 14 is provided. This seal comprises an annular array of bristles 1 5 secured between two side plates 1 6 and 1 7 and welded together at 1 8 to form a solid outer layer. It will be noted that the side plate 1 7 covers almost the entire radial depth of the bristles; this is necessary to support the bristles against the pressure exerted by the higher pressure gas in the space12.
The side plate 16, on the other hand, covers a smaller proportion of the bristles so that the bristles are not trapped between the side plates to such an extent as to preclude their movement. As shown in Figure 1 the bristles appear to extend radially; this is only for ease of illustration, and in fact as shown in Figure 2 the bristles, although lying mainly in a plane perpendicular to the seat axis, extend at an angle to the radial so that each bristle in effect lies along part of a spiral. Theinclination of the bristles is arranged to be such as to cause them to extend in the direction of the rotation of the shaft 11.
In order to cause the bristles to engage sealingly with the shaft, a degree of interference is arranged between the inner ends of the bristles and the outer surface 19 of the shaft. Thus the diameter of the annulus described by the ends of the bristles when they are undistorted is arranged to be less than the diameter of the shaft 11. If the difference between these diameters is 'I', there is said to be an amount of interference of I units.
When the seal is assembled on the shaft, the distortion of the bristles will cause them to press against the outer surface 1 9 of the shaft and thus to provide a seal.
The arrangement so far described, which represents the prior art situation, operates very satisfactorily when it is new. However, the various differential expansions and the continuous rub between the bristles and the surface of the shaft 11 eventually wears down the bristles until these clear the surface of the shaft. The seal then becomes leaky to a degree which depends upon the clearance.
In order to improve the life of the seal, in the present invention an elevated portion 20 is formed on that part of the surface 19 with which the bristles engage. The portion 20 is in this case formed as a sprayed-on patch of ceramic materialMetco and it extends fully across that annulus of surface 19 engaged by the bristles 15 but has only a small circumferential extent. The height of the portion 20 above the surface 19 is arranged to 'd' units, the value of d being calculated in accordance with the principles set out below.
In order to understand the effect of the raised portion 20 it is necessary to understand the condition of the seal in operation and its response to transient disturbances. Obviously the response of the individual bristles which make up the seal will depend upon their dimensions. With the common types of seal using bristles made of nickel base superalloy and of a few thousandths of an inch thickness, the individual bristles in their free state would move in response to the disturbance caused by the raised portion quickly enough to follow the surface closely. However, in the brush seal the bristles are not completely free to move, but are pressed one against another by the considerable forces produced by the differential pressure across the seal. Thus in the illustrated embodiment the bristles are forced against the side plate 1 7.
Calculation of the forces applied to the individual bristles in an operative seal show that (a) there is an optimum bristle length, and (b) a bristle of optimum length, when deformed from its rest position, will only resiliently recover a fraction (say one eighth) of the amount of deformation. It is this latter property which enables the present invention to operate.
Thus if we consider a bristle of optimum length or longer, with a degree of interference fit of I units and a raised portion having a height of d units, the total deformation of each bristle as the raised portion passes it will be (I+d). Each bristle will then recover by approximately (lid) 8If we make this recovery equal to d, the bristles will recover until they just touch the surface 1 9 of the shaft 11. If the above relation is correct, this means thatl+d8 orId=7Although it should be emphasized that this is an approximate relationship, it does enable the height of the raised portion to be approximately predetermined to enable a desired running clearance to be approximated. It should be noted 5 that the above analysis strictly only relates to the case of bristles of the optimum length or longer.
It should also be noted that wear of the bristles wil in any case occur to a certain extent. However, the amount of rubbing and wear will decrease as the length of the rub path is considerably reduced.
Use of a ceramic material for the raised portion 20 will reduce the friction between the bristles and the raised portion and will further increase the life of the seal.
It will be noted that although the embodiment described has an inwardly facing annular brush seal, the invention could be used for the outwardly facing versions or for axially facing face seals.