This is a Continuation in Part of commonly assigned co-pending patent application Ser. No. 09/347,680 entitled “On Axle Tire Inflation System” filed Jul. 2, 1999, now abandoned the teachings of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to on-axle tire inflation systems, and more particularly to an on-axle tire inflation system utilizing an axle and oil cap assembly air distribution system for trucks and other vehicles. 
     2. Description of the Related Art 
     One problem associated with pneumatic tires, especially in tractor trailer systems, involves maintaining proper or appropriate tire air pressure. A low pressure situation in a tire can often go undetected for a long period of time. While it is prudent to periodically check tire inflation pressure with an air pressure gauge, such checks are sporadic at-best. A low pressure tire can cause many problems, as can, a high pressure tire. 
     The concept of providing an on-axle tire inflation system, or what is known as a central tire inflation system (CTIS) or automatic tire inflation system (ATIS), is well known in the art. Such systems remotely or automatically inflate a pneumatic tire rotatably mounted onto the axle of a vehicle via an air distribution system. Typically, an on-board source of pressurized air, such as compressed air from the vehicle air brake compressor and/or compressed air reservoir, is used to fill the tire and maintain tire pressure. In this manner, the pneumatic tire may be inflated without the need to remove the tire from the axle. Some systems may also provide for the deflation of the tire or air pressure monitoring. 
     Such on-axle tire inflation systems have been incorporated into many types of vehicles such as tractor-trailers and off-road vehicles. By incorporating such systems into the vehicle, tire pressure associated problems may be alleviated. These systems usually include a rotary union coupling air from a fixed member to a member rotating with the wheels. These rotary unions are key components and often subject to premature failure. 
     However, such prior art systems are generally complicated and/or cumbersome. Additionally, in the case of truck trailers, most truck trailers are provided with a lubrication compartment at the end of the axle for containing lubricant for the wheel bearings. Such systems may include an oil sight glass and an oil plug centered on the site glass for filling and inspecting the lubricant level in the lubrication compartment. A tire inflation system must therefore also incorporate or accommodate such a lubrication system. 
     Some prior art solutions also fixe critical components outside the hubcap thereby exposing these components making them susceptible to damage and requiring unnecessary maintenance and repair. An effective automatic tire inflation system reduces tire maintenance. However, it is counter productive to install an inflation system that creates additional maintenance and/or repair. There is desired an improved tire inflation system that has an improved rotary union, is easy to install and requires little maintenance. 
     SUMMARY OF THE INVENTION 
     The present invention achieves technical advantages as a tire inflation system having a rotary union seated and protected in a hollow axle, which system is easily mounted, and required little maintenance. 
     In a preferred embodiment thereof, the present invention is a tire inflation system for a vehicle having pneumatic tires mounted onto a hollow axle. The tire inflation system includes a source of compressed air, coupled to a hollow axle, an air interface device having a rotary union, and a oil cap assembly adapted to carry at least one tire. The air interface system is advantageously adapted to have the rotary union protected within the hollow axle to protect this key component from outside elements and failures, such as oil leaks and brake dust. A unique wedge mechanism secures the air interface system against the inner wall of the hollow axle, providing for quick installation and maintaining integrity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a front plan view of one side of an axle and a sectional view of a oil cap assembly that is mountable with respect to the axle in accordance with a first preferred embodiment of the present invention; 
     FIG. 2 is a sectional view of the rim of FIG. 1; 
     FIG. 3 is a front view of the rim of FIG. 2; 
     FIG. 4 is a rear view of the rim of FIG. 2; 
     FIG. 5 is an enlarged, side sectional view of the bearing cap of the rim assembly of FIG. 1; 
     FIG. 6 is a front view of the bearing cap of FIG. 5; 
     FIG. 7 is a rear view of the bearing cap of FIG. 5; 
     FIG. 8 is a side sectional view of a oil cap assembly having an air interface system according to a second preferred embodiment that is mountable within the hollow axle; 
     FIG. 9 is an exploded side view of all components extended for ease of component description for the embodiment shown in FIG. 8; 
     FIG. 10 is a 3-dimensional view of one side and end of the hubcap and all components interior to the hubcap and axle of the embodiment of FIG. 8; and 
     FIG. 11 is a side sectional view of the oil cap assembly mounted to a hollow axle with the rotary union being protected within the hollow axle. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrate a preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and more particularly to FIG. 1, there is shown one side of a vehicle axle, generally designated  12 , having a rotor  14  or the like fixedly mounted thereon and internal bore or hollow  18 . Threads, indicated at  20 , are also provided at end  16  of axle  14 . Axle  14  is a depiction of one end of a conventional axle of the type typically provided on trailers of tractor-trailer rigs (not shown). However, it should be appreciated that the present invention is not limited to tractor-trailers, but may be applied to any axle/pneumatic tire vehicle. 
     Coupled to axle  12  via fitting  21  is air supply system  22  that provides pressurized air from a remote air tank or reservoir  23  to bore  18  of axle  12 . Tank  23  represents an on-board source of pressurized air such as compressed air from the vehicle air brake compressor and/or compressed air reservoir. However, it should be understood that the source of pressurized air is not limited to these examples, and could be any type of on-board compressed air source or external compressed air source with the appropriate coupling interface. Air supply system  22  includes incoming air supply line or conduit  24  which is in fluid communication with remote air tank or reservoir  23  or other source of pressurized air. Incoming air supply line  24  is coupled to the inlet of on/off valve  26  such as a ball valve which is in turn coupled to the inlet of air regulator  28 . 
     Air regulator  28  is adjustable either manually or automatically, to provide air at a regulated pressure. At the outlet of air regulator  28  is air control orifice  30  that is coupled to axle air line  32  which is in turn coupled to fitting  21  affixed to axle  12 . Thus, incoming pressurized air is directed through line  24 , into air regulator  28 , and then into bore  18  of axle  12  via line  32  at a pressure determined by air regulator  28  and air control orifice  30  and suitable for inflation pressure of a pneumatic tire. 
     Mounted to axle  12  and rotor  14  is oil cap assembly which carries at least one (1) pneumatic tire (not shown) and preferably two (2) pneumatic tires (not shown) mounted thereon in a conventional side-by-side manner as is typical in tractor-trailer rigs. Oil cap assembly  34  generally includes rim body  36 , bearing or rotary union seal cap  44 , and lubrication retention/seal assembly  50 . Retention/seal assembly  50  includes gasket  52 , sight glass  53 , retainer ring  54 , and plug  55  which together provide sealing for lubrication within rim body  36  and a visual inspection of the lubrication level therein. Gasket  52  is generally annular in shape and rests upon annular surface or ledge  56  of rim body  36 . Disposed over gasket  52  is sight glass  53  for allowing visual inspection of the lubrication level. Retainer ring  54  is disposed over sight glass  53  and is secured to rim body  36  via -screws (not shown) that are received in threaded bores  57 . Retainer ring  54  also holds plug  55 . 
     Referring now to FIGS. 2-4 rim body  36  is depicted. Rim body  36  is generally tubular-shaped with a center or central hub portion  40  having a first radial spoke  58 , a second radial spoke  59 , an elongated projection  60  extending along an axial direction relative first and second spokes  58  and  59 , and a short projection  61  extending along an axial direction relative first and second spokes  58  and  59  opposite elongated projection  60 . Rim body  36  along with center hub portion  40  defines internal cavity  42  that is open at one end. Rim body  36  additionally includes four radially outwardly projecting fins  38  for aiding in the retention of the tires (not shown). 
     Elongated projection  60  has internal bore  64  with graphite  63  that retains O-ring  66  and washer  67 . Positioned within internal bore  64  is spring  68  that abuts washer  67  and is retained on one end by NPT plug  70 , preferably being a ⅜ NPT plug. Disposed at the end of projection  60  is a square knob  65 . Extending through first spoke  58  and in communication at one end with bore  64  is first spoke bore  72 . 
     At the other end of first spoke bore  72  is a first air filter  74  and first quick connect  76 . Extending through second spoke  59  and in communication at one end with bore  64  is second spoke bore  78 . At the other end of second spoke bore  78  is a second air filter  80  and second quick connect  82 . Thus, there is fluid communication between cavity  42  and first and second quick connects  76  and  82 . Additionally, rim body  36  includes a peripheral, radially extending skirt  84  having a plurality of mounting bores  85 . Screws or the like (not shown) are used to attach rim body  36  to rotor  14  via mounting bores  85 . 
     As depicted in FIG. 1, bearing or union seal cap  44  is disposed adjacent knob  65  and thus bore  64 . Referring now to FIGS. 5-7 bearing or union seal cap  44  is depicted. Bearing cap  44 , preferably made of a suitable metal, is defined by bearing cap body  45  of an octagonal shape. Bearing cap body  45  has center hub  88  having bore  90  therethrough and outer portion  96  that radially surrounds center hub  88 . Threads  98  are disposed on an inner wall of outer portion  96  which are sized to correspond to threads  20  of axle  12 . Circumferential groove  93  is disposed in an outer surface of center hub  88  which holds O-ring  92 . Bearing cap body  45  further includes knob  94  on rear surface  95  through which bore  90  extends. 
     With reference again to FIG. 1, the manner of connection and operation of the present invention will be hereafter described. Rim body  36  is mounted to rotor  14  via studs, bolts or screws through bores  85  in peripheral skirt  84 . Preferably, rim body  36  is mounted to rotor  14  via studs  81  and through bores  85  have a tapered contour to allow easy aligning and attachment therebetween. At the same time, bearing cap  44  is threadedly received onto axle end  16  such that center hub  45  is received into bore  18  of axle  12 . O-ring  92  provides sealing when center hub  45  is received into bore  18 . When rim body  36  is mounted to rotor  14  and bearing cap  44  is threadedly received onto axle end  16  bore  48  of bearing cap  44  is in fluid communication with bore  18  of axle  12 . Additionally, knob  94  of bearing cap  44  abuts knob  65  such that bore  48  is in fluid communication with bore  64 . Thus, compressed air from tank  23  flows through line  24 , into pressure regulator  28 , through line  32  and into axle bore  18 . The compressed air then travels into bores  48  and  64 , into spoke bores  72  and  78 , through air-filters  74  and  80  then quick connects  76  and  82  respectively. Coupled to quick connect  74  is air conduit  77  that feeds an inner tire (not shown) of the twin tire pair such as on a tractor-trailer by connection to the valve stem (not shown) of the tire. Coupled to quick connect  82  is air conduit  83  that feeds an outer tire (not shown) of the twin tire pair such as on a tractor-trailer again by connection to the valve stem (not shown) of the tire. Since air conduits  77  and  83  are coupled to the valve stems of the mounted tires, constant air pressure may be maintained within the tires without the need for periodic checking. Of course, the principles of the present invention may be utilized for a single tire of a single rim system, or may be adapted for multiple tires. 
     More particularly, air conduits  77  and  83  are each connected to a quick connect assembly  110  (the quick connect assembly  110  connected with air conduit  83  being shown partially fragmented for illustration). Each quick connect assembly  110  includes mating male and female connectors, similar to conventional pneumatic fittings. However, each quick connect assembly also includes a one-way check valve  112  which only allows air to flow toward the tire. Thus, if the air pressure within the tire becomes low, the pressure differential allows higher pressure air to flow into the tire. Moreover, each quick connect assembly can be disconnected from the tire without losing pressure from within air conduit  77  or  83 . Each quick connect assembly also includes a female threaded portion  114  which crews directly onto a valve stem of the tire, and a projection  116  which holds the valve within the valve stem in an open position. 
     Referring now to FIG. 8, in view of FIG. 1, there is shown generally at  200  an on-axle tire air interface and inflation system mountable to rotor  14  according to a second preferred embodiment of the present invention. System  200  is seen to include a generally cylindrical oil cap  202  tapering from a proximal end  204  to a distal end  206  and having an annular outer surface  208  as shown. The oil cap  202  is seen to have an axially aligned passageway  210  in fluid communicating with a pair of radially extending passageways  212  and  214 , as shown, for communicating compressed air from a pneumatic source  23  (see FIG. 11) to quick connect/disconnects  110 . 
     The tire inflation system  200  according to this embodiment derives technical advantages in that the air interface device including a rotary union “RU” is sealingly disposed within the hollow axle and is thus not susceptible to damage from oil leaks or other failures of the oil cap assembly or the braking mechanism (not shown). 
     Referring to FIG.  8  and FIG. 9, the system  200  is seen to include a compression fitting and retainer  220  having a threaded counter bore  222  threadively adapted and secured to a threaded axial member  224  extending from the inner surface of the oil cap  202  as shown. Compression fitting  220  is seen to have an axial opening  226  extending therethrough receiving in a frictional secured arrangement a flexible nylon tubing  230 . Tubing  230  has a passageway  232  extending along the length thereof and is in fluid communication at one end with cavity  210  of cap  202 . The inner diameter of fitting opening  226  and the outer diameter of compression sleeve  230  are both preferably about ¼ inches. 
     A compression sleeve  240  is seen to be secured about a proximal end of tubing  230  at proximal end thereof. A generally cylindrical rigid seal shaft  242  has an air passageway  244  extending along the length thereof, as shown. The distal end of shaft  242  is seen to have a reduced diameter portion  246  having an outer diameter such that the distal end of shaft  242  can be frictionally inserted and sealingly secured within the opening  232  of tubing  230 . Shaft  242  is preferably comprises of a rigid material such as aluminum or other suitable rigid materials. 
     Still referring to FIG.  8  and FIG. 9, an annular carbon graphite seal  250  has a distal surface  252  adapted to abut and frictionally engage a proximal surface  254  of shaft  242 . As will be discussed in more detail shortly, graphite seal  250  remains stationary and abuts against rotating shaft  242  at a Rotary Union (RU) and minimizes friction therebetween due to graphite material characteristics. The opening defined through graphite seal  250  interfaces with and communicates air pressure therethrough to the rotating shaft passageway  244 . A cylindrical piston shown at  260  has a passageway  262  extending along the length thereof and a pair of O rings  264  defined in annular groves defined about piston  260 . Piston  260  is seen to have a distal end of reduced diameter at  266  which securingly and sealingly is disposed within the proximal recess  254  of graphite seal  250 . The O rings  264  provide a primary and secondary seal within bolt  282 , and preferably have a {fraction (5/16)}th inch diameter as will be discussed in more detail shortly. 
     An annular O ring seal is shown at  270  and has a diameter such that it can be securingly and sealingly coupled about the proximal end of piston  260 , and seated within a recess  280  of a threaded bolt  282 . Bolt  282  is also seen to have an inner shouldered portion shown at  288  adapted to abut against the proximal surface of graphite seal  250 , as shown in FIG.  8 . Radial seal  270  provides an air tight seal about the proximal end of piston  260 , and also an airtight seal within the distal end  280  of the bolt  282  such that air is communicated along the length thereof through the respective passageways without leaking. The proximal outer surface of bolt  280  is threaded, as shown at  284 , and is adapted to threadably receive a wedge nut shown at  290 . Wedge nut  290  has a threaded inner surface  292  and an annular tapered outer surface  294  tapering towards the distal end thereof. 
     Wedge nut  290  forms a sub-part of an axle plug assembly generally shown at  300 . The other parts of the axle plug assemble  300  include a tapered plug expander  302 , a flanged resilient seal  304 , an O ring  306 , and a wedge plate shown at  308 . Wedge plate  308  has an inwardly tapered inner surface  310  commensurate with the outer tapered surface  294  of wedge nut  292 , as shown. The seal  304  also has a tapered inner wall shown at  312  which is commensurate with a tapered outer surface  314  of the tapered plug expander  302 . Both the tapered plug expander  302 , the seal  304 , the wedge plate  308  and the wedge nut  290  all have an opening axially extending therethrough to facilitate the sealed communication of pneumatic air pressure from the axle  12  to the oil cap passageways  212  and  214 , as shown in the assembled view in FIG.  8 . 
     Shown in FIG. 11 is the vehicle air supply system  200  receiving pressurized air from the remote air tank or reservoir  23  via the hollow axle  12 . The air supply  23  is regulated to provide a supply of air at a constant pressure, manually or automatically set based on desired pneumatic tire pressure. The air output of the regulator is directed through the fitting  21  into a bore  326  in the axle  12 . The air flows from the entry point near the center of the axle  12  in an outward direction toward the outer ends of the axle  12 , to the assembly  200  and ultimately to the tires (not shown) via quick connect/disconnects  110 . 
     Installation 
     As seen in FIGS. 8,  9 ,  10  and  11 , components  290 ,  302 ,  304 ,  306  and  308  comprise the axle plug assembly  300  which provides a fixed plug at the end of the axle  12  so that air traveling through the hollow axle (from a point at the center of the axle toward the outer ends of the axle) may only escape the axle through the opening  286  in the center of bolt  282 . This axle plug assembly  300  differs from other plug seals in its ease of installation. 
     To install the axle plug assembly  300 , bolt  282  comes shipped with parts  302 , 304 ,  306  and  308  assembled on bolt  282  and with wedge nut  290  screwed loosely onto bolt  282 , but fictionally in place with respect to plate  308 . To install the axle plug assembly  300 , the assembly  300  is inserted into the respective end of the axle  12 . Bolt  282  is turned until secure within the axle  12 . As bolt  282  is tightened, wedge nut  290  responsively forces wedge plate  308  to expand outwardly and create a first secure seal against the internal axle wall  330 . Wedge plate  308  is also laterally forced to abut against seal  304 . Seal  304  is produced from a highly expandable plastic compound such as ST  801 . As wedge plate  308  is forced laterally against seal  304 , bolt  282  pulls expander  302  into seal  304  and engages tapered wall  312  such that it is forced outwardly to expand over the tapered portion  302  of plug expander  302 , forming a primary air seal. O-ring  306  provides a secondary airtight seal against the axle inner wall, shown at  330 . The result is an easily installed, airtight and secure seal at the end of each axle  12 . Air needed to inflate the tires must travel through the opening in the center of bolt  282  from the center of the axle axially toward the outer ends of the axle. 
     Compression fitting and retainer  220  secures the flexible nylon tubing  230  to the oil cap  202 . Nylon tubing  230  is a flexible tube which provides an aid in obtaining perpendicularity between the air channel  260  entering the cap  202  and the rotary union “RU” disposed inside the axle  12 , the rotary union “RU” consisting of rotating shaft  242  and carbon graphite seal  250 . The flexible tube  230  helps provide an airtight seal at the rotary union “RU”. When the rim  202  is installed over the end of the axle  12 , the rotating shaft  242  penetrates seal  270  and is prepared to make contact with carbon graphite seal  250 . 
     Air enters the center of the axle  12  from pneumatic source  23  via tubing  24  into chamber  328  of the axle  12 . The air applies pressure to piston  260 , which air pressure forces piston  260  to move axially on O-rings  264  in the direction towards the ends of the axle  12 . Piston  260  ceases to move axially when the carbon rotating shaft  242  comes in contact with and abuts with graphite seal  250 . The air pressure against piston  260  creates an airtight seal at the rotary union “RU”. As the wheels (not shown) on the vehicle turn, the carbon graphite seal  250  remains fixed while seal shaft  242  rotates flush against the carbon graphite seal  250 , while providing a sealed passageway for the air to move from the stationary axle chamber  328  to the rotating cap  202  with an airtight seal at the union “RU”. 
     The rotary union “RU” is advantageously placed in the passageway  328  of axle  12  in order to eliminate damage to the rotary union seal “RU” from oil leaks and brake dust, or from mechanical shearing, which damage is possible with other prior devices placed on the outside of the oil cap. 
     Compressed air travels from an air tank or reservoir  22 , through the fitting  21  at approximately the center of an axle(s)  12 , through the axle bore  326 , through bolt  282  permanently fixed in the end of the axle bore  12 , through the rotary union members  242  and  250 , and through flexible tube  230 . 
     The compressed air then travels through passageways  212  and  214  and through air conduit  77  and  83 . Coupled to air conduit  77  is the quick connect  110  that feeds an inner tire (not shown) of the twin tire pair such as on a tractor-trailer by connection to the valve stem (not shown) of the tire. Coupled to air conduit  83  is quick connect  110  that feeds an outer tire (not shown) of the twin tire pair by connection to the valve stem (not shown) of the tire. Since air conduits  77  and  83  are coupled to the valve stems of the mounted tires, constant air pressure may be maintained within the tires without the need for periodic checking. Of course, the principles of the present invention may be utilized for a single tire of a single rim system, or may be adapted for multiple tires. 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.
A vehicle air supply system ( 200) providing pressurized air from an axle ( 12) of a vehicle to rotating tires to maintain a desired pneumatic pressure. The air supply system is adapted to be sealingly disposed within the hollow axle of the vehicle, having a rotary union (RU) disposed therein protecting the rotary union from external conditions such as oil leaks or brake shavings. The air supply system includes an axle plug assembly ( 300) which is seated within the hollow axle ( 12), and then having resilient members expanded therewithin to provide an air tight seal. The axle plug assembly includes a slidable piston member ( 260) is axially movable within a plug assembly bolt ( 282) to facilitate adapting the device to vehicles having different axle lengths. A graphite seal ( 250) is provided to minimize the friction between a rotating shaft coupled to the oil cap ( 202), and the stationary axle plug assembly.