Patent Publication Number: US-11384507-B2

Title: Turn-driving apparatus for work machine

Description:
TECHNICAL FIELD 
     The present invention relates to a turning drive apparatus provided in a work machine such as a hydraulic excavator. 
     BACKGROUND ART 
     On a work machine equipped with a turning body is mounted a turning drive apparatus for turning the turning body. For example, a hydraulic excavator is provided with a drive apparatus for hydraulically turning the upper turning body, the drive apparatus including a hydraulic pump for discharging hydraulic fluid, and a hydraulic motor (turning motor) that is operated to turn the upper turning body by supply of the operating fluid to the hydraulic motor. For such a turning drive apparatus, it is important to efficiently turn the upper turning body having a large moment of inertia. 
     For example, Patent Literature 1 discloses a drive apparatus capable of performing relief cut control for reducing a relief loss to improve drive efficiency. The relief cut control is a control of operating the capacity of a variable displacement hydraulic pump so as to secure a flow rate necessary for turning the turning body while minimizing a relief flow rate which is a flow rate of hydraulic fluid flowing through the relief valve. Specifically, the relief cut control includes calculating the sum of a minimum relief flow rate and a turning-speed flow rate, as a target pump flow rate, and determining the pump capacity of the hydraulic pump for providing the pump flow rate equal to the target pump flow rate. The minimum relief flow rate is a minimum required relief flow rate to secure the relief pressure necessary for driving the turning body, and the turning-speed flow rate is a flow rate of hydraulic fluid actually flowing through the turning motor that is turning the turning body, the flow rate corresponding to the turning speed. 
     An apparatus that performs the above-described relief cut control has a problem of difficulty in securing turning acceleration enough to satisfy the operator&#39;s requirement at the start of the turning because of the characteristics of the variable displacement hydraulic pump. Specifically, the variable displacement hydraulic pump has such a characteristic that the higher the pump capacity (for example, the larger the tilt angle), the higher the volumetric efficiency ηv is obtained, whereas the relief cut control restricts the pump capacity at the time of turning start with a turning speed of 0 or extremely low to a capacity corresponding to a minimum required pump flow rate or a small capacity close thereto, which hinders high volumetric efficiency from being provided at the time of turning start. This involves the disadvantage of taking time to raise the actual pump pressure to the pressure necessary for activating the rotating body. 
     The volumetric efficiency ηv is the ratio of the actual discharge flow rate Q to the theoretical discharge flow rate Qth of the hydraulic pump (ηv=Q)/Qth), and the theoretical discharge flow rate Qth is represented by the product of the displacement volume V corresponding to the set tilt angle and the number of pump revolutions (for example, the number of engine revolutions) N (Qth=V×N). The difference between the theoretical discharge flow rate Qth and the actual discharge flow rate Q corresponds to a loss due to internal leakage of the pump. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP-A 2016-31125 
     SUMMARY OF INVENTION 
     An object of the present invention is to provide a turning drive apparatus for hydraulically turning a turning body included in a work machine, the apparatus being capable of securing high acceleration performance at the start of the turning with a reduced relief loss. 
     Provided is a turning drive apparatus installed in a work machine, which includes a machine body, a turning body turnably mounted on the machine body, and an engine that generates a power for driving the turning body, to hydraulically turn the turning body, the apparatus including: a variable displacement hydraulic pump that is driven by the engine to discharge hydraulic fluid; a turning motor composed of a hydraulic motor that is operated by supply of hydraulic fluid from the hydraulic pump to the hydraulic motor to turn the turning body; a turning control device operated by application of a turning command operation to the turning control device to allow hydraulic fluid to be supplied from the hydraulic pump to the turning motor to turn the turning body; a relief valve provided in a relief flow path for releasing hydraulic fluid discharged from the hydraulic pump to a tank and configured to open so as to restrict a pump pressure, which is a pressure of hydraulic fluid supplied to the turning motor, to a preset pressure or less; a turning speed detector that detects a turning speed of the turning body; and a flow rate control device that changes a pump capacity, which is a capacity of the hydraulic pump, when the turning command operation is applied to the turning control device, to thereby control a pump flow rate, which is a flow rate of the hydraulic fluid discharged from the hydraulic pump. The flow rate control device includes: a turning-speed flow rate calculation part that calculates a turning-speed flow rate, which is a flow rate of hydraulic fluid to be made to flow through the turning motor in accordance with the turning speed detected by the turning speed detector when the turning body is turned; a relief-cut-control target flow rate calculation part that calculates a relief-cut-control target pump flow rate which is a target value of the pump flow rate, on the basis of a sum of the turning-speed flow rate and a minimum relief flow rate, which is a relief flow rate of the hydraulic fluid flowing through the relief valve and a minimum flow rate necessary for securing the pump pressure necessary for opening the relief valve to start the turning body; and a pump capacity command part that inputs a pump capacity command for changing the pump capacity so as to provide the relief-cut-control target pump flow rate that is calculated by the relief-cut-control target pump flow rate calculation part. The relief-cut-control target pump flow rate calculation part and the pump capacity command part are configured to make the pump capacity larger than the pump capacity corresponding to the sum of the minimum relief flow rate and the turning-speed flow rate at a time of turning start in which the turning command operation is applied to the turning control device and the turning speed is lower than a setting turning speed that is preset. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view of a hydraulic excavator that is a work machine according to an embodiment of the present invention; 
         FIG. 2  is a circuit diagram showing a turning drive apparatus installed in the hydraulic excavator; 
         FIG. 3  is a block diagram showing a functional configuration of a controller included in the turning drive apparatus; and 
         FIG. 4  is a flowchart showing an arithmetic control operation executed by the controller shown in  FIG. 3 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     There will be described a preferred embodiment of the present invention with reference to the drawings. 
       FIG. 1  shows a hydraulic excavator as a work machine according to an embodiment of the present invention. The hydraulic excavator includes: a lower traveling body  1  which is a machine body; an upper turning body which is a turning body mounted on the lower traveling body  1  so as to be turnable around a turning axis X; and a work attachment  3  mounted on the upper turning body  2 . 
     The work attachment  3  includes a boom  4 , an arm  5 , a bucket  6 , and a plurality of extendable hydraulic cylinders, namely, a boom cylinder  7 , an arm cylinder  8 , and a bucket cylinder  9 . The boom  4  has a proximal end to be connected to the upper turning body  2  rotatably in a derricking direction and a distal end opposite to the proximal end. The arm  5  has a proximal end rotatably connected to the distal end of the boom  4  and a distal end opposite to the proximal end, the bucket  6  rotatably attached to the distal end of the arm  5 . The boom cylinder  7  is interposed between the boom  4  and the upper turning body  2  so as to cause the boom  4  to be derricked by the expansion and contraction motions thereof. Similarly, the arm cylinder  8  is interposed between the boom  4  and the arm  5  so as to cause the arm  5  to be rotationally moved by the expansion and contraction motions thereof, and the bucket cylinder  9  is interposed between the arm  5  and the bucket  6  so as to cause the bucket  6  to be rotationally moved by the expansion and contraction motions thereof. 
       FIG. 2  is a circuit diagram showing a turning drive apparatus according to the present embodiment. The turning drive apparatus is an apparatus for hydraulically turning the upper turning body  2  to the lower traveling body  1  by use of an engine  10  mounted on the hydraulic excavator as a power source, the apparatus including a hydraulic pump  20 , a turning motor  30 , a turning control device  40 , a relief valve  50 , a plurality of sensors, and a controller  60 . 
     The hydraulic pump  20  is connected to the output shaft of the engine  10  and driven by the engine  10  to thereby suck and discharge hydraulic fluid in the tank. The hydraulic pump  20  is a variable displacement type. Specifically, the hydraulic pump  20  includes a pump body configured to have an adjustable capacity and a pump regulator  22  attached thereto. The pump regulator  22  is operated by input of a pump capacity command from the controller  60  so as to change a pump capacity, which is the capacity of the pump body. The pump capacity command is a signal that specifies a target pump capacity qpt, and the pump regulator  22  operates the pump body to adjust the actual pump capacity to the target pump capacity qpt. 
     The turning motor  30  is a hydraulic motor which is operated by supply of hydraulic fluid from the hydraulic pump  20  to the hydraulic motor to turn the turning body. Specifically, the turning motor  30  includes an output shaft connected to the upper turning body  2  and a motor body which is operated by supply of operating fluid to the motor body to rotate the output shaft. The turning motor  30  has a right turning port  32 A and a left turning port  32 B. The turning motor  30  is configured to be operated by supply of hydraulic fluid to the right turning port  32 A so as to discharge hydraulic fluid through the left turning port  32 B while turning the upper turning body  2  rightward and, conversely, to be operated by supply of hydraulic fluid to the left turning port  32 B so as to discharge hydraulic fluid through the right turning port  32 A while turning the upper turning body  2  leftward. The turning motor  30  turns the upper turning body  2  at the speed corresponding to the flow rate of hydraulic fluid flowing through the turning motor  30 . 
     The turning control device  40  is operated by a turning command operation that is applied to the turning control device  40  by an operator to allow hydraulic fluid to be supplied from the hydraulic pump to the turning motor to turn the turning body. The turning control device  40  according to the present embodiment includes a turning control valve  42  and a turning operation valve  43 . 
     The turning control valve  42  is interposed between the hydraulic pump  20  and the turning motor  30  and operated so as to switch the direction in which hydraulic fluid is supplied from the hydraulic pump  20  to the turning motor  30  and so as to change the flow rate of the hydraulic fluid. 
     The turning control valve  42  shown in  FIG. 2  is composed of a pilot-operated directional selector valve having a right turning pilot port  42   a  and a left turning pilot port  42   b . With no input of pilot pressure to any of the right turning and the left turning pilot ports  42   a  and  42   b , the turning control valve  42  is kept in a neutral state (the central position in  FIG. 2 ) to block the communication between the hydraulic pump  20  and the turning motor  30 . By input of the pilot pressure (right turning pilot pressure) to the right turning pilot port  42   a , the turning control valve  42  is opened so as to shift from the neutral state to a right turning state (the left position in  FIG. 2 ) by the stroke corresponding to the magnitude of the pilot pressure. Specifically, the turning control valve  42  is opened so as to allow the hydraulic fluid discharged from the hydraulic pump  20  to be supplied to the right turn port  32 A of the turning motor  30  at the flow rate corresponding to the magnitude of the pilot pressure. By input of the pilot pressure (left turning pilot pressure) to the left turning pilot port  42   b , conversely, the turning control valve  42  is opened so as to shift from the neutral state to a left turning state (the right position in  FIG. 2 ) by the stroke corresponding to the magnitude of the pilot pressure. Specifically, the turning control valve  42  is opened so as to allow the hydraulic fluid discharged from the hydraulic pump  20  to be supplied to the left turn port  32 B of the turning motor  30  at the flow rate corresponding to the magnitude of the pilot pressure. 
     The turning operation valve  43  constitutes a turning operation device that is operated by the turning command operation applied thereto to thereby apply a pilot pressure corresponding to the turning command operation to the turning control valve  42  to operate it. Specifically, the turning operation valve  43  includes a turning operation lever  45  and a turning pilot valve  46 . 
     The turning operation lever  45  is an operation member provided in an operation room included in the upper turning body  2 . The turning operation lever  45  allows a turning command operation, for example, an operation for tilting the turning operation lever  45 , to be applied to the turning operation lever  45  by an operator, being connected to the turning pilot valve  46  so as to cause the turning pilot valve  46  to be opened in conjunction with the tilt of the turning operation lever  45 . 
     The turning pilot valve  46  is interposed between a not-graphically-shown pilot hydraulic source (for example, a pilot pump that is driven by the engine  10 ) and the right turning and left turning pilot ports  42   a  and  42   b  of the turning control valve  42 , and opened in response to the turning command operation applied to the turning operation lever  45  to thereby allow the pilot pressure to be supplied to one of the right turning and the left turning pilot ports  42   a  and  42   b  from the pilot hydraulic source. Specifically, by the turning command operation applied to the turning operation lever  45 , the turning pilot valve  46  is opened so as to allow the pilot pressure corresponding to the magnitude of the turning command operation to be supplied to the pilot port corresponding to the direction of the turning command operation out of the right turning and the left turning pilot ports  42   a  and  42   b.    
     The relief valve  50  is provided in a relief flow path  52  and operated so as to open and close the relief flow path  52 . The relief flow path  52  is a flow path providing direct connection between the pump line and the tank line to let the hydraulic fluid discharged from the hydraulic pump  20  released to the tank so as to bypass the turning control valve  42 . The relief valve  50  is opened so as to restrict the pump pressure Pp, which is the pressure of hydraulic fluid discharged from the hydraulic pump  20 , to a relief setting pressure Prf that is preset or less. Specifically, the relief valve  50  is opened to the maximum opening when the primary pressure thereof (i.e., the pump pressure Pp) becomes equal to or higher than the relief setting pressure Prf to open the relief flow path  52  at the maximum opening area, thereby inhibiting the rise of the pump pressure Pp beyond the relief setting pressure Prf. 
     The controller  60 , which is, for example, composed of a microcomputer having an arithmetic control function, serves as a flow control device according to the present invention. Specifically, the controller  60  has a function of changing a pump capacity qp, which is the capacity of the hydraulic pump  20 , in response to the application of the turning command operation to the turning operation valve  43  to thereby control a pump flow rate Qp which is the flow rate of hydraulic fluid discharged from the hydraulic pump  20 . 
     The plurality of sensors, which are disposed to input information for enabling the controller  60  to execute the flow control to the controller  60 , includes an engine speed sensor  14 , a pump pressure sensor  24 , a turning speed sensor  34 , a right turning pilot pressure sensor  44   a , and a left turning pilot pressure sensor  44   b . The engine speed sensor  14  detects the number of engine revolutions Ne corresponding to the rotational speed of the engine  10 . The pump pressure sensor  24  is a pressure sensor that detects the pump pressure Pp. The turning speed sensor  34  is a turning speed detector that detects the turning speed SL of the upper turning body  2  driven by the turning motor  30 . The right turning and left turning pilot pressure sensors  44 A and  44 B are pressure sensors that detect the right turning pilot pressure Psa and the left turning pilot pressure Psb applied to the turning control valve  42  from the turning operation valve  43 , respectively (in other words, detect the direction and magnitude of the turning command operation applied to the turning operation valve  43 ). Each of the sensors  14 ,  24 ,  34 ,  44 A and  44 B generates a detection signal, which is an electric signal corresponding to the physical quantity to be detected, and inputs the detection signal to the controller  60 . 
     As functions for controlling the pump flow rate Qp, the controller  60  includes, as shown in  FIG. 3 , a turning-speed flow rate calculation part  62 , a relief-cut-control target pump flow rate calculation part  63 , which is referred to as “RCC target pump flow rate calculation part  63 ” in the below description and  FIG. 3 , a positive-control target pump flow rate calculation part  64 , which is referred to as “PC target pump flow rate calculation part  64 ”, in the below description and  FIG. 3 , a horsepower-control target pump flow rate calculation part  65 , which is referred to as “HC target pump flow rate calculation part  65 ” in the below description and  FIG. 3 , and a pump capacity command part  66 . There will be described the arithmetic control operations executed by them with additional reference to the flowchart shown in  FIG. 4 . 
     When the turning command operation is applied to the turning operation valve  43  (YES in step S 1 ), the turning-speed flow rate calculation part  62  calculates a turning-speed flow rate Qs 1  (step S 2 ), and the RCC target pump flow rate calculation part  63  calculates a relief-cut-control target pump flow rate Qc 1  (referred to as “RCC target pump flow rate Qc 1 ” in the below description and  FIG. 4 ) based on the turning-speed flow rate Qs 1  (Step S 3   a ). In parallel with this, the PC target pump flow rate calculation part  64  calculates a positive-control target pump flow rate Qct (referred to as “PC target pump flow rate Qc 2 ” in the below description and  FIG. 4 ) (Step S 3   b ), and the HC target pump flow rate calculation part  65  calculates a horsepower-control target pump flow rate Qc 3  (referred to as “HC target pump flow rate Qc 3 ” in the below description and  FIG. 4 ) (Step S 3 C). 
     The turning-speed flow rate Qs 1  calculated in the step S 2  is the flow rate of hydraulic fluid to be made to flow through the turning motor  30  during the turning of the upper turning body  2  in response to the turning speed SL detected by the turning speed sensor  34 . The turning-speed flow rate calculation part  62  calculates the product of the turning speed SL and the motor capacity qm of the turning motor  30  as the turning-speed flow rate Qs 1  (Qs 1 =SL×QM). 
     The RCC target pump flow rate Qc 1  calculated in step S 3   a  is the target pump flow rate calculated for execution of the relief cut control. The relief cut control is a control of operating the pump capacity qp of the hydraulic pump  20  so as to secure a flow rate necessary for turning the upper turning body  2  while minimizing a relief flow rate which is the flow rate of hydraulic fluid flowing through the relief valve  50 . Accordingly, the RCC target pump flow rate Qct is basically calculated based on the sum of the minimum relief flow rate Qrf and the turning-speed flow rate Qs 1 , the minimum relief flow rate Qrf being the minimum required relief flow rate to secure the pump pressure Pp necessary for opening the relief valve  50  to start the upper turning body  2 . 
     However, the ratio of the actual discharge flow rate Q to the theoretical discharge flow rate Qth of the hydraulic pump, namely, the volumetric efficiency ηv of the hydraulic pump  20 , is decreased with a decrease in the pump capacity; therefore, if the RCC target pump flow rate Qc 1  is restricted to a flow rate substantially equal to the minimum relief flow rate Qrf at the time of turning start in which the turning speed SL is extremely low, it takes time to increase the actual pump pressure Pp to the pressure necessary for starting the turning body, in spite of application of a large turning command operation by an operator, because high volumetric efficiency ηv cannot be obtained, which prevents the acceleration required by the operator from being satisfied. 
     In view of this, the RCC target pump flow rate calculation part according to the present embodiment is configured to set the turning start flow rate Qst with a positive value (&gt;0) only at the time of turning start, specifically, only when a turning operation command is applied to the turning operation valve  43  (YES in step S 1 ) and the turning speed SL detected by the turning speed sensor  34  is less than a predetermined setting turning speed SLo, as shown in step S 3   a  of  FIG. 4 , and configured to calculate a value obtained by adding the turning start flow rate Qst to the sum of the minimum relief flow rate Qrf and the turning-speed flow rate Qs 1 , as the relief-cut-control target pump flow rate Qc 1 . 
     This calculation operation of the RCC target pump flow rate Qc 1  only has to increase the RCC target pump flow rate Qc 1  by the amount of the turning start flow rate Qst only at the time of turning start and set the RCC target pump flow rate Qc 1  to the sum of the minimum relief flow rate Qrf and the turning-speed flow rate Qs 1  at the time of normal turning with the turning speed SL equal to or higher than the setting turning speed SLo, while the calculation procedure for obtaining the result is not limited. The calculation of the RCC target pump flow rate Qc 1  at the time of normal turning may be achieved, for example, by either setting the turning start flow rate Qst only at the time of turning start to make it included in the RCC target pump flow rate Qc 1  or making the RCC target pump flow rate Qc 1  always include the turning start flow rate Qst while setting the turning start flow rate Qst to 0 at the time of normal turning (SL≥SLo). 
     As shown in step S 3   a  in  FIG. 4 , the RCC target pump flow rate calculation part  63  according to the present embodiment sets the turning start flow rate Qst so as to decrease the turning start flow rate Qst with an increase in the turning speed SL. This makes it possible to set a large pump capacity qp, when the turning speed SL is low even at the time of turning start to thereby make the large pump capacity qp large and secure a high volumetric efficiency ηv corresponding thereto, and makes it possible to reduce the RCC target pump flow rate Qc 1  with an increase in the turning speed SL, which lowers the requirement for acceleration, thereby increasing the priority of reducing the relief loss. 
     More specifically, the RCC target pump flow rate calculation part  63  according to the present embodiment sets the turning start flow rate Qst so as to continuously decrease the turning start flow rate Qst to zero with an increase in the turning speed SL to the setting turning speed SLo. This prevents the pump capacity qp from being abruptly changed by an increase in the turning speed SL across the setting turning speed SLo, thereby allowing smoother turning drive to be performed. 
     The turning start flow rate Qst may be either calculated based on a pre-prepared calculation formula with respect to the relationship between the turning speed SL and the turning start flow rate Qst or determined by use of a pre-prepared map with respect to the relationship. Alternatively, it is also possible to always keep the turning start flow rate Qst at the time of starting turning a constant value. 
     The PC target pump flow rate Qc 2  calculated in step S 3   b  is a target pump flow rate that is calculated for execution of a positive control, which is a control of increasing the pump capacity qp with an increase in the turning command operation. Specifically, the PC target pump flow rate calculation part  64  calculates the PC target pump flow rate Qc 2  based on a pilot pressure corresponding to the turning command operation, that is, a larger pilot pressure out of the right turning and the left turning pilot pressures Psa and Psb, by use of an arithmetic expression or a map prepared in advance with respect to the relationship between the pilot pressure and the PC target pump flow rate Qc 2 , that is, a characteristic in which the PC target pump flow rate Qc 2  is increased with an increase in the turning pilot pressure Psa or Psb as shown in step S 3   b  in  FIG. 4 . 
     The HC target pump flow rate Qc 3  calculated in step S 3   c  is a target pump flow rate that is calculated for execution of a horsepower control, which is a control of limiting the pump flow rate Qp so as to keep the product of the pump pressure Pp and the pump flow rate Qp under a horsepower curve that is determined on the basis of the capacity of the engine  10 . The HC target pump flow rate calculation part  65  calculates the HC target pump flow rate Qc 3  based on a curve preset with respect to the relationship between the pump pressure Pp and the HC target pump flow rate Qc 3  (for example, a curve that is shown in step S 3   c  in  FIG. 4  and corresponds to the horsepower curve). 
     After the calculation of the target pump flow rates Qc 1 , Qc 2 , and Qc 3 , the pump capacity command part  66  of the controller  60  selects the lowest one of the target pump flow rates Qc 1 , Qc 2 , and Qc 3  and sets the selected one to the final target pump flow rate Qpt (step S 4 ). In other words, the final target pump flow rate Qpt is determined with priority to the lowest one of the target pump flow rates Qc 1 , Qc 2 , and Qc 3 . Furthermore, the pump capacity command part  66  calculates a value obtained by dividing the thus determined final target pump flow rate Qpt by the number of engine revolutions Ne detected by the engine speed sensor  14  as the target pump capacity qpt, then generating a pump capacity command for bringing the actual pump capacity qp close to the target pump capacity qpt and inputting the pump capacity command to the pump regulator  22  of the hydraulic pump  20  (step S 5 ). 
     Thus can be performed such a pump flow rate control as to bring the pump flow rate Qp of the hydraulic pump  20  close to the final target pump flow rate Qpt. This enables the pump capacity command part  66 , in the case where a large turning command operation (i.e., an operation requiring to start turning of the upper turning body  2  with high acceleration) is applied to the turning operation valve  43 , while the upper turning body  2  is stopped, to give priority to the RCC target pump flow rate Qc 1  for the determination of the final target pump flow rate Qpt, to make the final target pump flow rate Qpt larger than the sum of the minimum relief flow rate Qrf and the turning-speed flow rate Qs 1  by the turning start flow rate Qst (in other words, to make the actual relief flow rate larger than the minimum relief flow rate Qrf) to thereby execute the pump flow rate control that satisfies the above acceleration requirement while basically executing the relief cut control. 
     In contrast, in the case where a small turning command operation is applied to the turning operation valve  43  to give priority to the PC target pump flow rate Qc 2  for the determination of the final target pump flow rate Qpt, that is, in the case where high acceleration is not required, the pump capacity command part  66  can maximumly reduce the relief loss by restricting the final target pump flow rate Qpt to a low flow rate corresponding to the turning command operation. 
     Besides, in the case where the HC target pump flow rate Qc 3  is lower than either of the target pump flow rates Qc 1  and Qc 2 , the pump capacity command part  66  gives priority to the HC target pump flow rate Qc 3 , whichever the RCC target pump flow rate Qc 1  or the PC target pump flow rate Qc 2  is lower, thereby preventing inconvenience such as engine stop due to excessive horsepower requirement. 
     The present invention is not limited to the above-described embodiment. The present invention also encompasses, for example, the following modes. 
     (A) Setting of Turning Start Flow Rate Qst 
     In the case where the turning start flow rate Qst is set for calculating the RCC target pump flow rate in the present invention, the value thereof can be suitably set in consideration of the characteristics (especially volume efficiency) of the hydraulic pump. Besides, the setting turning speed SLo which corresponds to the upper limit turning speed at the time of turning start can also be freely set according to the preference of an operator or the characteristics of the work machine (the moment of inertia of the upper turning body  2 , the characteristics of the hydraulic pump, the hydraulic motor, etc.). 
     Although the value of the RCC target pump flow rate Qc 1  at the time of turning start in the example shown in  FIG. 4  is set to a large one enough to be larger than the value at the setting turning speed SLo, the turning start flow rate Qst may be set so that the value of the RCC target pump flow rate Qc 1  at the time of the turning start is approximately equal to the value at the setting turning speed SLo (for example, a constant value) or kept smaller than the value. 
     Furthermore, the effect of securing sufficient acceleration at the time of turning start can be achieved also by means other than the setting of the turning start flow rate Qst. For example, at the time of turning start, not making the turning start flow rate Qst included in the RCC target pump flow rate Qc 1  but adding a preset correction amount to the target pump capacity qpt calculated on the basis of the RCC target pump flow rate Qc 1  also makes it possible to secure high acceleration performance at the time of turning start. 
     (B) Hydraulic Pump 
     The hydraulic pump according to the present invention may be one that is not dedicated to the turning motor but used also for driving other hydraulic actuators. Also in this case, giving priority to the relief cut control at least at the time of turning start enables the effect of the present invention to be provided. 
     (C) Turning Control Device 
     The turning control device according to the present invention is not limited to the combination of the turning control valve  42  and the turning operation valve  43 . The turning control device can also be constituted by, for example, a solenoid valve interposed between the pilot hydraulic source and the pilot ports  42   a  and  42   b  of the turning control valve  42  to change pilot pressure, an electric lever device allowing a turning command operation to be applied thereto and generating a turning command signal which is an electric signal corresponding to the turning command operation, and a pilot pressure operation unit that inputs a pilot pressure command signal to the solenoid valve so as to cause a pilot pressure corresponding to the turning command signal to be input to the pilot ports  42   a  and  42   b.    
     (D) Pump Flow Rate Control Other than Relief Cut Control 
     The present invention can be broadly applied to any case where the pump flow control to be executed includes at least a relief cut control. The present invention encompasses, for example, a mode where only the relief cut control is executed while no positive control or no horsepower control is executed, and a mode where another control is executed with the relief cut control instead of or in addition to the positive control and the horsepower control. In a mode, such as the latter mode, where a plurality of controls are executed, including at least the relief cut control and the positive control, generating a pump capacity command with priority to a lower target pump flow rate out of the relief-cut-control target pump flow rate and the positive control target pump flow rate enables both securing high acceleration performance at the time of start of turning and reducing a relief loss to be achieved. “Giving priority to a lower target pump flow rate of the relief-cut-control target pump flow rate and the positive-control target pump flow rate” intends to define a relative relationship between the two target pump flow rates, not intending to exclude a mode of generating a pump capacity command based on a minimum target pump flow rate that is lower than either of the relief-cut-control target pump flow rate and the positive-control target flow rate (for example, the horsepower-control pump flow rate) like the above embodiment. 
     As described above, there is provided a turning drive apparatus for hydraulically turning a turning body included in a work machine, the apparatus being capable of securing high acceleration performance at the start of the turning with a reduced relief loss. 
     Provided is a turning drive apparatus installed in a work machine, which includes a machine body, a turning body turnably mounted on the machine body, and an engine that generates a power for driving the turning body, to hydraulically turn the turning body, the apparatus including: a variable displacement hydraulic pump that is driven by the engine to discharge hydraulic fluid; a turning motor composed of a hydraulic motor that is operated by supply of hydraulic fluid from the hydraulic pump to the hydraulic motor to turn the turning body; a turning control device operated by application of a turning command operation to the turning control device to allow hydraulic fluid to be supplied from the hydraulic pump to the turning motor to turn the turning body; a relief valve provided in a relief flow path for releasing hydraulic fluid discharged from the hydraulic pump to a tank and configured to open so as to restrict a pump pressure, which is a pressure of hydraulic fluid supplied to the turning motor, to a preset pressure or less; a turning speed detector that detects a turning speed of the turning body; and a flow rate control device that changes a pump capacity, which is a capacity of the hydraulic pump, when the turning command operation is applied to the turning control device, to thereby control a pump flow rate, which is a flow rate of the hydraulic fluid discharged from the hydraulic pump. The flow rate control device includes: a turning-speed flow rate calculation part that calculates a turning-speed flow rate, which is a flow rate of hydraulic fluid to be made to flow through the turning motor in accordance with the turning speed detected by the turning speed detector when the turning body is turned; a relief-cut-control target flow rate calculation part that calculates a relief-cut-control target pump flow rate, which is a target value of the pump flow rate, on the basis of a sum of the turning-speed flow rate and a minimum relief flow rate, which is a relief flow rate of the hydraulic fluid flowing through the relief valve and a minimum flow rate necessary for securing the pump pressure necessary for opening the relief valve to start the turning body; and a pump capacity command part that inputs a pump capacity command for changing the pump capacity so as to provide the relief-cut-control target pump flow rate that is calculated by the relief-cut-control target pump flow rate calculation part. The relief-cut-control target pump flow rate calculation part and the pump capacity command part are configured to make the pump capacity larger than the pump capacity corresponding to the sum of the minimum relief flow rate and the turning-speed flow rate at a time of turning start in which the turning command operation is applied to the turning control device and the turning speed is lower than a setting turning speed that is preset. 
     This turning drive apparatus, making the actual pump capacity larger than the pump capacity corresponding to the sum of the minimum relief flow rate and the motor flow rate at the time of the start of the turning to thereby increase the volumetric efficiency, that is, giving more priority to the securement of the volumetric efficiency than the reduction of the relief loss at the time of starting turning, enables high acceleration performance to be secured, while basically executing a relief cut control of securing a pump flow rate required for turning the turning body at a current turning speed with a reduced relief flow rate. 
     As a specific mode for rendering the pump capacity large, the relief-cut-control target pump flow rate calculation part is preferably configured to set a turning start flow rate for increasing the pump capacity at the time of turning start and configured to calculate the relief-cut-control target pump flow rate on the basis of a flow rate obtained by adding the turning start flow rate to the sum of the minimum relief flow rate and the turning-speed flow rate at the time of turning start. This embodiment allows the pump capacity at the time of turning start to be properly increased by a simple arithmetic operation of adding the turning start flow rate to the sum of the minimum relief flow rate and the turning-speed flow rate for the calculation of the target pump flow rate at the time of turning start. 
     More specifically, it is preferable that the turning start flow rate be set so as to be decreased with an increase in the turning speed. This makes it possible to secure a large pump capacity and a high volumetric efficiency by setting a large turning start flow rate especially when the turning speed is low even at the time of turning start, and to increase the priority of reducing the relief loss by reducing the target pump flow rate with a decrease in the requirement for acceleration due to an increase in the turning speed. 
     In this case, the turning start flow rate is preferably set so as to be continuously decreased to zero with an increase in the turning speed to the setting turning speed. This prevents the pump capacity from being abruptly changed by an increase in the turning speed across the setting turning speed, thereby enabling smoother turning drive to be performed. 
     It is preferable that the flow rate control device further includes a positive-control pump flow rate calculation part that calculates a positive-control target pump flow rate for increasing the pump capacity with an increase in the turning command operation applied to the turning control device, and that the pump capacity command part is configured to generate the pump capacity command with priority to a lower target pump flow rate of the relief-cut-control target pump flow rate and the positive control target pump flow rate. This configuration, reducing the pump capacity to give priority to the positive-control target pump flow rate, allows the reduction of the relief loss to be prioritized, when the turning command operation applied to the turning control device is small, that is, when high acceleration is not required.