BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates generally to radial lip seals for use with relatively rotatable members such as wheel axle applications, and in particular to lip seals wherein the contacting lip is formed of two dissimilar materials. 
     2. Related Prior Art 
     Lip-type fluid seals typically employ a flexible sealing member that is fixed to an outer metallic case and includes one or more sealing lips along a radially inward edge thereof. The selection of the sealing member material is determined by a number of factors, including the environment in which the seal will be operating. Elastomeric materials, such as rubber, are often employed as the sealing lip material in applications where the lip seal is exposed to a dirty environment. For example, seals used in heavy equipment applications for sealing axle bearings are exposed on the air or atmosphere side of the seal to various contaminants which would be harmful to the bearings, including water, salt, and abrasives such as dust, mud, sand, etc. In other applications, contaminants may be present in lubricant on one side of the seal. In either case, elastomers are generally preferred as the sealing lip material over other materials, such as polytetrafluoroethylene (PTFE) materials, as the elastomers are far more resistant to wear under such contaminated conditions than those of the PTFE materials, which generally perform poorly. 
     Environmental conditions which favor PTFE materials over the elastomers for the selection of the sealing member include low and high temperature and/or dry or low lubricant running conditions. The PTFE materials are generally more wear resistant than elastomers under such conditions. 
     Difficulties arise, however, when seals operate in environments that favor neither material. For example, in the heavy equipment application described above, it is not uncommon for the seal to be subjected not only to a contaminated environment, but also to low or high temperature and/or low lubricity conditions. In the past, the elastomers were selected over PTFE materials, as the elastomers are more tolerant of low or high temperature and/or low lubricity conditions than are the PTFE materials of a contaminated running condition. Thus, there is a need in the industry for a lip seal that performs well in a contaminated environment and low or high temperature and/or low lubricity operating conditions. 
     SUMMARY OF THE INVENTION AND ADVANTAGES 
     A fluid seal assembly for a relatively rotatable member having a cylindrical sealing surface comprises a rigid annular case, a sealing member formed of elastomeric material fixed to the case including an annular seal lip having a contaminated side and an opposite side and an annular elastomeric lip portion disposed on the contaminated side having an annular elastomeric lip edge supported for sealing engagement with the sealing surface of the relatively rotatable member. The invention is characterized by the sealing lip including an insert formed of PTFE material bonded to the elastomeric lip portion in axially adjacent relation thereto on the opposite side of the sealing lip having an annular PTFE lip edge supported for simultaneous sealing engagement with the sealing surface of the relatively rotatable member. 
     According to a further aspect of the invention, a unitized fluid seal assembly is provided having an outer metal case, an inner wear sleeve, a sealing member having a body formed of elastomeric material fixed to the case including a radially inwardly projecting annular sealing lip having a contaminated side and an opposite side and an elastomeric lip portion having an annular elastomeric lip edge disposed on the contaminated side in sealing engagement with the wear sleeve. The invention is characterized by the sealing lip including a PTFE insert bonded to the lip portion and having an annular PTFE lip edge disposed on the opposite side in simultaneous sealing engagement with the wear sleeve and sealed from the contaminated side by engagement of the elastomeric lip portion with the wear sleeve. 
     The invention also contemplates a method of manufacturing such fluid seals wherein a PTFE insert is bonded to an elastomeric lip portion to provide a PTFE lip portion. The elastomeric and PTFE lip portions are trimmed to provided common axially adjacent elastomeric and PTFE lip edges disposed on the contaminated and opposite sides, respectively of the seal and supported for simultaneous sealing engagement with a sealing surface of a relatively rotatable member. 
     The invention thus provides a fluid seal having a sealing lip with a composite elastomeric/PTFE lip edge. The elastomeric lip edge is disposed on the contaminated side of the lip and the PTFE lip edge is disposed on the opposite side of the lip and are support for simultaneous contact with the corresponding sealing surface of a relatively rotatable member, such as a wear sleeve. 
     Such a composite seal configuration is particularly well suited in applications where the operating conditions favor neither a lip seal fabricated of only elastomeric or PTFE material. In a heavy equipment axle bearing seal application, the elastomeric lip edge on the contaminated side of the sealing lip is exposed to contaminants during operation, while the PTFE lip edge is on the opposite side shielded from the contaminating atmosphere by the elastomeric lip edge. In operating conditions where, in addition to the contaminating environment, there exists also low or high temperature and/or low lubricity operating conditions, the PTFE lip edge comes into play to maintain a good fluid seal with the sealing surface of the relatively rotatable member. The simultaneous contact of the wear-resistant PTFE lip edge advantageously limits the degree of wear of the elastomeric lip edge to that of the PTFE lip edge, thereby enabling the elastomeric lip edge to continue shielding the PTFE lip edge from contaminants even under unfavorable conditions of extreme temperatures and/or low lubricity conditions. 
     A further advantage of the PTFE/elastomeric lip edge combination is that the PTFE material has a higher modulus of elasticity than that of the elastomeric material. During operation, reversible hydrodynamic surface deformations are imparted to the elastomeric lip portion to assist in the exclusion of contaminants. It has the further advantage of providing such hydrodynamic aids without having to form them during the manufacturing process. This simplifies the manufacture of such seals, particularly those requiring bi-directional hydrodynamic capabilities. Still a further advantage is that the bi-modulus imparted hydrodynamic surface deformations are not permanent like those of the molded, embossed, or cut type, which are prone to entrapping contaminants thereby reducing the effect of the hydrodynamic features and in some cases causing premature wear of the seal and/or scoring of the sealing surface of the relatively rotatable member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other advantages and features of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 is an enlarged radial sectional view of a portion of a composite bi-modulus lip seal constructed according to a presently preferred embodiment of the invention and shown installed in a wheel axle application; 
     FIG. 2 is a further enlarged fragmentary sectional view of a portion of the lip seal of FIG. 1; 
     FIG. 3 is an enlarged fragmentary section view of a mold tool used in manufacturing the composite fluid seal of the invention; and 
     FIG. 4 is a further enlarged radial section view of the portion of the seal assembly formed in the mold tool of FIG. 3 shown in its as-molded and trimmed condition. 
    
    
     DETAILED DESCRIPTION 
     Referring now in more detail to the drawings, there is shown in FIG. 1 a lip-type fluid seal assembly generally designated  10 , having a rigid annular case or carrier  12  in the preferred form of a cylindrical cup-shaped metallic shell having a cylindrical wall  14  and a radial flange portion  16 . 
     An annular sealing member, generally indicated at  18 , is fixed to the flange portion  16  of the case  12  and may be formed of any suitable elastomeric material, such as silicone, polyacrylate, fluoroelastomer, ethylene acrylic, or nitrile elastomer. The member  18  has an elastomeric body portion  20  fixed to the flange  16 , an elastomeric flex section or neck  22 , and an annular elastomeric head section  24 . An annular sealing lip  26  extends radially from the head  24  and includes a contaminated side  28  and an axially opposite side  30 , corresponding to the orientation of the seal in its operating environment for a given application. The opposite side  30  is that side of the sealing lip  26  which faces a fluid or grease reservoir or protected air environment from which the seal is intended to protect from contaminants, and the contaminated side  28  is that side of the sealing lip  26  from which the contaminants, such as dust, dirt, sand, road grime, salt, water, etc. are intended to be excluded from entering the fluid reservoir past the sealing lip  26  and may be a contaminated air environment or a contaminated lubricant or fluid. For simplicity, in the description of the embodiment that follows, the contaminated side  28  will be referenced as the air side  28 , and the other protected side  30  will be referenced as the oil side  30 , it being understood that the contaminating environment can be other than air. 
     The sealing lip  26  is of a composite construction and includes an elastomeric lip portion  32  provided on the air side  28  of the sealing lip  26  and, according to the invention, a nonelastic insert  34  formed of polytetrofluoroethylene (PTFE) material bonded to the elastomeric lip portion  32  in axially adjacent relation thereto on the fluid side  30  of the sealing lip  26 . 
     The elastomeric lip portion  32  includes an annular elastomeric lip edge  33 , and the PTFE insert  34  includes a PTFE lip portion  36  with an annular PTFE lip edge  37 . The elastomeric lip edge  33  is disposed on the air side  28  of the sealing lip  26 , while the PTFE lip edge  37  is disposed on the fluid side  30  in axially spaced relation to the air side  28 . The elastomeric lip edge  37  extends circumferentially continuously so as to shield the PTFE lip edge  37  from the air side  28 . The PTFE lip edge  37  is likewise preferably circumferentially continuous and as such the sealing lip  26  is provided with a composite contact sealing surface having a continuous band of elastomeric material on the air side and a continuous band of PTFE material on the oil side. The elastomeric and PTFE materials are intimately bonded and as such they are joined across a common interface  38  such that the elastomeric lip edge  33  transitions without interruption into the PTFE lip edge  37  in the axial direction. 
     The elastomeric and PTFE lip edges  33 ,  37  are supported by the sealing member  18  in such manner as to simultaneously sealingly engage a cylindrical sealing surface  40  of a relatively rotatable member  42 . According to a preferred embodiment of the invention, the relatively rotatable member  42  comprises a unitized wear sleeve  42  of the well known type having a cylindrical body or sleeve section  44 , the inner periphery  46  of which is dimensioned to fit snuggly on a shaft  48 , and the outer periphery of which serves as the sealing surface  40  upon which the lip edges  33 ,  37  run, as illustrated in FIGS. 1 and 2. The wear sleeve  40  is curved outwardly at its ends to provide radially outwardly extending end flanges  50 ,  52  on the fluid and air side  30 ,  32  of the sealing lip  26 , respectively. In this way, the wear sleeve  40  is joined with the case  12  and sealing member  18  as a unitized assembly. 
     The sealing member  18  is molded with an annular spring retention groove  54  adapted to receive and retain an annular coil or garter spring  56  in conventional manner. The spring  56  acts to bias the head section  24  and thus the lip edges  33 ,  37  of the sealing lip  26  constantly toward compressive sealing engagement with the sealing surface  40 . 
     FIGS. 3 and 4 illustrate a method of manufacturing the seal assembly  10  of the invention. A multiple part mold tool  58  has parts  60 ,  62  thereof adapted to engage the outer surface  64  of the side wall  14  of the case  12  and the oil side surface  66  of the flange portion  16 , respectively. An opposing upper mold part  68  has a surface  70  thereof adapted to engage the air side  72  of the flange portion  16  such that, when the mold parts are closed, the case  12  becomes firmly clamped in the mold tool with an extended portion  74  of the flange  16  projecting into a molding cavity  76  of the mold tool  58  provided between opposing contoured surfaces of the upper mold part  68  and that of a lower mold part  78 . 
     With the mold tool  58  open, the case  12  is inserted and an annular washer or wafer  80  of the PTFE material, which preferably has been etched on at least its interface surface  38 , is positioned in the mold  58  to serve as the insert  34  with the etched side  82  exposed and an opposite side  84  supported on an upper surface  86  of the lower mold part  78 . 
     A ring of uncured elastomer is loaded in the cavity  76  of the mold tool  58 , and the mold tool  58  thereafter closed, it being understood that the mold tool  58  is suitably heated prior to molding to cause the elastomeric material to flow and bond with the PTFE wafer  80 . As shown in FIG. 3, the upper surface  86  of the lower mold part  78  is formed with a series of concentric ridges or teeth  87  which bite into the PTFE wafer  80  when the mold  58  is closed to support the wafer  80  against movement. The wafer  80  used in the process is of a simple, flat washer configuration with the opposite sides  82 ,  84  thereof being substantially parallel and planer. A shown in FIG. 3, the etched side  82  of the wafer  80  is exposed in the cavity  76 , as is a small portion of the opposite side  84  and an outer edge  88  of the wafer  80 . 
     Upon closing the mold  58 , the uncured elastomer material is caused to flow and fill the unoccupied region of the cavity  76 , and in doing so encapsulates the projecting portion  74  of the flange  16  of the case, and the projecting sides  82 ,  84  and edge  88  of the wafer  80 . Following molding, the elastomer is heat cured in conventional manner to set the elastomer. The sealing lip  26  is then trimmed across the elastomeric and PTFE regions while in its free state (i.e., in its unflexed condition prior to engagement with the wear sleeve  42 , illustrated by broken chain lines in FIG. 2) to provide the common, composite contact surface made up of the elastomeric and PTFE lip edges  33 ,  37  for simultaneously engaging the sealing surface  40  of the wear sleeve  42  when in an installed condition, as illustrated in solid lines in FIG.  2 . It will be appreciated by those skilled in the art and from FIG. 2 that the sealing lip  26  is initially smaller in diameter than that of the wear sleeve  44  and upon installation with the wear sleeve  44  is caused to expand somewhat as the head  24  rotates radially outwardly upon flexing of the neck  22  about an angle of rotation B from the free state to the installed condition as the sealing lip  26  engages the sealing surface  40 . 
     The trimming operation is illustrated schematically in FIG. 4, wherein after curing, the sealing member  18  is loaded in its free state in a fixture (not shown) with the pre-trimmed PTFE insert  34  and elastomer lip portion  32  supported generally horizontally and the elastomeric portion  32  facing upwardly. The trimming of the sealing lip  26  is along trim line  90  in FIG. 4, leaving the common coplanar lip edges of the elastomeric and PTFE portions  33 ,  37 . The trim plane or line  90  is at an angle A that is determined in part by the angle of rotation B of the head  24  which may vary from one application to another. It is preferred, however, that the trim angle A be equal to or greater than the angle of rotation B of the head section  24 , such that when the seal assembly  10  is installed, the elastomeric and PTFE lip edge portions  33 ,  37  sealingly engage the sealing surface  40  simultaneously, as illustrated in FIGS. 1 and 2. The trim angle A is selected to cause the elastomeric lip edge  33  to project radially inwardly slightly beyond that of the PTFE lip edge  37 , such that when installed, the relatively softer elastomeric portion  33  is caused to be biased further toward engagement with the sealing surface  40  than that of the PTFE lip edge  37  and thereby is caused to wear during an initial break-in period at a rate greater than that of the relatively harder PTFE lip edge  37  until the elastomeric lip edge  33  is reduced to the diameter of the PTFE lip edge  37 , at which point the portions  33 ,  37  will wear together at the slower rate of the PTFE material. In this way, the elastomeric lip edge portion  32  acts initially as a sacrificial seal, assuring that the PTFE lip edge  37  is protected from exposure to contaminants during the initial break-in period, and continues thereafter to protect the PTFE lip edge  37  from such contaminants during the service life of the seal  10  by wearing together with the PTFE lip edge portion  37 . In the example shown, the trim angle A may be about 7° inwardly of an axial plane P of the axially extending side wall  14  to achieve the stated objectives. 
     The sealing member  18  further includes an annular projection  92  that is spaced radially from the sealing surface  40  of the wear sleeve  42  to provide, in conjunction with the wear sleeve  42 , a labyrinth seal. Also formed on the oil side  30  of the sealing member  18  are hydrodynamic pumping flutes  94  of conventional type which may be molded integrally with the sealing member  18 . 
     The end flange  52  of the wear sleeve  42  on the air side  28  of the assembly  10  may include a secondary annular sealing lip  96  in contact with the side wall  14  of the case  12  to assist in excluding contaminants from entering the interior of the seal assembly  10 . 
     In operation, the wear sleeve  42  is press fit onto a shaft  48 , such as the shaft or spindle of a large equipment wheel axle to provide a static seal therebetween, and the case  12  fit into a cup housing  100  of an adjacent component  102  such as a hub. The seal assembly  10  is oriented so that the air side  28  of the sealing lip  26  faces toward the exterior environment side of the seal, and the fluid side  30  faces axially inwardly toward the protected sealed area, such as the sealed region about a wheel bearing (not shown), with the seal  10  operating to protect the sealed bearing area from contaminants present on the air side  28  of the seal  10 . 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims wherein parenthetical reference numerals are merely for convenience and are not in any way to be limiting, the invention may be practiced as otherwise than as specifically described.
A lip-type fluid seal ( 10) for a shaft includes a composite sealing lip ( 26) having an elastomeric portion ( 32) and a PTFE portion ( 36). The elastomeric portion ( 32) includes a lip edge ( 33) that extends circumferentially continuously to seal the adjacent lip edge ( 37) of the PTFE portion ( 36) from any contaminants on the elastomeric side ( 28) of the seal ( 10). The PTFE portion ( 32) has a greater hardness and modulus of elasticity which serves to limit the wear of the elastomeric portion ( 32) and to induce hydrodynamic surface deformations on the elastomeric lip portion ( 32) during operation to enhance the exclusion of contaminants. The PTFE lip edge ( 37) performs well under low temperature and/or low lubricity conditions and its relative hardness protects the elastomeric lip edge from excessive wear.