Abstract:
A face seal device for use with components rotating at a high rotational speed includes a non-rotating face seal ring ( 5 ) and a rotary face seal ring ( 11 ) mounted in a loose fit on and rotatable together with the rotary component. The rotary face seal ring includes a sealing face ( 6 ) which, in operation, is essentially radially aligned for cooperation with an opposite sealing face ( 16 ) of the non-rotating face seal ring. The rotary face seal ring is formed of at least a base portion ( 20 ) and an extension portion ( 21 ). The extension portion provides an additional mass and has an areal center of gravity axially spaced from the areal center of gravity of the base portion An axial spacing z between the areal centers of gravity and the mass m of the extension portion are in specific relationship with each other, whereby in operation a torque directed towards the non-rotating face seal ring is applied to the rotary face seal ring. This torque arises from the centrifugal forces that are a function of the mass of the extension portion and the spacing between the areal centers of gravity.

Description:
[0001]    The following disclosure is based on German Utility Model Application No. 200 19 881.5, filed on Nov. 23, 2000, which is incorporated into this application by reference.  
         FIELD OF AND BACKGROUND OF THE INVENTION  
         [0002]    The invention relates to a face seal device for use with components rotating at a high rotational speed.  
           [0003]    The invention relates, in particular, to a face seal device for use with turbine engines e.g. in aircraft construction. In applications of this type, it is not permissible to use pairings of ceramic material for the face seal rings although these would be preferable, basically, because of their high wear-resistance properties. Consequently, it is pairings of a steel material that receive prime consideration for the rotary face seal ring, whilst the static face seal ring may consist of a carbon material. At the high rotational speeds involved in such types of applications, and despite the absence of direct contact, the face seal rings may experience a heating effect due to the friction between the cooperating face seal rings and the gaseous or liquid medium in the sealing clearance (molecular friction) to such an extent as to lead to thermally induced distortions or deformations, especially in the rotating face seal ring, whence the geometry of the sealing clearance could be adversely affected. In particular, the sealing clearance may open in the manner of a wedge (formation of a so-called V-gap) thereby resulting in considerably higher leakages.  
         OBJECTS OF THE INVENTION  
         [0004]    One object of the invention is to provide an improved face seal device suitable for high rotational speeds while having minimized leakage. Another object of the invention is to provide a face seal device having a rotary face seal ring which, while it can be made of any suitable material including materials permissible in aircraft construction, avoids detrimental effects in regard to the geometry of the sealing clearance caused by thermally induced distortions of the rotary face seal ring at high rotational speeds.  
         SUMMARY OF THE INVENTION  
         [0005]    These and other objects are solved by a face seal device according to the present invention for use with components rotating at a high rotational speed. According to one formulation of the invention, such a face seal device includes a non-rotating face seal ring and a rotary face seal ring, wherein the rotary face seal ring is formed for mounting in a loose fit on the rotary component for common rotation therewith. The rotary face seal ring includes a sealing face which, in operation, is essentially radially aligned for cooperation with an opposite sealing face of the non-rotating face seal ring. The rotary face seal ring is formed from of least two sub-portions including a base portion and an extension portion. The extension portion creates an additional mass and has an areal center of gravity axially spaced from the areal center of gravity of the base portion such that the axial spacing z between the areal centers of gravity and the mass of the extension portion satisfy the following equation:  
         m   *   z     =     k   *     10     -   2       *   Δ                 T   *         (     D   -   d     )     *     b   2         r   *     n   2         *   q             k   =     α   *   E                           
 
           [0006]    where and where q is in a range between 0.5 and 2.0, preferably between 0.7 and 1.5, and most preferably between 0.8 and 1.3. For the meaning of the symbols used in this equation, reference is made to the following description of a preferred embodiment of the invention.  
           [0007]    The at least two sub-portions, which together form the rotary face sealing ring are preferably formed as a single piece or, at least, are rigidly connected with each other. The axial spacing between the center of gravity of the extension portion and that of the base portion is arranged to be such that the rotary face seal ring will be subjected to a specifically oriented torque in correspondence with the spacing between the centers of gravity and the centrifugal forces effective on the extension portion. A thermally induced deformation of the rotary face seal ring is thereby effectively countered and a compensating effect for a change in the geometry of the sealing clearance caused by such a deformation is obtained, so that the desired geometry of the sealing clearance will be maintained even at high rotating speeds even if the rotary face seal ring consists of a steel material which is subjected to incomparably greater thermal distortion than ceramic material due to the different thermal conductivity and co-efficient of thermal expansion of these materials. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The invention as well as embodiments and advantages thereof are described below in greater detail, by way of example, with reference to the drawings in which:  
         [0009]    [0009]FIG. 1 is a longitudinal sectional view of a face seal device according to a preferred embodiment of the invention, installed in an equipment requiring sealing, and;  
         [0010]    [0010]FIG. 2 is a fragmentary sectional view of the rotary face seal ring of the face seal device of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0011]    The invention will be explained in more detail hereinafter with the aid of a preferred embodiment as shown in FIGS. 1 and 2. A seal casing of a face seal device according to the invention bears the reference  1  and can be mounted in a boring  4  of a housing  2  (depicted only in a fragmentary manner in the drawing) of an equipment e.g. a turbine engine for an aircraft. A shaft  3 , e.g. the shaft of a turbine wheel, extends through the boring  4  to the exterior. A rotationally stationary face seal ring  5  is mounted axially moveably in the seal casing  1  for cooperation with a rotary face seal ring  11 . Provision is further made for a biasing element, e.g. in the form of a biasing spring  7  or a plurality of peripherally distributed biasing springs which are supported at one end on the stationary face seal ring  5  and on the seal casing  1  at the other end thereof, for producing a bias force for urging the stationary face seal ring  5  against the rotary face seal ring  11 . The stationary face seal ring  5  is held such that it cannot rotate relative to the seal casing  1 . Although other mechanisms could be provided, the present embodiment uses a dog-pin  9  which projects axially from the seal casing  1  and engages in an axial recess  8  in the stationary face seal ring  5  such as to prevent a rotation thereof.  
         [0012]    A sleeve  10  is suitably mounted on the shaft  3  such that it cannot rotate relative thereto. The rotary face seal ring  11  is mounted on the sleeve  10  in a loose-fit and is caused to rotate therewith by means of a drive-pin  13  projecting axially from a flange-like end portion  12  of the sleeve  10  and engaging in a boring  14  which is formed in a stepped end face  15  of the rotary face seal ring  11  facing the flange-like end portion  12 . The dimensions of the drive-pin  13  and the boring  14  are such that, although they will enable torque to be transferred between the sleeve  10  and the rotary face seal ring  11 , there will be sufficient free-play between the parts as to allow the rotary face seal ring  11  to be mounted in a loose-fit on the sleeve  10 .  
         [0013]    As is shown, the flange-like end portion  12  of the sleeve  10  overlaps only a small radial portion of the rotary face seal ring  11  near the outer end  15  thereof so that the rear part of the rotary face seal ring  11  is held on the sleeve  10  in such a way that it is essentially free of axial support.  
         [0014]    The mutually facing, inner end faces of the face seal rings  5 ,  11  form essentially radially aligned sealing faces  6 ,  16  between which, in operation, a sealing clearance is produced, which separates the sealing faces  6 ,  16  so that they do not contact each other. By contrast, the sealing faces  6 ,  16  are urged together in sealing engagement by the bias force of the spring  7  when the shaft  3  is stationary so as to prevent a medium from leaking along the region between the sealing faces  6 ,  16  when the sealing device is in a stationary state. If so desired, pumping structures may be formed in one of the sealing faces  6 ,  16  so as to pump a medium e.g. the medium being sealed, between the sealing faces and thereby assist in the formation of a sealing clearance. Pumping structures of this type are known to a person skilled in the art and thus do not need to be described here in detail.  
         [0015]    As illustrated in particular in FIG. 2, the rotary face seal ring  11  includes a base portion  20  and an extension portion  21 . The base portion  20  preferably has an essentially rectangular cross-section. The radial and axial dimensions of the base portion  20  are determined by an external diameter D, an internal diameter d and an axial width b. The inner peripheral surface  17  of the rotary face seal ring  11  is mounted in a loose fit on an outer radially projecting peripheral surface portion  18  of the sleeve  10 . Provision is made for a secondary seal e.g. in the form of an O-ring  19 , which is inserted in an annular groove in the outer peripheral surface portion  18  of the sleeve  10  so as to produce a sealing connection between the peripheral surfaces  17 ,  18  of the rotary face seal ring  11  and the sleeve  10 .  
         [0016]    Preferably, the extension portion  21  may, as illustrated, likewise have a rectangular cross-sectional configuration, but, if so desired, it may be designed to have any other arbitrary cross-sectional shape. The extension portion  21  preferably is an integral part on a side of the face seal ring  11  remote from the sealing face  16 . Accordingly, the face seal ring  11  is preferably made in one-piece form. However, the extension portion  21  could also be a separate part which is mounted to the base portion  20  in a suitable manner e.g. by welding or by adhesion, and it could consist of the same or of a different steel material than that of the base portion  20 . The external diameter of the extension portion  21  may correspond to that of the base portion  20  but it could also differ therefrom.  
         [0017]    The virtual axial boundary between the base portion  20  and the extension portion  21  is indicated by a dashed line  22  in FIG. 2 of the drawings. The areal center of gravity of the base portion  20  is referenced S whilst that of the extension portion  21  is referenced s. The radial spacing between the areal center of gravity s of the extension portion  21  and the central longitudinal axis of the face seal device is referenced r whilst the mutual axial spacing between the areal centers of gravity S and s is referenced z.  
         [0018]    The extension portion  21  provides an additional mass m for producing centrifugal forces during the operation of the face seal device, acting in a radial plane through the areal center of gravity s. Due to the axial spacing z between the areal center of gravity s and the areal center of gravity S of the base portion  20 , a torque is produced by the centrifugal forces, which is effective in a direction such that the rotary face seal ring  11  will be rotated or tilted against the stationary face seal ring  5 . Moreover, this action is not hindered, given the loose-fit mounting of the rotary face seal ring  11  relative to the sleeve  10 .  
         [0019]    Experiments have shown that the mass m of the extension portion  21  and the spacing z of the centers of gravity, required for essentially compensating any tilting of the rotary face seal ring  11  with the resultant undesired formation of a V-gap, can be determined to a near approximation by the following equation:  
         m   *   z     =     k   *     10     -   2       *   Δ                 T   *         (     D   -   d     )     *     b   2         r   *     n   2         *   q             k   =     α   *   E                           
 
         [0020]    In the equation:  
                                           ΔT   =   the temperature difference between the sealing face 16 and the               end face 15 of the face seal ring 11 [K],       n   =   the rotational speed [I/s],       D   =   the external diameter of the base portion 20 [m],       d   =   the internal diameter of the base portion 20 [m],       b   =   the width of the base portion 20 [m],       E   =   the modulus of elasticity of the material of the face seal ring               [N/m 2 ],       α   =   the co-efficient of thermal expansion of the material of the face               seal ring [I/K],       r   =   the radial spacing of the areal centre of gravity s of the               extension portion 21 [m],       z   =   the mutual axial spacing between the areal centres of gravity S               and s [m]       m   =   the mass of the extension portion 21 [kg].               The factor q should be in a range between 0.5 and 2.0,               preferably between 0.7 and 1.5, and most preferably between 0.8               and 1.3.                  
 
         [0021]    The manner in which a face seal device of the above-mentioned type functions is as follows.  
         [0022]    Due to the frictional heat (molecular friction) occurring in the sealing clearance between the sealing faces  6 ,  16  at high rotational speeds of the shaft  3 , e.g. at rotational speeds of 10,000 min −1  and more, despite lack of contact, the rotary face seal ring  11  experiences a heating and shows a temperature gradient from the sealing face  16  to the opposite outer end face  15 . The temperature gradient causes the rotary face seal ring  11  to experience a thermally induced distortion or deformation. The distortion is particularly marked if the rotary face seal ring  11  consists of a steel material having a comparatively low heat conductivity λ and a high co-efficient of thermal expansion α. In a steel material, the ratio of α/λ is large in comparison to that of ceramic materials such as SiC or WC. Consequently, if there is a similar development of heat, a rotary face seal ring  11  consisting of a steel material will experience greater distortions than a rotary face seal ring consisting of a ceramic material. The deformation of the rotary face seal ring  11  produced at high rotational speeds due to the thermally induced distortions manifests itself in the form of a corresponding alteration in the configuration of the sealing clearance, in that the sealing clearance is opened at an angle to form a wedge-shaped gap. The consequences of a sealing clearance being altered in this manner would be increased leakage. In accordance with the invention, this is countered by utilizing the centrifugal forces to which the extension portion  21  is subjected at high rotational speeds so as to apply a radial force to the rotary face seal ring  11 . This radial force exerts a torque through a lever arm z, thereby causing the face seal ring  11  to experience a rotation which is effective in the sense of a reduction in the angle of the sealing clearance so that the opening of the sealing clearance produced by the thermally induced distortion can be compensated. This compensating torque becomes larger, the higher the rotational speed and, thus, the greater the thermal distortion. By appropriate selection of the mass m of the extension portion  21  and the axial spacing z between the centers of gravity in correspondence with the equation mentioned hereinabove, the compensating effect produced by the torque resulting from the centrifugal force can be optimized and predetermined, within a permissible range of rotational speeds, for each face seal device, so that a full, or a virtually full compensation effect can be achieved for the thermally induced deviations from a desired configuration of the sealing clearance.  
         [0023]    Although the invention has been described hereinabove on the basis of a special design for use in connection with the construction of turbines used in aircraft, in which field there are certain reasons for ceramic materials not being permissible for the face seal rings so that a steel material has to be used therefor, it should be understood that the invention is not restricted to face seal rings made of such a material. Rather, the invention can likewise be used, to advantage, for face seal devices utilizing face seal rings of other materials e.g. ceramic materials such as SiC or sintered materials such as WC.  
         [0024]    The above description of the preferred embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures disclosed. It is sought, therefore, to cover all such changes and modifications as fall within the spirit and scope of the invention, as defined by the appended claims, and equivalents thereof.