Abstract:
A hydraulic circuit for a vehicle having at least one selectively actuated hydraulic assist motor. The hydraulic circuit includes a first loop that includes, in serial order, a first hydraulic pump, a first hydraulic line, at least one hydraulic motor and a second hydraulic line. A second hydraulic pump and a third valve is used to control the flow of low pressure fluid from the second pump to the hydraulic motor casing. The low pressure fluid through the third valve maintains the motor pistons retracted when the motor is not being used. For actuating the motor, the third valve is actuated for disconnecting the second pump and relieving pressure from the motor casing. Low pressure is then provided to the first and second lines for extending the motor pistons. The first and second valves are then actuated for isolating the first and second lines from one another, and the first pump is used to power the motor. The first, second and third valves are advantageously solenoid cartridge valves.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to vehicles having a primary transmission and a hydraulic motor that provides a drive assist and, more particularly, a hydraulic circuit including such a hydraulic assist motor. 
         [0003]    2. Description of the Related Art 
         [0004]    Many large vehicles include a mechanical transmission as the primary driver of the vehicle and use hydraulic assist motors to selectively drive additional wheels. For example, such a vehicle may have rear wheels driven by the mechanical transmission and front steerable wheels that are selectively driven by hydraulic assist motors. When being driven on a paved road, such a vehicle will typically employ only the mechanical transmission. When the vehicle must be driven in poor traction or off-road conditions, e.g., when on a construction site, it can be beneficial to employ the hydraulic assist motors to provide the vehicle with an additional set of driven wheels. 
         [0005]    As a general rule, the hydraulic assist motors employed in such vehicles are subject to damage if the pistons of the motors are not retracted when the motors are de-activated and the vehicle is in motion, e.g., traveling on a paved road. While various hydraulic circuits are known for use with such selectively actuated hydraulic motors, such circuits often rely on highly complex valves that require extensive custom machining and, thus, can be quite expensive. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a hydraulic circuit with a plurality of valves for controlling the operation of at least one hydraulic motor wherein the valves may take the form of relatively simple and inexpensive solenoid cartridge valves. 
         [0007]    The invention comprises, in one form thereof, a hydraulic circuit ( 22 ,  23 ) that includes at least one hydraulic motor ( 24 ) and a first hydraulic discharge pump ( 62 ,  162 ). The motor ( 24 ) defines a motor cavity ( 54 ) and includes at least one piston ( 40 ) at least partially disposed within a piston bore ( 42 ) and movable between an extended position and a retracted position ( 40 R). The piston ( 40 ) permits free rotation of the motor ( 24 ) when in the retracted position. The first hydraulic pump ( 62 ,  162 ) has a selectively variable discharge. Hydraulic fluid can be recirculated through a first loop within the circuit ( 22 ). The first loop includes, in serial order, the first hydraulic pump ( 62 ,  162 ), a first hydraulic line ( 94 ), said hydraulic motor ( 24 ) and a second hydraulic line ( 96 ). A third hydraulic line ( 98 ) provides fluid communication between the first hydraulic line ( 94 ) and a first valve ( 86 ). A fourth hydraulic line ( 118 ) provides fluid communication between the second hydraulic line ( 96 ) and a second valve ( 88 ). A fifth hydraulic line (lines  126 ,  130 ,  132  combined) provides fluid communication between the first and second valves ( 86 ,  88 ). The first valve ( 86 ) has an open position ( FIG. 2 ) permitting bi-directional fluid communication between the third ( 98 ) and fifth ( 126 ,  130 ,  132 ) hydraulic lines and a closed position ( FIG. 5 ) preventing fluid communication from the third hydraulic line ( 98 ) to the fifth hydraulic line ( 126 ,  130 ,  132 ). The second valve ( 88 ) has an open position ( FIG. 2 ) permitting bi-directional fluid communication between the fourth ( 118 ) and fifth ( 126 ,  130 ,  132 ) hydraulic lines and a closed position ( FIG. 5 ) preventing fluid communication from the fourth ( 118 ) hydraulic line to the fifth hydraulic line ( 126 ,  130 ,  132 ). A sixth hydraulic line ( 100 ,  101 ) receives a hydraulic fluid discharge from either a constant source of low pressure hydraulic fluid or a second hydraulic pump ( 64 ,  65 ) and provides fluid communication between the second hydraulic pump ( 64 ,  65 ) and a third valve ( 84 ,  85 ). A seventh hydraulic line ( 120 ,  142 ) provides fluid communication between the third valve ( 84 ,  85 ) and the motor cavity ( 54 ). The third valve ( 84 ,  85 ) has a first position ( FIG. 2 ,  FIG. 6 ) providing fluid communication from the sixth hydraulic line ( 100 ,  101 ) to the seventh hydraulic line ( 120 ,  142 ) and a second position ( FIG. 3 ) preventing fluid communication from the sixth hydraulic line ( 100 ,  101 ) to the seventh hydraulic line ( 120 ,  142 ). When the first hydraulic pump ( 62 ,  162 ) is discharging at no greater than a minimal discharge rate and the third valve ( 84 ,  85 ) is in its first position, hydraulic fluid pressure within the motor cavity ( 54 ) moves the at least one piston ( 40 ) into its retracted position. The at least one piston ( 40 ) is movable into its extended position by moving the third valve ( 84 ,  85 ) into its second position and positioning each of the first and second valves ( 86 ,  88 ) in their open positions and operating the second pump ( 64 ,  65 ). The motor  24  may be driven/is actuable after extending the at least one piston ( 40 ) by moving each of the first and second valves ( 86 ,  88 ) into their closed positions and operating the first hydraulic pump ( 62 ,  162 ) at a discharge rate greater than said minimal discharge rate. 
         [0008]    In some embodiments, the hydraulic circuit ( 22 ,  23 ) also includes a hydraulic fluid storage vessel ( 60 ) wherein the second pump ( 64 ,  65 ) receives fluid (via lines  102 ,  140 ) from the storage vessel ( 60 ). An eighth hydraulic line ( 122 ,  123 ) provides fluid communication between the motor bearings and/or cavity ( 54 ) and the storage vessel ( 60 ). 
         [0009]    In other embodiments, the hydraulic circuit ( 22 ,  23 ) further includes a ninth hydraulic line ( 124 ,  125 ) providing fluid communication between the third valve ( 84 ,  85 ) and the storage vessel ( 60 ) wherein the third valve ( 84 ,  85 ) provides fluid communication between the seventh ( 120 ,  144 ) and ninth ( 124 ,  125 ) hydraulic lines when the third valve ( 84 ,  85 ) is in its second position. 
         [0010]    In still other embodiments, the hydraulic circuit ( 22 ,  23 ) includes a fourth valve ( 90 ). The fourth valve ( 90 ) has an open position allowing fluid communication from the fifth hydraulic line ( 126 ,  130 ,  132 ) to the ninth hydraulic line ( 124 ,  125 ) and a closed position preventing fluid communication from the fifth hydraulic line ( 126 ,  130 ,  132 ) to the ninth hydraulic line ( 124 ,  125 ). The fourth valve ( 90 ) is positioned in its open position when the at least one piston ( 40 ) is in its retracted position. The fourth valve ( 90 ) is moved to its closed position when extending the at least one piston ( 40 ) and actuating the at least one hydraulic motor ( 24 ). 
         [0011]    In yet additional embodiments, the hydraulic circuit ( 22 ,  23 ) includes a one-way check valve ( 92 ) disposed between the fifth ( 126 ,  130 ,  132 ) and sixth ( 100 ,  101 ) hydraulic lines. The one-way check valve ( 92 ) allows fluid communication from the sixth hydraulic line ( 100 ,  101 ) to the fifth hydraulic line ( 126 ,  130 ,  132 ) and prevents fluid communication from the fifth hydraulic line ( 126 ,  130 ,  132 ) to the sixth hydraulic line ( 100 ,  101 ). 
         [0012]    The first ( 86 ), second ( 88 ), third ( 84 ,  85 ) and fourth ( 90 ) valves may advantageously take the form of solenoid activated valves. 
         [0013]    In still other embodiments of the invention, the circuit ( 22 ,  23 ) may include a tenth hydraulic line ( 108 ,  125 ) providing fluid communication between the sixth hydraulic line ( 100 ,  101 ) and a storage vessel ( 60 ) wherein a pressure relief valve ( 66 ,  67 ) is disposed in the tenth hydraulic line ( 108 ,  125 ) and releases hydraulic fluid from the sixth hydraulic line ( 100 ,  101 ) toward the storage vessel ( 60 ) when the fluid pressure within the sixth hydraulic line ( 100 ,  101 ) exceeds a predetermined threshold value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The above mentioned and other features of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
           [0015]      FIG. 1  is a cross sectional view of a hydraulic motor. 
           [0016]      FIG. 2  is a schematic view of a vehicle with a hydraulic system and hydraulic power assist motors wherein the pistons of the motors have been retracted for free-wheeling operation. 
           [0017]      FIG. 3  is a schematic view of the hydraulic system of  FIG. 2  wherein the hydraulic pressure within the motor cases has been released to the hydraulic tank. 
           [0018]      FIG. 4  is a schematic view of the hydraulic system of  FIG. 2  wherein the pistons of the hydraulic motors have been extended. 
           [0019]      FIG. 5  is a schematic view of the hydraulic system of  FIG. 2  wherein the hydraulic motors are operating. 
           [0020]      FIG. 6  is a schematic view of an alternative hydraulic system having hydraulic power assist motors. 
       
    
    
       [0021]    Corresponding reference characters indicate corresponding parts throughout the several views. Although the exemplification set out herein illustrates embodiments of the invention, in several forms, the embodiments disclosed below are not intended to be exhaustive or to be construed as limiting the scope of the invention to the precise forms disclosed. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    A vehicle  20  having a hydraulic circuit  22  that includes two hydraulic motors  24  is schematically depicted in  FIG. 2 . The depicted vehicle  20  is a truck having an internal combustion engine  26  that drives wheels  30  by means of a mechanical power train  28 . Engine  26  powers hydraulic circuit  22  through a schematically depicted power-take-off (“PTO”) shaft  32 . As discussed in greater detail below, hydraulic circuit  22  selectively powers hydraulic motors  24 . When actuated, hydraulic motors  24  drive wheels  34 . 
         [0023]    When vehicle  20  is operating, power train  28  and wheels  30  provide the primary driving wheels for vehicle  20  while motors  24  and wheels  34  provide a pair of auxiliary drive wheels for use when vehicle  20  is being operated in poor traction conditions, on rough terrain, or in other circumstances in which providing additional drive wheels is advantageous. In the illustrated embodiment, wheels  34  are steerable wheels while wheels  30  are non-steerable wheels. The present invention, however, may be used with a variety of other vehicles. For example, hydraulic motors  24  can also be used with non-steerable wheels and the primary drive system of the vehicle could employ a hydraulic circuit with hydraulic motors located between engine  26  and primary drive wheels  30  rather than a mechanical power train. 
         [0024]    After discussing the general structure and operation of hydraulic motors  24 , the structure and operation of hydraulic circuit  22  within which hydraulic motors  24  are located will be discussed. 
         [0025]    The illustrated hydraulic motors  24  used in hydraulic circuit  22  are conventional hydraulic motors and a cross sectional view of hydraulic motors  24  is provided in  FIG. 1 . Motor  24  includes a cylinder block  36  mounted on the spindle  38  of a steering knuckle. Cylinder block  36  is fixed to spindle shaft  38  with splines that prevent cylinder block  36  from rotating. Camming pistons  40  are mounted within piston bores  42  located in cylinder block  36  and have a rolling cam member  44  mounted on their projecting end. Rolling cam members  44  are engaged with cam ring  46  and drivingly rotate cam ring  46  during operation of hydraulic motor  24 . A valve assembly (not shown) located at the distal end of spindle shaft controls the flow of hydraulic fluid to the individual piston bores  42  located within cylinder block  46  to thereby control the operation of motor  24 . 
         [0026]    Cam ring  46  is secured to a conventional wheel hub assembly (not shown) by inserting fasteners through holes  48  in cam ring  46 . A wheel  34  is mounted on the wheel hub assembly whereby operation of each of the hydraulic motors  24  drivingly rotates one of the wheels  34 . 
         [0027]    As can be seen in  FIG. 1 , the radially inner surface  50  of cam ring  46  includes a series of sloped camming surfaces  52 . When inner surface  50  is engaged with rolling cam members  44 , camming pistons  40  reciprocate within bores  42  as rolling cam members  44  travel across sloped camming surfaces  52 . An internal motor valve assembly controls the flow of high pressure hydraulic fluid to bores  42  and the return of low pressure hydraulic fluid from bores  42 . 
         [0028]    During operation of motor  24 , high pressure hydraulic fluid will be supplied to some of the bores  42  forcing the pistons  40  and rolling cam members  44  associated with those bores  42  radially outwardly while other pistons  40  and cam members  44  are being forced radially inwardly and expelling relatively low pressure hydraulic fluid. As one having ordinary skill in the art will understand, this will drivingly rotate camming ring  46  and wheel  34  mounted thereon. By altering the sequence in which piston bores  42  receive the high pressure hydraulic fluid, camming ring  46  can be driven in either rotational direction and thereby selectively rotate wheels  34  in either the forward or reverse direction. 
         [0029]    As discussed in greater detail below, the hydraulic fluid for driving pistons  40  is supplied to and returned from motors  24  through hydraulic lines  94 ,  94   a  and  96 ,  96   a.  When high pressure hydraulic fluid is supplied to motors  24  through lines  94 ,  94   a  and reduced pressure hydraulic fluid returned through lines  96 ,  96   a,  motors  24  will rotate cam rings  46  and wheels  34  in one direction, e.g., forward. When high pressure hydraulic fluid is supplied to motors through lines  96 ,  96   a  and reduced pressure hydraulic fluid returned through lines  94 ,  94   a,  motors  24  will rotate cam rings  46  and wheels  34  in the opposite direction, e.g., reverse. 
         [0030]    It is sometimes desirable to have the ground engaging wheels  34  secured to motors  24  rotate freely without having hydraulic motors  24  powering the rotation of wheels  34 . During such periods of free wheel rotation or “roading,” pistons  40  are retracted so that rolling cam members  44  do not interfere with the rotation of cam ring  46 . As discussed in greater detail below, retraction of pistons  40  is accomplished by reducing the flow and pressure of the hydraulic oil communicated to all of the piston bores  42  while maintaining the hydraulic oil within motor case cavities  54  at a greater pressure. Hydraulic lines  120 ,  120   a  are in communication with motor case cavity  54  and, as discussed in greater detail below, are used to regulate the pressure of hydraulic fluid within motor case cavities  54 . In  FIG. 1 , pistons  40  are illustrated in their extended positions in solid lines. The retracted position of one of the pistons is depicted in dashed lines designated  40 R. 
         [0031]    When free-wheeling, the pressure of the oil located between camming ring  46  and cylinder block  36  is regulated not only to maintain the pressure at a value that keeps pistons  40  retracted but also to replace the volume of oil seeping from motor  24 . The motor seepage oil cools and lubricates bearings (not shown) located on spindle shaft  38 . After lubricating the bearings, this seepage oil is advantageously returned by internal passages to lines  122 ,  122   a  by which it is conveyed to tank  60 . The circulation of seepage oil occurs when vehicle  20  is operating with motors  24  in a free-wheeling configuration with pistons  40  retracted and when motors  24  are operating and driving wheels  34 . 
         [0032]    The general structure of hydraulic circuit  22  will now be discussed with reference to  FIG. 2 . Hydraulic circuit  22  includes a pump module  56 , a control valve module  58  and an oil tank  60 . The pump and control valve modules  56 ,  58  comprise a plurality of individual components and can be installed as modular units  56 ,  58  in vehicle  20  or the separate components forming modules  56 ,  58  can be installed individually. 
         [0033]    Pump module  56  includes a reversible variable displacement pump  62  and a charge pump  64  that are powered by PTO shaft  32 . PTO shaft  32  drives pumps  62 ,  64  whenever engine  26  is operating. Charge pump  64  is a constant displacement pump and discharges hydraulic oil at a substantially constant pressure. In the illustrated embodiment, charge pump  64  discharges hydraulic oil at a pressure that remains substantially constant at about 400 psi. Variable displacement pump  62  discharges oil at variable quantities and pressures. 
         [0034]    Hydraulic lines  94 ,  96  act as the inflow and discharge lines for variable displacement pump  62 . The illustrated variable displacement pump  62  is a selectively reversible pump. Thus, pump  62  has two operating conditions: one in which line  96  is the inflow line and line  94  is the discharge line and another one in which line  94  is the inflow line and line  96  is the discharge line. 
         [0035]    Lines  94  and  96  lead to ports on each of the hydraulic motors  24  either directly or through branch lines  94   a,    96   a.  The pumps  24  are arranged in parallel. Lines  94 ,  96  and  94   a,    96   a  are in communication with the internal valve assembly within motors  24  that feed and drain hydraulic fluid from cylinder bores  42 . As mentioned above, when high pressure hydraulic fluid is provided to motors  24  through hydraulic lines  94 ,  94   a  and reduced pressure hydraulic fluid is returned to pump  62  through lines  96 ,  96   a,  pumps  24  will operate in a first rotational direction. When pump  62  is reversed and high pressure fluid is provided to motors  24  through hydraulic lines  96 ,  96   a  and reduced pressure hydraulic fluid is returned to pump through lines  94 ,  94   a,  motors  24  will operate in the opposite rotational direction. Because the hydraulic fluid is returned directly to pump  62  from motors  24 , hydraulic system  22  is referred to as a closed system. 
         [0036]    Hydraulic line  94  (and pump  62 ) is also in communication with valve cartridge  86  through hydraulic line  98 . Similarly, hydraulic line  96  (and the opposite side of pump  62 ) is in communication with valve cartridge  88  through hydraulic line  118 . The purpose of hydraulic lines  98 ,  118  and valve module  58  is discussed in greater detail below. 
         [0037]    Charge pump  64  receives hydraulic fluid from tank  60  through hydraulic line  102  and discharges hydraulic fluid into hydraulic line  100 . Hydraulic line  100  connects charge pump  64  with valve cartridge  84 . Valve cartridge  84  is also in fluid communication with motor case cavities  54  of motors  24  through hydraulic lines  120 ,  120   a  and with tank  60  through hydraulic line  124 . 
         [0038]    Hydraulic line  104  is in communication with discharge line  100  and provides fluid communication to pressure relief valve  66  via hydraulic line  106 . Hydraulic oil flowing through pressure relief valve  66  enters hydraulic line  108  which conveys the hydraulic oil toward tank  60 . In the illustrated embodiment, pressure relief valve  66  is set such that the pressure within discharge line  100  will not exceed a pressure of approximately 50 psi. Charge pump  64  has a substantially constant discharge rate with a discharge pressure of approximately 400 psi. When pump  64  is discharging fluid into line  100 , a fraction of this discharge flow will, thus, flow through pressure relief valve  66  into hydraulic line  108 . 
         [0039]    Hydraulic fluid entering hydraulic line  108  passes through oil cooler  76  and oil filter  80  before entering tank  60 . Oil cooler  76  includes an internal bypass valve  78  that allows hydraulic fluid to bypass cooler  76  and continue flowing toward tank  60  if cooler  76  becomes clogged. Similarly, oil filter  80  includes an internal bypass valve  82  that allows hydraulic fluid to bypass filter  80  and continue flowing toward tank  60  if filter  80  becomes clogged. It is also noted that hydraulic line  109  conveys seepage oil from pump  62  to line  108  where it enters the flow of hydraulic oil being conveyed to tank  60 . The volume of hydraulic fluid conveyed through oil seepage lines  109  and lines  122 ,  122   a  is relatively minimal compared with the volumes of hydraulic oil conveyed through the discharge ports of pumps  62 ,  64 . 
         [0040]    The discharge flow from charge pump  64  is also communicated through discharge line  100  and line  104  to hydraulic lines  110  and  112  and, thus, one-way by-pass valves  74  and  70 . By-pass valve  74  is positioned parallel with pressure relief valve  72  with both valves  74  and  72  positioned between hydraulic line  94  (via hydraulic line  114 ) and hydraulic line  100  (via hydraulic line  104 ). Pressure relief valve  72  is positioned to relieve the pressure within hydraulic line  94  if it exceeds a predetermined threshold, e.g., a pressure that would damage pump  62  or motors  24 . Pressure relief valve  72  will allow the flow of hydraulic fluid from line  94  into hydraulic line  104  where it can flow through hydraulic line  106 , pressure relief valve  66  and into hydraulic line  108  which will convey the fluid to tank  60 . If the pressure within line  94  becomes excessively low relative to hydraulic line  100 , one-way valve  74  will allow the passage of hydraulic fluid from line  100  through lines into hydraulic line  94  via the interconnecting hydraulic lines  104 ,  110  and  114 . 
         [0041]    Similarly, by-pass valve  70  is positioned parallel with pressure relief valve  68  with both valves  70  and  68  positioned between hydraulic line  96  (via hydraulic line  116 ) and hydraulic line  100  (via hydraulic line  104 ). Pressure relief valve  68  is positioned to relieve the pressure within hydraulic line  96  if it exceeds a predetermined threshold, e.g., a pressure that would damage pump  62  or motors  24 . Pressure relief valve  68  will allow the flow of hydraulic fluid from line  96  into hydraulic line  104  where it can flow through hydraulic line  106 , pressure relief valve  66  and into hydraulic line  108  which will convey the fluid to tank  60 . If the pressure within line  96  becomes excessively low relative to hydraulic line  100 , one-way valve  70  will allow the passage of hydraulic fluid from line  100  into hydraulic line  96  via the interconnecting hydraulic lines  104 ,  112  and  116 . 
         [0042]    As mentioned above, hydraulic line  118  is in fluid communication with valve cartridge  88 . Hydraulic line  126  extends between valve cartridge  88  and valve cartridge  90 . Valve cartridge  90  is, in turn, in fluid communication with hydraulic line  124  through hydraulic line  128 . Hydraulic line  130  extends between line  126  and hydraulic line  100  and includes a one-way check valve  92 . Hydraulic line  132  provides fluid communication between hydraulic line  130  and valve cartridge  86 . Check valve  92  is positioned so that it prevents hydraulic fluid from lines  126  and  132  from entering hydraulic line  100 . If the pressure within hydraulic line  100  exceeds the pressure within line  130 , check valve  92  will open allowing hydraulic fluid from line  100  to enter line  130 . Once such fluid has entered line  130 , the flow of the fluid will depend, in part, on the positions of valves  86 ,  88  and  90 . 
         [0043]    Valve cartridges  84 ,  86 ,  88  and  90  are commonly available conventional valve cartridges. The use of valve cartridges  84 ,  86 ,  88  and  90  in the arrangement depicted in  FIGS. 2-6 , allows the operation of motors  24  to be controlled without the use of a highly complex and, thus expensive, custom valve. Valve cartridge  84  includes first and second valve arrangements  84   a,    84   b,  a solenoid  84   c  and a biasing member  84   d.  Hydraulic lines  100 ,  120  and  124  will be in communication with either valve arrangement  84   a  or  84   b.  Biasing member  84   d  urges cartridge valve  84  into a position wherein valve arrangement  84   b  is in communication with lines  100 ,  120  and  124  while the activation of solenoid  84   c  will place valve arrangement  84   a  into communication with hydraulic lines  100 ,  120 ,  124 . Valve arrangement  84   a  provides fluid communication between lines  120  and  124  and terminates hydraulic line  100 . Valve arrangement  84   b  provides fluid communication between lines  100  and  120  and terminates hydraulic line  124 . 
         [0044]    Valve cartridges  86 ,  88  and  90  each have a common structure with two valve arrangements  86   a,    86   b;    88   a,    88   b;    90   a,    90   b,  a solenoid  86   c,    88   c,    90   c  and a biasing member  86   d,    88   d,    90   d.  The biasing members  86   d,    88   d,    88   d  of valve cartridges  86 ,  88 ,  90  respectively urge valve arrangements  86   a,    88   a,    90   a  into communication with the hydraulic lines connected with the cartridges while activation of solenoids  86   c,    88   d,    90   d  will place valve arrangements  86   b,    88   b,    90   b  into communication with the hydraulic lines connected with the cartridges. Each of the valve cartridges  86 ,  88  and  90  are connected with two hydraulic lines and valve arrangements  86   a,    88   a,    90   a  provide fluid communication between the two hydraulic lines. Valve arrangements  86   b,    88   b,    90   b  each include a one-way check valve. Valve arrangement  86   b  only allows fluid flow from line  132  to line  98 , valve arrangement  88   b  only allows fluid flow from line  126  to line  118  and valve arrangement  90   b  only allows fluid flow from line  128  to line  126 . Valves  86 ,  88  and  90  are referred to herein as being “open” when valve arrangements  86   a,    88   a,    90   a  are being employed and “closed” when valve arrangements  86   b,    88   b,    90   b  are being employed. 
         [0045]    The operation of hydraulic circuit  22  and its control of motors  24  will now be discussed with reference to  FIGS. 2-5 . Turning first to  FIG. 2 , this figure illustrates hydraulic circuit  22  when motors  24  are in a free-wheeling condition with pistons  40  and rolling cam members  44  retracted so that they do not engage inner surface  50  of cam ring  46  as cam ring  46  rotates about cylinder block  36 . In the situation depicted in  FIG. 2 , i.e., when motors  34  are in a free-wheeling condition, the discharge flow of variable displacement pump  62  will be negligible. Charge pump  64  will, however, be operating and discharging hydraulic fluid at approximately 400 psi. Relief valve  66  will reduce the pressure of the hydraulic fluid within line  100  to approximately 50 psi and re-circulate some of the hydraulic fluid to charge pump  64  through line  108 , tank  60  and line  102 . Valve arrangement  84   b  of valve cartridge  84  is in communication with hydraulic lines  100 ,  120  and  124  and thereby allows hydraulic fluid from line  100  to enter lines  120  and  120   a.  Lines  120 ,  120   a,  in turn, communicate the fluid from charge pump  64  to motor case cavity  54  where it retains pistons  40  and cam members  44  in their retracted positions. Hydraulic fluid entering cavities  54  is returned in part to tank  60  through hydraulic lines  122 ,  122   a.    
         [0046]    As can also be seen in  FIG. 2 , valve arrangements  86   a,    88   a,    90   a  are all in communication with the hydraulic lines respectively connected with valve cartridges  86 ,  88 ,  90 . Thus, lines  94 ,  98 ,  132 ,  130 ,  126 ,  118  and  96  form a loop with variable displacement pump  62 . It is also noted that the position of valve cartridge  90  (in combination with the position of valve cartridges  86 ,  88 ) allows fluid within lines  94 ,  96  to be in communication with tank  60 . As can also be seen from  FIG. 2 , lines  94 ,  98 ,  132 ,  130 ,  126 ,  128 ,  124 ,  118  and  96  are all in communication with tank  60  when valves  86 ,  88  and  90  are in the configuration depicted in  FIG. 2 . As discussed above, the fluid discharged from pump  64 , will flow to motor case cavities  54  through lines  100  and  120  and will return to tank  60  through lines  104 ,  106  and  108 . Another fraction of the flow of fluid discharged from pump  64  will flow into lines  94  and  96  through one-way relief valves  70 ,  74  and  92 . For the fluid to pass through these three relief valves, the pressure within the receiving lines must be slightly less than in lines  110 ,  112  or  100 . The fluid passing through these valves will also experience a slight drop in pressure. The greater pressure in motor case  54  relative to bore cavities  42  reflects this pressure drop across one-way valves  70 ,  74 ,  92 . In other words, the fluid discharged by pump  64  “deadheads” at motor cavities  54  after passing through line  100 , valve  84  and line  120 / 120   a  while the fluid discharged from pump  64  can only enter bores  42  after passing through one of check valves  70 ,  74  or  92  which ensures that the fluid within bores  42  is at a slightly lower pressure than the fluid in cavities  54 . As mentioned above, pump  62  is not supplying high pressure hydraulic fluid to either of lines  94  or  96  in the condition illustrated in  FIG. 2 . 
         [0047]    The operating condition illustrated in  FIG. 2  is maintained until it is desired to engage motors  24 . To avoid damage to motors  24  a multi-step process is used to transition between the “roading” or free-wheeling operating condition illustrated in  FIG. 2  and the condition depicted in  FIG. 5  in which motors  24  are driving wheels  34 . (Wheels  34 , engine  26 , PTO shaft  32 , mechanical power train  28  and wheels  30  have been omitted from  FIGS. 3-5  for purposes of graphical clarity.) 
         [0048]      FIG. 3  illustrates the first step in the transition process from free-wheeling to hydraulic motor actuated conditions. This transition process is implemented using a conventional electronic controller (not shown). Such controllers are typically found in vehicles employing hydraulic drive assist motors and, as one having ordinary skill in the art will understand, such controllers can be programmed to operate solenoids  84   c,    86   d,    88   d,    90   d  and pump  62  in the manner described below to implement the transition process from a free-wheeling mode to a power assist mode and from a power assist mode to a free-wheeling mode. 
         [0049]    As can be seen in  FIG. 3 , solenoid  84   c  of valve cartridge  84  has been activated and valve arrangement  84   a  now connects hydraulic fluid line  120  with line  124  and hydraulic line  100  in communication with charge pump  64  has been capped. In this configuration, the fluid pressure within motor case cavities  54  will fall as fluid within cavities  54  drains to tank  60  due to the fluid communication between cavities  54  and tank  60  provided by hydraulic lines  120 ,  124 . As a result, the pressure within motor cavities  54  and cylinder bores  42  will equalize. The hydraulic fluid being discharged by charge pump  64  will be re-circulated primarily through a loop defined by line  100 , line  106 , line  108 , cooler  76 , filter  80 , tank  60  and line  102 . Some of the fluid discharged by pump  64  will also flow through check valves  92 ,  70  and  72  and will return to tank  60  through lines  124 ,  122  after passing through valve  90 . After circuit  22  has had sufficient time to release the motor case pressure to tank  60 , the transition process will move to the condition shown in  FIG. 4 . It is also possible, however, for the transition process to move to the condition shown in  FIG. 4  while the pressure within cavity  54  is still be relieved. 
         [0050]      FIG. 4  represents the conditions under which pistons  40  are extended and rolling cam members  44  are re-engaged with inner surface  50  of camming ring  46 . In this step of the process, cartridge valve  90  is “closed,” i.e., valve arrangement  90   b,  with its one-way valve, is placed in communication with lines  126  and  128  thereby preventing hydraulic fluid from line  126  from entering lines  128  and  124  and returning to tank  60 . Cartridge valves  84 ,  86  and  88  remain in the same positions as in  FIG. 3 . Pump  62  is discharging no more than a minimal flow of fluid in the condition shown in  FIG. 4 . Charge pump  64  is always operating when vehicle  20  is running and, thus, is still discharging fluid. 
         [0051]    Because valve  84  does not allow fluid in line  100  to enter line  120 , fluid discharged from pump  64  entering line  100  will pass through one-way check valve  92 . With both valve cartridges  86 ,  88  being in an “open” position, i.e., with valve arrangements  86   a  and  88   a  providing fluid communication between lines  98  and  132  and between lines  118  and  126  respectively, fluid flowing from line  100  into line  130  and then lines  126  and  132  will pass through valves  86  and  88  and enter lines  98  and  118 . After entering lines  98  and  118  the fluid will be in communication with lines  94 / 94   a  and  96 / 96   a.    
         [0052]    Because valve cartridges  86 ,  88  are each in an “open” position, fluid within line  98 , and lines  94 / 94   a  which are in fluid communication therewith, and line  118 , and lines  96 / 96   a  which are in fluid communication therewith, will all substantially equalize. Thus, the fluid pressure within lines  94 / 94   a  and  96 / 96   a  will also be increased and motors  24  will experience a fluid pressure increase in piston bores  42 . This increase in the fluid pressure will extend pistons  40  and re-engage rolling cam members  44  with camming surface  50 . Because the increased pressure in lines  94 / 94   a  is substantially equivalent to the increased pressure in lines  96 / 96   a,  motors  24  will not experience a pressure differential between lines  96  and  94  or between lines  96   a  and  94   a  and the increased fluid pressure in piston bores  42  will not cause motors  24  to rotate due to hydraulic pressure. The motors  24  may, however, be rotating due to motion of the vehicle. 
         [0053]      FIG. 5  represents the condition in which motors  24  are operating and driving the rotation of wheels  34 . After extending pistons  40 , as depicted in  FIG. 4 , cartridge valves  86  and  88  are “closed” to separate the hydraulic lines supplying high pressure hydraulic fluid to motors  24 . The discharge of pump  62  is then increased. to actuate motors  24 . Cartridge valves  84  and  90  remain in the same position shown in  FIG. 4 . To “close” valves  86  and  88 , solenoids  86   c  and  88   c  are activated and valve arrangements  86   b  and  88   b  are placed in communication with hydraulic lines  98  and  132  and lines  118  and  126  respectively. Valve arrangement  86   b  prevents the flow of fluid from line  98  (which in communication with pump  62  via line  94 ) to line  132  while valve arrangement  88   b  prevents the flow of fluid from line  118  (which is in communication with the opposite side of pump  62  via line  96 ) to line  126 . 
         [0054]    After closing valves  86 ,  88 , hydraulic lines  94 / 94   a  and  96 / 96   a  form a closed loop that includes pump  62  and motors  24  with motors  24  being arranged in parallel. Depending upon which direction pump  62  is pumping either line  94 / 94   a  will be a high pressure discharge line with line  96 / 96   a  being a relatively low pressure return line, or, line  96 / 96   a  will be a high pressure discharge line with line  94 / 94   a  being a relatively low pressure return line. The pressure differential and fluid flow across motors  24  in this situation will cause motors  24  to rotate and drive wheels  34 . The direction of rotation of motors  24  will depend upon the direction in which pump  62  is operating. 
         [0055]    Seepage oil and oil from cavity  54  is returned to tank  60  through lines  109 ,  122 ,  122   a  and  124  in the motor operation condition depicted in  FIG. 5 . Charge pump  64  draws hydraulic oil from tank  60  and replaces the seepage losses by introducing hydraulic oil into the closed loop formed by pump  62 , hydraulic lines  94 / 94   a  and  96 / 96   a  and motors  24  into the return line through either valve  70  or valve  74  depending upon the direction of operation of pump  62 . The hydraulic oil will enter the closed loop through valve  70  and into hydraulic line  96  if hydraulic line  96  is acting as the low pressure inlet line to pump  62  and it will enter the closed loop through valve  74  and into hydraulic line  94  if hydraulic line  94  is acting as the low pressure inlet line to pump  62 . 
         [0056]    When it is desirable to stop the operation of motors  24  and return to the free-wheeling condition depicted in  FIG. 2 , the discharge volume of pump  62  is first reduced to a negligible amount to reduce the fluid pressure within piston bores  42 . Valve cartridges  86 ,  88  and  90  are then opened (placing valve arrangements  86   a,    88   a,    90   a  in communication with their respective hydraulic lines) to allow the pressure within lines  94 / 94   a  and  96 / 96   a  to substantially equalize and drain to tank  60  through line  124 . Valve  84  is then shifted to place valve arrangement  84   b  into communication with hydraulic lines  100 ,  120  and  124 . Valve arrangement  84   b  allows hydraulic fluid discharged from charge pump  64  to enter motor case cavities  54  through lines  100  and  120 / 120   a  and thereby force the retraction of pistons  40  and rolling cam members  44  and placing circuit  22  in the condition shown in  FIG. 2 . 
         [0057]      FIG. 6  illustrates a slightly modified version of the hydraulic circuit shown in  FIGS. 2-5 . The most significant difference between the hydraulic circuit  23  shown in  FIG. 6  and the hydraulic circuit  22  of  FIGS. 2-5  is that variable displacement pump  162  and charge pump  164  illustrated in  FIG. 6  are only operated when it is desirable to have pump  162  operating. A second charge pump  65  is operated whenever the vehicle engine  26  is running. 
         [0058]      FIG. 6  illustrates the free-wheeling condition. In this condition, pumps  162  and  164  will not be operating and only charge pump  65  will be operating. Charge pump  65  draws hydraulic fluid from tank  60  through line  140  and discharges it into hydraulic line  101 . Line  101  is in communication with hydraulic line  125  and valve cartridge  85 . Hydraulic line  125  includes a pressure relief valve  67  that limits the pressure within line  101  to approximately 50 psi. Fluid passing through valve  67  passes through oil cooler  76  and oil filter  80  and is returned to tank  60 . 
         [0059]    With regard to the control valves for circuit  23 , valves  86 ,  88 ,  90  and  92  all operate in the same manner as described above with reference to  FIGS. 2-5 . Valve cartridge  85  and its connection with the hydraulic circuit, however, do differ somewhat from valve cartridge  84  and its connections. Hydraulic lines  142 ,  142   a  provide fluid communication between valve  85  and motor case cavities  54 . When valve cartridge  85  is in the position shown in  FIG. 6  (with valve arrangement  85   b  in communication with lines  101 ,  142  and  144 ), lines  142 ,  142   a  will communicate hydraulic fluid from line  101  to cavities  54  to retain pistons  40  in their retracted positions within motors  24 . Hydraulic lines  123  and  123   a  are similar to lines  122 ,  122   a  of  FIGS. 2-5  and return seepage oil from motors  24  to tank  60 . Hydraulic line  144  extends between line  123  and valve  85  and, when valve cartridge  85  is shifted to place valve arrangement  85   a  into communication with lines  101 ,  142  and  144 , lines  144  and  142  will provide communication between cavities  54  and tank  60  in addition to lines  123  and  123   a.    
         [0060]    Hydraulic lines  146  and  148  are oil seepage lines that are in communication with line  125  and return pump lubricating seepage to tank  60  while hydraulic line  150  is used to provide variable displacement pump  162  with hydraulic oil to replace seepage and other losses when pump  162  is operating. With regard to the replacement of seepage losses, it is noted  FIG. 6  provides a simplified schematic view of charge pump  164  and pump  162  and omits showing check valves similar to valves  70  and  74  through which fluid to replace seepage losses can be communicated to pump  162 . When pump  162  is operating, hydraulic lines  94 / 94   a  and  96 / 96   a,  pump  162  and motors  24  form a closed loop in the same manner discussed above with reference to  FIGS. 2-5 . 
         [0061]    When making the transition from the free-wheeling condition depicted in  FIG. 6  to a condition where motors  24  are operating and driving wheels  34 , circuit  23  undergoes similar steps to those discussed above with reference to  FIGS. 2-5 . Initially, valve  85  is shifted to place valve arrangement  85   a  into communication with lines  101 ,  142  and  144 . This terminates the flow of hydraulic fluid from line  101  to motor case cavities  54 . Termination of this fluid flow causes the motor case cavities  54  to drain to tank  60  through lines  123 ,  123   a  and through lines  142 / 142   a.  Lines  142 / 142   a  will be in communication with tank  60  through valve  85 , line  144  and line  123 . After the fluid pressure within cavities  54  has been reduced, valve  90  is closed to increase the pressure within bores  42  and extend pistons  40 . 
         [0062]    After pistons  40  have been extended, valves  86  and  88  are closed to separate the forward and reverse lines  94 / 94   a  and  96 / 96   a  feeding motors  24 . Pumps  162 ,  164  are then actuated. A clutch (not shown) is disposed between pumps  162 ,  164  and the PTO shaft to selectively activate and deactivate pumps  162 ,  164 . The discharge volume of pump  162  is then raised thereby raising the pressure in either hydraulic lines  94 / 94   a  or in hydraulic lines  96 / 96   a  depending upon the direction in which pump  162  is operating. As a result of the hydraulic fluid flow through lines  94 / 94   a  and  96 / 96   a,  motors  24  will begin operating and driving wheels  34  in either a forward or reverse direction depending upon the operating direction of pump  162 . 
         [0063]    While the illustrated embodiments employed charge pumps in combination with variable displacement pumps  62 ,  162 , alternative embodiments could utilize other constant sources of low pressure hydraulic fluid instead of charge pumps. 
         [0064]    While this invention has been described as having an exemplary design, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles.