Abstract:
The present invention provides a rotary seal including a first part having an axial sealing lip and a second part having an axial counterface against which the axial sealing lip bears. According to the invention, a rotational one of the first part and the second part is provided with a reservoir for retaining a volume of grease at a location radially inward of the axial sealing lip. The reservoir is designed such that the movement of grease under the action of centrifugal force is prevented, while the movement of base oil, which bleeds from the volume of grease retained in the reservoir, is allowed. As a result, the supply of base oil to the sealing contact takes place for a longer period of time, which extends the life of the seal.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a rotary seal having at least one axial sealing lip that bears against an axially oriented counterface. More specifically, the invention relates to a grease-lubricated seal of this kind, whereby the seal is adapted for improved lubrication of a contact interface between the axial sealing lip and the counterface. 
       BACKGROUND 
       [0002]    An example of a rotary seal with an axial sealing lip is known from US 2010/0066030. This document describes a sealing device for a wheel bearing arrangement that is adapted for rotation of the bearing inner ring. The device comprises an elastomeric sealing element, mounted to a stationary part of the bearing arrangement, and a metal slinger, mounted to a rotational part of the bearing arrangement. The elastomeric element has a radial lip that bears against a cylindrical surface of the slinger and further has an axial lip that bears against a flange surface of the slinger. The primary function of the axial lip is to prevent the entry of contaminants into the bearing. A wheel bearing must operate in an environment where grit and moisture are heavily present, and the axial lip is therefore in tight contact with the flange surface of the slinger, to ensure that no ingress occurs. Under dynamic conditions, the resulting sliding contact generates friction and heat, meaning that lubrication is essential for preventing early failure of the seal. 
         [0003]    In US 2010/0066030, a solution is proposed for improving the lubrication of the axial contact interface. The seal is provided with a grease lubricant and a non-contact part of the axial lip comprises a grease holding portion for holding grease that is to be supplied to a contact part of the lip. The grease holding portion may be provided in the form of concavities on the surface of the non-contact part, or annular or radial grooves. 
         [0004]    Grease is a semi-solid substance that typically comprises a base oil that is held within a thickener matrix. Under hydrodynamic lubrication conditions, it is base oil from the grease that forms a lubricant film for preventing direct contact between the elastomeric lip and the metal counterface. When grease is supplied to a sliding contact interface, as taught by the above document, the grease will be sheared. 
         [0005]    This shearing mechanism is one of the mechanisms by which base oil is released from grease, but it also has the effect of increasing frictional losses. 
         [0006]    Consequently, there is room for improvement. 
       DISCLOSURE OF THE INVENTION 
       [0007]    The present invention is based on an improved understanding of grease lubrication in rotary seals. 
         [0008]    In grease lubricated seals, the lubrication condition, or film thickness, is assumed to be determined by the availability of lubricant near the sealing contact. The present inventor has found that the available lubricant predominantly stems from grease that is present on a rotating part of the seal. Over time, the grease on the rotating part loses its base oil and eventually becomes incapable of providing an oil film thickness that adequately separates the sealing contact. The time at which this inadequate film thickness occurs can therefore be extended by increasing the amount of grease on the rotating part of the seal. 
         [0009]    Thus, the present invention resides in a rotary seal comprising a first part having an axial sealing lip and a second part having an axial counterface against which the axial sealing lip bears, whereby one of the first and second parts is rotational with respect to the other of the first and second parts. According to the invention, the rotational part is provided with a reservoir for retaining a volume of grease at a location radially inward of the axial sealing lip. The reservoir is designed such that the movement of grease under the action of centrifugal force is prevented, while the movement of base oil, which bleeds from the volume of grease retained in the reservoir, is allowed. As a result, the supply of base oil to the sealing contact takes place for a longer period of time, which extends the life of the seal. 
         [0010]    In a first embodiment of the invention, the second part of the seal is rotational. The second part may be a slinger comprising a cylindrical part and a radial flange part. 
         [0011]    In one example, the reservoir is formed by a bend in the flange part, which creates a retention surface for retaining the volume of grease in a radial direction. Suitably, the retention surface extends in an axial direction at an angle of less than 40 degrees relative to a rotation axis of the seal. The angle of the retention surface may be adapted depending on the operating speed of the rotational part (i.e. the magnitude of the centrifugal force acting on the grease volume). For example, in low-speed applications, an angle of between 20 and 40 degrees may be used. In high-speed applications, an angle of less than 20 degrees is preferable. As a result, an axial component of the centrifugal force acting on the volume of grease is insufficient to cause sideways movement of the grease, but is sufficient to allow side-flow of base oil. In some applications, to prevent movement of the grease, the retention surface may be parallel to the axis of rotation, so that the axial component of the centrifugal force is zero. In such applications, a pressure differential created within the rotating grease is sufficient to cause side flow of the base oil. 
         [0012]    In a second example of the first embodiment, the reservoir comprises an overhanging lip, so that the volume of grease is retained in a radial direction and in an axial direction. The reservoir may be a separate part that is moulded to or adhesively fixed to the second part of the seal. To allow the movement of base oil, the reservoir further comprises channels. The channels may be grooves provided in the lip, which allow side-flow of oil out of the reservoir. The channels may also be through-holes provided in the reservoir which allow base oil to flow in a radially outward direction. In a still further embodiment, the reservoir is made of a porous material, whereby the pores in the reservoir act as channels for the base oil. 
         [0013]    Preferably, the channels have a width of less than 1 mm, so that grease cannot escape from the reservoir via the channels. The number of channels and the size of the channels is selected depending on the volume of base oil that is advantageously supplied to the sealing contact. 
         [0014]    In a second embodiment of a seal according to the invention, the first part of the seal is rotational. The reservoir is then suitably moulded into an elastomeric element on which the axial sealing lip is provided. The reservoir may comprise an overhanging lip and further comprise channels, as described above. Alternatively, the reservoir may be formed by a roughened retention surface on the elastomeric element. Again, the roughened retention surface preferably extends in an axial direction at an angle of less than 40 degrees relative to the rotation axis. In high speed applications, the angle is preferably less than 20 degrees. 
         [0015]    A seal according to the invention can be used in any rotational application for sealing a gap between an outer component that is mounted coaxially around an inner component. The first part of the seal can comprise the reservoir according to the invention and can be mounted to the inner component or to the outer component, depending on which component is rotational in use. Similarly, the second part of the seal can comprise the reservoir and can be mounted to the inner component or to the outer component. As mentioned, the second part of the seal can be a slinger. The second part can also be an integral part of an assembly in which the first part is mounted. For example, in a wheel bearing unit adapted for inner ring rotation, the part having the counterface for the axial sealing lip can be a wheel mounting flange of the bearing unit. The wheel mounting flange then comprises the reservoir feature, which may be moulded on or adhesively attached. Alternatively, depending on the material of the wheel mounting flange, the reservoir feature may be cast into the flange, or machined into the flange after e.g. a metal forging process. 
         [0016]    A seal according to the invention is provided with a grease lubricant. The grease may be provided in a cavity of the seal, in an amount greater than the volume of grease retained in the reservoir. In an advantageous development, the reservoir is specifically filled with grease, such that the retained volume of grease represents at least 80% of the total volume of grease in the seal. Since the grease in the reservoir is the main source of base oil for lubricating the sealing contact, the amount of grease used can be significantly reduced while, at the same time, seal life is extended. 
         [0017]    Other advantages will become apparent from the following detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a part cross-sectional view of a conventional seal; 
           [0019]      FIG. 2  is a part cross-sectional view of an example of a seal according to a first embodiment of the invention, in which the reservoir feature is provided in a rotational slinger component; 
           [0020]      FIG. 3  is a part cross-sectional view of a further example of a seal according to the first embodiment; 
           [0021]      FIG. 4  is a part cross-sectional view of an example of a seal according to a second embodiment of the invention, in which the reservoir feature is provided in a rotational lip component; 
           [0022]      FIG. 5   a  is a part cross-sectional view of a further example of a seal according to the second embodiment; 
           [0023]      FIG. 5   b  is a perspective view of part of the rotational lip component of  FIG. 5   a.    
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 1  shows an example of part of a conventional seal. The seal  100  has a first part  110  comprising an elastomeric element  112  bonded to a metal casing element  118 . A second part  120  of the seal consists of a slinger having a radial flange part  125  and a cylindrical part  122 . The elastomeric element  112  has an axial sealing lip  115 , which bears against an axially inner surface  127  of the radial flange part. This axially inner surface will be referred to as an axial counterface. The elastomeric element  112  of the first part also has a radial sealing lip that bears against a radial counterface on the cylindrical part  122  of the slinger  120 . 
         [0025]    The seal shown in  FIG. 1  is suitable for use in a wheel bearing unit adapted for inner ring rotation. Typically, the metal casing element  118  is mounted to a stationary outer ring of the bearing unit and the slinger  120  is mounted to a rotational inner ring. For measuring rotational speed of the inner ring, an axially outer surface of the flange part  125  of the slinger is provided with a magnetized rubber moulding  129 . A changing magnetic field is picked up by e.g. a Hall sensor and the associated rotational speed data is fed to a vehicle control system such as ABS. 
         [0026]    The main purpose of the seal is to protect the functionality of the bearing unit. The seal both retains lubricant within the bearing cavity and prevents the ingress of contaminants such as moisture and grit. The axial sealing lip  115  is particularly important for preventing the entry of contaminants and is therefore generally in tight contact with the axial counterface  127 . To ensure that tight contact is maintained, the first part of the seal may further comprise a garter spring  140 . Friction is therefore generated when the slinger  120  is rotating. To reduce the friction, the seal  100  is provided with a grease lubricant  160  in a cavity  145  between the axial sealing lip  115  and the cylindrical part  122  of the slinger  120 . 
         [0027]    The lubrication of the sealing contact between the axial sealing lip  115  and the axial counterface  127  will now be discussed. Under rotational conditions, most of the grease  160  will be thrown onto the stationary elastomeric element  112  and a relatively small amount of grease  160  will remain on the slinger, as indicated in  FIG. 1 . The grease comprises base oil held in a thickener matrix. The grease on the slinger will slowly release base oil for lubrication of the sealing contact, whereby the resulting oil flow is driven by the large centrifugal forces that result from the rotational movement. The stationary grease on the elastomeric element will also release oil, but on a much longer timescale, due to the absence of these forces. 
         [0028]    The released oil forms an oil film under hydrodynamic lubrication conditions, which separates the axial sealing lip and the axial counterface. Over time, film thickness decreases due to loss of oil. When a feed rate of released oil becomes insufficient to replenish the sealing contact, the film thickness will reach a critical value at which the axial sealing lip and the axial counterface are no longer adequately separated. Wear and loss of sealing function will then soon take place. 
         [0029]    The present inventor has found that the time until the critical thickness value is reached can be significantly lengthened by increasing the volume of grease on the rotating part of the seal which is available for supplying base oil to the sealing contact. As a result, seal life can be extended. 
         [0030]    Thus, in a seal according to the invention, the rotating part of the seal comprises a reservoir feature for holding a volume of grease. The reservoir is adapted to prevent the movement of grease under the action of centrifugal force, but to allow the movement of base oil, which bleeds from the grease retained in the reservoir. 
         [0031]    An example of a first embodiment of a seal according to the invention is shown in  FIG. 2 . The seal  200  is suitable for use in a wheel bearing unit adapted for inner ring rotation as described above. The seal comprises a first part  210  having an elastomeric element  212  with an axial sealing lip  215 , whereby the first part  210  is identical to the first part of the seal shown in  FIG. 1 . The second part  220  of the seal is formed by a slinger which has a radial flange part  225  and a cylindrical part  222 . A sealing contact is defined between the axial sealing lip  215  and an axial counterface  227  on the radial flange part, and the slinger rotates about a rotational axis  250 . According to the invention, the slinger is provided with a reservoir  230  for retaining a volume of grease  260  at a location radially inward of the sealing contact. In this example, the reservoir  230  is formed by a bend in the flange part  225  of the slinger, such that the flange part further comprises an axially extending surface  235 . This surface, which will be referred to as a retention surface, acts as an overhang for radially retaining a volume of grease  260 . 
         [0032]    In this example, the seal is filled with an amount of grease in a cavity  245  between the axial sealing lip  215  and the cylindrical part  222  of the slinger. Under the action of centrifugal force, much of the grease will be thrown onto the stationary elastomeric element  212 . Some of the grease—the grease volume  260 —will be thrown onto the overhanging retention surface  235  on the flange part  225  of the slinger. Suitably, the retention surface  235  extends at an angle a of less than 40 degrees relative to the rotational axis  250 . In the example of  FIG. 2 , the angle is approximately 25 degrees. As a result, the centrifugal force acting on the grease volume  260  has a relatively small axial component, which is insufficient to allow sideways movement of the grease but which allows side flow of base oil from the grease. Upon reaching the edge of the retention surface  235 , the base oil will then flow in a radially outward direction towards the sealing contact. 
         [0033]    Comparing the inventive seal of  FIG. 2  with the conventional seal of  FIG. 1 , it is evident that the inventive seal retains a greater volume of grease on the rotational slinger. As explained above, this leads to longer seal life. 
         [0034]    A further example of a seal according to the invention comprising a slinger with a reservoir feature is shown in  FIG. 3 . The seal  300  again comprises a first part  310  having an elastomeric element with an axial sealing lip  315  which is in sealing contact with a flange part  325  of the slinger  320 . In this example, the reservoir  330  is formed by a moulding provided on an axially inner surface of the flange part  325 . As with the example depicted in  FIG. 2 , the grease reservoir  330  has an overhanging surface which retains a volume of grease  360  in a radial direction. Additionally, the grease reservoir  330  comprises a lip  332  which retains the grease in an axial direction. The lip  332  also prevents the movement of base oil. Therefore, to allow base oil to move in a radially outward direction towards the sealing contact, the reservoir  330  comprises a plurality of through holes  337  which extend from a radially inner surface of the reservoir to a radially outer surface of the reservoir. Suitably, the through-holes have a diameter of less than 1 mm, so that grease from the grease volume  360  is unable to pass through. 
         [0035]    In the example depicted in  FIG. 3 , the moulded grease reservoir  330  is formed as part of the process in which magnetized rubber  329  (for speed detection) is moulded to the axially outer side of the flange part  325  of the slinger. Suitably, the flange part comprises openings  328  which allow the rubber to extend to the axially inner side of the flange part  325 , to form the reservoir  330 . The reservoir in this example is therefore discontinuous. As will be understood, the reservoir may also be formed by a continuous part that is moulded to or adhesively fixed to the axially inner side of the flange part. 
         [0036]    In the examples discussed thus far, the second part of the seal (the part comprising the axial counterface) has formed the rotating part of the seal. In a second embodiment, the first part of the seal forms the rotational part. A first example of such a seal according to the invention is shown in  FIG. 4 . 
         [0037]    The second part of the seal  400  is again formed by a slinger  420 , which has an axially oriented counterface  427  on the flange part  425 . The first part  410  of the seal comprises a metal casing element to which an elastomeric element  412  is bonded. The elastomeric element has an axial sealing lip  415 , which is in sealing contact with the counterface  427 . Further, the elastomeric element  412  comprises a reservoir  430  according to the invention, which retains a volume of grease  460  at a location radially inward of the sealing contact. The grease volume  460  in this example is specifically provided on the reservoir  430 , and constitutes the large majority of the grease within the seal. 
         [0038]    In this example, the reservoir  430  comprises a retention surface  435  which is roughened to promote the adhesion of grease. Part of the retention surface extends in an axial direction, essentially parallel to a rotational axis of the seal. The volume of grease  460  is therefore retained in a radial direction and will not move in a sideways direction under the action of centrifugal force. A pressure differential created within the grease under rotational conditions allows the side-flow of base oil, which will ultimately flow towards the sealing contact. 
         [0039]    A second example of a seal according to the invention, whereby the first part of the seal  500  comprises a reservoir, is shown in  FIG. 5   a . In this second example, the reservoir  530  is moulded into the elastomeric element  512  of the first part  510  and comprises an overhanging lip  532 . Thus, a volume of grease  560  is retained on the elastomeric element in a radial direction as well as in an axial direction. To allow the movement of base oil from the grease volume  560 , the lip  532  is provided with grooves  537 . A detail of the elastomeric element  512 , showing the axial sealing lip  515  and the overhanging lip  532  of the grease reservoir, is depicted in  FIG. 5   b.    
         [0040]    The grooves  537  extend from an axially inner side of the lip  532  to an axially outer side of the lip, thereby allowing the side flow of base oil, which will ultimately flow in a radially outward direction towards the sealing contact. Suitably, the grooves have a width of less than 1 mm, to that grease is unable to pass through. 
         [0041]    The invention is not to be regarded as being limited to the embodiments described above, a number of additional variants and modifications being possible within the scope of the subsequent patent claims. 
       REFERENCE NUMERALS 
       [0000]    
       
           100  Conventional seal 
           110  First part of seal 
           112  Elastomeric element 
           115  Axial sealing lip 
           118  Metal casing element 
           120  Second part of seal (slinger) 
           122  Cylindrical part of slinger 
           125  Flange part of slinger 
           127  Axial counterface 
           129  Magnetized rubber moulding 
           140  Garter spring 
           145  Grease cavity 
           160  Grease 
           200  Seal according to invention 
           210  First part of seal 
           212  Elastomeric element 
           215  Axial sealing lip 
           120  Second part of seal (slinger) 
           222  Cylindrical part of slinger 
           225  Flange part of slinger 
           227  Axial counterface 
           230  Reservoir feature 
           235  Retention surface 
           245  Grease cavity 
           250  Axis of rotation 
           260  Grease volume in reservoir 
         α Angle of retention surface relative to rotation axis 
           300  Seal according to invention 
           310  First part of seal 
           315  Axial sealing lip 
           320  Second part of seal (slinger) 
           325  Flange part of slinger 
           328  Opening in flange part 
           329  Magnetized rubber moulding 
           330  Reservoir feature 
           332  Overhanging lip 
           337  Through hole 
           260  Grease volume in reservoir 
           400  Seal according to invention 
           410  First part of seal 
           412  Elastomeric element 
           415  Axial sealing lip 
           420  Second part of seal (slinger) 
           425  Flange part of slinger 
           427  Axial counterface 
           430  Reservoir feature 
           435  Roughened retention surface 
           460  Grease volume in reservoir 
           500  Seal according to invention 
           510  First part of seal 
           515  Axial sealing lip 
           520  Second part of seal (slinger) 
           527  Axial counterface 
           530  Reservoir feature 
           532  Overhanging lip 
           537  Groove in overhanging lip 
           560  Grease volume in reservoir