Patent Publication Number: US-2022232775-A1

Title: Work vehicle having a cutter assembly with a pre-loaded gear train and method of controlling same

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     Not applicable. 
     STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     FIELD OF THE DISCLOSURE 
     This disclosure relates to work vehicles for cutting crop and, in particular, to the cutter assemblies thereof. 
     BACKGROUND OF THE DISCLOSURE 
     In the hay and forage industry, agricultural windrowers are configured to cut crop material from the ground and arrange the cut material in windrows for later processing (e.g., by a separate baler). A windrower may include a header having a wide cutter assembly thereon that extends across a path of travel of the machine. The cutter assembly includes an arrangement of gear-driven, rotary cutters that function to cut the crop material, with one or more motors (e.g., hydraulic motors) driving the gears. The cut crop material is then provided to a conditioner assembly in the header, which may act to crimp the crop after it is cut and redirect the crimped crop to form it into a uniform windrow. 
     SUMMARY OF THE DISCLOSURE 
     A work vehicle for cutting crop material is disclosed. The work vehicle includes a header supported by a chassis of the vehicle, with the header including a cutter assembly. The cutter assembly includes, in turn, a cutter bar frame, a series of rotary cutters mounted on the cutter bar frame and arranged in a lengthwise direction, and a gear train having gears coupled to the series of rotary cutters to transfer power thereto. The gear train having a first gear and a second gear. The work vehicle also includes a cutter control system having a first motor coupled to the first gear of the gear train to provide power to the gear train, a second motor coupled to the second gear of the gear train to provide power to the gear train, and a controller, including a processor and memory architecture, operably connected to the first motor and the second motor to control operation thereof. The cutter control system drives the first gear at a first speed via the first motor and drives the second gear at a second speed via the second motor, with the second speed being different than the first speed to pre-load the gear train into enmeshing engagement with each other in one rotational direction. 
     A method of controlling a cutter assembly in a header of a work vehicle for cutting crops is further disclosed. The method includes providing a cutter assembly having a series of rotary cutters coupled to a gear train having a first gear and a second gear and providing a first motor and a second motor to drive the first gear and the second gear, respectively, with the first and second motors operated by a controller. The method also includes driving the first gear at a first speed with the first motor and driving the second gear at a second speed with the second motor, with the first speed being different than the second speed to pre-load the gear train into enmeshing engagement with other in one rotational direction. 
     The details of one or more embodiments are set-forth in the accompanying drawings and the description below. Other features and advantages will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       At least one example of the present disclosure will hereinafter be described in conjunction with the following figures: 
         FIG. 1  is a perspective view of an example work vehicle in the form of a self-propelled windrower, with a top portion of a header housing removed, and that includes a cutter assembly and associated cutter control system, in accordance with an embodiment; 
         FIG. 2  is a perspective view of the cutter assembly included in the windrower of  FIG. 1   
         FIG. 3  is an underside view of a portion of the cutter assembly of  FIG. 2 , illustrating a portion of a gear train included in the cutter assembly; 
         FIG. 4  is a schematic view of an example hydraulic circuit for a cutter control system associated with the cutter assembly of  FIGS. 2 and 3 ; 
         FIG. 5  is a schematic view of another example hydraulic circuit for a cutter control system associated with the cutter assembly of  FIGS. 2 and 3 ; 
         FIG. 6  is a schematic view of another example hydraulic circuit for a cutter control system associated with the cutter assembly of  FIGS. 2 and 3 ; 
         FIG. 7  is a schematic view of another example hydraulic circuit for a cutter control system associated with the cutter assembly of  FIGS. 2 and 3 ; 
         FIG. 8  is a schematic view of another example hydraulic circuit for a cutter control system associated with the cutter assembly of  FIGS. 2 and 3 ; 
         FIG. 9  is a schematic view of another example hydraulic circuit for a cutter control system associated with the cutter assembly of  FIGS. 2 and 3 ; 
         FIG. 10  is a schematic view of another example hydraulic circuit for a cutter control system associated with the cutter assembly of  FIGS. 2 and 3 ; 
         FIG. 11  is a schematic view of another example hydraulic circuit for a cutter control system associated with the cutter assembly of  FIGS. 2 and 3 ; and 
         FIG. 12  is a schematic view of an example electrical circuit for a cutter control system associated with the cutter assembly of  FIGS. 2 and 3 . 
       Like reference symbols in the various drawings indicate like elements. For simplicity and clarity of illustration, descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the example and non-limiting embodiments of the invention described in the subsequent Detailed Description. It should further be understood that features or elements appearing in the accompanying figures are not necessarily drawn to scale unless otherwise stated. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are shown in the accompanying figures of the drawings described briefly above. Various modifications to the example embodiments may be contemplated by one of skill in the art without departing from the scope of the present invention, as set-forth the appended claims. 
     Overview 
     As previously noted, agricultural windrowers include a header having a wide cutter assembly thereon that extends across a path of travel of the machine, with one common style of cutter assembly including an arrangement of gear-driven, rotary cutters that function to cut the crop material. The rotary cutters extend generally along a length of the cutter assembly, with a gear train coupled to the rotary cutters to provide a driving power thereto. In many cutter bar assemblies, the gear train is driven by a single motor from one end of the cutter assembly, but some cutter bar assemblies have a dual motor configuration where the gear train is driven from both ends of the cutter assembly. In either configuration, the gears in the gear train are designed to have “clearance” or “gear backlash” between the gear teeth, which is necessary to prevent jamming and provide for smooth rotation of the meshed gears, while also may minimizing noise and preventing overheating of the gears. Because there is backlash provided between the gear teeth, a gear can be rotated a slight amount relative to its adjacent gear. When a series of gears are put together, as with the gear train of the cutter assembly, the amount of rotational movement of the last gear in the drive train relative to the first gear can be significant. 
     The above-described relative motion between the gears may cause issues when the windrower is operated. That is, when the windrower is cutting crop, one end of the cutter assembly can be loaded more heavily than the other end. In a single motor embodiment, if the end furthest from the motor is loaded less heavily than the end nearest to the motor, the gears on the far end can “over-run” or turn slightly faster than the motor because they have momentum. When this happens, the gear teeth are momentarily loaded in the reverse rotational direction, and a frequent reversal in the gear loading direction will result in gear “chatter” in the gear train. In a dual motor embodiment, crop loading on the gear train can similarly cause the motors on each of the opposing ends of the cutter assembly to be alternately loaded more heavily than the other motor. This causes the gears to be loaded up in one rotational direction when a gear driven by the first motor on one end of the cutter assembly is turning faster than a gear driven by the second motor on the other end of the cutter assembly and causes the gears to be loaded up in the other rotational direction when the gear driven by the second motor turns faster than the gear driven by the first motor. Again, when this happens, the gear teeth are alternately loaded in differing directions, resulting in gear chatter. Undesirably, this gear chatter in the cutter assembly can cause the gears to wear out sooner than desired 
     To prevent the gear train from being alternately loaded up in differing directions during operation and reduce the likelihood of gear chatter, a work vehicle cutter assembly with a pre-loaded gear train and an associated control method are provided. Specifically, a cutter control system operates to drive a first gear of the gear train at a first speed and drive a second gear of the gear train at a different second speed to generate torque wind-up on the gears, which causes the gears to remain biased in one rotational direction during operation of the cutter assembly. This torque wind-up or pre-loading of the gears inhibits the gears from alternately being loaded in different rotational directions during operation of the cutter assembly, thereby reducing or eliminating gear chatter and reducing the wear on the gears associated therewith. 
     According to example embodiments, the cutter control system operates to control first and second motors that drive the first and second gears, respectively. The cutter control system operates the second motor at a speed that is slightly lower than the speed of the first motor and maintains the second motor at a lower speed during operation to pre-load the gears of the gear train in one rotational direction. 
     In certain embodiments, the cutter control system is a hydraulic circuit that provides hydraulic oil to a first hydraulic motor and a second hydraulic motor of the cutter assembly, to drive the respective hydraulic motors. The hydraulic circuit controls the amount of hydraulic oil provided to the first and/or second hydraulic motors to controllably operate the first motor at a first speed and the second motor at a second speed, where the first speed is higher than the second speed. The first motor thereby operates to apply a main driving force to one end of the gear train while the second motor operates to apply a braking force to the other end of the gear train, with a torque wind-up being generated in the gear train to pre-load the gears thereof into enmeshing engagement with each other in one rotational direction (i.e., directionally pre-load the gears). The hydraulic circuit can control the amount of hydraulic oil provided to the first and/or second hydraulic motors via any of a number of hydraulic circuit arrangements, including by use of: an orifice or orifice valve that restricts hydraulic fluid flow to an inlet of one of the hydraulic motors, variable displacement pumps that selectively control hydraulic fluid flow to the hydraulic motors, and/or a priority valve that increases hydraulic fluid flow to an inlet of one of the hydraulic motors, as nonlimiting examples. 
     In other implementations, the cutter control system is an electric control system where motor drives provide a controlled power to a first electric motor and a second electric motor of the cutter assembly, to drive the respective electric motors. The motor drives operate through associated power electronics to control the voltage and/or current applied to the electric motors to controllably operate the first motor at a first speed and the second motor at a second speed, where the first speed is higher than the second speed. The first motor thereby operates to apply a main driving force to one end of the gear train while the second motor operates to apply a braking force to the other end of the gear train, with a torque wind-up being generated in the gear train to pre-load the gears thereof into enmeshing engagement with each other. 
     Example embodiments of a work vehicle with a cutter assembly and associated cutter control system according to this disclosure will now be described in conjunction with  FIGS. 1-12 . By way of non-limiting examples, the following describes the cutter assembly and cutter control system as incorporated into a self-propelled windrower. The following examples notwithstanding, the cutter assembly and cutter control system can be incorporated into other types of work vehicles, mower assemblies, or machines that include an elongated gear train therein and that could benefit from protection against gear chatter in the gear train. It is therefore recognized that aspects of the invention are not meant to be limited only to the specific embodiments described hereafter. 
     Example Embodiment(s) of a Work Vehicle with a Cutter Assembly Having a Pre-Loaded Gear Train 
     With initial reference to  FIG. 1 , a self-propelled windrower  20  is illustrated that is operable to mow and collect standing crop in a field, condition the cut crop as it moves through the machine (e.g., to improve its drying characteristics), and then return the conditioned material to the field in a windrow or swath. The windrower  20  includes a main chassis  22  supported on driven right and left front wheels  24 R and  24 L, respectively and on right and left caster mounted rear wheels, of which only a right rear wheel  26 R is shown. Carried on a forward end region of the chassis  22  is a cab  28 . Mounted on the chassis  22  behind the cab  28  is a casing  30  within which is located a power source (not shown) such as an internal combustion engine. A harvesting header  32  is coupled so as to be supported by the forward end of the chassis  22 . Operator controls (not shown) are provided in the cab  28  for operation of the windrower  20 , including the attached harvesting header  32 . 
     The harvesting header  32  includes an outer housing  33 , a top portion of which is removed in  FIG. 1  to illustrate various internal components of the header  32 . Positioned within housing  33  is a cutter assembly  34  that delivers cut crop to a following crop converging auger  36  that directs crop rearward into a discharge passage for further processing by a crop conditioning arrangement including upper and lower crop conditioning rolls  42  and  44 , respectively. Conditioned crop is expelled to the rear by the conditioning rolls  42  and  44  and is formed into a windrow by upright right and left, windrow forming panels (not shown) which are supported by a top wall of an open-bottomed frame  46  located between the front wheels  24 R and  24 L. 
     In certain embodiments, a controller  48  may also be provided. The controller  48  may be in electrical (or other) communication with various devices of the windrower  20 , to control various aspects of the operation of the windrower. In particular, the controller  48  may communicate with components in header  32  to control operation of cutter assembly  34 . The controller  48  may be configured as a computing device with one or more processors and memory architectures, as a hard-wired computing circuit (or circuits), as a hydraulic or electrohydraulic control device, and so on. 
     As shown in greater detail in  FIG. 2 , the cutter assembly  34  includes a cutter bar frame  50  that forms a base of the assembly  34 . Arranged along the cutter bar frame  50  is a series of rotary cutters  52  (e.g., ten cutters, as in  FIG. 2 ) that extend across the path of travel of the windrower  20 , with each cutter  52  being rotatable about its own upright axis. For the sake of convenience, the ten cutters  52  in  FIG. 2  will be denoted by the letters (a)-(j), beginning with the left most cutter in the series as viewed from the front of the machine. The cutters  52  are rotatably supported on an elongated, flat gear case  54  that extends underneath the cutters  52  for the full length thereof, with the gear case  54  mounted to a top side of the cutter bar frame  50 . The gear case  54  includes openings or pockets  56  therein, as shown in  FIG. 3 , and contains a train of flat spur gears—i.e., gear train  58  including gears  60 —a portion of which is shown in  FIG. 3 . The gear train  58  extends the length of the cutter assembly  34 , with the gears  60  operably engaged with one another to distribute driving power between one another and to the cutters  52 ( a )- 52 ( j ). Each of the cutters  52  includes a generally elliptical, formed blade carrier  62  and a pair of blades  64  mounted at opposite ends of the carrier  62 . As shown in  FIG. 2 , all of the cutters  52  are ninety degrees out of phase with one another inasmuch as the circular paths of travel of the blades  64  of adjacent cutters  52  overlap one another and must be appropriately out of phase to avoid striking each other. Due to the positive mechanical drive connection between the group of rotary cutters  52  through the spur gears  60 , such cutters  52  always remain properly in phase with one another. 
     A shaft assembly  66  is coupled to each of the outermost rotary cutters  52 ( a ),  52 ( j ) on opposing ends of the cutter assembly  34  and that projects upwardly from the cutter to define the axis of rotation thereof, with the outermost gears  60  of the gear train  58  driven by the shaft assemblies  66  for rotation therewith, as shown in phantom in  FIG. 2 . While shaft assemblies  66  are shown coupled to outermost rotary cutters  52 ( a ),  52 ( j ), it is recognized that the shaft assemblies  66  could be coupled to other rotary cutters, such as rotary cutters  52 ( b ),  52 ( i ), for example. Each shaft assembly  66  is centered within a cage  68  that provides protection thereto, such as by preventing crop from becoming wrapped up on the shaft assembly  66 . Additionally, each shaft assembly  66  includes a lower universal joint (not shown) housed within the cage  68  that provides for coupling of the shaft assembly  66  to a drive shaft  70 . The drive shaft  70  extends upward out of cage  68  and projects into a right-angle gearbox  72 . Inside the gearbox  72 , the drive shaft  70  operably connects with a horizontal output shaft  74  that ultimately drives the auger  36  and pair of conditioning rolls  42  and  44  ( FIG. 1 ) via a belt and pulley drive and a transmission box. 
     According to example embodiments described in detail here below, a cutter control system is provided to selectively drive gears  60 ( a ),  60 ( b ) at opposing ends of the gear train at different speeds. The cutter control system is described as including a drive arrangement  76  with rotary motors  78 ,  80 , along with an associated controller and control system or circuit (hydraulic or electrical circuit) that collectively operate to pre-load the gears  60  of the gear train  58 , via driving of gears  60 ( a ),  60 ( b ) at different speeds. In various embodiments, the motors  78 ,  80  may be in various configurations, including hydraulic motors or electric motors, as primary examples, although mechanically driven motors are also envisioned. 
     More specifically, the drive arrangement  76  drives the shafts  70  at each of opposing ends of cutter assembly  34 , along with the various components of the header  32  that derive power therefrom, including the rotary cutters  52  of cutter assembly  34 . Each of the motors  78 ,  80  is carried on an elevated platform  82  that is coupled onto the topside of gearbox  72 , so that the motors  78 ,  80  are disposed high above the crop handling region of the header  32 . Projecting downwardly from each motor  78 ,  80  is drive shaft  70 , which extends through platform  82  and into gearbox  72 , before passing on down to the associated shaft assembly  66  and to the gear train  58  to transfer power thereto. The two motors  78 ,  80  cooperatively drive and share the load of all of the rotary cutters  52  of the cutter assembly  34 , with the gear train  58  of the cutter assembly  34  receiving driving input power from the motors  78 ,  80 . This means, for example, that the gear  60 ( a ) (shown in phantom in  FIG. 2 ) associated with the cutter  52 ( a ) does not need to bear all the loading from the other gears in the gear train  58  since approximately one half that loading is directed to the gear  60 ( b ) (shown in phantom in  FIG. 2 ) associated with the rotary cutter  52 ( j ) at the opposite end of the gear train  58 . 
     In operation of the cutter assembly  34 , it is recognized that it is desirable to drive the opposing ends of the gear train  58  (e.g., the outermost gears  60 , to which shaft assemblies  66  are coupled) at different speeds, with a first gear  60 ( a ) at one end and a second gear  60 ( b ) at the other end driven at different speeds during operation of the windrower  20 . That is, driving of second gear  60 ( b ) at a speed that is lower than the speed of first gear  60 ( a )—or conversely driving of first gear  60 ( a ) at a speed that is lower than the speed of second gear  60 ( b )—pre-loads the gears  60  of the gear train  58  into enmeshing engagement with each other in one rotational direction. Maintaining such a speed relationship between the gears  60 ( a ),  60 ( b ), such as maintaining gear  60 ( b ) at a speed slower than gear  60 ( a ), prevents the gears  60  from alternately being loaded in different directions during operation of the cutter assembly  34 , thereby eliminating gear chatter and reducing wear on the gears  60 . 
     In example embodiments described in further detail here below, motors  78 ,  80  are operated at different speeds to provide for the gears  60 ( a ),  60 ( b ) being driven at different speeds. That is, motor  80  is operated at a speed that is lower than the speed of motor  78 —or conversely motor  78  is operated at a speed that is slightly lower than the speed of motor  80 —to provide the differential speeds of gears  60 ( a ),  60 ( b ). It is recognized, however, that in alternative embodiments, the driving of the gears  60 ( a ),  60 ( b ) at different speeds could be achieved in manners other than via the operation of motors  78 ,  80  at different speeds. For example, motors  78 ,  80  could be run at the same speed, but with a reduction gear (not shown) positioned between one of the motors  78 ,  80  and its respective driven gear  60 ( a ),  60 ( b ) in the gear train  58  to result in one of the gears being driven at a slower speed. 
     In certain example embodiments, a cutter control system is provided in the form of a hydraulic circuit that drives a pair of rotary hydraulic motors  78 ,  80  in drive arrangement  76 . Referring now to  FIG. 4 , and with continued reference to  FIGS. 1-3 , an example embodiment of a hydraulic circuit  86  is illustrated for controlling operation of the hydraulic motors  78 ,  80 . The hydraulic circuit  86  is illustrated as a closed-loop, hydrostatic system and is operable by hydraulic fluid, i.e., hydraulic oil, to enable running of motors  78 ,  80  according to a desired operation. An onboard platform pump  88  is powered by a motor  90  (e.g., electric motor) that may be driven by the engine of the windrower  20  to provide for circulation of hydraulic oil within the circuit  86 , with the platform pump  88  being mechanically driven by the motor  90 . The platform pump  88  is preferably a pressure-compensated, load-sensitive pump that includes a swash plate  92  (denoted schematically for purposes of illustration by the arrow associated with the pump) that may be adjustably stroked or destroked to change its angular position and correspondingly adjust the output flow rate of oil therefrom as measured, for example, in gallons per minute. The variable flow output from platform pump  88  is achieved via electronic displacement control of the pump, with increases and decreases in the output flow providing more or less flow to the motors  78 ,  80  to adjust the speed of the rotary cutters  52  on cutter assembly  34 . Other general components of the hydraulic circuit  86  include relief valves  94 , a charge pump  96 , and a charge pressure relief  98 , consistent with as known in a hydrostatic hydraulic circuit. 
     A controller  100  is provided in hydraulic circuit  86  to control operation of selected components therein, with it understood that controller  100  could be incorporated into the controller  48  of  FIG. 1  or provided as a separate controller. The controller  100  may be configured as computing devices with associated processor devices  100 ( a ) and memory architectures  100 ( b ), as hydraulic, electrical or electro-hydraulic controllers, or otherwise. As such, the controller  100  may be configured to execute various computational and control functionality with respect to the hydraulic circuit  86  and may be in electronic or hydraulic communication with various components therein. For example, in hydraulic circuit  86 , controller  100  provides for variable flow output from platform pump  88  via electronic displacement control thereof, with increases and decreases in the output flow providing more or less flow to the motors  78 ,  80  to adjust the speed of the rotary cutters  52  on cutter assembly  34 . In various embodiments, controller  100  may also communicate with actuators, sensors, valves and other devices within the hydraulic circuit. 
     In the hydraulic circuit  86 , a high-pressure line  102  leads from the platform pump  88  to a tee connection  104 , where one fluid path  106  leads to the motor  78  and another fluid path  108  leads to the motor  80 . A mechanical-type flow divider  110  (e.g., rotary style flow divider) is positioned at tee connection  104  to divide a flow of hydraulic oil provided from platform pump  88 . In the illustrated embodiment, the flow divider  110  operates to provide a 50-50 split of the hydraulic oil to the fluid paths  106 ,  108 , such that equal amounts of hydraulic oil flow along the fluid paths toward the motors  78 ,  80 . Return lines  112  lead from the motors  78 ,  80  back to another tee connection  114 , with a single return line  116  going to the backside of the pump  88 . A case drain line  118  is also connected to each of motors  78 ,  80  and leads to a reservoir  120  that stores low pressure hydraulic oil. Hydraulic oil from return lines  112  and case drain lines  118  flows into/through relief valves  94 , charge pump  96 , charge pressure relief  98  and reservoir  120  in a known manner to remove any oversupply of oil to the pump  88  and to provide cooling for the pump. 
     As shown in  FIG. 4 , to provide for differential operation of the motor  78  and the motor  80  in cutter assembly  34 , hydraulic circuit  86  includes an orifice valve (or more generally an “orifice”)  122  in fluid path  108  that operates to restrict or control the flow of hydraulic oil provided to the motor  80 . The orifice  122  is positioned along a secondary fluid path  124  that is parallel to the motor  80  (i.e., positioned across motor  80 ), such that a portion of hydraulic oil in fluid path  108  is diverted from an inlet  126  of motor  80  to flow along the secondary fluid path  124  and through orifice  122 , thereby bypassing motor  80  and being routed to join return line  112  at the outlet  128  of motor  80 . In the illustrated embodiment, the orifice  122  provides a restricted flow therethrough in a fixed amount, such that the amount of hydraulic oil diverted from the inlet  126  of motor  80  remains unchanged during operation of the motors  78 ,  80  via hydraulic circuit  86 . As an example, orifice  122  may be set such that the flow of hydraulic oil therethrough results in the flow of hydraulic oil to the inlet  126  of motor  80  being 5-10% less than the flow of hydraulic oil provided to motor  78 . This diverting or restriction of hydraulic fluid to motor  80  results in the motor operating at a reduced speed as compared to motor  78 , with the differential speed of motors  78 ,  80  resulting in the gears  60 ( a ),  60 ( b ) being driven at different speeds and the gears  60  of the gear train  58  being pre-loaded into enmeshing engagement with each other in one rotational direction to prevent gear chatter, as described in detail previously. 
     Referring now to  FIG. 5 , a hydraulic circuit  130  is provided according to another embodiment. The hydraulic circuit  130  is substantially similar to the hydraulic circuit  86  of  FIG. 4 , and thus common components of the circuit are identified consistent with those in  FIG. 4 . In hydraulic circuit  130 , the mechanical-type flow divider  110  divides a flow of hydraulic oil provided from platform pump  88  between fluid paths  106 ,  108 , such that equal amounts of hydraulic oil flow along the fluid paths toward the motors  78 ,  80 . In the embodiment of  FIG. 5 , a (variable) orifice valve  132  is positioned in fluid path  108  that operates to restrict the flow of hydraulic oil provided to the motor  80 . Specifically, the orifice valve  132  is positioned along a secondary fluid path  124  that is parallel to the motor  80 , such that a portion of hydraulic oil in fluid path  108  is diverted from the inlet  126  of motor  80  to flow along the secondary fluid path  124  and through orifice valve  132 , thereby bypassing motor  80  and being routed to join return line  112  at the outlet  128  of motor  80 . The orifice valve  132  is configured to selectively restrict or control the flow of hydraulic oil therethrough by a desired amount and may be an electro-hydraulically controlled valve, for example. 
     Controller  100  of the hydraulic circuit  130  is operatively connected to the orifice valve  132  to adjust the valve and control the amount of hydraulic oil that flows therethrough. Adjustment of the orifice valve  132  may be performed responsive to inputs received by the controller  100  of one or more operational parameters that are measured during operation of the hydraulic circuit  130  and the motors  78 ,  80 . For example, sensors may be included in hydraulic circuit that measure one or more of the speed of motors  78 ,  80  and pressure(s) within the hydraulic circuit (and the load on the motors  78 ,  80 )—with speed sensors  138  and pressure sensors  140  generally indicated in dashed lines in  FIG. 5 , as non-limiting examples. The controller  100  may implement a transfer function that adjusts the orifice valve  132  responsive to these received inputs. For example, the controller  100  may adjust the orifice valve  132  as the cutter assembly  34  is loaded up on one side or as the load drops down. In a case where the load drops on motor  78  and the load rises on motor  80 , for example, the controller  100  will open the orifice valve  132  by an increased amount in order that more hydraulic oil is diverted from motor  80 , such that motor  80  continues to run slower than motor  78  and a directional pre-load on the gears is maintained in a constant fashion—i.e., so that motor  78  continues to provide a main driving force on gear train  58  and motor  80  provides a braking force. As an example, orifice valve  132  may be adjusted such that the flow of hydraulic oil therethrough results in the flow of hydraulic oil to the inlet  126  of motor  80  being maintained at 5-10% less than the flow of hydraulic oil provided to motor  78 . 
     Referring now to  FIG. 6 , a hydraulic circuit  142  is provided according to another embodiment. Again, the hydraulic circuit  142  is substantially similar to the hydraulic circuit  86  of  FIG. 4 , and thus common components of the circuit are identified consistent with those in  FIG. 4 . In hydraulic circuit  142 , the mechanical-type flow divider  110  divides a flow of hydraulic oil provided from platform pump  88  between fluid paths  106 ,  108 , such that equal amounts of hydraulic oil flow along the fluid paths toward the motors  78 ,  80 . In the embodiment of  FIG. 6 , a (variable) orifice valve  144  is positioned in fluid path  108  that operates to restrict the flow of hydraulic oil provided to the motor  80 . The orifice valve  144  is positioned along a bypass fluid path  146  that is parallel to the motor  80 , with a portion of hydraulic oil in fluid path  108  being diverted from the inlet  126  of motor  80  to flow along the bypass fluid path  146  and through orifice valve  144 , thereby bypassing motor  80  and being routed or dumped directly into the case drain line  118  and back to reservoir  120 . 
     Similar to the orifice valve  132  of  FIG. 5 , the orifice valve  144  is configured to selectively restrict or control the flow of hydraulic oil therethrough by a desired amount. In one embodiment, the orifice valve  144  is an electronically controlled valve in operable communication with controller  100 . The controller  100  operates to adjust the orifice valve  144  responsive to inputs thereto of one or more operational parameters (motor speed, hydraulic circuit pressure(s), etc.), with a transfer function of the controller  100  adjusting the orifice valve  144  responsive to these received inputs. As an example, orifice valve  144  may be adjusted such that the flow of hydraulic oil therethrough results in the flow of hydraulic oil to the inlet  126  of motor  80  being maintained at 5-10% less than the flow of hydraulic oil provided to motor  78 . By varying the fluid flow through orifice valve  144 , hydraulic circuit  142  ensures that motor  80  continues to run slower than motor  78  and that a directional pre-load on the gears  60  is maintained in a constant fashion to pre-load the gears  60  of the gear train  58  into enmeshing engagement with each other in one rotational direction. 
       FIG. 7  illustrates another embodiment of a hydraulic circuit  150 . Again, the hydraulic circuit  150  is substantially similar to the hydraulic circuit  86  of  FIG. 4 , and thus common components of the circuit are identified consistent with those in  FIG. 4 . In hydraulic circuit  150 , the mechanical-type flow divider  110  divides a flow of hydraulic oil provided from platform pump  88  between fluid paths  106 ,  108 , such that equal amounts of hydraulic oil flow along the fluid paths toward the motors  78 ,  80 . In the embodiment of  FIG. 7 , a (variable) orifice valve  152  is positioned in fluid path  108  that operates to bleed off a portion of the flow of hydraulic oil provided to the second motor  80 . The orifice valve  152  is positioned in a bleed-off fluid path  154  that is parallel to or across the flow divider  110 . Upon passing through the flow divider  110 , a portion of hydraulic oil in fluid path  108  is bled off and diverted back to high-pressure line  102  upstream from tee connection  104 . Accordingly, the flow of hydraulic oil in fluid path  108  provided to the inlet  126  of motor  80  can be reduced or restricted as compared to the flow of hydraulic oil in fluid path  106  that is provided to the motor  78 . 
     Similar to the orifice valves of  FIGS. 4 and 5 , the orifice valve  152  is configured to selectively restrict or control the flow of hydraulic oil therethrough by a desired amount. In one embodiment, the orifice valve  152  is an electronically controlled valve in operable communication with controller  100 . The controller  100  operates to adjust the orifice valve  152  responsive to inputs thereto of one or more operational parameters (motor speed, hydraulic circuit pressure(s), etc.), with a transfer function of the controller  100  adjusting the orifice valve  152  responsive to these received inputs. By varying the fluid flow through orifice valve  152 , hydraulic circuit  150  ensures that motor  80  continues to run slower than motor  78  to pre-load the gears  60  of the gear train  58  into enmeshing engagement with each other in one rotational direction. 
     Referring now to  FIG. 8 , a hydraulic control system or hydraulic circuit  156  is provided according to another embodiment. The hydraulic circuit  156  includes a pair of onboard platform pumps  158 ,  160  powered by a motor  90  that may be driven by the engine of the windrower  20  to provide for circulation of hydraulic oil within the hydraulic circuit  156 . Each of the platform pumps  158 ,  160  may be configured as a variable displacement pump operable to provide a controlled amount of hydraulic oil to a respective motor  78 ,  80  to run the motor according to a desired operation. Each of the pumps  158 ,  160  may, for example, be a pressure-compensated, load-sensitive pump that includes a swash plate  162  that may be adjustably stroked or destroked to change its angular position and correspondingly adjust the output flow rate of oil therefrom. Other general components of the hydraulic circuit  156  include relief valves  94 , charge pump  96 , and charge pressure relief  98 . 
     The hydraulic circuit  156  includes a high-pressure line  102  leading from each pump  158 ,  160  to its respective motor  78 ,  80 , such that a first high-pressure fluid path  106  leads to the motor  78  and a second high-pressure fluid path  108  leads to the motor  80 . Return lines  112  lead from the motors  78 ,  80  back to the backside of each of the respective pumps  158 ,  160 . A case drain line  118  is also connected to each of motors  78 ,  80  and leads to a reservoir  120  that stores low pressure hydraulic oil. 
     The variable flow output from each pump  158 ,  160  is achieved via electronic displacement control of the pumps  158 ,  160 , with increases and decreases in the output flow providing more or less flow to the motors  78 ,  80 , to adjust the speed of the rotary cutters  52  on cutter assembly  34 . For providing such electronic displacement control, controller  100  is provided in the hydraulic circuit  156  that is operatively connected to the variable displacement pumps  158 ,  160  to control the output flow rate of hydraulic oil therefrom. Controller  100  is programmed to control the output flow from the pumps  158 ,  160  such that the output flow from pump  158  is always greater than the output flow from pump  160 . The controller  100  may adjust the output flow from the pumps  158 ,  160  responsive to inputs received by the controller  100  in the form of an operator input (e.g., via controls in the cab  28 ) and/or one or more operational parameters that are measured during operation of the hydraulic circuit  156  and the motors  78 ,  80 . For example, sensors may be included in hydraulic circuit  156  that measure one or more of the motor speed of motors  78 ,  80  and pressure(s) within the hydraulic circuit  156 —with speed sensors  138  and pressure sensors  140  generally indicated in dashed lines in  FIG. 8 , as non-limiting examples. The controller  100  may adjust the output flow rate of one or more of the pumps  158 ,  160  responsive to the received inputs and to maintain a leader-follower relationship between the speed of the motors  78 ,  80 , i.e., that motor  80  always runs slower than motor  78 , to maintain a pre-load on the gears  60  in one rotational direction and thereby prevent gear chatter. As an example, one or more of the variable displacement pumps  158 ,  160  may be adjusted such that the flow of hydraulic oil provided to motor  80  is 5-10% less than the flow of hydraulic oil provided to motor  78 . 
     Another hydraulic circuit  166  is illustrated in  FIG. 9 , where a high-pressure line  102  leads from a platform pump  88  to a tee connection  104 , with one fluid path  106  leading to the motor  78  and another fluid path  108  leading to the motor  80 . A priority valve  168  is positioned in fluid path  106  to control the flow of hydraulic oil to motor  78 , with priority valve  168  functioning to give motor  78  priority to motor  80  regarding a flow of hydraulic oil thereto. The priority valve  168  may be configured as an electronically controlled valve in operable communication with controller  100 . As previously described, controller  100  may adjust the flow through priority valve  168  responsive to inputs received by the controller  100  in the form of one or more operational parameters that are measured during operation of the hydraulic circuit  166  and the motors  78 ,  80 . In operation of hydraulic circuit  166 , controller  100  controls the setting of priority valve  168  such that the flow of hydraulic oil provided to motor  78  through the valve is greater (e.g., 5-10% greater) than the flow of hydraulic oil provided to motor  80 , with motor  78  thus running at an increased speed as compared to motor  80 . 
     Another hydraulic circuit  170  is illustrated in  FIG. 10 , where a high-pressure line  102  leads from a platform pump  88  to a tee connection  104  having a flow divider  172  positioned thereat to divide a flow of hydraulic oil provided from platform pump  88 . In the illustrated embodiment, the flow divider  172  divides a flow of hydraulic oil from platform pump  88  between a fluid path  106  that leads to the motor  78  and a fluid path  108  that leads to the motor  80 . The flow divider  172  is configured to provide a differential flow between the two fluid paths  106 ,  108 , with the flow divider  172  directing a greater flow of hydraulic oil to motor  78  and a lesser flow of hydraulic oil to motor  80 . The division of hydraulic oil between fluid paths  106 ,  108  may be set at a fixed amount by flow divider  172 , with the flow of hydraulic oil provided to motor  80  being 5-10% less than the flow of hydraulic oil provided to motor  78  as an example amount. Motor  80  is thereby caused to run slower than motor  78 , which applies a pre-load on the gears  60  of cutter assembly  34  in one rotational direction. 
     Another hydraulic circuit  174  is illustrated in  FIG. 11 , where motors  78 ,  80  are provided as variable displacement motors (e.g., variable displacement axial piston motors) that may be selectively controlled to vary the speeds therebetween. A high-pressure line  102  leads from a platform pump  88  to a tee connection  104  having a flow divider  110  positioned thereat to divide a flow of hydraulic oil provided from platform pump  88  between fluid path  106  that leads to the motor  78  and fluid path  108  that leads to the motor  80 . The flow divider  110  operates to provide a 50-50 split of the hydraulic oil to the fluid paths  106 ,  108 , such that equal amounts of hydraulic oil flow to each motor  78 ,  80 . The operation of variable displacement motors  78 ,  80  to drive the rotary cutters  52  of cutter assembly  34  at a desired speed is achieved via electronic displacement control by controller  100 . Controller  100  is programmed to control the speed of motors  78 ,  80  such that the speed of motor  78  is always greater than the speed of motor  80 . The controller may adjust the speeds of the variable displacement motors  78 ,  80  responsive to inputs received by the controller  100  in the form of an operator input (e.g., via controls in the cab  28 ) and/or one or more operational parameters that are measured during operation of the hydraulic circuit and the motors  78 ,  80  (motor speeds and pressure(s) within the hydraulic circuit  174 ). The controller  100  may adjust the speed of one or more of the motors  78 ,  80  responsive to the received inputs and to maintain a constant relationship between the speed of the motors  78 ,  80 , i.e., that motor  80  always runs slower than motor  78 , to pre-load the gears  60  of the gear train  58  into enmeshing engagement with each other in one rotational direction 
     According to other embodiments, the motors  78 ,  80  in cutter assembly  34  ( FIG. 3 ) are in the form of rotary electric motors  78 ,  80 . Referring now to  FIG. 12 , and with continued reference to  FIGS. 1-3 , an electric control system  180  for driving and controlling operation of the electric motors  78 ,  80  is illustrated. The electric control system  180  receives power from the mechanical motion of the engine in windrower  20  and converts and conditions that power into an electric power suitable for use by the electric motors  78 ,  80 . In electric control system  180 , a generator  182  converts mechanical energy from the windrower engine to electric power in the form of alternating current (AC) power. The AC power output from generator  182  is provided to a pair of motor drives  184 ,  186 , with motor drive  184  providing a controlled power input to motor  78  and motor drive  186  providing a controlled power input to motor  80 . 
     In the illustrated embodiment, each of motor drives  184 ,  186  is an adjustable speed drive (ASD) designed to receive an AC power input from the generator  182 , rectify the AC input, and perform a DC/AC conversion of the rectified segment into a three-phase alternating voltage of variable frequency and amplitude that is supplied to its associated electric motor  78 ,  80 . In operation, AC power input from the generator  182  is fed to a rectifier bridge  188  that converts the AC power input to a DC power, such that a DC link voltage is present between rectifier bridge  188  and a switch array  190 . The DC link voltage is then buffered or smoothed by a DC link capacitor bank  192  and provided to switch array  190 , which includes a series of IGBT switches (for example) and anti-parallel diodes that collectively form a PWM inverter  194 . PWM inverter  194  controls IGBT switches to synthesize variable-frequency, variable-amplitude DC voltage waveforms that are delivered to its associated motor  78 ,  80  following a constant Volts-per-Hertz or vector controls with or without speed/position sensors algorithm. In this regard, the motor drives  184 ,  186  provide voltage regulation in steady state and fast dynamic step load response over a full load range. 
     As shown in  FIG. 12 , controller  100  in electric control system  180  is operatively coupled with the motor drives  184 ,  186  to provide control functions thereto. Controller  100  is programmed to operate motor drives  184 ,  186  to provide a controlled power (controlled voltage and/or current) to motors  78 ,  80  to control the speed (or torque) of the motors for driving rotary cutters  52 , such as according to operator inputs (e.g., via controls in the cab  28 ) received thereby. Additionally, controller  100  is programmed to operate motor drives  184 ,  186  to provide a controlled power to motors  78 ,  80  to operate the motors such that the speed (or torque) of motor  78  is always greater than the speed (or torque) of motor  80 , such as by a value of 5-10%. To maintain motor  78  at a higher speed (or torque) than motor  80 , controller  100  receives one or more operational parameters that are measured during operation of the motors  78 ,  80 . Sensors may be included in electric control system  180  that measure one or more of the speed or torque of motors  78 ,  80 —with speed sensors  198  and torque sensors  200  generally indicated in dashed lines in  FIG. 12 . The controller  100  may adjust the speed (or torque) of one or more of motors  78 ,  80  responsive to the received inputs and to maintain a leader-follower relationship between the motors  78 ,  80  (e.g., that motor  80  always runs slower than motor  78 ) to pre-load the gears  60  of the gear train  58  into enmeshing engagement with each other in one rotational direction and thereby prevent gear chatter. 
     A dual motor drive of a cutter assembly included in a harvesting header may be controlled according to a number of methods. A first motor of the dual motor drive is caused to operate at a speed that is greater than a speed of the second motor, and this speed differential between the motors is maintained during operation of the header such that such that the first motor always applies a main driving force to the gear train of the cutter assembly and the second motor always applies a secondary driving force (i.e., braking force) to the gear train of the cutter assembly. The speed differential between the motors causes a directional pre-load to be applied onto the gears of the gear train, and this directional pre-load is maintained during operation of the header. Chatter between the gears is thus prevented, thereby reducing wear on the gears and increasing the longevity thereof. 
     Embodiments include a hydraulic control system (hydraulic circuit) that controls operation of first and second hydraulic motors of the cutter assembly. Methods for controlling operation of the hydraulic motors of the cutter assembly may be implemented by any of the hydraulic circuits illustrated in  FIGS. 4-11 . In embodiments, the flow of hydraulic oil to the first motor and second motor of the cutter assembly is controlled via the use of valves (a fixed or variable orifice valve or a priority valve) that restrict the flow of oil to one of the motors or prioritize the flow of oil to one of the motors, to achieve the differential flow of hydraulic oil to the motors. In other embodiments, the flow of hydraulic oil to the first motor and second motor of the cutter assembly is controlled via the use of one or more variable displacement pumps that provide an increased flow of oil to one of the motors, to achieve the differential flow of hydraulic oil to the motors. In other embodiments, the flow of hydraulic oil to the first motor and second motor of the cutter assembly is controlled via the use of a flow divider that diverts differing amounts of oil to the two motors, to achieve the differential flow of hydraulic oil to the motors. In other embodiments, the first motor and second motor of the cutter assembly are variable displacement motors that are operated according to a differential electronic displacement control scheme, such that the first motor operates at a higher speed than the second motor. In embodiments, dynamic control of the hydraulic motors is enabled via the acquisition and analysis of one or more operational parameters of the hydraulic circuit (motor speed, system pressure, etc.), to maintain a desired differential speed relationship between the motors. 
     Other embodiments have an electric control system that controls operation of first and second electric motors of the cutter assembly. Methods for controlling operation of the electric motors of the cutter assembly may be implemented by the electric control system illustrated in  FIG. 12 . Power provided to the first and second motors is controlled (via operation of motor drives associated with the electric motors) to operate the motors such that the speed (or torque) of the first motor is always greater than the speed (or torque) of the second motor. In embodiments, dynamic control of the electric motors is enabled via the acquisition and analysis of one or more operational parameters of the motors (motor speed, motor torque), to maintain a desired differential speed relationship between the motors. 
     ENUMERATED EXAMPLES 
     The following examples are provided, which are numbered for ease of reference. 
     1. A work vehicle for cutting crop material includes a header supported by a chassis of the vehicle, with the header including a cutter assembly. The cutter assembly includes, in turn, a cutter bar frame, a series of rotary cutters mounted on the cutter bar frame and arranged in a lengthwise direction, and a gear train having gears coupled to the series of rotary cutters to transfer power thereto. The work vehicle also includes a cutter control system having a first motor coupled to a first gear of the gear train to provide power to the gear train, a second motor coupled to a second gear of the gear train to provide power to the gear train, and a controller, including a processor and memory architecture, operably connected to the first motor and the second motor to control operation thereof. The cutter control system drives the first gear at a first speed via the first motor and drives the second gear at a second speed via the second motor, with the second speed being different than the first speed to pre-load the gears of the gear train into enmeshing engagement with each other in one rotational direction. 
     2. The work vehicle of example 1, wherein the cutter control system operates the first motor at a first motor speed to apply a main driving force to the first gear and operates the second motor at a second motor speed that is lower than the first motor speed to apply a braking force to the second gear, the main driving force and the braking force generating torque wind-up in the gear train to pre-load the gears. 
     3. The work vehicle of example 1, wherein the first motor comprises a first hydraulic motor and the second motor comprises a second hydraulic motor, and wherein the cutter control system comprises a hydraulic circuit that provides hydraulic oil to the first hydraulic motor along a first fluid path and to the second hydraulic motor along a second fluid path to drive the respective first and second hydraulic motors. 
     4. The work vehicle of example 3, wherein the hydraulic circuit is configured to deliver a first flow of the hydraulic oil to the first hydraulic motor and a second flow of hydraulic oil to the second hydraulic motor to cause the first hydraulic motor to operate at the first motor speed and the second hydraulic motor to operate at the second motor speed. 
     5. The work vehicle of claim 4, wherein the hydraulic circuit includes a flow restriction of the hydraulic oil to the second hydraulic motor. 
     6. The work vehicle of example 4, wherein the hydraulic circuit further includes a fluid pump to circulate the hydraulic oil, a flow divider to divide the hydraulic oil received from the fluid pump between the first fluid path and the second fuel path, and an orifice positioned in the second fluid path downstream or across from the flow divider to divert a portion of the hydraulic oil in the second fluid path from an inlet of the second motor. 
     7. The work vehicle of example 6, wherein the orifice comprises an orifice valve that is selectively controllable to vary an amount of the hydraulic oil that is diverted from the inlet of the second motor. 
     8. The work vehicle of example 7, wherein the controller is programmed to receive an input from one or more sensors in the cutter control system comprising one or more of motor speed, motor load, and hydraulic circuit pressure, and adjust the orifice valve based on the input. 
     9. The work vehicle of example 3, wherein the hydraulic circuit comprises a first pump to provide a first flow of the hydraulic oil along the first fluid path and a second pump to provide a second flow of the hydraulic oil along the second fluid path, wherein the second flow provided by the second pump is less than the first flow provided by the first pump to cause the first hydraulic motor to operate at the first motor speed and the second hydraulic motor to operate at the second motor speed. 
     10. The work vehicle of example 3, wherein the hydraulic circuit comprises a fluid pump to circulate the hydraulic oil along the first fluid path and the second fluid path and a priority valve positioned in the first fluid path and operable to selectively increase a flow of hydraulic oil to the first hydraulic motor as compared to a flow of hydraulic oil to the second hydraulic motor, to thereby cause the first hydraulic motor to operate at the first motor speed and the second hydraulic motor to operate at the second motor speed. 
     11. The work vehicle of example 2, wherein the first motor comprises a first electric motor and the second motor comprises a second electric motor, and wherein the cutter control system comprises motor drives configured to provide controlled power to the first and second electric motors to operate the first electric motor at the first motor speed and operate the second electric motor at the second motor speed. 
     12. A method of controlling a cutter assembly in a header of a work vehicle for cutting crops includes providing a cutter assembly having a series of rotary cutters coupled to a gear train having a first gear and a second gear and providing a first motor and a second motor to drive the first gear and the second gear, respectively, with the first and second motors operated by a controller. The method also includes driving the first gear at a first speed with the first motor and driving the second gear at a second speed with the second motor, with the first speed being different than the second speed to pre-load the gears of the gear train into enmeshing engagement with other in one rotational direction. 
     13. The method of example 12, wherein driving the first gear at the first speed comprises operating the first motor at a first motor speed to apply a main driving force to the first gear and wherein driving the second gear at the second speed comprises operating the second motor at a second motor speed that is lower than the first motor speed to apply a braking force to the second gear, the main driving force and the braking force generating torque wind-up in the gear train to pre-load the gears. 
     14. The method of example 13, wherein the first motor comprises a first hydraulic motor and the second motor comprises a second hydraulic motor, and wherein the method comprises delivering a first flow of hydraulic oil to the first hydraulic motor along a first fluid path and delivering a second flow of hydraulic oil to the second hydraulic motor along a second fluid path to cause the first hydraulic motor to operate at the first motor speed and the second hydraulic motor to operate at the second motor speed. 
     15. The method of example 12, wherein the first motor comprises a first electric motor and the second motor comprises a second electric motor, and wherein the method comprises providing, via the controller, controlled power to the first and second electric motor to operate the first electric motor at the first motor speed and operate the second electric motor at the second motor speed. 
     Conclusion 
     The foregoing has thus provided a work vehicle for cutting crop material featuring a header with a cutter assembly having a series of rotary cutters driven by a gear train that receives power from first and second motors of a cutter control system. The cutter control system operates to drive a first gear of the gear train at a first speed via the first motor and drive a second gear of the gear train at a second speed via the second motor, with the second speed being different than the first speed to pre-load the gear train into enmeshing engagement with each other in one rotational direction, thereby reducing or eliminating gear chatter and reducing the wear on the gears associated therewith. 
     As used herein, the singular forms “a”, “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the disclosure. Explicitly referenced embodiments herein were chosen and described to best explain the principles of the disclosure and their practical application, and to enable others of ordinary skill in the art to understand the disclosure and recognize many alternatives, modifications, and variations on the described example(s). Accordingly, various embodiments and implementations other than those explicitly described are within the scope of the following claims.