Patent Publication Number: US-2016245311-A1

Title: Hydraulic Pressure Circuit and Working Machine

Description:
TECHNICAL FIELD 
     The present invention relates to a hydraulic pressure circuit having an accumulator and a working machine on which the hydraulic pressure circuit is mounted. 
     BACKGROUND ART 
     In a working machine, pressurized oil discharged from a boom hydraulic cylinder during a boom lowering operation is accumulated in an accumulator and pressurized oil relieved from a swinging hydraulic motor during acceleration or deceleration of the swinging is also accumulated in the accumulator (for example, see Patent Literature 1). 
     Patent Literature 1: Japanese Patent Application Publication No. 2010-84888 
     Since the speed of a boom hydraulic cylinder decreases when accumulation of pressure in an accumulator progresses so that the accumulator pressure increases, the accumulator cannot accumulate to a high pressure level and has to abandon energy. Thus, it is not possible to recycle the energy efficiently. 
     Moreover, when a circuit is switched to cope with a decrease in the speed of the boom hydraulic cylinder, a shock occurs during the circuit switching, which deteriorates operability. Thus, it is not desirable to switch the circuit to cope with the decrease in the speed. 
     Further, when the boom hydraulic cylinder cooperates with other actuators during the pressure accumulation, oil may be consumed by an actuator having a low pressure level, and the boom may be slowly lowered and may stop. 
     DISCLOSURE OF THE INVENTION 
     The present invention has been made in view of the above problem, and an object thereof is to provide a hydraulic pressure circuit and a working machine capable of solving a problem that the speed of a hydraulic pressure cylinder decreases when an accumulator pressure increases without switching a circuit, which may deteriorate operability, and recycling energy efficiently. 
     An invention according to claim  1  is a hydraulic pressure circuit including: a main pump driven by an engine; a hydraulic pressure cylinder including a piston operated with an operating fluid supplied from the main pump and one chamber and another chamber partitioned by the piston; an accumulator that accumulates the operating fluid pushed from the one chamber of the hydraulic pressure cylinder; and an assist pump that sucks in the operating fluid from the accumulator when pressure accumulation in the accumulator progresses such that an accumulator pressure increases. 
     An invention according to claim  2  is the hydraulic pressure circuit according to claim  1 , in which the assist pump pressurizes the operating fluid sucked from the accumulator and supplies the pressurized operating fluid to the other chamber of the hydraulic pressure cylinder. 
     An invention according to claim  3  is the hydraulic pressure circuit according to claim  1  or  2 , in which the hydraulic pressure circuit further includes a pressure reducing valve that reduces hydraulic pressure supplied from at least one of the assist pump and the accumulator to a predetermined pressure level; and a pilot circuit that uses operating fluid pressure connected to the pressure reducing valve as a pilot source pressure. 
     An invention according to claim  4  is a working machine including: a vehicle body; a working unit mounted on the vehicle body; and the hydraulic pressure circuit according to any one of claims  1  to  3 , provided in the hydraulic pressure cylinder that operates the working unit. 
     An invention according to claim  5  is the working machine according to claim  4 , in which the working unit includes a boom rotated in a vertical direction, wherein the hydraulic pressure cylinder is a boom cylinder that moves the boom in the vertical direction. 
     According to the invention disclosed in claim  1 , when the pressure accumulation of the accumulator that accumulates the operating fluid pushed from one chamber of the hydraulic pressure cylinder progresses so that the accumulator pressure has increased and the speed of the hydraulic pressure cylinder has decreased, the operating fluid supplied to the accumulator is consumed by the assist pump to thereby suppress an increase in the accumulator pressure. Thus, it is possible to diminish a decrease in the speed of the hydraulic pressure cylinder without switching a circuit and to prevent the occurrence of a shock during the switching, which may occur when a circuit is switched to cope with the decrease in the speed of the hydraulic pressure cylinder. 
     According to the invention disclosed in claim  2 , since the assist pump motor supplies the operating fluid to the other chamber of the hydraulic pressure cylinder, it is possible to reduce the amount of operating fluid supplied from the main pump. Thus, it is possible to suppress an adverse effect on other hydraulic pressure actuators that share the main pump, and to secure the ability to cooperate with the other hydraulic pressure actuators. Moreover, since the energy that has to be consumed in order to maintain the operating speed of the hydraulic pressure cylinder can be recycled efficiently by the assist pump motor, it is possible to suppress energy loss. 
     According to the invention disclosed in claim  3 , since the pressure reducing valve reduces the hydraulic pressure supplied from at least one of the assist pump and the accumulator and uses the same as pilot source pressure, it is possible to eliminate the use of a conventional pilot pump. 
     According to the invention disclosed in claim  4 , the operating fluid supplied to the accumulator mounted on the working machine is consumed by the assist pump to suppress an increase in the accumulator pressure. Thus, it is possible to diminish the decrease in the speed of the working unit and to prevent the occurrence of a shock during the switching, which may occur when a circuit is switched to cope with the decrease in the speed of the working unit. 
     According to the invention disclosed in claim  5 , since a shock occurs during the switching and the operability deteriorates if a circuit is switched to cope with the decrease in the boom lowering speed, by decreasing the amount of the operating fluid pushed from the boom cylinder and accumulated in the accumulator so that the operating fluid is consumed by the assist pump, it is possible to prevent the decrease in the boom lowering speed and to suppress energy loss. Moreover, since the energy that has to be consumed in order to maintain the boom lowering speed can be recycled efficiently by the assist pump motor, it is possible to suppress energy loss. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating an embodiment of a hydraulic pressure circuit according to the present invention; 
         FIG. 2  is a circuit diagram illustrating a switching state of the hydraulic pressure circuit; 
         FIG. 3A  is a circuit diagram illustrating a pressure accumulation state of a swinging motor of the hydraulic pressure circuit and  FIG. 3B  is a circuit diagram illustrating an example where a pilot circuit uses the accumulated pressure; and 
         FIG. 4  is a perspective view illustrating an embodiment of a working machine according to the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, the present invention will be described in detail according to an embodiment illustrated in  FIGS. 1 to 4 . 
     As illustrated in  FIG. 4 , a vehicle body  1  of an excavator HE as a working machine includes a lower traveling body  2  and an upper swinging body  3  provided on the lower traveling body  2  so as to be swingable by a swinging motor  3   m.  A machine chamber  4  in which an engine, a pump, and the like are mounted, a cab  5  for protecting an operator, and a working unit  6  are mounted on the upper swinging body  3 . 
     The working unit  6  has a configuration in which a base end of a boom  7  rotated in an vertical direction by two boom cylinders  7   c   1  and  7   c   2  as hydraulic pressure cylinders arranged in parallel is supported by the upper swinging body  3 , a stick  8  rotated in a front-rear direction by a stick cylinder  8   c  is supported by a distal end of the boom  7 , and a bucket  9  rotated by a bucket cylinder  9   c  is supported by a distal end of the stick  8 . The boom cylinders  7   c   1  and  7   c   2  are arranged in parallel in relation to the same boom  7  and perform the same operation simultaneously. 
       FIG. 1  illustrates an engine power assist system which accumulates the potential energy of the working unit  6  in an accumulator with the aid of the boom cylinder  7   c   1  and accumulates the kinetic energy of the upper swinging body  3  in the accumulator with the aid of the swinging motor  3   m  to use the energy in assisting the engine power. 
     Next, a circuit configuration of this system will be described. The boom cylinders  7   c   1  and  7   c   2  are partitioned into one chamber  7   ch  positioned closer to the head side and the other chamber  7   cr  positioned closer to the rod side by a single-rod-type piston  7   cp  that operates with operating oil pressure. 
     An assist pump motor  15  that serves as a pump having a motor function is connected directly or via gears to a main pump shaft  14  of main pumps  12  and  13  driven by the engine  11  mounted in the machine chamber  4 . The main pumps  12  and  13  and the assist pump motor  15  have a swash plate capable of variably adjusting a pump/motor displacement (piston stroke) by adjusting the swash angle (tilt angle). The swash angles (tilt angles) are controlled by regulators  16 ,  17  and  18  and are detected by swash angle sensors  16 φ,  17 φ, and  18 φ, and the regulators  16 ,  17 , and  18  are controlled by electromagnetic valves. For example, the regulators  16  and  17  of the main pumps  12  and  13  can be controlled automatically by negative flow control pressure (so-called negative control pressure) guided by a negative flow control passage  19   nc  and can be controlled with signals other than the negative control pressure by electromagnetic switching valves  19   a  and  19   b  of a negative flow control valve  19 . 
     The main pumps  12  and  13  discharge operating oil as operating fluid sucked up from a tank  21  to passages  22  and  23 , and the pump discharge pressures thereof are detected by pressure sensors  24  and  25 . An output passage  27  drawn from one side of a main boom control valve  26  for controlling the boom cylinders  7   c   1  and  7   c   2  and an output passage  29  drawn from a sub-boom control valve  28  among pilot-operated direction/flow rate control valves connected to the main pumps  12  and  13  are connected to a boom energy recovery valve  31  as a composite valve by a passage  30 . 
     The boom energy recovery valve  31  is a composite valve in which the functions of a plurality of circuits switching an accumulation circuit A and a regeneration circuit B illustrated in  FIG. 1  and a circuit, which is not illustrated, for guiding the pressurized operating oil supplied from the main pumps  12  and  13  during a boom raising operation toward the head side of the two boom cylinders  7   c   1  and  7   c   2  are incorporated into a single block. The head-side ends of the two boom cylinder  7   c   1  and  7   c   2  are connected to the boom energy recovery valve  31  by passages  32  and  33 , respectively. 
     The other output passage  34  drawn from the main boom control valve  26  is connected to one of the boom cylinders—the boom cylinder  7   c   1 , and a pressure sensor  35  that detects a rod-side pressure of the boom cylinder is provided in the rod-side end. The rod-side ends of the two boom cylinders  7   c   1  and  7   c   2  arranged in parallel can communicate with each other with the aid of a bypass passage  36 , and the communication between the rod-side ends of the boom cylinders  7   c   1  and  7   c   2  can be blocked by an electromagnetic separation valve  37  provided in the middle of the bypass passage  36 . The rod-side end of the boom cylinder  7   c   2  is connected to the boom energy recovery valve  31  by a passage  38 . 
     The output passage  27  drawn from the one side of the main boom control valve  26  can communicate with the other output passage  34  via an electromagnetic switching valve  39  and a check valve  40 . Moreover, a pressure sensor  41  is provided on the discharge side of the assist pump motor  15  so as to detect the discharge pressure of the assist pump motor  15 , an electromagnetic switching valve  43  is provided in the discharge passage  42 , and a passage  45  that passes through a check valve  44  is connected to the output passage  34 . 
     The discharge passage  42  of the assist pump motor  15  branches into three passages  46 ,  47 , and  48 . The passage  46  is connected to an electromagnetic unload valve  49 , and the connection of the electromagnetic unload valve  49  extends from tank passages  50  and  51  to a spring check valve  52  and then to the tank  21  via and an oil cooler  53  or a spring check valve  54 . The passage  47  is connected to a tank passage  50  via a relief valve  55 . 
     The passage  48  is connected to an accumulator passage  62  in which a plurality of first accumulators  61  are provided via an electromagnetic switching valve  57 , a check valve  58 , and a passage  59 , and a pressure sensor  63  that detects pressure accumulated in the first accumulator  61  is connected to the accumulator passage  62 . The accumulator passage  62  is connected to a passage  66  via an electromagnetic regeneration valve  64  and a check valve  65 . The passage  66  extends from the tank  21  and is connected to an intake-side passage  68  connected to an intake port of the assist pump motor  15  via a check valve  67 . A pressure sensor  69  that detects an intake-side pressure of the assist pump motor is provided in the intake-side passage  68 . 
     The assist pump motor  15  has a function of switching the electromagnetic regeneration valve  64  to a communicating position when accumulation in the first, accumulator  61  progresses and the accumulator pressure has increased to a predetermined value to suck in the operating oil from the first accumulator  61  to thereby reduce an increase in the pressure of the accumulator  61  and pressurize the sucked operating oil and supply the same to the rod side chamber  7   cr  of the boom cylinder  7   c   1 . 
     The boom energy recovery valve  31  includes a pilot-operated main switching valve  71 . The main switching valve  71  controls supply of pilot pressure with the aid of an electromagnetic switching valve  72  to thereby switch the relation between the passages  73 ,  74 ,  75 , and  76 . 
     The passage  73  is connected to one port of one of drift reduction valves—a drift reduction valve  77 —and an external passage  32  drawn from the head-side end of the boom cylinder  7   c   1  is connected to the other port of the drift reduction valve  77  via a passage  78 . The drift reduction valve  77  controls opening/closing and an opening degree of ports by controlling pilot pressure in a spring chamber with the aid of a pilot valve  79 . A passage  81  branched from the passage  30  is connected to the passage  73  via a check valve  82 . 
     The passage  74  is connected to the passage  30  and is also connected to one port of the other one of the drift reduction valves—a drift reduction valve  83 . An external passage  33  drawn from the head-side end of the other boom cylinder  7   c   2  is connected to the other port of the drift reduction valve  83  via an inner passage  84 . The drift reduction valve  83  controls opening/closing and an opening degree of ports by controlling a pilot pressure in spring chamber with the aid of a pilot valve  85 . 
     The pilot valves  79  and  85  allow the spring chambers of the drift reduction valves  77  and  83  to communicate with the passages  78  and  84  or a passage  86  to the tank  21 . 
     The passage  75  branches into a check valve  87 , a spring check valve  88 , and a passage to a variable throttle valve  89 . A passage that passes through the check valve  87  is connected to an external passage  38  and an inner passage  90 . A relief valve  91  and a check valve  92  are provided between the passage  90  and the passage  78 , and a relief valve  93  and a check valve are provided between the passage  90  and the passage  84 . Further, a pressure sensor  95  and an adjustment valve  96  are provided between the passage  78  and the passage  84 , and a pressure sensor  97  and an adjustment valve  98  are provided between the passage  84  and the passage  90 . The spring check valve  88  and the variable throttle valve  89  are connected to the tank passage  50  via a passage  99 . 
     The passage  76  is connected to the passage  59  via a passage  105  that passes through a check valve  104 , and the pressure of the passage  105  is detected by a pressure sensor  106 . A passage branched from the passage  105  is connected to the tank passage  50  via a relief valve  107 , a passage  108 , and the passage  99 . The passage  108  communicates with the passage  105  via the check valve  109 , and the passage  105  is connected to the passage  108  via an electromagnetic switching valve  110 . 
     As illustrated in  FIG. 1 , the accumulation circuit A is a circuit which extends from the passage  32  drawn from the head-side end of one of the boom cylinders—the boom cylinder  7   c   1 —and reaches the first accumulator  61  via the passage  78 , the drift reduction valve  77 , the passage  73 , the main switching valve  71 , the check valve  104 , and the passage  105  in the boom energy recovery valve  31 . The accumulation circuit A has, as illustrated in  FIG. 2 , a function of accumulating the oil pushed from the head side of the boom cylinder  7   c   1  in the accumulator  61 . 
     As illustrated in  FIG. 1 , the regeneration circuit B is a circuit which extends from the passage  33  drawn from the head-side end of the other boom cylinder  7   c   2  and reaches the rod-side end of the other boom cylinder  7   c   2  via the passage  84 , the drift reduction valve  83 , the passage  74 , the main switching valve  71 , the passage  75 , the check valve  87 , and the passage  38  in the boom energy recovery valve  31 . The regeneration circuit B has a function of regenerating the oil pushed from the head side of the boom cylinder  7   c   2  and supplying the same to the rod side of the boom cylinder  7   c   2 . 
     Opposing relief valves  114  and  115  and opposing check valves  117  and  118  are provided between the passages  112  and  113  of the motor driving circuit C that connects the swinging motor  3   m  and a swinging control valve  111  that controls the swinging direction and speed of the swinging motor  3   m.  A makeup passage  116  having a tank passage function of returning the oil discharged from the motor driving circuit C to the tank  21  and a makeup function of making up for the operating oil to the motor driving circuit C is connected between the relief valves  114  and  115  and between the check valves  117  and  118 . Operating oil is supplied from the makeup passage  116  to a side where there is a possibility of the occurrence of vacuum in the passages  112  and  113  via the check valves  117  and  118  with pressure which does not exceed the spring biasing pressure of the spring check valve  52 . 
     Further, the passages  112  and  113  of the motor driving circuit C communicate with a swing energy recovery passage  121  via check valves  119  and  120 . The passage  121  is connected to a passage  123  via a sequence valve  122  in which source pressure on an inlet side rarely changes with back pressure on an outlet side and is also connected to a second accumulator  125  via a passage  124 . The pressure associated with the second accumulator  125  is detected by a pressure sensor  126 . The passage  123  is connected to the accumulator passage  62  of the first accumulator  61  by a passage  129  that passes through a check valve  128  and an electromagnetic switching valve  127 . The passage  129  is connected to the tank passage  50  via a relief valve  130 , and the second accumulator  125  is connected to the tank passage  51  via a relief valve  131 . 
     In the circuit configuration described above, the swash angle sensors  16 φ,  17 φ, and  18 φ, the pressure sensors  24 ,  25 ,  35 ,  41 ,  63 ,  69 ,  95 ,  97 ,  106 , and  126  input the detected swash angle signals and the pressure signals to an in-vehicle controller (not illustrated). Moreover, the electromagnetic switching valves  39 ,  43 ,  57 ,  72 ,  110 , and  127 , the electromagnetic unload valve  49 , and the electromagnetic regeneration valve  64  are turned on and off according to a driving signal output from the in-vehicle controller (not illustrated) or switched by a proportional operation according to the driving signal. Moreover, the boom control valves  26  and  28 , the swinging control valve  111 , and other hydraulic actuator control valves (not illustrated) (including traveling motor, stick cylinder, and bucket cylinder control valves and the like) are pilot-operated by a manual operating valve (so-called a remote control valve) which is lever-operated or pedal-operated by an operator in the cab  5 , and the pilot valves  79  and  85  of the drift reduction valves  77  and  83  are also pilot-operated in an interlinked manner. 
     Hereinafter, the contents of the functions controlled by the in-vehicle controller will be described. 
     (Engine Power Assisting Function) 
     An engine power assisting function of the hydraulic pressure circuit having the above-described configuration will be described. 
       FIGS. 1 and 2  illustrate a circuit state when a boom lowering operation of lowering the boom  7  is performed. The operating oil discharged from the assist pump motor  15  functioning as a pump is pressurized and supplied to the rod side of one of the boom cylinders—the boom cylinder  7   c   1 —via the electromagnetic switching valve  43 . The operating oil pushed from the head side of the boom cylinder  7   c   1  to the passages  32  and  78  is controlled so as to flow from the passage  73  to the passage  76  via the drift reduction valve  77  of the boom energy recovery valve  31  by the main switching valve  71 . The operating oil is accumulated in the first accumulator  61  via the passages  105  and  59 . 
     At the same time, the operating oil pushed from the head side of the other boom cylinder  7   c   2  to the passages  33  and  84  is controlled so as to flow from the passage  74  to the passage via the drift reduction valve  83  of the boom energy recovery valve  31  by the main switching valve  71  and is regenerated on the rod side of the boom cylinder  7   c   2  via the check valve  87  and the passage  38 . 
     In this manner, the boom energy recovery valve  31  performs accumulation in the first accumulator  61  during the boom lowering operation and regeneration on the rod side of the boom cylinder  7   c   2  at the same time with the aid of the main switching valve  71  and the drift reduction valve  77  and  83 . 
       FIG. 1  illustrates a circuit state in which the assist pump motor  15  functions as a hydraulic pump while consuming the hydraulic pressure energy accumulated in the accumulator  61 . When the electromagnetic regeneration valve  64  is switched to the communicating position, the assist pump motor  15  functioning as a hydraulic pump sucks in the operating oil accumulated in the first accumulator  61 . In this case, since the electromagnetic switching valve  43  is switched to the communicating position, the operating oil discharged from the assist pump motor  15  is pressurized and supplied to the rod side of the boom cylinder  7   c   1 , and the boom  7  is lowered with strong force. 
       FIG. 2  illustrates a circuit state where the assist pump motor  15  functions as a hydraulic pump concurrently with the accumulation of pressure in the accumulator  61 . The electromagnetic switching valve  43  is at the communicating position and the electromagnetic regeneration valve  64  is switched to the blocking position, whereby the operating oil is accumulated in the accumulator  61  and the operating oil sucked from the tank  21  by the assist pump motor  15  is pressurized and supplied to the rod side of the boom cylinder  7   c   1 . 
     Moreover, when the boom raising operation (not illustrated) of raising the boom  7  is performed, the main switching valve  71  of the boom energy recovery valve  31  is switched to stop the accumulation of pressure in the first accumulator  61  and regeneration of pressure on the rod side of the boom cylinder  7   c   2 , and the operating oil supplied from the main pumps  12  and  13  to the passage  30  via the boom control valves  26  and  28  is controlled so as to flow from the passage  74  to the passage  73  by the switched main switching valve  71  and is guided from the passages  73  and  30  to the head side of both boom cylinders  7   c   1  and  7   c   2  via the drift reduction valves  77  and  83 . Moreover, operating fluid is returned from the rod side of the boom cylinders  7   c   1  and  7   c   2  to the tank  21  via the output passage  34  and the boom control valve  26 . 
     In this manner, the engine power assisting function accumulates the head-side pressure of one of the boom cylinders—the boom cylinder  7   c   1 —in the first accumulator  61  and regenerates the head-side pressure of the other one of the boom cylinders—the boom cylinder  7   c   2 —on the rod side of the boom cylinder  7   c   2 . 
     (Boom Speed Compensating Function) 
     Next, a boom speed compensating function will be described. 
     The boom speed compensating function is a function of allowing the assist pump motor  15  to consume the accumulator pressure of the first accumulator  61  to suppress a pressure increase and pressurizing the operating oil and supplying the operating oil from the assist pump motor  15  to the chamber  7   cr  on the rod side of the boom cylinder  7   c   1  in order to solve a problem that the boom lowering speed decreases when the pressure of the first accumulator  61  is high (that is, a problem that the operating speed of a stick cylinder  8   c,  a bucket cylinder  9   c,  or a swinging motor  3   m  that cooperates with the boom cylinders  7   c   1  and  7   c   2  increases). 
     In order to realize the boom speed compensating function, the electromagnetic switching valve  43  provided in the middle of the passage  45  capable of communicating with the assist pump motor  15  and the rod side of the boom cylinder  7   c   1  is switched to the communicating position during the boom lowering operation as illustrated in  FIG. 1 . In this way, the operating oil is preferentially supplied from the assist pump motor  15  to the chamber  7   cr  on the rod side of the boom cylinder  7   c   1  associated with pressure accumulation of the first accumulator  61  via the passages  45  and  34 . Moreover, the electromagnetic regeneration valve  64  provided in the middle of the passages  62  and  66  capable of communicating with the first accumulator  61  and the assist pump motor  15  is switched to the communicating position. In this way, the operating oil supplied to the first accumulator  61  is sucked by the assist pump motor  15  to thereby diminish an increase in the accumulator pressure. 
     The effect of the boom speed compensating function will be described. 
     The assist pump motor  15  performs a pumping action when the operating oil is accumulated in the first accumulator  61  via the electromagnetic switching valve  57  and when the pressurized oil is supplied to the rod side of the boom cylinder  7   c   1  as illustrated in  FIGS. 1 and 2 . 
     When the accumulator pressure of the first accumulator  61  detected by the pressure sensor  63  is low or moderate, the electromagnetic switching valve  43  is opened and the electromagnetic regeneration valve  64  is closed as illustrated in  FIG. 2 , whereby the assist pump motor  15  supplies the operating oil sucked from the tank  21  to the rod side of the boom cylinder  7   c   1 . Moreover, the potential energy of the heavy working unit  6  during the boom lowering operation is converted into hydraulic pressure pushed from the head side of the boom cylinder  7   c   1 , and the pressure is effectively accumulated in the first accumulator  61  via the drift reduction valve  77 , the main switching valve  71 , and the like. 
     When the accumulator pressure of the first accumulator  61  has reached a high pressure level, the electromagnetic regeneration valve  64  disposed between the assist pump motor  15  and the first accumulator  61  is opened as illustrated in  FIG. 1  so that the pressurized oil supplied to the first accumulator  61  from the chamber  7   ch  on the head side of the boom cylinder  7   c   1  is consumed by the assist pump motor  15  as suction oil. In this way, it is possible to suppress an increase in the accumulator pressure of the first accumulator and to secure the boom lowering speed. Moreover, it is possible to secure the ability to cooperate with other hydraulic actuators such as the swinging motor  3   m.    
     The advantageous effects of the boom speed compensating function will be described. 
     If the boom speed compensating function is not provided, when the pressure of the first accumulator  61  increased when cooperating with the boom cylinders  7   c   1  and  7   c   2  and other hydraulic actuators, a high boom cylinder rod pressure is required to lower the boom  7 . However, in the conventional open center circuit, the operating oil discharged from the main pump  12  flows into a hydraulic actuator having a lower load and the operating oil is not supplied to the rod side of the boom cylinder  7   c.  As a result, the boom  7  is not lowered. Since the boom speed compensating function is provided, it is possible to supply the operating oil discharged from the assist pump motor  15  exclusively to the rod side of the boom cylinder  7   c   1  and to perform the boom lowering operation even if the pressure of the first accumulator  61  is high to a certain extent. 
     Moreover, when the pressure of the first accumulator  61  increases, the rod-side pressure of the boom cylinder  7   c   1  increases, and thus, a problem that the boom  7  is not lowered occurs in particular in a folded posture of the working unit  6  wherein the inertial moment of the working unit  6  decreases. One solution to this problem is to return the drift reduction valves  77  and  83  and the main switching valve  71  of the boom energy recovery valve  31  to the neutral position to cut the pressure accumulation in the first accumulator  61  when the pressure of the first accumulator  61  reaches to a high pressure level. In this case, a pressure shock or an abrupt speed change occurs during the boom operation and an operability problem occurs. 
     Thus, when the pressure of the first accumulator  61  exceeds a predetermined threshold, the boom energy recovery valve  31  is not switched and the accumulation of the oil in the first accumulator  61 , having returned from the head side of the boom cylinder  7   c   1  and reached the first accumulator  61  is diminished, and the oil is consumed by the assist pump motor  15  as illustrated in  FIG. 1 . In this way, it is possible to prevent an abrupt change in the circuit switching and to suppress an abnormal pressure increase in the first accumulator  61 . Thus, the pressure on the head side of the boom cylinder  7   c   1  can be returned effectively to the intake port side of the assist pump motor  15 , which leads to energy saving. 
     (Swing Energy Recovery Function) 
       FIG. 3  illustrates a swing energy recovery function. The sequence valve  122  in which the source pressure on the inlet side rarely changes with the back pressure on the outlet side is employed in order to absorb driving energy before the accumulator pressure exceeds the setting pressure of the relief valves  114  and  115  when the rotation of the swinging motor  3   m  is accelerated so that the accumulator pressure does not exceed the relief setting pressure and to absorb the braking energy discharged outside from the passages  112  and  113  of the motor driving circuit C when the rotation stops so that the braking energy is accumulated in the second accumulator  125  as hydraulic pressure energy. The operating oil leaking from the sequence valve  122  when rotation accelerates and decelerates is recovered and accumulated in the second accumulator  125 . 
     Further, in order to reduce energy loss as much as possible, the electromagnetic switching valve  127  that opens and closes the passage  129  between the first and second accumulators  61  and  125  is provided so that the accumulator pressure is also discharged from the second accumulator  125  when the pressure discharged from the first accumulator  61  ha reached a pressure level equal to the pressure of the second accumulator  125 . 
     That is, in order to improve energy recovery efficiency and to reduce pressure drop as much as possible, the electromagnetic switching valve  127  is provided between the first and second accumulators  61  and  125  having different pressure levels. 
     Moreover, as illustrated in  FIG. 3A , the driving energy and the braking energy relieved from the relief valves  114  and  115  when the rotation of the swinging motor  3   m  is accelerated and stopped are accumulated in the second accumulator  115  which takes out the energy and converts the same into pressure before the relief valves  114  and  115  acts, whereby the relieved swing energy is recovered. In this case, the electromagnetic switching valve  127  is closed and the operating oil leaking from the sequence valve  122  during acceleration and deceleration is recovered and accumulated in the second accumulator  125 . 
     Although not illustrated in the drawings, since vacuum may occur on the upstream side of the swinging motor  3   m,  the electromagnetic unload valve  49  is opened from the start point of a swinging operation to detect an amount of arm operation and an operation speed of a swing operation lever, the swash angle of the assist pump motor  15  is controlled according to the detection values, and an amount of oil corresponding to the operation amount and the operation speed of the swing operation lever is supplied from the assist pump motor  15  to a passage in the motor driving circuit C where there is a possibility of the occurrence of vacuum via the electromagnetic unload valve  49 , the tank passages  50  and  51 , and the makeup passage  116 . 
     Moreover, as illustrated in  FIG. 3B , the electromagnetic switching valve  127  is opened and the operating oil pressure accumulated in the second accumulator  125  is discharged and supplied to the passage  62  of the first accumulator  61 . 
       FIG. 3B  illustrates an example in which the assist pump motor  15  is driven as a pump, the electromagnetic switching valve  57  is opened to supply the operating oil sucked up from the tank  21  to the first accumulator  61 , and the hydraulic pressure obtained by the assist pump motor  15 , the first accumulator  61 , and the second accumulator  225  is used as a pilot pressure. 
     (Pilot Backup Function) 
     That is,  FIG. 3B  illustrates a pilot backup function realized by the assist pump motor  15  and the accumulators  61  and  125 . A pilot backup circuit is formed such that a pressure reducing valve  135  is connected to the passage  134  drawn from the accumulator passage  62  to which pressurized oil is supplied from the assist pump motor  15  and the accumulators  61  and  125 , the pilot circuit  138  is connected to the pressure reducing valve  135  via a filter  136  and a filter protecting spring check valve  137 , and the pilot pressure is supplied to the pilot circuit  138 . 
     The pilot circuit  138  is a circuit that pilot-operates the main control valves  26 ,  28 , and  111 , the pilot valves  79  and  85 , and the like, for example, and supplies a predetermined pilot pressure set to the pressure reducing valve  135  to the pilot circuit  138  as a pilot source pressure. 
     The pressurized pilot oil is based on the supply from the first and second accumulators  61  and  125  and is supplemented by the operating oil supplied from the assist pump motor  15  as illustrated in  FIG. 3B  when the pressure sensors  63  and  126  detect a decrease in the pressure energy accumulated in the accumulators  61  and  125 . 
     After the engine starts, the accumulation of pressure in the first accumulator  61  is performed immediately by the assist pump motor  15 , and predetermined pressure set to the pressure reducing valve  135  is also supplied to the pilot circuit  138  via the pressure reducing valve  135 . 
     Since this pilot backup function eliminates the need of the conventional pilot pump, it is possible to suppress the cost. 
     Next, the advantageous effects of the embodiment will be described. 
     As illustrated in  FIG. 1 , when the pressure accumulation of the first accumulator  61  that accumulates the operating oil pushed from one chamber  7   ch  of the boom cylinder  7   c   1  progresses so that the accumulator pressure has increased and the lowering speed of the boom cylinders  7   c   1  and  7   c   2  has decreased, the operating oil supplied to the accumulator  61  is consumed by the assist pump motor  15  to thereby suppress an increase in the accumulator pressure as illustrated in  FIG. 1 . Thus, it is possible to diminish a decrease in the speed of the boom cylinders  7   c   1  and  7   c   2  without switching a circuit and to prevent the occurrence of a shock during the switching, which may occur when a circuit is switched to cope with the decrease in the speed of the boom cylinders  7   c   1  and  7   c   2 . 
     As illustrated in  FIG. 2 , since the assist pump motor  15  supplies the operating oil to the chamber  7   cr  on the rod side of the boom cylinder  7   c   1 , it is possible to reduce the amount of operating oil supplied from the main pumps  12  and  13 . Thus, it is possible to suppress an adverse effect on other hydraulic actuators such as the swinging motor  3   m  that shares the main pumps  12  and  13  and to secure the ability to cooperate with the other hydraulic actuators. 
     As illustrated in  FIGS. 3A and 3B , since the pressure reducing valve  135  reduces the hydraulic pressure supplied from at least one of the assist pump motor  15  and the accumulator  61  and uses the same as pilot source pressure, it is possible to eliminate the use of the conventional pilot pump. 
     The operating oil supplied to the first accumulator  61  mounted on the excavator HE is consumed by the assist pump motor  15  to suppress an increase in the accumulator pressure. Thus, it is possible to diminish the decrease in the speed of the working unit  6  and to prevent the occurrence of a shock during the switching, which may occur when a circuit is switched to cope with the decrease in the speed of the working unit  6 . 
     That is, since a shock occurs during the switching if a circuit is switched to cope with the decrease in the boom lowering speed, by decreasing the amount of the operating oil pushed from the boom cylinder  7   c   1  and accumulated in the first accumulator  61  so that the operating oil is consumed by the assist pump motor  15 , it is possible to prevent a decrease in the boom lowering speed and to suppress energy loss. 
     INDUSTRIAL APPLICABILITY 
     The present invention is industrially applicable to business operators associated with manufacturing and selling hydraulic pressure circuits or working machines. 
     EXPLANATION OF REFERENCE NUMERALS 
     HE: Excavator as working machine 
       1 : Vehicle body 
       6 : Working unit 
       7 : Boom 
       7   c   1 : Boom cylinder as hydraulic pressure cylinder 
       7   cp:  Piston 
       7   ch:  One chamber 
       7   cr:  The other chamber 
       11 : Engine 
       12 ,  13 : Main pump 
       15 : Assist pump motor as assist pump 
       61 : Accumulator 
       135 : Pressure reducing valve 
       138 : Pilot circuit