Abstract:
A shaft seal, which is pre-stressed by a spring, has a main body element and a projection, which is adjacent thereto. The projection is designed as a sacrificial element which, when the seal is used correctly, abrades because of the friction with the opposite surface and, in the process, polishes the opposite surface. The radial cross-sectional profile of the projection is selected in so that the sealing-lip supporting surface is not substantially enlarged with increasing abrasion of the projection, thereby a predetermined surface pressure at the sealing-lip supporting surface of the projection is essentially maintained over the service life of the seal.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a shaft seal, arranged between a shaft and a housing, having at least one sealing lip which bears with a sealing-lip supporting surface against the shaft surface moving relative to the seal. A predetermined surface pressure is applied to the sealing lip and shaft surface interface by a pre-stressing element.  
           [0003]    2. Background and Summary of the Invention  
           [0004]    In the prior art, a multitude of sealing arrangements for providing a seal between two components moving relative to each other has been disclosed. The sealing arrangement seals spaces containing different media or pressure conditions-from each other.  
           [0005]    [0005]FIG. 1 shows diagrammatically, by way of example, such a sealing arrangement according to the prior art used in a field of use, which is frequently encountered. In the case illustrated, a gap between a shaft  1  and a housing part  2  is shown, through which shaft  1 , rotating relative to the housing  2 , is guided. These are to be sealed in such a manner that a medium contained in a space  3  cannot pass into a space  4 , and vice versa. Arranged in space  3  is a shaft bearing  5 , for example a grooved ball bearing, which supports shaft  1  in the opening of the housing  2  and is lubricated by a suitable lubricant. In contrast, space  4  is exposed to environmental influences and so spray water and dirt can pass into it, the intention being to prevent them from entering into space  3 .  
           [0006]    To mutually seal spaces  3  and  4 , there is provided in the gap, which is defined by a shaft surface  7  and a surface  8  of the housing  2 , a shaft seal. Shaft seal  10  consists of an elastomeric sealing material and has essentially a U-shaped cross-sectional configuration. A first limb  11  of the shaft seal  10  bears in a sealing manner against surface  8  of housing  2  and is secured thereon—for example, by means of compression. A second limb  12  is in sealing contact with shaft surface  7 . First and second limbs  11  and  12 , respectively, are connected to each other by a base section  13 . Furthermore, an L-shaped stiffening element is provided for reinforcing first limb  11  and base section  13 . In addition, shaft seal  10 , according to FIG. 1, has a protective lip  15  which is arranged at that end of second limb  12  which is adjacent to base section  13 , and comes to bear against shaft surface  7 .  
           [0007]    Second limb  12  bears with its outer end against shaft surface  7  at a predetermined contact pressure force, with the result that shaft surface  7  can rotate relative to shaft seal  10  and can also move in the axial direction. The contact pressure force is determined by the restoring force, which depends on the elastic properties of the seal material and the pre-stressing of second limb  12  with respect to shaft surface  7 , and/or by the tangential force of a helical tension spring  16  which is inserted into shaft seal  10  at the outer end of second limb  12  and presses the outer end of second limb  12  against shaft surface  7  with a predetermined force.  
           [0008]    A variant of shaft seal  10  according to FIG. 1, in which a protective lip  15  is not provided, is illustrated in cross section on an enlarged scale in FIG. 2. In particular, a sealing lip  20 , formed at the outer end of second limb  12 , can be seen in FIG. 2.  
           [0009]    A body element  21  of sealing lip  20  is defined by two side surfaces  22  and  23  tapering toward each other. Body element  21  has an essentially triangular cross section, against which spring  16  presses, as is indicated by an arrow in FIG. 2. At the tip of body element  21 , sealing lip  20  bears with an annular, relatively narrow sealing-lip supporting surface  24  against shaft surface  7 .  
           [0010]    Grinding seals of this type, such as shaft seal  10  described above, provide a reliable seal, particularly if the wear of sealing lip  20  is small on account of the surface quality of the shaft surface  7  and/or on account of the lubrication of the sealing edge or sealing-lip supporting surface  24 . In contrast, increased friction of the seal on opposite surface  7  has a disadvantageous effect on account of the temperature increase associated therewith and the effects of wear caused as a result. The wear leads to a reduced sealing action of shaft seal  10 , which, as will be explained in greater detail with reference to FIGS. 3A to  3 C,  4  and  5 , depends substantially on the surface pressure at sealing-lip supporting surface  24 .  
           [0011]    [0011]FIGS. 3A, 3B and  3 C each illustrate profiles of sealing lips  27 ,  28  and  29 , which are arranged on a sealing body  25 , bearing against an opposite surface  26 . They differ in “sharpness”. These profiles have been used for the measurements illustrated in FIGS. 4 and 5. The profiles of FIGS. 3A, 3B and  3 C differ in each case merely by the different point radii R 1 , R 2  and R 3  of sealing lips  27 ,  28  and  29 , respectively. Sealing lip  27 , according to FIG. 3A, has a point radius R 1 =0.1 mm; sealing lip  28 , according to FIG. 3B, has a point radius of R 2 =0.2 mm; and, sealing lip  29 , according to FIG. 3C, has a point radius of R 3 =0.3 mm. The remaining parameters of the profiles: the rectangular cross-sectional shape of sealing body  25  with a height HO and a length LO, the overall length L 1 , the 30° angle with respect to the axis of symmetry of the side surfaces  22  and  23  of the sealing lips, and the sealing materials are essentially identical for all of the profiles of FIGS. 3A to  3 C.  
           [0012]    [0012]FIG. 4 shows the distributions of the surface pressures in the sealing gap X R1 , X R2  and X R3  for the different point radii R 1 , R 2  and R 3 , respectively. FIG. 5 illustrates the rise in the maximum value of the surface pressure in the sealing gap Y R1 , Y R2  and Y R3  for the point radii R 1 , R 2  and R 3 , respectively, as a function of compression distance.  
           [0013]    The curves according to FIGS. 4 and 5 show a marked dependence of the surface pressure in the sealing gap on the point radius, the maximum value of the surface pressure decreasing with increasing point radius R 1   → R 2   → R 3 . In the case of relatively small point radii, a relatively large surface pressure is obtained. Accordingly, sealing action improves with the geometry otherwise unchanged. Laboratory tests also show that the sealing lips having the smallest point radii have the highest seal tightness. It should also be noted that the variants having the smallest point radii achieve the greatest surface pressures with, at the same time, the smallest reaction forces.  
           [0014]    In summary, it follows from this that seal tightness of a sealing arrangement having a resilient sealing material depends substantially on the surface pressure in the sealing gap, which in turn depends on the “sharpness” of the sealing lip and the contact pressure force.  
           [0015]    The wear of sealing lip  20 , which influences the surface pressure of sealing-lip supporting surface  24 , is therefore critical to the service life of the sealing arrangement or shaft seal  10 . The wear depends on the relative speed of sealing lip  20  with respect its respective shaft surface  7 , on the roughness of shaft surface  7  bearing against sealing lip  20 , and on the wear properties of the sealing material.  
           [0016]    The roughness of shaft surface  7  is reduced over time by sealing lip  20  rubbing against it, since the sealing lip grinds in a running surface on shaft  1 . Even after a short running time, sealing lip  20  produces a finely polished region on shaft surface  7 . Subsequently, sealing lip  20  is subject to a greatly reduced wear or virtually no at all. In the case of structures with small axial relative displacements of shaft surface  7 , this region is very narrow. A structure of this type can be achieved if, for example, shaft seal  10  on shaft  1  is situated directly next to a fixed shaft bearing, such as, for example, a tapered roller bearing.  
           [0017]    In the case of structures in which shaft seal  10  on shaft  1  is far away from a fixed bearing of a fixed/moveable bearing arrangement, a relative displacement between shaft  1  and shaft seal  10  in the axial direction of shaft  1  may occur. On account of different coefficients of thermal expansion of the shaft material and the housing material, axial relative displacements occur in the case of changing external temperatures during warm-up operation. Furthermore, axial relative displacements between shaft  1  and shaft seal  10  are caused by axial forces, which can arise due to intermeshing. These axial forces cause deformation of the housing and of the bearing cover and the bearing plate of a shaft bearing and thus also deformation between shaft seal  10  and a fixed bearing. This has a disadvantageous effect on the sealing arrangement since the deformation leads to axial displacement of the running surface of sealing lip  20  on shaft  1 .  
           [0018]    Such axial displacement of shaft  1  causes the running surface of sealing-lip supporting surface  24  on shaft surface  7  to change, with the result that sealing lip  20  is constantly in frictional contact with various circumferential points of shaft surface  7 . This has the result that, in comparison to a structure without significant axial displacement of shaft  1 , a substantially larger region of shaft surface  7  has to be ground in.  
           [0019]    An axial relative movement between sealing lip  20  and shaft  1  causes increased wear as a function of the magnitude and frequency of the axial displacement. As a consequence of such wear, a completely rounded, worn contour of an originally pointed sealing lip can arise, as illustrated in FIG. 6.  
           [0020]    [0020]FIG. 6 shows, from left to right, various states  1 , II, III and IV of sealing lip  20 .  
           [0021]    State  1  refers to sealing lip  20  when new, with an originally pointed sealing-lip supporting surface  24 . The narrow region on shaft  1  is polished by sealing-lip supporting surface  24  and is indicated by a line in FIG. 6. State II shows sealing lip  20  with a sealing-lip supporting surface  24 ′, said lip having already been worn to a certain extent without there being axial play of shaft The region polished by the sealing lip  20  corresponds essentially to the width of sealing-lip supporting surface  24 ′.  
           [0022]    When there is axial play of the shaft  1 , after a short running time, state III arises in which a sealing supporting surface  24 ″ has formed. The region polished by sealing  20  is indicated on the shaft  1  and is wider than sealing-lip supporting surface  24 ″.  
           [0023]    This region is wider because of the axial play of shaft  1 . This is associated with a relatively pronounced wear or relatively pronounced widening of the originally pointed sealing-lip supporting surface  24  (state  1 ). After a long running time state IV finally arises, in which the wear of sealing lip  20  has produced a greatly widened sealing-lip supporting surface  24 ′″. The region polished by sealing lip  20  is wider than sealing-lip supporting surface  24 ′″.  
           [0024]    The seal tightness of the sealing arrangement is disadvantageously affected in going from I → II → III → IV in two respects: firstly, there is enlargement of sealing supporting surface  24   →   24 ″ →   24 ′″. Secondly, the material thickness of sealing lip  20  is reduced perpendicular with respect to the sealing-lip supporting surface (indicated in FIG. 6 by means of a dashed line parallel to shaft surface  7 ), as a result of which the contact pressure force produced by the elasticity of the sealing-lip material and by the pre-stressing of spring  16  is reduced. The consequence of this is a pronounced loss of surface pressure and therefore of seal tightness.  
           [0025]    The wear also depends on supplying the contact point with lubricant. For this purpose, the opposite running surface or shaft surface  7  is generally greased in advance. However, during installation of shaft seal  10  or of shaft  1  the grease is wiped away, with only a small amount of grease remaining, for example, in the chamber between protective lip  15  and sealing lip  20  in FIG. 1. Furthermore, the lubricating grease is increasingly pushed to the outside from the region of the contact surface or running surface of shaft seal  10  by a constant axial movement.  
           [0026]    In the prior art, it is known to polish opposite surface  7 , against which sealing lip  20  bears, to reduce the surface roughness to reduce wear of sealing lip  20 . The sealing lip then no longer has to ensure a corresponding grinding-in process. However, a surface treatment of this type is subject to process fluctuations and requires additional machining time and increased costs.  
           [0027]    Another known approach is avoidance of combined rotational and linear movement in the sealing region. For example, DE 198 39 485 A1 discloses a sealing arrangement in which a sealing lip bears against a sleeve rotating together with the shaft. In the event of axial movement, the seal is carried along via a bearing to the shaft, with the result that the running surface of the sealing lips on the sleeve does not change. However, a structure of this type is very complex and requires the shaft bearing to be connected to the seal. Thus, the actual seal cannot be used independently of the shaft bearing.  
           [0028]    DE 198 31 523 A1 discloses a further possibility of avoiding at least small axial movements of an opposite surface relative to a sealing lip. An axial projection, which has the sealing lip, on a sealing body, is pre-stressed in the axial direction by means of a compression spring which is arranged between the sealing body and a stop situated opposite the latter. The axial compression spring is intended to absorb high-frequency, small-amplitude oscillations of the opposite surface in the axial direction without relative movement between the sealing lip and the opposite surface. However, the costs of the seal are significantly increased by the additional, axial compression spring.  
           [0029]    It is also known to treat the surface of the sealing lip to reduce friction as disclosed, for example, in DE 199 49 205 A1, in which treatment of a moveably arranged sealing-lip surface with halogens is described.  
           [0030]    Furthermore, it is known in the prior art to coat the sealing lip with a friction-reducing material. DE 198 39 502 A2 shows, for example, a sealing lip coated with PTFE.  
           [0031]    However, reducing the friction of the sealing lip, according to the prior art, requires coating of or a hardening treatment of the sealing-lip material. Both processes increase production costs of the seal. Moreover, the coating may become detached from the base material of the sealing lip. Furthermore, a hardening treatment is subject to process-induced fluctuations in quality.  
           [0032]    Drawbacks of prior approaches are overcome by a shaft seal with at least one sealing lip which bears with a sealing-lip supporting surface against an opposite surface moving relative to the seal at a predetermined surface pressure applied by a pre-stressing element. The sealing lip has a main body element and at least one projection, which is supported by and adjacent to the main body element. The projection bears against the opposite surface and is a sacrificial element, which abrades due to friction with the opposite surface. In the process, the projection polishes the opposite surface. The radial cross-sectional profile of the projection is such that the sealing-lip supporting surface is not substantially enlarged by abrasion thereby maintaining a predetermined surface pressure at the sealing-lip surface of the projection over a service life of the seal.  
           [0033]    An advantage of the present invention is that because the sealing-lip supporting surface does substantially enlarge, the surface pressure does not diminish. Thus, over the life of the seal, seal tightness is maintained.  
           [0034]    Yet another advantage of a seal, according to the present invention, is that relatively small demands can be placed on the opposite surface moving relative to the seal or on the running surface of the seal. Because of the shape of the projection, some axial movement can be tolerated.  
           [0035]    Another advantage of the present invention is that it is inexpensive to produce. The opposite surface need not be polished to such a high degree as prior systems. The application of a lubricating material is also not critical to the seal.  
           [0036]    The invention makes provision for the projection to be a sacrificial element, which abrades due to friction with the opposite surface. In the process, the projection polishes the opposite surface. In this case, the radial cross-sectional profile of the projection is selected in such a manner that the sealing-lip supporting surface is not substantially enlarged with increasing abrasion of the projection. Thus, a predetermined surface pressure at the sealing-lip supporting surface of the projection is essentially maintained over the service life of the seal. This means that the projection is of rather more pointed design in comparison with the main body element. In contrast, the main body element is of such a design—is preferably wider that the projection is supported and thus protected against lateral movement.  
           [0037]    An essential concept of the present invention therefore resides in recognizing that abrasion of the seal occurs and ensuring that the cross-sectional geometry due to abrasion causes a smaller enlargement of the sealing-lip supporting surface than is the case in prior art seals. As a result, the surface pressure remains essentially the same—apart from a slight decrease due to the abrasion-induced change in the seal radius, with the result—as explained above—that reliable sealing is ensured with long-term stability.  
           [0038]    Preferably, the radial extent of the projection is selected so that abrasion of the projection over the service life of the seal essentially stops due decrease in friction on account of the increasingly more finely polished opposite surface. The precise dimension of the radial extent of the projection also depends, inter alia, on how much axial displacement between the seal and opposite surface are to be reckoned with and how long it takes until a sufficiently fine polish is achieved over the entire region of axial displacement. The longer the projection, the wider it should be to provide sufficient stability with respect to lateral movement. The radial extent of the main body element is preferably greater than the radial extent of the projection. This ensures that the main body element, with its relatively large volume, ensures the necessary stiffness of the sealing lip, thereby preventing lateral movement of the sealing lip.  
           [0039]    The width profile of the seal can preferably be selected in such a manner that—if the seal is not yet abraded—the width in the region of the projection increases at increasing distance from the opposite surface to a smaller extent than the width in the region of the main body element increases at increasing distance. As a result, in this case the projection is of more pointed design than the main body element.  
           [0040]    Alternatively, the projection can also have an essentially constant width or can even taper at increasing distance. It is particularly advantageous if the width of the projection decreases at increasing distance from the sealing-lip supporting surface. As a result, a loss in surface pressure due to a loss in material because of the decrease in the supporting surface of the sealing lip can is compensated In addition, the opposite surface is ground smooth at the beginning to a relatively wide track, and so, in consequence, there is less wear of the sealing lip following the abrasion.  
           [0041]    In an advantageous refinement, the main body element can have an essentially triangular cross-sectional configuration with two main side surfaces tapering toward the projection. In the case of such a construction of the sealing lip, the projection can simply be integrally formed on the main body element.  
           [0042]    According to a further aspect, the sealing lip consists of an essentially elastic or resilient sealing material. Furthermore, the main body element and the projection are preferably designed as a single piece. Since the main body element has substantially more volume than the projection, the elasticity of the main body element essentially comes into play during the contact pressure force, and the contact pressure force decreases only negligibly during wear of the projection. Furthermore, such a construction of a sealing lip can be realized in an extremely simple manner for various sealing arrangements without additional components, coatings, or the like being required.  
           [0043]    According to a further advantageous refinement of the invention, at least one of a grinding material is provided in the direction parallel to the sealing-lip supporting surface adjacent to at least the projection. A grinding material of this type assists or accelerates the grinding-in process, with the result that the opposite surface is polished more rapidly, thereby substantially stopping the abrasion of the sealing lip.  
           [0044]    Furthermore, according to one embodiment, at least one layer of a supporting material can be provided in the direction parallel to the sealing-lip supporting surface adjacent to at least the projection, said supporting material preferably being softer than the sealing material, at least in the region of the projection. This prevents the sealing lip from moving laterally in the region of the projection. By virtue of the fact that the supporting material is softer, and therefore more elastic than the sealing material of the projection, the surface pressure is concentrated on the sealing-lip supporting region of the projection, with the result that the surface pressure is increased or maintained.  
           [0045]    According to a further preferred refinement, the projection has at least one initial sealing lip with the sealing-lip supporting surface bearing against the opposite surface, and at least one additional sealing lip with an additional sealing-lip supporting surface spaced apart from the opposite surface. In this case, the opposite surface is first ground-in by the initial sealing lip. The additional sealing lip then comes into play after appropriate wear of the initial sealing lip. The sealing action of the sealing lip is improved on account of a smaller sealing-lip supporting surface of the additional sealing lip in comparison to the worn away initial sealing lip. To reinforce this effect, a plurality of additional sealing lips and/or a plurality of initial sealing lips may be provided.  
           [0046]    In the case of a sealing arrangement having a sealing lip according to the invention, the sealing action can be maintained in spite of wear. In addition, the quality requirements which have to be met by the processing of the opposite surface to reduce the friction of the sealing lip on the opposite surface can be reduced, since, according to the invention, the sealing lip itself grinds in and polishes its running surface on account of a predetermined amount of initial wear. A seal according to the invention is therefore more robust with respect to tolerances during the processing or treatment of the opposite surface. Finally, the extended durability of the seal means that maintenance costs can be reduced.  
           [0047]    The invention will be explained in greater detail below by way of example with reference to the figures, in which the same reference numbers are used in the figures for identical or essentially identical elements. In the figures:  
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0048]    The advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Detailed Description, with reference to the drawings wherein:  
         [0049]    [0049]FIG. 1 is a partial cutaway view of an example of the use of a shaft seal according to the prior art;  
         [0050]    [0050]FIG. 2 is an enlarged sectional view of the shaft seal FIG. 1, according to prior art;  
         [0051]    [0051]FIGS. 3A, 3B and  3 C show profiles of sealing lips with different point radii;  
         [0052]    [0052]FIG. 4 is a graph representing the distribution of the surface pressures in the sealing gap for the profiles according to FIGS. 3A, 3B and  3 C;  
         [0053]    [0053]FIG. 5 is a graph representing the relationship of the surface pressure with increasing compression of the profiles according to FIGS. 3A, 3B and  3 C;  
         [0054]    [0054]FIG. 6 is a sectional illustration of a sealing lip according to the prior art which bears on a shaft, in various states of wear; and  
         [0055]    [0055]FIGS. 7 through 13 illustrate sectional views of exemplary embodiments of sealing lips according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0056]    [0056]FIG. 7 shows a sealing lip  40 A, according to the present invention, of a first exemplary embodiment of a seal  10 A. Sealing lip  40 A is, like the sealing lip  20  of FIG. 2, pressed against the opposite surface or shaft surface  7  of a shaft  1 , which rotates relative to the seal, by a helical tension spring  16 . Other pre-stressing elements may also be used within the scope of the present invention. Alternatively, pre-stressing can be obtained merely by the elasticity of the rubber material with a spring  16  or the like being omitted.  
         [0057]    Sealing lip  40   a  has a main body element  41   a,  which has an essentially triangular cross-sectional, and has two main side surfaces  42 A and  43 A tapering toward each other at an obtuse angle. Spring  16  produces a contact pressure force bearing against the base side of main body element  41   a  between main side surfaces  42   a  and  43   a.    
         [0058]    Opposite spring  16 , a projection  45  is coupled to main body element  41  a.  
         [0059]    Projection  45  bears with a sealing-lip supporting surface  46  against opposite surface Projection  45  and main body element  41  a are formed as a single piece from a resilient sealing material. In addition to the contact pressure force produced by spring  16 , part of the contact pressure force arises from the elastic deformation of the sealing-lip material, which is produced by compressing or deflecting the sealing-lip material.  
         [0060]    Projection  45  can be formed together with main body element  41  a in one working step by known seal-production processes. Alternatively, projection  45  can be formed on sealing lip  40   a  by removal of sealing-lip material.  
         [0061]    The width of projection  45 , as measured in the cross section of the sealing lip  40   a  parallel to the sealing-lip supporting surface  46 , is constant. The width does not increase or decrease with increasing vertical distance from the sealing-lip supporting surface  46 ; whereas, the sealing lip  40   a  rapidly widens with increasing distance in the region of main body element  41   a.    
         [0062]    The vertical extent or height of the web-shaped projection  45  in relation to its width is selected so that projection  45  cannot move laterally due to the contact pressure force arising by means of spring  16  and the elasticity of the sealing-lip material.  
         [0063]    Over the service life of seal  10   a,  sealing-lip supporting surface  46  does not substantially widen due to the wear occurring on opposite surface  7  due to abrasion. Only a slight decrease in the surface pressure, on account of the loss of the sealing-lip material in the region of projection  45 , and of a reduced contact pressure force associated therewith, is to be expected.  
         [0064]    Abrasion and, therefore, wear of sealing lip  40   a  is reduced with increased grinding in of the seal on opposite surface  7  and polishing of opposite surface  7  by means of sealing lip  40   a.  Projection  45 , or a part of it, is sacrificed to polish the running surface of sealing lip  40   a  on opposite surface  7  for the purpose of reducing friction and wear, particularly when there is axial play of shaft  1 . However, since sealing-lip supporting surface  46  remains essentially constant during this initial abrasion and the contact pressure force decreases only slightly owing to the small loss of material in the region of projection  45 , sealing lip  40   a  essentially retains its surface pressure and therefore its sealing action. The durability of seal  10   a  is therefore increased as a whole.  
         [0065]    In the case of a further exemplary embodiment of a seal  10   b,  shown in FIG. 8, a sealing lip  40   b,  according to the invention, has a main body element  41   b  which primarily forms the volume of sealing lip  40   b  and has essentially a triangular cross section with two main side surfaces  42   b  and  43   b  tapering toward each other at an obtuse angle.  
         [0066]    Opposite spring  16 , which produces the contact pressure force, sealing lip  40   b  has a projection  55 , which is adjacent to the main body element  41   b.  Projection  55  is of wedge-shaped design and widens as a function of distance from main body element  41   b.    
         [0067]    During abrasion of sealing lip  40   b,  sealing-lip supporting surface  56  is therefore reduced, with the result that a reduction of the surface pressure, which reduction is based on the loss of material, can be compensated for by sealing lip  40   b,  which becomes sharper due to abrasion. Sealing-lip supporting surface  56 , which is wider at the initially, causes the running surface of sealing lip  40   b  on opposite side  7  to be ground in, particularly when there is axial play of shaft  1 , over a wider track than required by the sealing lip which has been made narrower due to the abrasion in the region of the projection.  
         [0068]    [0068]FIG. 9 illustrates a cross section a further exemplary embodiment of a seal  10   c  with a sealing lip  40   c,  according to the invention. As in the case of the preceding exemplary embodiments, sealing lip  40   c  has a main body element  41   c  which is defined essentially by two main side surfaces  42   c  and  43   c  tapering toward each other and has a generally triangular cross-sectional configuration and a projection  65  which is coupled to the point of main body element  41   c.  Projection  65  bears with a sealing-lip supporting surface  66  against opposite surface  7  of shaft  1 .  
         [0069]    Wedge-shaped projection  65  has a first side surface  67  which constitutes a continuous extension of the first main side surface  42   c  of main body element  41   c.  A second side surface  68  of projection  65 , which surface is adjacent to the second main side surface  43   c,  is curved concavely inward, with the result that the width of projection  65 , measured parallel to sealing-lip supporting surface  66  increases with increasing distance from sealing-lip supporting surface  66 . However, this increase in width of projection  65  turns out to be substantially smaller than the increase in the width of main body element  41   c  defined by main side surfaces  42   c  and  43   c.    
         [0070]    Sealing-lip supporting surface  66  of seal  10   c  is enlarged slowly during abrasion of sealing lip  40   c.  The polishing or grinding in of sealing lip  40   c  can therefore also be achieved in this exemplary embodiment with a small structural outlay.  
         [0071]    Referring now to FIG. 10, a projection  75  of a sealing lip  40   d  of a seal  10   d,  according to an aspect of the present invention, widens at a distance from a sealing-lip supporting surface  76  to a smaller extent than a main body element  41   d  of sealing lip  40   d,  which element is defined by two main side surfaces  42   d  and  43   d.    
         [0072]    Projection  75 , which is of wedge-shaped design, is adjoined to main body element  41   d  of sealing lip  40   d.  A first side surface  77  of projection  75  forms the extension of main side surface  42   d.  A second side surface  78  of projection  75  is adjacent to second main side surface  43   d  at an angle. The relative angle between side surfaces  77  and  78  of projection  75  is more acute than the relative angle of main side surfaces  42   d  and  43   d.  Sealing-lip supporting surface  76  is enlarged slowly with abrasion of projection  57  in comparison to abrasion in the region of main body element  41   d.  The result is that the sealing effect of sealing lip  40   d  is retained over the service life.  
         [0073]    [0073]FIG. 11 illustrates an exemplary embodiment of a seal  10   e,  according to the invention, having a sealing lip  40   e,  in which embodiment two main side surfaces  42   e  and  43   e  define a main body element  41   e  and extend continuously from two initial sealing lips  83  and  84  of a projection  85  by means of two outer side surfaces  81  and  82 .  
         [0074]    The two initial sealing lips  83  and  84  bear against opposite surface  7  and together form an initial sealing-lip supporting surface  86 . Between the two initial sealing lips  83  and  84 , projection  85  has an additional sealing lip  87  with a narrow or sharp sealing-lip supporting surface  88 . This additional sealing-lip supporting surface  88  is spaced apart from sealing-lip supporting surface  86 .  
         [0075]    Initially, only initial sealing lips  83  and  84  bear with sealing-lip supporting surface  86  on opposite surface  7  and are exposed there to the abrasion. The entire width of initial sealing lips  83  and  84  increases with increasing distance from initial sealing-lip supporting surface  86  to a smaller extent than the width of main body element  41   e.  When the two outer initial sealing lips  83  and  84  have ground shaft surface  7  smooth and have been worn, additional sealing lip  87  arranged between, them comes into use. The sharp-edged nature of sealing-lip supporting surface  88  of additional sealing lip  87  provides a higher surface pressure than at initial sealing lips  83  and  84 . Since opposite surface  7  is ground smooth by outer initial sealing lips  83  and  84 , the sharp-edged nature of additional sealing lip  87 . Therefore, seal tightness of seal  10   e  is retained.  
         [0076]    Yet, a further embodiment, illustrated in FIG. 12, of a seal  10   f,  according to the invention, having a sealing lip  40   f  has a main body element  41   f,  the main side surfaces  42   f  and  43   f  of which are extended continuously into two outer side surfaces  91  and  92  from an initial sealing lip  93  and an additional sealing lip  94  of a projection  95 .  
         [0077]    Initial sealing lip  93  has a wedge-shaped cross-sectional configuration similar to projection  75  of the sealing lip  40   d  of FIG. 10 and bears with an initial sealing-lip supporting surface  96  against opposite surface  7 . Additional sealing lip  94  tapers in a pointed manner and has a sealing-lip supporting surface  97 , which is spaced apart in the vertical direction from the initial sealing-lip supporting surface  96 .  
         [0078]    Additional sealing lip  94  does not initially come into contact with opposite surface  7 . Only when projecting initial sealing lip  93  has been subjected to appropriate wear on opposite surface  7 , by abrasion, does additional sealing lip  94  come to rest with its sharp sealing-lip supporting surface  97  against opposite surface  7  polished by initial sealing lip  93 . The sharp-edged nature of additional sealing lip  94  ensures increased surface pressure. Because of the widely polished running surface on shaft surface  7 , this sharp-edge provides a durable seal tightness of seal  10   e.    
         [0079]    A sealing lip  40   g,  which is illustrated in FIG. 13, of a seal  10   g,  according to the invention, is formed with a main body part  41  g having two main side surfaces  42   g  and  43   g  and a web-shaped projection  105  consisting of resilient sealing material. Furthermore, sealing lip  40   g  has two layers  101  and  102  consisting of a grinding material, which layers are arranged adjacent to main side surfaces  42   g  and  43   g  and butt against projection  105 . Layers  101  and  102  rest with supporting surfaces  103  and  104  against opposite surface  7  parallel with and adjacent to a sealing-lip supporting surface  106  of projection  105 .  
         [0080]    The grinding material of layers  101  and  102  is particularly suitable for grinding opposite surface  7  smooth and therefore reduces the wear of sealing lip  40   g  by polishing of the running surface on opposite surface  7 , particularly when there is axial play of shaft  1 . The durability of seal  10   g  can therefore be significantly increased.  
         [0081]    Of course, other sealing-lip forms may also be considered within the scope of the invention such as, for example, a different number of initial and additional sealing lips and a changed arrangement thereof as long as a maximum overall width of the projection essentially retains a required surface pressure and therefore seal tightness of the seal, and the sealing lip has sufficient stiffness, so that it does not move laterally.  
         [0082]    Lateral movement can be avoided by at least one layer of a supporting material provided adjacent to the sealing-lip projection. The supporting material is preferably softer than the sealing-lip material and supports the sealing lip in the region of the projection against swinging laterally. Alternatively, both supporting material layers and grinding material layers can be provided.  
         [0083]    As with the shape of the projection, various configurations can also be considered for the shape of the main body element such as, for example, a rectangular cross section of sufficient width or a rounded cross-sectional shape. In particular, the shape of the main body element has to be suitable for obtaining sufficient rigidity of the sealing lip and for ensuring a sufficient contact pressure force.  
         [0084]    Although exemplary embodiments illustrate shaft seals, the invention may be used for different seals in which an axial and/or rotary relative movement between a sealing lip or sealing-lip supporting surface and an opposite surface occurs.  
         [0085]    While several examples for carrying out the invention have been described, those familiar with the art to which this invention relates will recognize alternative designs and embodiments for practicing the invention. Thus, the above-described embodiments are intended to be illustrative of the invention, which may be modified within the scope of the following claims.