Abstract:
A hub seal for installation on the wheel end of a heavy duty trailer or truck is disclosed which includes an annular casing, an elastomeric sealing ring supported within the annular casing and including an axially extending sealing lip, an annular sleeve mounted coaxial with the annular casing, the annular sleeve having an axially extending sealing surface, and a spring for biasing the axially extending sealing lip against the axially extending sealing surface such that a substantial portion of the sealing lip is in sealing engagement with the sealing surface over a substantial contact area.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The subject application is a continuation-in-part of copending U.S. application Ser. No. 08/957,807 which was filed on Oct. 24, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention relates to a hub seal, and more particularly, to a hub seal having a machinable thrust ring which reacts to axial loads imposed during installation of the hub seal on the wheel end of a vehicle, and which is partially removed through shearing action during the break-in period of the seal. The hub seal includes a lay-down lip seal formed from an elastomeric material. 
     2. Background of the Related Art 
     In tractor and trailer wheel end axle assemblies, particularly those employed on large trailers, semi-trailers and tractors, the wheel is mounted on a fixed axle for rotation thereabout. The hub of the wheel defines a lubricant chamber about the end of the axle in association with the wheel bearings. A quantity of lubricant is maintained in the lubricant chamber to continuously bathe the bearings. A hub cap is used to enclose the lubricant chamber and a shaft seal is fit onto the axle to effect a dynamic seal between the axle and the lubricant chamber. 
     Shaft seals are well known in the art, and generally include a wear member or sleeve and a seal member or casing. The casing is mounted to rotate with the wheel hub relative to the sleeve member. The casing has an outer diameter which is designed to provide an interference fit with the wheel hub and the sleeve is dimensioned to be press fit onto the wheel axle. An early example of a unitized shaft seal in which the wear member and seal member are constructed as a one-piece assembly is disclosed in U.S. Pat. No. 3,685,841 to Keller. Later examples of semi-unitized shaft seals in which the wear member and seal member can be disassembled for repair and inspection are disclosed in U.S. Pat. Nos. 4,208,057 and 4,252,329 to Messenger. 
     It is known in the prior art, as illustrated in each of the above-identified patents, to provide the seal member with elastomeric bumper elements that serve to maintain the axial spacing of the wear member and seal member, as well as absorb the axial load exerted upon the seal member when the seal assembly is press fit onto the axle shaft. Upon installation, the bumper elements are compressed and during the initial break-in period of the seal assembly, the bumper elements are partially worn away to provide a minimal running clearance between the wear member and the seal member. 
     During the break-in period, the elastomeric bumper elements generate significant resistance to rotation between the sleeve and casing. This resistance generates heat and must be overcome by increasing the torque applied between the sleeve and casing. The increased heat can have an adverse effect on the integrity of the seal and the increase in torque has an adverse effect on vehicle efficiency. In addition, the particulate material worn away from the bumper elements can contaminate the bearing chamber and cause damage to the seal assembly. 
     U.S. Pat. No. 5,015,001 to Jay discloses a seal assembly which includes an annular bumper element formed from a fusible material which transitions via frictional heating from a solid to a liquid during the break-in period. During break-in, it can take a considerable amount of time and applied torque to sufficiently elevate the temperature of the bumper element to a induce a phase change. This can have an adverse effect on vehicle efficiency. Clearly, there is a need in the art for a hub seal of the type having a bumper element that can be removed during the break-in period without causing an increase in torque and without generating excessive heat. 
     It is also known in the art of hydrodynamic shaft seals to provide a radial lip element in sealing contact with the axial sealing surface of a wear sleeve to prevent lubricating fluid from migrating along the sealing surface. In general, there are two types of radial lip elements employed in shaft seals. The first type provides one or more thin bands of contact between the sealing lip and the sealing surface of the wear sleeve. Examples of such seals are disclosed in U.S. Pat. No. 4,695,063 to Schmitt et al., U.S. Pat. No. 4,844,480 to Gralka, and U.S. Pat. No. 4,906,009 to Saitoh and U.S. Pat. No. 5,427,387 to Johnston. 
     The second type provides a deformable lip element which, upon installation, is deflected from its normal orientation into sealing contact with a circumferential area of the axial sealing surface of the wear sleeve. Deformable radial lip seals of this type are commonly referred to as “lay-down” lip seals and are typically fabricated from a synthetic resin material such as polytetrafluoroethylene (PTFE). Examples of such seals are disclosed in U.S. Pat. No. 4,591,168 to Hölzer and U.S. Pat. No. 5,209,499 to Ruff et al. 
     When a sealing lip fabricated from a material such as PTFE is deformed during assembly and subsequently installed in an operating environment in which it is subjected to significant mechanical stresses, material degradation and a loss of resiliency can occur rapidly. It would be desirable therefore, to increase the service life of a hydrodynamic shaft seal by providing a lay-down lip seal fabricated from an elastomeric material which could be biased into sealing engagement with the sealing surface of the wear sleeve. It would also be desirable to provide such a lip seal with structures to increase the hydrodynamic pumping effect of the sealing element. Examples of lip seals with such structures are disclosed in U.S. Pat. No. 4,441,722 to Pichler and U.S. Pat. No. 4,783,086 to Bras et al. 
     SUMMARY OF THE INVENTION 
     The subject invention is directed to a hub seal for installation on the wheel end of a heavy duty trailer or truck which overcomes the inadequacies of prior art hub seals. The hub seal of the subject invention includes an annular casing which defines a radially outer circumferential canal at an outboard side thereof. A thrust ring defined by polymeric material is disposed within the circumferential canal and is dimensioned and configured to support axial loads imposed upon the hub seal during installation. The thrust ring is preferably defined by a continuous ring of polymeric material, although it is envisioned that the ring may be segmented or discontinuous. Those skilled in the art will readily appreciated that the adhesive characteristics of the polymeric material from which the thrust ring is formed facilitates bonding of the thrust ring into the circumferential canal. Preferably, a circumferential lip projects axially into the circumferential canal to increase the bonding surface area thereof. 
     The hub seal further includes an annular wear sleeve which is mounted coaxial with the annular casing. The annular sleeve includes a deflector ring coated with a releasing material to prevent bonding of the thrust ring to the deflector ring. The deflector ring is provided with means for mechanically removing at least a portion of the thrust ring upon relative rotation of the casing and sleeve so as to create a running clearance between the deflector ring and the thrust ring. 
     Preferably, the deflector ring includes a circumferential planar surface which extends generally parallel to the plane of the thrust ring to mechanically remove at least a portion of the thrust ring by shearing action. Alternatively, the deflector ring may include a plurality of circumferentially spaced apart tangentially extending flats formed in a radially outer portion thereof for mechanically removing at least a portion of the thrust ring. It is also envisioned that the deflector ring could include a plurality of circumferentially spaced apart radially inwardly extending notches formed in a radially outer portion thereof to mechanically remove at least a portion of the thrust ring. 
     In a preferred embodiment of the subject invention, a circumferential retaining lip forms a radially inner wall of the circumferential canal. This lip is angled radially outwardly to minimize running torque and direct the movement of material, mechanically removed from the thrust ring by the deflector ring, in a radially outward direction. 
     The subject invention is also directed to a method of fabricating a hub seal with a machinable thrust ring which includes the steps of providing an annular casing defining a circumferential canal, injecting a polymeric material into the circumferential canal to form a machinable thrust ring, providing an annular sleeve having a deflector ring configured to mechanically remove at least a portion of the thrust ring, and unitizing the annular casing and the annular sleeve so as to position the deflector ring in opposing relationship to the machinable thrust ring. Preferably, the step of injecting the polymeric material into the circumferential canal includes the step of bonding the polymeric material within the canal, and the method further includes the step of coating the deflector ring with a releasing material which prevents bonding of the thrust ring to the deflector ring. 
     The subject invention is further directed to a hub seal which includes an annular casing, an elastomeric sealing ring supported within the annular casing and including a sealing lip, an annular sleeve mounted coaxial with the annular casing, the annular sleeve having an axially extending sealing surface, and an annular garter spring for biasing the sealing lip against the axially extending sealing surface such that a substantial portion of the sealing lip is in sealing engagement with the sealing surface over a substantially continuous contact area. 
     Preferably, the sealing lip and the sealing surface are in sealing engagement with one another over a substantially continuous contact area having an axial length of about between 0.020 inches and 0.120 inches. The elastomeric sealing ring is a preferably a composite structure which includes an elastomeric sealing portion and a metallic support portion. In such an instance, the elastomeric sealing portion and the metallic support portion can be bonded to one another, or molded integral with one another. The elastomeric sealing ring may also be configured as a two-part structure in which the elastomeric sealing portion and a metallic support portion are mechanically interfaced with one another to form an integrated structure. 
     In a preferred embodiment of the subject invention, the sealing lip of the elastomeric sealing portion includes a sealing surface having a plurality of spaced apart projections which provide a hydrodynamic pumping aide during relative rotation of the casing and the sleeve. The spaced apart projections have a generally trapezoidal configuration, with the lateral sides thereof inclined with respect to the axis of the hub seal. 
     These and other features of the hub seal of the subject invention and the method of fabrication of the hub seal of the subject invention will become more readily apparent to those having ordinary skill in the art from the following detailed description of the invention taken in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that those having ordinary skill in the art to which the subject invention appertains will more readily understand how to make and use the hub seal of the subject invention, preferred embodiments of the sealing device will be described in detail hereinbelow with reference to the drawings wherein: 
     FIG. 1 is a perspective view of a hub seal constructed in accordance with a preferred embodiment of the subject invention; 
     FIG. 2 is an exploded perspective view of a hub seal constructed in accordance with a preferred embodiment of the subject invention, wherein the deflector ring has a planar surface for machining the thrust ring provided on the outboard side of the casing; 
     FIG. 3 is a side elevational view in cross-section of a portion of the wheel end of a vehicle with the hub seal of the subject invention sealingly mounted therein; 
     FIG. 4 is a side elevational view in cross-section of the hub seal of the subject invention prior to break-in wherein the thrust ring is wholly intact; 
     FIG. 5 is a side elevational view in cross-section of the hub seal of the subject invention shortly after break-in commences with the thrust ring partially removed; 
     FIG. 6 is a side elevational view in cross-section of the hub seal of the subject invention subsequent to break-in with a sufficient portion of the thrust ring removed so as to create a running clearance between the deflector ring and the thrust ring; 
     FIG. 7 is an exploded perspective view of yet another hub seal constructed in accordance with a preferred embodiment of the subject invention, wherein the deflector ring includes a plurality of circumferentially spaced apart radially inwardly extending notches formed in a radially outer portion thereof for machining the thrust ring; 
     FIG. 8 is an exploded perspective view of another hub seal constructed in accordance with a preferred embodiment of the subject invention in which the deflector ring includes a plurality of circumferentially spaced apart tangentially extending flats formed in a radially outer portion thereof for machining the thrust ring; 
     FIG. 9 is a cross-sectional view of another embodiment of a hub seal assembly constructed in accordance with a preferred embodiment of the subject invention; 
     FIG. 10 is an enlarged localized cross-sectional view of the hub seal illustrated in FIG. 9 which includes a composite sealing ring subassembly; 
     FIG. 11 is a cross-sectional view of the sealing ring subassembly illustrated in FIG. 10; 
     FIG. 12 is a cross-sectional view of another hub seal assembly constructed in accordance with a preferred embodiment of the subject invention; 
     FIG. 13 is an enlarged localized cross-sectional view of the hub seal assembly of FIG. 12; 
     FIG. 14 is a cross-sectional view of the lip seal cartridge which forms part of the hub seal assembly of FIG. 13; 
     FIG. 15 is an enlarged localized cross-sectional view of the lip seal cartridge illustrated in FIG. 14; 
     FIG. 16 is an elevational view of a portion of the lip seal of the subject invention illustrating a particular geometric configuration of the hydrodynamic pumping aide provided on the sealing surface thereof; 
     FIG. 17 is an elevational view of a portion of the lip seal of the subject invention illustrating another geometric configuration of the hydrodynamic pumping aide provided on the sealing surface thereof; and 
     FIG. 18 is an elevational view of a portion of the lip seal of the subject invention illustrating yet another geometric configuration of the hydrodynamic pumping aide provided on the sealing surface thereof. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now in detail to the drawings wherein like reference numerals identify similar structural elements of the subject invention, there is illustrated in FIG. 1 a two-part semi-unitized shaft seal assembly constructed in accordance with a preferred embodiment of the subject invention and designated generally by reference numeral  10 . As illustrated in FIG. 3, hub seal  10  is adapted and configured to provide a dynamic seal between the interior bore  12  of a wheel hub  14  and an axle shaft  16 . Roller bearings  20  are disposed within wheel hub  14  for supporting the rotation of the wheel hub relative to axle shaft  16 . Wheel hub  14  defines a chamber for containing a volume of lubricating fluid which continuously bathes roller bearings  20 . Shaft  10  serves to seal the lubricant chamber. 
     Referring now to FIG. 2 in conjunction with FIG. 3, shaft seal  10  includes an outer annular casing  22  and an inner metallic wear sleeve  24 . Casing  22  and wear sleeve  24  are assembled together to form a two-part, separable, semi-unitized seal assembly, as best seen in FIG.  1 . Wear sleeve  24  includes an axially extending portion  26  and a radially outwardly extending portion  28 . The axially extending portion  26  is dimensioned and configured to be press-fit onto the outer diameter of axle shaft  16 . The radially outwardly extending flange portion  28  of wear sleeve  24  defines an annular deflector ring which opposes the outboard side of the annular outer casing  22  when the casing and sleeve are assembled. 
     Referring to FIG. 3 in conjunction with FIG. 4, the outer casing  22  of shaft seal  10  includes an elastomeric radially outer hub engagement ring  30 , a primary metallic reinforcing ring  32 , a primary elastomeric sealing ring  34 , and a secondary metallic reinforcing ring  36 . The outer hub engagement ring  30  is formed with a pair of axially spaced apart circumferential compression ribs  35  and  37  which are configured to be compressed, as much as 50% of their radial height, against the surface of the interior bore  12  of wheel hub  14  during installation. The hub engagement ring  30  is bonded to the outer surface of the axially extending portion  32   a  of the primary reinforcing ring  32 . The primary sealing ring  34  includes a radially extending portion  34   a  positioned against the interior surface of the radially extending portion  32   b  of primary reinforcing ring  32  and an axially extending portion  34   b  which is biased radially inwardly by an annular garter spring  38 . Garter spring  38  urges the axially extending portion  34   b  of sealing ring  34  into sealing contact with the radially outer surface of the axially extending portion  26  of wear sleeve  24  to effect a seal therebetween. As a result, the axially extending portion  34   b  of the elastomeric sealing ring  34  is in continuous sealing engagement with the outer surface of the axially extending portion  26  of wear sleeve  24  over a substantial contact area. 
     With continuing reference to FIG. 4, the secondary reinforcing ring  36  of hub seal  10  includes a radially extending portion  36   c  and a radially extending lip portion  36   a  which are connected by an angled axially extending portion  36   b.  The radially extending portion  36   a  supports garter spring  38  in its operative position, and the radially depending lip portion  36   c  compresses the radially extending portion  34   a  of primary sealing ring  34  against the interior surface of the radially extending portion  32   b  of primary reinforcing ring  32 . The primary reinforcing ring  32  includes a radially depending lip portion  32   c  which engages a radially inset land area  36   d  formed between the axially extending portion  36   b  and radially extending portion  36   a  of secondary reinforcing ring  36 . The engagement of these two structure facilitates integration of the outer casing  22  of shaft seal  10 . 
     An elastomeric rib structure  40 , preferably formed integral with hub engagement ring  30 , is bonded to the outer surface of the radially extending portion  32   b  of primary reinforcing ring  32 . Rib structure  40  defines a flexible lip  42  configured to decrease the running torque between the deflector ring  28  and the outboard side of the casing  32  during vehicle operation. Rib structure  40  further defines a flexible retaining lip  44  delimiting the inner wall of a circumferential canal  46 , the outer wall of which is delimited by an axial extension  30   a  of hub engagement ring  30 . 
     A polymeric thermoplastic material, such as, for example, polypropylene, is deposited into the circumferential canal  46 , preferably by means of a conventional injection process, to form a thrust ring  50  upon hardening and setting. The polymeric material which forms thrust ring  50  has adhesive characteristics which serve to effect bonding of the material to the structural surfaces defining the circumferential canal  46 . In addition, an upstanding lip  48  projects into canal  46  from race structure  40  to increase the bonding surface area for the polymeric/adhesive material of thrust ring  50 . 
     Thrust ring  50  is configured to act as a conventional spring-back bumper element to absorb the axial load exerted on shaft seal  10  when it is press-fit onto axle shaft  16  during installation. In accordance, with a preferred embodiment of the subject invention, thrust ring  50  is a machinable sacrificial structure configured to be partially sheared away during the break-in period of shaft seal  10 . More particularly, the deflector ring  28  of wear sleeve  24 , is configured to mechanically remove at least a portion of thrust ring  50  during break-in to create a running clearance between the deflector ring  28  and the thrust ring  50 . Specifically, deflector ring  28  has a generally planar surface  28  which is initially compressed against thrust ring  50  during installation, and which, through rotation during the break-in period of the shaft seal, functions to machine away a portion of the thrust ring by shearing action. 
     During the break-in period, there is an initial increase in start-up torque due to the presence of the thrust ring  50 . However, after only a few revolutions of the wheel hub, the torque applied to the shaft seal rapidly decreases as the thrust ring is machined away to create a running clearance. In addition, because the running clearance is created in a relatively short period of time, there is not a substantial increase in frictional heat generation in the area of the thrust ring. Moreover, during the break-in period, the temperatures at the interface between the thrust ring and the deflector ring will remain well below the melting point of the polymeric material from which the thrust ring is formed, thereby maintaining the integrity of the shaft seal. 
     In a preferred embodiment of the subject invention, because the material from which thrust ring  50  is formed has adhesive characteristics, which are enhanced as the temperature of the material increases, a lubricant, in the form of a releasing material, such as, for example, Krytox™ is provided on the planar machining surface of deflector ring  28 , in the form of a coating designated by reference numeral  60 , to prevent material parted from the thrust ring  50  from bonding with the deflector ring  28  during the break-in period (See FIG.  2 ). As set forth hereinabove, the radially inner wall of the circumferential canal  46  is delimited by a retaining lip  44 . This structure is angled in a radially outward direction to direct the movement of material parted from thrust ring  50  in a radially outward direction, thereby preventing contamination of shaft seal  10 . In addition, retaining lip  44  serves to decrease running torque in conjunction with flexible lip  42 . 
     Referring now to FIGS. 4 through 6, there is illustrated, in sequential order, a rendering of the manner in which a portion of thrust ring  50  is mechanically removed by the deflector ring  28  when the outer casing  22  and wear sleeve  24  rotate relative to one another during the break-in period of shaft seal  10 . More particularly, as illustrated in FIG. 4, during installation of the shaft seal and prior to break-in, thrust ring  50  remains wholly intact, but in a slightly compressed state in response to the forces exerted thereupon by the deflector ring during installation. At such a time, a desirable degree of spacing is maintained between the casing and the deflector ring. Then, as illustrated in FIG. 5, when break in begins, so does the mechanical removal of the thrust ring  50  by the machining surface of the deflector ring  28 . 
     As illustrated in FIG. 6, after only a few wheel revolutions, a minimal running clearance is created between thrust ring  50  and defector ring  28 , and the torque applied to the shaft seal falls to a normal running level, minimized by flexible lip  42  and retention lip  44 . During the break-in period, the temperature of the shaft seal in the area of thrust ring  50  remains well below the melting point of the polymeric material from which the thrust ring is formed, thus ensuring the integrity of the shaft seal of the subject invention. 
     Referring now to FIGS. 7 and 8, there are illustrated two other shaft seals constructed in accordance with a preferred embodiment of the subject invention and designated generally by reference numerals  100  and  200 , respectively. Shaft seal  100  includes an outer casing  122  which is constructed in the same manner as outer casing  22  of shaft seal  10 . However, the wear sleeve  124  of shaft seal  100  differs from the wear sleeve  24  of shaft seal  10  in that it includes four circumferentially spaced apart radially inwardly extending notches  128   a  through  128   d  formed in a radially outer portion of the deflector ring  128 , as illustrated in FIG. 7, for mechanically removing at least a portion of a machinable thrust ring provided on the outboard side of outer casing  122 . Similarly, as illustrated in FIG. 8, the wear sleeve  224  of shaft seal  200  differs from the wear sleeve  24  of shaft seal  10  in that it includes four circumferentially spaced apart tangentially extending flats  228   a  through  228   d  formed in a radially outer portion of the deflector ring  128  for mechanically removing at a least a portion of a machinable thrust ring provided on the outboard side of outer casing  222 . 
     Referring now to FIG. 9, there is illustrated another hub seal constructed in accordance with a preferred embodiment of the subject invention and designated generally by reference numeral  300 . Hub seal  300  includes an outer annular casing  322  and an inner metallic wear sleeve  324 . As in the previously described embodiments, the wear sleeve and casing form a two-part, semi-unitized seal assembly for use on the wheel end of a vehicle. Moreover, the wear sleeve is designed to be press-fit onto the outer diameter of a vehicle axle shaft. 
     As best seen in FIG. 10, the outer casing  322  of hub seal  300  includes a composite sealing ring  334  which includes an elastomeric sealing portion  335  and a circumferential metallic support portion  337 . The two structural portions of sealing ring  334  are preferably molded to one another during an injection molding process. Alternatively, the two structural components of composite sealing ring  334  may be bonded all to one another using conventional techniques. The elastomeric sealing portion  335  of composite sealing ring  334  includes a radial sealing lip  339  of the type commonly refereed to as a lay-down lip seal which is dimensioned and configured to sealingly engage the outer surface of the axially extending portion  326  of wear sleeve  324  over a substantial contact area having an axial length of about between 0.020 inches and 0.120 inches. An annular garter spring  338  biases sealing lip  339  into sealing engagement with the outer surface of the axially extending portion  326  of wear sleeve  324  by providing radially inwardly directed forces. 
     As best seen in FIG. 10, the outer casing  322  of hub seal  300  includes an outer hub engagement ring  330  and an inner secondary reinforcing ring  336 . Hub engagement ring  330  includes a radially extending portion  330   a,  an axially extending portion  330   b  and a radially depending portion  330   c.  Reinforcing ring  336  includes a radially extending portion  336   a,  an axially extending portion  336   b  and radially depending portion  336   c.    
     In the assembled condition illustrated in FIG. 10, the radially depending portion  330   c  of engagement ring  330  mechanically interfaces with the radially extending portion  336   a  of reinforcing ring  336 . In addition, the axially extending base portion of composite sealing ring  335  is interposed within the gap that exists between the radially depending portion  336   c  of reinforcing ring  336  and the radially extending portion  330   a  of engagement ring  330 , together with a compressible abutment ring  341  which acts as a shim for the radially depending portion  336   c.  As in each of the previously described embodiments, hub seal  300  includes a machinable thrust ring  350  which is deposited in a circumferential canal  346  formed in the outer casing which serves as a sacrificial bumper element that is mechanically removed by the radially extending portion  328  of wear sleeve  324  during the break-in period of the seal assembly. 
     Referring now to FIG. 11, the composite sealing ring  335  is molded in such a manner so as provide the lay-down sealing lip  339  with a particular trim height, approach angle, molded lip angle and trim angle to maximize the extent of the axial sealing contact at the interface between the elastomeric sealing lip and the sealing surface of the wear sleeve. A molded lip seal constructed in this manner has a trimmed inboard end surface providing a radiused leading edge  345 , as illustrated, for example, in FIG.  10 . In a preferred embodiment of the subject invention, the trim height of the lip is about 0.339±0.008 inches, the approach angle is about between 20° and 30°, the molded lip angle is about 100°±0.5° and the trim angle is about 20°±0.003° relative to a plane extending perpendicular to the axis of the seal. Also in a preferred embodiment of the subject invention, the sealing lip  339  at its smallest diameter is about between 0.020 inches and 0.100 inches smaller than the axial sealing member with which it makes contact, such that a radial force component exists between the two contacting elements which causes the sealing lip to lay down on the axial sealing member over a portion of its length. 
     With continuing reference to FIG. 11, the inner sealing surface of sealing lip  339  is provided with a plurality of spaced apart projections  375  which serve as hydrodynamic pumping aides to facilitate the movement of lubricant fluid on the axle shaft in an axial direction toward and back into the bearing chamber of the wheel hub. Projections  375  are molded integral with the sealing lip and preferably have a trapezoidal geometry with the lateral sides thereof being inclined at an angle with respect to the axis of the hub seal assembly. Alternatively, the projections can have triangular or rod-like geometries, as shown, for example in FIGS. 16 through 18, and can be attached to one another by a raised bridging structure. In a preferred embodiment of the subject invention, the spaced apart projections are located at about between 0.020 inches and 0.075 inches from the radially inner edge of the sealing lip. Preferably, the projections are raised about 0.002 inches from the surface of the sealing lip. 
     Referring now to FIGS. 12 and 13, there is illustrated another hub seal assembly constructed in accordance with a preferred embodiment of the subject invention and designated generally by reference numeral  400 . Hub seal assembly  400  includes a stepped annular casing  422  and a wear sleeve  434 . Casing  422  has first and second casing portions  422   a  and  422   b  which are overlaid exteriorly by an elastomeric hub engagement shell  430 , which, in part, defines the radially outer boundary of a circumferential canal  446  within which may be deposited a machinable thrust ring of the type described in connection with the previously described embodiments of the invention. 
     The first portion of casing  422  encloses an annular lip seal cartridge which is designated generally by reference numeral  500  and separately illustrated in FIGS. 14 and 15. Lip seal cartridge  500  includes an outer support ring  530  and a cooperative inner reinforcing ring  536 . Outer support ring  530  includes a radially extending portion  530   a,  an axially extending portion  530   b  and a radially depending portion  530   c,  while inner reinforcing ring  536  includes a radially extending portion  536   a  and an axially extending portion  536   b.  The radially depending portion  530   c  of support ring  530  mechanically interfaces with a shoulder portion  536   c  of reinforcing ring  536  to facilitate unitization of lip seal cartridge  500 . 
     With continuing reference to FIG. 15, in conjunction with FIG. 13, cartridge  500  further includes an elastomeric sealing ring  534  which includes a base portion  537  and a radial sealing lip  539 . In contrast to the composite sealing ring  334  of hub seal assembly  300  described hereinabove, the base portion  537  of elastomeric sealing ring  534  is secured to the radially inner end of the radially extending portion  530   a  of support portion  530  by conventional bonding methods known in the art. The radial sealing lip  539  is molded in such a manner, so as to form, what is commonly referred to as a lay-down lip seal. Moreover, it is dimensioned and configured in such a manner so as to sealingly engage the outer surface of the axially extending portion  426  of wear sleeve  434  over a substantially continuous contact area. As in the previously described embodiments, an annular garter spring  438  is provided to bias the elastomeric sealing lip  539  into sealing engagement with the outer surface of the axially extending portion  426  of wear sleeve  424  by providing radially inwardly directed forces. 
     Referring to FIG. 14, the inner sealing surface of radial sealing lip  539  is provided with a plurality of spaced apart projections  575  which serve as hydrodynamic pumping aides to facilitate the movement of lubricant fluid on the axle shaft in axial direction toward and back into a lubricant reservoir. Projections  575  are molded integral with sealing lip  539  and have a trapezoidal geometry. As in the previous embodiment, the molded projections can have alternative geometries, as illustrated for example in FIGS. 16 through 18. 
     Although the hub seal of the subject invention has been described with respect to a preferred embodiment, it is apparent that modifications and changes can be made thereto without departing from the spirit and scope of the invention as defined by the appended claims.