You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
This is a divisional of application Ser. No. 13/169,369 filed on Jun. 27, 2011 and issued as U.S. Pat. No. 8,683,720 on Apr. 1, 2014, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to a hydraulic suspension hitch system. In an embodiment, the present disclosure relates specifically to a hydraulic suspension hitch system for a work machine, such as a wheel tractor-scraper machine. 
     BACKGROUND 
     A wheel tractor-scraper is a machine employed in various industries, such as agriculture, construction and mining to load, haul, eject and spread layers of earth. Such machines are particularly suited for applications such as roadway construction and site preparation, where material needs to be removed or added while creating or maintaining grade and hauling occurs over a distance. Conventional wheel tractor-scrapers typically include a tractor portion having a forward frame member that supports an operator station and a propulsion power source operatively coupled to the drive wheels of the machine. An articulated joint couples the tractor portion to the rear scraper portion of the machine. The scraper portion has a rear frame member that supports both a bowl for collecting and hauling material, and the rear wheels. During operation, the bowl is typically lowered to engage the ground along a cutting edge that is driven forward by the machine, thus, scraping the earth and loading the bowl. These machines may have an earth-moving work tool, such as an elevator, conveyor, auger, or spade, associated with the bowl to facilitate penetration, loading, and/or unloading of the material to be transported. 
     One problem with wheel tractor-scrapers is that the articulated joint that couples the tractor portion to the rear scraper portion of the machine may transfer a great deal of pitch and bounce causing shock and vibration to propagate through the machine to the operator when the machine contacts bumps and/or holes along a driving path. 
     The disclosure of U.S. Pat. No. 3,311,389 provides a system for control of pitch and bounce in tractor-trailer machines. Thus, in the &#39;389 patent a tractor-trailer hitch is shown in which vertical motion is permitted, but limited and cushioned by a hydraulic cylinder connected between the units and associated with gas over oil accumulators and a fixed restrictive element (e.g., a fixed fluid orifice) to provide the desired spring rate. Given that this system is to reduce shock and vibration, it is desired to keep the hydraulic cylinder&#39;s rod situated near a centered extension point during travel time so that it may extend or retract as the wheels of the scraper portion engage bumps or holes in the driving path. To slow movement of the cylinder rod so that it is less likely to “top out” or “bottom out” when bumps or holes in the driving path are engaged, a fixed restriction is provided in the fluid line, thus slowing flow of fluid to or from the cylinder. Shortcomings of the &#39;389 patent were improved upon by U.S. Pat. No. 3,430,657, which provides a balancing of gravitational forces in a vertically disposed valve spool. 
     However, these references both only provide a fixed restrictive element, which may have only one ideal loading weight to minimize vibration and shock propagated through the machine. Thus, it is desirable to provide a system that improves upon these and other shortcomings of an articulated hitch system, as discussed above, and allows for tuned vibration damping at multiple loading levels of the machine. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present disclosure provides a suspension system. In an embodiment, the suspension system includes a hydraulic cylinder having an extendable rod. The hydraulic cylinder is fluidly coupled to a fluid pump. The suspension system also includes a control valve fluidly coupled between the hydraulic cylinder and the fluid pump. The control valve adjusts an extension length of the rod. In addition, the suspension system includes a variable orifice fluidly coupled between the hydraulic cylinder and the control valve. The variable orifice is configured to adjust resistance to extension and retraction of the rod. Furthermore, the suspension system includes a first fixed restrictive element fluidly coupled in series with the variable orifice to dissipate energy as the rod extends, and a second fixed restrictive element fluidly coupled in series with the variable orifice to dissipate energy as the rod retracts. 
     In another aspect, the present disclosure provides a cushion hitch that couples a towed vehicle to a drive vehicle. In an embodiment, the hitch includes a linkage system having first and second pivotable links configured to couple a towed vehicle to a drive vehicle. The hitch also includes a suspension system coupled to the linkage system. The suspension system is configured to adjust the ride height of the towed vehicle with respect to the drive vehicle. In an embodiment, the suspension system includes a hydraulic cylinder with an extendable rod. The hydraulic cylinder and the rod are coupled between the first and second pivotable links. A control valve is fluidly coupled between the hydraulic cylinder and the fluid pump. The control valve is configured to adjust an extension length of the rod. A variable orifice is fluidly coupled between the hydraulic cylinder and the control valve and is configured to adjust resistance to extension and retraction of the rod. A first fixed restrictive element is fluidly coupled in series with the variable orifice to dissipate energy as the rod extends and a second fixed restrictive element is fluidly coupled in series with the variable orifice to dissipate energy as the rod retracts. 
     In a further aspect, the present disclosure provides a tractor-scraper machine. The tractor-scraper machine includes a tractor drive vehicle having a propulsion system and a hydraulic fluid pump. The tractor-scraper also includes a towed scraper. Furthermore, the tractor-scraper includes a cushion hitch that couples the towed scraper to the tractor drive vehicle. In an embodiment, the cushion hitch includes first and second pivotable links coupled between the towed scraper and the tractor drive vehicle. A suspension system is coupled to the cushion hitch. The suspension system includes a hydraulic cylinder with an extendable rod. The hydraulic cylinder and the rod are coupled between the first and second pivotable links. A control valve is fluidly coupled between the hydraulic cylinder and the fluid pump. The control valve is configured to adjust an extension length of the rod. A variable orifice is fluidly coupled between the hydraulic cylinder and the control valve and is configured to adjust resistance to extension and retraction of the rod. A first fixed restrictive element is fluidly coupled in series with the variable orifice to dissipate energy as the rod extends, and a second fixed restrictive element is fluidly coupled in series with the variable orifice to dissipate energy as the rod retracts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a side view of an embodiment of a work machine, illustrated here as a tractor-scraper, having a cushion hitch hydraulic suspension system. 
         FIG. 2  illustrates an enlarged portion of the work machine of  FIG. 1 , showing an embodiment of the cushion hitch hydraulic suspension system in schematic form. 
         FIG. 3  illustrates a schematic diagram of an embodiment of a fluid circuit for the cushion hydraulic suspension system of  FIG. 2 . 
         FIG. 4  illustrates a graph of an operating range for fluid flow rate vs. pressure drop for an embodiment of the fluid circuit of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates generally to a hydraulic suspension hitch system. In an embodiment, the present disclosure relates specifically to a hydraulic suspension hitch system for a work machine, such as a wheel tractor-scraper machine. 
       FIG. 1  illustrates an elevating wheel tractor-scraper  10  having a tractor portion  11  (e.g., a drive vehicle), with a front frame section  12 , and a scraper portion  13  (e.g., a towed vehicle), with a rear frame section  14 , that are pivotally coupled through an articulation hitch  16 . Steering may be provided by one or more steering cylinders  32  (actuators) (one shown) mounted between the tractor portion  11  and scraper portion  13  on opposing sides of the scraper  10 . 
     The front frame section  12  supports a power source  20  and a cooling system (not shown). The power source  20  is operatively connected through a transmission (not shown) to become a propulsion system to drive front wheels  24  located on opposite sides of scraper  10  for primary propulsion of the scraper  10 . The front frame section  12  may also support an operator station  18  for primary control of the scraper  10  during operations of the scraper  10 . 
     The rear frame section  14  supports a bowl  28  and rear wheels  26 . The bowl  28  may also include a work tool  30 , such as an elevator  34 . In another embodiment, the bowl  28  may include an auger, a conveyor, and/or a spade, to facilitate penetration, loading, and/or unloading of the material to be transported by scraper  10 . 
     Power source  20  is a propulsion system that may include an engine such as a diesel engine, a gasoline engine, a gaseous fuel powered engine such as a natural gas engine, or any other type of engine. Power source  20  may alternatively include a non-combustion source of power such as a fuel cell, a power storage device, an electric motor, or other similar mechanism. 
     To propel the scraper  10 , power source  20  may be operatively coupled with front wheels  24  using a transmission (not shown), torque converter (not shown), gear box (not shown), transfer case (not shown), differential (not shown), drive shaft (not shown), reduction gear arrangement, and/or any other devices configured to transmit power from power source  20  to the wheels  24 . 
     In an alternative embodiment, scraper  10  may include an electric or hydraulic drive (not shown). For example, power source  20  may be operatively connected to a pump (not shown), such as a variable or fixed displacement hydraulic pump. The pump may produce a stream of pressurized fluid directed to one or more motors (not shown) associated with wheels  24  for the primary means of propulsion. Alternatively, power source  20  may be drivably connected to an alternator or generator (not shown) configured to produce an electrical current used to power one or more electric motors (not shown) for driving the front wheels  24 . 
     In addition to driving the front wheels  24 , power source  20  may be configured to supply power to a work tool  30  employed by the scraper  10  to penetrate and/or transfer material into or out of bowl  28 , or to perform other operations. For example, in one embodiment, a transmission (not shown) is connected to a fluid pump (not shown). The pump may be fluidly connected through one or more supply and/or return lines (not shown) to supply a flow of pressurized fluid to a hydraulic motor (not shown), which is in turn operatively connected to power work tool  30 . In one embodiment, work tool  30  is an elevator  34 . Elevator  34  generally includes a series of parallel, horizontally disposed flights  36  connected to a drive chain  38 . The drive chain  38  is operatively connected to rotational sprockets, an elevator drive shaft, and/or an elevator motor. 
     Throughout the specification, use of the terms supply and return in the alternative, or shown as “supply/return” should be understood to refer to the fact that the system may include a reversible pump that may be employed to change the direction of flow within particular conduits, in one direction acting as a supply, and in the other acting as a return line. 
     Wheel tractor-scrapers may be employed in push-pull operations, wherein a first tractor scraper is either pulled or pushed by a second machine, for example, a track-type dozer or another wheel tractor-scraper, during the loading process. Wheel tractor-scrapers are often provided with hitches or push bars to facilitate these operations. Some large wheel tractor-scrapers are provided with an additional, rear mounted engine  40  or other secondary propulsion power source system operatively connected to drive the rear wheels  26  of the machine  10  (e.g., twin-engine scrapers), making these machines better suited for handling adverse terrain and worksite conditions. Other alternatives provide a fluid operated rear wheel drive assist system on the machines. 
       FIG. 2  illustrates an enlarged portion of an embodiment of the work machine (scraper  10 ) of  FIG. 1 , showing an embodiment of the articulation cushion hitch hydraulic suspension system  16  in schematic form.  FIG. 3  illustrates a schematic diagram of an embodiment of a fluid circuit for the cushion hydraulic suspension system of  FIG. 2 . Front frame section  12  of tractor  11  is coupled to rear frame section  14  of scraper  13  via first  42  and second  44  pivotable links. In an embodiment, the hitch  16  includes a vertical link  45  coupling first  42  and second  44  pivotable links to rear frame section  14  using one or more vertical pivot pins  48 . 
     First  42  and second  44  pivotable links are formed of a rigid material, such as steel, iron, or other high tensile strength metallic material. In the alternative, it is contemplated that first  42  and second  44  pivotable links may be formed of a composite material, such as carbon graphite, Kevlar, or other high strength materials. First  42  and second  44  pivotable links couple front frame section  12  with rear frame section  14  at hitch pivot  46  locations. Pivot  46  locations may include a pin (not shown) or other holding device that passes through openings (not shown) in links  42 ,  44  and also through openings (not shown) in frame sections  12 ,  14 . First  42  and second  44  pivotable links may be situated in non-parallel planes, thus substantially creating a trapezoidal shape with first  42  and second  44  pivotable links, front frame section  12  and vertical link  45 . As should be readily understood, such a configuration of the hitch  16  allows for a pivoting motion along multiple planes and axis between tractor  11  and scraper  13 . 
     A hydraulic cylinder assembly  50 , having an extendable rod  52 , is disposed between and coupled to first  42  and second  44  pivotable links. As shown in  FIG. 2 , an embodiment provides that hydraulic cylinder assembly  50  has a body portion that couples with a forward or tractor portion pivot  46  and the extendable rod  52  couples with a rearward or scraper portion pivot  46 . However, it should be understood that the orientation of hydraulic cylinder assembly  50  may be inverted and/or switched to the other of the pivots  46  so long as first  42  and second  44  pivotable links are configured to pivot between the tractor  11  and the scraper  13 . As extendable rod  52  extends, a ride height of scraper  13  is raised. Conversely, as extendable rod  52  retracts, the ride height of scraper  13  is lowered due to pivoting of first  42  and second  44  pivotable links at pivots  46 . 
     Extension/retraction of extendable rod  52  is provided via a pressurized fluid (e.g., a hydraulic fluid) being controlled by a control valve  62 . To extend extendable rod  52  of hydraulic cylinder assembly  50 , control valve  62  receives pressurized fluid from a fluid pump  63  that is powered by power source  20  or by some other power source. Fluid pump  63  passes the pressurized fluid through a fluid line  64 , a variable orifice assembly  66 , hydraulic cylinder assembly  50 , fluid lines  68  and  69 , and then to a fluid tank return/holding tank  70 . As should be understood, the fluid system (e.g., a hydraulic fluid system) is generally a closed loop fluid system where the operable fluid is pressurized to perform a work function and then is returned to be used again. As will be explained in more detail below, variable orifice assembly  66  is a system that influences a speed at which extendable rod  52  extends and contracts. 
     To retract extendable rod  52 , control valve  62  essentially reverses flow of the pressurized fluid through hydraulic cylinder assembly  50 . One embodiment and one state of operation for control valve  62  is shown in a schematic view provided in  FIG. 3  having a variety of fluid valves and fluid lines. Other embodiments of control valve  62  may also be used with the present disclosure. However, for sake of brevity, control valve  62  is described for one embodiment with the schematic diagram of  FIG. 3 , which should be readily understood by those having ordinary skill in the art, and is not explained further. 
     Control valve  62  may lock extendable rod  52  into a fully retracted location, a fully extended location, and/or at any location in between. For example, when filling bowl  28 , extendable rod  52  may be locked fully retracted, thus lowering bowl  28  and work tool  30 . In addition, control valve  62  may position extendable rod  52  at a given extension length or ride height and yet also allow the extendable rod  52  to “float” (e.g., retract and extend) as wheels  24 ,  26  engage bumps and/or holes in a driving path, which in turn, causes first  42  and second  44  pivotable links to pivot at pivot points  46 . While driving wheel tractor scraper  10  it may be desirable to loosely hold extendable rod  52  in a somewhat middle location to absorb shock provided by bumps and holes along the driving path. 
     One or more accumulators  72  are fluidly coupled with the control valve  62  via a fluid line  74 . Accumulator  72  is a fluid tank having a free-floating piston, bladder, or other device that divides accumulator  72  into different chambers. One chamber is for the pressurized fluid and one chamber is for a compressible gas (e.g., nitrogen). Accumulator  72  receives the pressurized fluid in the fluid chamber, which displaces the piston or bladder, thus compressing the gas in the gas chamber. Accordingly, accumulator  72  provides compliance to the pressurized fluid. For example, if the wheel tractor scraper  10  hits a bump or hole while driving, extendable rod  52  is likely to be forced to extend or retract very quickly as shock of the bump or hole is transferred between tractor  11  and scraper  13 . This passes the pressurized fluid through control valve  62  and to or from accumulator  72 . Thus, accumulator  72  absorbs a significant amount of this shock/energy rather than passing it between tractor  11  and scraper  13 , as would happen if articulation hitch  16  was rigid. 
     Turning again to  FIG. 3 , the disclosure provides an embodiment of a variable orifice assembly  66 . In an embodiment, variable orifice assembly  66  includes a variable orifice  76 . A controller  78  provides operation signals such as electrical, pressurized fluid, or other communication signals to variable orifice  76 . The operation signals cause the variable orifice  76  to increase and/or decrease a size of a fluid passageway through variable orifice  76 . This changing of size of the fluid passageway influences or otherwise controls a resistance to the pressurized fluid flowing through the variable orifice  76 . This, in turn, influences or otherwise controls the damping or energy dissipated by the extension and retraction of extendable rod  52  of hydraulic cylinder assembly  50 . Accordingly, an extension and retraction rate for extendable rod  55  can be tuned to provide an optimized hitch rigidity and an optimized machine efficiency, (e.g., more rigid hitch), an optimized operator ride comfort (e.g., less rigid hitch), or anywhere in between. 
     If, variable orifice  76  does not respond to control signals from controller  78  or otherwise fails to operate or is not active, the fluid opening may float anywhere along an operating range for the variable orifice. Accordingly, this situation would not provide the tuned operation for articulation hitch  16 . Thus, first  80  and second  86  choke/check valves are fluidly coupled in series with variable orifice  76 . First choke/check valve  80  includes a first fixed orifice  82  and a first check valve  84  in parallel with one another, and in series with variable orifice  76 . Similarly, second choke/check valve  86  includes a second fixed orifice  88  and a second check valve  90  in parallel with one another, and in series with variable orifice  76 . First check valve  84  and second check valve  90  may be biased check valves requiring fluid to reach a pre-determined pressure to open the valve and pass through. 
     In an embodiment, variable orifice assembly  66  is configured to receive variable orifice  76 , first check valve  84 , second check valve  90 , and has first  82  and second  88  fixed orifices formed in a block manifold. However, components of variable orifice assembly  66  may also be formed using individual components fluidly coupled together. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure provides wheel tractor-scraper  10  that includes an articulation hitch  16 . In operation, the scraper  10  loads, hauls, and ejects earth, thus efficiently moving loads of earth from one location to another location. Articulation hitch  16  is configured as a cushion hitch, which greatly reduces shock and vibration felt by an operator of scraper  10 . Hydraulic cylinder assembly  50  (including extendable rod  52 ) and pivotable links  42 ,  44  enable articulation hitch  16  to adjust a height of scraper  13  relative to tractor  11  and also to absorb energy transfer between tractor  11  and scraper  13  as control valve  62  permits extendable rod  52  to extend and retract, thus causing pivotable links  42  and  44  to pivot about pivot points  46 . Control valve  62  may be employed to keep extendable rod  52  at a somewhat middle extension length during driving times to provide hydraulic cushion suspension for hitch  16 . 
     Variable orifice  76  provides a tuned operation for extension and retraction speeds of extendible rod  52  as instructed by controller  78 . Controller  78  may employ variable orifice  76  to provide different damping rates for different operations of wheel tractor scraper  10 . For example, when loading or unloading bowl  28 , variable orifice  76  may be smaller, causing articulation hitch  16  to be more rigid. Conversely, when driving wheel tractor scraper  10 , variable orifice  76  may be larger, causing articulation hitch  16  to be less rigid, and thus providing a smoother ride for the operator. 
     However, in the event that variable orifice  76  does not respond to instructions from controller  78 , first  80  and second  86  choke/check valves provide fixed tuning for extension and retraction speeds for extendable rod  52 . Specifically, if variable orifice  76  is not active or is not operational, fluid pressure at variable orifice assembly  66  may force variable orifice  76  to be at a fully open (e.g., least restrictive) state and may consequently allow a fluid flow rate through variable orifice assembly  66  that is larger than desired. Thus, in an embodiment, first  82  and second  88  fixed orifices provide a flow rate that is lower (e.g., more restrictive) than the non-active flow rate of variable orifice  76 . Flow rates of first  82  and second  88  fixed orifices may differ from one another. This may be explained by following flow of fluid through variable orifice assembly  66 . In addition,  FIG. 4  illustrates a graph of an operating range for fluid flow rate vs. pressure drop (DP) for an embodiment of the fluid circuit of  FIG. 3 . In  FIG. 4 , line A represents the maximum flow rate vs. pressure drop (DP) when variable orifice  76  is operating properly and responding to instructions from controller  78 . However, line B represents flow rate vs. pressure drop (DP) in the event that variable orifice  76  is not operating properly or is not responding to instructions from controller  78 . It should be understood that the values provided in  FIG. 4  are representative of an embodiment, and other values may be used with the systems of the present disclosure. 
     When extendable rod  52  is extended, fluid flows from control valve  62  through fluid line  64 , through first fixed orifice  82  (shown as D in  FIG. 4 ), through variable orifice  76  (shown as C in  FIG. 4 ), and through second fixed orifice  88  (shown as F in  FIG. 4 ) to cylinder  50 , then to fluid line  68  and back to control valve  62 . Once fluid pressure reaches a desired pressure, the fluid will overcome biasing pressure of second check valve  90  and also flow through second check valve  90  (shown as E in  FIG. 4 ). First check valve  84  forces fluid to flow through first fixed orifice  82  when extendable rod  52  is extending. 
     Conversely, when extendable rod  52  is retracted, fluid flows from control valve  62  through fluid line  68 , through cylinder  50 , through second fixed orifice  88  (shown as F in  FIG. 4 ), through variable orifice  76  (shown as C in  FIG. 4 ), and through first fixed orifice  82  (shown as D in  FIG. 4 ), then to fluid line  64  and back to control valve  62 . Once fluid pressure reaches a desired pressure, the fluid will overcome biasing pressure of first check valve  84  and will also flow through first check valve  84  (shown as G in  FIG. 4 ). Second check valve  90  forces fluid to flow through second fixed orifice  88  when extendable rod  52  is retracting. 
     Thus, flow rates for extension and retraction of extendible rod  52  may be individually tuned even when variable orifice  76  is not active or is not responsive to communication signals from controller  78 . 
     It should be understood that the above description is intended for illustrative purposes only. In particular, it should be appreciated that all methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 
     While aspects of the present disclosure have been particularly shown and described with reference to the embodiments above, it will be understood by those skilled in the art that various additional embodiments may be contemplated by modification of the disclosed machines, systems and methods without departing from the spirit and scope of what is disclosed. Such embodiments should be understood to fall within the scope of the present invention as determined based upon the claims below and any equivalents thereof.

Summary:
The present disclosure provides a hitch suspension system that reduces a transfer of energy (e.g., provides a smoother ride) between two bodies hitched together, such as between a towing vehicle and a towed vehicle/trailer. In an embodiment, the suspension system includes a hydraulic cylinder with an extendable rod in fluid communication with a fluid pump. A control valve is fluidly coupled between the hydraulic cylinder and the fluid pump and is configured to adjust an extension length of the rod. Additionally, a variable orifice that adjusts resistance to extension and retraction of the rod is fluidly coupled between the hydraulic cylinder and the control valve. A first fixed restrictive element is fluidly coupled in series with the variable orifice to dissipate energy ad the rod extends and a second fixed restrictive element is fluidly coupled in series with the variable orifice to dissipate energy as the rod retracts.