Abstract:
A gearbox for a drive line irrigation system in which the input and output shafts of the gearbox are sealed with a compressed and sintered silicon and carbide face seal. The face seal is contained within the gearbox and compressed between a gearbox end cap and a shaft bearing, obviating the need for external loading by bellows or similar spring elements.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of power transmission for use in mechanized irrigation equipment, and more particularly to the wheel drive gearboxes that drive the tires supporting and propelling the irrigation system. 
     2. Description of the Related Art 
     Center pivot and linear irrigation systems are utilized to irrigate large sections of land. Typically, such systems are capable of watering a quarter section of land, i.e., 160 acres, or more. A center pivot irrigation system is an approximately one-quarter mile mechanized pipe that rotates around a fixed pivot point. When activated, the irrigation pipe rotates around the pivot, emitting liquids over the area. The length of time required for a 360 degree rotation of the irrigation pipe typically range from 12 hours up to several days. Linear irrigation systems are comprised of similar irrigation structure, but move linearly across sections of land to be irrigated, typically by wire guidance. 
     In conventional irrigation systems, the irrigation pipe spans are 120 to 180 foot lengths supported by a wheeled tower at each intersecting span. Sprinklers are located at spaced intervals either along the length of the span or along a water conduit parallel to the span. The wheels of each tower are normally positioned perpendicular to the span such that the system moves laterally. When constructed, wheels tracks are established to follow a prescribed path, either circular for center pivot systems or linear for linear systems. Each tower is typically provided with one or more drive motors operable to move the tower. Generally, power from the motors is transferred to the wheels via a wheel drive gearbox connected to each wheel. Most conventional systems either utilize an electric gearmotor mechanically connected between the pair of wheel drive gearboxes, or hydraulic or electric motors directly coupled to the wheel drive gearboxes. 
     Agricultural wheel drive gearboxes are generally comprised of a gearbox housing in which is mounted a power input shaft having a worm gear defined thereon. A spur gear is axially attached to a power output shaft which is transversely mounted in the gearbox relative to the power input shaft such that the teeth of the spur gear engage the worm gear. A lubricant, such as heavy industrial grease or oil is disposed within the gearbox. Seals are provided at the points where the power shafts protrude from the gearbox. These prior art seals commonly consist of one or more o-rings or similar rubber sealing elements disposed around the shaft in sealing contact therewith. The standard operating pressure for such a seal is about 10-15 psi. Typically, these wheel drive gearboxes are not designed to withstand the full range of hazardous environmental conditions to which irrigation systems are likely to be exposed, such as moisture, dirt, insects and temperature extremes. Particularly vulnerable in these gearboxes are the oil seals located at the power shaft exit points. Such seals are utilized to maintain an oil bath for the internal gears and are therefore pressurized. As such, maintaining the integrity of the seal and the internal pressure within the gearboxes under the extreme working conditions of irrigation systems becomes crucial. These oil seals are most commonly lipped oil seals formed of nitrile rubber. 
     Generally, in order to maintain internal oil bath or lubricant pressure within the gearbox, lipped oil seals rely upon either a fixed polymer or nitrile lip having a face or contact surface compressed against the steel surface of a rotating shaft. Prior art irrigation drive line gearbox seals might also be mechanically compressed, i.e. loaded, to enhance sealing pressure by such means as a bellows or spring mechanism to urge the sealing elements into contact with one another. Without this “preloading”, the gearbox will likely experience leakage at the seal face, especially as internal gearbox pressures rise. In this regard, prior art irrigation drive line gearboxes are likely to require an expansion chamber or vent plug to allow oil to completely submerge the worm gear while not pushing the seals out of the gearbox, i.e., creating excess pressure at the sealing interface. In another iteration of these prior art seals, an expanded gear housing is provided above the worm gear, thereby increasing the air cavity to allow some pressure compensation (thermal expansion) and preventing a blow-out of the seal. One drawback to the above-described nitrile rubber lip seals is that such seals fatigue much more quickly under the high internal pressures which may be generated in these types of gearboxes. 
     Specifically, these oil seals become wear points along the contact line between the lip face and the rotating metal drive shaft. Generally, all prior art polymer or nitrile seal lips harden and erode with age such that the intersecting metal surfaces are subject to environmentally induced corrosion and wear from the rotating seal. This creates particular vulnerability to the wheel drive gearboxes. As the seals wear, they eventually lose elasticity and sealing contact, resulting in a loss of internal gearbox pressure. Once internal pressure is lost, the gearbox lubricant leaks from the internal bearings and gears of the wheel drive, contaminating the internal gearbox environment, which may result in rapid total failure of the wheel drive gearbox. 
     As mentioned above, wheel drives exposed to agricultural environments are subject to many different hazards such as moisture, corrosive liquids including liquidized manure, the full range of soil and mineral conditions, insects and temperature extremes which could range from continental polar to equatorial. All of these hazards can damage the integrity of the irrigation system wheeldrives, and in particular, the lipped or cartridge oil seals used to maintain pressure within the gearbox. This is especially true for these types of prior art seals since their incorporation into a irrigation drive line gearbox leaves them at least partially exposed to the external environment. 
     More particularly as to these prior art wheel drive gearboxes, they are unable to maintain extreme case pressure arising from the varied operating environments without creating a gearcase vacuum pressure during cooling. These variances in case pressure are a contributing factor to premature failure in lipped and cartridge type seals. Further, the gearcase vacuum created when cooling may draw hygroscopic moisture into the gearcase, further contaminating the lubricant and resulting in accelerated gear and bearing wear. 
     Typically, irrigation systems are placed into service upon completion of construction and initial inspection. Thereafter, they are infrequently examined or maintained. As a result, when a wheel drive gearbox loses lubrication, it is unlikely to be detected. Loss of lubricant may result in seizure of the gears within the gearbox and when the gearbox has seized, the entire irrigation drive train and watering system may be subject to automated shutdown. One solution is to more frequently check and maintain the gear drives to the irrigation system. However, it is often difficult to determine when a seal will ultimately fail by visual inspection. Therefore increased inspection may not reasonably assure detection. Furthermore, in that such irrigation systems are otherwise automated and located in remote areas, such a solution is time consuming and expensive. In addition, since the system must be shut down to safely inspect the gear drive seals, there is a down time associated with such a procedure. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide the means and apparatus to hermetically seal the gearcase from the environment. 
     It is also an object of the invention to maintain a high-pressure gear drive. 
     It is a further object of the invention to provide a gear drive that requires much less maintenance than prior gear drive. 
     It is a further object of the invention to provide a gear drive that is less likely to experience oil seal failure than prior art gear drives. 
     These and other objects are achieved through the wheel drive gearbox rotating oil seal system of the present invention in which the gearbox is hermetically sealed, such that the gearcase and components within can withstand the full range of extreme environmental and operating conditions without lubricant loss or failure during the stated design life. The rotating oil seal system provides a mechanical face seal comprised of compressed and sintered silicon and carbide elements whose surfaces create a sealing interface under a preloaded tension established during assembly. This ceramic-carbide face seal is functionally constructed within the gearbox housing, eliminating direct contact from solar radiation, especially ultraviolet radiation. The mechanical elements are made from inert silicates immune to metallic oxidation and wear. The preload of the sealing elements allow internal or external pressures to exceed 3 atmospheres without fatiguing or failing the components. 
     The above described invention is advantageous over the prior art because it is functionally impervious to environmental and application related failure modes, with the only wear surface being the interface between the silicon and carbide rotating elements, obviating the need for constant attention as is necessary with the prior art devices. In addition, the above described invention protects the gearbox&#39;s internal lubricant from a vacuum condition intake of hygroscopic moisture and other environmental hazards such as fertilizers, pesticides, fungicides, and liquidized effluent from the temperature extremes that could result in lubricant loss or failure. Further, the system can be easily retrofit the existing gear drives in the market. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation view of a center pivot irrigation span. 
     FIG. 2 is an elevation view of the invention installed on the tower of a center pivot irrigation span. 
     FIG. 3 is a first cut-away side view of the invention. 
     FIG. 4 is a second cut-away front view of the invention. 
     FIG. 5 is a blown-up, cut-away vie of the face seal. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the detailed description of the invention, like numerals are employed to designate like parts throughout. Various items of equipment, such as fasteners, fittings, etc., may be omitted to simplify the description. However, those skilled in the art will realize that such conventional equipment can be employed as desired. 
     Although the protection system of the current will be described in relation to a center pivot irrigation system, those skilled in the art will understand that the invention is also applicable to gear drive gearboxes or other drive train or power transmission systems used with other irrigation system uses, one such non-limiting example being linear or lateral irrigation systems, or other oil field or industrial, or agricultural application. 
     With reference to FIG. 1, a center pivot irrigation system  10  of the kind known in the art generally comprises an approximately one-quarter mile length of mechanized irrigation pipe  12  supported and propelled by motorized wheel towers  14  rotating on a center pivot point (not shown). Typically, the pipe spans are approximately 120 to 180 feet in length and utilize 6 to 10 inch diameter steel pipe irrigating waters or other fluids. Drive motors  16  are utilized to provide forward or reverse propel movement of the irrigation system. Attached to the pipe  12  are sprinkler or other liquid emitters  18 . 
     FIG. 2 more specifically illustrates the drive train  20  for each support tower. Specifically, the mechanized drive train  20 , which is supported on tower chassis  14 , includes an electric gearmotor  16  and primary gearbox  22  acting as the primary drive unit. Extending from primary gearbox  22  to each wheel  24  is a drive shaft  26 . At wheel  24 , drive shaft  26  attaches to a wheel drive gearbox  28  which powers wheel hub  30  on which is mounted wheel  24 . Typically drive shaft  26  will be attached to wheel drive gearbox  28  via a universal joint or coupler (not shown). 
     With reference to FIGS. 3 and 4, a wheel drive gearbox  28  of the present invention is shown. Gearbox  28  is formed of an outer housing  32  in which is mounted an output bearing  34  used to rotatingly secure an output shaft  36  within housing  32 . Output shaft  36  includes a first end  38  and a second end  40 , wherein said second end  40  extends from said outer housing  32  through a housing cap  42  having an output aperture  44  defined therein. Mounted on the second end  40  of output shaft  36  is an output spindle  46  comprising a tubular member  48  attached to said output shaft  36  and a wheel hub  30  concentrically attached to said tubular member  48 . Adjacent the first end  38  of output shaft  36  is mounted worm gear  50 . Worm gear  50  includes a cylindrical collar  52  around which is disposed a circular flange  54  having teeth  56  mounted around the periphery of the flange  54 . Rotation of worm gear  50  results in rotation of output spindle  46  and wheel hub  30 . 
     An input shaft  58  having a first end  60  and a second end  62  is also rotatingly mounted within housing  32 . Input shaft  58  is supported within housing  32  by input bearings  64  and is provided with worm threads  66  along its length. The second end  62  of input shaft  58  extends from housing  32  through a housing cap  68  having an input aperture  70  therein, and is disposed for engagement by drive shaft  26  (FIG.  2 ). Input shaft  58  is mounted within housing  32  so that the axis of input shaft  58  is perpendicular to the axis of output shaft  36  and the worm threads  66  of input shaft  58  engage the teeth  56  of worm gear  50 . 
     A ceramic face mechanical seal  72  is provided adjacent the input shaft  58  and output shaft  36  where each shaft extends from housing  32 . Specifically, adjacent the output shaft  36 , seal  72  is fitted within housing cap  42  and adjacent the input shaft  58 , seal  72  is fitted within cap  68 . Seal  72  is also mounted so that it is substantially flush with the adjacent bearing  34 ,  64 , and in sealing contact with the shaft around which said seal  72  is mounted. As seen in the Figures, adjacent both output aperture  44  and input aperture  70 , seal  72  is mounted within gearbox housing  32 , and specifically cap  42 ,  68 , thereby protecting seal  72  from the exterior environment. Those skilled in the art will understand that by mounting seal  72  between the cap, bearing and shaft, the seal  72  of the present invention is preloaded and does not require external spring elements for loading as do the general face seals of the prior art. It is this preloading that boosts the seal&#39;s pressure capacity and results in the achieved benefits for the gearbox of the present invention. 
     With reference to FIG.  5  and turning more particularly to seal  72 , it is formed of a compressed and sintered silicon carbide and is comprised of a cap element  78  and a bearing element  80 . Cap element  78  includes a cup portion  82  and a face portion  84  wherein cup portion  82  mounts within a journal  86  bored into cap  46 ,  68 . Bearing element  80  includes a boot portion  88  and a face portion  90 , wherein bearing element  80  is mounted around shaft  36  and said boot portion  88  is compressed against the adjacent bearing. As seen in FIGS. 3 and 4, when mounted in this fashion, the face portion  84  of cap element  78  and the face portion  90  of bearing element  80  are urged into sealing contact with one another at a seal interface. 
     The above described system will inhibit vegetation or other debris from damaging gearbox and gear motor seals or compromising the drive train of various agricultural and industrial equipment. Furthermore, the system assures internal pressure of the gear drive of such equipment. While standard prior art seals will generally only hold internal gearbox pressures of 10-15 psi, internal gearbox pressures in excess of 60 psi have been experienced in irrigation drive line gearboxes of the present invention. Those skilled in the art will understand that although the protection system has been specifically described for use with center pivot irrigation systems, the protection system is also suitable for other equipment. 
     While certain features and embodiments of the invention have been described in detail herein, it will be readily understood that the invention encompasses all modifications and enhancements within the scope and spirit of the following claims.