Abstract:
A turf care machine includes a base mounted on a plurality of wheels, a support structure affixed to and extending from the base for supporting a turf machine operator interface, and a hydraulic control system for regulating the speed and direction of the mower. The hydraulic system includes: an engine with a drive shaft; a single hydraulic pump driven by the engine; a first valve set having an inlet connected to the pump outlet; a first hydraulic wheel motor connected to the first valve set; a second valve set connected to the first valve set and to a second hydraulic wheel motor. This drive system has the advantage of using only one pump for driving both drive wheels in a seemingly independent dual path mode of operation, thereby minimizing cost and the number of fluid linkages and parts in such a system.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority of U.S. provisional application Serial No. 60/139,231 filed Jun. 14, 1999, the entire contents of which are hereby incorporated by reference into the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to hydraulic drive systems for self-propelled turf care equipment, and more particularly to a dual-path reversible hydrostatic drive system for a steerable turf care machine having a single variable volume hydraulic pump as a source of hydraulic drive power. 
     2. Discussion 
     Commercial turf care machines are typically self-propelled and are often equipped with drive mechanisms for both driving and steering the wheels of the machines. These drive mechanisms typically include a mechanism for transferring the driving torque from a source of power such as an internal combustion engine to at least two of the machine wheels, thereby turning the machine wheels and assisting the machine operator in both propelling and maneuvering the piece of equipment. 
     While both hydraulic and mechanical drive mechanisms have been commonly used in the past, hydraulic drive mechanisms are being implemented in turf care machines on an ever increasing basis due to decreases in the price of such systems. Specifically, hydrostatic drive mechanisms have long been known to exhibit desirable characteristics such as allowing a machine operator to quickly switch between forward, neutral and reverse modes of operation, and to provide for efficient steering of the machines as well. Dual path hydrostatic drives are used on both zero-turn riding mowers and zero-turn walk-behind mowers for a number of years, as is shown in U.S. Pat. No. 3,616,869 to Rilling and U.S. Pat. No. 4,920,733 to Berrios, which are both hereby incorporated by reference. These drive systems feature a dedicated hydraulic pump and hydraulic motor combination for each of the two driving wheels. Typically, the pumps and motors are located near the driving wheels, and the two pumps are driven by a belt powered by a pulley mounted to the output shaft of the internal combustion engine. 
     One desirable feature of these dual path machines is their ability to turn and maneuver with considerable agility. This is a direct result of each driving wheel being controlled by the operator independently of the other driving wheel. So, while one wheel is being driven forward at a desired speed, the other wheel can be driven, for example, in the opposite direction at the same speed, thereby allowing the turf care machine to turn on its own axis. Commercial riding and walk-behind mowers having a dual path hydrostatic drive system are available from more than a half-dozen different companies. 
     A turf care machine operator manipulates a typical dual path hydrostatic drive mechanism through a plurality of levers located on an operator/turf machine interface such as a handle bar structure. These operator control systems typically include both traction controls and speed controls to enhance machine operability and maneuverability. A typical traction control for a dual path hydrostatically-driven machine provides the machine operator with selective control between forward, neutral and reverse modes of operation for each driven wheel, while a typical speed control provides the operator with an overall control for regulating the maximum forward speed at which the machine can be operated. State-of-the art traction and speed controls are disclosed in U.S. Pat. No. 5,343,678 to Stuart and U.S. Pat. No. 5,651,241 to Wegner, which are both hereby incorporated by reference. They are also available on various mid-size commercial mowers including those from Textron Turf Care &amp; Specialty Products of Johnson Creek, Wis., such as the Ransomes® Bob-Cat® mid-size mowers Model No. 942215 (a riding mower) and Model No. 930301 (a walk-behind mower), the operations and details of which are respectively described in publicly available Ransomes operator and service manuals. 
     While the present dual path hydrostatic drive systems that are presently available on commercial turf care machines typically exhibit sufficient performance characteristics, such systems normally require a hydraulic fluid reservoir as well as a pair of reversible variable displacement pumps, thus increasing turf care machine cost. It would therefore be desirable to provide more economic arrangements for the hydraulic drive system which facilitates ease of operation, improves efficiency and/or reduces manufacturing costs. 
     As such, a first object of the present invention is to provide an improved system for regulating/controlling a hydrostatic drive mechanism of a conventional turf care machine. 
     A second object of the present invention is to provide an improved system for regulating/controlling the hydrostatic drive system of a turf care machine that allows the operator to adjust the power to the drive system in a way which helps reduce heating of the hydraulic fluid and improves system efficiency. 
     A third object of the present invention is to provide a system and method for regulating/controlling the hydrostatic drive of a turf care machine that requires only one single pump for driving both drive wheels in a seemingly independent dual path mode of operation, thereby minimizing cost and the number of fluid linkages and parts in such a system. 
     A fourth object of the present invention is to provide an economically efficient dual-path hydrostatic drive system which has a reduced number of external hydraulic connections for improved system reliability. 
     SUMMARY OF THE INVENTION 
     To achieve one or more of the foregoing objects, and in accordance with the teachings of the present invention, a single-pump hydraulic drive system for a steerable, self-propelled wheeled vehicle is provided that is designed and adapted for regulating the operation of a self-propelled mower or other piece of turf care equipment or similar kind of the machine. The system of the present invention finds particular utility for midsize commercial mowers and similar kinds of steerable self-propelled wheeled equipment, typically having a net weight of about 140 kg (about 310 pounds) to about 500 kg (about 1100 pounds), which are manually steered and must often be manipulated around obstacles during operation. 
     A manually-steered self-propelled wheeled vehicle, such as a commercial mower, according to one embodiment of the present invention, is implemented with a single-pump hydraulic drive system having a pair of control valve sets for controlling the volume and direction of fluid from the single hydrostatic pump to a pair of wheel drive motors. The single hydraulic pump, which is preferably a variable volume pump, functions to provide hydraulic fluid power required to drive the wheels. The pump preferably includes a manually-operated maximum volume adjustment mechanism, such as a movable machine speed lever connected to a rotatable trunnion shaft or other volume control input means on the pump, for selectively setting the desired maximum pump displacement at any desired level, from a fraction of the pump&#39;s output, such 20%, all the way up to 100%. Via this machine speed lever, an operator can select the maximum drive speed at which the mower will operate. If desired, this speed control mechanism can be operatively connected in a combined manner on the turf machine so that the speed control lever limits the top speed of the traction controls. The operator traction controls typically include independently-actuated left and right control levers, typically operated by the user&#39;s left and right hands, which are mechanically connected via independent linkages to the conventional control valve operators mounted on the first and second valve sets, which valve sets will shortly be described. 
     The hydraulic pump is connected to a first valve set, which regulates the volume and direction of fluid to the left hydraulic wheel motor. The left wheel motor mechanically drives the mower&#39;s left driving wheel, and thus controls the direction and speed of rotation of the left driving wheel. Hydraulic fluid may also be partially or completely shunted or bypassed around the left hydraulic wheel motor by the first valve set, in those operating situations where the left driving wheel is not to be driven at all or is not to be driven at a speed as great as the speed of the right driving wheel. The hydraulic fluid, after traveling back from the left wheel motor into the first valve set, or after being shunted through the first valve set, is then transferred through a suitable fluid conduit to a second valve set. The second valve set regulates the direction and flow of the fluid to the lawn mower&#39;s right wheel drive motor. This motor is turn mechanically drives the mower&#39;s right driving wheel, and thus controls the direction and speed of rotation of the right driving wheel. Hydraulic fluid may also be partially or completely shunted around the right hydraulic wheel motor by the second valve set, in those situations where the right driving wheel is not to be driven at all or is not to be driven at a speed as great as the speed of the left driving wheel. After the hydraulic fluid leaves the second valve set, it recirculates through a suitable fluid conduct back to the inlet side of the single hydraulic pump. 
     The first and second valve sets preferably each contain at least one control valve for regulating the volume and direction of hydraulic fluid discharged therefrom. Preferably each control valve has two outlet ports which are directly piped to the associated wheel drive motor. Each control valve also preferably has a pressure inlet port, a pilot bypass port, and a discharge port. One preferred control valve is an infinitely-adjustable five-port three-position proportional directional control valve for selectively directing fluid to its associated wheel motor. The control valve also preferably has a three-position closed center valve spool arrangement. In the forward and reverse positions, the control valve directs hydraulic fluid to the wheel motor in order to respectively run the wheel motor in the forward or reverse direction. Because the control valve is proportional and infinitely positionable, the flow of hydraulic fluid can be adjusted as desired to achieve any desired rate of speed of the associated wheel, including no speed (that is no rotation), which occurs when the valve spool is in its closed center position. 
     The left and right operator traction control levers and linkages provided on the mower may take on any conventional or suitable form for providing independent mechanical movements to the control operators of the first and second valve sets. Typically, the control operators of the valve sets will be adjustable between full forward, neutral and full reverse positions. As a first example, on a walk-behind mower employing a single-pump hydrostatic drive system of the present invention, these control levers may be hand grip levers of the type conventionally found on walk-behind mowers having a two-pump dual-path hydraulic drive system. As a second example, on a riding mower where the operator is seated, the control levers may be a pair of hand-operable, pivotally mounted, spring-returned-to-neutral levers positioned along side of, or in between the knees of, the seated operator. As a third example, on a stand-up riding or platform mower, the operator traction control levers may be a pair of hand levers or coupling members that are independently movable and preferably are arranged to pivot about a single axis. The control levers may also be limited in their pivotal movement in at least one direction by a lawn mower handle structure upon which they are mounted. As a fourth example, left and right foot-operated control pedals may be provided. In all of these examples, the control operator of the first valve set receives its command as to the desired direction and volume of the fluid flow into the left wheel drive motor from the left-hand (or left foot) traction control lever through the left-hand linkage. Similarly, the control operator of the second valve set receives its command as to the desired direction and volume of fluid flow into the right wheel drive motor from the right-hand (or right-foot) traction control lever through the right-hand linkage. In this manner, the direction and speed of the rotation of the left and right driving wheels may be accurately controlled. Through manipulation of this pair of controls, the left wheel can go in a forward direction at any desired speed while the right wheel is going in a rearward direction at any desired speed (or vice versa). This functionality enables the operator to turn the mower at an effective zero turning radius, if so desired, just like in traditional dual-path machines. 
     At least two further variations of the first embodiment of the present invention are contemplated. The control valve of each valve set that determines fluid volume and direction may be a spring-returned to closed-center neutral position valve, meaning the hydraulic flow to the associated wheel motor is zero in the absence of external forces applied to shift the valve away from its centered neutral position. Preferably, suitable spring members are provided internally within the control valve to shift its spool to a closed-center neutral position. Alternatively, the control valve may be spring-returned to one of its full open positions. In either variation, the traction control levers and/or their associated linkages may also be provided with spring members to return the control valve to a neutral position when the mower operator releases the traction control levers. Alternatively, as is common with some commercial walk-behind mowers, the traction control levers and/or their associated linkages may be provided with spring members to return the control valve to a full open position, such as full forward, when the mower operator releases the traction control levers. 
     A second embodiment or aspect of the single-pump hydrostatic drive system of the present invention for steerable self-propelled wheeled vehicles is provided. This second aspect includes all of the same components found in the first embodiment of the invention. It also includes, within each control valve set, a pressure-operated bypass valve, which is also sometimes called a slave-operated volume compensation valve. These bypass valves are preferably infinitely-adjustable three-port two-position pilot-operated control valves that are spring-returned to an open position in the absence of back pressure in a pilot fluid line typically connected to normal outlet port of the bypass valve. Each bypass valve functions to divert all or a portion of the fluid around its respective control valve within its valve set when a particular wheel is not being driven at all or is not being driven at its near maximum rate of speed so as to match the particular pump output flow. These slave-operated bypass valves include opposed pilot lines connected respectively to the inlet port and the pilot bypass port of the associated control valve. Thus, the nearly instantaneous pressures experienced within the fluid conduits connected to the inlet port and the bypass port serve to control the position of the slave-operated bypass valve. When a sufficient pressure backup is detected in the conduit leading to the inlet port of the control valve, the bypass valve diverts part or all of the fluid flow to the discharge port of its valve set. In this manner, the energy-wasting, heat-generating circulation of the hydraulic fluid through the bypass passages and bypass port of the directional control valve is avoided, and the overall speed of response of the hydraulic drive system as a whole is improved. According to this aspect of the present invention, the bypass valves are preferably mounted directly to or adjacent to their associated control valve. Or both valves can be mounted adjacent to one another on a common pre-ported valve mounting plate or manifold block to achieve a single integrated valve set that minimizes the number of external fluid connections that have to be made to each valve set. In addition, if desired, the single pump and the first and second valve sets may be mounted upon a larger common hydraulic manifold block, base or mounting plate to further minimize the number of external fluid connections between these three sets of hydraulic components within the hydrostatic drive system of the present invention. In all versions of the present invention, the valve sets are preferably positioned in relatively close proximity to the hydraulic wheel motors, which typically directly drive the axles of the driving wheels of the turf care machine. 
    
    
     Additional aspects, features, objects and advantages of the present invention will become apparent from a reading of the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings and appended claims. 
     BRIEF DESCRIPTION OF THE FIGURES 
     The drawings form an integral part of the description of the preferred embodiments and are to be read in conjunction therewith. Like reference numerals designate the same or similar components or features in the various Figures, where: 
     FIG. 1 is a perspective view of a typical walk-behind turf machine incorporating the single-pump hydrostatic drive system of the present invention; 
     FIG. 2 is an elevational view of the FIG. 1 walk-behind turf machine as viewed from the rear, showing the single hydraulic pump and two valve sets mounted on a common hydraulic manifold bolted to the rear deck of the mower; 
     FIG. 3 is an enlarged perspective view of the operator control area of the handle bar structure showing the T-bar engine speed control lever and the centrally located speed control lever for drive pump to set the maximum drive speed; 
     FIG. 4 is a diagrammatic plan view showing one possible lay-out of the various hydraulic and mechanical components of the single-pump dual-path hydrostatic drive system of the present invention generally located on the rear deck of a commercial mower; 
     FIG. 5 is a detailed hydraulic diagram of the hydrostatic drive system of the present invention particularly useful for a dual-path walk-behind commercial mower showing the spools of the two control valves being spring-returned to their full-forward open position, and also showing the hydraulic interconnections among the various hydraulic components; and 
     FIG. 6 is a hydraulic drawing of the hydrostatic drive system of the present invention, just like that of FIG. 5, except that the directional control valves of the valve sets are spring-returned to a center position, which is particularly useful for riding mowers. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, FIG. 1 shows a typical piece of turf care equipment, namely a zero-turn walk-behind mower, generally at  2  upon which the hydrostatic drive control system of the present invention is implemented. Conventional elements of the turf care equipment  2  are fully described in publicly available Ransomes Hydro Mid-Size Power Units Parts Manual, P/N-2308093 dated March 1995. Although the present invention is described as being implemented in association with a hydrostatic drive mechanism in a turf care machine, it should be understood and appreciated that the hydrostatic drive control mechanism of the present invention is also adaptable for use in any piece of machinery in which a hydrostatic drive mechanism is implemented or may be used. 
     With reference to FIGS. 1,  2  and  3 , the turf machine  2  includes a rear deck frame or base  54  having a handlebar support structure  196  rigidly mounted to and extending upwardly from the base  54 . A front frame or mower deck  212  under which belts and cutting blades (not shown) are located extends forwardly from the rear deck or base. The cutter drive belts and rotary grass-cutting blades are implemented under the mower deck as shown and described in publicly available Ransomes Mid-Size Cutter Decks Parts Manual, P/N-2308096. A power source, such as a conventional internal combustion engine shown generally at  200 , is mounted to base  54  and provides power to the various components of the machine in any conventional or suitable manner including those well known in the art, such as flexible belt drives or hydraulic pump and motor drives. Turf care machine  2  also includes a fuel tank  198  mounted on the handlebar support structure  196  for storing fuel used by engine  200 . In addition a master pump speed control lever  199  is provided near the top of handle bar structure  196  for setting and/or adjusting the maximum displacement of variable volume  10 . Lever  199  is connected via a conventional mechanical linkage  203  to the displacement control operator  205  projecting from pump  10 . Also provided on the top plate  209  of handlebar structure  196  is a motor speed control lever  211  connected by a conventional mechanical linkage, such as a Bowden cable arrangement (not shown), to the throttle lever (not shown) of motor  200 . 
     Turf care machine  2  includes left rear and right rear driving wheels  62 ,  82  rotatably attached to base  54  and two front caster-style non-driven wheels  198  attached to mower deck  212 . Specifically, rear wheels  62 ,  82  are preferably attached to and driven by drive mechanisms, including hydraulic drive wheel motors  60 ,  80  shown in FIG. 3, in a manner that will be described in more detail below. Associated with each hydraulic wheel motor is a valve set  20 ,  64 . Each valve set  20 ,  64  is mechanically coupled to control levers  190  found on handle bar structure  174 . Also associated with these handle bars  174  are lockable, thumb-operated neutral latch mechanisms  84 . These conventional neutral latch mechanisms are of the type as that disclosed in the aforementioned Ransomes Hydro Mid-Size Power Unit Parts Manual. Alternatively, an operator control arrangement of the type described in U.S. Pat. No. 5,343,678 to Stuart may be utilized, and the disclosure of that patent is hereby incorporated herein by reference. The Stuart patent discloses a pair of hand levers or coupling members that are independently movable and preferably are arranged to pivot about a single axis. 
     Referring to FIG. 4, turf machine  2  includes an internal combustion engine  200 , typically having a vertical output shaft, coupled to an concentrically-mounted optional first clutch assembly  204 . Assembly  204  may be an electrical or mechanical clutch. Engine  200  is drivingly connected to a variable volume hydraulic pump  10  via first flexible drive belt  208 . Alternatively first flexible belt  208  may be driven with conventional tensioner pulley and/or idler pulley arrangement. Either arrangement may be utilized to translate the mechanical rotary power of the output shaft of the engine to a pulley concentrically mounted to the input shaft of pump  10 . Also shown concentrically mounted on the engine output shaft is the second optional clutch assembly  206 . Assembly  206  is coupled to the cutter blade assembly  210  by drive belt  209 . As a first alternative, belt  209  may be coupled via a tensioner pulley and idler pulley arrangement to provide selectively applied mechanical rotary power to the rotary mower blades. As a second alternative, a second hydraulic pump (not shown), driven directly or by belt from the mechanical rotary power of the output shaft of the engine, may be used to power one or more hydraulic motors to drive conventional rotary cutting blades or conventional reel or cylindrical-style cutting blades (not shown). These cutter drive alternatives are well-known and need not be further described. 
     As shown in FIGS. 2 and 4, variable volume pump  10  is preferably centrally mounted on the rear portion of deck  54 , and first and second valve sets  20 ,  64  are preferably mounted on deck  54  on either side of pump  10 . Pump  10  and valve sets  20 ,  64  are preferably mounted on a common hydraulic manifold  21  to minimize external hydraulic connections between the various ports of pump  10  and the various ports of the individual valves within and between valve sets  20 ,  64 . In this manner, the possibility of leakage from external fittings and lines is minimized, as is required assembly time. Appropriate internal passages for main and pilot fluid flows through the manifold can readily be provided using techniques well known in the art of hydraulic valving and hydraulic manufacture. The hydraulic output from pump  10  is coupled through an appropriately-sized and secured fluid conduit to inlet port  12  of first valve set  20 . This conduit, like most of the other fluid lines in the hydraulic circuit diagram, may be an external fluid line or may be an internally plumbed passage provided in manifold  21 . First valve set  20  also has outlet ports  14  and  16  in the fluid communication with hydraulic wheel motor  60 , and a discharge port  18 . The conduits between outlet ports  14  and  16  and motor  60  may be plumbed with rigid hollow fluid-carrying lines, such as tubing or pipe. Or all or part of these lines may be a flexible hydraulic line, such as braided hydraulic hose with metallic pressure-crimped end fittings with appropriate threaded fasteners. 
     Similarly, second valve set  64 , which is identical in terms of function to first valve set  20 , has hydraulic output from port  18  coupled through an appropriately-sized and secured fluid conduit to inlet port  66  of second valve set  20 . Once again, this conduit, like most of the other fluid lines in the hydraulic circuit diagram, may be an external fluid line or may be an internally plumbed passage provided in manifold  21 . Second valve set  64  also has outlet ports  68  and  70  in the fluid communication with hydraulic wheel motor  80 , and a discharge port  72 . The fluid connection between outlet ports  68  and  70  and wheel motor  80  may be plumbed in the manner described for the fluid lines connected to outlet ports  14  and  16 . 
     As best seen in FIG. 5, first valve set  20 , which functions to regulate the amount and direction of fluid to left wheel drive motor  60 . Valve set  20  has a volume control valve  22  and an optional slave-operated bypass valve  40 . As shown, valve  22  is preferably an infinitely-adjustable directional control valve having three distinct internal valve spool positions, namely a distinct full forward position represented by (rectangular) envelope  24 , a distinct neutral or blocked-center position represented by envelope  26 , and a distinct full reverse position, represented by envelope  28 . First control valve  22  has a first or inlet port  30 , a bypass port  38 , and a discharge port  36 . Valve  22  also has first and second outlet ports  32  and  34  which are in respective fluid communication with and correspond to first and second outlet ports  14  and  16 . When optional slave-operated bypass valve  40  is not provided, inlet port  30  normally is in fluid communication with and may be regarded as corresponding to inlet port  12 . Discharge port  36  is in fluid communication with discharge port  18  of first valve set  20 . Directional control valve  22  is connected to control lever  190  and return spring  50 . The position of the valve spool within valve  22  is determined by the position of manual operator  190 , which is connected to left-hand control lever. Pilot bypass outlet  38  is connected to second pilot control inlet  48  of the optional slave-operated volume compensation control valve  40 . 
     As can be seen in FIG. 5, the forward position represented by envelope  24  of valve  20  connects (i.e., establishes open fluid communication between) inlet port  30  to outlet port  32  and connects outlet port  34  to discharge port  36 . In addition, there is a fluid path from inlet port  30  to pilot port  38  in envelope  24 . In the center position represented envelope  26 , outlet ports  32  and  34  are blocked, pilot port  38  is blocked, and inlet port  30  and discharge port  36  are in fluid communication, all as shown. Accordingly, when valve  22  is in its center position, the fluid flowing into inlet port  38  will flow immediately out of outlet port  36  (subject to any pressure drops caused by restricted size passages and orifices), and the fluid will not be directed to or reach hydraulic motor  60 . Similarly, the fluid in the lines leading to motor  60  will be blocked, and thus motor  60  and the left wheel connected thereto will effectively be hydraulically braked. In the reverse position represented by envelope  28 , inlet port  30  is connected to outlet port  34  and output port  32  is connected to discharge port  36 . In addition, pilot port  38  is in fluid communication with inlet port  30 . Accordingly, when control valve  22  is in either its full forward position  24  or its full reverse position  28 , pilot line  38  will be at substantially the same pressure as is inlet port  30 . 
     A slave-operated bypass or volume compensation valve  40  may optionally be provided within first valve set  20 . If valve  40  is not provided, then inlet port  12  is piped directly to inlet port  30 , as previously noted. As shown in Figure valve  40  is preferably an infinitely-adjustable proportional control valve having two basic positions or envelopes  47  and  49 , three main ports  42 ,  44  and  45 , two pilot ports  46  and  48 , and a drain line port  52 . Specifically, valve  40  has a first inlet port  42 , a first outlet port  44 , and second outlet port  45 , and a first pilot inlet port  46  and a second pilot inlet port  48 . In the first distinct or at-rest position of valve  40 , which is represented by envelope  47 , inlet port  12  is connected to first outlet port  44 , and second outlet port  45  is blocked, as shown. In the second distinct or fully shifted position represented by envelope  49 , inlet valve  12  is connected to second output port  45 , and first output port  44  is blocked, as shown. Return spring  50  operates to bias valve  40  into its nominal or at-rest position  47 , as does sufficient hydraulic pressure on second pilot port  48 , provided that it is able to overcome the hydraulic pressure if any being delivered to pilot port  46 . Hydraulic pressure on first pilot port  46  which is sufficiently greater than the combined force of the hydraulic pressure at pilot port  48  and the effective pressure generated by return spring  50 , causes valve  40  to shift from its at-rest position or to the position represented by envelope  49 . Thus, as can be readily understood, pilot ports  46  and  48  function in opposed relation to automatically actuate or bias the valve spool of slave-operated compensation valve  40  to one position or the other. In its first or fully at-rest position  47 , bypass valve  40  functions to direct hydraulic fluid from pump  10  delivered to inlet port  12  to outlet port  44  and onto inlet port  30  of valve  22 . In its second fully-actuated position  49 , bypass valve  40  functions to bypass hydraulic fluid being delivered to inlet port  12  to discharge port  18 , thus bypassing directional control valve  22  and left wheel drive motor  60  altogether. 
     Valve  40  may also assume intermediate positions between positions  47  and  49  if the combined hydraulic forces and its return spring force are sufficiently closely balanced to permit the valve spool to partially shift. In such a blended position, a portion of the fluid being delivered from pump  10  to inlet  12  will be sent through valve  22  hydraulic motor  60 , while a portion will be-diverted to discharge ports  45  and  18  so as to go directly to inlet port  66  of second valve set  64 . 
     Control valve  22  and bypass valve  40  may each be provided with a drain line to the low point  52  in the hydraulic system, as shown, if needed. Low point  52  normally will be at the inlet or suction side of pump  10 . These optional drain connections help recirculate fluid which has leaked past the valve spools of valve  20  and valve  40 . 
     Also shown in FIG. 5, is an optional two-port, two-position bypass valve  100  connected in parallel with pump  10 . Bypass valve  100  is a manually operated valve that has two positions or envelopes  104  and  108 . During normal operation of the hydrostatic system, valve  100  is closed, as shown by envelope  104 . When opened, as shown by envelope  108 , valve  100  allows hydraulic fluid to flow freely around pump  10 . At times this may prove convenient when all power on the mower is off, and the mower must be moved manually, since this will allow hydraulic fluid in wheel motors  60  and  80  to circulate in a complete loop through the hydraulic lines of hydraulic drive system  5 . 
     In operation, both valves  22  and  40  are capable of assuming and an almost infinite number of intermediate positions between the distinct valve spool positions. In such blended positions, there is some fluid crossover between certain ports. Specifically, with regard to valve  20 , in an intermediate position between fully-forward position  24  and center position  26 , there is a portion of the fluid flowing into inlet port  30  being passed to outlet port  32  and thereafter to wheel motor  60 , while another portion of the fluid into inlet port  30  is being diverted to discharge port  36 . In a similar manner, when the valve  22  is in a blended position partially between the center position  26  and full reverse position  28 , a portion of the fluid flow to inlet port  30  is sent to outlet port  34 , and a portion of that fluid flow to inlet port  30  is bypassed to discharge port  36 . These two blended positions just described are similar, except that the fluid flow through wheel motor  60  is an opposite direction. In each of these blended positions, the relative amount of fluid being directed to wheel motor  60  versus the amount bypassed to discharge port  36  is determined by the precise position of the valve spool, as control by manual operator  190 . 
     Those skilled in the art should appreciate that the energy in the hydraulic fluid under pressure that is not being utilized in wheel motor  60  or that is not being lost through pressure drops in first valve set  20  or in its associated conduits is available to drive wheel motor  80 . Due to the substantially identical construction of first valve set  20  and second valve set  64 , the fluid pressures between the two valve sets and their respective hydraulic wheel motors tend to equalize very nicely in a reasonably steady-state condition when the mower is traveling forward over reasonably level ground in a substantially straight line. To the extent that any unequal hydraulic pressures may exist that are undesirable, the operator of the mower, by making minor manual adjustments to the relative position of the traction control levers, can easily make compensations so as to keep the mower tracking in a substantially straight line. Such minor corrections to the traction control levers will normally be made instinctively without any real effort of a part of an experienced mower operator, much like an experienced car driver will make slight changes to the steering wheel in order to keep driving a car in a substantially straight line down any given the road without having to think about those steering corrections. 
     The operation of the FIG. 5 drive system will now be further described to help further explain the functioning of the single-pump, dual-path drive system which is inherently shown therein. With reference to FIG. 5, consider operation of the mower with the left-hand traction control lever for the left wheel drive in its neutral position and the right-hand traction control lever for the right wheel drive being in its full forward position shown. The left traction control lever  190  is raised slightly to place the first control valve  22  in the neutral position against biasing spring  50 . As a result, hydraulic pressure from inlet port  30  of valve  22  is diverted to discharge outlet port  36 , thereby bypassing left drive motor  60 . Also, with valve  22  in its neutral position (envelope  26 ), hydraulic pressure on outlet port  38  and pilot port  48  will promptly be relieved via controlled leakage to drain  52 . As a result, pressure in inlet port  30  will be sufficient to shift slave-operated valve  40  from its at-rest position  47  to full bypass position  49 . This shifting changes the flow of hydraulic fluid from the variable volume pump  10  so as to divert the flow away from valve  22 , and subsequently the left drive wheel  60 , directly to discharge port  18  of valve set  20 , where it then enters inlet port  66  or second valve set  64 . With directional valve  74  in its full forward position, the pressurized fluid is directed through bypass valve  76  to port  86  of value  74 , and onto and through second hydraulic wheel motor  80 . 
     As may be appreciated from FIG. 5, when control lever  190  is further moved so as to place valve  22  into its full reverse position represented by envelope  28 , pressure is applied through bypass port  38  so as to allow the slave-operated bypass valve  40  to shift back into its first position  47 . This allows fluid to once again flow through first outlet port  44  of slave-operated valve  40  into inlet port  30  of control valve  22 . As can be seen in FIG. 5, in the full reverse position  28 , the fluid flows from inlet  30  through second output port  16  so as to drive left wheel motor  60  in a reverse direction. The flow leaves left drive motor  60  and enters port  32  of control valve  22  at and is directed to the discharge port  36 . Subsequently, the fluid flows through discharge port  18  to inlet port  66  of second valve set  64 . The slave-operated valve  40  provides an efficient means to effectively divert fluid energies at very low pressure drops away from control valve  22 , and hence wheel motor  60 , to second volume control valve  74  and subsequently right drive motor  80 . 
     The functioning of the right valve set  64  is identical to that described for left valve set  20 , with the exception that diverted fluids are not directed toward the left wheel  60 , but instead are directed back to variable volume pump  10 . 
     FIG. 6 shows a hydraulic diagram for another embodiment of the single pump hydrostatic drive system of the present invention. FIG. 6 is similar to the FIG. 5 hydraulic diagram, except that control valves  22  and  74  are spring-returned to a neutral position, and an optional air-to-oil heat exchanger  215  is provided at the inlet line leading to pump  10 . By way of background, there are a number of conventional mowers, including sit-down riding mowers and standup platform mowers, which utilize control linkages and/or reversible pumps which return to neutral to provide a braking action upon the driving wheels with the control levers and linkages have been released. For example, in the aforementioned &#39;678 patent to Stuart, left and right operator levers, called coupling members, are independently pivotable control assemblies that the operator of the mower can grasp with his left and right hands. These assemblies or members are pivotably supported along a common axis by a lawn mower handlebar structure that extends from the rear of a midsize commercial mower. 
     The handle bar shown in that patent serves to limit the forward movement of these coupling members. Pushing the right coupling member or lever forward causes the right rear drive wheel of the mower to be driven forwardly. Pushing the left coupling member forward causes the left rear drive wheel of the mower to be driven forwardly. Upon being released by the operator, the coupling members are returned automatically by a spring force generated by one or more coiled springs to a neutral configuration, wherein the driving forces to the wheels are stopped. Simultaneously, a spring force causes a brake arm to engage a brake band which tightens upon a brake drum, thereby preventing movement of the lawn mower. The braking force on either wheel can be increased by pivoting its respective coupling member in a counterclockwise direction, When the mower is placed in reverse, the lower bar of the coupling member can be used to control the drive of the mower, so the operator movement of the member will correspond to the direction of desired movement. Using the hydraulic system of the present invention, band brakes are not necessary since the hydraulic fluid, once trapped in the lines by the closed center valves  22  and  74  shown in FIG. 5, will not permit the driving wheels to turn. Thus, the control valves  22  and  74  shown in FIG. 5, with their spring-returned to closed center configuration, will inherently provide a braking action as is often desired for sit-down riding mowers and stand-up riding platform mowers. Manually operated shutoff valves shown in parallel with wheel motors  60  and  80  in FIG. 5 are normally closed when the hydraulic system of FIG. 5 is being operated as a self-propelled dual-path hydraulic drive system. When a riding mower using the FIG. 5 system has all power turned off, the mower can be manually moved by simply opening these two manual shutoff valves. When opened, these shutoff valves allow hydraulic fluid to recirculate freely in a loop through the shutoff valve and its respective wheel motor. 
     While the above description discusses two embodiments of the present invention, it will be understood that the description is exemplary in nature and is not intended to limit the scope of the invention. For example, the invention, while disclosed for use in a commercial walk-behind lawn mower, may also be used for other steerable self-propelled commercial lawn equipment, such as a stand-up or platform mowers, aerators, spreaders and sprayers. Also, other types of operator-controlled, steerable self-propelled mobile equipment that traditionally has used a two-pump dual-path hydrostatic drive system may also beneficially use the single-pump system of the present invention. Such steerable self-propelled mobile equipment includes but is not limited to large vacuum cleaners, cement buggies, pipe and steel rod carriers, sweepers, floor and pavement scrubbers, skid-steer loaders, and agricultural equipment such as windrowers. The present invention, in its broader aspects, will therefore be understood to be applicable for use with such equipment, and also to be susceptible to modification, alteration, and variation by those skilled in the art without deviating from the fair scope of the invention as defined in the following claims.