Abstract:
A gear for use in a positive displacement pump wherein the gear lobe outer ends are the outer ends of metallic ribs embedded in each lobe and having extended wear properties and good sealing properties.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a gear for use in a positive displacement pump wherein the gear has extended wear properties and good sealing properties  
         [0002]     Rotary gear pumps and rotary gear lobe pumps consist of a housing, sometimes referred to as a case, with two gears rotating on shafts inside the case. The rotating gears create a vacuum on one side of the pump case, drawing materials into the intake side of the pump. The materials are then carried in the cavities of the lobes to the discharge side of the pump. Pressure created on the discharge side of the pump forces the materials out a discharge port.  
         [0003]     Lobe pumps and gear pumps are both rotary gear pumps. The most commonly used distinction between the two is that in a lobe pump the gears are both turned by external shafts. In a rotary pump one gear is driven by a shaft and the other gear, referred to as a driven gear, is driven by interaction with the gear driven by the shaft. Such pumps may contain gears having from two to about 12 or more lobes. The use of from about 3 to about 8 lobes is more typical.  
         [0004]     Rotary gear pumps and rotary gear lobe pumps are frequently used to pump substances that are difficult to pump due to thickness of the material being pumped or due to the liquids containing solids. Some substances to be pumped can also be caustic and abrasive. The gears in the pump case are subject to wear or corrosion, or both. In most cases the life of the pump is a function of the durability of the outer surfaces of the gears and inner surfaces of the case.  
         [0005]     The choice of materials from which the gears and the case are made can determine how well the pump functions, how well the pump wears and how well the pump resists corrosion or rust. The pump cases are usually made from cast steel. Pump cases can also be made from various other materials, including stainless steel, stainless steel alloys, metallic alloys selected for specific properties, such as corrosion resistance to particular fluids, and also from rubber or plastic materials. Pump cases may also have replaceable inner wear surfaces referred to as liners or wear plates. The replaceable wear surfaces may be made from hardened steel alloys or stainless steel or other of the materials mentioned above.  
         [0006]     The pump gears are made from various materials, including hardened steel alloys, stainless steel, other metals and metallic alloys, and rubber or plastic materials. Gears may also be made with metal inner structures and molded rubber or plastic-outer surfaces, including the lobes of the gear. The wear surfaces of this type of gear are the rubber or plastic materials of the outer surface. The abrasion resistance and the corrosion resistance are functions of the durability of the rubber or plastic material.  
         [0007]     In some applications, steel gears are more durable, particularly with respect to abrasion, than molded rubber or plastic gears. Molded rubber or plastic gears typically seal better than steel gears and therefore can develop more pressure. Rubber gears also run quieter. Steel gears are typically more expensive to manufacture.  
         [0008]     Accordingly, a continuing search has been made for gears which have improved abrasion and corrosion resistance and which also pump more efficiently than steel gears.  
       SUMMARY OF THE INVENTION  
       [0009]     According to the present invention, improved abrasion resistance and pumping efficiency is accomplished by a gear for use in a positive displacement pump comprising: a cylindrical body, including a coaxially positioned opening configured to mount the cylindrical body on a mating shaft; a plurality of metallic ribs equally spaced about and mounted on the cylindrical body; and, a plastic or rubber material positioned around an outside of the cylindrical body and on sides of the metallic ribs to form lobes on the gear, the lobes having their outer ends formed by the outer ends of the metallic ribs and their first and second ends formed by first and second ends of the metallic ribs.  
         [0010]     The invention further comprises a gear for use in a positive displacement pump, the gear comprising: a cylindrical body, including a coaxially positioned opening configured to mount the cylindrical body on a mating shaft; a plurality of metallic ribs equally spaced about and mounted on the cylindrical body; and, a plastic or rubber material positioned around an outside of the cylindrical body and on sides of the metallic ribs to form lobes on the gear, the lobes having outer ends formed of the metallic ribs.  
         [0011]     The invention also comprises a gear for use in a positive displacement pump, the gear comprising: a cylindrical body, including a coaxially positioned opening configured to mount the cylindrical body on a mating shaft; a plurality of metallic ribs equally spaced about and mounted on the cylindrical body; and, a plastic or rubber material positioned around an outside of the cylindrical body and on sides of the metallic ribs to form lobes on the gear, the lobes having exposed first and second ends of the metallic ribs on the first and second ends of the gear. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1 . is a schematic diagram of a prior art positive displacement lobe pump;  
         [0013]      FIG. 2 . is a schematic diagram of a prior art positive displacement lobe pump gear;  
         [0014]      FIG. 3 . is a schematic diagram of an metal skeleton for a prior art pump gear;  
         [0015]      FIG. 4 . is a cross-sectional view of a prior art gear, including a rubber or plastic lobe molded over a metal skeleton as shown in  FIG. 3 ;  
         [0016]      FIG. 5  is a side view of the gear shown in  FIG. 4 ;  
         [0017]      FIG. 6  is an end view of a gear according to the present invention; and,  
         [0018]      FIG. 7  is a side view of the gear of  FIG. 6 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     In the description of the Figures, the same numbers will be used throughout to refer to the same or similar components.  
         [0020]     In  FIG. 1 a  gear pump  10  is shown. Gear pump  10  includes a pump case  12 , which includes an outer cover  14  and an inlet  16  through which fluids are passed into the pump as shown by arrows  18  and an outlet  20  through which fluids are discharged from the pump as shown by arrows  22 . A first gear  24  includes first gear lobes  26  and is mounted on a shaft  28  and rotates as shown by arrow  29 . A second gear  30  is mounted on a shaft  36  for rotation as shown by arrow  37 . The operation of such gear pumps is well known to those skilled in the art. Fluids are drawn into the pump by the creation of a vacuum at inlet  16  with the pumped material which is typically a liquid which may contain particulates and the like being passed as shown by arrows  29  and  37  to discharge  20 . The discharged fluid can be at a relatively high pressure. Pressures as high as 200 psi or higher are not uncommon. A first end of the gear case is shown at  36 .  
         [0021]     In  FIG. 2 a  prior art gear  40  is shown. Gear  40  includes lobes  42  having lobe tips  44  and having a shaft receptacle opening  46 , which typically includes a key way  48  to secure gear  40  to a shaft (not shown). Gears of this configuration are well known and can be formed of steel or metals, metal alloys or rubber or plastic or of rubber or plastic with a metal skeleton to provide greater strength to the gears. As discussed previously, the metal gears tend to be less effective at close contact to effectively seal with mating gears. Further the metal gears are very noisy and in some instance may be more vulnerable to corrosion and the like. By contrast they are more durable with respect to abrasive wear than are rubber and plastic gears. Rubber and plastic gears as noted previously are also well known.  
         [0022]     In  FIG. 3 a  metal skeleton for a prior art gear is shown. A shaft receptacle  46 , including a key way  48 , is shown in a steel cylinder  52 . Steel ribs  50  are welded or otherwise joined to the outside of cylinder  52  at evenly spaced locations. This skeleton is typically then placed in a mold where a rubber or plastic material is injected into the mold to form the gear as shown in  FIG. 4 . The steel ribs  50  extend outwardly toward lobe tips  44  but do not extend outside and are not exposed at lobe tips  44 . As shown, outer ends  54  of the ribs end well short of the tips  44  of the lobes.  
         [0023]      FIG. 5  shows a side view of the gear of  FIG. 4 . First ends  56  and  58  of the gear are typically covered with the same rubber or plastic used to cover the steel cylinder and steel ribs. A tip  44  of a lobe is shown along with the sides of three lobes.  
         [0024]     In  FIG. 6  an embodiment of the present invention is shown. In this embodiment the steel ribs extend to form the outer lobe tips. The gears are produced in the same way with the gear being produced by molding or otherwise forming the rubber or plastic around the steel ribs and steel cylinder. The ribs extend outwardly as described previously and are evenly spaced around steel cylinder  52 .  
         [0025]     The lobes are formed of rubber or plastic material positioned on the outsides of the steel ribs. The outer tips  44  of the lobes as described constitute the outer ends  54  of the ribs. Accordingly, a steel surface is positioned at the outside edge of each lobe to provide increased wear while the sides of the lobes are formed of a rubber or plastic material which provides better sealing between the gears and much quieter operation.  
         [0026]     Typically the ribs are of a thickness from about ¼ to about ⅜ of an inches and are desirably formed of a relatively hard material such as T316 stainless steel or stainless steel covered with hardened chrome coating or the like. Any suitable metal or other metal alloy could be used for the fabrication of the gears, such as hardened 17-4 stainless steel, bronze and steel. Other metals and metal alloys could also be used. Similarly, the materials used to produce a rubber or plastic coating on the lobes may be selected from materials such as buna rubber, buna-N rubber, fluoroelastomers, such as copolymers of vinylidene fluoride and hexafluoropropylene, urethane and a variety of plastic materials.  
         [0027]     The suitable materials are by no means restricted to those cited above and any material which is suitable for the production of gears or coated gears is suitable for the practice of the present invention.  
         [0028]      FIG. 7  is a side view of the gear of  FIG. 6  is shown. The width of the rib is shown at  60  and as noted is desirably from about ¼ to about ⅜ of an inch. In the embodiment shown in  FIG. 7 , the ends  56  and  58  of the gear include exposed ends of the ribs in each lobe. In the embodiment shown, the sides and the ends of the ribs are exposed on respectively the ends of the gear and the outside of the lobes. It will be understood that the ends of the ribs may be covered with rubber or plastic material with only the tips of the lobes having their tips formed by the edges of the steel ribs. Similarly, it may be desirable in some instances to have the ends of the lobes covered with rubber or plastic material but have the ends of the ribs exposed on the ends of the gear. Such embodiments are well within the scope of the present invention, although it is preferred that both the ends and the tips include exposed metallic edges of the ribs.  
         [0029]     As indicated previously, any suitable metallic material can be used to form the ribs. Stainless steel is particularly suitable since it is relatively corrosion resistant and is relatively hard. As indicated previously, hardened chrome or other hardened surfaces may be applied to the stainless steel surface within the scope of the present invention. It is preferred that both the tips of the lobes and the ends of the gear include the exposed edges or sides of the ribs in combination.  
         [0030]     As well known to those skilled in the art, close tolerances are necessary in such pumps to ensure efficient pumping. Typically a new gear when placed in a pump is, for example, about 5.073 inches in diameter and fits into a case which is approximately 5.075 inches in diameter. Clearly, these close spacings require close tolerances for the gear. They also illustrate the necessity for close tolerances after pumping for a period of time when the gear surfaces may have been worn away.  
         [0031]     As indicated previously, the pump cases may include replaceable linings and the like. Such variations are well known to those skilled in the art. It is also well known that positive displacement pump gears require frequent replacement. According to the present invention, extended life is achieved since the outer tips of the lobes are of steel or other suitable hard abrasion resistant metal material. Further, the gears of the present invention run much quieter than all metal gears and seal better with other gears in mating engagement as shown in  FIG. 1 . The gear of the present invention is more effective in pumping than the previous all metal gears. Similarly, the gear of the present invention has advantages over the rubber or plastic coated gears since the outer surfaces of the gear of the present invention are of a hard, abrasion resistant material which provides extended life. The gear of the present invention offers extended life and more efficient and quieter operation than any of the prior art gears.  
         [0032]     While the present invention has been described by reference to certain of its preferred embodiments, it is pointed out that the embodiments described are illustrative rather than limiting in nature and that many variations and modifications are possible within the scope of the present invention. Many such variations and modifications may be considered obvious and desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments.