Abstract:
A centering mechanism for a hydraulic pump, a hydraulic pump assembly and a method of assembling a hydraulic pump assembly. The pump assembly generally includes a hydraulic pump, a control arm, and a centering mechanism. The pump generally includes a pump housing, a pump mechanism operable to control a flow of hydraulic fluid through the housing, the pump mechanism having a neutral condition in which fluid does not flow through the housing, a trunnion cap connectable to the housing, the trunnion cap and the housing cooperating to house the pump mechanism, and an input shaft extending along an axis and through the trunnion cap, the shaft being rotatable to operate the pump mechanism. The control arm is connected to the shaft, and movement of the control arm causes rotation of the shaft. The centering mechanism may generally include a first bracket fixable to the housing, a second bracket adjustably fixable to the first bracket, and biasing structure operable to return the control arm to a centered position when an operating force is not applied to the control arm. The second bracket is adjustable relative to the first bracket to an adjusted position such that the centered position corresponds to the neutral condition of the pump mechanism, the second bracket being fixable in the adjusted position. Fasteners fix the first bracket and the trunnion cap to the pump housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a Section 371 National Stage Application of International Application No. PCT/US2007/077182, filed Aug. 30, 2007 and published as WO 2008/028007 A2 on Mar. 6, 2008, which claims priority to U.S. Provisional Patent Application Serial No. 60/824,300, filed Sep. 1, 2006, the entire contents of which is hereby incorporated by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure is related to power machines. More particularly, this disclosure is related to power machines having a hydraulic drive system. 
     SUMMARY 
     Power machines can utilize a hydraulic system (sometimes known as a hydrostatic system) to supply power to drive the power machine. For example, a conventional skid steer loader has a hydraulic pump that provides hydraulic oil to a hydraulic drive motor causing the hydraulic drive motor to be actuated. The hydraulic drive motor has an output that is transmitted to one or more axles to drive wheels that cause the power machine to move. One type of power machine, a skid steer loader, has a pair of hydraulic pumps, one for each side of the machine, to provide drive power to each side of the machine independently. 
     The conventional hydraulic pump of the type implemented in a power machine has an input or pintle shaft that extends from a pump housing and is coupled to an internal mechanism such as a swash plate located within the pump housing. The input shaft is actuable to cause the internal mechanism or swash plate to move within the hydraulic pump. The swash plate has a neutral or center position. When the swash plate is in the neutral position, the hydraulic pump is not providing any hydraulic oil to the hydraulic motor. 
     An operator has access to drive control actuators that are operably coupled to the input shafts of the hydraulic pumps. When the operator engages the drive control actuator, the input shaft of the hydraulic motor is actuated, causing the internal mechanism or swash plate to move from the neutral position, thereby allowing the hydraulic oil to be pumped out of the hydraulic pump to the hydraulic motor. When the drive control actuators are not engaged, the input shaft is urged to the neutral position by a pump centering mechanism that engages the input shaft. 
     Pump centering mechanisms can be adjusted to ensure that the input shaft returns to the neutral position, as opposed to returning to a position that is slightly off of the neutral position. In such a case, the power machine may creep in a forward or reverse direction when the operator is not engaging the drive control actuators. Adjustments to the pump centering mechanism may be relatively small and can be difficult to make. 
     Because it may be necessary to adjust the pump centering mechanism, what is needed is a pump centering mechanism that is easy to adjust. Such a mechanism should be easy to access when the hydraulic pump has been installed within the power machine and should be capable of accepting minor adjustments in a consistent manner. 
     In some independent aspects, the invention provides a centering mechanism for a hydraulic pump. The pump generally includes a pump housing and an input shaft extending along an axis, the pump having a neutral condition in which hydraulic fluid does not flow through the pump, a control arm being connected to the shaft, movement of the control arm controlling operation of the pump. The centering mechanism may generally include a bracket assembly and biasing structure operable to return the control arm to a centered position when an operating force is not applied to the control arm. The bracket assembly may include a first bracket fixable to the housing and defining threaded holes, a second bracket adjustably fixable to the first bracket member, the second bracket defining slots associated and partially alignable with the threaded holes, and adjusting fasteners, each adjusting fastener extending through an associated slot and threadable in an associated threaded hole to adjustably fix the second bracket to the first bracket. The second bracket is adjustable relative to the first bracket such that the centered position corresponds to the neutral condition of the pump, the second bracket being fixable in the position by the adjusting fasteners. 
     In some independent aspects, the invention provides a hydraulic pump assembly. The pump assembly generally includes a hydraulic pump, a control arm, and a centering mechanism. The pump generally includes a pump housing, a pump mechanism operable to control a flow of hydraulic fluid through the housing, the pump mechanism having a neutral condition in which fluid does not flow through the housing, a trunnion cap connectable to the housing, the trunnion cap and the housing cooperating to house the pump mechanism, and an input shaft extending along an axis and through the trunnion cap, the shaft being rotatable to operate the pump mechanism. The control arm is connected to the shaft, and movement of the control arm causes rotation of the shaft. 
     In such aspects, the centering mechanism may generally include a first bracket fixable to the housing, a second bracket adjustably fixable to the first bracket, and biasing structure operable to return the control arm to a centered position when an operating force is not applied to the control arm. The second bracket is adjustable relative to the first bracket to an adjusted position such that the centered position corresponds to the neutral condition of the pump mechanism, the second bracket being fixable in the adjusted position. Fasteners fix the first bracket and the trunnion cap to the pump housing. 
     In some independent aspects, the invention provides a method of assembling a hydraulic pump assembly. The pump assembly generally includes a hydraulic pump, a control arm, and a centering mechanism. The pump includes a pump housing, a pump mechanism operable to control a flow of hydraulic fluid through the housing, the pump mechanism having a neutral condition in which fluid does not flow through the housing, a trunnion cap, and an input shaft extending along an axis, the shaft being rotatable to operate the pump mechanism. Movement of the control arm causes rotation of the shaft. The centering mechanism generally includes a first bracket, a second bracket, and biasing structure operable to return the control arm to a centered position when an operating force is not applied to the control arm. 
     In such aspects, the method may generally include the acts of positioning the pump mechanism at least partially in the housing; positioning the trunnion cap on the housing to substantially enclose the pump mechanism; providing fixing fasteners; with the fixing fasteners, fixing the first bracket and the trunnion cap to the housing, the shaft extending through the trunnion cap; providing adjusting fasteners; with the adjusting fasteners, connecting the first bracket and the second bracket; connecting the control arm to the shaft; loosening the adjusting fasteners to unfix the second bracket from the first bracket; moving the second bracket relative to the first bracket to an adjusted position such that the centered position corresponds to the neutral condition of the pump; and tightening the adjusting fasteners to thereby fix the second bracket to the first bracket in the adjusted position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a power machine of the type in which the present disclosure may be implemented illustrating a side and rear view of the power machine. 
         FIG. 2  is a perspective view of the power machine of  FIG. 1  illustrating a front and side view of the power machine. 
         FIG. 3  is a block diagram illustrating a hydraulic drive system of the type implemented in the power machine of  FIG. 1 . 
         FIG. 4  is a perspective view of a tandem hydraulic pump assembly illustrating a centering mechanism of one illustrative embodiment coupled to an input shaft of one of the hydraulic pumps. 
         FIG. 5  is another perspective view of the tandem hydraulic pump assembly of  FIG. 4 . 
         FIG. 6  an exploded diagram illustrating the centering mechanism of  FIG. 4 . 
         FIG. 7A  is a plan view of an illustrative embodiment of a first bracket of the centering mechanism of  FIG. 6 . 
         FIG. 7B  is a cross-sectional view of the first bracket of  FIG. 7A  taken along line  7 B- 7 B. 
         FIG. 7C  is a side elevational view of the first bracket of  FIG. 7A  with a cross-sectional view of a feature configured to accept a threaded fastener. 
         FIG. 8A  is a plan view of the second bracket of the centering mechanism of  FIG. 6 , which is configured to engage the first bracket. 
         FIG. 8B  is a cross-sectional view of the second bracket of  FIG. 8A  taken along line  8 B- 8 B. 
         FIG. 8C  is a side elevational view of the second bracket of  FIG. 8A  viewed from line  8 C- 8 C. 
         FIG. 9A  is a plan view of a control arm that is configured to engage the input shaft of the hydraulic pump of  FIG. 4 . 
         FIG. 9B  is a side elevation view of the control arm of  FIG. 9A  viewed from line  9 B- 9 B, illustrating an aperture configured to accept a threaded fastener. 
         FIG. 10  is a cross-sectional view of the right drive pump of the tandem hydraulic pump assembly of  FIG. 4  taken along a centerline axis of the input shaft and illustrating the positioning of the centering mechanism and control arm relative to the input shaft of the right drive pump. 
         FIG. 11A  is a plan view of a centering arm of the centering mechanism of  FIG. 6 . 
         FIG. 11B  is a perspective view of the centering arm of  FIG. 11A . 
         FIG. 11C  illustrates a pair of centering arms positioned adjacent one another as in the centering mechanism of  FIG. 6 . 
         FIG. 12  is a perspective view of a tandem hydraulic pump assembly illustrating a centering mechanism of an alternative illustrative embodiment coupled to an input shaft of one of the hydraulic pumps. 
         FIG. 13  is another perspective view of the tandem hydraulic pump assembly of  FIG. 12 . 
         FIG. 14  an exploded diagram illustrating the centering mechanism of  FIG. 12 . 
         FIG. 15  is a plan view of an alternative illustrative embodiment of a second bracket of the centering mechanism of  FIG. 14 . 
         FIG. 16  is a plan view of an alternative illustrative embodiment of a first bracket of the centering mechanism of  FIG. 14 . 
     
    
    
     Before any features and at least one embodiment of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the components set forth in the following description and claims or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     The use of “including”, “having”, and “comprising” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The use of letters to identify elements of a method or process is simply for identification and is not meant to indicate that the elements should be performed in a particular order. 
     DETAILED DESCRIPTION 
     A power machine  10 , of the type in which incorporation of the present disclosure is useful, is illustrated generally in  FIGS. 1 and 2 . As shown, power machine  10  includes a main frame assembly  16 , lift arm assembly  30  and operator compartment  40 . A pair of wheels  12 , which are mounted to stub axles  14 , extend from both sides of main frame  16 . 
     Lift arm assembly  30  is mounted to upright members  20  of main frame assembly  16 . As shown, lift arm assembly  30  includes a pair of lift arms  32 , which overlie wheels  12 . Lift arms  32  are attached to each other by a cross member  33 , and are pivotally mounted at a rearward end to upright members  20 . Lift arm assembly  30  is configured to be pivotally attached to an attachment such as bucket  34 . Lift arm assembly  30  is raised and lowered with respect to main frame assembly  16  by actuating a pair of lift cylinders  36 . Each of the lift cylinders  36  has a first end pivotally mounted to one of upright members  20  and a second end pivotally mounted to one of lift arms  32 . Bucket  34  is rotated with respect to lift arms  32  in a known manner by actuating one or more bucket tilt cylinders (not shown). 
     Operator compartment  40  is defined and partially enclosed by a cab  42 . Cab  42  includes side panels  44 , overhead panel  46 , rear panel  48 , and seat pan  52  upon which seat  54  is mounted. Cab  42  is an integral unit and is pivotally mounted at its rear to main frame assembly  16 . Cab  42  is positioned above an engine compartment (not shown) that is located within the main frame assembly  16 . Drive control actuators  58 , which, in the illustrated embodiment are pivotable levers, are positioned within the operator compartment  40 . By manipulating each of the drive control actuators  58 , such as by moving them in a forward or rearward direction, the operator can control a hydraulic drive system, located in the engine compartment and described in more detail below. The hydraulic drive system causes the power machine  10  to move in a forward or reverse direction. 
     In the illustrative embodiment, shown in  FIGS. 1 and 2 , power machine  10  is a skid steer loader, and an operator uses drive control actuators  58  to control both the movement and the steering of the power machine  10 . Power machine  10  is not limited by any particular feature of the skid steer loader shown in  FIGS. 1 and 2 . As one example, the drive control actuators  58  need not be pivotable levers but can be any type of actuation device. In addition, power machine  10  can be any type of vehicle that incorporates a hydraulic drive system, such as a mini excavator, a wheeled loader, a utility vehicle, to name a few non-limiting examples. 
       FIG. 3  is a block diagram illustrating a hydraulic drive system  80  suitable for use in power machine  10 . Hydraulic drive system  80  includes a hydraulic pump assembly  60 , which, in the illustrative embodiment, includes a left drive pump  62  and a right drive pump  64 . For the purposes of this disclosure, the left drive pump  62  powers the drive on the left hand side of the power machine  10 , and the right drive pump  64  powers the drive on the right hand side of the power machine  10 . A drive control actuator  58 , located in the operator compartment  40  (shown in  FIG. 2 ), is coupled to each of the left drive pump  62  and the right drive pump  64  via links  22 . Links  22 , in the illustrated embodiment, include a rigid link operably coupled to both the drive control actuator  58  and one of the left and right drive pumps  62  and  64 . Actuation of one of the drive control actuators  58  in a forward or reverse direction is communicated via one of the links  22  to left drive pump  62  or to right drive pump  64 . 
     When the left drive pump  62  has been actuated by its corresponding drive control actuator  58 , the left drive pump  62  pumps hydraulic oil into the hydraulic motor  66 A via a hydraulic link  70  such as a hose. Hydraulic motor  66 A is operatively coupled to a transfer mechanism  68 , which in turn is coupled to a pair of axles  14 A and  14 B. Oil flow into the hydraulic motor  66 A causes the hydraulic motor  66 A to provide a rotational force to the transfer mechanism  68 . Transfer mechanism  68 , in turn, causes the axles  14 A and  14 B to rotate in a forward or reverse direction depending upon the direction of the oil flow into the hydraulic motor  66 A. Axles  14 A and  14 B are coupled to wheels  12 A and  12 B, which turn with the axles  14 A and  14 B to cause the power machine  10  to move. 
     Transfer mechanism  68  can be any suitable structure capable of transmitting an output of the hydraulic motor  66 A to the axles  14 A and  14 B. For example, the transfer mechanism  68  can include an assembly of gears and chains configured to operably couple both of the axles  14 A and  124 B to the output of hydraulic motor  66 A to drive the axles  14 A and  14 B in tandem. Alternatively, any other structure can be provided to transfer the output of the hydraulic motor  66 A to either axle  14 A or axle  14 B, or both. 
     Similarly, the right drive pump  64  is coupled to a hydraulic motor  66 B via a hydraulic link  72 . Hydraulic motor  66 B has an output that is coupled to a transfer mechanism  69 . Transfer mechanism  69 , in turn, is coupled to axles  14 C and  14 D. Axles  14 C and  14 D are coupled to wheels  12 C and  12 D. Thus, actuation of the drive control actuator  58  in communication with right drive pump  64  causes oil to be pumped, via hydraulic link  72 , into hydraulic motor  66 B. Depending on the direction of oil pumped into hydraulic motor  66 B, the wheels  12 C and  12 D will be driven in a forward or reverse direction. Transfer mechanism  69  can also be any suitable structure capable of transmitting an output of the hydraulic motor  66 B to the axles  14 C and  14 D. 
     The drive system  80  illustrated in  FIG. 3  is shown for illustrative purposes only. Other drive systems may be incorporated into power machine  10 . For example, power machine  10  can include a hydraulic motor dedicated to each of the wheels on the machine. Thus, each wheel can be independently driven by one of the left and right drive pumps. Similarly, the hydraulic pump assembly can have a single hydraulic drive pump that controls either the front two or rear two wheels for a two-wheel drive power machine  10 . Alternatively still, the front wheels and rear wheels can each be driven together by a hydraulic pump assembly having a single hydraulic drive pump or tandem hydraulic drive pumps to provide four-wheel drive. 
       FIGS. 4 and 5  illustrate a hydraulic pump assembly  60  of the type described above with respect to  FIG. 3 . Hydraulic pump assembly  60  includes left drive pump  62  and right drive pump  64 . A front side  61  of the hydraulic pump assembly  60  is shown in  FIG. 4 , and a back side  63  of the hydraulic pump assembly  60  is shown in  FIG. 5 . Each of the left drive pump  62  and right drive pump  64  has a housing  67  with a pair of ports  92  therein, which are configured to be coupled via hydraulic links  70  and  72  to hydraulic motors  66 A and  66 B, respectively, as is shown in  FIG. 3 . Hydraulic oil is pumped under pressure through ports  92  from each of the left drive pump  62  and the right drive pump  64  to their respective hydraulic motors  66 A and  66 B. The direction of the hydraulic flow from the ports  92  depends on whether the respective drive pump has been actuated in a forward or reverse direction. 
     On the back side  63  of the hydraulic drive pump system  60 , a port  94  is shown between the left drive pump  62  and the right drive pump  64 . Port  94  is an inlet, which is configured to be coupled to a hydraulic oil supply (not shown). The hydraulic oil supply provides oil to each of the left drive pump  62  and the right drive pump  64 . In addition, a pair of ports  96  is shown. Each of the ports  96  are adapted to be coupled to a hydraulic reservoir (not shown) to return oil from the respective hydraulic drive pumps to the reservoir. 
     Left drive pump  62  has a pintle arm or input shaft  88  that extends through a trunnion cap  95  that is fastened to the housing  67  of the left drive pump  62 . Input shaft  88  engages an internal mechanism such as a swash plate (not shown) located inside the housing  67 . The input shaft  88  is rotatable to cause the internal mechanism to move and direct oil within the left drive pump  62 . Input shaft  88  has a centered or neutral position. In the neutral position, the swash plate is positioned so that no oil is pumped out of the ports  92 , and thus, the wheels  12 A and  12 B are not driven by the left drive pump  62 . In one illustrative embodiment, rotating the input shaft  88  in a clockwise direction will cause the internal mechanism to move and direct oil through the ports  92  to hydraulic motor  66 A to cause wheels  12 A and  12 B to move in a forward direction. Rotating the input shaft  88  in a counter-clockwise direction will cause the wheels  12 A and  12 B to move in a reverse direction. 
     Right drive pump  64  is similarly configured with an input shaft  88  ( FIG. 10 ) that extends through a trunnion cap  95  and is coupled to an internal mechanism such as a swash plate (not shown). Right drive pump  64  is shown in  FIGS. 4 and 5  with a pintle lever or control arm  102  attached to the input shaft  88 . Control arm  102  is also adapted to be coupled to link  22  ( FIG. 3 ). Control arm  102  thus transfers an operating force transmitted from the drive control actuator  58  through link  22  to the input shaft  88  to cause the input shaft  88  to rotate when such a force is applied. 
     A centering mechanism  100  is attached to the right drive pump  64 . Centering mechanism  100  engages the control arm  102  to provide a centering force to assist the control arm  102  to move the input shaft  88  to the neutral position when no operating force is applied to the control arm  102  from the drive control actuator  58 . It is to be understood that a control arm  102  and centering mechanism  100  of the type attached to the right drive pump  64  is also to be attached to the left drive pump  62 . The hydraulic pump assembly  60  is shown in  FIGS. 4 and 5  with just one centering mechanism  100  for illustrative purposes only. 
     Each centering mechanism  100  includes a first bracket  116 . The first bracket  116  is adapted to be fixedly attached to the trunnion cap  95 . Each of the left drive pump  62  and the right drive pump  64  have a trunnion cap  95 , and thus a first bracket  116  is attached to each trunnion cap  95 . Fasteners  98 , which are engaged with the pump housing  67  to secure the trunnion cap  95  to the pump housing  67 , are removed, and first bracket  116  is positioned upon the trunnion cap  95 . Both the trunnion cap  95  and the first bracket  116  are then secured to the housing  67  by a plurality of fixing fasteners  124  that extend through apertures  122  in the first bracket  116  as well as through the trunnion cap  95 . 
     A second bracket  104  is mounted onto the first bracket  116 . Second bracket  104  is rotatably adjustable with respect to the first bracket  116 . Second bracket  104  includes a generally planar body or primary portion  105  and a tab  114 , which extends angularly away from the generally planar primary portion  105 . Primary portion  105  is aligned so that when the second bracket  104  is mounted onto the first bracket  116 , the primary portion  105  is positioned adjacent to the first bracket  116 , and the tab  114  extends away from the first bracket  116 . Second bracket  104  has a pair of slots  130  that extend through the primary portion  105  and though each of which an adjusting fastener  132  extends to engage the first bracket  116  to secure the second bracket  104  to the first bracket  116 . The slots  130  allow for some adjustment of the second bracket  104  with respect to the first bracket  116  when the fasteners  132  are not firmly in place. When the fasteners  132  are firmly in place, the second bracket  104  is securely fastened to the first bracket  116 . 
     Control arm  102  is configured to be positioned adjacent to the second bracket  104  and be secured to the input shaft  88 . First centering arm  106  and second centering arm  108  are positioned adjacent the control arm  102 . A bushing  148 , which is fastened by a fastener  150  to the input shaft  88 , captures the first and second centering arms  106  and  108  between the bushing  148  and the control arm  102 . The bushing  148  also provides a rotating fulcrum for the first and second centering arms  106  and  108  so that they are rotatable with respect to the input shaft  88 . 
     Each of the first centering arm  106  and the second centering arm  108  extend away from the input shaft  88  and are positioned so that they are on opposite sides of tab  114 . A coil spring  112  is attached to each of the first centering arm  106  and the second centering arm  108 . The coil spring  112  exerts a force on each of the first centering arm  106  and the second centering arm  108  that tends to pull the two centering arms  106  and  108  together. When no other force is acting upon the first centering arm  106  and the second centering arm  108 , they are pulled together until each of the centering arms  106  and  108  engages tab  114 . 
     A fastener  110  extends into the control arm  102  so that it is positioned between and is capable of engaging the first and second centering arms  106  and  108 . When the control arm  102  moves from the neutral or centered position, for example, towards the front side  61  of hydraulic pump assembly  60 , the fastener  110  rotates with the control arm  102  in a clockwise direction and engages centering arm  108 . The force applied by the coil spring  112  against centering arm  108  is overcome and the centering arm  108  is rotated away from the tab  114  along with the control arm  102 . When forces, such as the actuation of the drive control actuator  58  that can act on the control arm  102 , are removed, the coil spring  112  urges the second centering arm  108  toward the first centering arm  106  until the second centering arm  108  engages tab  114 . 
     When tab  114  is properly positioned and the first centering arm  106  and the second centering arm  108  are positioned to engage the tab  114 , the centering arms  106  and  108  urge the control arm  102  to move the input shaft  88  into the neutral position. Adjustment of the second bracket  104  with respect to the first bracket  116 , therefore, rotates tab  114 , which defines the position of the input shaft  88  when no other force is acting upon the control arm  102 . Thus, if the tab  114  is properly adjusted, the input shaft  88  will return to the neutral position when no other force is acting upon the control arm  102 . As described above, the second bracket  104  can be adjusted with respect to the first bracket  116  to position the tab  114  so that it is properly positioned. 
       FIG. 6  is an exploded view of centering mechanism  100  and control arm  102 . First bracket  116  (also shown in  FIGS. 7A-7C ) has a plurality of apertures  122 , which are positioned to be aligned with similar apertures in the trunnion cap  95  so that fasteners  124  can extend through the first bracket  116  and the trunnion cap  95  to secure both components to the housing  67 . First bracket  116  includes a pair of flanges  117 , which are positioned to extend beyond the outer perimeter of trunnion cap  95 . A boss  118  extends into each of the flanges  117 . Each boss  118  is adapted to accept a threaded fastener  132  to secure the second bracket  104  to the first bracket  116 . In one illustrative embodiment, boss  118  is extruded into the first bracket  118  and is provided with a thread to accept threaded fastener  132 . However, the boss  118  can be formed in any manner and need not be provided with threads. 
     First bracket  116  also includes a formation  120  with an aperture  119  extending therethrough to allow the first bracket  116  to be fitted over the input shaft  88 . The aperture  119  is large enough so that the first bracket  116  does not engage the input shaft  88 . The formation  120  includes a lip  121 , which is shaped to engage the second bracket  104  so that the second bracket  104  can be positioned properly with respect to the first bracket  116  and the input shaft  88 . 
     The second bracket  104  (also shown in  FIGS. 8A-8C ) is configured to be positioned adjacent and be attached to the first bracket  116 . Second bracket  104  includes a protrusion  133  formed into the generally planar primary portion  105  of the second bracket  104 . Protrusion  133  can be extruded into the second bracket  104  and includes an aperture  134  that is sized so that the protrusion  133  fits over the feature  120  and engages the lip  121  on the first bracket  116 . The second bracket  104  is thus centered on the first bracket  116  and is capable of rotating on the feature  120 . The relationship between the lip  121  and the protrusion  133  (shown in  FIG. 10 ) centers the second bracket  104  relative to the first bracket  116  and the input shaft  88 , thereby preventing the second bracket  104  from moving off center when it is being adjusted. 
     The second bracket  104  further includes a plurality of slotted apertures  128 . The slotted apertures  128  are positioned to fit over the fasteners  124 , which hold the first bracket  116  to the housing  67 . This allows the second bracket  104  to be able to rotate with respect to the first bracket  116  without any interference from the fasteners  124 . 
     Second bracket  104  further includes a pair of slots  130  each of which are sized to accept a fastener  132 . Fasteners  132  are also configured to engage threaded boss  118  in the first bracket  116  to secure the second bracket  104  to the first bracket  116 . When the fasteners  132  are not snuggly fitted onto the second bracket  104 , the second bracket  104  is capable of rotating with respect to the first bracket  116  within the confines of slots  130  to properly position tab  114 . When the fasteners  132  are snuggly tightened, the second bracket  104  is firmly held in position with respect to the first bracket  116 . 
     Tab  114  includes an aperture  115  extending therethrough. Aperture  115  is configured to accept a tool such as a screwdriver or other similar instrument. By inserting an instrument into the aperture  115  when the fasteners  132  are not snugly tightened to the second bracket  104 , the second bracket  104  can be easily rotated in one direction or the other to find a proper position for the tab  114 . 
     Control arm  102  (also illustrated in  FIGS. 9A-9B ) is positioned adjacent the second bracket  104 . Control arm  102  includes an aperture  140  that is sized and shaped to accept and be engaged with the input shaft  88 . Control arm  102  also includes a slot  144  that extends from aperture  140  to an outer surface  136  of the control aim  102 . Slot  144  divides a portion of the control arm  102  into first and second fingers  160  and  162 , respectively. A cross bore  164  extends through first finger  160  and into second finger  162 . Cross bore  164  is configured to accept a fastener  142 . Fastener  142  is capable of engaging the cross bore  164  so that it is fixedly attached to the control arm  102 . When fastener  142  is engaged with control arm  102 , tightening the fastener  142  causes the control arm  102  to deform slightly at the slot  144  to snuggly fit the control arm  102  onto the input shaft  88 . Control arm  102  also includes a linkage engagement member  138 , which is configured to accept and be attached to link  22 . 
     Control arm  102  is thus rotatable with respect to the first and second brackets  116  and  104 . When a force from the drive control actuator  58  is transmitted via link  22  to the control arm  102 , the control arm  102  rotates towards the forward direction  61  or the reverse direction  63 . The control arm  102  thus rotates the input shaft  88  with respect to the casting  67 , causing the internal mechanism to move and direct oil to the particular hydraulic motor through the orifices  92 . 
     First and second centering arms  106  and  108  are positioned adjacent the control arm  102 . Each of the first and second centering arms  106  and  108  has an aperture  109  extending through a first end  111  of the respective arms. The aperture  109  in each of the first and second centering arms  106  and  108  is large enough to fit over the input shaft  88  without engaging the input shaft  88 . Bushing  148  provides a retaining force onto the first and second centering arms  106  and  108  to hold the centering arms  106  and  108  in position with respect to the control arm  102 . Spacers  146  are positioned between the control arm  102  and the first centering arm  106  as well as between the first centering arm  106  and the second centering arm  108 . Another spacer  146  is positioned between the second centering arm  108  and the bushing  148 . Spacers  146  prevent metal-to-metal contact between the control arm  102 , first and second centering arms  106  and  108  and bushing  148 . 
     Returning again to  FIG. 6 , each of the first and second centering arms  106  and  108  has a member  150  on a second end of the centering arm  106  and  108  adapted to accept and secure coil spring  112 . Spring  112  is positioned between the first and second centering arms  106  and  108  and acts to pull the first and second centering arms  106  and  108  toward each other. A fastener  110  is fitted into the control arm  102  at an aperture  107 . The fastener  110  is positioned so that it is capable of engaging either the first centering arm  106  or the second centering arm  108  when the control arm  102  rotates with respect to the first and second brackets  116  and  104 . Thus, the fastener  110 , which moves with the control arm  102  acts against the spring  112  to separate the first centering arm  106  from the second centering arm  108 . 
     When a force from the drive control actuator  58  is removed, the spring  112  tends to pull the first centering arm  106  and the second centering arm  108  together until they are both engaging the tab  114  of the second bracket  104 . It is to be understood that depending on the direction of rotation of control arm  102 , fastener  110  will engage either the first centering arm  106  or the second centering arm  108 . 
       FIGS. 11A-11C  illustrate the first and second centering arms  106  and  108  in more detail. In the illustrative embodiment, the first centering arm  106  and the second centering arm  108  are identical or nearly identical. The first and second centering arms  106  and  108  include an aperture  166  on a second end that is capable of accepting member  150  to provide an attachment point on each of the first and second centering arms  106  and  108  for coil spring  112 . As shown in  FIG. 6 , member  150  can be a fastener system, such as a nut and bolt arrangement, that is attached at the aperture  166 . The first and second centering arms  106  and  108  are shown aligned together in  FIG. 11C . 
       FIGS. 12-16  illustrate an alternative illustrative embodiment of a portion of the centering mechanism  100 . In  FIGS. 12-16 , the pump assembly  60  and the centering mechanism  100  are similar to that described above with respect to  FIGS. 1-11C . Common elements have the same reference number, and modified elements have the same reference number“′”. 
       FIG. 15  illustrates an alternative construction of the second bracket  104 ′, and  FIG. 16  illustrates an alternative construction of the first bracket  116 ′. In the alternative construction, the second bracket  104 ′ defines three adjusting slots  130 ′, and the first bracket  116 ′ correspondingly defines three bosses  118 ′. As shown in  FIG. 14 , three adjusting fasteners  132 ′ are provided to adjustably connect the second bracket  104 ′ to the first bracket  116 ′. 
     In the illustrated alternative embodiment, the tab  114 ′ of the second bracket  104 ′ defines a pair of apertures  115 ′. A tool (or more than one tool) may engage one or both of the apertures  115 ′ and be used to adjust the second bracket  104 ′ relative to the first bracket  116 ′. 
     In the illustrated alternative embodiment, the bosses  118 ′ defined in the first bracket  116 ′ do not depend below the lower surface of the first bracket  116 ′. Also, in the illustrated alternative embodiment, the second bracket  104 ′ and the first bracket  116 ′ are not provided with the cooperating protrusion  133  and lip  121 , described above. It should be understood, however, that such structure may be provided for this alternative embodiment. With these modifications, the first bracket  116 ′ and the second bracket  104 ′ (with the exception of the tab  114 ′) are substantially planar. 
     The illustrative embodiments provide for a centering system on a hydraulic drive pump that is easy to adjust. Merely by temporarily loosening fasteners  132  and engaging aperture  115  to move or rotate the second bracket  104  with respect to the first bracket  116 , the centering mechanism  100  can be easily adjusted so that that it is properly positioned. Thus, when there is no force applied on the control arm  102  by the operator through drive control actuators  58 , the centering mechanism  100  will urge the input shaft  88  to a neutral position. The arrangement allows for an easily adjustable centering mechanism that is amenable to small adjustments. 
     Although the present disclosure has been described with reference to the preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the disclosure.