Abstract:
A power transfer unit coupling two otherwise separate hydraulic systems for bidirectional transfer of hydraulic power therebetween without transfer of fluid between the two systems. Control of the power transfer unit is effected using only hydraulic pressures, and static operation of the unit is maintained without power transfer until a determined pressure differential between the coupled systems is achieved to reduce wear and increase service life of the unit. Once dynamic power transferring operation of the unit is initiated, a pressure differential lower than the determined level is maintained between the two systems.

Description:
BACKGROUND OF THE INVENTION 
     The field of the present invention is reversible hydraulic motor-pump units. More particularly, the present invention relates to power transfer units wherein two reversible hydraulic motor-pump units are coupled for torque transfer therebetween. Each one of the motor-pump units is associated with a separate hydraulic system having its own main high-pressure pump and fluid reservoir. By means of the power transfer unit, hydraulic power may be borrowed from one system for conversion into mechanical power by one of the motor-pump units, and then converted by the other motor-pump unit into hydraulic power which is supplied to the other of the two hydraulic systems. 
     It is conventional in modern aircraft to provide a plurality of separate hydraulic systems by which the various control functions of the aircraft may be performed. For example, the hydraulic systems of the aircraft may be used to move and selectively position control surfaces such as the slats or flaps of the wing, and to raise and lower the aircraft landing gear. In order to improve the level of flight safety, the hydraulic systems may be in redundant relationship with respect to performing some control functions. In order to provide such plural and partially redundant hydraulic systems while also minimizing the weight required for such systems, it is common to provide hydraulic power transfer units between the plural systems. These conventional hydraulic power transfer units provide for borrowing of hydraulic power from one system in order to meet a need in a coupled system which is beyond the supply capability of the primary high-pressure pump of that system which is borrowing power. Additionally, it is necessary that the hydraulic power transfer units prevent transfer of fluid between the coupled systems such that a failure of one system does not incapacitate a coupled system. 
     However, it is recognized in the field that conventional hydraulic power transfer units have several shortcomings. Among the shortcomings is a tendency for conventional units to operate too frequently. That is, a relatively low level of hydraulic pressure differential between two coupled hydraulic systems will result in conventional power transfer units operating in order to minimize the pressure differential between the coupled systems. Such overly frequent operation results in increased wear and shortened service life for conventional power transfer units. Another recognized shortcoming of conventional power transfer units is the possibility of failure of one portion of the power transfer unit resulting in failure of both of the coupled systems due to fluid leakage between the two systems. 
     Those conventional power transfer units which provide for bi-directional transfer of power between coupled hydraulic systems have in many cases also employed relatively complex electro-hydraulic control systems. Such complexity is undesirable because it provides additional failure modes for the power transfer unit. The necessity of providing electrical power to such units is also a disadvantage. 
     SUMMARY OF THE INVENTION 
     In view of the above, it is a primary object of the present invention to provide a hydraulic power transfer unit which will not operate until a predetermined pressure differential exists between the two hydraulic systems which are coupled by the power transfer unit. An additional object of the present invention is to provide a power transfer unit of the above character which once operating will maintain a pressure differential between the coupled hydraulic systems which is lower than the predetermined pressure differential necessary to begin operation of the power transfer unit. 
     Yet another object of the present invention is to provide a power transfer unit of the above-described character wherein leakage of fluid between the two hydraulic systems coupled by the power transfer unit is positively prevented. 
     Still another object of the present invention is to provide a power transfer unit using entirely hydraulic control derived from one of the two hydraulic systems coupled by the power transfer unit. 
     Accordingly, the present invention provides a power transfer unit having a first reversible fluid motor-pump unit of selectively variable displacement and a second reversible fluid motor-pump unit of fixed displacement. Each of the motor-pump units has respective high-pressure and low-pressure inlet/outlet ports as well as an input/output shaft by which mechanical power may be delivered to or derived from the motor-pump unit. The first and second motor-pump units are coupled via their respective input/output shafts for torque transfer therebetween with attendant reversal of rotational direction dependent upon which of the motor-pump units is operating as a pump and which is operating as a motor. Each of the first and second motor-pump units is associated with a separate respective fluid source means each having a respective primary high-pressure pump providing relatively higher pressure fluid to the high-pressure inlet/outlet port of a respective one of the motor-pump units and a comparatively lower pressure fluid to the low-pressure inlet/outlet port of the respective one of the motor-pump units. Fluid pressure responsive control means is provided which is responsive to the comparatively higher pressure of both of the two fluid source means such that onset of operation of the power transfer unit to transfer power in either direction between the two coupled hydraulic systems is delayed until a predetermined fluid pressure differential exists between the two hydraulic systems. The fluid pressure responsive control means is also operative once power transfer is initiated between the two coupled hydraulic systems to maintain a selected fluid pressure differential therebetween which is less than the predetermined fluid pressure differential necessary to begin operation of the power transfer unit. 
     Other objects and advantages of the present invention will be apparent to those skilled in the pertinent art from a reading of the following detailed description of a single preferred embodiment of the invention taken in conjunction with the drawing FIGURES comprising a part of the present disclosure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     FIG. 1 schematically depicts a power transfer unit according to the present invention coupling two otherwise separate hydraulic systems each having a charging pump and primary high-pressure pump drawing fluid from a respective reservoir for supply to a respective load; 
     FIG. 2 depicts schematically and partially in cross section a power transfer unit according to the present invention; and 
     FIG. 3 depicts a portion of FIG. 2 enlarged to better illustrate detail thereof. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Generally referenced by the numeral 10 in the FIG. 1 are a pair of coupled hydraulic systems wherein many of the components thereof may be duplicated in each of the two hydraulic systems. Because of duplication of components in each of the two systems, reference numerals which are used to refer to a component of the system illustrated on the left-hand portion of FIG. 1 which is duplicated on the right-hand portion of FIG. 1 are also employed on the right-hand portion of the FIGURE with a prime added thereto. 
     Viewing now the left-hand portion of FIG. 1, it will be seen that a reservoir 12 is provided wherein a store of hydraulic fluid 14 is received. Fluid 14 flows from reservoir 12 to a charging pump 16 via a conduit 18. The charging pump 16 provides the fluid pressurized to an intermediate level via a conduit 20 to a primary or main high-pressure pump 22. The pump 22 provides high-pressure fluid to a load 24 via a conduit 26. The load 24 may comprise any of a variety of motors or actuators which are driven by high-pressure hydraulic fluid selectively under the control of manual or automatic devices. During operation of the system 10, the load 24 has a pressure fluid absorption or consumption characteristic which varies markedly dependent upon the number and size of actuators, motors, and other devices which are drawing fluid from the conduit 26 at any one particular time. Relatively lower pressure fluid is exhausted from the load 24 via a conduit 28 which couples with conduit 20 intermediate of the charging pump 16 and high-pressure pump 22. A relief valve 30 couples conduit 28 to the reservoir 12 such that the pressure in ccnduit 20 and 28 is limited to about 150% of the design discharge pressure of charging pump 16. 
     Interconnecting the two hydraulic systems described immediately above is a power transfer unit 32. The power transfer unit 32 includes a first high-pressure inlet/outlet port 34 and first low-pressure inlet/outlet port 36 which are respectively connected with the conduits 26 and 20 via conduits 38 and 40. Similarly, the power transfer unit 32 includes second high-pressure inlet/outlet port 42 and second low-pressure inlet/outlet port 44 which are respectively connected with the conduits 26&#39; and 20&#39; via conduits 46 and 48. 
     During operation of the system 10, all of the pumps 16, 16&#39;, 22 and 22&#39; are driven such that high-pressure fluid is supplied at equal design pressure levels via the conduits 26, 26&#39; to the respective loads 24, 24&#39;. However, in the event that one of the loads 24 or 24&#39; exceeds the pumping capability of its respective main high pressure pump 20 or 22&#39;, the fluid pressure level in the associated conduit 26, 26&#39; will drop below the design pressure range for the hydraulic system. In such an event, the power transfer unit 32 is operative to &#34;borrow&#34; hydraulic power from the other of the two hydraulic systems. Alternatively, in the event that one of the hydraulic systems is disabled, for example, because of failure of its charge pump 16 or main high-pressure pump 22, the loads associated with that hydraulic system may still be operated, albeit at a lower level of speed or power consumption by transferring, via the power transfer unit 32, hydraulic power from the one of the two systems which still is fully functioning. 
     Considering now FIG. 2, it will be seen that the power transfer unit 32 includes a first variable-displacement motor-pump unit 50 which is of the axial piston swashplate type. The motor-pump unit 50 includes a rotational barrel 52 defining a plurality of axially extending bores 54 wherein are received a like plurality of axially reciprocal plunger units 56. The plungers 56 engage shoe members 58 which are in sliding engagement with a variableangle swashplate member 60. The barrel member 52 is journaled by bearings 62 and 64 which engage shaft portions 66 of the motor pump unit 50. 
     The power transfer unit 32 also includes a second fixed-displacement motor-pump unit 68 of bent axis type. The second motor-pump unit 68 includes a rotatable barrel portion 70 which defines a plurality of axially extending bores 72 reciprocally receiving a like plurality of axially reciprocal plunger members 74. The barrel member 70 is journaled by a pair of axially spaced apart bearing members 76 and 78 and is rotationally driven by a drive shaft member 80 having constant velocity universal joints on each end thereof. The universal joints drivingly engaging the barrel member 70 and a socket member 82, respectively, to couple these members for rotation in unison. Socket member 82 is drivingly connected with the shaft portion 66 of motor-pump unit 50, and defines a plurality of radially outwardly extending drive arms 84, matching in number the plurality of bores 72. Each one of the plunger members 74 is drivingly connected with a respective one of the drive arms 84 by a connecting rod 86 each having spherical termination ends thereon which are received in ball-and-socket relationship at a respective one of the drive arms 84, and with a respective one of the plunger members 74. In order to complete this preliminary description of the power transfer unit 32 it must be noted that a radially outer and axially extending surface 88 of the socket member 82 defines a sealing surface against which a pair of back-to-back fluid seals 90 and 92 are engaged. Further, the power transfer unit 68 includes a case which is only schematically depicted partially in FIG. 2, but which will be understood to receive and support the component parts of the unit. As a consequence, fluid transfer between the first fluid motor-pump 50 and the second fluid motor-pump 68 is positively prevented. 
     It will be understood viewing FIG. 2 that when high-pressure fluid is supplied to the conduits 38 and 46 as described hereinabove, each of the fluid motor-pump units 50 and 68 tends to operate as a motor and to drive the other of the fluid motor-pump units. However, because the fluid motor-pump units 50 and 68 are coupled to oppose one another and have substantiaI static friction, there will exist within a certain range of fluid pressures within the conduits 38 and 46 a static torque balance between the fluid motor-pump units. However, when the fluid pressure differential between the conduits 38 and 46 exceeds the above-mentioned range, one of the fluid motor-pump units 50 or 68 will begin to operate as a motor and to drive the other of the fluid rotor-pump units in its pumping mode of operation. Under such conditions, the driving motor-pump unit will receive pressurized fluid at the respective conduit 38 or 46 and discharge spent fluid via the respective conduit 40 or 48. The motor-pump unit which is being driven in a pumping mode will receive relatively lower-pressure fluid via the conduit 40 or 48 and will discharge this fluid pressurized via the respective conduit 46 or 38. It will be further recognized that the static torque balance between the fluid motor-pump units 50 and 68 is greatly influenced by the angular position of the swashplate member 60. Also, this angular position greatly influences the operating speed and torque versus pressure characteristic of the power transfer unit 32 in operation. 
     In order to control the angular position of the swashplate member 60, an elongate control arm 90 is attached thereto. At its outer end, the control arm 94 defines oppositely disposed arcuate surfaces 96 and 98. The arcuate surfaces 96 and 98 are received between the precisely spaced apart opposite ends of control plungers 100 and 102. Each of the plungers 100 and 102 defines an operative part of respective control assemblies 104 and 106. Each of the control assemblies 104 and 106 is similar in construction, although they may differ in effective fluid pressure-responsive area. Viewing the control assemblies 104 and 106, it will be seen that each includes a respective coil compression spring 108, 110 extending between a rear wall of the control assembly and respective annular moveable spring seat members 112, 114. The spring seat members 112, 114 each respectively engage in annular radially inwardly extending portion of the 116, 118 of the respective control assembly as well as an annular radially outwardly extending collar part 120, 122 of the respective plunger 100, 102. Consequently, a rest position is defined for the control arm 94 wherein each of the spring seats 112, 114 is in engagement with the respective annular portion 116, 118 as well as its respective collar part 120, 122. In the rest position of the lever 94, the swashplate member 60 defines a selected angle with respect to a perpendicular from the shaft 60. Consequently, the first motor pump unit 50 defines at the rest position of lever 94 a selected effective fluid displacement per rotation of the shaft 66, and also has a selected characteristic of static and dynamic torques verses fluid pressure and operating speed, respectively. 
     Also included by power transfer unit 32 is a fluid pressure responsive control valve apparatus 124, viewing now FIGS. 2 and 3. The control valve apparatus 124 includes a housing 126 defining a stepped bore 128 therein. Received within the step bore 128 are a pair of sleeve members 130, 132, respectively receiving a plunger member 134, and a spool valve member 136. At the left end of the control valve assembly 124 the sleeve 130 and plunger 134 cooperate with the housing 126 to define a chamber 138 receiving a coil compression spring 140 and a spring seat member 142. At its right end the member 142 bears against the plunger member 134. A port 144 opens to chamber 138 and communicates therefrom to the high pressure conduit 46 of the second motor-pump unit 68 via conduit 146. Similarly, at the right end of the control valve apparatus 124 the housing 126 cooperates with a cap member 148 to define a chamber 150 wherein is received a respective coil compression spring 152 extending between the cap member 148 and a spring seat member 154. The spring seat 154 bears upon the right end of the spool valve 136 to bias the latter into engagement with the plunger member 134. The housing 126 also defines ports 156 and 158 which respectively communicate separately with the interior or case cavities of the respective first and second motor-pump units 50 and 68 via conduits 160 and 162. A port 164 defined by the housing 126 communicates with the high pressure conduit 38 of the first fluid motor-pump unit 50 via a respective conduit 166. 
     Within the housing 126 the sleeves 130 and 132 cooperate to define an annular chamber 168 communicating with the port 164 and conduit 166. The sleeve 130 defines a radially extending notch 170 at the end thereof abutting sleeve member 132. The notch 170 communicates pressurized fluid from conduit 166 to a chamber 172 defined intermediate of the ends of the plunger member 134 and valve member 136. The housing 126 also defines a passage 174 communicating from the chamber 168 to the chamber 150. Consequently, the spool valve member 136 is exposed at both of its ends to pressure fluid communicated via conduit 166 from the high-pressure port 38 of motor-pump unit 50. Similarly, the sleeve member 132 cooperates with housing 126 to define an annular chamber 176 communicating with port 158 and conduit 162. The housing 128 also defines a passage 178 communicating chamber 176 with an annular chamber 180 circumscribing the sleeve member 130. The chamber 180 is matched by like annular chamber 182 communicating with the passage 156 and conduit 160. 
     Viewing now the sleeve member 132 and spool valve member 136 in greater detail, it will be seen that the spool valve member 136 is slidably and sealingly received within the sleeve member 132. The spool valve meter 136 defines a pair of axially spaced apart lands 184 and 186 which in a centered position thereof align with axially and radially extending slot-like passages 188 and 190 defined by the sleeve member 136. An axially extending groove portion of the spool valve member 192 extends between the lands 184 and 186 and define as a radial clearance with sleeve member 132. A radially extending passage 194 communicates through the sleeve member radially outwardly of the groove portion 192 to the annular chamber 176. The slots 188 and 190 are of narrow circumferential extent, and the axial ends thereof precisely align in sealing relationship with the axial ends of the lands 184 and 186. Radially outwardly of the slots 188 and 190, the housing 126 cooperates with sleeve member 136 to define respective annular chambers 200 and 202. The chamber 200 is communicated via a port 204 and conduit 206 with the control assembly 104. Similarly, chamber 202 is communicated by a port 208 and conduit 210 with the control assembly 106. Recalling the structure of the control assemblies 104 and 106, it will be seen that the conduit 206 opens into a chamber 212 cooperatively defined by the plunger 100 and the remainder of control assembly 104. Similarly, the conduit 210 opens into a chamber 214 cooperatively defined by the plunger 102 and the remainder of control assembly 106. 
     In order to complete this description of the control valve apparatus 124, it must be noted that the sleeve member 130 cooperates with the plunger member 134 to define an annular chamber 216 which communicates with the chamber 180 via a radially extending passage 218. Similarly, the plunger member 134 cooperates with sleeve member 130 to define an annular chamber 220 communicating with the chamber 182 via a radially extending passage 222. Intermediate of the ends of the plunger member 134 and spaced along the length thereof, the plunger member 134 defines a plurality of radially extending and circumferentially continuous grooves 224 which cooperate with the sleeve member 130 to define labyrinth seals. 
     Having observed the structure of the hydraulic system 10 and of the power transfer unit 32 thereof, attention may now be given to its operation. With all of the pumps 16, 16&#39;, 22 and 22&#39; operating, the conduits 26 and 26&#39; are charged with high-pressure fluid at substantially equal pressures according to the design of the hydraulic system 10. As the pressure fluid absorptions of the loads 24 and 24&#39; vary as various load items thereof are valved in and out of operation, the fluid flow rates and pressures within the conduits 26 and 26&#39; varies. Spent fluid from each of the loads 24 and 24&#39; is returned via the respective conduits 28 and 28&#39; to the conduits 20, 20&#39; intermediate of the charging pumps 16, 16&#39; and the main high-pressure pumps 22, 22&#39;. The relief valves 30 and 30&#39; operate to limit the pressure within conduits 20 and 20&#39; to about 150% of the design output pressure of the charging pumps 16 and 16&#39;. Accordingly, the conduits 40 and 48 communicating with the power transfer unit 32 are also maintained at a pressure between the design discharge pressure of the charging pumps 16, 16&#39; and the relief pressure value of the respective relief valves 30 and 30&#39;. 
     While the conduits 26 and 26&#39; are charged to pressure levels which are substantially at the design pressure level for the hydraulic system 10, the motor-pump units 50 and 68 of power transfer unit 32 will not be operating despite fluid pressure variation within conduits 26 and 26&#39; which are within a limited and predetermined range. Such is the case because the motor-pump units 50 and 68 are connected via the shaft portions 66 and 80 in opposing torque relationship. Even though the torque produced by one of the motor-pump units 50 and 68 may exceed that opposing torque produced by the other of the motor-pump units, the torque differential between the two units is not sufficient to overcome the static friction, or breakaway torque, required to start the two units into rotation. However, even while the two motor-pump units 50 and 68 are static, the control valve apparatus 124 is effective to actuate the control assemblies 104 and 106 in preparation for beginning of operation of the motor-pump units 50 and 68. 
     By way of example only, the load 24 may be exceeding the pumping capacity of high-pressure pump 22 such that the pressure in conduit 26 is lower than that in conduit 26&#39;, but the pressure differential therebetween is not sufficient to begin operation of the motor-pump units 50 and 68. The relatively lower fluid pressure in conduit 26 is communicated via conduit 38 to port 34 and therefrom via conduit 166 into chambers 170, and thence via passage 174 to chamber 150. On the other hand, the comparatively higher fluid pressure from conduit 26&#39; is communicated via conduit 46 to port 42 and thence via conduit 146 to chamber 183 at the left end of control valve apparatus 124. Accordingly, the pressure differential between chamber 138 and chamber 172 is effective to shift the plunger member 134 and spool valve member 136 slightly rightwardly in opposition to spring 152 viewing FIG. 2. Rightward movement of the spool valve member 136 shifts the land 186 rightwardly with respect to radially extending slot 190 to communicate chamber 172 with passage 190, and to communicate high-pressure fluid therefrom to conduit 210 via port 208. The high-pressure fluid communicated via conduit 210 to control assembly 106 is effective in chamber 214 to urge plunger member 102 rightwardly. Simultaneously, land 184 of spool valve member 136 is shifted slightly rightwardly with respect to passage 188 to communicate chamber 212 of control assembly 104 with the case of motor-pump unit 50 via the flow path defined by features 206, 204, 200, 188, 194, 176, 158, and 162. Therefore, fluid within chamber 212 of control assembly 104 is drained to the relatively low pressure established by charging pump 16 and relief valve 30. When the force effective on plunger 102 is sufficient to overcome the preload of spring 108, the lever 94 is moved rightwardly an amount dependent upon the spring rate of spring 108. 
     Viewing the motor-pump unit 50 in greater detail, in will be seen that rightward movement of lever 94 in response to the above-described sequence of events results in a movement of the swashplate member 60 from its rest position toward a position of decreased displacement for the motor-pump unit 50. Consequently, the resisting torque generated by motor-pump unit 50 is decreased while the driving torque generated by the motor-pump unit 68 retains its previous level. Consequently, the power transfer unit 32 is prepared for the beginning of operation with the motor-pump unit 68 operating as a motor driving motor-pump unit 50 in a pumping mode. Should the pressure differential between the conduits 26 and 26&#39; reach the predetermined level whereat the breakaway torque of the motor-pump units 50 and 68 is exceeded by the torque differential therebetween, the latter units will begin operating with motorpump unit 68 driving motor-pump unit 50 to pump fluid from conduit 40 to conduit 38 to assist in maintaining the pressure level in conduit 26 approximately at the design pressure level. Once the motor-pump units 50, 68 of power transfer unit 32 begin operation, the difference between the static friction of the components of the power transfer unit and the dynamic frictions effective therein during operation results in the pressure differential maintained between the conduits 26 and 26&#39; being less then that pressure differential which is necessary to begin operation of the power transfer unit. Accordingly, during operation of the power transfer unit, the control valve apparatus 24 modulates the position of control lever 94, and displacement of variable displacement motor-pump unit 50 in the range extending between the decreased displacement position thereof, and that displacement which is defined at the rest position of the swashplate member 60 and control lever 94. 
     On the other hand, in the event that the load 24&#39; exceeds the pumping capacity of primary high-pressure pump 22&#39; such that the pressure in conduit 26&#39; is lower than that in conduit 26, a higher effective pressure will prevail in chamber 172 of the control valve apparatus 124 than that prevailing in the chamber 138. Consequently, the plunger member 134 will be shifted slightly leftwardly in opposition to compression spring 140 while the compression spring 152 acting through the spring seat member 154 urges the spool valve member 136 to follow plunger member 134. Such leftward movement of the spool valve member 136 results in communication between chamber 150 and port 204 and conduit 206 extending therefrom to control assembly 104. Also, such leftward shifting of the spool valve member 136 results in communication of conduit 210 from control apparatus control assembly 106 communicating via port 208 and passage 190 with the axially extending clearance between the groove portion 192 of spool valve 136 and sleeve 132 to drain fluid via passage 194 to conduit 162 communicating with the case of motor-pump unit 50. As a result, the control lever 94 is shifted leftwardly in opposition to compression spring 110 of the control assembly 106 according to the preload and spring rate thereof. Such shifting of control lever 94 moves the swashplate member 60 angularly to a position increasing the effective displacement and static torque of motor-pump unit 50 in preparation for operation thereof as a motor. 
     The increase in effective displacement of motor-pump unit 50 as described above results in this motor-pump unit generating a greater effective driving torque. On the other hand, the resisting torque generated by motor-pump unit 68 is decreased by the relatively lower fluid pressure effective in conduit 26&#39; and communicating thereto via conduit 46. When the pressure differential between conduit 26 and 26&#39; reaches the determined level necessary to overcome the break away torque requirement set by static frictions within the power transfer unit 32, operation thereof begins with motor-pump unit 50 operating as a motor driving motor-pump unit 68 in a pumping mode of operation. Consequently, the motor-pump unit 68 receives fluid via conduit 48 and delivers this fluid pressurized via conduit 46 to the conduit 26&#39; to assist in meeting the demands of the load 24&#39;. During such operation of the power transfer unit, the control valve assembly 124 acts to modulate the position of control lever 94 and of swashplate member 60 in the range extending from the maximum displacement position therefore to the rest position previously described. 
     In view of the above, it will be seen that when the power transfer unit is in operation with either one of the motor-pump units 50 and 68 driving the other, the control lever 94 and swashplate member 60 is modulated between the rest position thereof and either the minimum displacement position or maximum displacement position therefor according to the direction of operation of the power transfer unit. That is, if the power transfer unit is operating to transfer power from the left-hand side of the system illustrated in FIG. 1 to the right-hand side thereof, the swashplate member 60 is positioned in a range extending from the maximum displacement position thereof to the rest position therefore. On the other hand, if the power transfer unit 32 is operating to transfer power from the right-hand side of the hydaulic system illustrated in FIG. 1 to the left-hand side thereof, then the swashplate member 60 of motor-pump unit 50 is modulated in a range extending from the minimum effective displacement position therefore to the rest position. In view of this, it will be seen that when the load demand of either load 24 or 24&#39; decreases such that the associated primary high-pressure pump 22 or 22&#39; is able to meet the design pressure requirement for the hydraulic system 10, the control lever 94 and swashplate member 60 will be modulated to the rest position therefore. Consequently, operation of the power transfer unit 32 will continue until such time as the torque differential between the two motor pump units falls below the total of dynamic frictional torque effective within the power transfer unit 32 and the resisting torque of that unit which is being operated in the pumping mode. When this stall condition is reached, operation of the motor-pump units 50 and 68 of the power transfer unit 32 will cease. However, the control valve apparatus 124 and control assemblies 104 and 106 will be continuously operative to move the control lever 94 and swashplate member 60 away from the rest position thereof in anticipation of once again beginning operation of the motor-pump units of power transfer unit 32 as pressure levels in the conduits 26 and 26&#39; vary dynamically in response to variations of pressure fluid utilization effective within loads 24 and 24&#39;. 
     Having described my invention in sufficient detail to allow one skilled in the art to make and use same, I desire to protect my invention under applicable law according to the following claims. Several modifications will suggest themselves to those skilled in the art. For example, the singular preferred embodiment herein depicted and described is predicated upon coupling hydraulic systems of equal design operating pressures. However, it is considered easily within the skill of the art to couple systems of unequal design pressures by means of the present invention altered in the relative size and pressure responsive areas of component parts thereof as necessary. Such modification, and others, are intended to be encompassed by the appended claims. While my invention has been depicted and described by reference to a singular preferred embodiment thereof, such reference is not intended to imply a limitation upon the invention, and no such limitation is to be inferred. I desire to limit my invention only according to the scope and spirit of the following claims, which also provide additional definition of the invention.