Abstract:
The present invention provides a dynamic seal for enclosing a radial gap between coaxial, relatively rotatable inner and outer members. The dynamic seal includes an inner seal part, mountable to the inner member; an outer seal part, mountable to the outer member; and a counterface part radially suspended between the inner and outer seal parts and coaxial therewith. Each of the inner and outer seal parts provides a set of first and second sealing elements, which bear against oppositely oriented axial surfaces of the counterface part. The counterface part is thus axially retained in both directions by the inner seal part at a radially inner contact location, and is axially retained in both directions by the outer seal part at a radially outer contact location.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a seal for enclosing a radial gap between relatively rotatable coaxial components. More particularly, the invention is directed to such a seal which is able to compensate for positional variations while maintaining sealing contact. 
       BACKGROUND TO THE INVENTION 
       [0002]    A common application for a radial seal is to seal the radial gap between a shaft and e.g. a bearing housing. The seal typically comprises an annular casing that is mounted in a bore of the bearing housing, and further comprises an elastomeric seal lip that is bonded to the metal casing. The seal lip bears against the shaft and makes sliding contact with a shaft counterface during rotation. Especially at high speeds, the shaft may rotate with a slight eccentricity, known as dynamic run-out, which in effect changes the radial gap between the counterface and the housing. To ensure that the seal lip remains in contact with the counterface, the seal is generally mounted with a very small clearance and the lip is spring-loaded. The radial load on the seal lip generates additional friction and the seal typically experiences uneven wear around the lip circumference. 
         [0003]    One solution for removing the radial load on a seal is to execute the seal with radially overlapping parts and axially oriented seal contact surfaces. An example of a seal of this kind is disclosed in U.S. Pat. No. 4,348,031. The document describes a rotary fluid seal having a sealing ring arranged between axially opposed counterfaces of relatively rotatable members. The sealing ring has a pair of divergent seal lips which bear against the counterfaces, but are able to float freely between the relatively rotatable members. Upon relative rotation of the members, the sealing ring is frictionally driven into rotation by one or other of the members. 
         [0004]    There is still room for improvement. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The present invention resides in a dynamic seal for enclosing a radial gap between coaxial, relatively rotatable inner and outer members. The dynamic seal comprises an inner seal part, mountable to the inner member; an outer seal part, mountable to the outer member; and a counterface part radially suspended between the inner and outer seal parts and coaxial therewith. Each of the inner and outer seal parts comprises a set of first and second sealing elements which bear against oppositely oriented axial surfaces of the counterface part. The counterface part is thus axially retained in both directions by the inner seal part at a radially inner contact location, and is axially retained in both directions by the outer seal part at a radially outer contact location. 
         [0006]    The counterface part is not connected to either of the inner seal part or the outer seal part, and is thus a floating part, free of radial load. Relative radial movements between the inner and outer seal parts can therefore be accommodated in a manner that does not increase friction and wear. 
         [0007]    Further, the axial retention of the counterface part by each of the inner and outer seal parts means that the seal has two potential dynamic contact locations. Let us assume that the outer member and the outer seal part are rotational during dynamic sealing conditions. If the friction at the radially outer contact location is greater than the friction at the radially inner contact location, the counterface part will rotate with the outer seal part and the sealing elements of the inner seal part will be in sliding contact with the counterface part. If the friction at the radially inner contact location becomes greater than at the radially outer contact location, then the counterface part will “stick” to the inner seal part and the rotational sealing elements of the outer seal part will be in sliding contact with the counterface part. 
         [0008]    A dynamic seal according to the invention is thus able to adopt a second dynamic contact location before the friction at a first dynamic contact location becomes excessively high. Wear is thus prevented, which prolongs the life of the seal. 
         [0009]    In a first embodiment, at least one of the first and second sealing elements of the inner and outer seal parts are axially enclosed between oppositely oriented axial surfaces of the counterface part. In one example, the counterface part comprises a ring with an outer U-shaped section at the radially outer circumference and an inner U-shaped section at the radially inner circumference. The outer seal part then comprises a flange that extends into the outer U-shaped section of the counterface part. The first and second sealing elements may be executed as lips that bear against the inner axial surfaces of the outer U-shaped section. Likewise, the inner seal part comprises a flange that extends into the inner U-section of the counterface part and which has first and second seal lips that bear against the inner axial surfaces of the inner U-section. 
         [0010]    In a second embodiment, the counterface part is axially enclosed by the first and second sealing elements of at least one of the inner and outer seal parts. In a preferred example, the counterface part is formed by a flat ring. A radially outer edge of the flat ring extends into a cavity of the outer seal part, which is enclosed by the first and second sealing elements of the outer seal part. Likewise, a radially inner edge of the flat ring extends into a cavity of the inner seal part, which is enclosed by the first and second sealing elements of the inner seal part. Advantageously, each cavity is filled with a lubricant such as grease, to lubricate whichever set of sealing elements are in sliding contact with the axial surfaces of the flat ring. 
         [0011]    The counterface part may be made of a metal, ceramic or polymer material. The axial surfaces of the counterface part which are in contact with the sealing elements may advantageously be provided with a low-friction coating, to minimise seal wear. In some examples, the counterface part comprises a breathable material that prevents moisture from entering the sealed environment, but permits moisture to escape. 
         [0012]    The sealing elements of the inner and outer seal parts are preferably made from an elastomeric material such as Acryl-Nitrile-Butadiene Rubber, Ethylene Propylene Rubber, Fluorocarbon Rubber or Silicone Rubber. Suitably, each sealing element comprises a lip section having an axially oriented surface that is at least partly in contact with the counterface part. The sealing element may further comprise an arm section that is bonded to e.g. a casing element. In some examples, the lip section is made of a first elastomeric material and the arm section is made of a second elastomeric material, which has a lower elastic modulus that the first elastomeric material. The arm section may thus provide the sealing element with flexibility, while the lip section has better wear resistance. 
         [0013]    In a further development of the second embodiment, the dynamic seal is adapted to permit an axial displacement of the counterface part relative to the inner and outer members, in a manner which maintains effective sealing between the counterface part and each set of first and second sealing elements. 
         [0014]    Suitably, each of the first and second sealing elements of the inner seal part comprises an inner lip section that is contact with an axial surface of the counterface part and further comprises an arm section with a flexure that resiliently urges the lip section against the corresponding axial surface. Likewise, each of the first and second sealing elements of the outer seal part comprises an outer lip section that is contact with an axial surface of the counterface part and further comprises an arm section with a flexure that resiliently urges the lip section against the corresponding axial surface. The flexure in each arm section is further adapted to serve as a hinge point about which each arm section rotates in response to an axial displacement of the counterface part relative to the inner and outer members. As a result, the lip section of each sealing element of the inner and outer seal parts remains parallel to the counterface part, to maintain an effective sealing contact. 
         [0015]    The axial deflection may occur as the result of a pressure build up within e.g. a sealed bearing. A seal according to the further development of the invention may thus be used to compensate for pressure differentials. 
         [0016]    The seal may be adapted to accommodate an axial deflection of 0.1-0.8 mm, depending on the expected pressure differential. Suitably, when an axial deflection towards one axial side of the seal is to be accommodated, the sealing elements of the inner and outer seal at that axial side have an appropriate stiffness that permits the deflection. 
         [0017]    Preferably, a seal according to the invention has radial and axial symmetry. 
         [0018]    Advantageously, the lip section of each sealing element may comprise a plurality of micro-lips formed by protrusions that extend from an axial surface of the lip section. The micro-lips have a small contact area in comparison with the axial surface as whole, to minimize contact friction. 
         [0019]    In one example of a dynamic seal according to the further development of the invention, the lip section of at least one sealing element comprises at least one first micro-lip that is in contact with the counterface part when this part is in a central (axially undeflected) position. The lip section further comprises at least one second micro-lip that is arranged with an axial gap to the counterface part. The axial gap is less than or equal to a maximum axial deflection that the seal is designed to accommodate. Thus, when the aforementioned axial deflection takes place, the at least one second micro-lip comes into contact with the counterface part, to enhance the sealing effect. 
         [0020]    A seal according to the invention has further advantages, which will become apparent from the following detailed description and accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0021]    In the following, the invention is described with reference to the accompanying drawings, in which: 
           [0022]      FIG. 1  shows a radial cross-section through a first example of a seal according to the invention; 
           [0023]      FIG. 2  shows a radial cross-section through a second example of a seal according to the invention; 
           [0024]      FIG. 3  shows a radial cross-section through a third example of a seal according to the invention. 
           [0025]      FIG. 4  shows a radial cross-section through a fourth example of a seal according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    Radial seals are employed to enclose a radial gap between coaxial, relatively rotatable components, such as a bearing housing and a shaft. The seal serves to retain lubricant within the bearing and to exclude external contaminants, such as moisture and dirt. The seal typically has one or more seal lips which, in use of the seal, are in sliding contact with a counterface. To ensure effective sealing, the lip(s) need to remain in contact with the counterface at all times. Preferably, contact is ensured in a manner that minimises friction and wear. 
         [0027]    An example of a radial seal according to the invention is depicted in  FIG. 1 . 
         [0028]    The seal  100  comprises an inner seal part  110  and an outer seal part  120  which are respectively mounted to a shaft  10  and a housing  20  in the depicted example. The inner seal part  110  comprises first  111  and second  112  axially spaced, sealing elements which extend in a radially outward direction from an inner casing  113  that is mounted on the shaft  10 . Likewise, the outer seal part  120  comprises first  121  and second  122  axially spaced sealing elements which extend in a radially inward direction from an outer casing  123  mounted to the housing  20 . In this example, the sealing elements are made of an elastomeric material such as NBR. 
         [0029]    The seal  100  further comprises an annular counterface part  130  that is arranged in a radial gap between the inner and outer seal parts  110 ,  120  and that is axially arranged between the first and second sealing elements  111 ,  112  of the inner seal part and between the first and second sealing elements  121 ,  122  of the outer seal part  120 . The counterface part  130  may be an essentially flat ring made of a metallic or polymeric or ceramic material and has oppositely oriented first and second axial surfaces  131 ,  132  which serve as counterfaces of the seal  100 . 
         [0030]    In this example, the first and second sealing elements  111 ,  112  of the inner seal part  110  have an inner arm section  114  that extends in axial direction towards the counterface part  130 . Likewise, the first and second sealing elements  121 ,  122  of the outer seal part  120  have an outer arm section  124  that extends in axial direction towards the counterface part  130 . The inner and outer arm sections  114 ,  124  provide the respective sealing elements with stiffness. Further, the inner first and second sealing elements  111 ,  112  have an inner lip section  115  that extends in a radial direction and bears against the first and second axial surfaces  131 ,  132  of the counterface part  130  respectively. Likewise, the outer first and second sealing elements  121 ,  122  have an outer lip section  125  that extends in a radial direction and bears against the first and second axial surfaces  131 ,  132  of the counterface part  130  respectively. 
         [0031]    Thus, a radially inner portion of the counterface part  130  is axially retained between the inner lip sections  115  and a radially outer portion of the counterface part is axially retained between the outer lip sections  125 . 
         [0032]    The counterface part  130  is axially retained by the inner and outer seal parts  110 ,  120 , but is not fixed to either. Therefore, friction dictates whether the inner lip sections  115  or the outer lip sections  125  are in sliding contact with the first and second axial surfaces  131 ,  132  of the counterface part. Let us assume that the shaft  10  is the rotational component. If a first amount of friction between the outer lip sections  125  and the counterfaces  131 ,  132  is greater than a second amount of friction between the rotating inner lip sections  115  and the counterfaces, then the rotating inner lip sections  115  will be in sliding contact. However, if the first amount of friction increases due to e.g. breakdown of a lubricating film, and becomes higher than the second amount, then the counterface part  130  will rotate with the inner seal part  110 , and the outer lip sections  125  will be in sliding contact with the counterfaces  131 ,  132 . 
         [0033]    Thus, the seal  100  automatically “selects” the sliding contact interface with the lowest friction, thereby minimising wear and extending seal life. 
         [0034]    Preferably, the axial surface of the inner and outer lip sections  115 ,  125  that bears against the counterface part  130  has a number of micro-lips  116  formed by protrusions separated by recesses  117 . Peripheral edges of the micro-lips  116  have a surface area that is significantly less than that of the aforementioned axial surface of the lip sections, meaning that low friction is generated during dynamic sealing conditions. In addition, the recesses  117  between the micro-lips  116  can serve as a reservoir for lubricant, to further reduce friction in the sliding contacts. 
         [0035]    As may be seen from  FIG. 1 , the inner part  110  of the seal has a cavity  119  enclosed by the first and second sealing elements  111 ,  112  and a radially outer surface of the seal casing  113 . Similarly, the outer part  120  of the seal has a cavity  129  enclosed by the outer first and second sealing elements  121 ,  122  and a radially inner surface of the outer casing  123 . Advantageously, the cavities  119 ,  129  are provided with a grease for lubricating the sliding contacts between the lip sections  115 ,  125  of the seal and the first and second axial surfaces  131 ,  132  of the counterface part. 
         [0036]    As may further be seen, the counterface part  130  extends into the cavity  119  of the inner seal part  110 , with a radial gap to the radially outer surface of the inner casing  113 , and extends into the cavity  129  of the outer seal part  120 , with a radial gap to the radially inner surface of the outer casing  123 . Thus, the counterface part  130  is suspended between the inner  115  and outer lip sections  125  of the inner and outer seal parts respectively. As a result, there is no radial load on the seal, which is therefore able to accommodate variations in the radial gap between the shaft  10  and housing  20 , due to e.g. dynamic run-out, without causing uneven wear of the inner or outer seal lips. 
         [0037]    In many radial seal applications, the seal must be able to withstand a pressure differential. Especially at high speeds, high pressure can build up within a bearing, which has an effect on the seal contact(s). In a further development of the invention, the seal is adapted to allow for an axial displacement of the counterface part, while maintaining effective sealing contact. 
         [0038]    An example of a seal according to the invention with this capability is depicted in  FIG. 2 . 
         [0039]    Again, the seal comprises a counterface part  130  which is axially retained and radially suspended between first and second sealing elements  211 ,  212  of an inner seal part  210  and first and second sealing elements  221 ,  222  of an outer seal part  220 . In this example, both of the inner and outer seal parts  210 ,  220  are formed from two separate pieces which are connected together. This facilitates assembly of the seal  200 , given that the counterface part  130  radially overlaps both the inner and outer seal parts. The inner seal part has first and second casing elements  213   a,    213   b  and the outer seal part  220  has first and second casing elements  223   a,    223   b,  which are joined together in axial direction after the counterface part has been arranged between the sealing elements  211 ,  212 ,  221 ,  222 . 
         [0040]    Each sealing element is at least partly made from a resilient material such as rubber. The first and second sealing elements  211 ,  212  of the inner seal part  210  have an arm section  214  and a lip section  215 . Each inner arm section  214  extends from the respective inner casing part  213   a,    213   b  in a radially outward direction and comprises a bend or flexure  218 . The lip section  215  extends from the flexure  218  in radially inward direction and in an axial direction towards the counterface part  230 . In other words, the lip section  215  of the inner first and second sealing elements  211 ,  212  is angled relative to the first and second axial surfaces of the counterface part  130 . Suitably, the lip sections  215  are angled so as to exert an axial force on the counterface part, due to the resiliency of each sealing element. Further, the lip section  215  comprises three micro-lips  216   a ,  216   b,    216   c  in this example, with recesses  217  in between, whereby peripheral edges of the micro-lips are in contact with first and second axial surfaces  231 ,  232  of the counterface part  230 . 
         [0041]    The first and second sealing elements  221 ,  222  of the outer seal part  220  have a corresponding arm section  224 , flexure  228  and lip section  225 . 
         [0042]    Let us assume that due to high pressure within the seal  200 , an axial force F acts on the counterface part  230  in the direction indicated by the arrow. The seal is adapted to permit a small amount of axial displacement of the counterface part  230  relative to the inner and outer members. Depending on the application, the permitted axial displacement can be between 0.1 and 0.8 mm. 
         [0043]    The counterface part  130  presses against the lip section  215 ,  225  of the first sealing elements  211 ,  221  of the inner and outer seal parts respectively. The flexure  218  in each first sealing element acts as a hinge point about which the respective lip section  215 ,  225  rotates, to enable the axial displacement of the counterface part  230 . Further, the flexure  218  in the second sealing element  212 ,  222  of the inner and outer seal parts respectively acts as a hinge point about which the respective inner and outer lip sections  215 ,  225  rotate, due to the resiliency of the sealing elements  212 ,  222 . As a result, the micro-lips  216   a,    216   b ,  216   c  of the lip section  215 ,  225  of each second sealing element  212 ,  222  remain in contact with the second axial surface  232 . In other words, the inner and outer lip sections  215 ,  225  remain parallel with the axial surfaces  131 ,  132  of the counterface part  130 . 
         [0044]    In addition, the stiffness of each lip section  215 ,  216  is adapted to permit axial displacement. The stiffness depends on the thickness of the lip section. Taking the lip section  215  of the first sealing element  211  of the inner seal part  210  as an example, the radially outermost micro-lip  216   a  has a greater axial width than the intermediate micro-lip  216   b,  which in turn has a greater axial width than the innermost micro-lip  216   c.  Thus, the main body of the lip section  215  has an essentially constant thickness, and a corresponding stiffness which permits axial displacement. 
         [0045]    An advantage of a such a seal according to the invention is that pressure compensation is possible. A further example of a radial seal that permits pressure compensation within e.g. a sealed bearing is depicted in  FIG. 3 . 
         [0046]    Again, the seal comprises a counterface part  130  which is axially retained and radially suspended between first and second sealing elements  311 ,  312  of an inner seal part  310  and first and second sealing elements  321 ,  322  of an outer seal part  320 . Each sealing element is at least partly made from a resilient material such as rubber. The first and second sealing elements  311 ,  312  of the inner seal part  310  have a flexible arm section  314  and a lip section  315  that bears against first and second axial counterfaces  131 ,  132  of the counterface part. The flexible arm section  314  has a V-shaped flexure  318 , which acts as a spring to urge the lip section  315  against the respective counterface. The first and second sealing elements  321 ,  322  of the inner seal part  320  also have a flexible arm section  324  and a lip section  325  that bears against the first and second axial counterfaces  131 ,  132 . Again, the flexible arm sections  324  have a V-shaped flexure  318 , which acts as a spring to urge the outer lip sections  325  against the respective counterface. As explained above, the flexure  318  in each sealing element  311 ,  312 ,  321 ,  322  also acts as a hinge point, to enable the lip sections  315 ,  325  to remain parallel to the counterface part when an axial displacement of the counterface part  130  occurs 
         [0047]    Further, each lip section  315 ,  325  comprises a number of first micro-lips  316   a  (four in the depicted example) which are in contact with the respective counterfaces  331 ,  332 , when the counterface part  230  is in an undeflected position. Each lip section  315 ,  325  also has a number of second micro-lips  316   b  (two in the depicted example) with an axial gap to the counterface part  330 . The width of the gap is less than or equal to the maximum axial displacement for which the seal is designed. The lip sections are designed such that when a pressure acting on the counterface part  130  causes it to move in the direction of e.g. the first sealing elements  311 ,  321  of the inner and outer seal parts, the corresponding lip sections  315 ,  325  bend, so that the second micro-lips  316   b  also come into contact with the counterface part  130 . The sealing effect is thus enhanced at that axial side of the seal, given that six micro-lips are now in contact with the counterface part. 
         [0048]    In the embodiments of a seal according to the invention depicted thus far, the counterface part has axially outer surfaces that are retained between the first and second sealing elements of the inner and outer seal parts. An alternative embodiment is depicted in  FIG. 4 , in which the counterface part is retained at axially inner surfaces thereof, which enclose the first and second sealing elements. 
         [0049]    Again, the seal  400  has an inner seal part  410  mounted to a shaft  10  and an outer seal part  420  mounted to a housing  20 . The counterface part  430  is radially suspended between the inner and outer seal parts and in this example has a first U-shaped annular cavity  433   a  at its inner circumference and has a second U-shaped annular cavity  433   b  at its outer circumference. The inner seal part  410  has a radially extending flange  414  that extends into the first cavity  433   a,  with a radial gap to the circumferential surface of the first cavity. Likewise, the outer seal part  420  has a radially extending flange  424  that extends into the second cavity  433   b,  with a radial gap to the circumferential surface of the second cavity. Further, the flange  414  of the inner seal part has first and second seal lips  411 ,  412  that respectively bear against first and second inner axial surfaces  431   a,    432   a  of the first cavity  433   a.  Likewise, the flange  424  of the outer seal part has first and second seal lips  421 ,  422  that respectively bear against first and second inner axial surfaces  431   b,    432   b  of the second cavity  433   b.  As before, the counterface part  430  is free of radial load and the seal  400  can accommodate dynamic run-out without increased friction and wear. 
         [0050]    In this embodiment, the counterface part  420  is suitably formed from two separate sections which are joined together axially after the inner and outer seal parts  410 ,  420  have been arranged against one of the separate sections. 
         [0051]    A number of aspects/embodiments of the invention have been described. It is to be understood that each aspect/embodiment may be combined with any other aspect/embodiment. The invention may thus be varied within the scope of the accompanying patent claims.