Abstract:
A slide ring seal for sealing a rotatable shaft includes a non-rotating slide ring formed as a cylinder having a variable inner diameter that defines a profile forming a cross-section of the slide ring and corresponding to a matching profile of the shaft for engagement therewith.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a slide ring seal for sealing a rotatable shaft and including a non-rotating slide ring. The present invention also relates to a non-rotating method of mounting of the slide ring of a slide ring seal on the rotatable shaft.  
         [0003]     2. Description of the Prior Art  
         [0004]     In many cases, slide ring seals must be used for sealing rotatable shafts. E.g., at high pressure differences across the seal, when only a small leakage must be insured, at high environmental temperatures and high circumferential speeds, it is necessary to use slide ring seals for sealing the rotatable shafts. According to the state of the art, the basic structure of such seals consists of two annular or ring-shaped parts of which one is mounted on the rotatable shaft and the other one is mounted in a housing section that surrounds the shaft. The end surfaces of the two parts, which are made as smooth as possible, are close-fitted with each other, with a so-called sealing clearance formed therebetween. As narrow as possible sealing clearance provides for a best sealing effect. In practice, the width of a sealing clearance amounts to several microns. In principle, the difference between radial and axial seals is defined by the orientation of the sealing clearance. In axial seals, the sealing clearance extends in a radial direction, and in radial seals, the sealing clearance extends in an axial direction. The sealing effect depends more on the clearance width than the clearance length. In axial seals, the clearance width is more easily controlled. In particular, in axial seals, a more narrow clearance can be achieved than in radial seals. Therefore, the axial slide ring seals are used more widely (see Miiller, Nau, Handbook of Sealing Technology).  
         [0005]     In order to insure a sealing effect an axial force should press the stationary and rotatable slide rings against each other. German Patent DE-PS 37 40 694 discloses a slide ring seal in which the axial force is generated by a spring.  
         [0006]     In order to retain the slide rings on the shaft and in the housing and, simultaneously, to obtain a good sealing, other components are necessary. As a retaining component an elastomeric component, which is expensive to produce, can be used. An elastomeric component having a special shape in order to be able to withstand the required medium and high pressure differences is disclosed in German Patent DE-PS 41 15 155.  
         [0007]     An important fact is the selection of a material of the slide ring. When selecting a material of a slide ring, tribological considerations have to be considered. In addition the temperature stability and the surface quality are important considerations.  
         [0008]     German Publication DE-05 100 56 102 discloses adaptation of a slide ring to special condition by selection of an appropriate material (in this case, of silicon carbide and carbon-silicon carbide composition).  
         [0009]     Slide rings of the state of the art include a large number of separate components which should be combined with each other to form a seal. This requires large volumes as the total ratio of a sealing surface and the required volume is unfavorable. The excess volume complicates mounting of the slide rings in apparatuses and machines where often a compact mass with a maximum sealing effect is required. The large number of parts makes their manufacture and mounting time-consuming and expensive.  
         [0010]     Because of the friction between the slide rings and insignificant cooling, a large amount of heat is generated. The heat should be removed in a controlled manner. In Japanese Publication JP-1120486, this problem is solved by selection of the material of the slide ring. However, this solution is not always applicable. When the above-discussed slide ring seals are used in dynamical or positive-displacement machines for treating reactive gases, some components of the seal can have a small resistance against the reactive gas loads, and the adaptation of the seals becomes difficult.  
         [0011]     It is known to cut a slide ring with a saw in three or more ring segments and then suitable ring segments are joined together, e.g., by using a spring tension ring or placing the segments in a suitable metal mounting. When a ring is cut with saw, some amount of ring material is removed. Therefore, it is not any more possible to form a full circle of ring segments which are formed by cutting a slide ring blank. Therefore, to produce a slide ring, ring segments of several blanks are used or, alternatively, the segments are subjected to an appropriate treatment. In both cases, the manufacturing costs increase.  
         [0012]     Accordingly, an object of the invention is to provide a slide ring seal as compact as possible and which, at the same time, would insure the best possible sealing effect.  
       SUMMARY OF THE INVENTION  
       [0013]     This and other objects of the present invention, which will become apparent hereinafter, are achieved according to the invention by providing a non-rotating slide ring formed as a cylinder having a variable inner diameter that defines a profile forming a cross-section of the slide ring and corresponding to a matching profile of the shaft for engagement therewith, and by providing a method of mounting the slide ring on the shaft and which includes breaking the slide ring at least in two parts, mounting the at least two parts about the shaft, and securing the at least two parts from separation from each other.  
         [0014]     A slide ring seal according to the present invention insures a very high tightness with as few components as possible and with using a minimal volume. The construction of the seal is noticeably simplified in comparison with those of the state of the art, which noticeably reduces manufacturing, mounting and maintenance costs. The materials, which are used for forming the slide rings, permit the use of the inventive slide ring seal in a harmful environment, e.g., in dynamic or positive displacement machines for delivery of reactive gases. For selection of the materials of the slide rings, in addition to consideration of tribological characteristics, a requirement that the used materials had as small plastic deformation as possible during breaking also should be taken into consideration. In addition, with a slide ring seal according to the present invention, the influence of the thermal effect, which is produced during the operation of a machine the inventive slide ring seal is used in, in particular, the influence of the material expansion is reduced to a minimum. The seal has a very small harmful volume which permits to use the seal in machine in which the harmful volume is critical, e.g., in vacuum pumps. In vacuum pumps, in particular, the shock pressure resistance plays an increased role. The advantage of the inventive slide ring seal also consists in that it can well withstand shock pressures. The inventive slide seal ring has an increased service life because the load is distributed over several surfaces. Therefore, the pressure of the surfaces against each other which is generated upon application of axial forces is reduced, whereby the abrasion is also reduced. The axial forces can be obtained in different ways, e.g., by using a spring, pressure difference between sealed from each other chambers, or an elastomeric ring. By proper selection of the number of sealing surfaces, operational and sealing characteristics can be optimized.  
         [0015]     According to the method of the present invention, the parts after being mounted on the shaft, are pressed against each other with suitable retaining means, and the shaft, together with the slide ring mounted thereon, is mounted in the machine housing. Because breaking is not accompanied by removal of material from break surfaces, the separate ring segments can be again joined together, without any additional treatment. The surfaces are not smooth but are rather irregular. Therefore, the slide ring, which is formed of broken parts is those tight than a ring formed sawed parts. The inventive method insures easy mounting and dismounting of seal and does not produce any refuse as all of the broken ring segments are used. Thereby, the costs of the rings is reduced. Simultaneously, the replacement of a defective seal is noticeably simplified, which also reduces the costs.  
         [0016]     The combination of the inventive slide ring and the inventive method permits mounting of the seal at arbitrary locations, which makes forming of, e.g., undercuts possible.  
         [0017]     The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The drawings show:  
         [0019]      FIG. 1 a  perspective view of a slide ring seal according to the present invention with a variable inner diameter and which is mounted between a rotatable shaft and a housing;  
         [0020]      FIG. 2 a  cross-sectional view illustrating mounting of the slide ring seal according to the present invention in a vacuum pump;  
         [0021]      FIG. 3 a  cross-sectional view illustrating different profiles of an inner contour of a slide ring according to the present invention;  
         [0022]      FIG. 4 a  cross-sectional view illustrating a slide ring seal according to the present invention and formed of several segments;  
         [0023]      FIG. 5 a  perspective view of a slide ring seal according to the present invention illustrating the structure of the sealing surfaces; and  
         [0024]      FIG. 6 a  perspective view illustrating mounting of seal ring according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]     A slide ring seal according to the present invention, which is shown in  FIG. 1 , includes a slide ring  105  that is arranged between a rotatable shaft  101  and a housing  103 . The sliding ring  105  is held in place by an elastomeric ring  107 . The slide ring seal seals chambers  111  and  113  from each other. A spring  109  applies to the slide ring  105  a force directed rightwardly in  FIG. 1 . The slide ring  105  has a variable inner diameter that defines a profile  115 . The rotatable part, the shaft  101 , has a matching profile  116 , with the slide ring profile  115  and the shaft profile  116  engaging with each other. This is achieved, e.g., with a shaft element  117  engaging in the slide ring  105 , as shown in  FIG. 1 . While in  FIG. 1 , the shaft element  117  is shown as an integral part of the shaft  101 , it can constitute a component of a separate part mounted on the shaft  101 . Axial forces that act on the slide ring  105 , press the slide ring  105  against radial surfaces  119 , with a clearance being formed therebetween. A surface, a surface normal of which extends parallel to the shaft axis, is designated as a radial surface.  
         [0026]     The radial surfaces are formed of a material which is selected based on tribological considerations. In the embodiment considered here, this material is steel. The slide ring  105  is formed of carbon, ceramics, or other breakable material. Because the shaft  101  has the shaft element  117  projecting into the slide ring  105 , the ring  105  cannot be pushed onto the shaft  101 . Therefore, the slide ring  105 , in accordance with the inventive method, is broken into several ring pieces. These pieces are then arranged about the shaft  101 . Thereafter, the elastomeric ring  107  is pushed over the mounted ring pieces, e.g., two, holding them together. Dependent on the requirements, the ring pieces can be glued with each other at the breaking surfaces with a suitable glue, which further increases the sealing effect. The rubbed-off material, which is formed on the inner surfaces of the slide ring during functioning of the seal, accumulates in chamber  121 .  
         [0027]     Frictional forces, which act between the slide ring  105  and radial surfaces  119  generate heat that causes increase in temperature and a resulting longitudinal expansion of the element  117  of the shaft  101 . The longitudinal expansions of the shaft  101  and the slide ring  105 , which is caused by existing thermal condition, because of the difference in materials the shaft  101  and the slide ring  105  are made of, are not the same. As a result, the axial clearance between the shaft element  117  and the slide ring profile  115  is reduced. The thermal expansion should not lead to the reduction of the sealing clearance or to a run-on of the parts at both sides. Therefore, the thermal expansion should be taken into consideration. Accordingly, the width y of the shaft element  117  and the size x of the recess of the profile  115  are so selected that the difference therebetween is larger than the longitudinal expansion at a maximal temperature that is expected during operation.  
         [0028]     The distance of the radial surfaces  119  from each other can be maintained during the manufacturing process only to a limited extent. However, this does not present a problem for the slide ring according to the present invention because during a short, in comparison with their service life, response time, self-optimization takes place. At the start of the operation, a sealing clearance is formed only on one of the surfaces. At this surface, in this phase of the operation, abrasion is increased because of high forces acting on the slide ring. Therefore, an excessive amount of material is removed. In a short while, the slide ring becomes adapted to the shape of the radial surfaces  119  and to the distance therebetween.  
         [0029]      FIG. 2  shows the use of the inventive slide ring seal in a vacuum pump, e.g., in a two-shaft positive displacement pump. The shaft  201  of the pump supports a rotary piston  221  which is arranged in the housing  203 . In the pump, the compression/expansion chamber  211  should be sealed from the chamber  213  in which the drive is located. A symmetrical slide ring  205  is arranged between the shaft  201  and the housing  203 . The slide ring  205  is held in the housing  203  with an elastomeric ring  207 . The slide ring  205  has a variable inner diameter that defines the profile  215 . The axial force can bias the slide ring  205  against the surfaces  219   a  or  219   b , dependent on the direction in which the force acts. In the embodiment, shown in  FIG. 2 , this force is produced by a pressure difference of pressures prevailing in the drive chamber  213  and the compression/expansion chamber  211 , with vacuum prevailing in the compression/expansion chamber  211  and with the drive chamber  213  being under pressure which is slightly below the atmospheric pressure. When the pressures in the chambers  211  and  213  are reversed, the seal still functions adequately because of its symmetricity. With a reversed pressure ratio, the force acts, in the plane of the drawing, rightwardly, with the sealing effect being applied to the surfaces  219   b . Furthermore, the symmetrical mounting of the seal ring  205  is facilitated by the fact that no predetermined orientation should be observed.  
         [0030]     The sealing effect of the slide ring according to the present invention is noticeably improved in comparison with the slide ring seals of the state of the art because more sealing surfaces act simultaneously in a compact space. Therefore, the pressure drop across separate surfaces is respectively smaller than in case of a single sealing surface. The formation of the profile insures a labyrinth-like sealing.  
         [0031]     Dependent on the application, subjecting the seal to the action of a seal gas may be desirable. To this end, there is provided a bore  225  in the housing  203  and a bore  227  in the slide ring  205  itself. Alternatively, it is possible to form the slide ring of a porous material, so that the seal gas can penetrate through the pores of the slide ring. Still further, it is possible to use two slide rings according to the present invention arranged axially one after another and axially spaced from each other, with the seal gas being introduced into the gap between the two slide rings.  
         [0032]     For manufacturing of slide rings, particularly for use in vacuum pumps, an electrographitized artificial carbon is used. The use of this material provides for adaptation to high environmental temperatures. The maximal compatible environmental temperature then would depend only on the material of the static seal  207 .  
         [0033]     The profiles ( 115 ,  215 ) are not limited to those described above. Other possible profiles are shown in  FIG. 3 . E.g., a saw-tooth-shaped profile shown in  FIG. 3   a  also can be used. It is also possible to form the recesses  320  with different depths or width, as shown in  FIG. 3   b . It is further possible to form the recesses without increase in radius ( FIG. 3   c ). Rather, the radius of the slide ring  330  is reduced at locations  331 .  
         [0034]     Advantageously, the profile includes radial surfaces  333  ( FIG. 3   d ), i.e., surfaces the normals  335  of which extend parallel to the shaft axis  337 .  
         [0035]     An inventive effect is also achieved with the profile shown in  FIG. 3   d  and formed as a step-shaped profile. With the profile of  FIG. 3   d , separate steps act as separate seal surfaces.  
         [0036]     The profile, which is defined by the inner diameter of the slide ring, can also so be formed that the sealing effect is achieved with the axial force acting in both of opposite axial directions. Also, a saw tooth-shaped profile  341 , which is shown in  FIG. 3   e  and which engages a saw-tooth matching profile  343 , is one of possible embodiments implementing the inventive idea. When the saw-tooth shape is selected, the thermal expansion of the components should be taken into account, and care should be taken to provide a corresponding free space  345 .  
         [0037]     A slide ring with a variable inner diameter according to the present invention can be also formed as shown in  FIG. 4 . In the embodiment shown in  FIG. 4 , the slide ring  405  is formed of several, preferably but not necessarily identical, segments  407  which are oriented relative to each other by axial projections  409 . Sections of a ring  413 , which is pushed over a shaft  401 , engage in recesses  411 . An elastomeric ring  415  seals the ring  405  against a housing  403 . The sections of the ring  413  are sealed against the shaft  401  with seal rings  417  such as, e.g., elastomeric rings. The axial forces in such a slide ring can be generated by the elastomeric ring, a spring, or by a pressure difference of the chambers which are to-be-sealed from each other. This embodiment likewise provides a compact structure with a high sealing effect.  
         [0038]     A further advantageous embodiment of a slide ring seal according to the present invention is shown in  FIG. 5 . In the embodiment shown in  FIG. 5 , sealing surfaces  519  of a slide ring seal  505 , which extend transverse to the rotational axis, are provided with flutes  507 . The flutes  507  increase the sealing clearance and thereby reduce the wear. The flutes or grooves can also be provided in seals discussed above. Particularly advantageously, the flutes or grooves can be provided on symmetrical rings on both sides of the seal, whereby they provide for self-centering of the seal. They also provide for a high circumferential speed.  
         [0039]      FIG. 6  illustrates a method of mounting of a slide ring  605  according to the present invention.  FIG. 6   a  shows a slide ring  605  before mounting it on a shaft. On an end surface of the ring  605 , predetermined breaking points can be provided. To this end, at predetermined locations, the end surface is slightly slit or sawed. In a further step, the slide ring  605  is purposely broken, and two halves  610  and  612  are produced, as shown in  FIG. 6   b . Then, the two ring halves  610  and  612  are mounted on shaft  601 , as shown in  FIG. 6   c , with the break surfaces abutting each other. Finally, the two halves  610  and  612  are secured against separation, e.g., by an elastomeric ring  607 , as shown in  FIG. 6   d . Advantageously, the break surfaces  630  of the two ring halves  610  and  612  are glued with a suitable glue, whereby an additional sealing effect is achieved.  
         [0040]     Though the present invention was shown and described with references to the preferred embodiments, such are merely illustrative of the present invention and are not to be construed as a limitation thereof, and various modifications of the present invention will be apparent to those skilled in the art. It is, therefore, not intended that the present invention be limited to the disclosed embodiments or details thereof, and the present invention includes all variations and/or alternative embodiments within the spirit and scope of the present invention as defined by the appended claims.