Abstract:
A pump gear is provided. The pump gear includes a first shaft portion having a first end and a second end with an integral gear portion formed therebetween. The first shaft portion is made from a first tool steel material. A second shaft portion having a third end brazed to the first end. The second shaft portion has at least one integral drive spline formed adjacent one end. The second shaft portion is made from a second tool material. The first tool steel material has a vanadium content by weight of greater than 9% and the second tool steel material has a vanadium content by weight of less than or equal to 1%.

Description:
BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to a pump gear arrangement and in particular to a pump gear arrangement having a bi-metal shaft. 
     Pump gears are used in a variety of applications such as in fuel pumps for turbine engines. The pump gear typically includes a shaft having a spur gear and a drive spline formed on one end. To reduce cost and weight, pump gears are typically formed from a single unitary material. The pump gear is typically formed for example from a single piece of tool steel alloy bar stock. In general, the drive spline is a relatively small feature that is formed using conventional machining practices allowing it to be integrally and cost effectively formed on the pump gear shaft. 
     The pump gear may also provide additional functions such as provisions for mounting and driving centrifugal pumps, electrical generators, and auxiliary fuel or hydraulic pumps. These additional functions required a longer pump gear which incorporates drive splines, keys, and threads to retain these auxiliary components. These features are difficult to machine in the wear resistant materials needed for the pump gear. In addition, the cost of the pump gear material is typically over ten times the cost of the tools steels used for shafts, splines, and threaded joints. Reduced cost for raw materials and machining of splines and threads can be achieved joining wear resistant pump gear materials to conventional tool steel. 
     Due to the desire to form the drive spline as an integral feature, the machinability of material often drives the material selection. However, the desire for good machinability conflicts with a desired wear resistance property for the spur gear. In applications such as aircraft engines, the pump gear is operating in low lubricity jet fuel at high temperatures. This environment has been found increase the wear on the spur gear. While a higher wear resistance material could be used, this would increase the size, weight and cost of the pump gear since the drive spline may not be easily machined using these materials due to its small size. As a result, the desire for a light and small pump gear impacts operational life and increases maintenance costs. 
     Accordingly, while existing pump gears are suitable for their intended purposes the need for improvement remains, particularly in providing a pump gear having improved gear wear resistance while maintaining machinability of the drive spline and threaded features to retain additional components integrated with the pump gear. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one aspect of the invention, a pump gear is provided. The pump gear includes a first shaft having a first end and a second end with an integral gear portion formed therebetween, the first shaft being made from a first tool steel material. A second shaft is arranged having a third end brazed to the first end, the second shaft having at least one integral drive spline, the second shaft being made from a second tool steel material. Wherein the first tool steel material has a vanadium content by weight of greater than 9% and the second tool steel material has a vanadium content by weight of less than or equal to 1%. 
     According to another aspect of the invention, another pump gear is provided. The pump gear includes a first shaft having a first end and a second end with an integral gear portion formed therebetween. The first shaft is made from a first steel alloy comprising by weight 2.45% carbon, 5.25% chromium, 81.25% iron, 1.30% molybdenum, and 9.75% vanadium. A second shaft is arranged having a third end brazed to the first end. The second shaft has at least one integral drive spline, the second shaft being made from a second steel alloy comprising by weight 0.40% carbon, 5.20% chromium, 91.15% iron, 1.30% molybdenum, 1.0% silicon, and 0.95% vanadium. 
     According to yet another aspect of the invention, A method of fabricating a pump gear is provided. The method includes forming a first shaft from a first steel alloy comprising by weight 2.45% carbon, 5.25% chromium, 81.25% iron, 1.30% molybdenum, and 9.75% vanadium. A gear is formed on the first shaft, the gear having teeth. A second shaft is formed from a second steel allow comprising by weight 0.40% carbon, 5.20% chromium, 91.15% iron, 1.30% molybdenum, 1.0% silicon, and 0.95% vanadium. A first end of the first shaft is coupled to a second end of the second shaft. 
     These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a perspective view of a pump gear according to an embodiment of the invention; 
         FIG. 2  is a side view of the pump gear of  FIG. 1 ; 
         FIG. 3  is an end view of the pump gear of  FIG. 1 ; 
         FIG. 4  is a sectional view of the pump gear of  FIG. 1 ; 
         FIG. 5  is a sectional view of a pump having the pump gear of  FIG. 1 ; and, 
         FIG. 6  is a sectional view of a pump having a third integral shaft. 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Pump gear systems are often used in demanding operating environments involving weight, space and lubrication constraints. Embodiments of the present invention provide advantages improving operational life and reliability in a pump gear system. Embodiments of the invention provide further advantages in providing a pump gear that allows for machinability of features with decreased gear tooth wear in high temperature jet fuel applications. 
     An exemplary pump gear  20  is shown in  FIGS. 1-4  having a first shaft portion  22  and a second shaft portion  24 . In one embodiment, the pump gear  20  may be used in a fuel pump device such as that used in an aircraft engine for example. The first shaft portion  22  includes a first end  23  and a second end  25 . Formed adjacent the first end is an integral internal drive spline  26 . In the exemplary embodiment, conventional machining processes are used to form the drive spline  26  having a 13 tooth, 0.735-inch (9.525 millimeter) pitch diameter and a 30-degree pressure angle. The drive spline  26  adapts the pump gear  20  to mate with and transfer rotational energy from a power source (not shown). 
     The first shaft portion  22  further includes a cylindrical portion  28  having a larger diameter than adjoining cylindrical surface  30  to form a step surface  32 . The step surface  32  forms a stop that the second shaft portion  24  rests against when the shaft portions  22 ,  24  are assembled. The first shaft portion  22  further includes a central bore  34  that extends therethrough. 
     In the exemplary embodiment, the first shaft portion  22  is made from a tool steel alloy that comprises by weight about 0.40% carbon, 5.20% chromium, 91.15% iron, 1.30% molybdenum, 1.0% silicon, and 0.95% vanadium. The tool steel alloy may be tool steel designated H13 by the American Iron and Steel Institute (AISI) or designated A681 by the American Society for Testing and Materials (ASTM). In one embodiment, the first shaft portion  22  may be made from Nu-Die® V (AISI H13) hot work tool steel manufactured by Crucible Industries. It has been found that low vanadium tool steels, such as less than 1% by weight vanadium for example, provides advantages in allowing the small drive spline  26  to be machined using conventional manufacturing processes. 
     The second shaft portion  24  includes a first end  40  and an opposing second end  42 . The first end  40  includes a recess  36  sized to receive the cylindrical surface  30 . The second shaft portion  24  further includes a spur gear  38  integrally formed on the second shaft portion  24 . A first bearing journal  44  is arranged between the spur gear  38  and the first end  40 . A second bearing journal  46  is arranged between the spur gear  38  and the second end  42 . The second shaft portion  24  also includes a bore  48  that is coaxial with the central bore  34 . 
     In the exemplary embodiment, the second shaft portion  24  is made from a tool steel alloy that comprises by weight about 2.45% carbon, 5.25% chromium, 81.25% iron, 1.30% molybdenum, and 9.75% vanadium. The tool steel alloy used to fabricate the second shaft portion may be may be designated AISI A11 or may be CPM® 10V tool steel manufactured by Crucible Industries. It has been found that that the AISI A11 tool steel having a high vanadium content, such as greater than 9% vanadium for example, provides advantages in higher wear resistance compared to AISI H13 tool steel when operating in low lubricity fuel at high temperatures. 
     To fabricate the pump gear  20 , the first shaft portion  22  is machined from AISI H13 tool steel with the drive spline  26  formed integrally in the shaft. Similarly, the second shaft portion  24  is formed from AISI A11 tool steel with the spur gear  38  formed integrally in the shaft. The cylindrical surface  30  on the second end  25  is inserted into the recess  36  and the first shaft portion  22  is joined to the second shaft portion  24  by brazing to form a bi-metal shaft arrangement. Once the shaft portions  22 ,  24  are joined, the bores  34 ,  48  are formed co-axially with the centerline of the pump gear  20 . In the exemplary embodiment, the cylindrical surface  30  has a diameter of 0.5770 inches (14.656 millimeters) +/−0.0005 inches (0.0127 millimeters) and the overlap between the first shaft portion  22  and the second shaft portion  24  is a ratio of 1.08 (+/−0.2) the diameter of the surface  30 . 
     Referring now to  FIG. 5 , an embodiment of a pump  50  is shown. The pump  50  includes a first housing  52  and a second housing  54 . The first housing  52  may include one or more journals  56 ,  58  that are sized to receive the journal bearings  44 ,  46  respectively. In the exemplary embodiment, the journals  56 ,  58  are sized to allow the pump gear  20  to float within the first housing  52 . Arranged in parallel with the second shaft portion  24  is a second pump gear  60 . The second pump gear  60  includes a gear portion  62  that cooperates with the spur gear  38  to increase the pressure of the fluid being moved. In one embodiment, the second end  42  extends beyond the second housing  52  to allow the pump gear  20  to be coupled to one or more accessory devices  63 . 
     The first shaft portion  22  is arranged substantially within the second housing  54 . Coupled to the first shaft portion  22  is a first stage pump section  64 . In one embodiment, the pump section  64  is a helical impeller. Adjacent the first stage pump section  64  is a second stage pump section  66 . In operation, the fluid being pumped is transferred through the first stage pump section  64  and second stage pump section  64  to the spur gear  38 . The spur gear  38  cooperates with the gear portion  62  to increase the pressure of the fluid. Coupled to the first shaft portion  22  is a transmission system  68  that is coupled to an energy source  70  such as a motor for example. The transmission system  68  drives the pump gear  20  to operate the pump. It should be appreciated that the transmission system  68  may include gears, pulleys or belts to selectively operate the pump  50 . 
     Referring to  FIG. 6 , another embodiment of a pump  72  is shown. In this embodiment, the second portion  24  is formed from a gear shaft  76  and an end shaft  74 . The end shaft  74  may be made from a tool steel alloy that comprises by weight about 0.40% carbon, 5.20% chromium, 91.15% iron, 1.30% molybdenum, 1.0% silicon, and 0.95% vanadium. The tool steel alloy may be tool steel designated H13 by the American Iron and Steel Institute (AISI) or designated A681 by the American Society for Testing and Materials (ASTM). In one embodiment, the end shaft  74  may be made from Nu-Die® V (AISI H13) hot work tool steel manufactured by Crucible Industries. 
     The end shaft  74  includes a first diameter  76  that is sized to be received within a recess in the gear shaft  76 . A center bore  78  may be formed on the end  80 . In the exemplary embodiment, the first diameter  76  has a diameter of 0.5770 inches (14.656 millimeters) +/−0.0005 inches (0.0127 millimeters) and the overlap between the gear shaft  76  and the end shaft  74  is a ratio of 1.08 (+/−0.2) the diameter of the diameter  76 . 
     The end shaft  74  has a second end  82  that may be adapted to couple with one or more accessory devices  63 . It should be appreciated that by forming the end shaft  74  from tool steel will further reduce the weight of the pump gearing system. 
     The gear shaft  76  is made from a tool steel alloy that comprises by weight about 2.45% carbon, 5.25% chromium, 81.25% iron, 1.30% molybdenum, and 9.75% vanadium. The tool steel alloy used to fabricate the gear shaft  76  may be may be designated AISI A11 or may be CPM® 10V tool steel manufactured by Crucible Industries. In the exemplary embodiment, the end shaft  74  is coupled to the gear shaft  76  by brazing. 
     It should be appreciated that the forming of a bi-metal pump gear provides advantages in allowing the small drive spline to be machined integrally with the shaft while providing a spur gear that as improved wear resistance when operating in low lubricity fuel at high temperatures. As a result, the pump gear fits within existing space envelopes and is lighter and less expensive when compared to a similarly performing pump gear fabricated from a single tool steel alloy. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.