Patent Publication Number: US-2005138924-A1

Title: Hydraulic drive apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation-in-part of U.S. Non-Provisional patent application Ser. No. 09/652,090, filed Aug. 31, 2000 and incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention relates to a hydraulic drive apparatus used for hydraulic shovels, cranes, asphalt finishers, machine tools and the like. More particularly, the invention relates to a hydraulic drive apparatus that controls a pressure of working oil supplied from a hydraulic pump to be higher than a load pressure of a hydraulic motor by a predetermined pressure.  
      A technique to be described hereunder is known for a hydraulic pressure circuit for controlling an electro-hydraulic servomechanism.  
      In  FIG. 5 , reference numeral  1  is an operation lever which inputs a desired operation position by varying an angle of the lever; and  2  is an operation-position signal outputting means which generates a voltage (operation position signal) depending on an input angle of the operation lever  1  and outputs the voltage to a controller  3 . The operation position input by the operation lever  1  is converted by the operation-position signal outputting means  2  into the operation-position signal to be output to the controller  3 .  
      A feedback signal is fed back to the controller  3 , from a speed detector  4  which is attached to an output portion of a hydraulic motor  16  to detect a speed (an output rotational number) of the output portion.  
      The hydraulic pump  12  feeds working oil from a tank  11  to the hydraulic motor  16 . During the flow of the working oil from the tank  11  to the hydraulic motor  16 , the working oil is selected in its flow or its flow is controlled by a directional control valve  15 . That is, the directional control valve  15  controls an operation of the hydraulic motor  16 .  
      An operation-position signal output from the operation-position signal outputting means  2  and a feedback signal output from the speed detector  4  are input the controller  3 . The controller  3  compares the input operation position signal with the input feedback signal, and controls the directional control valve  15  so that the hydraulic motor  16  operates in accordance with the operation position that is input by the operation lever  1 . Accordingly, the hydraulic motor  16  operates in accordance with the operation position input by the operation lever  1 .  
      A pressure of the working oil supplied from the hydraulic pump  12  is set at a constant value corresponding to the maximum load pressure independently of a quantity of the working oil used by the hydraulic motor  16 , by a combination of a pressure-compensating-function-added servo regulator  13  and a relief valve  14 . Accordingly, the hydraulic motor  16  is operable for any arbitrary load pressure.  
      However, the hydraulic drive apparatus discussed above has the following problem. Even if the load pressure to the hydraulic motor  16  is extremely smaller than the pressure of the working oil supplied from the hydraulic pump  12 , the pressure of the working oil supplied from the hydraulic pump  12  is always constant. Therefore, the drive horsepower of the hydraulic pump  12  is wastefully consumed.  
      Accordingly, an object of the present invention is to drive a hydraulic motor without wastefully consuming the drive horsepower of a hydraulic pump.  
      A hydraulic drive apparatus in which a plurality of hydraulic motors are driven through a plurality of control valves by one hydraulic pump as disclosed in JP-A-11-36374 has been known for the hydraulic drive apparatus used for general purpose construction machines, such as hydraulic shovel.  
      An example of the arrangement of the hydraulic drive apparatus will be described with reference to an accompanying drawing.  
      In a hydraulic drive apparatus  500  shown in  FIG. 15 , when an operation position is input to an operation lever  610 , an electric motor  630  is driven and rotated in accordance with the operation position input to the operation lever  610 . The control valve  650  is designed such that when a rotational speed difference is produced between the electric motor  630  and the hydraulic motor  601 , the control valve  650  adjusts a quantity of working oil supplied from a hydraulic pump  502  to the hydraulic motor  601  to reduce the speed difference between the electric motor  630  and the hydraulic motor  601  by driving and rotating the hydraulic motor  601 . Therefore, when the electric motor  630  is rotated, the hydraulic motor  601  rotates following up the rotation of the electric motor  630 , through the action of the control valve  650 . A control valve, such as the control valve  650 , which drives and rotates the hydraulic motor in accordance with the rotational speed difference between the electric motor and the hydraulic motor, will be referred to as an electro-hydraulic servovalve.  
      When the operation position is input to an operation lever  710 , an operation oil flows through operation oil passages  711  and  712  in accordance with the operation position input to the operation lever  710 , and a control valve  750  changes its valve or spool position by the flow of the operation oil. When the control valve  750  moves its position, a quantity of the working oil supplied from the hydraulic pump  502  to a hydraulic motor  701  is controlled, so that the hydraulic motor  701  is driven and rotated. Such a hydraulic motor has the control valve  750 , exclusive of the electro-hydraulic servovalve, arranged such that it drives and rotates the hydraulic motor  710  in accordance with the operation position input to the operation lever  710  will be referred to as a general purpose valve.  
      Generally, the electro-hydraulic servovalve is higher in control accuracy than the general purpose valve.  
      However, the hydraulic drive apparatus  500  mentioned above suffers from the following problem: When an electro-hydraulic servovalve is used as one of the plurality of control valves in order to improve the control accuracy of a specific rotary shaft, it is difficult to simultaneously drive those hydraulic motors.  
      For example, in a case of simultaneously driving and rotating the hydraulic motor  601  and the hydraulic motor  701 , if a load of the hydraulic motor  701  is smaller than that of the hydraulic motor  601 , the working oil supplied from the hydraulic pump  502  flows through the control valve  750  (i.e., the general purpose valve), but does not flow through the control valve  650  (i.e., the electro-hydraulic servovalve). Therefore, the hydraulic motor  701  can be driven and rotated, but the hydraulic motor  601  is not rotated.  
      Accordingly, an object of the present invention is to provide a hydraulic drive apparatus which is provided with an electro-hydraulic servovalve and at least one general purpose valve, and is capable of simultaneously operating a plurality of hydraulic motors.  
     BRIEF SUMMARY OF THE INVENTION  
      Briefly stated, one embodiment of the present invention is directed to a hydraulic drive apparatus for driving and rotating a drive rotary member driven and rotated by hydraulic pressure. The apparatus comprises a working oil supplying unit for supplying working oil to drive and rotate the drive rotary member; and a rotation control unit for controlling a quantity of the working oil supplied from the working oil supplying unit to the drive rotary member so that the drive rotary member is driven and rotated as desired, the rotation control unit including: an operation position inputting unit for inputting an operation position; an operation-position signal outputting unit for generating and outputting an operation position signal depending on the operation position input by the operation position inputting unit; a drive signal outputting unit for computing and converting the operation position signal output from the operation-position signal outputting unit into a drive signal output therefrom; an electric motor driven and rotated at a speed and a quantity of rotation depending on the drive signal output from the drive signal outputting unit; and a working oil control unit for controlling a quantity of the working oil supplied from the working oil supplying unit to the drive rotary member so that the drive rotary member is driven and rotated depending on rotation of the electric motor; a drive oil pressure detecting unit for detecting a pressure of the working oil for driving and rotating the drive rotary member, and generating and outputting a drive oil pressure signal depending on the pressure thus detected; a supplying oil pressure detect unit for detecting a pressure of the working oil supplied from the working oil supplying unit to the working oil control unit, and generating and outputting a supplying-oil pressure signal depending on the pressure thus detected; a pump discharge pressure control unit for controlling a pressure of the working oil supplied from the working oil supplying unit to the working oil control unit to be equal to or lower than a set pressure; an oil pressure control unit for receiving the supplying-oil pressure signal output from the supplying oil detect unit and the drive oil pressure signal output from the drive oil pressure detecting unit, and outputting a pressure signal to the pump discharge pressure control unit, thereby controlling the pressure of the working oil supplied from the working oil supplying unit to be higher, by a predetermined pressure, than the pressure of the working oil for driving and rotating the drive rotary member; a supplying oil quantity control unit for controlling a quantity of the working oil that the working oil supplying unit supplies; and a supplying oil quantity signal outputting unit for receiving the operation position signal output from the operation position signal outputting unit, generating a supplying oil quantity signal from the operation position signal, and outputting the supplying oil quantity signal to the supplying oil quantity control unit, thereby controlling the quantity of the working oil supplied to the supplying oil quantity control unit by the working oil supplying unit.  
      A second embodiment of the present invention is directed to a hydraulic drive apparatus for driving and rotating a drive rotary member driven and rotated by hydraulic pressure. The apparatus comprises a working oil supplying unit for supplying working oil to drive and rotate the drive rotary member; and a rotation control unit for controlling a quantity of the working oil supplied from the working oil supplying unit to the drive rotary member so that the drive rotary member is driven and rotated as desired, the rotation control unit including: an operation position inputting unit for inputting an operation position; operation-position signal outputting unit for generating and outputting an operation position signal depending on the operation position input by the operation position inputting unit; a drive signal outputting unit for computing and converting the operation position signal output from the operation-position signal outputting unit into a drive signal output therefrom; an electric motor driven and rotated at a speed and a quantity of rotation depending on the drive signal output from the drive signal outputting unit; and a working oil control unit for controlling a quantity of the working oil supplied from the working oil supplying unit to the drive rotary member so that the drive rotary member is driven and rotated depending on rotation of the electric motor; a drive oil pressure detecting unit for detecting a pressure of the working oil for driving and rotating the drive rotary member, and generating and outputting a drive oil pressure signal depending on the pressure thus detected; a supplying oil pressure detect unit for detecting a pressure of the working oil supplied from the working oil supplying unit to the working oil control unit, and generating and outputting a supplying-oil pressure signal depending on the pressure thus detected; a main relief valve for regulating a pressure of the working oil supplied from the working oil supplying unit to the working oil control unit to be equal to or lower than a set pressure; an electromagnetic relief valve for varying the set pressure of the main relief valve by varying a set pressure thereof; an oil pressure control unit for receiving the supplying-oil pressure signal output from the supplying oil detect unit and the drive oil pressure signal output from the drive oil pressure detecting unit, and outputting a position signal to the electromagnetic relief valve to vary the set pressure of the electromagnetic relief valve and thus the set pressure of the main relief valve, thereby controlling the pressure of the working oil supplied from the working oil supplying unit to be higher, by a predetermined pressure, than the pressure of the working oil for driving and rotating the drive rotary member; a supplying oil quantity control unit for controlling a quantity of the working oil that the working oil supplying unit supplies; and a supplying oil quantity signal outputting unit for receiving the operation position signal output from the operation-position signal outputting unit, generating a supplying oil quantity signal from the operation position signal, and outputting the supplying oil quantity signal to the supplying oil quantity control unit, thereby controlling the quantity of the working oil supplied to the supplying oil quantity control unit by the working oil supplying unit.  
      A third embodiment of the present invention is directed to a hydraulic drive apparatus for driving and rotating a drive rotary member driven and rotated by hydraulic pressure. The apparatus comprises: a working oil supplying unit for supplying working oil to drive and rotate the drive rotary member; and a rotation control unit for controlling a quantity of the working oil supplied from the working oil supplying unit to the drive rotary member so that the drive rotary member is driven and rotated as desired, the rotation control unit including: an operation position inputting unit for inputting an operation position; an operation-position signal outputting unit for generating and outputting an operation position signal depending on the operation position input by the operation position inputting unit; a drive signal outputting unit for computing and converting the operation position signal output from the operation-position signal outputting unit into a drive signal output therefrom; an electric motor driven and rotated at a speed and a quantity of rotation depending on the drive signal output from the drive signal outputting unit; and a working oil control unit for controlling a quantity of the working oil supplied from the working oil supplying unit to the drive rotary member so that the drive rotary member is driven and rotated depending on rotation of the electric motor; a drive oil pressure detecting unit for detecting a pressure of the working oil for driving and rotating the drive rotary member, and generating and outputting a drive oil pressure signal depending on the pressure thus detected; a supplying oil pressure detect unit for detecting a pressure of the working oil supplied from the working oil supplying unit to the working oil control unit, and generating and outputting a supplying-oil pressure signal depending on the pressure thus detected; a main relief valve for regulating a pressure of the working oil supplied from the working oil supplying unit to the working oil control unit to be equal to or lower than a set pressure; an electromagnetic relief valve for varying the set pressure of the main relief valve by varying a set pressure thereof; and an oil pressure control unit for receiving the supplying-oil pressure signal output from the supplying oil detect unit and the drive oil pressure signal output from the drive oil pressure detecting unit, and outputting a position signal to the electromagnetic relief valve to vary the set pressure of the electromagnetic relief valve and thus the set pressure of the main relief valve, thereby controlling the pressure of the working oil supplied from the working oil supplying unit to be higher, by a predetermined pressure, than the pressure of the working oil for driving and rotating the drive rotary member, wherein when the pressure of the working oil detected by the drive oil pressure detecting unit is equal to or higher than a predetermined pressure, the oil pressure control unit feeds the position signal of a predetermined value to the electromagnetic relief valve.  
      A fourth embodiment of the present invention is directed to a hydraulic driving apparatus comprising: a drive rotary member driven by hydraulic pressure; a pump for supplying working oil; a working oil control unit connecting the pump to the drive rotary member with oil passages, controlling a quantity of the working oil supplied from the pump to the drive rotary member; an operation position inputting unit for inputting an operation position; an operation position signal outputting unit for outputting an operation position signal according to the operation position input by the operation position inputting unit; an electric motor electrically connected to a drive signal outputting unit and operatively coupled with the working oil control unit, thereby being driven at a predetermined speed or quantity of rotation according to the drive signal output from the drive signal outputting unit; a pump discharge pressure control unit electrically connected to an oil pressure control unit and operatively coupled to the pump, controlling a pressure of the working oil discharged from the pump; a supplying oil quantity control unit electrically connected to a supplying oil quantity signal outputting unit and operatively coupled with the pump for controlling a quantity of the working oil that the pump supplies; a supplying oil pressure detect unit in communication with a first oil passage connecting the pump to the working oil control unit, the supplying oil pressure detect unit detecting a pressure of the working oil supplied from the pump to the working oil control unit, and outputting a supplying-oil pressure signal to the oil pressure control unit; a drive oil pressure detecting unit in communication with a second oil passage connecting the working oil control unit to the drive rotary member, the drive oil pressure detecting unit detecting a pressure of the working oil for driving the drive rotary member, and generating and outputting a drive oil pressure signal to the oil pressure control unit; wherein the drive signal outputting unit computes and converts the operation position signal output from the operation-position signal outputting unit into a drive signal output therefrom; wherein the oil pressure control unit receives the supplying-oil pressure signal output from the supplying oil detect unit and the drive oil pressure signal output from the drive oil pressure detecting unit, and outputs a pressure signal to the pump discharge pressure control unit, so that a pressure of the working oil supplied from the pump is controlled to be higher, by a predetermined pressure, than the pressure of the working oil for driving the drive rotary member; and wherein the supplying oil quantity signal outputting unit receives the operation signal output from the operation-position signal outputting unit, and outputs a supplying oil quantity signal to the supplying oil quantity control unit, corresponding to a quantity of the working oil that the pump supplies.  
      A fifth embodiment of the present invention is directed to a method for controlling a hydraulic drive apparatus having a drive rotary member driven by a drive pressure of a working fluid, a pump outputting the working fluid at a supply pressure, and a working fluid control unit controlling a quantity of the working fluid supplied from the pump to the drive rotary member, the working fluid being supplied through fluid passages comprising a supply line connecting the pump to the working fluid control unit and a drive line connecting the working fluid control unit to the drive rotary member, the method comprising the steps of: determining the drive pressure of the working fluid in the drive line; determining the supply pressure of the working fluid in the supply line; providing a pressure relief unit in fluid communication with the supply line, the pressure relief unit having a controllable pressure set point; determining a differential pressure between the drive pressure and the supply pressure; and controlling the pressure set point based on the differential pressure to maintain the supply pressure greater by a predetermined pressure than the drive pressure.  
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
      The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.  
      In the drawings:  
       FIG. 1  is a hydraulic pressure circuit diagram showing a hydraulic-drive apparatus which is a first embodiment of the present invention;  
       FIG. 2  is a hydraulic pressure circuit diagram showing a hydraulic drive apparatus which is a second embodiment of the present invention;  
       FIG. 3  is a hydraulic pressure circuit diagram showing a 20 hydraulic drive apparatus which is a third embodiment of the present invention;  
       FIG. 4  is a hydraulic pressure circuit diagram showing a hydraulic drive apparatus which is a fourth embodiment of the present invention;  
       FIG. 5  is a hydraulic pressure circuit diagram showing a related hydraulic drive apparatus;  
       FIG. 6  is hydraulic pressure circuit diagram showing a hydraulic drive apparatus which constitutes a fifth embodiment;  
       FIG. 7  is hydraulic pressure circuit diagram showing a hydraulic drive apparatus which constitutes a sixth embodiment;  
       FIG. 8  is hydraulic pressure circuit diagram showing a hydraulic drive apparatus which constitutes a seventh embodiment;  
       FIG. 9  is hydraulic pressure circuit diagram showing a hydraulic drive apparatus which constitutes an eighth embodiment;  
       FIG. 10  is hydraulic pressure circuit diagram showing a hydraulic drive apparatus which constitutes a ninth embodiment.  
       FIG. 11  is hydraulic pressure circuit diagram showing a hydraulic drive apparatus which constitutes a tenth embodiment;  
       FIG. 12  is hydraulic pressure circuit diagram showing a hydraulic drive apparatus which constitutes an eleventh embodiment.  
       FIG. 13  is hydraulic pressure circuit diagram showing a hydraulic drive apparatus which constitutes a twelfth embodiment;  
       FIG. 14  is hydraulic pressure circuit diagram showing a hydraulic drive apparatus which constitutes a thirteenth embodiment;  
       FIG. 15  is hydraulic pressure circuit diagram showing a related hydraulic drive apparatus; and  
       FIGS. 16   a - 16   d  show the ideal and actual supply oil pressure and drive oil pressure for a hydraulic drive apparatus, such as a winch, lifting a load under various control scenarios. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present disclosure relates to the subject matter contained in Japanese patent application Nos. Hei. 11-246183 (filed on Aug. 31, 1999), 2000-007386 (filed on Jan. 17, 2000) and 2000-004948 (filed on Jan. 13, 2000), which are expressly incorporated herein by reference in their entireties. The preferred embodiments of the present invention disclosed herein are described below with reference to the accompanying drawings.  
     FIRST EMBODIMENT  
       FIG. 1  is a hydraulic pressure circuit diagram showing a hydraulic drive apparatus which constitutes a first embodiment of the present invention.  
      An arrangement of the hydraulic drive apparatus of the first embodiment will first be described.  
      In  FIG. 1 , reference numeral  41  designates a tank for storing oil;  43  is a hydraulic motor as a drive rotary member, driven and rotated by hydraulic pressure; and  42  is a hydraulic pump as working oil supplying means for driving and rotating the hydraulic motor  43 .  
      Reference numeral  20  designates a rotation control means which controls a quantity of the working oil supplied from the hydraulic pump  42 , and supplies the quantity controlled working oil to the hydraulic motor  43 , thereby driving the hydraulic motor  43  at a desired rotational speed. The rotation control means  20  comprises an operation lever  21  as operation position inputting means to which an operation position is input, an operation-position signal outputting means  22  for generating and outputting an operation position signal, which is dependent on the operation position input to the operation lever  21 , a drive signal output circuit  23 A as drive signal outputting means for computing the operation position signal output from the operation-position signal outputting means  22  into a drive signal, and outputting the drive signal, an electric motor  24  as an electrically driven motor being driven and rotated in accordance with a speed and a quantity of rotation, both being defined by the drive signal output from the drive signal output circuit  23 A, and a directional control valve  25  as working oil control means which drives and rotates the hydraulic motor  43  in accordance with a rotation of the electric motor  24  by controlling a quantity of the working oil supplied from the hydraulic pump  42  and supplying the quantity-controlled working oil to the hydraulic motor  43 .  
      The directional control valve  25  and the hydraulic motor  43  are coupled together by screw means in order to vary an opening degree of the valve in accordance with a speed difference between the hydraulic motor  43  and the electric motor  24 .  
      Reference numeral  45  designates a main relief valve which regulates a hydraulic pressure of the working oil, which is supplied from the hydraulic pump  42  to the directional control valve  25 , to be a set pressure or lower. Reference numeral  46  designates an electromagnetic relief valve for varying the set pressure of the main relief valve  45  by varying a set pressure thereof.  
      Reference numerals  47  and  48  are pressure gauges as drive oil pressure detecting means. Each of those pressure gauges detects a pressure of the working oil for driving and rotating the hydraulic motor  43 , and generates a drive oil pressure signal depending on the detected pressure. Reference numeral  49  is a pressure gauge as supply oil pressure detecting means, which detects a pressure of the working oil supplied from the hydraulic pump  42  to the directional control valve  25 , and generates and outputs a supplying-oil pressure signal depending on the detected pressure.  
      Reference numeral  50  is a check valve for preventing the reverse flow of the working oil, and reference numeral  51  is a pump regulator as supplying oil quantity control means, which controls a quantity of the working oil supplied by the hydraulic pump  42 .  
      Reference numeral  23 B is an oil pressure control circuit as oil pressure control means. The oil pressure control circuit  23 B receives a supplying-oil pressure signal output from the pressure gauge  49  and drive oil pressure signals output from the pressure gauges  47  and  48 . The oil pressure control circuit  23 B feeds current to the electromagnetic relief valve  46  to vary a set pressure of the electromagnetic relief valve  46  to thereby vary a set pressure of the main relief valve  45 . Consequently, the oil pressure control circuit  23 B executes such a control that the pressure of the working oil supplied from the hydraulic pump  42  is higher, by a predetermined pressure, than the pressure of the working oil for driving and rotating the hydraulic motor  43 , as further discussed below.  
      Reference numeral  23 C is a supplying oil quantity signal output circuit as supplying oil quantity signal outputting means which computes an operation position signal output from the operation-position signal outputting means  22  into a supplying oil quantity signal and outputs it.  
      Incidentally, the drive signal output circuit  23 A, the oil pressure control circuit  23 B and the supplying oil quantity signal output circuit  23 C are accommodated in a housing  23 .  
      Reference numerals  31  to  37  designate signal transmission paths; reference numerals  61  to  66  are oil passages; and numeral  67  is a pilot oil passage.  
      An operation of the thus constructed hydraulic drive apparatus of the first embodiment will be described.  
      Description will be given about an operation of the hydraulic drive apparatus to drive and rotate the hydraulic motor  43  in accordance with an operation position input by the operation lever  21 .  
      An operator changes an angle of the operation lever  21  to input an operation position to the hydraulic drive apparatus. Then, the operation-position signal outputting means  22  outputs an operation position signal as an electric signal, which is dependent on the operation position input to the operation lever  21 . Subsequently, the operation-position signal output from the operation-position signal outputting means  22  is input to the drive signal output circuit  23 A by way of the signal transmission path  31 . Upon receipt of the signal, the drive signal output circuit  23 A computes the operation-position signal received from the operation-position signal outputting means  22 , and outputs a drive signal to the electric motor  24 . The drive signal output from the drive signal output circuit  23 A is then input to the electric motor  24  via the signal transmission path  32 . As a result, the electric motor  24  is driven and rotated at a speed and a quantity of rotation, which are defined by the drive signal received from the drive signal output circuit  23 A.  
      The electric motor  24 , directional control valve  25  and hydraulic motor  43  make up an electro-hydraulic servomechanism, not shown. Therefore, the hydraulic motor  43  is rotated following up the rotation of the electric motor  24 .  
      Description will be given about an operation of the electro-hydraulic servomechanism, which is made up of the electric motor  24 , directional control valve  25  and hydraulic motor  43 .  
      When an angle difference θ 3  is produced between an input angle θ 1  of the electric motor  24  and an output angle θ 2  of the hydraulic motor  43 , a mechanical mechanism (not shown) opens a port of the directional control valve  25  in accordance with the angle difference θ 3 . When the port of the directional control valve  25  is opened, the working oil is supplied from the hydraulic pump  42  to the hydraulic motor  43  and the hydraulic motor  43  is driven and rotated since the working oil for driving and rotating the hydraulic motor  43  has been fed from the tank  41  to the directional control valve  25  through the oil passages  61  to  63  by the hydraulic pump  42 . When the hydraulic motor  43  is driven and rotated, the mechanical mechanism (not shown) reduces the angle difference θ 3 . When the input angle θ 1  is larger than the output angle θ 2 , the output angle θ 2  is increased through the rotation of the hydraulic motor  43  till the angle difference θ 3  is reduced to zero. When the input angle θ 2  is larger than the output angle θ 1 , the output angle θ 1  is reduced through the rotation of the hydraulic motor  43  till it is reduced to zero.  
      Accordingly, so long as the angle difference θ 3  is present, the hydraulic motor  43  is driven and rotated by the servomechanism, while following up the rotation of the electric motor  24 .  
      As described above, the electric motor  24  is driven and rotated in accordance with the operation position that is input by operating the operation lever  21  by the operator, and the hydraulic motor  43  is driven and rotated following up the rotation of the electric motor  24 . Accordingly, the hydraulic motor  43  is driven and rotated in accordance with the operation position that is input through the operation lever  21  by the operator. An operation of a featured portion of the present invention will be described.  
      The working oil for driving and rotating the hydraulic motor  43  by the hydraulic pump  42  is supplied from the tank  41  to the directional control valve  25  through the oil passages  61  to  63 . The working oil that is supplied to the directional control valve  25  is supplied through an oil passage  64  to the hydraulic motor  43  by the directional control valve  25 , and thereafter it is returned to the directional control valve  25  via an oil passage  65 , or it is supplied through the oil passage  65  to the hydraulic motor  43 , and then it is returned to the directional control valve  25  via the oil passage  64 , or it is not supplied to the hydraulic motor  43  since its passage connected to the hydraulic motor  43  is interrupted. When the working oil is supplied to the hydraulic motor  43  and then returned to the directional control valve  25 , the working oil is returned to the tank  41  via an oil passage  66  to the tank  41 .  
      A pressure of the working oil supplied from the hydraulic pump  42  to the directional control valve  25  is controlled to be higher than the pressure of the working oil for driving and rotating the hydraulic motor  43  by a maximum pressure of 20 kg/cm 2 .  
      A control of the pressure of the working oil supplied from the hydraulic pump  42  to the directional control valve  25  will now be described.  
      The inputting and outputting of the oil pressure control circuit  23 B which dominantly function in the control will be described.  
      The drive oil pressure signals output from the pressure gauges  47  and  48  are input through signal transmission paths  33  and  34  to the oil pressure control circuit  23 B. Further, a supplying-oil pressure signal output from the pressure gauge  49  is input to the oil pressure control circuit  23 B through a signal transmission path  35 . The drive oil pressure signals are respectively generated by the pressure gauges  47  and  48  depending on pressures of the working oils in the oil passages  64  and  65  which are detected by the pressure gauges  47  and  48 . The supplying-oil pressure signal is generated by the pressure gauges  49  depending on a pressure of the working oil in an oil passage  63 , which is detected by the pressure gauge  49 .  
      The oil pressure control circuit  23 B feeds current to the electromagnetic relief valve  46  through a signal transmission path  36 .  
      The oil pressure control circuit  23 B varies a pressure of the working oil supplied from the hydraulic pump  42  to the directional control valve  25  in the following way by varying the current output therefrom.  
      When the oil pressure control circuit  23 B varies the current that is output through the signal transmission path  36  to the electromagnetic relief valve  46 , a set pressure of the electromagnetic relief valve  46  varies since the set pressure of the electromagnetic relief valve  46  is determined by the current input thereto. Since the pressure of the pilot oil in the pilot oil passage  67  is equal to the set pressure in the electromagnetic relief valve  46 , the pressure of the pilot oil also varies with variation of the set pressure of the electromagnetic relief valve  46 . The set pressure of the main relief valve  45  is determined by the pressure of the pilot oil. Therefore, when the pressure of the pilot oil varies, the set pressure of the main relief valve  45  also varies. Further, the main relief valve  45  controls the pressure of the working oil that is supplied from the hydraulic pump  42  to the directional control valve  25  to be the set pressure or lower. When the set pressure of the main relief valve  45  varies, the pressure of the working oil supplied from the hydraulic pump  42  to the directional control valve  25  also varies.  
      As described above, the oil pressure control circuit  23 B is able to vary the pressure of the working oil supplied from the hydraulic pump  42  to the directional control valve  25  by varying the current fed to the electromagnetic relief valve  46 .  
      Then, the control for the working oil that is supplied from the hydraulic pump  42  to the directional control valve  25  will be described while tracing its control phases.  
      The pressures of the working oils flowing through the oil passages  64  and  65  are detected by the pressure gauges  47  and  48  respectively; the drive oil pressure signals representative of those detected pressures are generated; and those signals are output to the oil pressure control circuit  23 B through the signal transmission paths  33  and  34 . The pressure of the working oil in the oil passage  63  is detected by the pressure gauge  49 ; a supplying-oil pressure signal representative of the detected pressure is generated; and the signal is output to the oil pressure control circuit  23 B through the signal transmission path  35 .  
      The oil pressure control circuit  23 B receives a supplying-oil pressure signal from the pressure gauge  49 , and drive oil pressure signals from the pressure gauges  47  and  48 . Then, by using a higher one of the drive oil pressure signals (i.e., the drive signal output from the pressure gauge  47  or  48  which detects a pressure higher than that detected by the other pressure gauge  47  or  48 ) and the supplying-oil pressure signal of the pressure gauge  49 , the oil pressure control circuit  23 B determines the current to be fed to the electromagnetic relief valve  46  so that the pressure of the working oil received from the hydraulic pump  42  is higher than the pressure of the working oil for driving and rotating the hydraulic motor  43  by 20 kg/cm 2  at maximum. Then, the oil pressure control circuit feeds the current to the electromagnetic relief valve  46  through the signal transmission path  36 . By using this current, as described above, the oil pressure control circuit  23 B varies the pressure of the working oil supplied from the hydraulic pump  42  to the directional control valve  25 .  
      The reason why the drive oil pressure signal indicative of a higher pressure is selected from the drive oil pressure signals output from the pressure gauges  47  and  48  when the oil pressure control circuit  23 B determines the current to be fed to the electromagnetic relief valve  46  is that a higher one of the hydraulic pressures detected by the pressure gauges  47  and  48  is the pressure of the working oil for driving and rotating the hydraulic motor  43 .  
      By repeating the above-mentioned process constantly, the pressure of the working oil supplied from the hydraulic pump  42  is controlled to be higher, by a maximum pressure of 20 kg/cm 2 , than the pressure of the working oil for driving and rotating the hydraulic motor  43 .  
      The reason for using the pressure of the working oil for driving and rotating the hydraulic motor  43  as detected by the drive oil pressure gauges  47 ,  48  for closed loop feedback control of the pressure of the working oil supplied by the hydraulic pump  42  may best be understood with reference to  FIGS. 16   a - 16   d.    
      Referring to  FIG. 16   a,  for the purpose of illustrating the operation of the oil pressure control circuit  23 B ( FIG. 1 ), reference will now be made to the schematic representation of three phases (wind-up start  5   a,  elevation of the load  6  at constant velocity  5   b,  and wind-up complete  5   c ) of a winding-up operation of a winch coupled to a hydraulic motor as depicted therein.  
       FIG. 16   b  is an ideal representation of pump supply-oil pressure as a function of time and shows for each of the three phases of the winding-up operation of  FIG. 16   a  the corresponding pump supply-oil pressure when the pump supply-oil pressure is not controlled (curve  49   a ) and when the pump supply-oil pressure is controlled by closed loop feedback control (curve  49   b ). The abscissa of  FIG. 16   b  is in registry with the layout of the three phases depicted in the schematic of  FIG. 16   a , such that the mid portion of the curves  49   a,    49   b  corresponds to the elevation of the load  6  at constant velocity phase  5   b  of the winding-up operation. The abscissa in  FIGS. 16   c  and  16   d  are similarly positioned.  
      Referring to the curve  49   a,  the pump supply-oil pressure is shown to be constant throughout all three phases of the winding-up operation. Winches, driven by a conventional hydraulic drive, such as the drive shown in  FIG. 5  discussed above, or alternatively, a hydraulic drive apparatus with the hydraulic pressure circuit of  FIG. 1  in which the state of the electromagnetic control valve  46  is held constant, have a theoretical ideal pump supply-oil pressure that is constant.  
      In a conventional system, such as the system of  FIG. 5 , the winch is controlled by adjusting the flow rate of the working oil driving the hydraulic motor  16  coupled to the winch with the directional control valve  15  in accordance with the operating position input by the operation lever  1 . The pressure of the working oil supplied from the hydraulic pump  12  is set at a constant value corresponding to a maximum load pressure independently of a quantity of the working oil used by the hydraulic motor  16 , by a combination of a pressure-compensating-function-added servo regulator  13  and a relief valve  14 . Accordingly, the hydraulic motor  16  is operable for any arbitrary load pressure. Even if the load pressure to the hydraulic motor  16  is extremely small in comparison to the pressure of the working oil supplied from the hydraulic pump  12 , the pressure of the working oil supplied from the hydraulic pump  12  is always constant.  
      Referring to the curve  49   b,  the profile of the pump supply-oil pressure curve that is shown is an ideal profile generated by controlling the pump supply-oil pressure throughout all three phases of the winding-up operation by closed loop feedback control of the pressure of the working oil supplied by the hydraulic pump. More specifically, referring to  FIG. 1 , the pressure differential of the working oil generated between the hydraulic motor side of the oil controller  25  which is relieved and the hydraulic pump side of the oil controller  25  is used to control the electromagnetic relief valve  46  which in turn varies the a set pressure of the main relief valve  45 . Accordingly, the pump supply-oil pressure is controlled in accordance with the actual load applied to the winch.  
      Referring now to  FIG. 16   c , the theoretical profiles of the supply oil pressure curves  49   a ,  49   b  of  FIG. 16   b  are reproduced for comparison to actual supply oil pressure profiles for the winding-up operation of  FIG. 16   a  under three different conditions: an uncontrolled supply-oil pressure profile  49   c , an open-loop supply-oil pressure profile  49   d , and a closed-loop supply-oil pressure profile  49   e . The uncontrolled supply-oil pressure profile  49   c  represents the actual supply-oil pressure when the supply-oil pressure is not controlled and corresponds to the condition in which the state of the electromagnetic relief valve  46  is constant. The open-loop supply-oil pressure profile  49   d  represents the actual supply-oil pressure when the supply-oil pressure is controlled without using the supply-oil pressure and corresponds to the condition in which the electromagnetic relief valve  46  is controlled solely by load pressure. In other words, the pilot pressure in line  67  of  FIG. 1  is controlled based on the load pressure as determined from the drive-oil pressure gauges  47 ,  48  without using the supply-oil pressure to calculate the pilot pressure, since the supply-oil pressure is applied to open the pressure relief valve  45  against the pilot pressure. The closed-loop supply-oil pressure profile  49   e  represents the actual supply-oil pressure when the supply-oil pressure is controlled using the supply-oil pressure and corresponds to the oil pressure control circuit  23 B of the present invention as shown in  FIG. 1 . More specifically, by providing the supply oil pressure gauge  49  to detect the actual output pressure of the hydraulic pump  42  and by sending the detected supply oil pressure to the oil pressure control unit  23 B, stable control of the hydraulic pump supply-oil pressure may be achieved by the closed loop feedback control discussed above. In other words, by using the actual output pressure of the hydraulic pump  42  as detected by the supply oil pressure gauge  49 , the oil pressure control circuit  23 B controls the position of the electromagnetic valve  46  which, in turn, controls the set point of the relief valve  45 , thereby achieving stable control of the hydraulic pump oil supply pressure.  
      A comparative analysis of the supply-oil pressure profiles  49   a - 49   e  of  FIG. 16   c  shows that, in theory, keeping the supply-oil pressure constant at all times (profile  49   a ), e.g., at the maximum pressure required to drive the winch at the start of the winding-up operation, is undesirable because the energy loss is considerable as compared the theoretical required pump supply-oil pressure (profile  49   b ). In order to reduce such energy loss, timely control of the pump supply-oil pressure in accordance with the actual load applied to the hydraulic motor  43  is desirable. A comparison of the actual supply-oil pressure when the supply-oil pressure is not controlled (profile  49   c ) to the theoretical required pump supply-oil pressure (profile  49   b ) shows that, in practice, not controlling the supply-oil pressure also is undesirable because the energy loss is considerable as compared the theoretical required pump supply-oil pressure (profile  49   b ). Open-loop control of the supply-oil pressure as shown in profile  49   d  also is undesirable as the control system is unstable. However, when closed-loop control of the supply-oil pressure is employed (profile  49   e ), control of the hydraulic motor  43  and winch is stable and the supply-oil pressure generally approaches the theoretical profile  49   b.    
      Referring to  FIG. 16   d,  the change in drive oil pressure as a function of time for the three phases of the lifting operation of  FIG. 16   a  is shown for the different control scenarios for the supply oil pressure shown in  FIG. 16   c  as discussed above. In theory, when the supply oil pressure is constant (profile  49   a ) the corresponding drive oil pressure (profile  47   a ) also is constant. Also in theory, the ideal drive oil pressure (profile  47   b ) corresponding to the closed loop control of the supply oil pressure (profile  49   b ) varies in accordance with the actual load applied to the winch. The actual uncontrolled drive oil pressure (profile  47   c ) corresponds to the uncontrolled supply-oil pressure (profile  49   c ) and represents the actual drive-oil pressure when the supply-oil pressure is not controlled, in other words, when the state of the electromagnetic relief valve  46  is constant. The open-loop drive oil pressure (profile  47   d ) corresponds to open-loop supply-oil pressure profile  49   d  and represents the actual drive-oil pressure when the electromagnetic relief valve  46  is controlled solely by load pressure. The closed-loop drive oil pressure (profile  47   e ) corresponds to the closed-loop supply-oil pressure (profile  49   e ) when the supply-oil pressure is controlled by providing the supply oil pressure gauge  49  to detect the actual output pressure of the hydraulic pump  42  and by sending the detected supply oil pressure to the oil pressure control unit  23 B.  
      From the above discussion, an artisan of ordinary skilled in the hydraulic control circuit art will understand that for a supply-oil pressure controlled by a control circuit only having a load responsive pressure relief valve, such as the open-loop control discussed above, the supply oil pressure diverges significantly from the desired supply oil pressure. Further, the artisan will understand that closed-loop feedback control of the supply oil pressure will overcome this problem. More specifically, when the error rate calculated from the difference between the actual and desired supply oil pressure is greater than a predetermined value, control of the pilot pressure in line  67  of  FIG. 1  will decrease the error rate as further discussed below.  
      In the first preferred embodiment of the closed-loop control of the present invention, the current that the oil pressure control circuit  23 B feeds to the electromagnetic relief valve  46  via the signal transmission path  36  when the higher one of the hydraulic pressures detected by the pressure gauges  47  and  48  is equal to or higher than a predetermined pressure, is set to be 0 A (zero A). This current setting causes the electromagnetic relief valve  46  to open, that is, the main relief valve  45  is opened due to the opening of the electromagnetic relief valve  46  to establish an unload state, so that the working oil flows from the main relief valve  45  to the tank  41 , and the hydraulic pump  42  receives no load. Accordingly, when the hydraulic drive apparatus is used for the winch, and the hydraulic motor  43  is forcibly driven and rotated by an external load as in the case of winding-down of the winch, the main relief valve  45  may be placed to an unloading state to such an extent that no cavitation occurs in the working oil supplied to the hydraulic motor  43 . As a result, there is eliminated the wasteful consumption of the drive horsepower of the hydraulic pump  42 .  
      Since the required unloading state level is merely to such an extent that no cavitation occurs in the working oil supplied to the hydraulic motor  43 , the current that the oil pressure control circuit  23 B feeds to the electromagnetic relief valve  46  via the signal transmission path  36  may be set to be a predetermined current other than the current 0 A.  
      In the hydraulic drive apparatus of the embodiment, the electro-hydraulic servomechanism carries out such a control that when the electric motor  24  stops, the hydraulic motor  43  also stops. Accordingly, when the electric motor  24  stands still and the hydraulic motor  43  receives an external load to rotate, the port of the directional control valve  25  is opened and closed to drive and rotate the hydraulic motor  43  to resist the load applied. If the check valve  50  is not provided at the position shown in  FIG. 1 , the hydraulic motor  43  will run uncontrollably when the hydraulic motor  43  is placed to an unloading state since in this case, the working oil flows in the reverse direction, viz., the working oil flows from the directional control valve  25  through the oil passage  63  to the hydraulic pump  42 . If the set pressure of the main relief valve  45  is set to be larger than the load pressure in order to prevent the reverse flow, the drive horsepower of the hydraulic pump  42  is wastefully consumed. In this connection, if the check valve  50  is provided at the position shown in  FIG. 1  as in the first embodiment, the main relief valve  45  can be put in an unloaded state without any reverse flow of the working oil, and therefore it is possible to eliminate the wasteful consumption of the drive horsepower of the hydraulic pump  42 .  
      Examples where the hydraulic motor  43  receives an external load while the electric motor  24  stops are: when the hydraulic drive apparatus is applied to the driving machine of a service car, it stops on a sloping road; when the hydraulic drive apparatus is applied to a rotary machine of the service car, the rotating operation stops on a sloping surface; and when the hydraulic drive apparatus is applied to the winch, the winch is stopped.  
      In the hydraulic drive apparatus of the first embodiment, a quantity of the working oil that the hydraulic pump  42  supplies is preferably controlled to be about 5% larger than the necessary quantity of the working oil for the hydraulic motor  43 . Therefore, the flow loss of the hydraulic pump  42  is small.  
      The control of the quantity of the working oil that the hydraulic pump  42  supplies will be described hereunder.  
      A quantity of the working oil necessary for driving the hydraulic motor  43  (this quantity will be referred to merely as a necessary quantity) can be obtained from the rotational speed (rotational number) and stroke volume of the hydraulic motor  43 . The hydraulic motor  43  follows up the electric motor  24  in rotation. Therefore, the rotational speed of the hydraulic motor  43  is equal to the rotational speed of the electric motor  24 . In the embodiment, the stroke volume of the hydraulic motor  43  takes a design value. The rotational speed of the electric motor  24  is determined by an operation position signal output from the operation-position signal outputting means  22 .  
      Accordingly, the supplying oil quantity signal output circuit  23 C computes a necessary quantity of the hydraulic motor  43  by using an operation-position signal output from the operation-position signal outputting means  22  and a known stroke volume of the hydraulic motor  43 ; generates a supplying oil quantity signal so that the quantity of the working oil supplied from the hydraulic pump  42  is 5% larger than the computed necessary quantity of the hydraulic motor  43 ; and transmits the generated signal to the pump regulator  51  through a signal transmission path  37 . Upon receipt of the supplying oil quantity signal, the pump regulator  51  controls the quantity of the working oil supplied by the hydraulic pump  42  by operating a pump capacity (cc/rev) in accordance with the supplying oil quantity signal.  
      In this way, the quantity of the working oil that the hydraulic pump  42  supplies is controlled to be 5% higher than the necessary quantity of the hydraulic motor  43 .  
      In the first embodiment, the quantity of the working oil that the hydraulic pump  42  supplies is controlled to be 5% higher than the necessary quantity of the hydraulic motor  43 . It is evident that the quantity of the working oil that the hydraulic pump  42  supplies may be controlled to be higher than the necessary quantity of the hydraulic motor  43  by a percentage value other than 5%. The quantity of the working oil that the hydraulic pump  42  supplies may be controlled remotely by use of the pump regulator  51  and the supplying oil quantity signal output circuit  23 C.  
      The pump regulator  51  is not limited to the regulator of the type illustrated in  FIG. 1 , but may be of the hydraulic pilot type, the electromagnetic proportional type, the electrically driven type, or the like.  
      In the hydraulic drive apparatus of the first embodiment, the drive signal output circuit  23 A, the oil pressure control circuit  23 B and the supplying oil quantity signal output circuit  23 C are all contained in a housing  23 . Since the housing  23  is used, there is no chance that the hydraulic drive apparatus is made complicated as a whole. Further, the electrical components including the drive signal output circuit  23 A, oil pressure control circuit  23 B and supplying oil quantity signal output circuit  23 C are gathered into the housing. This makes it easy to repair.  
      In the present embodiment, the pressure of the working oil supplied from the hydraulic pump  42  is controlled to be higher than the pressure of the working oil for driving and rotating the hydraulic motor  43  by a maximum pressure of 20 kg/cm 2 . It is evident that the pressure difference is not limited to 20 kg/cm 2  but may be any of other suitable values than it.  
      In the electro-hydraulic servomechanism mentioned above, if the directional control valve  25  is not provided with a throttle, the hydraulic motor  43  follows up the electric motor  24  in rotation in a quick responsive manner, but it is easy to vibrate. In the hydraulic drive apparatus of the embodiment, the directional control valve  25  includes a large throttle. Therefore, the hydraulic motor  43  smoothly follows up the electric motor  24  in rotation, although its responsive time may be long. In the hydraulic-drive apparatus of the embodiment, the throttle of the directional control valve  25  is arranged in the meter-out of the  25 A side and in the meter-in of the  25 B side. Accordingly, the hydraulic drive apparatus of the embodiment is used in a case where the hydraulic motor  43  constantly receives an external load only in the winding-down direction (indicated by an arrow D in  FIG. 1 ), as in the winch.  
     SECOND EMBODIMENT  
       FIG. 2  is a hydraulic pressure circuit diagram showing a hydraulic drive apparatus which is a second embodiment of the present invention.  
      A basic construction of the hydraulic drive apparatus of the second embodiment is substantially the same as of the first-embodiment except that the throttle of the directional control valve  25  is arranged in the meter-in of the  25 A side and in the meter-out of the  25 B side. The hydraulic drive apparatus is used in a case where the hydraulic motor  43  constantly receives an external load only in the direction of an arrow U in  FIG. 2 .  
     THIRD EMBODIMENT  
       FIG. 3  is a hydraulic pressure circuit diagram showing a hydraulic drive apparatus which is a third embodiment of the invention.  
      A basic construction of the hydraulic drive apparatus of the third-embodiment is substantially the same as of each of the first and second embodiments except that the throttle of the directional control valve  25  is arranged in the meter-in and meter-out of the  25 A side and in the meter-in and meter-out of the  25 B side. The hydraulic drive apparatus is used in a case where the hydraulic motor  43  receives external loads in directions of arrows U and D in  FIG. 3 .  
     FOURTH EMBODIMENT  
       FIG. 4  is a hydraulic pressure circuit diagram showing a hydraulic drive apparatus which is a fourth embodiment of the present invention.  
      An arrangement of the hydraulic drive apparatus of the embodiment will be described.  
      The arrangement of the hydraulic drive apparatus of this embodiment is substantially the same as of each of the first to third embodiments except the arrangement to be described hereunder.  
      Reference numeral  143  indicates a hydraulic motor as an additional drive rotary member to be driven and rotated by hydraulic pressure.  
      Reference numeral  120  designates a rotation control means which controls a quantity of the working oil supplied from the hydraulic pump  42  to the hydraulic motor  143 , thereby driving and rotating the hydraulic motor  143  as desired. The additional rotation control means  120  is made up of an operation lever  121 , an additional operation-position signal outputting means  122 , a drive signal output circuit  23 A, an electric motor  124 , and a directional control valve  125 . The operation lever  121  serves as additional operation position inputting means and is able to input a desired operation position. The additional operation-position signal outputting means  122  generates an operation position signal in accordance with an operation position input to the operation lever  121 . The drive signal output circuit  23 A serves as additional drive signal outputting means and computes and converts an operation position signal output from the additional operation-position signal outputting means  122  into a drive signal. The electric motor  24  serves as an additional electric motor and is driven and rotated at a speed and a quantity of rotation, both being defined by the drive signal output from the drive signal output circuit  23 A. The directional control valve  125  serves as additional working oil control means, and controls a quantity of the working oil supplied from the hydraulic pump  42 , and supplies the thus quantity-controlled working oil to the hydraulic motor  143 , thereby driving and rotating the hydraulic motor  143  in accordance with rotation of the electric motor  124 .  
      The drive signal output circuit  23 A also serves as drive signal outputting means as in the hydraulic drive apparatus of each of the first to third embodiments.  
      Reference numerals  147  and  148  represent pressure gauges as additional drive oil pressure detecting means. Each of those pressure gauges detects a pressure of the working oil for driving and rotating the hydraulic motor  143 , and generates and outputs a drive oil pressure signal depending on the pressure thus detected.  
      In the embodiment, an oil pressure control circuit  23 B as oil pressure control means receives a supplying-oil pressure signal output from the pressure gauge  49 , drive oil pressure signals output from the pressure gauges  47  and  48 , and drive oil pressure signals output from the pressure gauges  147  and  148 . The oil pressure control circuit  23 B feeds current to the electromagnetic relief valve  46 , and varies a set pressure of the main relief valve  45  by varying a set pressure of the electromagnetic relief valve  46  whereby it carries out such a control that a pressure of the working oil supplied from the hydraulic pump  42  is set to be higher, by a predetermined pressure, than a pressure of the working oil for driving and rotating the hydraulic motor  43 .  
      The supplying oil quantity signal output circuit  23 C as supplying-oil quantity signal outputting means receives an operation position signal output from the operation-position signal outputting means  22 , and an operation position signal output from the additional operation-position signal outputting means  122 , and computes those signals into a supplying oil quantity signal, and outputs the resultant signal.  
      Reference numerals  131  and  132  stand for signal transmission paths for transmitting signals, and numerals  164  and  165  represent oil passages for transmitting working oil.  
      The pressure sensors  47  to  49 , and  147  and  148  are electrically connected to the oil pressure control circuit  23 B through signal transmission paths not shown.  
      A computer  180  is electrically connected to the drive signal output circuit  23 A, oil pressure control circuit  23 B, and supplying oil quantity signal output circuit  23 C through signal transmission paths not shown.  
      An operation of the hydraulic drive apparatus of this embodiment will be described.  
      The operation of this hydraulic drive apparatus is substantially the same as that of each of the hydraulic drive apparatuses of the first to third embodiments.  
      The additional rotation control means  120  is capable of rotating the hydraulic motor  143  as desired similarly to the rotation control means  20  which drives and rotates the hydraulic motor  43  as desired.  
      The hydraulic drive apparatus of this embodiment controls the quantity of the working oil supplied from the hydraulic pump  42  so that it is larger than the sum of the necessary quantities of the hydraulic motors  43  and  143  by 5%, in the similar manner as the hydraulic drive apparatus of each of the first to third embodiments does. Therefore, the flow loss of the hydraulic pump  42  is small.  
      The hydraulic drive apparatus of this embodiment controls the pressure of the working oil supplied from the hydraulic pump  42  so that it is higher, by a maximum pressure of 20 kg/cm 2 , than a higher one of the pressures of the working oils for driving the hydraulic motors  43  and  143 , in the similar manner as the hydraulic drive apparatus of each of the first to third embodiments does. Therefore, there is eliminated the wasteful consumption of the drive horsepower of the hydraulic pump  42 .  
      In the hydraulic drive apparatus of this embodiment, the computer  180  has various functions. Examples of the functions are: it outputs a signal to the drive signal output circuit  23 A by way of a signal transmission path (not shown) to set rotational speeds of the electric motors  24  and  124 ; it outputs a signal to the oil pressure control circuit  23 B by way of a signal transmission path (not shown) to set a set pressure of the electromagnetic relief valve  46 ; and it outputs a signal to the supplying oil quantity signal output circuit  23 C via a signal transmission path to set a quantity of working oil discharged from the hydraulic pump  42 .  
      In this embodiment, the hydraulic motor  43  is driven and rotated in response to the operation position signal input from the operation lever  21 , and the hydraulic motor  143  is driven and rotated in response to the operation position input from the operation lever  121 . Alternatively, those hydraulic motors  43  and  143  may be driven and rotated in response to an operation position input from one operation lever.  
      While in this embodiment, the hydraulic drive apparatus includes two electro-hydraulic servovalves, it may include three or more number of electro-hydraulic servovalves.  
     5TH EMBODIMENT  
       FIG. 6  is a hydraulic pressure circuit diagram showing a hydraulic drive apparatus  100 , which is a fifth embodiment of the present invention.  
      An arrangement of the hydraulic drive apparatus  100  of the embodiment will be described.  
      In  FIG. 6 , the hydraulic drive apparatus  100  includes a hydraulic pump  102  for supplying working oil from a tank  101  through an oil passage  111  to an oil passage  112 .  
      The hydraulic drive apparatus  100  includes a first hydraulic motor  201  and a second hydraulic motor  301 . The first hydraulic motor  201  receives working oil from the hydraulic pump  102  through one of oil passages  113  and  114  and the oil passage  112 , and is driven and rotated by the supplied working oil. The second hydraulic motor  301  receives working oil from the hydraulic pump  102  through the oil passages  112 ,  118 ,  119  and  120  and an oil passage  122  or  123 , and is driven and rotated by the supplied working oil.  
      The hydraulic drive apparatus  100  further includes a first operation unit  200  which is made up of a first operation lever  210  as first operation position input means which receives an operation position, and generates and outputs an operation position signal indicative of the input operation position, a drive signal output circuit  220 A as drive signal outputting means which receives the operation position signal from the first operation lever  210  by way of a signal transmission path  16 , computes the operation position signal into a drive signal, and outputs the drive signal, an electric motor  230  which receives a drive signal through a signal transmission path  162  from the drive signal output circuit  220 A, and is driven and rotated at a rotational speed defined by the drive signal received, and a first control valve  250  which allows the working oil supplied from the hydraulic pump  102  to the first hydraulic motor  201  to flow therethrough, adjusts a quantity of the working oil flowing therethrough in accordance with the rotational speeds of the electric motor  230  and the first hydraulic motor  201 , and thereby adjusts a quantity of the working oil supplied from the hydraulic pump  102  to the first hydraulic motor  201 , whereby the first hydraulic motor  201  is driven and rotated in accordance with the operation position input to the first operation lever  210 .  
      In the hydraulic drive apparatus, a drive rotation of the first hydraulic motor  201  is reduced in speed by a speed reduction mechanism  202  and thereafter is transmitted to a load  203 . As described above, the first control valve  250  is constructed such that it allows the working oil supplied from the hydraulic pump  102  to the first hydraulic motor  201  to flow therethrough, adjusts a quantity of the working oil flowing therethrough in accordance with the rotational speeds of the electric motor  230  and the first hydraulic motor  201 , and thereby adjusts a quantity of the working oil supplied from the hydraulic pump  102  to the first hydraulic motor  201 . To be more specific, the first control valve  250  is coupled to the rotary shaft of the electric motor  230  and to the rotary shaft of the first hydraulic motor  201  by means of screws. When a rotational speed difference is produced between the electric motor  230  and the first hydraulic motor  201 , the first control valve adjusts a quantity of the working oil flowing therethrough, viz., a quantity of the working oil supplied from the hydraulic pump  102  to the first hydraulic motor  201 , so that the rotational speed of the electric motor  230  is equal to that of the first hydraulic motor  201 . In this state, the first hydraulic motor  201  is driven and rotated. The working oil having rotated the first hydraulic motor  201  passes through the other of the oil passages  113  and  114 . And it passes through the first control valve  250  again and further through an oil passage  115 , and returns to the tank  101 .  
      The first control valve  250  constitutes an electro-hydraulic servovalve.  
      The hydraulic drive apparatus  100  includes a second operation unit  300 , which is made up of a second operation lever  310  as second operation position inputting means to which an operation position is input, and a second control valve  350  which allows the working oil supplied from the hydraulic pump  102  to the second hydraulic motor  301  to flow therethrough, adjusts a quantity of the working oil flowing therethrough in accordance with an operation position input to the second operation lever  310 , whereby the second hydraulic motor  301  is driven and rotated in accordance with an operation position input to the second operation lever  310 .  
      The oil passages  120  and  121  intercommunicate with each other through a check valve  170 . A drive rotation of the second hydraulic motor  301  is reduced in speed by a speed reduction mechanism  302 , and transmitted to a load  303 . The working oil having rotated the second hydraulic motor  301  passes through the other of the oil passages  122  and  123 , and again through the second control valve  350 . Further, it passes through an oil passage  124  and returns to the tank  101 . Of the working oil flowing through the oil passage  119 , working oil other than the working oil supplied to the second hydraulic motor  301  is supplied again to the tank  101  via the second control valve  350  and an oil passage  125 .  
      The second control valve  350  constitutes a general purpose valve.  
      The hydraulic drive apparatus  100  includes a pressure sensor  131  as supplying oil detect means which detects a pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250 , and generates a supplying-oil pressure signal dependent on the pressure, and pressure sensors  132  and  133  as drive oil pressure detecting means which detects a pressure of the working oil for driving and rotates the first hydraulic motor  201  and generates and outputs a drive oil pressure signal that is dependent on the pressure. The pressure sensors  131 ,  132  and  133  are arranged so as to detect pressures of the working oil flowing through the oil passages  112 ,  113  and  114 .  
      The hydraulic drive apparatus  100  also includes a pressure control valve  140  as first working oil adjusting means, a pressure control valve  150  as second working oil pressure adjusting means and a pressure adjust signal output circuit  220 B as pressure adjust signal outputting means. The pressure control valve  140  receives working oil from the hydraulic pump  102  via the oil passages  112  and  116 , allows the supplied working oil to flow therethrough, and adjusts a pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  to be below a set pressure. The pressure control valve  150  allows the working oil supplied from the hydraulic pump  102  to the second control valve  350  to flow therethrough, and adjusts a pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  to be below a set pressure. The pressure adjust signal output circuit  220 B receives an operation position signal from the first operation lever  210  via the signal transmission path  161 , a supplying-oil pressure signal from the pressure sensor  131  via a signal transmission path, and a drive oil pressure signal from each of the pressure sensors  132  and  133  via signal transmission paths (not shown). The pressure adjust signal output circuit  220 B judges, by the input operation position signal, whether the first hydraulic motor  201  drives and rotates the first hydraulic motor  201  or stops the rotation of the first hydraulic motor  201 . When the first operation unit  200  drives and rotates the first hydraulic motor  201 , the pressure adjust signal output circuit computes a pressure of the working oil necessary for driving and rotating the first hydraulic motor  201  by use of the input supplying-oil pressure signal and the drive oil pressure signal. The pressure adjust signal output circuit generates a pressure adjust signal which causes the pressure control valve  150  to adjust a set pressure of the pressure control valve  150  so that the set pressure is higher, by a predetermined pressure, than a pressure of the working oil necessary for driving and rotating the first hydraulic motor  201 . In the embodiment, the predetermined pressure is 20 Kg/cm 2 . When the first operation unit  200  stops the rotation of the first hydraulic motor  201 , the pressure adjust signal output circuit generates a pressure adjust signal which causes the pressure control valve  150  to adjust a set pressure thereof to such a pressure (0 Kg/cm 2  in the embodiment) at which the pressure control valve  150  allows the working oil to freely flow. Then, the pressure adjust signal output circuit outputs the generated pressure adjust signal through a signal transmission path  163  to the pressure control valve  150 .  
      The working oil having passed through the pressure control valve  140  is returned to the tank  101 , through an oil passage  117 .  
      The pressure control valve  150  includes a main relief valve  151  and an electromagnetic relief valve  152 . The main relief valve  151  adjusts a pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  to a set pressure or smaller, by flowing the working oil supplied from the hydraulic pump  102 . The electromagnetic relief valve  152  adjusts a set pressure of the main relief valve  151  by adjusting a set pressure thereof in accordance with a pressure adjust signal as input thereto through the signal transmission path  163 .  
      As shown in  FIG. 6 , the first control valve  250 , pressure control valve  140  and pressure control valve  150  are arranged so that the pressures of the working oil supplied thereto are equal to one another.  
      The hydraulic drive apparatus  100  includes a pump regulator  180 , a pressure sensor  134 , and a supplying-oil adjust signal output circuit  220 C. The pump regulator  180  as supplying oil quantity adjusting means adjusts a quantity of the working oil that the hydraulic pump  102  supplies. The pressure sensor  134  as operation position detecting means detects an operation position input to the second operation lever  310 , and generates and outputs an operation position detect signal defined by the operation position. The supplying-oil adjust signal output circuit  220 C as supplying oil quantity adjusting signal outputting means receives an operation position signal from the first operation lever  210  by way of the signal transmission path  161 , and an operation position detect signal from the pressure sensor  134  by way of a signal transmission path (not shown) The supplying-oil adjust signal output circuit judges, by the input operation position detect signal, whether the second operation unit  300  drives and rotates the second hydraulic motor  301  or stops the rotation of the second hydraulic motor  301 . When the second operation unit  300  drives and rotates the second hydraulic motor  301 , the supplying-oil adjust signal output circuit generates a supplying-oil adjust signal which causes the pump regulator  180  to adjust a quantity of the working oil that the hydraulic pump  102  supplies to a predetermined quantity, e.g., a maximum oil quantify of the hydraulic pump  102  in the embodiment. When the second operation unit  300  stops the rotation of the second hydraulic motor  301 , the supplying-oil adjust signal output circuit computes a quantity of the working oil necessary for driving and rotating the first hydraulic motor  201  in accordance with an operation position signal input thereto through the signal transmission path  161 . The supplying-oil adjust signal output circuit generates a supplying-oil adjust signal which causes the pump regulator  180  to adjust a quantity of the working oil that the hydraulic pump  102  supplies to be larger than that of the working oil necessary for driving and rotating the first hydraulic motor  201  by a predetermined quantity (5% in the embodiment). Then, the supplying-oil adjust signal output circuit outputs the generated oil quantity adjust signal through the signal transmission path  164  to the pump regulator  180 .  
      The drive signal output circuit  220 A, pressure adjust signal output circuit  220 B or supplying-oil adjust signal output circuit  220 C are accommodated in a control unit  220 .  
      As described above, the second control valve  350  is arranged so as to adjust a quantity of the working oil flowing therethrough in accordance with an operation position input to the second operation lever  310 . More specifically, the second operation lever  310  is arranged to supply a quantity of the working oil, which is dependent on an operation position as input thereto. The second control valve  350  receives operation oil from the second operation lever  310  via the operation oil passage  311  or  312 , and adjusts a quantity of the working oil flowing therethrough in accordance with the quantity of the operation oil supplied. As described above, the pressure sensor  134  detects an operation position input to the second operation lever  310 . To be more specific, the pressure sensor  134  detects a pressure of the operation oil supplied from the second operation lever  310  to the second control valve  350  by detecting an oil pressure which is the higher of the pressures of the operation oils supplied to the operation oil passages  311  and  312 . The pressure sensor  134  further detects an operation position input to the second operation lever  310 , which is proportional to a pressure of the operation oil supplied from second operation lever  310  to the second control valve  350  by detecting a pressure of the operation oil supplied from the second operation lever  310  to the second control valve  350 .  
      An operation of the hydraulic drive apparatus. 100  thus constructed will be described.  
      A case (referred to as a first case) where the first operation unit  200  drives and rotates the first hydraulic motor  201  and the second operation unit  300  stops the rotation of the second hydraulic motor  301  will first be described.  
      The hydraulic pump  102  feeds working oil from the tank  101  to the oil passage  112  via the oil passage  111 .  
      In the first case, the first operation unit  200  drives and rotates the first hydraulic motor  201  in accordance with an operation position input to the first operation lever  210 . Description to follow is an operation of the hydraulic pump when the first operation unit  200  drives and rotates the first hydraulic motor  201  in accordance with an operation position input to the first operation lever  210 .  
      An operation position is input to the first operation lever  210 . Upon receipt of the operation position, the first operation lever  210  generates an operation position signal that is dependent on the input operation position. The generated operation position signal is output from the first operation lever  210  to the signal transmission path  161 , and input to the drive signal output circuit  220 A via the signal transmission path  161 .  
      The drive signal output circuit  220 A computes the operation position signal as input thereto into a drive signal. In turn, the drive signal output circuit  220 A outputs the drive signal to the signal transmission path  162 . The electric motor  230  is driven and rotated at a rotational speed defined by the drive signal received. When a rotational speed difference is produced between the electric motor  230  and the first hydraulic motor  201 , the first control valve  250 , as described above, adjusts a quantity of the working oil flowing therethrough, viz., a quantity of the working oil supplied from the hydraulic pump  102  to the first hydraulic motor  201  via one of the oil passages  113  and  114  so that the rotational speeds of the electric motor  230  and the first hydraulic motor  201  are equal to each other, and in this state drives and rotates the first hydraulic motor  201 . Therefore, the first hydraulic-motor  201  rotates a rotational speed substantially equal to that of the electric motor  230 . When the first hydraulic motor  201  is driven and rotated, the first hydraulic motor  201  is reduced in speed by the speed reduction mechanism  202 , and then the rotation reduced in speed is transmitted to the load  203 .  
      In this way, the first operation unit  200  drives and rotates the first hydraulic motor  201  in accordance with the operation position input to the first operation lever  210 .  
      The working oil for driving and rotating the first hydraulic motor  201  passes through the other of the oil passages  113  and passage  114 , and then through the first control valve  250 , and returned to the tank  101  via the oil passage  115 .  
      In the first case, the second operation unit  300  stops the rotation of the second hydraulic motor  301 . Therefore, the second control valve  350  is at a neutral position. After passing the pressure control valve  150 , the working oil goes again to the tank  101  by way of the oil passages  119 , second control valve  350  and oil passage  125 .  
      In the first case, a quantity of the working oil that the hydraulic pump  102  supplies is set to be 5% larger than that of the working oil for driving and rotating the first hydraulic motor  201 . An operation of the hydraulic drive apparatus to set a quantity of the working oil supplied by the hydraulic pump  102  to be 5% larger than a quantity of the working oil for driving and rotating the first hydraulic motor  201 , will be described.  
      The operation position signal is input to the supplying-oil adjust signal output circuit  220 C via the signal transmission path  161 .  
      A supplying-oil pressure signal output from the pressure sensor  131  and an operation position detect signal output from the pressure sensor  134  are input to the supplying-oil adjust signal output circuit  220 C via a signal transmission path, not shown.  
      The supplying-oil adjust signal output circuit  220 C judges that the second operation unit  300  stops the rotation of the second hydraulic motor  301 , from the operation position detect signal as input, and computes a quantity of the working oil necessary for driving and rotating the first hydraulic motor  201  in accordance with the input operation position signal. Specifically, a capacity of the first hydraulic motor  201  is already known. As already described, the rotational speed of the first hydraulic motor  201  is determined by the operation position signal output from the first operation lever  210 . Therefore, the supplying-oil adjust signal output circuit  220 C multiplies a known capacity of the first hydraulic motor  201  by a rotational speed of the first hydraulic motor  201  that is obtained from the operation-position signal, thereby producing a quantity of the working oil necessary for driving and rotating the first hydraulic motor  201 . Then, the supplying-oil adjust signal output circuit  220 C generates a supplying-oil adjust signal which causes the pump regulator  180  to adjust a quantity of the working oil that the hydraulic pump  102  supplies so that a quantity of the working oil that the hydraulic pump  102  supplies is 5% larger than of the working oil necessary for driving and rotating the first hydraulic motor  201 .  
      The supplying-oil adjust signal that is generated by the supplying-oil adjust signal output circuit  220 C is output to a signal transmission path  164 , from the supplying-oil adjust signal output circuit  220 C, and is input to the pump regulator  180  by way of the signal transmission path  164 . The pump regulator  180  having received the supplying-oil adjust signal adjusts a quantity of the working oil that the hydraulic pump  102  supplies so that a quantity of the working oil that the hydraulic pump  102  is 5% larger than the working oil necessary for driving and rotating the first hydraulic motor  201 .  
      In this way, a quantity of the working oil that the hydraulic pump  102  supplies is 5% larger than that of the working oil necessary for driving and rotating the first hydraulic motor  201 .  
      In the first case, the supplying-oil pressure signal that is input to the supplying-oil adjust signal output circuit  220 C is no used by the supplying-oil adjust signal output circuit  220 C in order to generate a supplying-oil adjust signal.  
      In the first case, a pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  is set to be higher than that of the working oil necessary for driving and rotating the first hydraulic motor  201  by 20 kg/cm 2  or lower. Description will be given hereunder about an operation of the hydraulic drive apparatus to set the pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  to be higher than that of the working oil necessary for driving and rotating the first hydraulic motor  201  by 20 kg/cm 2  or lower.  
      An operation position signal is input from the first operation lever  210  to the pressure adjust signal output circuit  220 B, from the signal transmission path  161 .  
      The pressure sensor  131  detects a pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250 , and generates a supplying-oil pressure signal defined by the detect pressure. The generated supplying-oil pressure signal is output from the pressure sensor  131 to the signal transmission path, and through the signal transmission path to pressure adjust signal output circuit  220 B.  
      Each of the pressure sensors  132  and  133  detects a pressure of the wording oil for driving and rotating the first hydraulic motor  201 , and generates a drive oil pressure signal defined by the detected pressure. The generated drive oil pressure signal is output to the signal transmission path (not shown) from each of those sensors  132  and  133 , and passes through the signal transmission path to the pressure adjust signal output circuit  220 B.  
      The pressure adjust signal output circuit  220 B judges, by the input operation position signal, that the first operation unit  200  drives and rotates the first hydraulic motor  201 , and computes a pressure of the working oil necessary for driving and rotating the first hydraulic motor  201  by use of the input drive oil pressure signal. In the embodiment, the pressure adjust signal output circuit  220 B receives the drive oil pressure signals from the pressure sensors  132  and  133 . The pressure adjust signal output circuit  220 B judges that the pressure, which is the larger of the pressures obtained from the two drive oil pressure signals, is the pressure of the working oil for driving and rotating the first hydraulic motor  201 . The pressure adjust signal output circuit  220 B obtains a pressure of the working oil, which is supplied from the hydraulic pump  102  to the first control valve  250 , from the supplying-oil pressure signal. The pressure adjust signal output circuit  220 B generates a pressure adjust signal for adjusting a set pressure of the pressure control valve  150  so that a pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  is higher than a pressure of the working oil necessary for driving and rotating the first hydraulic motor  201  by 20 kg/cm 2  or lower.  
      The pressure adjust signal is generated by the pressure adjust signal output circuit  220 B and output to the signal transmission path  163 , and to the pressure control valve  150  via the signal transmission path  163 . That is, the pressure adjust signal is input to the electromagnetic relief valve  152 . The electromagnetic relief valve  152  having received the pressure adjust signal adjusts the set pressure in accordance with the input pressure adjust signal, so that it adjusts the set pressure of the main relief valve  151  to be higher than the pressure of the working oil necessary for driving and rotating the first hydraulic motor  201  by a 20 kg/cm 2 . The main relief valve  151  adjusts a pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  to be lower than the set pressure, i.e., a pressure being higher than the pressure of the working oil necessary for driving and rotating the first hydraulic motor  201  by 20 kg/cm 2  or lower.  
      In this way, the pressure of the working oil that is supplied from the hydraulic pump  102  to the first control valve  250  is set to be higher than the pressure of the working oil necessary for driving and rotating the first hydraulic motor  201  by 20 kg/cm 2  or lower. The set pressure of the pressure control valve  140  is set to be sufficiently high. In the first case, the set pressure of the pressure control valve  150  never exceeds the set pressure of the pressure control valve  140 . The working oil never flows through the pressure control valve  140 .  
      As described above, in the first case, a quantity of the working oil that the hydraulic pump  102  supplies is set to be 5% larger than the quantity of the working oil necessary for driving and rotating the first hydraulic motor  201 . A pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  is set to be higher than a pressure of the working oil necessary for driving and rotating the first hydraulic motor  201  by 20 kg/cm 2  or lower.  
      A case (referred to as a second case) where the first operation unit  200  stops the rotation of the first hydraulic motor  201  and the second operation unit  300  drives and rotates the second hydraulic motor  301 , will be described hereunder.  
      The hydraulic pump  102  supplies the working oil from the tank  101  to the oil passage  112  via the oil passage  111 .  
      In the second case, an operation position for stopping the rotation of the first hydraulic motor  201  is input to the first operation lever  210 . When the operation position for stopping the rotation of the first hydraulic motor  201  is input to the first operation lever  210 , the drive signal output circuit  220 A, as in the first case, outputs a drive signal to the electric motor  230  to stop its rotation. When the electric motor  230  stops its rotation, the first hydraulic motor  201 , as described above, rotates at a rotational speed substantially equal to that of the electric motor  230 . Therefore, the first hydraulic motor  201  also stops its rotation.  
      In the second case, the second operation unit  300  drives and rotates the second hydraulic motor  301  in accordance with the operation position input to the second operation lever  310 . Description will be given about an operation of the hydraulic drive apparatus when the second operation unit  300  drives and rotates the second hydraulic motor  301  in accordance with an operation position input to the second operation lever  310 .  
      An operation position is input to the second operation lever  310 . When the operation position is input to the second operation lever  310 , working oil flows through the operation oil passage  311  or  312  in accordance with the operation position input to the second operation lever  310 . Then, the second control valve  350  adjusts a quantity of the working oil that comes in through the oil passages  121 , in accordance with the operation oil flowing through the operation oil passage  311  or  312 . The working oil having passed through the second control valve  350  passes through one of the oil passages  122  and  123 , and drives and rotates the second hydraulic motor  301 . When the second hydraulic motor  301  is driven to rotate, a rotation of the second hydraulic motor  301  is reduced in speed and transmitted to the load  303 .  
      In this way, the second operation unit  300  drives and rotates the second hydraulic motor  301  in accordance with the operation position input to the second operation lever  310 .  
      After driving and rotating the second hydraulic motor  301 , the working oil passes the other of the oil passages  122  and  123  and through the second control valve  350 , and returns to the tank  101  by way of the oil passage  124 .  
      In the second case, a pressure of the working oil supplied from the hydraulic pump  102  to the second control valve  350  is set to be below a set pressure of the pressure control valve  140 . An operation of the hydraulic drive apparatus to set a pressure of the working oil supplied from the hydraulic pump  102  to the second control valve  350  below a set pressure of the pressure control valve  140  will be described.  
      As described in the first case, the operation position signal, supplying-oil pressure signal drive oil pressure signal are input to the pressure adjust signal output circuit  220 B.  
      The pressure adjust signal output circuit  220 B judges from the input operation position signal that the first operation unit  200  stops the rotation of the first hydraulic motor  201 . The pressure adjust signal output circuit  220 B generates a pressure adjust signal which sets a set pressure of the pressure control valve  150  at 0 kg/cm 2 , and inputs the generated pressure adjust signal to the pressure control valve  150  by way of the signal transmission path  163 .  
      Accordingly, a pressure of the working oil supplied from hydraulic pump  102  to the first control valve  250 , viz., a pressure of the working oil supplied from the hydraulic pump  102  to the second control valve  350 , is a set pressure of the pressure control valve  140  or lower.  
      In this way, a pressure of the working oil supplied from the hydraulic pump  102  to the second control valve  350  is a set pressure of the pressure control valve  140  or lower.  
      In the second case, the supplying-oil pressure signal and the drive oil pressure signal that are input to the pressure adjust signal output circuit  220 B are not used by the pressure adjust signal output circuit  220 B for generating the pressure adjust signal.  
      The-working oil having flowed through the pressure control valve  140  is supplied to the tank  101  via the oil passage  117 . In the second case, a quantity of the working oil that the hydraulic pump  102  supplies is set to be a maximum oil quantity at which the hydraulic pump  102  is capable of supplying the working oil. An operation of the hydraulic drive apparatus to set a quantity of the working oil that the hydraulic pump  102  supplies to be a maximum oil quantity at which the hydraulic pump  102  is capable of supplying the working oil, will be described.  
      As described in the first case, the operation position signal is input through the signal transmission path  161  to the supplying-oil adjust signal output circuit  220 C, and the supplying-oil pressure signal and the operation position detect signal are input to the same by way of a signal transmission path, not shown.  
      The supplying-oil adjust signal output circuit  220 C judges, by the input operation position detect signal, that the second operation unit  300  drives and rotates the second hydraulic motor  301 , and computes the maximum oil quantity at which the hydraulic pump  102  is capable of supplying the working oil in accordance with the input supplying-oil pressure signal. The supplying-oil adjust signal output circuit  220 C computes the maximum oil quantity at which the hydraulic pump  102  is capable of supplying the working oil, by use of a pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250 , the pressure being presented by the supplying-oil pressure signal, and the limited horsepower of the engine for driving the hydraulic pump  102 . Further, the supplying-oil adjust signal output circuit  220 C generates a supplying-oil adjust signal, which causes the pump regulator  180  to adjust a quantity of the working oil that the hydraulic pump  102  supplies so that a quantity of the working oil that the hydraulic pump  102  supplies is equal to the maximum oil quantity at which the hydraulic pump  102  is capable of supplying the working oil.  
      The supplying-oil adjust signal generated by the supplying-oil adjust signal output circuit  220 C is output from the supplying-oil adjust signal output circuit  220 C to the signal transmission path  164 , and input to the pump regulator  180  via the signal transmission path  164 . The pump regulator  180 , which receives the supplying-oil adjust signal, adjusts a quantity of the working oil that the hydraulic pump  102  supplies so that a quantity of the working oil that the hydraulic pump  102  supplies is equal to the maximum oil quantity at which the hydraulic pump  102  is capable of supplying the working oil.  
      In this way, the quantity of the working oil that the hydraulic pump  102  supplies is set to be the maximum oil quantity at which the hydraulic pump  102  is capable of supplying the working oil.  
      In the second case, the operation position signal input to the supplying-oil adjust signal output circuit  220 C is not used by the supplying-oil adjust signal output circuit  220 C to generate the supplying-oil adjust signal.  
      As described above, in the second case, the pressure of the working oil supplied from the hydraulic pump  102  to the second control valve  350  is below the set pressure of the pressure control valve  140 . The quantity of the working oil that the hydraulic pump  102  is set at the maximum oil quantity at which the hydraulic pump  102  can supply the working oil.  
      A case (referred to as a third case) where the first operation unit  200  drives and rotates the first hydraulic motor  201 , and the second operation unit  300  drives and rotates the second hydraulic motor  301 , will be described.  
      The hydraulic pump  102  supplies the working oil from the tank  101  through the oil passage  111  to the oil passage  112 .  
      In the third case, as in the first case, the first operation unit  200  drives and rotates the first hydraulic motor  201  in accordance with the operation position input to the first operation lever  210 .  
      In the third case, as in the first case, the second operation unit  300  drives and rotates the second operation lever. 310  in accordance with the operation position input to the second operation lever  310 .  
      In the third case, when a pressure of the working oil in the oil passages  119  is lower than the set pressure of the pressure control valve  150 , a pressure of the working oil supplied through the oil passage  112  to the first control valve  250  is higher than a pressure of the working oil necessary for driving and rotating the first hydraulic motor  201  by 20 kg/cm 2  or lower. When a pressure of the working oil in the oil passages  119  is higher than the set pressure of the pressure control valve  150 , a pressure of the working oil supplied through the oil passage  112  to the first control valve  250  is substantially equal to a pressure of the working oil supplied through the oil passages  121  to the second control valve  350 , and equal to a set pressure of the pressure control valve  140  or lower. This will be described in detail.  
      In the third case, the first operation unit  200  drives and rotates the first hydraulic motor  201 . Therefore, as in the first case, a set pressure of the pressure control valve  150  is set to be higher than a pressure of the working oil necessary for driving and rotating the first hydraulic motor  201  by 20 kg/cm 2 , by the pressure adjust signal output circuit  220 B. Specifically, when a pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  is higher than a pressure of the working oil necessary for driving and rotating the first hydraulic motor  201  by 20 kg/cm 2 , the pressure adjust signal output circuit  220 B causes the pressure control valve  150  to open the passage, and hence flows the working oil from the oil passage  118  to the oil passage  119  and lowers the pressure of the working oil.  
      Accordingly, when a pressure of the working oil in the oil passages  119  is lower than a set pressure of the pressure control valve  150 , a pressure of the working oil supplied through the oil passage  112  to the first control valve  250  is lower than the set pressure of the pressure control valve  150 , viz., lower than the pressure of the working oil necessary for driving and rotating the first hydraulic motor  201  by 20 kg/cm 2  or lower. When the pressure of the working oil in the oil passages  119  is higher than the set pressure of the pressure control valve  150 , the pressure of the working oil in the oil passage  118  is substantially equal to the pressure of the working oil in the oil passages  119 . Therefore, a pressure of the working oil supplied through the oil passage  112  to the first control valve  250  is substantially equal to the pressure of the working oil, and equal to the set pressure of the pressure control valve  140  or lower.  
      In the third case, the second operation unit  300  drives and rotates the second hydraulic motor  301 . Accordingly, as in the second case, a quantity of the working oil is set at the maximum oil quantity at which the hydraulic pump  102  can supply the working oil, by the supplying-oil adjust signal output circuit  220 C.  
      As described above, in the third case, when the pressure of the working oil in the oil passages  119  is lower than the set pressure of the pressure control valve  150 , the pressure of the working oil supplied through the oil passage  112  to the first control valve  250  is higher than the pressure of the working oil necessary for driving and rotating the first hydraulic motor  201  by 20 kg/cm 2  or lower. When the pressure of the working oil in the oil passages  119  is higher than the set pressure of the pressure control valve  150 , the pressure of the working oil supplied through the oil passage  112  to the first control valve  250  is substantially equal to the pressure of the working oil supplied through the oil passages  121  to the second control valve  350 , and equal to the set pressure of the pressure control valve  140  or lower. A quantity of the working oil that the hydraulic pump  102  supplies is set at the maximum oil quantity at which the hydraulic pump  102  can supply the working oil.  
      In the embodiment, in the first and third cases, the set pressure of the pressure control valve  150  is higher than the pressure of the working oil necessary for driving and rotating the first hydraulic motor  201  by 20 kg/cm 2 . In the embodiment, the predetermined pressure may be selected within a range of any other suitable values than 20 kg/cm 2  if the range does not give rise to the wasting of the drive horsepower of the hydraulic pump  102 . In the second case, the set pressure of the pressure control valve  150  is set at 0 kg/cm 2 , but it may be selected within a range of other pressure values than 0 kg/cm 2  if the range allows the working oil in the oil passages  118  to substantially and freely flow to the oil passages  119 .  
      In the embodiment, in the first case, a quantity of the working oil that the hydraulic pump  102  supplies is 5% larger than of the working oil necessary for driving and rotating the first hydraulic motor  201 . The predetermined quantity may be selected within a range of other values of percentage than 5% if the range ensures substantially low flow loss. In the second and third cases, the quantity of the working oil supplied to the hydraulic pump  102  is set at the maximum oil quantity at which the hydraulic pump  102  can supply the working oil. However, it is not always exactly equal to the maximum oil quantity, but it may be near the maximum oil quantity and any value of quantity if it depends on a speed at which the second hydraulic motor  301  is driven and rotated.  
      In the embodiment, the second control valve  350  of the general purpose valve moves its position in response to the operation oil. The second control valve  350 , which is constructed such that its position is moved by the operation oil, may take any other construction if it allows its position to be moved in response to the operation position input to the second operation lever  310 .  
      While taking the form of the pressure sensor  134  in the embodiment, the operation position detecting means may take any other means if it is able to detect an operation position input to the second operation lever  310 .  
     SIXTH EMBODIMENT  
       FIG. 7  is a hydraulic pressure circuit diagram showing a hydraulic drive apparatus  100 , which is a sixth embodiment of the present invention.  
      In the fifth embodiment, as recalled, the pressure control valve  150  of the hydraulic drive apparatus  100  is made up of the main relief valve  151  and the electromagnetic relief valve  152 . In the hydraulic drive apparatus  100  of the sixth embodiment, the pressure control valve  150  consists of only an electromagnetic relief valve  153 . The remaining construction of the sixth embodiment is substantially the same as of the hydraulic drive apparatus  100  of the fifth embodiment.  
      Accordingly, an operation of the hydraulic drive apparatus  100  of the present embodiment is similar to that of the hydraulic drive apparatus  100  of the fifth embodiment.  
     SEVENTH EMBODIMENT  
       FIG. 8  is a hydraulic pressure circuit diagram showing a hydraulic drive apparatus  100  which is a seventh embodiment of the present invention.  
      An arrangement of the hydraulic drive apparatus  100  of the present embodiment will first be described.  
      The arrangement of the hydraulic drive apparatus  100  of the present embodiment is substantially the same as of the hydraulic drive apparatus  100  of the fifth embodiment except its arrangement to be described hereunder.  
      The hydraulic drive apparatus  100  of the embodiment uniquely includes an additional hydraulic motor  401  between the oil passage  125  and the tank  101 . The additional hydraulic motor  401  is driven and rotated by working oil supplied thereto from the hydraulic pump  102  through a route of the oil passages  112 ,  118  and  119 , the second control valve  350 , and the oil passages  125 ,  181  and  182 , and an oil passage  183  or  184 .  
      The hydraulic drive apparatus  100  includes an additional operation unit  400  which is made up of an additional operation lever  410  as additional operation position inputting means to which an operation position is input, and an additional control valve  450  which allows the working oil supplied from the hydraulic pump  102  to-the additional hydraulic motor  401  to flow therethrough, adjusts a quantity of the working oil flowing therethrough in accordance with an operation position input by the additional operation lever  410 , and adjusts a quantity of the working oil supplied from the hydraulic pump  102  to the additional hydraulic motor  401 , whereby the additional hydraulic motor  401  is driven in accordance with the operation position input to the additional operation lever  410 .  
      The oil passages  181  and  182  intercommunicate with each other through a check valve  171 . A rotation of the additional hydraulic motor  401  is reduced in speed by a speed reduction means  402  and transmitted to a load  403 . The working oil have driven and rotated the additional hydraulic motor  401  passes through the additional hydraulic motor  401 , passes again through the oil passage  183  or  184 , and returns to the tank  101  via the oil passage  185 . Of the working oil flowing through the oil passage  125 , the working oil other than that supplied to the additional hydraulic motor  401  flows through the additional control valve  450  and an oil passage  186 , and returns to the tank  101 .  
      The additional control valve  450  is a general purpose valve.  
      The additional control valve  450  is arranged such that a quantity of the working oil flowing therethrough is adjusted in accordance with the operation position input to the additional operation-lever  410 . To be more specific, the additional operation lever  410  supplies a quantity of the working oil, which depends on the input operation position. The additional control valve  450  receives the operation position from the additional operation lever  410  through operation oil passage  411  or  412 , and adjusts the working oil flowing therethrough in accordance with a quantity of the operation oil supplied.  
      A pressure sensor  135  as operation position detecting means detects an operation position input to the additional operation lever  410 . Specifically, the pressure sensor  135  detects a pressure of the operation oil, which is the higher of the operation oils flowing through operation oil passages  411  and  412 , and detects a pressure of the operation oil supplied from the additional operation lever  410  to the additional control valve  450  by the former pressure detection.  
      As recalled, in the fifth embodiment, the supplying-oil adjust signal output circuit  220 C judges by the operation position detect signal input thereto, that the second operation unit  300  stops the rotation of the second hydraulic motor  301 . In this connection, the present embodiment, the supplying-oil adjust signal output circuit  220 C judges, by each of the operation position detect signal input thereto, that the second operation unit  300  and the additional operation unit  400  stop the second hydraulic-motor  301  and the additional hydraulic motor  401 , respectively. Also in the fifth embodiment, the supplying-oil adjust signal output circuit  220 C judges, by the operation position detect signal input thereto, that the second operation unit  300  stops the rotation of the second hydraulic motor  301 .  
      In this connection, in the embodiment, the supplying-oil adjust signal output circuit  220 C judges, by each position detect signal input thereto, that either of the second operation unit  300  and the additional operation unit  400  drives and rotates either of the second hydraulic motor  301  and the additional hydraulic motor  401 .  
      Accordingly, the operation of the hydraulic drive apparatus  100  of the present embodiment is substantially the same as of the hydraulic drive apparatus  100  of the fifth embodiment except that the additional hydraulic motor  401  may be operated.  
      In the hydraulic drive apparatus  100  of the present embodiment, it is impossible to simultaneously operate the second hydraulic motor  301  and the additional hydraulic motor  401 .  
     EIGHTH EMBODIMENT  
       FIG. 9  is a hydraulic pressure circuit diagram showing a hydraulic drive apparatus  100  which is an eighth-embodiment of the present invention.  
      An arrangement of the hydraulic drive apparatus  100  of this embodiment will be described.  
      The arrangement of the hydraulic drive apparatus  100  of this embodiment is substantially the same as of the hydraulic drive apparatus  100  of the seventh embodiment except that while the oil passage  181  in the hydraulic drive apparatus  100  of the seventh embodiment communicates with the oil passage  125 , the oil passage  181  in the hydraulic drive apparatus  100  of this embodiment communicates with the oil passages  119 .  
      An operation of the hydraulic drive apparatus  100  of the embodiment will be described.  
      The operation of the hydraulic drive apparatus  100  of this embodiment is substantially the same as of the hydraulic drive apparatus  100  of the seventh embodiment except that the second hydraulic motor  301  and the additional hydraulic motor  401  may simultaneously be operated.  
     NINTH EMBODIMENT  
       FIG. 10  is a hydraulic pressure circuit diagram showing a hydraulic drive apparatus  100  which constitutes a ninth embodiment of the invention.  
      An arrangement of the hydraulic drive apparatus  100  of this embodiment will be described.  
      The arrangement of the hydraulic drive apparatus  100  of this embodiment is substantially the same as of the hydraulic drive apparatus  100  of the fifth embodiment except, its arrangement to be described hereunder.  
      In the fifth embodiment, the oil passage  118  and the oil passage  119  communicate with each other through the pressure control valve  150 . In this embodiment, the oil passage  118  and the oil passage  119  allow the working oil, which is supplied from the hydraulic pump  102  to the second control valve  350 , to flow therethrough, and communicate with each other through a flow control valve  190  as working oil quantity adjust means, which adjusts a quantity of the working oil flowing therethrough.  
      The hydraulic drive apparatus  100  of the fifth embodiment includes the pressure adjust signal output circuit  220 B and the supplying-oil adjust signal output circuit  220 C. The hydraulic drive apparatus  100  of this embodiment includes an oil quantity adjust signal output circuit  220 D as oil quantity adjust signal outputting means. The oil quantity adjust signal output circuit  220 D receives an operation position signal from the first operation lever  210 , and generates a working oil quantity adjust signal for causing the flow control valve  190  to adjust a quantity of the working oil flowing therethrough in accordance with the operation position signal, and further generates a supplying oil quantity adjust signal for causing the pump regulator  180  to adjust a quantity of the working oil that the hydraulic pump  102  supplies. The oil quantity adjust signal output circuit  220 D outputs the generated working oil quantity adjust signal to the flow control valve  190 , and the supplying oil adjust signal to the pump regulator  180 . In the hydraulic drive apparatus  100  of the embodiment, the drive signal output circuit  220 A and the oil quantity adjust signal output circuit  220 D are contained in the control unit  220 .  
      The pressure control valve  140  forms working oil pressure adjusting means, which receives the working oil from the hydraulic pump  102 , allows the working oil to flow therethrough, and adjusts a pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  to be below the set pressure. An operation of the hydraulic drive apparatus  100  of the present invention will be described.  
      A case (referred to as a first case) where the first operation unit  200  drives and rotates the first hydraulic motor  201 , and the second operation unit  300  stops the rotation of the second hydraulic motor  301 , will first be described.  
      In the first case, as in the fifth embodiment, the first hydraulic motor  201  is driven and rotated, by the first operation unit  200 , and the second hydraulic motor  301  is stopped in rotation by the second operation unit  300 .  
      In the first case, a pressure of the working oil supplied from the hydraulic pump  102  to-the first control valve  250  is controlled, by the pressure control valve  140 , to be the set pressure of the pressure control valve  140  or lower. The working oil having passed through the pressure control valve  140  is supplied again to the tank  101  via the oil passage  117 .  
      In the first case, a quantity of the working oil that the hydraulic pump  102  supplies is set to be 5% larger than a quantity of the working oil necessary for driving and rotating the first hydraulic motor  201 . A quantity of the working oil flowing through the flow control valve  190  is set to be 5% of a quantity of the working oil necessary for driving and rotating the first hydraulic motor  201 . Description will be given hereunder about an operation of the hydraulic drive apparatus to set a quantity of the working oil that the hydraulic pump  102  supplies to be 5% larger than a quantity of the working oil necessary for driving and rotating the first hydraulic motor  201 , and to set a quantity of the working oil flowing through the flow control valve  190  to be 5% of a quantity of the working oil necessary for driving and rotating the first hydraulic motor  201 .  
      The operation position signal is input from the first operation lever  210  to the oil quantity adjust signal output circuit  220 D via the signal transmission path  161 .  
      The supplying-oil pressure signal output from the pressure sensor  131 , and the operation position detect signal output from the pressure sensor  134  are input to the oil quantity adjust signal output circuit  220 D by way of a signal transmission path, not shown.  
      The oil quantity adjust signal output circuit  220 D judges, by the operation position detect signal input thereto, that the second operation unit  300  stops the rotation of the second hydraulic motor  301 . The oil quantity adjust signal output circuit, like the supplying-oil adjust signal output circuit  220 C in the fifth embodiment, computes a quantity of the working oil necessary for driving and rotating the first hydraulic motor  201  by use of the operation position signal input thereto. The oil quantity adjust signal output circuit  220 D generates a supplying-oil adjust signal which causes the pump regulator  180  to adjust a quantity of the working oil that the hydraulic pump  102  supplies so that a quantity of the working oil flowing through the flow control valve  190  is 5% larger than the quantity of the working oil necessary for driving and rotating the first hydraulic motor  201 . The oil quantity adjust signal output circuit further generates a working oil quantity adjust signal which causes a quantity of the working oil flowing through the flow control valve  190  so that a quantity of the working oil flowing through the flow control valve  190  is 5% of the quantity of the working oil necessary for driving and rotating the first hydraulic motor  201 .  
      The supplying-oil adjust signal generated is output from the oil quantity adjust signal output circuit  220 D to the signal transmission path  164 , and then to the pump regulator  180 . Upon receipt of the supplying-oil adjust signal, the pump regulator  180  adjusts a quantity of the working oil supplied from the hydraulic pump  102  so that the quantity of the working oil that the hydraulic pump  102  supplies is 5% larger than the quantity of the working oil necessary for driving and rotating the first hydraulic motor  201 .  
      The working oil quantity adjust signal generated is output from the oil quantity adjust signal output circuit  220 D to the signal transmission path  163  and then to the flow control valve  190 . Upon receipt of the working oil quantity adjust signal, the flow control valve  190  adjusts a quantity of the working oil flowing through the valve is 5% of the quantity of the working oil necessary for driving and rotating the first hydraulic motor  201 .  
      In this way, the quantity of the working oil that the hydraulic pump  102  supplies is set to be 5% larger than the quantity of the working oil necessary for driving and rotating the first hydraulic motor  201 . The quantity of the working oil flowing through the flow control valve  190  is set to be 5% of the quantity of the working oil necessary for driving and rotating the first hydraulic motor  201 .  
      In the first case, the supplying-oil pressure signal input to the oil quantity adjust signal output circuit  220 D is not used by the oil quantity adjust signal output circuit  220 D to generate the supplying-oil adjust signal and the working oil quantity adjust signal.  
      As described above, in the first case, the quantity of the working oil that the hydraulic pump  102  supplies is set to be 5% larger than the quantity of the working oil necessary for driving and rotating the first hydraulic motor  201 . The quantity of the working oil flowing through the flow control valve  190  is set to be 5% of the quantity of the working oil necessary for driving and rotating the first hydraulic motor  201 . Therefore, a sufficient amount of the working oil for driving the first hydraulic motor  201  is fed to the first hydraulic motor  201 .  
      Description will be given hereunder about a case (referred to as a second case) where the first operation unit  200  stops the rotation of the first hydraulic motor  201 , and the second operation unit  300  drives and rotates the second hydraulic motor  301 .  
      In the second case, as in the fifth embodiment, the first hydraulic motor  201  is stopped in rotation by the first operation unit  200 , and the second hydraulic motor  301  is driven and rotated by the second operation unit  300 .  
      Also in the second case, as in the first case, a pressure of the working oil fed from the hydraulic pump  102  to the first control valve  250  is controlled, by the pressure control valve  140 , to be the set pressure of the pressure control valve  140  or lower.  
      In the second case, a quantity of the working oil that the hydraulic pump  102  is set to be the maximum oil quantity at which the hydraulic pump  102  is capable of supplying the working oil, and the flow control valve  190  is set to allow the working oil to freely flow therethrough.  
      Description will next be given hereunder about an operation of the hydraulic drive apparatus  100  to set the quantity of the working oil supplied by the hydraulic pump  102  at the maximum oil quantity at which the hydraulic pump  102  can supply, and to set the flow control valve  190  so that it allows the working oil to freely flow therethrough.  
      The oil quantity adjust signal output circuit  220 D receives the operation position signal through the signal transmission path  161 , and the supplying-oil pressure signal and operation position detect signal through a signal transmission path, not shown.  
      The oil quantity adjust signal output circuit  220 D judges, by the operation position signal input thereto, that the first operation unit  200  stops the rotation of the first hydraulic motor  201 . The oil quantity adjust signal output circuit, as the supplying-oil adjust signal output circuit  220 C of the fifth embodiment so does, computes the maximum oil quantity at which the hydraulic pump  102  can supply the working oil by use of the supplying-oil pressure signal. And the oil quantity adjust signal output circuit  220 D computes a supplying-oil adjust signal which causes the pump regulator  180  to adjust a quantity of the working oil that the hydraulic pump  102  supplies so that a quantity of the working oil that the hydraulic pump  102  supplies is equal to the maximum oil quantity at which the hydraulic pump  102  can supply the working oil. Further, the oil quantity adjust signal output circuit generates a working oil quantity adjust signal for causing the flow control valve  190  to adjust a quantity of the working oil flowing therethrough so that the working oil flowing from the oil passages  118  through the flow control valve  190  to the oil passages  119  freely flows through the oil passages  119 .  
      The supplying-oil adjust signal is thus generated by the oil quantity adjust signal output circuit  220 D and output therefrom to the signal transmission path  164 , and then to the pump regulator  180 . Upon receipt of the supplying-oil adjust signal, the pump regulator  180  adjusts a quantity of the working oil that the hydraulic pump  102  supplies so that a quantity of the working oil that the hydraulic pump  102  supplies is equal to the maximum oil quantity at which the hydraulic pump  102  can supply the working oil.  
      The working oil quantity adjust signal is thus generated by the oil quantity adjust signal output circuit  220 D and output to the signal transmission path  163 , and input to the flow control valve  190  through the signal transmission path  163 . Upon receipt of the working oil quantity adjust signal, the flow control valve  190  adjusts a quantity of the working oil flowing through the flow control valve  190  so that the working oil flowing from the oil passages  118  through the flow control valve  190  to the oil passages  119  may freely flow into the flow control valve  190 .  
      In this way, the quantity of the working oil that the hydraulic pump  102  supplies is set to be the maximum oil quantity that the hydraulic pump  102  can supply the working oil, and the flow control valve  190  is set to allow the working oil to freely flow therethrough.  
      In the second case, the operation position signal input to the oil quantity adjust signal output circuit  220 D is not used by the oil quantity adjust signal output circuit  220 D to generate supplying-oil adjust signal.  
      As described above, in the second case, a quantity of the working oil that the hydraulic pump  102  supplies is set to be equal to the maximum oil quantity at which the hydraulic pump  102  can supply the working oil, and the flow control valve  190  is set to allow the working oil to freely flow therethrough. Accordingly, the maximum oil quantity at which the hydraulic pump  102  can supply the working oil is supplied to the second hydraulic motor  301 .  
      Description will now be given hereunder about a case (referred to as a third case) where the first operation unit  200  drives and rotates the first hydraulic motor  201  and the second operation unit  300  drives and rotates the second hydraulic motor  301 .  
      In the third case, the first operation unit  200  drives and rotates the first hydraulic motor  201  in accordance with the operation position input to the first operation lever  210 , and the second operation unit  300  drives and rotates the second hydraulic motor  301  in accordance with the operation position input to the second operation lever  310 .  
      Also in the third case, as in the first case, a pressure of the working oil supplied from the hydraulic pump  102  to first control valve  250  is controlled to be the set pressure of the pressure control valve  140  or lower by the pressure control valve  140 .  
      In the third case, the second operation unit  300  drives and rotates the second hydraulic motor  301 . Accordingly, as in the second case, a quantity of the working oil that the hydraulic pump  102  supplies is set to be equal to the maximum oil quantity at which the hydraulic pump  102  can supply the working oil, by the oil quantity adjust signal output circuit  220 D.  
      As described above, in the third case, the quantity of the working oil that the hydraulic pump  102  supplies is set to be equal to the maximum oil quantity at which the hydraulic pump  102  can supply the working oil, and the flow control valve  190  is set to allow the working oil to freely flow therethrough. The working oil is supplied to the second hydraulic motor  301  at the maximum oil quantity at which the hydraulic pump  102  can supply the working oil.  
      In the present embodiment, in the first case, the quantity of the working oil that the hydraulic pump  102  supplies is 5% larger than that of the working oil necessary for driving and rotating the first hydraulic motor  201 , and the quantity of the working oil that flows through the flow control valve  190  is 5% of the quantity of the working oil necessary for driving and rotating the first hydraulic motor  201 . The predetermined quantity of the working oil may take any value other than 5% if it falls within a range where the flow loss is substantially small. In the second and third cases, the quantity of the working oil that the hydraulic pump  102  supplies is equal to the maximum oil quantity of the working oil that the hydraulic pump  102  can supply. Quantity values near the maximum oil quantity suffices for the required quantity of the working oil that the hydraulic pump  102  supplies.  
      In the present embodiment, the additional hydraulic motor and the additional operation unit may be used in addition to the second operation unit  300  as the seventh or eighth embodiments to the fifth embodiment.  
     TENTH EMBODIMENT  
       FIG. 11  is a hydraulic pressure circuit diagram showing a hydraulic drive apparatus  100  which is a tenth embodiment of the invention.  
      An arrangement of the hydraulic drive apparatus  100  of this embodiment will first be described.  
      The arrangement of the hydraulic drive apparatus  100  of the embodiment is substantially the same as of the hydraulic drive apparatus  100  of the ninth embodiment except its arrangement to be described hereunder.  
      In the hydraulic drive apparatus  100  of the embodiment, the pressure control valve  140  consists of an electromagnetic relief valve  141 .  
      The hydraulic drive apparatus  100  of the embodiment includes a pressure adjust signal output circuit  220 E as pressure adjust signal outputting means. The pressure adjust signal output circuit  220 E receives a pressure adjust signal outputting means through a signal transmission path, not shown, from the pressure sensor  131 , drive oil pressure signals through a signal transmission path, not shown, from the pressure sensors  132  and  133 , and an operation position signal through the signal transmission path  161  from the first operation lever  210 , and an operation position detect signal through a signal transmission path (not shown) from the pressure sensor  134 . The pressure adjust signal output circuit  220 E judges, by the operation position detect signal input thereto, whether the second operation unit  300  drives and rotates the second hydraulic motor  301  or stops the rotation of it. When the second operation unit stops the rotation of the second hydraulic motor  301 , the pressure adjust signal output circuit computes a pressure of the working oil necessary for driving and rotating the first hydraulic motor  201  by use of the input supplying-oil pressure signal and the drive oil pressure signal. And the pressure adjust signal output circuit generates a pressure adjust signal which causes the pressure control valve  140  to adjust the set pressure of the pressure control valve  140  so that the set pressure of the pressure control valve  140  is a predetermined pressure (20 kg/cm 2  in this instance) higher than the pressure of the working oil necessary for driving and rotating the first hydraulic motor.  201 . When the second operation unit  300  drives and rotates the second hydraulic motor  301 , the pressure adjust signal output circuit generates a pressure adjust signal which causes the pressure control valve  140  to adjust the set pressure of the pressure control valve  140  so that the set pressure is a predetermined pressure, e.g., 350 kg/cm 2  in this embodiment. It outputs the generated pressure adjust signal to the pressure control valve  140 .  
      While the hydraulic drive apparatus  100  of the ninth embodiment includes the flow control valve  190 , the hydraulic drive apparatus  100  of this embodiment includes a flow control valve  191 , which can more reliably stop the flow of the working oil when comparing with the flow control valve  190  of the ninth embodiment.  
      An operation of the hydraulic drive apparatus  100  of this embodiment will be described.  
      The operation of the hydraulic drive apparatus  100  of this embodiment is substantially the same as of the hydraulic drive apparatus  100  of the ninth embodiment except its operation to be described below.  
      For the operation, the first case of the hydraulic drive apparatus will first be described.  
      In the first case, in the ninth embodiment, the oil quantity adjust signal output circuit  220 D sets the quantity of the working oil flowing through the flow control valve  190  to be 5% of the quantity of the working oil necessary for driving and rotating the first hydraulic motor  201 . In this connection, in this embodiment, the oil quantity adjust signal output circuit  220 D stops the flow of the working oil through the flow control valve  191 .  
      In the first case, a pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  is set to be higher than of the working oil necessary for driving and rotating the first hydraulic motor  201  by 20 kg/cm 2  or lower. Description will be given about an operation of the hydraulic drive apparatus to set a pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  to be higher than of the working oil necessary for driving and rotating the first hydraulic motor  201  by 20 kg/cm 2  or lower.  
      The operation position signal is input to the pressure adjust signal output circuit  220 E via the signal transmission path  161 . The operation position detect signal is input to the pressure adjust signal output circuit  220 E via a signal transmission path, not shown. The supplying-oil pressure signal is input to the pressure adjust signal output circuit  220 E via a signal transmission path, not shown. The drive oil pressure signal is input from-the pressure sensors  132  and  133  to the pressure adjust signal output circuit  220 E via a signal transmission path, not shown.  
      The pressure adjust signal output circuit  220 E judges, by the input operation position signal and the input operation position detect signal, that the second operation unit  300  stops  20  the rotation of the second hydraulic motor  301 . The pressure adjust signal output circuit computes a pressure of the working oil for driving and rotating the first hydraulic motor  201  by use of the input supplying-oil pressure signal and the input drive oil pressure signal, as the pressure adjust signal output circuit  220 B does so in the first case in the fifth embodiment. And it generates a pressure adjust signal for causing the pressure control valve  140  to adjust the set pressure thereof so that the set pressure is 20 kg/cm 2  higher than the pressure of the working oil necessary for driving and rotating the first hydraulic motor  201 .  
      The pressure adjust signal is thus generated and output to a signal transmission path  165 , and then to the pressure control valve  140  via the signal transmission path  165 . That is, the pressure adjust signal is input to the electromagnetic relief valve  141 . Upon receipt of the pressure adjust signal, the electromagnetic relief valve  141  adjusts the set pressure in accordance-with the input pressure adjust signal, and adjusts the pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  to be the set pressure or lower.  
      In this way, in the first case, the pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  is set to be higher than the pressure of the working oil necessary for driving and rotating the first hydraulic motor  201  by 20 kg/cm 2  or lower.  
      For the operation, the second and third cases will be described.  
      In those cases, the pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  is 350 kg/cm 2  or lower. An operation of the hydraulic drive apparatus to set the pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  to be 350 kg/cm 2  or lower will be described.  
      As referred to in the description of the first case, the operation position signal, the operation position detect signal, the supplying-oil pressure signal and the drive oil pressure signal are input to the pressure adjust signal output circuit  220 E.  
      The pressure adjust signal output circuit  220 E judges, by the input operation position signal, that the second operation unit  300  drives and rotates the second hydraulic motor  301 , generates-a pressure adjust signal to set the set pressure of the pressure control valve  140  to 350 kg/cm 2 , and inputs the generated pressure adjust signal through the signal transmission path  165  to the pressure control valve  140 .  
      Accordingly, in the second and third cases, a pressure of the working oil supplied from the hydraulic pump  102  to the first control valve  250  is the set pressure of the pressure control valve  140  or lower, viz., 350 kg/cm 2  or lower.  
      In the embodiment, in the first case, the set pressure of the pressure control valve  140  is 20 kg/cm 2  higher than the pressure of the working oil necessary for driving and rotating the first hydraulic motor  201 . The predetermined pressure may take any other value than 20 kg/cm 2  if it is substantially free from the wasting of the drive horsepower of the hydraulic pump  102 . In the second and third case, the set pressure of the pressure control valve  140  is 350 kg/cm 2 . It may take any other value than 350 kg/cm 2  if it is sufficiently larger than the pressure of the working oil necessary for driving and rotating the first hydraulic motor  201 .  
     ELEVENTH EMBODIMENT  
       FIG. 12  is a hydraulic pressure circuit diagram showing a hydraulic drive apparatus  100 , which is an eleventh embodiment of the invention.  
      While the pressure control valve  140  in the hydraulic drive apparatus- 100  of the tenth embodiment consists of the electromagnetic relief valve  141 , the pressure control valve  140  in the hydraulic drive apparatus  100  of the present embodiment includes a main relief valve  142  and an electromagnetic relief valve  143 . The remaining arrangement of the hydraulic drive apparatus of this embodiment is substantially the same as of the hydraulic drive apparatus  100  of the tenth embodiment.  
      An operation of the hydraulic drive apparatus  100  of this embodiment is substantially the same as of the hydraulic drive apparatus  100  of the tenth embodiment.  
     TWELFTH EMBODIMENT  
       FIG. 13  is a hydraulic pressure circuit diagram showing a hydraulic drive apparatus  100  which constitutes a twelfth embodiment of the invention.  
      An arrangement of the hydraulic drive apparatus  100  of this embodiment will first be described.  
      An arrangement of the hydraulic drive apparatus  100  of this embodiment is substantially the same as of the hydraulic drive apparatus  100  of the ninth embodiment except its arrangement to be described hereunder.  
      The hydraulic drive apparatus  100  of the embodiment includes an additional hydraulic motor  401  provided between the oil passage  125  and the tank  101 . The additional hydraulic motor  401  is driven and rotated by the working oil supplied from the hydraulic pump  102  by way of a route of the oil passage  112 ,  118  and  119 , second control valve  350 , and the oil passage  125 ,  181  and  182 , and the oil passage  183  or  184 .  
      The hydraulic drive apparatus  100  of the embodiment includes additional operation unit  400 , which also includes an additional operation lever  410  as additional operation position inputting means to which an operation position is input, and an additional control valve  450  which allows the working oil supplied from the hydraulic pump  102  to the additional hydraulic motor  401  to flow therethrough, adjusts a quantity of the working oil flowing through the additional control valve in accordance with the operation position input to the additional operation lever  410 , and adjusts, by the adjustment, a quantity of the working oil supplied from the hydraulic pump  102  to the additional hydraulic motor  401 , whereby the additional hydraulic motor  401  is driven and rotated in accordance with the operation position input to the additional operation lever  410 .  
      The oil passages  181  and  182  intercommunicate with each other through the check valve  171 . A rotation of the additional hydraulic motor  401  is reduced in speed by the speed reduction means  402  and then transmitted to the load  403 . The working oil having driven and rotated the additional hydraulic motor  401  passes through the oil passage  183  or  184 , and passes again through the additional control valve  450 , and returns to the tank  101  via the oil passage  185 . Of the working oils passing through the oil passage  125 , the working oils other than the additional hydraulic motor  401  passes through the additional control valve  450  and the oil passage  186 , and are fed again to the tank  101 .  
      The additional control valve  450  is arranged such that it adjusts a quantity of the working oil therethrough. Accordingly, an operation position input to the additional operation lever  410 . To be more specific, the additional operation lever  410  is arranged such that it supplies an operation oil that is dependent in amount on an operation position input thereto. The additional control valve  450  is arranged such that it receives the operation oil through the operation oil passage  411  or  412  from the additional operation lever  410 , and adjusts a quantity of the working oil flowing therethrough in accordance with a quantity of the operation oil supplied thereto.  
      The pressure sensor  135  as operation position detecting means detects the operation position input to the additional operation lever  410 . To be more specific, the pressure sensor  135  detects a pressure of the operation oil supplied from the additional operation lever  410  to the additional control valve  450  by detecting a pressure of the operation oil, which is higher of those oils passing through the operation oil passage  411  or  412 .  
      In the ninth embodiment, oil quantity adjust signal output circuit  220 D judges, by the input operation position detect signal, that the second operation unit  300  drives and rotates the second hydraulic motor  301 . In connection with this, in this embodiment, the oil quantity adjust signal output circuit  220 D judges, by the input operation position detect signal, that one of the second operation unit  300  and the additional operation unit  400  drives and rotates either of the second hydraulic motor  301  and the additional hydraulic motor  401 . Also in the ninth embodiment, the oil quantity adjust signal output circuit  220 D judges, by the input operation position, that the second operation unit  300  stops the rotation of the second hydraulic motor  301 . In this connection, in this embodiment, the oil quantity adjust signal output circuit  220 D judges, by the input operation position detect signal, that the second operation unit  300  and the additional operation unit  400  stop the rotation of the second hydraulic motor  301  and the additional hydraulic motor  401 , respectively.  
      An operation of the hydraulic drive apparatus  100  of this embodiment is substantially the same as of the hydraulic drive apparatus  100  of the ninth embodiment except that the additional hydraulic motor  401  is operated.  
      In the hydraulic drive apparatus  100  of the hydraulic drive apparatus  100  of this embodiment, it is impossible to simultaneously operate the second hydraulic motor  301  and the additional hydraulic motor  401 .  
     THIRTEENTH EMBODIMENT  
       FIG. 14  is a hydraulic pressure circuit diagram showing a hydraulic drive apparatus  100  which forms a thirteenth embodiment of the present invention.  
      An arrangement of the hydraulic drive apparatus  100  of thirteenth embodiment will first be described.  
      The arrangement of the hydraulic drive apparatus  100  of the embodiment is substantially the same as of the hydraulic drive apparatus  100  of the seventh embodiment in the following point. While the oil passage  181  of the hydraulic drive apparatus  100  communicates with the oil passage  125  in the twelfth embodiment, the oil passage  181  of the hydraulic drive apparatus  100  communicates with the oil passages  119  in the thirteenth embodiment.  
      An operation of the hydraulic drive apparatus  100  of this embodiment will be described.  
      The operation of the hydraulic drive apparatus  100  of the embodiment is substantially the same as of the hydraulic drive apparatus  100  of the twelfth embodiment except that it is impossible to simultaneously operate the second hydraulic motor  301  and the additional hydraulic motor  401 .  
      It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.