Abstract:
The present invention provides a less expensive driver having a common electric motor for individually driving a fluid pressure pump and a transport mechanism. The inventive driver ( 1 ) comprises a hydraulic cylinder ( 8 ), a hydraulic pump ( 6 ) for supplying a working fluid to the hydraulic cylinder ( 8 ), and a transport mechanism ( 3 ) for driving a pivot arm ( 89 ) to pivot the pivot arm ( 89 ). The driver ( 1 ) further comprises a common electric motor ( 4 ) for driving the hydraulic pump ( 6 ) and the transport mechanism ( 3 ), and switching means ( 5 ) for connecting the electric motor ( 4 ) switchably to the hydraulic pump ( 6 ) and to the transport mechanism ( 3 ), wherein one of the hydraulic pump ( 6 ) and the transport mechanism ( 3 ) connected to the electric motor ( 4 ) is driven by the electric motor ( 4 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a driver for driving a fluid pressure pump and a transport mechanism by a common electric motor, and to an automatic pallet changer having such a driver. 
     2. Description of the Prior Art 
     Some of machine tools such as machining centers have an automatic pallet changer for exchanging a pallet placed in a machining area where a workpiece is machined and a pallet placed in a working area where a workpiece is attached to and detached from the pallet. 
     The automatic pallet changer includes a pivot arm, and a driver including a hydraulic unit and a transport mechanism. The hydraulic unit includes a hydraulic cylinder for moving up and down the pivot arm, a hydraulic pump for supplying a working fluid to the hydraulic cylinder, and an electric motor for driving the hydraulic pump. The transport mechanism includes a plurality of gears for driving the pivot arm to pivot the pivot arm, and is driven by a servo motor provided separately from the hydraulic unit. 
     In the automatic pallet changer, the hydraulic pump is driven by the electric motor to actuate the hydraulic cylinder, whereby the pivot arm is moved up. Thus, the pallets respectively placed in the machining area and in the working area are received and retained by the pivot arm. In turn, the transport mechanism is driven by the servo motor to pivot the pivot arm, whereby one of the pallets is transported from the machining area to the working area and the other pallet is transported from the working area to the machining area. When the other pallet is located above a table in the machining area and the one pallet is located above a support base in the working area, the servo motor is stopped, and then the hydraulic pump is driven by the electric motor to move down the hydraulic cylinder and the pivot arm. Thus, the pallets retained on the pivot arm are respectively placed on the table in the machining area and on the support base in the working area. 
     In the conventional automatic pallet changer, however, the electric motor for actuating the hydraulic pump and the servo motor for driving the transport mechanism are separately provided. Therefore, the size of the driver and, hence, the size of the automatic pallet changer are increased, making it difficult to reduce the costs thereof. Further, the provision of the two electric motors requires separate driver circuits for controlling these electric motors, thereby preventing cost reduction. 
     It is an object of the present invention to provide a less expensive driver of a smaller size having a common electric motor for driving a fluid pressure pump and a transport mechanism, and to provide an automatic pallet changer having such a driver. 
     SUMMARY OF THE INVENTION 
     The driver according to the present invention comprises an actuator, a fluid pressure pump for supplying a working fluid to the actuator, and a transport mechanism for driving a movable body to move the movable body. The driver further comprises a common electric motor for driving the fluid pressure pump and the transport mechanism, and switching means for connecting the electric motor switchably to the fluid pressure pump and to the transport mechanism, wherein one of the fluid pressure pump and the transport mechanism connected to the electric motor is driven by the electric motor. 
     In the driver, the electric motor is connected to the fluid pressure pump by the switching means when the actuator is to be driven. By driving the fluid pressure pump by the electric motor, the working fluid is supplied to the actuator to drive the actuator. When the movable body is to be moved, the switching means switches the connection of the electric motor from the fluid pressure pump to the transport mechanism to connect the electric motor to the transport mechanism. Then, the transport mechanism is driven by the electric motor to move the movable body. In the driver, the switching means thus switches the connection of the electric motor to connect the electric motor selectively to the fluid pressure pump and to the transport mechanism, so that the fluid pressure pump and the transport mechanism can individually be driven. 
     In the driver, the fluid pressure pump and the transport mechanism are individually driven by the single common electric motor, which is switchably connected thereto by the switching means. This eliminates the need for providing separate electric motors for the fluid pressure pump and for the transport mechanism, allowing for size reduction and cost reduction of the driver. 
     The aforesaid driver is advantageously applicable to an automatic pallet changer. That is, the automatic pallet changer comprises: a pivot arm; an actuator for moving up and down the pivot arm to transfer pallets to/from the pivot arm in a machining area and in a working area; a fluid pressure pump for supplying a working fluid to the actuator; a transport mechanism for driving the pivot arm to pivot the pivot arm between the machining area and the working area; a common electric motor for driving the fluid pressure pump and the transport mechanism; and switching means for connecting the electric motor switchably to the fluid pressure pump and to the transport mechanism; wherein one of the fluid pressure pump and the transport mechanism connected to the common electric motor is driven by the common electric motor. 
     In the automatic pallet changer, the electric motor is first connected to the fluid pressure pump by the switching means for driving the fluid pressure pump. Thus, the actuator is driven to move up the pivot arm, whereby the pallets are received on the pivot arm in the machining area and in the working area. In turn, the switching means switches the connection of the electric motor from the fluid pressure pump to the transport mechanism to connect the electric motor to the transport mechanism for driving the transport mechanism. Thus, the pivot arm is driven to be pivoted, whereby one of the pallets is transported from the machining area to the working area and the other pallet is transported from the working area to the machining area. Thereafter, the electric motor is again connected to the fluid pressure pump by the switching means to drive the fluid pressure pump. Thus, the actuator is driven to move down the pivot arm, whereby the pallets are placed in the machining area and in the working area. 
     In the automatic pallet changer, the fluid pressure pump and the transport mechanism are individually driven by the single common electric motor, which is switchably connected thereto by the switching means. This eliminates the need for providing separate electric motors for the fluid pressure pump and for the transport mechanism, allowing for size reduction and cost reduction of the automatic pallet changer. 
     It is preferred to employ a stepping motor as the electric motor and to employ an electro-magnetic clutch as the switching means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view illustrating a machine tool having a driver and an automatic pallet changer according to the present invention; 
     FIG. 2 is a sectional view of the automatic pallet changer as seen in a direction A—A in FIG. 1; and 
     FIG. 3 is a block diagram illustrating a control system for the inventive automatic pallet changer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A driver and an automatic pallet changer having the driver in accordance with an embodiment of the present invention will hereinafter be described with reference to FIGS. 1 to  3 . An explanation will be given to a machine tool including the driver and the automatic pallet changer. FIG. 1 is a perspective view illustrating the machine tool. FIG. 2 is a sectional view of the automatic pallet changer as seen in a direction A—A in FIG. 1, and FIG. 3 is a block diagram illustrating a control system for the automatic pallet changer. 
     In FIG. 1, the machine tool  81  is, for example, a horizontal machining center (hereinafter referred to as “machining center  81 ”), and includes a bed  82 , a column  83 , a table  84 , a spindle head  85 , and an automatic pallet changer  86 . 
     As shown in FIG. 1, the column  83  is disposed upright on the bed  82  and guided by a linear guide so as to be reciprocally movable along the Y-axis. The table  84  is located in front of the column  83  on the bed  82 . The table  84  is guided by a linear guide so as to be reciprocally movable along the Z-axis. The table  84  supports a pallet  90  in a horizontally rotatable manner. The spindle head  85  is provided on the column  83  on the side of the table  84 . The spindle head  85  has a tool for machining a workpiece W and a main spindle for turning the tool at a high speed, and is guided by a linear guide so as to be reciprocally movable along the X-axis. 
     As shown in FIG. 1, the automatic pallet changer  86  has a partition plate  87 , which partitions a space in the machining center  81  into a machining area B where the workpiece W is machined on the table  84  and a working area C where the workpiece W is attached to and detached from the pallet. The automatic pallet changer  86  includes a pallet support mechanism  88 , a pivot arm  89 , and a driver  1  (shown in FIG.  2 ). 
     As shown in FIG. 1, the pallet support mechanism  88  is opposed to the table  84  in the working area C. The pallet support mechanism  88  has a support base  91  for supporting a pallet  90 , and the support base  91  is supported in a horizontally rotatable manner on a platform  98 . Rectangular clamping block  92  are respectively provided upright on the pallets  90  in the areas B and C. Workpieces W are fixed onto outer peripheral faces of the clamping block  92  by clamps  93 . 
     As shown in FIG. 1, the pivot arm  89  extends into the areas B and C. The pivot arm  89  includes pallet holders  94  provided on opposite sides thereof for holding the pallets  90 . The pivot arm  89  is moved up and down and pivoted by the driver  1  shown in FIG. 2 for exchanging the pallet  90  placed on the table  84  in the machining area B and the pallet  90  placed on the support base  91  in the working area C. 
     The construction of the driver  1  will more specifically be described with reference to FIG.  2 . In FIGS. 1 and 2, like components are denoted by like reference characters. 
     In FIG. 2, the driver  1  is disposed, for example, in and around the bed  82  of the machining center  81 , and includes a hydraulic unit  2 , a transport mechanism  3 , an electric motor  4 , switching means  5 , and a controller  61  (shown in FIG.  3 ). The driver  1  causes the hydraulic unit  2  to move up and down the pivot arm  89 , and causes the transport mechanism  3  to pivot the pivot arm  89 . 
     The hydraulic unit  2  includes a hydraulic pump  6 , a hydraulic circuit  7 , and an actuator  8 . The hydraulic pump  6  is a bi-directional pump capable of discharging a working fluid in two directions, and is connected to the hydraulic circuit  7 . The hydraulic pump  6  discharges the working fluid under pressure to supply the working fluid to the actuator  8  through the hydraulic circuit  7 . 
     The hydraulic circuit  7  includes main pipes  9 ,  10  connected to ports of the hydraulic pump  6 , a check valve  11 , a selector valve  12 , and a shuttle valve  13 . The main pipes  9 ,  10  are further connected to the actuator  8 . The check valve  11  is provided in the main pipe  9  to permit only passage of a stream of the working fluid flowing from the hydraulic pump  6 . The check valve  11  is connected to the main pipe  10  via a pilot pipe  15 , and adapted to be forcibly opened by a stream of the working fluid introduced through the pilot pipe  15 . The selector valve  12  is connected to the main pipes  9  and  10  via branch pipes  16  and  17 , respectively, in a position between the check valve  11  and the hydraulic pump  6 . The selector valve  12  is connected to an oil tank  18 , and has a de actuation position (a) at which communication between the branch pipes  16 ,  17  and the oil tank  18  is interrupted (the valve is closed) when the hydraulic unit  2  is deactuated. The selector valve  12  is further connected to the branch pipes  16  and  17  via pilot pipes  19  and  20 , respectively, for introduction of the working fluid into the selector valve  12 , and further has actuation positions (b) and (c) at which the communication between the branch pipe  16  and the oil tank  18  and the communication between the branch pipe  17  and the oil tank  18  are respectively allowed (the valve is opened). The shuttle valve  13  is provided in a connection pipe  21  connected to the main pipes  9 ,  10  in a position between the check valve  11  and the actuator  8 . The shuttle valve  13  introduces a higher pressure stream of the working fluid flowing through either of the main pipes  9 ,  10  into a pressure switch  14 . The pressure switch  14  detects the pressure of the working fluid introduced from the shuttle valve  13 , and outputs a detection signal to the controller  61  (shown in FIG.  3 ). 
     The actuator  8  is a hydraulic cylinder (hereinafter referred to as “hydraulic cylinder  8 ”) to be actuated by the working fluid, and is disposed in the bed  82 . The hydraulic cylinder  8  includes a piston shaft  22  and a piston  23 . The piston shaft  22  is fixedly provided upright in the bed  82 , and has a bush  24  provided at a distal end thereof. The piston  23  has a cup-like shape having an open end. The bush  24  of the piston shaft  22  is inserted into the piston  23  from the open end thereof, so that the piston  23  is fitted around the piston shaft  22  slidably with respect to the piston shaft  22 . The open end of the piston  23  is closed by a bush  25  slidably fitted around the piston shaft  22 . Thus, pressure chambers  26  and  27  are defined between the bushes  24  and  25  and between the bush  24  and the ceiling of the piston  23 , respectively, in the hydraulic cylinder  8 . The pressure chambers  26  and  27  are respectively connected to the main pipes  9  and  10  via communication channels formed in the piston shaft  22 . The hydraulic cylinder  8  supports the pivot arm  89  on the top of the piston  23  thereof. 
     In the hydraulic unit  2 , the working fluid is discharged into the main pipe  9  by driving the hydraulic pump  6 . The working fluid is introduced into the pressure chamber  27  through the check valve  11  and one of the communication channels of the piston shaft  22 , thereby moving up the piston  23  of the hydraulic cylinder  8 . Thus, the pivot arm  89  is moved up. Further, the working fluid is discharged into the main pipe  10  from the hydraulic pump  6  in the hydraulic unit  2 . The working fluid is introduced into the pressure chamber  26  through the other communication channel of the piston shaft  22  and, at the same time, introduced into the check valve  11  through the pilot pipe  15 , thereby forcibly opening the check valve  11 . Thus, the piston  23  is moved down while the working fluid is returned into the hydraulic pump  6  from the pressure chamber  27  through the main pipe  10  and the check valve  11 . Thus, the pivot arm  89  is moved down. 
     The transport mechanism  3  is a reduction gear mechanism provided in the bed  82  and includes, for example, three gear shafts  31  to  33  and a plurality of gears  34  to  38 . The respective gear shafts  31  to  33  are provided parallel to each other in the bed  82 . The gear shafts  31 ,  32  are rotatably supported by the bed  82 , and the gear shaft  33  is fixedly supported by the bed  82 . The gear shaft  31  projects below from the bed  82  to extend to the vicinity of the hydraulic pump  6 . The gear  34  is supported rotatably about the gear shaft  31  in the bed  82 , and the gears  35 ,  36  are fixedly supported around the gear shaft  32 . The gear  37  is supported rotatably about the gear shaft  33 , and the gear  38  is formed on an outer circumferential portion of the piston  23 . The gears  34  and  35 , the gears  36  and  37 , and the gears  37  and  38  are meshed with each other. 
     When the gear shaft  31  is rotated, the transport mechanism  3  transmits the rotational motion to the gear  38  of the piston  23  with the rotation speed being reduced by means of the respective gears  34  to  38 . Thus, the piston  23  of the hydraulic cylinder  8  is rotated to pivot the pivot arm  89 . 
     The electric motor  4  is, for example, a stepping motor (hereinafter referred to as “stepping motor  4 ”) whose rotation angle can be controlled with a high level of accuracy. A pulley  41  is provided around a rotation shaft of the stepping motor  4 . A transmission belt  43  is stretched between the pulley  41  and a pulley  42  fixedly supported by the gear shaft  31 . The stepping motor  4  is connected to the controller  61  (shown in FIG.  3 ). 
     The gear shaft  31  is rotated by the stepping motor  4  via the pulley  41 , the transmission belt  43  and the pulley  42  for driving the transport mechanism  3  and the hydraulic pump  6 . 
     The switching means  5  includes two electro-magnetic clutches  51 ,  52 . The electro-magnetic clutches  51 ,  52  each include a clutch disk  54 , a clutch cover  55  having a magnetization coil, and the like. The electro-magnetic clutch  51  is located adjacent the gear  34  in the bed  82 . The clutch disk  54  of the electro-magnetic clutch  51  is supported rotatably about the gear shaft  31  so as to be coupled to the gear  34 , and the clutch cover  55  is fixedly supported around the gear shaft  31 . The electro-magnetic clutch  52  is provided adjacent the hydraulic pump  6 . The clutch disk  54  of the electro-magnetic clutch  52  is fixedly supported around a drive shaft  56  of the hydraulic pump  6 , and the clutch cover  55  is fixed to a distal end of the gear shaft  31 . 
     The switching means  5  magnetizes the magnetization coil of the electro-magnetic clutch  51  to couple the clutch disk  54  to the clutch cover  55 , whereby the stepping motor  4  is connected to the transport mechanism  3 . When the electro-magnetic clutch  51  is demagnetized, the stepping motor  4  is disconnected from the transport mechanism  3 . Further, the switching means  5  magnetizes the magnetization coil of the electro-magnetic clutch  52  to couple the clutch disk  54  to the clutch cover  55 , whereby the connection of the stepping motor  4  is switched from the transport mechanism  3  to the hydraulic pump  6  to connect the stepping motor  4  to the hydraulic pump  6 . 
     As shown in FIG. 3, the controller  61  is connected to a driver circuit  62  for the stepping motor  4 . The driver circuit  62  outputs a pulse signal on the basis of a drive command of the controller  61  to control the driving of the stepping motor  4 . The controller  61  is further connected to the electro-magnetic clutches  51 ,  52  to control the magnetization and demagnetization of the respective electro-magnetic clutches  51 ,  52 . The controller  61  further controls the driving of the stepping motor  4  on the basis of the detection signal applied from the pressure switch  14 . 
     With reference to FIGS. 1 to  3 , an explanation will be given to the operations of the driver  1  and the automatic pallet changer  86  as well as to the operation of the machining center  81 . 
     In FIG. 1, the tool of the spindle head  85  is moved along the three axes (X-, Y- and Z-axes) by reciprocally moving the column  83 , the table  84  and the spindle head  85  in the machining center  81 , whereby the workpieces W on the table  84  are machined. The table  84  is horizontally rotated to locate each of the workpieces W fixed to the clamping block  92  in an opposed relation with respect to the tool for successively machining the workpieces W. In the working area C, a pallet  90  is mounted on the support base  91 , and workpieces W to be next machined are fixed to the clamping block  92  for a preparatory operation. 
     Upon completion of the machining in the machining center  81 , the automatic pallet changer  86  exchanges the pallet  90  in the machining area B and the pallet  90  in the working area C as shown in FIG.  1 . 
     The exchange of the pallets  90  is achieved by moving up and down and pivoting the pivot arm  89  by means of the driver  1  as shown in FIG.  2 . As shown in FIG. 3, the controller  61  outputs a magnetization command (electric power) to the electro-magnetic clutch  52  to magnetize the electric-magnetic clutch  52  for connection between the stepping motor  4  and the hydraulic pump  6 . The controller  61  outputs a drive command to the driver circuit  62  to drive the stepping motor  4  on the basis of a pulse signal of the driver circuit  62 . Thus, the stepping motor  4  is rotated in a regular direction, whereby the hydraulic pump  6  is driven for rotation thereof (in a normal direction) via the pulley  41 , the transmission belt  43 , the pulley  42 , the gear shaft  31  and the electro-magnetic clutch  52  as shown in FIG.  2 . At this time, the electro-magnetic clutch  51  is disconnected from the transport mechanism  3 , so that the gear shaft  31  is rotated relatively to the clutch disk  54  of the electro-magnetic clutch  51  and the gear  34 . Therefore, only the hydraulic pump  6  is driven for rotation thereof. 
     As shown in FIG. 2, the hydraulic pump  6  discharges the working fluid to the main pipe  9  under pressure. The working fluid opens the check valve  11 , and flows through the main pipe  9  thereby to be introduced into the pressure chamber  27  of the hydraulic cylinder  8 . Thus, the piston  23  of the hydraulic cylinder  8  is moved up together with the pivot arm  89 . At this time, the pivot arm  89  receives the pallets  90  (the workpieces W and the clamping block  92 ) from the table  84  in the machining area B and from the support base  91  in the working area C while being moved up. The working fluid discharged into the main pipe  9  is introduced into the selector valve  12  through the pilot pipe  19 . When the pressure of the working fluid increases to higher than a predetermined level, the selector valve  12  is switched from the de actuation position (a) to the actuation position (b) to connect the oil tank  18  to the branch pipe  17 . Thus, the working fluid contained in the oil tank  18  flows into the main pipe  10  thereby to be supplied into the hydraulic pump  6 . 
     Upon the reception of the pallets  90  on the pivot arm  89 , as shown in FIG. 3 the controller  61  cancels the drive command applied to the driver circuit  62  thereby to deactuate the stepping motor  4 . Thus, the driving of the hydraulic pump  6  is stopped, so that the supply of the working fluid into the pressure chamber  27  of the hydraulic cylinder  8  is stopped. At this time, the check valve  11  is closed, so that the piston  23  of the hydraulic cylinder  8  and the pivot arm  89  are retained at upper positions. The working fluid is also introduced into the shuttle valve  13  through the main pipe  9  and the connection pipe  21 . Thus, the pressure switch  14  detects the inside pressure of the pressure chamber  27  of the hydraulic cylinder  8 , and outputs a detection signal to the controller  61 . Upon reception of the detection signal inputted from the pressure switch  14 , the controller  61  judges that the pivot arm  89  and the like are moved down due to reduction in pressure, and rotates again the stepping motor  4  in the normal direction thereby to drive the hydraulic pump  6  for rotation thereof. Thus, the piston  23  of the hydraulic cylinder  8  is moved up together with the pivot arm  89  by the working fluid introduced into the pressure chamber  27 . 
     After the stepping motor  4  is deactuated, the controller  61  demagnetizes the electro-magnetic clutch  52  as shown in FIG. 3 thereby to disconnect the stepping motor  4  from the hydraulic pump  6 . At the same time, the controller  61  outputs a magnetization command (electric power) to the electro-magnetic clutch  51  to magnetize the electro-magnetic clutch  51 , whereby the connection of the stepping motor  4  is switched from the hydraulic pump  6  to the transport mechanism  3  to connect the stepping motor  4  to the transport mechanism  3 . Further, the controller  61  outputs a drive command to the driver circuit  62 , and drives the stepping motor  4  on the basis of a pulse signal of the driver circuit  62 . The pulse signal outputted from the driver circuit  62  causes the stepping motor  4  to rotate the pivot arm  89  by 180 degrees. Thus, as shown in FIG. 2 the stepping motor  4  drives the transport mechanism  3  for ration thereof via the pulley  41 , the transmission belt  43 , the pulley  42 , the gear shaft  31  and the electro-magnetic clutch  51 . At this time, the electro-magnetic clutch  52  is disconnected from the hydraulic pump  6 , so that the gear shaft  31  is rotated together with the electro-magnetic clutch  51  and the gear  34 . Thus, only the transport mechanism  3  is driven for rotation thereof. 
     As shown in FIG. 2, the transport mechanism  3  transmits the rotational motion from the stepping motor  4  to the piston  23  of the hydraulic cylinder  8  with the rotation speed being reduced by the gears  34  to  38 , whereby the piston  23  is rotated together with the pivot arm  89 . Thus, the pallet  90  in the working area C and the pallet  90  in the machining area B are transported into the machining area B and into the working area C, respectively, by the pivot arm  89  for exchange of the pallets  90 . The stepping motor  4  is rotated by a degree corresponding to 180-degree rotation of the pivot arm  89  and stopped on the basis of the pulse signal outputted from the driver circuit  62 . At the same time, the pivot arm  89  is rotated by 180 degrees and stopped, whereby the pallets  90  are positioned on the table  84  in the machining area B and on the support base  91  in the working area C with a higher level of accuracy. 
     As shown in FIG. 3, the controller  61  demagnetizes the electro-magnetic clutch  51  to disconnect the stepping motor  4  from the transport mechanism  3 , and outputs a magnetization command (electric power) to the electro-magnetic clutch  52 , whereby the connection of the stepping motor  4  is switched from the transport mechanism  3  to the hydraulic pump  6  to connect the stepping motor  4  to the hydraulic pump  6 . Further, the controller  61  outputs a drive command to the driver circuit  62 , and drives the stepping motor  4  on the basis of a pulse signal of the driver circuit  62 . Thus, the stepping motor  4  is rotated in a reverse direction, so that the hydraulic pump  6  is driven for rotation thereof (in a reverse direction) via the pulley  41 , the transmission belt  43 , the pulley  42 , the gear shaft  31  and the electro-magnetic clutch  52  as shown in FIG.  2 . At this time, the electro-magnetic clutch  51  is disconnected from the transport mechanism  3 , so that the gear shaft  31  is rotated relatively to the clutch disk  54  of the electro-magnetic clutch  51  and the gear  34 . Therefore, only the hydraulic pump  6  is driven for rotation thereof. 
     As shown in FIG. 2, the hydraulic pump  6  discharges the working fluid to the main pipe  10  under pressure. The working fluid flows through the main pipe  10  thereby to be introduced into the pressure chamber  26  of the hydraulic cylinder  8 . Further, the working fluid is introduced into the check valve  11  through the pilot pipe  15  thereby to forcibly open the check valve  11 . Thus, the working fluid is returned from the pressure chamber  27  of the hydraulic cylinder  8  to the main pipe  10 , whereby the piston  23  is moved down together with the pivot arm  89 . At this time, the pivot arm  89  transfers the pallets  90  onto the table  84  in the machining area B and onto the support base  91  in the working area C while being moved down. Thus, the exchange of the pallets is completed. The working fluid discharged into the main pipe  10  is introduced into the selector valve  12  through the pilot pipe  20 . When the pressure of the working fluid is increased to higher than a predetermined level, the selector valve  12  is switched from the actuation position (b) to the actuation position (c) to connect the oil tank  18  to the branch pipe  16 . Thus, the working fluid contained in the oil tank  18  flows out into the main pipe  9  thereby to be supplied to the hydraulic pump  6 . 
     When the pallet exchange is performed again, the pivot arm  89  is moved up to receive pallets  90  thereon, and rotated by 180 degrees in the reverse direction to transport the pallets  90  onto the table  84  in the machining area B and onto the support base  91  in the working area C. Then, the pivot arm  89  is moved down to place the pallets  90  in the respective areas B and C. When the pallet exchange is successively performed, the pivot arm  89  is not rotated by 360 degrees but rotated by 180 degrees in the normal and reverse directions. 
     In the driver  1  and the automatic pallet changer  86  according to the embodiment of the present invention, the connection of the common electric motor  4  is switched between the hydraulic pump  6  and the transport mechanism  3  by the switching means  5 , so that the hydraulic pump  6  and the transport mechanism  3  are individually driven by the single common electric motor  4 . This eliminates the need for providing separate electric motors for the hydraulic pump  6  and for the transport mechanism  3 , allowing for size reduction and cost reduction of the driver  1  and the automatic pallet changer  86  and, hence, allowing for size reduction and cost reduction of the machine tool. 
     The present invention is not limited to the driver  1  and the automatic pallet changer  86  shown in FIGS. 1 to  3 , but may be embodied in the following ways. 
     (1) The driver  1  is applied not only to the machine tool but also to any apparatus having a fluid pressure pump and a transport mechanism. 
     (2) The electric motor  4  is not limited to the stepping motor, but may be a servo motor or the like. 
     (3) The switching means  5  is not limited to the electro-magnetic clutches  51 ,  52 , but may be any switching means which is capable of connecting the electric motor  4  switchably to the hydraulic pump  6  and to the transport mechanism  3 . 
     (4) The transport mechanism  3  is not necessarily adapted to pivot the pivot arm  89 , but may be adapted to linearly move a movable body by a rack and a pinion. 
     (5) When the driver  1  is applied to the machine tool, the controller  61  may double as a controller for controlling the entire machine tool. 
     (6) The driver  1  is not limited to the combination of the hydraulic unit and the transport mechanism, but may be a combination of a pneumatic circuit and a transport mechanism. 
     (7) The number of the gears of the transport mechanism  3  and the construction of the hydraulic circuit  7  may be selected as desired.