Patent Publication Number: US-2022234396-A1

Title: Rotary union with energy harvesting structure

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 63/141,044 filed on Jan. 25, 2021 
    
    
     BACKGROUND 
     Technical Field 
     The disclosed subject matter relates to tire inflation systems for heavy-duty vehicles, such as tractor-trailers or semi-trailers. More particularly, the disclosed subject matter relates to a rotary union utilized in a tire inflation system for a heavy-duty vehicle. Still more particularly, the disclosed subject matter is directed to a rotary union for a heavy-duty vehicle tire inflation system that includes energy harvesting structure integrated into the rotary union for energizing electronic components associated with a wheel end of the heavy-duty vehicle, such as a wheel end sensor, thereby eliminating the need for disposable energy sources, such as batteries, and minimizing vehicle maintenance associated with such components, thus reducing vehicle downtime. The rotary union of the disclosed subject matter also eliminates the need for other energy saving strategies employed with such electronic components when disposable energy sources are utilized, such as limiting functionality of the electronic components under certain circumstances in order to maximize battery life, thus improving the overall functionality and life of the components. In addition, the energy harvesting structure is housed within and protected by the rotary union, and components within the wheel end assembly are protected from the energy harvesting structure, thereby minimizing potential damage to the energy harvesting structure of the rotary union and/or other components of the wheel end assembly if components of the energy harvesting structure become defective, as well as decreasing packaging space and overall vehicle weight, and thus decreasing the cost associated with employing energy harvesting structures in the wheel end of the heavy-duty vehicle. 
     The use of tire inflation systems in heavy-duty vehicles has been very popular for many years. Heavy-duty vehicles typically include trucks and tractor-trailers or semi-trailers, and trailers thereof. Reference herein is made to heavy-duty vehicles for the purpose of convenience, with the understanding that such reference includes trucks, tractor-trailers and semi-trailers, and trailers thereof. Each heavy-duty vehicle generally includes a frame, from which at least one axle is suspended. A wheel end assembly is rotatably mounted on each end of the axle. More specifically, each wheel end assembly typically includes a wheel hub rotatably mounted on a bearing assembly that in turn is immovably mounted on a respective one of each of the ends of the axle, commonly known as an axle spindle. In this manner, the bearing assemblies enable each wheel hub to rotate about a respective axle spindle. A hubcap is attached to the outboard end of the wheel hub and seals the outboard end of the wheel end assembly. One or more tires in turn are mounted on the wheel hub in a manner known in the art. All heavy-duty vehicles include multiple tires, each of which is inflated with a fluid or gas, such as air, to an optimum or recommended pressure. This optimum or recommended tire pressure typically is referred to in the art as the target inflation pressure or the target pressure. 
     However, it is well known that air may leak from a tire, usually in a gradual manner, but sometimes rapidly if there is a problem with the tire, such as a defect or a puncture caused by a road hazard. As a result, it is necessary to regularly check the air pressure in each tire to ensure that the tires are not significantly below the target pressure and thus under-inflated. Should an air check show that a tire is under-inflated, it is desirable to enable air to flow into the tire to return it to the target pressure. Likewise, it is well known that the air pressure in a tire may increase due to increases in ambient air temperature, so it is necessary to regularly check the air pressure in each tire to ensure that the tires are not greatly above the target pressure, and thus over-inflated. Should an air check show that a tire is over-inflated, it is desirable to enable air to flow out of the tire to return it to the target pressure. 
     The large number of tires on any given heavy-duty vehicle setup makes it difficult to manually check and maintain the target pressure for each and every tire. This difficulty is compounded by the fact that heavy-duty vehicles in a fleet may be located at a site for an extended period of time, during which the tire pressure might not be checked. Any one of these heavy-duty vehicles might be placed into service at a moment&#39;s notice, leading to the possibility of operation with under-inflated or over-inflated tires. Such operation may increase the chance of less-than-optimum performance and/or reduced life of a tire in service as compared to operation with tires at the target pressure, or within an optimum range of the target pressure. Moreover, should a tire encounter a condition during operation of the heavy-duty vehicle that causes the tire to become under-inflated, such as developing a leak from striking a road hazard, or over-inflated, such as increasing pressure from an increased ambient air temperature, the life and/or performance of the tire may be significantly reduced if the under-inflation or over-inflation continues unabated during continued operation of the heavy-duty vehicle. The potential for significantly reduced tire life typically increases in heavy-duty vehicles that travel for long distances and/or extended periods of time. 
     Such a need to maintain the target pressure in each tire, and the inconvenience to the vehicle operator to manually check and maintain a proper tire pressure that is at or near the target pressure, led to the development of tire inflation systems. Tire inflation systems attempt to automatically monitor the pressure in a vehicle tire, inflate the tire with air, and/or deflate the tire to maintain the target pressure in the tire during operation of the heavy-duty vehicle. Many of these tire inflation systems utilize rotary unions that transmit air from a pressurized axle or pneumatic line in fluid communication with an air source located on the vehicle, such as an air tank, to the rotating tires. The rotary union provides an interface between static components and the rotating wheel components. As a result, a rotary union typically is mounted in or near the outboard end of an axle spindle, and is in fluid communication with one or more outgoing pneumatic lines which pneumatically connect to a respective tire proximate the axle spindle. The rotary union in turn is in fluid communication with an air source located on the heavy-duty vehicle via a pneumatic line that is connected to and extends inboardly from the rotary union into the axle spindle and is connected to the air source. 
     One such rotary union is mounted on the interior of the hubcap attached to the outboard end of a wheel hub rotatably mounted on the axle spindle of the axle. Such rotary unions typically include a housing for mounting the rotary union to the hubcap and a stem with an inboard portion and an outboard portion. The inboard portion of the rotary union stem threadably engages a female hose connector of a pneumatic conduit or line of the tire inflation system extending through the axle. The outboard portion of the rotary union stem includes one or more bearings press-fit onto the outboard portion of the stem. The bearings in turn are press-fit into the housing, which is attached to an intermediate wall of the hubcap via suitable fasteners, such as bolts. The housing rotates about the outboard portion of the rotary union stem as the hubcap rotates during operation of the heavy-duty vehicle via the bearings. The outboard portion of the rotary union stem, which remains static, in turn is in fluid communication with a tire hose connected to the hubcap via pneumatic conduit means integrated/attached to the hubcap. 
     Electronic components are often employed with the wheel ends of heavy-duty vehicles, including components of the wheel end assemblies. For example, wheel end sensors attached to or incorporated into the wheel end assemblies of heavy-duty vehicles, such as a hubcap, are often utilized to sense and monitor conditions of the wheel end assembly to determine if issues with any of the wheel end assembly components have arisen, including components of a tire inflation system. For example, such wheel end sensors have been employed to monitor the temperature of the wheel end assembly, as a consistently high temperature may indicate a lack of lubricant or improper functioning of the bearing assembly. Such wheel end sensors have also been employed to monitor the vibration experienced in the wheel end assembly, as a consistently high level of vibration may also indicate improper functioning of the bearing assembly. In addition, such wheel end sensors have been employed to monitor humidity in the wheel end assembly, which may indicate excess moisture that may damage components, wheel speed and direction, and/or the revolution count of the wheel hub, which may be used to calculate the distance that the vehicle has traveled, based on tire size. Moreover, such wheel end sensors have been employed to monitor pressure within the tire(s) of the heavy-duty vehicle. 
     When electronic components, such as wheel end sensors, are employed with wheel end assemblies, they often utilize a disposable power source, such as batteries, to energize the electronic components. While generally suitable for their intended use, such disposable power sources eventually need replaced, which typically requires removal of the wheel end sensor from the wheel end, resulting in increased vehicle maintenance and downtime. In addition, in certain wheel end assembly configurations in which the wheel end sensor is disposed in the hubcap, removal of the sensor from the hubcap may expose the interior of the hubcap, potentially resulting in the entry of contaminants into the hubcap or escape of lubricant from the hubcap, and thus the wheel end assembly. Moreover, energy saving strategies are often employed with such electronic components to attempt to extend the life of the disposable power source associated with the components, such as reducing functionality of the components under certain driving conditions. While such energy saving strategies may extend the life of the disposable power source, they do so at the cost of undesirably reducing the functionality of the electronic component(s) under certain conditions. 
     Energy harvesting structures that generate electrical current to power electrical components associated with the heavy-duty vehicle have been incorporated into or integrated with components of wheel end assemblies, such as the hubcap, to attempt to eliminate the use of disposable power sources associated with such electronic components. Prior art energy harvesting structures incorporated into or integrated with components of wheel end assemblies often take up a considerable amount of space and are not feasible for use in modern wheel end assemblies due to packaging constraints. Moreover, such prior art energy harvesting structures often include numerous bulky components, and thus undesirably increase the overall weight and operating cost of the heavy-duty vehicle when employed. In addition, such prior art energy harvesting structures often are not sealed within and/or protected by the associated wheel end assembly components, which can potentially result in damage to the energy harvesting structures during operation of the heavy-duty vehicle and/or damage to other components of the wheel end assembly if components of the energy harvesting structures become defective. Moreover, the overall size and power of prior art energy harvesting structures incorporated into or integrated with components of wheel assemblies can potentially induce increased torque on the associated wheel end assemblies, and thus mounted wheels, which can result in power loss to the heavy-duty vehicle. 
     Thus, there is a need in the art for a rotary union for a heavy-duty vehicle tire inflation system that includes energy harvesting structure integrated into the rotary union for energizing electronic components associated with a heavy-duty vehicle, including the wheel end, such as a wheel end sensor, thereby eliminating the need for disposable energy sources, such as batteries, and minimizing vehicle maintenance associated with such electronic components, thus reducing vehicle downtime. There is also a need in the art for a rotary union that eliminates the need to employ other energy saving strategies with such components when disposable energy sources are utilized, such as limiting functionality of the components under certain circumstances, in order to maximize battery life, thus improving the overall functionality of the components. In addition, there is a need in the art for a rotary union with energy harvesting structure that is housed within and protected by the rotary union, thereby minimizing potential damage to the energy harvesting structure and/or other components of the wheel end assembly, as well as decreasing packaging space and overall vehicle weight, and thus decreasing the cost associated with employing energy harvesting structures in the wheel end of the heavy-duty vehicle. The rotary union with energy harvesting structure of the disclosed subject matter satisfies these needs and overcomes the above-described disadvantages, drawbacks, and limitations, and will now be described. 
     BRIEF DESCRIPTION OF THE DISCLOSED SUBJECT MATTER 
     An objective of the disclosed subject matter is to provide a component for a heavy-duty vehicle tire inflation system that includes energy harvesting structure integrated therein for energizing electronic components associated with a heavy-duty vehicle. 
     Another objective of the disclosed subject matter is to provide a component for a heavy-duty vehicle tire inflation system that eliminates the need for disposable energy sources, such as batteries, to energize electronic components associated with the heavy-duty vehicle, thus minimizing vehicle maintenance associated with such disposable energy sources and reducing vehicle downtime. 
     Yet another objective of the disclosed subject matter is to provide a component for a heavy-duty vehicle tire inflation system that eliminates the need to employ other energy saving strategies with electronic components associated with a heavy-duty vehicle when disposable energy sources are utilized, such as limiting functionality of the components under certain circumstances in order to maximize battery life, thus improving the overall functionality and life of the components. 
     Another objective of the disclosed subject matter is to provide a component for a heavy-duty vehicle tire inflation system with structure that houses and encapsulates energy harvesting structure, thereby minimizing potential damage to the energy harvesting structure and/or other components of the wheel end assembly if components of the energy harvesting structure become defective. 
     Yet another objective of the disclosed subject matter is to provide a component for a heavy-duty vehicle tire inflation system that includes energy harvesting structure with decreased packaging space and overall weight, thus decreasing the overall vehicle weight and the cost associated with employing energy harvesting structure with the heavy-duty vehicle. 
     These objectives and other are achieved by the rotary union with energy harvesting structure of the disclosed subject matter, which includes a static portion, the static portion remaining static during operation of the heavy-duty vehicle; a rotatable portion, the rotatable portion rotating with one or more rotating components of a wheel end of the heavy-duty vehicle during operation, at least one of the static portion and the rotatable portion being mounted to a component associated with the wheel end, at least one of the static portion and the rotatable portion being in fluid communication with an air source located on the heavy-duty vehicle, the rotary union being in fluid communication with at least one wheel of the wheel end and allowing pressurized air from the air source to flow to the at least one wheel; and energy harvesting structure integrated with the rotary union, the energy harvesting structure generating electricity during operation of the heavy-duty vehicle for energizing one or more electronic components of the heavy-duty vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Exemplary embodiments of the disclosed subject matter, illustrative of the best modes in which Applicant has contemplated applying the principles, are set forth in the following description and are shown in the drawings. 
         FIG. 1  is a fragmentary perspective view of a portion of an axle spindle of an axle and a wheel end assembly shown in cross-section, showing certain components of a tire inflation system, including a prior art rotary union, and a brake drum and tire rims mounted on a wheel hub of the wheel end assembly; 
         FIG. 2  is an exploded perspective view of a hubcap for a heavy-duty vehicle, viewed looking in an inboard direction, that incorporates tire inflation system components, including a prior art rotary union, and includes a wheel end sensor mounted in the hubcap; 
         FIG. 3  is a cross-sectional view of the hubcap and incorporated tire inflation system components shown in  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of a hubcap that incorporates components of a tire inflation system, including a first exemplary embodiment rotary union with integrated energy harvesting structure of the disclosed subject matter; 
         FIG. 5  is a perspective view of the hubcap and first exemplary embodiment rotary union shown in  FIG. 4 , viewed looking in an outboard direction; 
         FIG. 6  is an enlarged cross-sectional view of the first exemplary embodiment rotary union shown in  FIG. 4 , shown removed from the hubcap. 
         FIG. 7  is an enlarged perspective view of the first exemplary embodiment rotary union shown in  FIG. 4 , shown removed from the hubcap; 
         FIG. 8  is a perspective view of the first exemplary embodiment rotary union shown in  FIG. 6 , viewed looking in an inboard direction, showing the pneumatic distribution plate removed and showing energy harvesting structure integrated into the rotary union; 
         FIG. 9  is a perspective view of a second exemplary embodiment rotary union with integrated energy harvesting structure of the disclosed subject matter shown in cross-section; 
         FIG. 10  is a cross-sectional view the second exemplary embodiment rotary union with integrated energy harvesting structure shown in  FIG. 8 ; 
         FIG. 11  is a plan view of a hubcap with the second exemplary embodiment rotary union shown in  FIG. 9  mounted therein, viewed looking in an inboard direction; and 
         FIG. 12  is a cross-sectional view of the hubcap and second exemplary embodiment rotary union shown in  FIG. 11 , taken along line A-A, showing the orientation of the hubcap, second exemplary embodiment rotary union, and other components of a tire inflation system relative to each other. 
     
    
    
     Similar numerals and characters refer to similar components throughout the drawings. 
     DETAILED DESCRIPTION OF THE DISCLOSED SUBJECT MATTER 
     In order to better understand the rotary union with energy harvesting structure of the disclosed subject matter and the environment in which it operates, a heavy-duty vehicle wheel end assembly that incorporates components of a tire inflation system  40  is shown in  FIG. 1 , and is indicated generally at reference numeral  12 . In a heavy-duty vehicle (not shown), one or more axles  10  typically depend from and extend transversely beneath a frame (not shown) of the heavy-duty vehicle. Axle  10  includes a central tube (not shown) and a pair of axle spindles  14  (only one shown) attached to respective ends of the central tube by any suitable means, such as welding. A wheel end assembly  12  is mounted on each axle spindle  14  of axle  10 . Inasmuch as each of axle spindles  14  and their respective wheel end assemblies  12  are similar, for purposes of conciseness and clarity, only one axle spindle and its respective wheel end assembly will be described. 
     Wheel end assembly  12  includes a bearing assembly  13  with an inboard bearing  16  and an outboard bearing  18  mounted on the outboard end of axle spindle  14 . A spindle nut assembly  20  threadably engages the outboard end of axle spindle  14  and secures inboard bearing  16  and outboard bearing  18  in place. A wheel hub  22  of wheel end assembly  12  is rotatably mounted on inboard bearing  16  and outboard bearing  18  in a manner known in the art. 
     A hubcap  24  of wheel end assembly  12  is mounted on the outboard end of wheel hub  22  by a plurality of bolts  26 , each one of which passes through a respective one of a plurality of openings  28  formed in the hubcap, and threadably engages a respective one of a plurality of aligned threaded openings  30  formed in the wheel hub. In this manner, hubcap  24  closes the outboard end of wheel hub  22 , and thus wheel end assembly  12 . A main continuous seal  32  is rotatably mounted on the inboard end of wheel end assembly  12  and closes the inboard end of the wheel end assembly. In a typical heavy-duty vehicle dual-wheel configuration, a plurality of threaded bolts  34  are used to mount a brake drum  36  and a pair of tire rims  38  on wheel end assembly  12 . Each one of a pair of tires (not shown) is mounted on a respective one of tire rims  38 , as is known in the art. 
     As indicated above, wheel end assembly  12  incorporates components of tire inflation system  40 . More specifically, a central bore  48  is formed in axle spindle  14  of axle  10 , through which a pneumatic conduit  44  of tire inflation system  40  extends toward an outboard end of the axle spindle. Pneumatic conduit  44  is fluidly connected to and extends between an air source (not shown) located on the heavy-duty vehicle, such as an air tank, and a prior art rotary union  42  of tire inflation system  40 . Rotary union  42  is attached to a plug  50  that is press-fit in a machined counterbore  52  formed in central bore  48  of axle spindle  14  at an outboard end of the axle spindle, which facilitates the connection of pneumatic conduit  44 , which is a static component, to an air tube assembly  46  that rotates with the tires. 
     Air tube assembly  46  includes a first tube  54  that is fluidly connected at one of its ends to prior art rotary union  42  inside hubcap  24 , and is fluidly connected at its other end to a tee fitting  56 , which passes through the hubcap and is secured to the hubcap. Additional pneumatic conduits or tubes (not shown) are fluidly connected to and extend from each one of two outlets of tee fitting  56  outside of hubcap  24  to each one of a respective pair of tires mounted on rims  38 . In this manner, air passes from the air source located on the heavy-duty vehicle, through pneumatic conduit  44 , rotary union  42 , first air tube  54 , and tee fitting  56 , and to the tires. Alternatively, axle  10  may be pressurized, in which case pneumatic conduit  44  is not utilized, and rotary union  42  fluidly communicates directly with the pressurized air in central bore  48 . In such configurations, air tube assembly  46  is rotatably connected to rotary union  42  inside hubcap  24 , passes through and is secured to the hubcap, and pneumatically connects to the tires via suitable means, such as pneumatic conduits. 
     With reference to  FIGS. 2-3 , in order to further understand the rotary union with energy harvesting structure of the disclosed subjected matter and the environment in which it operates, a hubcap  176  which incorporates and accommodates components of a tire inflation system  170 , including a prior art rotary union  86 , and employs a wheel end sensor  300  ( FIG. 2 ) to monitor conditions in the associated wheel end assembly, is shown and will now be described. Hubcap  176  is of the type described in U.S. Pat. No. 9,132,704, which is assigned to Applicant of the disclosed subject matter, Hendrickson USA, L.L.C. 
     Hubcap  176  includes a cylindrical side wall  178 . Hubcap  176  further includes an intermediate wall  177  integrally formed with side wall  178 . Intermediate wall  177  extends perpendicular to side wall  178 . Intermediate wall  177  provides mounting support for components of tire inflation system  170 , which will be described in greater detail below. A radially-extending flange  180  is formed on an inboard end portion  179  of side wall  178 , and is formed with a plurality of bolt openings  182  ( FIG. 2 ) through which a plurality of bolts (not shown) are disposed to secure hubcap  176  to the outboard end of a wheel hub (not shown) of a wheel end assembly (not shown), such as wheel hub  22  of wheel end assembly  12  ( FIG. 1 ). More specifically, each one of the plurality of bolts passes through a respective one of plurality of bolt openings  182 , and threadably engages a respective one of a plurality of aligned threaded openings (not shown) formed in the outboard end of the wheel hub. Hubcap  176  also includes a discrete outboard wall  190  ( FIG. 2 ) to seal the outboard end of the hubcap, and thus the wheel end assembly, which will be described in detail below. 
     Hubcap  176  incorporates and accommodates mounting of components of tire inflation system  170 , including prior art rotary union  86 . Tire inflation system  170  includes a dual wheel valve assembly  172  of a type known in the art that is integrated into intermediate wall  177  of hubcap  176 . More specifically, and with reference to  FIG. 2 , dual wheel valve assembly  172  includes a pair of wheel valves  148 A and  148 B. Each wheel valve  148 A and  148 B is disposed within a respective wheel valve housing chamber  216 A and  216 B formed in intermediate wall  177  of hubcap  176  and is attached to the intermediate wall by suitable means, such as fasteners (not shown). In this manner, intermediate wall  177  of hubcap  176  acts as a dual wheel valve housing for wheel valves  148 A and  148 B. With reference to  FIG. 2 , hubcap  176  also includes a pair of cylindrical bores  222  formed through side wall  178  and into intermediate wall  177  approximately one hundred-eighty (180) degrees from one another, which enables optimum configuration for two tire hoses (not shown) directly connected to the cylindrical bores via respective couplings, with each hose extending to a respective one of a pair of tires in a heavy-duty vehicle dual-wheel configuration. 
     Each wheel valve  148 A and  148 B is a spring-biased diaphragm valve that remains open during normal operating conditions and is capable of isolating each tire in tire inflation system  170  from one or more tires that experience a significant pressure loss, such as if the tire is punctured, as is known. Each wheel valve  148 A and  148 B is also capable of isolating each tire from the other components of tire inflation system  170  if the system develops a leak that exceeds the inflation capacity of the system, as is also known. 
     With reference to  FIGS. 2-3 , tire inflation system  170  further includes a pneumatic distribution plate  204 . Pneumatic distribution plate  204  includes an outboard surface  206  that is disposed against an inboard surface  186  ( FIG. 3 ) of intermediate wall  177  of hubcap  176 . Pneumatic distribution plate  204  includes an inboard surface  208  ( FIG. 3 ) to which rotary union  86  is positioned against and attached. Pneumatic distribution plate  204  is attached to inboard surface  186  ( FIG. 3 ) of intermediate wall  177  of hubcap  176  via a plurality of fasteners (not shown) disposed through axial openings  205  ( FIG. 2 ) formed in the pneumatic distribution plate that threadably engage aligned axial openings (not shown) formed in the hubcap intermediate wall. With reference to  FIG. 3 , pneumatic distribution plate  204  includes a central recess  210  and a pair of supply openings  214  formed in the pneumatic distribution plate at the central recess. Each one of supply openings  214  fluidly communicates with a respective wheel valve  148 A,  148 B housed in intermediate wall  177  of hubcap  176 . 
     With reference to  FIGS. 2-3 , rotary union  86  includes a housing  84 . Housing  84  is formed with a mounting flange  85  for attaching rotary union  86  to pneumatic distribution plate  204  of tire inflation system  170 . More specifically, and with reference to  FIG. 2 , mounting flange  85  of housing  84  is formed with a plurality of openings  87  that align with corresponding openings  207  formed in pneumatic distribution plate  204 . A plurality of fasteners  188  are disposed through openings  87  of mounting flange  85  and threadably engage the corresponding openings  207  formed in pneumatic distribution plate  204  to secure housing  84  of rotary union  86  to the pneumatic distribution plate. With reference to  FIGS. 2-3 , an outboard extension  89  of mounting flange  85  seats within central recess  210  ( FIG. 3 ) of pneumatic distribution plate  204  when housing  84  is attached to the pneumatic distribution plate in the manner described above. A gasket  88  is disposed between mounting flange  85  of rotary union housing  84  and inboard surface  208  ( FIG. 3 ) of pneumatic distribution plate  204  to provide a seal between the rotary union housing and the pneumatic distribution plate. 
     Rotary union  86  includes a stem  90  with a threaded inboard portion  92 . Threaded inboard portion  92  of stem  90  engages a female hose connector (not shown) of a pneumatic conduit (not shown) of tire inflation system  170 , such as pneumatic conduit  44  described above ( FIG. 1 ). The pneumatic conduit in turn is connected to and in fluid communication with an air source (not shown) mounted on the heavy-duty vehicle, such as an air tank. Threaded inboard portion  92  of stem  90  can be connected to the pneumatic conduit by any threaded or non-threaded known pneumatic connection means, including threads, push-to-connect fittings, tube fittings, crimped fittings, friction fittings, hose clamps, and the like. 
     With reference to  FIGS. 2-3 , stem  90  of rotary union  86  further includes an outboard portion  98  that enables rotatable mounting of housing  84  of the rotary union. More specifically, to facilitate the rotatable mounting of housing  84  of rotary union  86  on outboard portion  98  of stem  90  of the rotary union, each one of a pair of bearings  102  is press-fit onto the outboard portion of the stem, and the outboard portion of the stem, with the bearings, is pressed into a mounting cavity  104  ( FIG. 3 ) formed in the housing. Bearings  102  enable hubcap  72  and attached rotary union housing  84  to rotate about stem  90 , which remains static. To provide an additional seal between stem  90  and rotary union housing  84 , an outboard groove  106  ( FIG. 3 ) is formed in the housing, and a rotary seal  108  is disposed in the groove on the outboard end of outboard portion  98  of the stem. With reference to  FIG. 3 , stem  90  is formed with a central bore  100 , which facilitates the passage of air through rotary union  86 . 
     With continued reference to  FIG. 3 , when rotary union  86  is attached inboard surface  208  of pneumatic distribution plate  204 , a supply cavity  212  is formed between the rotary union and the pneumatic distribution plate at central recess  210 . A pair of supply openings  214  are formed in pneumatic distribution plate  204  at central recess  210 , which enables air to flow from central bore  100  of stem  90 , through supply cavity  212 , and into the pneumatic distribution plate via the supply openings. More particularly, air flows form the air source located on the heavy-duty vehicle, through central bore  100  of stem  90 , through supply cavity  212 , and through supply openings  214  in pneumatic distribution plate  204 , which divide the air flow into two separate paths so that air flows into each wheel valve  148 A and  148 B. 
     When each wheel valve  148 A and  148 B is open, air flows from each respective wheel valve through a respective wheel valve port  218 A and  218 B formed in pneumatic distribution plate  204 , through a respective channel (not shown) formed in the pneumatic distribution plate, and out of the pneumatic distribution plate through a respective exit port  220 A and  220 B formed in the plate. Each exit port  220 A and  220 B of pneumatic distribution plate  204  is in fluid communication with a respective cylindrical bore  222  (only one shown— FIG. 2 ) formed in intermediate wall  177  of hubcap  176 , which in turn are fluidly connected to respective tires of the heavy-duty vehicle via a respective coupling (not shown) and pneumatic line (not shown). In the event of a significant pressure loss in one of the tires or in the pneumatic components of tire inflation system  170  that allows the pressure level in the pneumatic conduit to fall below the selected pressure setting, the spring bias of wheel valves  148 A and  148 B causes them to close, thus isolating each tire from the rest of the tire inflation system. 
     With reference to  FIG. 2 , hubcap  176  enables mounting of wheel end sensor  300  in the hubcap. Wheel end sensor  300  is of the type described in U.S. Pat. No. 9,933,337, which is assigned to Applicant of the disclosed subject matter, Hendrickson USA, L.L.C. Wheel end sensor  300  includes a sensor block  320  formed with a perimeter ring  321  for mounting the wheel end sensor  300  in hubcap  176 . Sensor block  320  also includes a component mounting block  327  integrally formed inside of perimeter ring  321 . Component mounting block  327  is formed with a plurality of different sized and shaped recesses (not shown) for receiving components of wheel end sensor  300 . A main circuit board  354  and a pair of batteries  324  for supplying electrical energy to the circuit board via a respective pair of wires  355  are attached to and housed within the recesses formed in component mounting block  327  by any suitable means known in the art. Main circuit board  354  includes sensor instrumentation (not shown) for sensing certain designated operational conditions and generates data signals in a known manner. Main circuit board  354  includes one or more processors  356  that receive the data signals from the sensor instrumentation to collect and processes the sensed data. Wheel end sensor  300  also includes a light emitting diode (LED) readout (not shown) operatively connected to main circuit board  354 , which provides a visual indicator of undesirable operating conditions that may require attention or service within the wheel end assembly, as programmed in the main circuit board. Alternatively, wheel end sensor  300  may include an integrated RF antenna operatively connected to main circuit board  354 , which is utilized to generate signals for wireless data transfer to a receiver that may or may not be visible to the vehicle operator during vehicle operation that indicates such undesirable operation conditions or for wireless data transfer to a remote receiver to enable central collection and analysis, such as a computer or smart phone. 
     Sensor block  320  is mounted in hubcap  176  utilizing a wheel end sensor mounting assembly  325  of the hubcap. Wheel end sensor mounting assembly  325  generally includes a retaining ring  326 , a ring-shaped first gasket  328 , and a ring-shaped second gasket  330 . First gasket  328  is disposed between an inboard surface  323  of perimeter ring  321  and an outboard end  200  ( FIGS. 2-3 ) of side wall  178  of hubcap  176 . First gasket  328  is formed with a plurality of circumferentially spaced openings  329 . Perimeter ring  321  is formed with a plurality of circumferentially spaced openings  336 , which extend axially through the perimeter ring. Openings  336  of perimeter ring  321  are circumferentially aligned with plurality of openings  329  of first gasket  328  and a plurality of circumferentially spaced threaded openings  240  formed in outboard end  200  of side wall  178  of hubcap  176 . 
     With continued reference to  FIG. 2 , outboard wall  190  of hubcap  176  seats in a circumferentially extending recess  322  formed in perimeter ring  321  so that its outboard surface is coplanar with the outboard surface of the perimeter ring. A gasket or an O-ring (not shown) is disposed between outboard wall  190  of hubcap  176  and recess  322  to provide a seal between the outboard wall and the recess to protect electronic components of wheel end sensor  300  from entry of contaminants. Outboard wall  190  can be tinted, transparent, or translucent to enable a vehicle operator to view the LED readout of main circuit board  354 , if employed, and/or enable visual inspection of components of wheel end sensor  300  and/or undesirable operating conditions within the wheel end assembly, including within hubcap  176 . 
     Second gasket  330  of wheel end sensor mounting assembly  325  is disposed between the inboard surface of retaining ring  326  and the coplanar junction of the outboard surface of outboard wall  190  of hubcap  176  and the outboard surface of perimeter ring  321  of sensor block  320 . Second gasket  330  is formed with a plurality of circumferentially spaced openings  331 , which are circumferentially aligned with openings  336  of perimeter ring  321 . Retaining ring  326  is formed with plurality of circumferentially spaced openings  332  which extend through the retaining ring and are circumferentially aligned with plurality of openings  331  of second gasket  330 . A plurality of bolts or other mechanical fasteners  333  are disposed through respective aligned openings  332  of retaining ring  326 , openings  331  of second gasket  330 , openings  336  of perimeter ring  321 , openings  329  of first gasket  328 , and threadably engage threaded openings  240  of outboard end  200  of side wall  178  of hubcap  176  to capture and secure wheel end sensor  300  in the hubcap. 
     While generally suitable for its intended purpose, wheel end sensor  300  employs batteries  324  for supplying electrical energy to circuit board  354  and powering the sensor instrumentation associated with the wheel end sensor. Batteries  324 , which are disposable power sources, eventually need replaced, which requires removal of wheel end sensor  300  from hubcap  176 , resulting in increased vehicle maintenance and cost, as well as downtime of the heavy-duty vehicle. In addition, removal of wheel end sensor  300  from the hubcap  176  may expose the interior of the hubcap, potentially resulting in entry of contaminants into the hubcap or escape of lubricant from the hubcap, and thus the wheel end assembly. While wheel end sensor  300  could employ energy saving strategies to attempt to extend the life of batteries  324 , for example, reducing functionality of certain components of the wheel end sensor under certain operational conditions of the heavy-duty vehicle, such strategies undesirably reduce the functionality of the wheel end sensor under the conditions. The rotary union with energy harvesting structure of the disclosed subject matter overcomes the above described disadvantages, drawbacks, and limitations and will now be described. 
     A first exemplary embodiment rotary union with energy harvesting structure of the disclosed subject matter is shown in  FIGS. 4-8  and is indicated generally at  700 . First exemplary embodiment rotary union  700  is shown employed with a tire inflation system  470  ( FIGS. 4-6 ) that includes components that are incorporated into or mounted on a hubcap  576  ( FIGS. 4-5 ), which also mounts a wheel end sensor  400  ( FIGS. 4-5 ). Hubcap  576  is of the type described in U.S. Pat. No. 9,132,704, which is assigned to Applicant of the disclosed subject matter, Hendrickson USA, L.L.C. 
     With reference to  FIGS. 4-5 , hubcap  576  is similar in structure and function to hubcap  176  described above, and generally includes a cylindrical side wall  578 . Hubcap  576  also includes a frustoconical transition portion  579  extending outboardly from side wall  578 . An intermediate wall  577  of hubcap  576  is integrally formed with frustoconical transition portion  579  and extends generally perpendicular to side wall  578 . Intermediate wall  577  provides mounting support for components of tire inflation system  470 , which will be described in greater detail below. Intermediate wall  577  is also formed with a central opening  575  ( FIG. 4 ), the importance of which will be described below. It is to be understood that other shapes and configurations of hubcap  576 , including side wall  578 , transition portion  579 , and/or intermediate wall  577  may be employed without affecting the overall concept or operation of the disclosed subject matter, such as an integrated dome or cone shape formed as one piece or multiple pieces, and/or adjusting the intermediate wall to be an outboard wall. 
     A radially-extending flange  580  is formed on the inboard end of side wall  578  of hubcap  576 , and is formed with a plurality of bolt openings  582  to enable bolts (not shown) to secure hubcap  576  to the outboard end of a wheel hub (not shown) of a wheel end assembly, such as wheel hub  22  ( FIG. 1 ) of wheel end assembly  12  ( FIG. 1 ) described above. In this manner, hubcap  576  defines an interior compartment  583 . It is to be understood that means known to those skilled in the art other than bolts may be used to secure hubcap  576  to the wheel hub, such as a threaded connection between the hubcap and the wheel hub, other types of mechanical fasteners, and/or a press-fit. With reference to  FIG. 4 , hubcap  576  also includes a discrete outboard wall  590  to seal the outboard end of the hubcap, and thus the wheel end assembly. With reference to  FIG. 4 , wheel end sensor  400  is mounted in hubcap  576 . Wheel end sensor  400  is similar to wheel end sensor  300  described above and is of the type described in U.S. Pat. No. 9,933,337, which is assigned to Applicant of the disclosed subject matter, Hendrickson USA, L.L.C. With reference to  FIGS. 4-5 , wheel end sensor  400  includes a sensor block  420  formed with a perimeter ring  421  for mounting the wheel end sensor in hubcap  576 . Sensor block  420  also includes a component mounting block  427  ( FIG. 4 ) integrally formed inside of perimeter ring  421 . Component mounting block  427  is formed with a plurality of different sized and shaped recesses (not shown) for receiving components of wheel end sensor  400 , including a main circuit board  454  ( FIG. 4 ) and related processors, an LED readout (not shown), an integrated RF antenna (not shown), if employed, and sensor instrumentation (not shown), which are attached to and housed within the recesses by any suitable means known in the art. 
     With reference to  FIG. 4 , sensor block  420 , and thus wheel end sensor  400 , is mounted in hubcap  576  utilizing a wheel end sensor mounting assembly  425 . Wheel end sensor mounting assembly  425  generally includes a retaining ring  426 , a ring-shaped first gasket  428 , and a ring-shaped second gasket  430 . First gasket  428  is disposed between an inboard surface  423  of perimeter ring  421  and an outboard end  500  of transition portion  579  of hubcap  576 . First gasket  428  is formed with a plurality of circumferentially spaced openings  429 . Perimeter ring  421  is formed with a plurality of circumferentially spaced openings  436  ( FIG. 4 ), which extend axially through the perimeter ring. Openings  436  of perimeter ring  421  are circumferentially aligned with plurality of openings  429  of first gasket  428  and a plurality of circumferentially spaced threaded openings  540  formed in outboard end  500  of transition portion  579  of hubcap  576 . 
     Outboard wall  590  of hubcap  576  seats in a circumferentially extending recess  422  formed in perimeter ring  421  so that its outboard surface is coplanar with the outboard surface of the perimeter ring. A gasket or an O-ring  434  is disposed between outboard wall  590  of hubcap  576  and recess  422  of perimeter ring  421  to provide a seal between the outboard wall and the recess to protect electronic components of wheel end sensor  400  from entry of contaminants, such as main circuit board  454 . Outboard wall  590  of hubcap  576  is tinted, transparent, or translucent to enable visual inspection of components of wheel end sensor  400 , such as the LED readout, if employed, to determine if undesirable operating conditions exist within the wheel end assembly, and/or check lubricant levels within hubcap  576 . 
     Second gasket  430  of wheel end sensor mounting assembly  425  is disposed between the inboard surface of retaining ring  426  of the wheel end sensor mounting assembly and the coplanar junction of the outboard surface of outboard wall  590  and the outboard surface of perimeter ring  421  of sensor block  420 . Second gasket  430  is formed with a plurality of circumferentially spaced openings  431 . Retaining ring  426  is formed with plurality of circumferentially spaced openings  432  which extend through the retaining ring and are circumferentially aligned with plurality of openings  431  of second gasket  430 . A plurality of bolts  424  or other mechanical fasteners are disposed through respective aligned openings  432  of retaining ring  426 , openings  431  of second gasket  430 , openings  436  of perimeter ring  421 , openings  429  of first gasket  428 , and threadably engage threaded openings  540  of outboard end  500  of transition portion  579  of hubcap  576  to capture and secure wheel end sensor  400  in the hubcap. It is to be understood that hubcap  576  and/or wheel end sensor mounting assembly  425  could include different components, configurations, and/or structures than that shown and described without affecting the overall concept or operation of the disclosed subject matter. 
     With reference to  FIGS. 4-5 , hubcap  576  incorporates and accommodates mounting of components of tire inflation system  470 , including first exemplary embodiment rotary union with energy harvesting structure  700 . Tire inflation system  470  is generally similar in structure and function to tire inflation system  170  described above, except that it employs first exemplary embodiment rotary union  700 . With reference to  FIG. 4 , tire inflation system  470  includes a dual wheel valve assembly  672  that is integrated into intermediate wall  577  of hubcap  576 . More specifically, dual valve assembly  672  includes a pair of wheel valves  648 A and  648 B. Each wheel valve  648 A and  648 B is disposed within a respective wheel valve housing chamber  516 A and  516 B formed in intermediate wall  577  of hubcap  576 . In this manner, intermediate wall  577  of hubcap  576  acts as a dual wheel valve housing for wheel valves  648 A and  648 B. With reference to  FIG. 5 , hubcap  576  also includes a pair of cylindrical bores  622  (only one shown) formed approximately one-hundred-eighty (180) degrees from one another in intermediate wall  577 , which enables optimum configuration for two tires hoses (not shown) directly connected to the cylindrical bores via respective couplings (not shown), with each hose extending to a respective one of a pair of tires in a heavy-duty vehicle dual-wheel configuration. Alternatively, a single tire hose (not shown) may be connected to one of cylindrical bores  622  via a respective coupling (not shown) and the other cylindrical bore plugged or sealed, with the single tire hose extending to and being connected to a single tire (not shown), such as a wide-based single tire, in a heavy-duty vehicle single-wheel configuration. In such heavy-duty vehicle single-wheel configurations, hubcap  576  may be formed with only a single cylindrical bore  622 , to which the single tire hose is connected via a coupling. 
     Each wheel valve  648 A and  648 B is a spring-biased diaphragm valve that remains open during normal operating conditions and is capable of isolating each tire in tire inflation system  470  from one or more tires that experience a significant pressure loss, such as if the tire is punctured. Each wheel valve  648 A and  648 B is also capable of isolating each tire from the other components of tire inflation system  470  if the system develops a leak that exceeds the inflation capacity of the system. 
     With reference to  FIGS. 4-6 , tire inflation system  470  further includes a pneumatic distribution plate  604 , which is generally similar in structure and function to pneumatic distribution plate  204  described above. Pneumatic distribution plate  604  includes an outboard surface  606  ( FIGS. 4 and 6 ) that is disposed against an inboard surface  586  ( FIGS. 4 and 6 ) of intermediate wall  577 . Pneumatic distribution plate  604  includes an inboard surface  608  to which first exemplary embodiment rotary union  700  is attached, as will be described below. With reference to  FIG. 5 , pneumatic distribution plate  604  is attached to inboard surface  586  of intermediate wall  577  of hubcap  576  via a plurality of fasteners  609  disposed through axial openings (not shown) formed in the pneumatic distribution plate that threadably engage aligned axial openings (not shown) formed in the hubcap intermediate wall. With reference to  FIGS. 4 and 6 , pneumatic distribution plate  604  includes a central recess  610  and a pair of supply openings  614  formed in the pneumatic distribution plate at the central recess. Each one of supply openings  614  of pneumatic distribution plate  604  fluidly communicates with a respective wheel valve  648 A and  648 B housed in intermediate wall  577  of hubcap  576 . 
     With reference to  FIGS. 4-8 , first exemplary embodiment rotary union  700  is employed as a component of tire inflation system  470 . Rotary union  700  includes a housing  784 . Housing  784  has a generally cylindrical/stepped shaped and is formed of a suitable rigid material, such as aluminum. With reference to  FIGS. 4 and 6 , housing  784  is formed with a first cavity  792  and second cavity  794 . With reference to  FIGS. 4-8 , housing  784  further includes a mounting flange  790  for attaching rotary union  700  to pneumatic distribution plate  604  of tire inflation system  470 . More specifically, and with particular reference to  FIGS. 7-8 , mounting flange  790  is formed with a plurality of openings  793  that align with a corresponding plurality of openings (not shown) formed in inboard surface  608  of pneumatic distribution plate  604 . With reference to  FIGS. 5 and 7-8 , a plurality of fasteners  791  ( FIG. 5 ) are disposed through openings  793  of mounting flange  790  and threadably engage the corresponding openings formed in inboard surface  608  of pneumatic distribution plate  604  to secure housing  784  of rotary union  700  to the pneumatic distribution plate. A gasket (not shown) may be disposed between housing  784  of rotary union  700  and inboard surface  608  of pneumatic distribution plate  604  to provide a seal between the rotary union housing and the pneumatic distribution plate. 
     With reference to  FIGS. 4-8 , first exemplary embodiment rotary union  700  includes a stem  786  with a threaded inboard portion  787  ( FIGS. 4-7 ) that engages a female hose connector (not shown) of a pneumatic conduit (not shown) connected to and in fluid communication with an air source (not shown) mounted on the heavy-duty vehicle, such as an air tank. It is to be understood that stem  786  could be connected to the pneumatic conduit by any suitable known pneumatic connection means, such as threaded or non-threaded means including threads, push-to-connect fittings, tube fittings, crimped fittings, friction fittings, hose clamps, and the like, without affecting the overall concept or operation of the disclosed subject matter. With reference to  FIGS. 4, 6, and 8 , stem  786  of rotary union  700  further includes an outboard portion  788  that enables rotatable mounting of housing  784 . Stem  786  is formed with a central bore  795  that is in fluid communication with the pneumatic conduit connected to threaded inboard portion  787 . Central bore  795  extends entirely through threaded inboard portion  787  and outboard portion  788  of stem  786 . It is to be understood that stem  786  can include other structure, shapes, and/or configurations than that shown and described without affecting the overall concept and operation of the disclosed subject matter. 
     With reference to  FIGS. 4 and 6 , to facilitate rotatable mounting of housing  784  of first exemplary embodiment rotary union  700  about outboard portion  788  of stem  786 , a pair of bearings  730  are press-fit on the stem outboard portion, and the stem outboard portion, with the bearings, is press-fit in first cavity  792  formed in the housing. Bearings  730  thus enable housing  784  attached to pneumatic distribution plate  604 , which in turn is attached to intermediate wall  577  of hubcap  576 , to rotate with the hubcap about stem  786 , which remains static. 
     In accordance with an important aspect of the disclosed subject matter, first exemplary embodiment rotary union  700  enables operation of the rotary union for use with tire inflation system  470 , as well as includes energy harvesting structure which takes advantage of the rotation of hubcap  576  and the attached rotary union to generate electricity for energizing wheel end sensor  400  and components thereof. More specifically, and with reference to  FIGS. 4, 6, and 8 , rotary union  700  includes an energy harvesting assembly  750  integrated into the rotary union. Energy harvesting assembly  750  includes a coil mount  752 . Coil mount  752  is generally annularly shaped and is disposed within second cavity  794  of housing  784  of rotary union  700 . Coil mount  752  is attached to mounting flange  790  of housing  784  via fasteners  753  ( FIG. 8 ), which are disposed through openings (not shown) formed in the coil mount and threadably engage corresponding openings (not shown) formed in recesses  785  ( FIG. 8 ) of the mounting flange. Coil mount  752  is formed with a plurality of radial arms  754  extending radially inwardly from the coil mount, which terminate to form a generally segmented central opening  756  ( FIG. 8 ), providing the coil mount with its generally annular shape. With reference to  FIG. 8 , a coil  755  formed of a suitable metal material is wound around each radial arm  754 , the importance of which will be described below. Each coil  755  preferably is formed of copper or other electrical winding material known in the art. 
     With reference to  FIGS. 4, 6, and 8 , energy harvesting assembly  750  further includes a generally annular stator  760 . Stator  760  is disposed within second cavity  794  of housing  784  of rotary union  700  such that it is positioned within central opening  756  ( FIG. 8 ) of coil mount  752 . Stator  760  includes an annular body  763  with a central opening  764  through which outboard portion  788  of stem  786  of rotary union  700  is disposed. Stator  760  is attached to outboard portion  788  of stem  786  by any suitable means, such as welds, threads, or press-fit. Stator  760  includes a plurality of magnets  762  ( FIGS. 6 and 8 ) attached to and circumferentially spaced about the radially outward end of the stator by any suitable means, such as adhesive. As shown, stator  760  includes eight magnets  762  attached to and spaced circumferentially about body  763 , but could include more or less magnets without affecting the overall concept or operation of the disclosed subject matter. As stator  760  is attached to outboard portion  788  of stem  786 , magnets  762  are statically mounted and positioned in a precise location adjacent the radially inward ends of radial arms  754  of coil mount  752 , the importance of which will be described below. 
     With reference to  FIGS. 4 and 6 , energy harvesting assembly  750  of first exemplary embodiment rotary union  700  further includes a power routing assembly  770 . Power routing assembly  770  includes a body portion  772 , which is seated within central recess  610  of pneumatic distribution plate  604  of tire inflation system  470  and extends inboardly from the pneumatic distribution plate such that it a partially disposed within second cavity  794  of housing  784 . Body portion  772  is formed with an exterior of a non-conductive material, such as a plastic, and an interior of a suitable electrically conductive material, such as steel, nickel plated beryllium copper, or a copper alloy. With particular reference to  FIG. 6 , a first O-ring  773  and a second O-ring  775  are positioned between body portion  772  of power routing assembly  770  and central recess  610  of pneumatic distribution plate  604  of tire inflation system  470  to provide seals between the power routing assembly and the pneumatic distribution plate. 
     With continued reference to  FIG. 6 , body portion  772  of power routing assembly  770  is formed with a radially extending recess  774 . With reference to  FIGS. 4 and 6 , a rectifying PC board  776  is disposed within recess  774 , the importance of which will be described in detail below. With particular reference to  FIG. 6 , rectifying PC board  776  is formed with a plurality of circumferentially spaced openings  777 . Openings  777  of rectifying PC board  776  align with corresponding openings  778  formed in body portion  772  and corresponding threaded openings  607  formed in inboard surface  608  of pneumatic distribution plate  604  of tire inflation system  470 . A plurality of fasteners  779  are disposed through respective aligned openings  777  of rectifying PC board  776  and openings  778  of body portion  772  of power routing assembly  770 , and threadably engage threaded openings  607  of inboard surface  608  of pneumatic distribution plate  604  to secure the PC board to the body portion. Rectifying PC board  776  is operatively connected to coils  755  of energy harvesting assembly  750  by any suitable means, such as a wire(s) (not shown). 
     With reference to  FIGS. 4 and 6 , power routing assembly  770  further includes a conductive rod  771  formed with body portion  772 . Like body portion  772  of power routing assembly  770 , rod  771  is formed with an exterior of a non-conductive material, such as a plastic, and an interior with a suitable electrically conductive material, such as steel, nickel plated beryllium copper, or a copper alloy. Rod  771  extends outboardly from body portion  772  and passes through a central opening  605  formed in pneumatic distribution plate  604  of tire inflation system  470  and central opening  575  ( FIG. 4 ) of intermediate wall  577  ( FIGS. 4-5 ) of hubcap  576  ( FIGS. 4-5 ). With reference to  FIG. 4 , power routing assembly  770  includes a power routing connector  780  that is attached to the outboard end of rod  771 . Power routing connector  780  is attached directly to main circuit board  454  of wheel end sensor  400  via a fastener  781 , the importance of which will be described below. 
     With particular reference to  FIG. 6 , power routing assembly  770  provides a flow path to route air from central bore  795  of stem  786  of first exemplary embodiment rotary union  700  and into pneumatic distribution plate  604  of tire inflation system  470 . More specifically, body portion  772  of power routing assembly  770  is formed with a pair of supply openings  782 . Each supply opening  782  is in fluid communication with a respective supply opening  614  formed in pneumatic distribution plate  604  and a supply cavity  783  formed between outboard portion  788  of stem  786  and body portion  772  of power routing assembly  770 . With reference to  FIGS. 4 and 6 , supply cavity  783  is sealed from second cavity  794  and first cavity  792  of housing  784  of rotary union  700  via a rotary seal  734  disposed on the outboard end of outboard portion  788  of stem  786 , such that it is positioned between the stem and body portion  772  of power routing assembly  770 . 
     In this manner, first exemplary embodiment rotary union  700  provides a sealed flow path that enables transfer of air from the air source mounted on the heavy-duty vehicle, through the pneumatic conduit, stem  786 , supply cavity  783 , supply openings  614  of pneumatic distribution plate  604 , and into each wheel valve  648 A and  648 B. When each wheel valve  648 A and  648 B is open, air flows from each respective wheel valve through a respective wheel valve port (not shown) formed in pneumatic distribution plate  604 , through a respective channel (not shown) formed in the pneumatic distribution plate, and out of the pneumatic distribution plate through a respective exit port (not shown) formed in the plate. Each of the exit ports of pneumatic distribution plate  604  is in fluid communication with a respective cylindrical bore  622  (only one shown— FIG. 5 ) formed in intermediate wall  577  of hubcap  576 , which in turn are connected to respective vehicle tires via respective couplings (not shown) and hoses (not shown). The sealed flow path provided by first exemplary embodiment rotary union  700  ensures that other energy harvesting components of energy harvesting assembly  750  of the rotary union, such as coil mount  752 , including coils  755 , and stator  760 , including magnets  762 , are not within the pressurized air path of the rotary union, thereby preventing potential damage to such components from the pressurized air during operation of tire inflation system  470 . 
     In addition, first exemplary embodiment rotary union  700  also is capable of generating electricity for energizing wheel end sensor  400  mounted in hubcap  576  during operation of the heavy-duty vehicle. More specifically, during operation of the heavy-duty vehicle, as hubcap  576  rotates, because housing  784  of rotary union  700  is attached to pneumatic distribution plate  604 , which in turn is attached to intermediate wall  577  of the hubcap, the housing also rotates. Consequently, coil mount  752 , which is attached to housing  784  of rotary union  700 , and thus coils  755  wound on radial arms  754  of the coil mount, rotate about magnets  762  attached to stator  760 , which remain static with stem  786  of rotary union  700 . As coils  755  rotate about magnets  762 , the close proximity of the coils and the magnets enables an AC current to be produced in the coils. As rectifying PC board  776  is in close proximity and operatively connected to coils  755 , the AC current generated is transmitted to the PC board, which in turn converts the AC current to DC current via one or more circuits (not shown) of the PC board. The DC current in turn is transferred from rectifying PC board  776 , through body portion  772  of power routing assembly  770  to which the PC board is attached, through rod  771  of the power routing assembly, and to power routing connector  780 . Because connector  780  is directly attached to main circuit board  454  of wheel end sensor  400 , the DC current generated by energy harvesting assembly  750  of rotary union  700  can be utilized to directly power the wheel end sensor and associated components, such as processors associated with the main circuit board, sensor instrumentation, the LED readout, and/or the integrated RF antenna, if employed. 
     In this manner, energy harvesting assembly  750  of first exemplary embodiment rotary union  700  is capable of generating electrical current to power wheel end sensor  400  and its associated components, thereby eliminating the need for disposable power sources, such as batteries, to power the wheel end sensor and its associated components. Moreover, as energy harvesting assembly  750  of first exemplary embodiment rotary union  700  is capable of generating electrical current to power wheel end sensor  400  and its associated components, the rotary union eliminates the need to employ energy saving strategies with wheel end sensor  400  to conserve energy, such as limiting functionality under certain circumstances in order to maximize battery life when batteries are employed by a wheel end sensor, thus improving the overall functionality of the wheel end sensor and associated components. 
     It is contemplated that the electrical current generated by energy harvesting assembly  750  of rotary union  700  could also be stored via an electrical energy storage device (not shown) operatively connected to the energy harvesting assembly, such as a capacitor, a super-capacitor, an ultra-capacitor, a battery and/or other energy storage means, to provide future power to wheel end sensor  400  and its associated components and/or other electrical components of the heavy-duty vehicle, for example, when the heavy-duty vehicle is stationary and no electrical current is being generated by the energy harvesting structure. It is further contemplated that the electrical current generated by energy harvesting assembly  750  of rotary union  700  could be utilized to power other components, processes, and/or systems of the heavy-duty vehicle, such as active pneumatic control systems, powering local display, support continuous wireless streaming of data, power speed and directional monitoring of wheels, and Antilock Braking System and stability event recognition, without affecting the overall concept or operation of the disclosed subject matter. It is to be understood that other types of configurations for coils  755  and magnets  762  could be employed by rotary union  700  to generate electrical current other than that shown and described may be employed without affecting the overall concept or operation of the disclosed subject matter. 
     In accordance with another important aspect of first exemplary embodiment rotary union  700 , energy harvesting components of energy harvesting assembly  750  are housed within and protected by housing  784  of the rotary union, and components within the wheel end assembly are protected from the energy harvesting components of the energy harvesting assembly. More specifically, and with particular reference to  FIGS. 4 and 6 , when housing  784  of rotary union  700  is attached to pneumatic distribution plate  604  of tire inflation system  470  in the manner described above, energy harvesting components of energy harvesting assembly  750 , including stator  760 , magnets  762 , coil mount  752 , and coils  755 , as well as rectifying PC board  776 , which facilitates conversion of the AC current generated by the energy harvesting assembly to DC current, are effectively encapsulated by the housing within second cavity  794 . In this manner, the energy harvesting components of energy harvesting assembly  750  are protected during operation. Moreover, if one or more of the energy harvesting components of energy harvesting assembly  750  were to become defective during operation, because they are encapsulated within housing  784  of rotary union  700 , there is virtually no risk that the components can damage other components within the wheel end assembly, such as other components of tire inflation system  470  and/or components within the wheel hub to which hubcap  576  is attached. 
     In this manner, first exemplary embodiment rotary union  700  minimizes potential damage to the energy harvesting components of energy harvesting assembly  750  during operation and/or other components of the wheel end assembly if components of the energy harvesting assembly become defective during operation. Moreover, because rotary union  700  enables the energy harvesting components of energy harvesting assembly  750  to be housed within housing  784  of the rotary union, the overall design of the rotary union, including the energy harvesting assembly, is relatively compact, thereby decreasing packaging space and overall vehicle weight, and thus decreasing the cost associated with employing energy harvesting structure in the wheel end assembly of the heavy-duty vehicle. The relatively compact energy harvesting assembly  750  of rotary union  700  is capable of powering wheel end sensor  400  and its associated components, and/or other electronic components associated with a wheel end of the heavy-duty vehicle, while minimizing torque induced on the associated wheel end assembly, and thus mounted wheel(s), by the energy harvesting structure. In addition, the energy harvesting components of energy harvesting assembly  750  of first exemplary embodiment rotary union  700  are sealed from the pressurized air path of the rotary union, thereby preventing potential damage to such components from the pressurized air. 
     Thus, first exemplary embodiment rotary union with energy harvesting structure  700  of the disclosed subject matter provides a functional rotary union for a tire inflation system that includes energy harvesting structure integrated into the rotary union that can energize electronic components associated with a wheel end of the heavy-duty vehicle, such as a wheel end sensor, thereby eliminating the need for disposable energy sources, such as batteries, and minimizing vehicle maintenance associated with such components, thus reducing vehicle downtime. First exemplary embodiment rotary union  700  also eliminates the need for other energy saving strategies employed with such electronic components when disposable energy sources are utilized, such as limiting functionality under certain circumstances in order to maximize battery life, thus improving the overall functionality of the components. In addition, energy harvesting structure of first exemplary embodiment rotary union  700  is housed within and protected by the rotary union, thereby minimizing potential damage to the energy harvesting structure and/or other components of the wheel end assembly, as well as decreasing packaging space and overall vehicle weight, and thus decreasing the cost associated with employing energy harvesting structures in the wheel end of the heavy-duty vehicle. 
     A second exemplary embodiment rotary union with energy harvesting structure of the disclosed subject matter is shown in  FIGS. 9-12 , and is indicated generally at reference numeral  800 . Second exemplary embodiment rotary union  800  is generally similar in structure and function to first exemplary embodiment rotary union  700 , except for the manner and associated structure by which it is mounted to a hubcap, the flow path of pressurized air through the second exemplary embodiment rotary union and associated structure, and the manner and associated structure by which energy harvested by the second exemplary embodiment rotary union is transferred to electronic components of the heavy-duty vehicle, as will be described in detail below. Similar to first exemplary embodiment rotary union  700 , second exemplary embodiment rotary union  800  is utilized with a tire inflation system  702  and a hubcap  976  ( FIGS. 11-12 ), which is capable of accommodating components of the tire inflation system and mounts wheel end sensor  400  (not shown with second exemplary embodiment rotary union  800 — FIGS. 9-12 ). It is to be understood that hubcap  976  could mount wheel end sensors with structures and/or functions different than wheel end sensor  400  without affecting the overall concept or operation of the disclosed subject matter. 
     With reference to  FIGS. 11-12 , hubcap  976  is generally similar in structure and function to hubcap  576  described above, except that the structure is modified to accommodate mounting of second exemplary embodiment rotary union  800 , which will be described in detail below. Hubcap  976  generally includes a cylindrical side wall  978 . Hubcap  976  also includes a frustoconical transition portion  979  ( FIG. 11 ) extending outboardly from side wall  978 . An intermediate wall  977  of hubcap  976  is integrally formed with transition portion  979  ( FIG. 11 ) and extends between side wall  978 . Intermediate wall  977  provides mounting support for components of tire inflation system  702  ( FIGS. 9-12 ), including rotary union  800  ( FIGS. 9-12 ), which will be described in greater detail below. Intermediate wall  977  is formed with a central opening  975  ( FIG. 12 ), the importance of which will also be described below. It is to be understood that other shapes and configurations of hubcap  976 , including side wall  978 , transition portion  979  ( FIG. 11 ), and/or intermediate wall  977  may be employed without affecting the overall concept or operation of the disclosed subject matter, such as an integrated dome or cone shape formed as one piece or multiple pieces. 
     Hubcap  976  includes a pair of bosses  974  that are each formed with a cylindrical bore  987  ( FIG. 12 ). Cylindrical bores  987  ( FIG. 12 ) are formed approximately one hundred-eighty degrees from one another and extend into intermediate wall  977 , which enables optimum configuration for two tire hoses (not shown) directly connected to the cylindrical bores via respective couplings (not shown), with each hose extending to a respective one of a pair of tires of a heavy-duty vehicle dual-wheel configuration. Alternatively, a single tire hose (not shown) may be connected to one of cylindrical bores  987  ( FIG. 12 ) via a respective coupling (not shown) and the other cylindrical bore plugged or sealed, with the single tire hose extending to and being connected to a single tire (not shown), such as a wide-based single tire, in a heavy-duty vehicle single-wheel configuration. In such heavy-duty vehicle single-wheel configurations, hubcap  976  may be formed with only a single boss  974  with a cylindrical bore  987  to which the single tire hose is connected via a coupling. 
     A radially-extending flange  980  is formed on the inboard end of side wall  978  of hubcap  976 , and is formed with a plurality of bolt openings  982  ( FIG. 11 ) to enable bolts (not shown) to secure hubcap  976  to the outboard end of a wheel hub (not shown) of a wheel end assembly (not shown), such as wheel hub  22  of wheel end assembly  12  ( FIG. 1 ) described above. In this manner, hubcap  976  closes the outboard end of the wheel hub, and thus wheel end assembly, and defines an interior compartment  983  ( FIG. 12 ). It is to be understood that means known to those skilled in the art other than bolts may be used to secure hubcap  976  to the wheel hub, such as a threaded connection between the hubcap and the wheel hub, other types of mechanical fasteners, and/or a press-fit. Hubcap  976  also includes a discrete outboard wall (not shown), such as outboard wall  590  described above, to seal the outboard end of the hubcap, and thus the wheel end assembly. Wheel end sensor  400  is mounted in hubcap  976  between an outboard end  984  of transition portion  979  ( FIG. 11 ) and the outboard wall of the hubcap. More specifically, and with reference to  FIG. 11 , outboard end  984  is formed with a plurality of circumferentially spaced threaded openings  985 , which are engaged by fasteners (not shown) for mounting wheel end sensor  400 . The outboard wall of hubcap  976  is secured to the outboard end of wheel end sensor  400  by suitable means, such as fasteners or welding. 
     With reference to  FIGS. 11-12 , hubcap  976  incorporates and accommodates mounting of components of tire inflation system  702 , including second exemplary embodiment rotary union  800 . Tire inflation system  702  is similar in structure and function to tire inflation system  470 , except that it includes a pneumatic distribution plate  704  ( FIG. 12 ) with structure to accommodate rotary union  800 , which will be described in detail below. Tire inflation system  702  includes a dual wheel valve assembly (not shown) that is integrated into intermediate wall  977  of hubcap  976 . More specifically, the dual valve assembly includes a pair of wheel valves (not shown) similar in structure and function to wheel valves  648 A and  648 B described above. Each wheel valve is disposed within a respective wheel valve housing chamber (not shown) formed in intermediate wall  977  of hubcap  976 . In this manner, intermediate wall  977  of hubcap  976  acts as a dual wheel valve housing for the wheel valves. 
     With reference to  FIG. 12 , tire inflation system  702  further includes pneumatic distribution plate  704 . Pneumatic distribution plate  704  is generally similar in function to pneumatic distribution plate  604  described above, except that it includes structure and is configured to accommodate second exemplary embodiment rotary union  800 . Pneumatic distribution plate  704  includes an outboard surface  706  that is disposed against an inboard surface  986  of intermediate wall  977 . Pneumatic distribution plate  704  is attached to inboard surface  986  of intermediate wall  977  of hubcap  976  via suitable means, such as fasteners (not shown). Pneumatic distribution plate  704  includes a central opening  710 , the importance of which will be described below. 
     Pneumatic distribution plate  704  includes a pair of pneumatic conduits  716 . Each pneumatic conduit  716  is in fluid communication with a respective cylindrical bore  987  of bosses  974  of hubcap  976  via a respective ancillary pneumatic passage  720  formed in the intermediate wall. An O-ring  721  is positioned between inboard surface  986  of intermediate wall  977  of hubcap  976  and outboard surface  706  of pneumatic distribution plate  704  about each respective pneumatic conduit  716  and ancillary pneumatic passage  720  to seal between the conduit and the passage. Each pneumatic conduit  716  is also in fluid communication with a respective wheel valve housed in intermediate wall  977  of hubcap  976 , which will be described in detail below. 
     Second exemplary embodiment rotary union  800  is employed as a component of tire inflation system  702 . With reference to  FIGS. 9-12 , rotary union  800  includes a housing  884 . Housing  884  has a generally cylindrical/stepped shaped and is formed of a suitable rigid material, such as aluminum. With reference to  FIG. 12 , an inboard portion of housing  884  of rotary union  800  is disposed through central opening  710  of pneumatic distribution plate  704 . An annular end plate  989  is positioned against an inboard surface  708  of pneumatic distribution plate  704  and the inboard end of housing  884  of rotary union  800 . End plate  989  forms a seal between pneumatic distribution plate  704  and housing  884  of rotary union  800  via an O-ring  713  positioned between the pneumatic distribution plate and the annular end plate and an O-ring  715  positioned between the inboard end of housing  884  of rotary union  800  and the annular end plate. End plate  989  includes a central opening  990 , the purpose of which will be described below. 
     With reference to  FIGS. 9-10 and 12 , housing  884  is formed with an inboardly facing first cavity  892  and an outboardly facing second cavity  894 . First cavity  892  and second cavity  894  are separated by a pneumatic passage  896  ( FIGS. 9-10 ) extending through housing  884 . With reference to  FIGS. 9-12 , housing  884  further includes a mounting flange  890 . Mounting flange  890  enables mounting of rotary union  800  to intermediate wall  977  of hubcap  976 . More specifically, housing  884  of rotary union  800  is disposed through central opening  975  ( FIG. 12 ) of intermediate wall  977 , such that mounting flange  890  is positioned on an outboard surface  991  ( FIGS. 11-12 ) of the intermediate wall. Mounting flange  890  is attached to intermediate wall  977  via a plurality of fasteners  981  ( FIG. 11 ) disposed through respective openings (not shown) formed in the mounting flange that threadably engage aligned threaded openings (not shown) formed in outboard surface  991  ( FIGS. 11-12 ) of the intermediate wall to secure rotary union  800  to hubcap  976 . Mounting flange  890  also enables attachment of rotary union  800  directly to wheel end sensor  400  by any suitable means, such as fasteners (not shown). 
     With reference to  FIGS. 9-12 , second exemplary embodiment rotary union  800  includes a stem  886  with a threaded inboard portion  887  ( FIGS. 9-10 and 12 ). With reference to  FIG. 12 , threaded inboard portion  887  is disposed through central opening  990  of end plate  989 . Threaded inboard portion  887  engages a female hose connector (not shown) of a pneumatic conduit (not shown) connected to and in fluid communication with an air source (not shown) mounted on the heavy-duty vehicle, such as an air tank. It is to be understood that stem  886  could be connected to the pneumatic conduit by any suitable known pneumatic connection means, such as threaded or non-threaded means including threads, push-to-connect fittings, tube fittings, crimped fittings, friction fittings, hose clamps, and the like, without affecting the overall concept or operation of the disclosed subject matter. Stem  886  of rotary union  800  further includes an outboard portion  888  on which housing  884  of the rotary union is rotatably mounted, as will be described in detail below. With reference to  FIGS. 9-10 and 12 , stem  886  is formed with a central bore  895  that is in fluid communication with the pneumatic conduit connected to threaded inboard portion  887 . Central bore  895  extends partially outboardly through outboard portion  888  of stem  886  and fluidly connects to a cross bore  897  positioned perpendicular to the central bore. Cross bore  897  is in fluid communication with pneumatic passage  896  ( FIGS. 9-10 ) of housing  884  such that it splits the flow path from central bore  895  into two separate flow paths, with each being directed to a respective opposite side of the pneumatic passage. It is to be understood that stem  886  can include other structure, shapes, and/or configurations than that shown and described without affecting the overall concept and operation of the disclosed subject matter. 
     Each pneumatic passage  896  ( FIGS. 9-10 ) is in fluid commination with a respective wheel valve housed within intermediate wall  977  of hubcap  976 , which in turn is in fluid communication with a respective cylindrical bore  987  of bosses  974  of the hubcap. More specifically, and with reference to  FIG. 12 , an annular channel  714  is formed between housing  884  of rotary union  800  and pneumatic distribution plate  704 . The inboard end of annular channel  714  is sealed from interior compartment  983  of hubcap  976  via end plate  989 , O-ring  713 , and O-ring  715 . Annular channel  714  is in fluid communication with the wheel valves housed in intermediate wall  977  of hubcap  976  via an annular channel  711  formed between the housing and a recess  988  formed in the intermediate wall. Annular channel  714  is continuous with annular channel  711 . An O-ring  717  is positioned between inboard surface  986  of intermediate wall  977  of hubcap  976  and outboard surface  706  of pneumatic distribution plate  704  about annular channels  714  and  711  to seal between the channels. The outboard end of annular channel  711  is sealed via an O-ring  725  disposed within an annular recess  789  ( FIG. 12 ) formed in intermediate wall  977  of hubcap  976  that is positioned between housing  884  of rotary union  800  and the intermediate wall. 
     With reference to  FIGS. 9-10 and 12 , second exemplary embodiment rotary union  800  includes a first rotary seal  898  disposed within first cavity  892  of housing  884  about outboard portion  888  of stem  886  such that the first rotary seal is positioned inboard of pneumatic passage  896 . Rotary union  800  includes a second rotary seal  899  disposed in second cavity  894  of housing  884  about outboard portion  888  of stem  886  such that the second rotary seal is positioned outboard of pneumatic passage  896 . 
     To facilitate the rotatable mounting of housing  884  of second exemplary embodiment rotary union  800  about outboard portion  888  of stem  886 , a pair of bearings  830  are press-fit on the stem outboard portion, and the stem outboard portion, with the bearings, is press-fit in second cavity  894  formed in the housing, such that the bearings are positioned adjacent to and outboard of second rotary seal  899 . Bearings  830  thus enable housing  884  attached to hubcap  976  to rotate with the hubcap about stem  886 , which remains static. 
     In accordance with an important aspect of the disclosed subject matter, second exemplary embodiment rotary union  800  enables operation of the rotary union for use with tire inflation system  702 , as well as includes energy harvesting structures which takes advantage of the rotation of hubcap  976  and the attached rotary union to generate electricity for energizing wheel end sensor  400 . More specifically, and with reference to  FIGS. 9-12 , rotary union  800  includes an energy harvesting assembly  850  integrated into the rotary union. Energy harvesting assembly  850  includes a coil mount  852 . Coil mount  852  is generally annularly shaped and is disposed within a third cavity  900  formed in mounting flange  890  of housing  884  of rotary union  800 . Coil mount  852  is attached to mounting flange  890  of housing  884  via fasteners (not shown), which are disposed through openings  853  ( FIGS. 9 and 11 ) formed in the coil mount and threadably engage corresponding openings (not shown) formed in recesses  885  ( FIGS. 9 and 11 ) of the mounting flange. Coil mount  852  is formed with a plurality of radial arms  854  extending radially inwardly from the coil mount, which terminate to form a generally segmented central opening  856  ( FIG. 9 ), providing the coil mount with its generally annular shape. A coil (not shown) formed of a suitable metal material is wound around each radial arm  854 , the importance of which will be described below. Each coil preferably is formed of copper or other electrical winding material known in the art. 
     Energy harvesting assembly  850  further includes a generally annular stator  860 . Stator  860  is disposed within third cavity  900  of mounting flange  890  of housing  884  such that it is positioned within central opening  856  of coil mount  852 . Stator  860  includes an annular body  863  with a central opening  864  through which outboard portion  888  of stem  886  of rotary union  800  is disposed. Stator  860  is attached to outboard portion  888  of stem  886  by any suitable means, such as welds or press-fit. Stator  860  includes a plurality of magnets  862  attached to and circumferentially spaced about the radially outward end of the stator by any suitable means, such as adhesive. Stator  860  includes eight magnets attached to and spaced circumferentially about body  863 , but could include more or less magnets without affecting the overall concept or operation of the disclosed subject matter. As stator  860  is attached to outboard portion  888  of stem  886 , magnets  862  are statically mounted and positioned in a precise location adjacent the radially inward ends of radial arms  854  of coil mount  852 , the importance of which will be described below. 
     Second exemplary embodiment rotary union  800  further includes a rectifying PC board  876  ( FIGS. 10 and 12 ). Rectifying PC board  876  is attached to the outboard surface of mounting flange  890  of housing  884  via any suitable means, such as fasteners (not shown). Rectifying PC board  876  is operatively connected to the coils of energy harvesting assembly  850  by any suitable means, such as a wire(s) (not shown). Rectifying PC board  876  is operatively connected to wheel end sensor  400  via suitable means, such as wires, the importance of which will be described below. 
     With reference to  FIGS. 9-10 and 12 , second exemplary embodiment rotary union  800  provides a flow path to route air from central bore  895  of stem  886  to other components of tire inflation system  702 . More specifically, second exemplary embodiment rotary union  800  provides a sealed flow path that enables transfer of air from the air source mounted on the heavy-duty vehicle, through the pneumatic conduit, through central bore  895  of stem  886 , cross bore  897 , pneumatic passage  896 , and into each wheel valve housed within intermediate wall  977  of hubcap  976 . With reference to  FIG. 12 , when each wheel valve is open, air flows from each respective wheel valve through annular channel  711 , annular channel  714 , a respective pneumatic conduit  716  of pneumatic distribution plate  704 , a respective ancillary pneumatic passage  720  formed in intermediate wall  977  of hubcap  976 , a respective cylindrical bore  987  formed in the intermediate wall, and ultimately into the respective connected vehicle tire. The sealed flow path provided by second exemplary embodiment rotary union  800  ensures that energy harvesting components of the rotary union, such as coil mount  852 , including the coils, and stator  860 , including magnets  862 , are entirely removed from the pressurized air path of the rotary union. 
     Exemplary embodiment rotary union  800  also provides a fluid path to route air from cylindrical bores  987  to rectifying PC board  876  ( FIGS. 10 and 12 ) to enable wheel end sensor  400  to measure operating conditions of tire inflation system  702 , such as the pressure within the tires connected to the cylindrical bores. More specifically, and with reference to  FIGS. 11-12 , mounting flange  890  of housing  884  of rotary union  800  is formed with a pair of pneumatic passages  889  that extend through the mounting flange. Each pneumatic passage  889  is in fluid communication with a respective auxiliary passage  922  ( FIG. 12 ) formed in intermediate wall  977  of hubcap  976 , which in turn is in fluid communication with a respective cylindrical bore  987  formed in the intermediate wall. With reference to  FIG. 12 , an O-ring  723  is positioned between outboard surface  991  of intermediate wall  977  of hubcap  976  and the inboard surface of mounting flange  890  of housing  884  of rotary union  800  about each respective pneumatic passage  889  and auxiliary passage  922  to seal between the pneumatic passage and the auxiliary passage. In this manner, air is capable of flowing from a respective tire, through cylindrical bore  987 , auxiliary passage  922 , pneumatic passage  889 , and to the inboard surface of rectifying PC board  876 , which in turn is capable of communicating information about operating conditions of tire inflation system  702  to wheel end sensor  400 , to which it is operatively connected. 
     In addition, second exemplary embodiment rotary union  800  also is capable of generating electricity for energizing wheel end sensor  400  mounted on hubcap  976  during operation of the heavy-duty vehicle. More specifically, during operation of the heavy-duty vehicle, as hubcap  976  rotates, because housing  884  of rotary union  800  is attached to intermediate wall  977  of hubcap  976 , the housing also rotates. Consequently, coil mount  852 , which is attached to housing  884  of rotary union  800 , and thus the coils of radial arms  854  of the coil mount, rotate about magnets  862  attached to stator  860 , which remain static with stem  886  of rotary union  800 . As the coils rotate about magnets  862 , the close proximity of the coils and the magnets enables an AC current to be produced in the coils. As rectifying PC board  876  is in close proximity and operatively connected to the coils, the AC current generated by energy harvesting assembly  850  is transmitted to the PC board, which in turn facilitates conversion of the AC current to DC current via one or more circuits (not shown) of the PC board. The DC current in turn is transferred from rectifying PC board  876  directly to wheel end sensor  400  and the DC current generated by energy harvesting assembly  850  of rotary union  800  can be utilized to directly power the wheel end sensor and associated components. 
     In this manner, energy harvesting assembly  850  of second exemplary embodiment rotary union  800  is capable of generating electrical current to power wheel end sensor  400  and its associated components, thereby eliminating the need for disposable power sources, such as batteries, to power the wheel end sensor and its associated components. Moreover, as energy harvesting assembly  850  of second exemplary embodiment rotary union  800  is capable of generating electrical current to power wheel end sensor  400  and its associated components, the rotary union eliminates the need to employ energy saving strategies with wheel end sensor  400  to conserve energy, such as limiting functionality under certain circumstances in order to maximize battery life when batteries are employed by a wheel end sensor, thus improving the overall functionality of the wheel end sensor and associated components. 
     It is contemplated that the electrical current generated by energy harvesting assembly  850  of rotary union  800  could also be stored via an electrical energy storage device (not shown) operatively connected to the energy harvesting assembly, such as a capacitor, a super-capacitor, an ultra-capacitor, a battery and/or other energy storage means to provide future power to wheel end sensor  400  and its associated components and/or other components of the heavy-duty vehicle, for example, when the heavy-duty vehicle is stationary and no electrical current is being generated by the energy harvesting structure. It is further contemplated that the electrical current generated energy harvesting assembly  850  of rotary union  800  could be utilized to power other components, processes, and/or systems of the heavy-duty vehicle, such as active pneumatic control systems, powering local display, support continuous wireless streaming of data, power speed and directional monitoring of wheels, and Antilock Braking System and stability event recognition, without affecting the overall concept or operation of the disclosed subject matter. It is to be understood that other types of configurations for the coils and magnets  862  other than that shown and described may be employed by rotary union  800  without affecting the overall concept or operation of the disclosed subject matter. It is to be further understood that while rectifying PC board  876  is shown attached to the outboard surface of mounting flange  890  of housing  884  of rotary union  800  so that energy harvesting structure of energy harvesting assembly  850  is fully encapsulated, it is contemplated that the rectifying PC board could be located remotely from rotary union  800 , such as integrated directly into a wheel end sensor and operatively connected to energy harvesting assembly  850  via one or more wires, without affecting the overall concept or operation of the disclosed subject matter. 
     In accordance with another important aspect of second exemplary embodiment rotary union  800 , energy harvesting components of energy harvesting assembly  850  are housed within and protected by housing  884  of the rotary union. More specifically, when housing  884  of rotary union  800  is attached to intermediate wall  977  of hubcap  976 , energy harvesting components of energy harvesting assembly  850 , including stator  860 , magnets  862 , coil mount  852 , and the coils, are effectively encapsulated by the housing within third cavity  900  of mounting flange  890 . In this manner, the energy harvesting components of energy harvesting assembly  850  are protected during operation. Moreover, if one or more of the energy harvesting components of energy harvesting assembly  850  were to become defective during operation, because they are encapsulated within housing  884  of rotary union  800 , there is virtually no risk that the components can damage other components within the wheel end assembly, such as other components of tire inflation system  702  and/or components within the wheel hub to which hubcap  976  is attached. 
     In this manner, second exemplary embodiment rotary union  800  minimizes potential damage to the harvesting components of energy harvesting assembly  850  during operation and/or other components of the wheel end assembly if components of energy harvesting assembly  850  become defective during operation. Moreover, because rotary union  800  enables the energy harvesting components of energy harvesting assembly  850  to be housed within housing  884  of the rotary union, the overall design of the rotary union, including the energy harvesting assembly, is relatively compact, thereby decreasing packaging space and overall vehicle weight, and thus decreasing the cost associated with employing energy harvesting structure in the wheel end of the heavy-duty vehicle. The relatively compact energy harvesting assembly  850  of rotary union  800  is capable of powering wheel end sensor  400  and its associated components, and/or other electronic components associated with a wheel end of the heavy-duty vehicle, while minimizing torque induced on the associated wheel end assembly, and thus mounted wheel(s), by the energy harvesting structure 
     Thus, second exemplary embodiment rotary union with energy harvesting structure  800  of the disclosed subject matter provides a functional rotary union for a tire inflation system that includes energy harvesting structure integrated into the rotary union that can energize electronic components associated with a wheel end of the heavy-duty vehicle, such as a wheel end sensor, thereby eliminating the need for disposable energy sources, such as batteries, and minimizing vehicle maintenance associated with such components, thus reducing vehicle downtime. Second exemplary embodiment rotary union  800  also eliminates the need for other energy saving strategies employed with such electronic components when disposable energy sources are utilized, such as limiting functionality under certain circumstances in order to maximize battery life, thus improving the overall functionality of the components. In addition, energy harvesting structure of second exemplary embodiment rotary union  800  is housed within and protected by the rotary union, thereby minimizing potential damage to the energy harvesting structure and/or other components of the wheel end assembly, as well as decreasing packaging space and overall vehicle weight, and thus decreasing the cost associated with employing energy harvesting structures in the wheel end of the heavy-duty vehicle. 
     It is to be understood that the rotary union with energy harvesting structure of the disclosed subject matter finds application in all types of tire inflation systems, hubcaps, heavy-duty axle spindles, wheel end assemblies, and vehicles known to those skilled in the art, including other types of tire inflation systems, hubcaps, wheel end assemblies, and vehicles than those shown and described herein and known to those skilled in the art, without affecting the concept or operation of the disclosed subject matter. It is also to be understood that other shapes and configurations for the rotary union with energy harvesting structure of the disclosed subject matter other than those shown and described above may be employed without affecting the overall concept or operation of the disclosed subject matter. In addition, while components of the energy harvesting structure of the disclosed rotary union are shown and described as being removed from the pressurized flow path within the rotary union, it is contemplated that some or all of such components could be within the pressurized flow path without affecting the overall concept or operation of the disclosed subject matter. 
     Accordingly, the rotary union with energy harvesting structure of the disclosed subject matter is simplified; provides an effective, safe, inexpensive, and efficient structure which achieves all the enumerated objectives; provides for eliminating difficulties encountered with the prior art; and solves problems and obtains new results in the art. 
     In the foregoing description, certain terms have been used for brevity, clearness and understanding; but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art, because such terms are used for descriptive purposes and are intended to be broadly construed. Moreover, the description and illustration of the disclosed subject matter is by way of example, and the scope of the disclosed subject matter is not limited to the exact details shown or described. 
     Having now described the features, discoveries and principles of the disclosed subject matter; the manner in which the rotary union with energy harvesting structure of the disclosed subject matter is used and installed; the characteristics of the construction and arrangement; and the advantageous, new and useful results obtained; the new and useful structures, devices, elements, arrangements, parts and combinations are set forth in the appended claims.