Abstract:
An earth-working machine includes a mobile chassis, a turntable rotatably mounted on the mobile chassis, a carriage rotatably mounted on the turntable in eccentric relation thereto, an earth-working mechanism mounted on the carriage, a source of a fluid pressure, a pair of first and second fluid motors for rotating the turntable and the carriage, respectively, and a fluid rate synchronizer composed of a pair of third and fourth fluid motors interconnected by a common output shaft for discharging amounts of fluid under pressure at a predetermined ratio, the source being operatively coupled to the third and fourth fluid motors, the first and second fluid motors being operatively coupled to the third and fourth fluid motors, respectively, whereby the turntable and the carriage will be angularly moved about their own axes at a constant angular displacement ratio. First and second position detectors are provided for respectively detecting a first angular position of the turntable with respect to the mobile chassis and a second angular position of the carriage with respect to the turntable. An electric control circuit controls solenoid-operated valves to stop the rotation of the first and second fluid motors when the first and second angular positions are reached.

Description:
This is a continuation of application Ser. No. 302,118 filed Jan. 24, 1989, now abandoned which is a continuation of application Ser. No. 060,984, filed Jun. 10, 1987, now abandoned, which is a continuation of application Ser. No. 705,998, filed Feb. 17, 1985, also abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an earth-working machine such as an excavator for digging ditches in road construction, and more particularly to an earth-working machine having a turntable and a carriage which are driven by respective hydraulic motors so as to be controllably angularly movable independently on a self-propelled mobile chassis for enabling an excavating mechanism on the carriage to turn in various angular ranges for avoiding interference with surrounding traffic and/or objects and providing wide working areas for the excavating mechanism. 
     Conventional earth-working machines or excavators include an excavating mechanism composed of a boom or bucket arm having a bucket on its distal end for trenching a ditch in a road. In operation, the material scooped by the bucket is transferred back by turning the boom around the machine. Since the boom is angularly moved through an semicircular angule range, the boom and the bucket as they move project laterally of the machine, resulting in the danger of interfering with surrounding traffic and/or objects. Therefore, a large working radius or range clear of any obstructions should be reserved around the machine for allowing safe swinging movement of the boom. This requirement however is difficult or even impossible to meet in situations where only relativey small or limited spaces are available for the machine. 
     To eliminate such a difficulty, there has been proposed an excavator having a turntable rotatably mounted on a mobile chassis and a carriage rotatably mounted on the turntable and supporting an excavating mechanism, the turntable and the carriage having shafts positioned out of coaxial relation. With this arrangement, the bucket on the boom is allowed to move over the chassis without appreciably projecting laterally thereof when the turntable and the carriage are turned about their shafts. Therefore, unwanted interference with traffic or objects is prevented around the machine, and the excavator can be placed in relatively small spaces for road construction or other earth-moving applications. The proposed excavator are however less resistant to vibrations and susceptible to adverse environments. Another problem is that the turntable and the chassis which are driven by respective hydraulic motors tend to be brought out of synchronism when they are operated for a long time. Therefore, it has been necessary to correct the relative angular positions of the turntable and the chassis by independently operating the hydraulic motors. Such angular positional adjustments have heretofore been carried out by a manual operation, which is time-consuming, troublesome, and not reliable sufficiently. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an earth-working machine having a turntable and a carriage mounted on the turntable and supporting an excavating mechanism, and a hydraulic circuit including a fluid rate synchronizer for supplying equal amounts of fluid from a hydraulic pressure source to respective hydraulic motors to rotate the turntable and the carriage at a constant angular displacement ratio. 
     Another object of the present invention is to provide an earth-working machine having a turntable and a carriage supporting thereon an excavating mechanism, and a position setting mechanism for setting the turntable and the carriage to relative angular positions, respectively. 
     According to the present invention, an earth-working machine includes a mobile chassis, a turntable rotatably mounted on the mobile chassis, a carriage rotatably mounted on the turntable in eccentric relation thereto, an earth-working mechanism mounted on the carriage, a source of a fluid pressure, a pair of first and second fluid motors for rotating the turntable and the carriage, respectively, and a fluid rate synchronizer composed of a pair of third and fourth fluid motors interconnected by a common output shaft for discharging amounts of fluid under pressure at a predetermined ratio, the source being operatively coupled to the third and fourth fluid motors, the first and second fluid motors being operatively coupled to the third and fourth fluid motors, respectively, whereby the turntable and the carriage will be angularly moved about their own axes at a constant angular displacement ratio. Since the first and second fluid motors can be controlled by the fluid rate synchronizer to rotate the turntable and carriage at the constant angular displacement ratio. The fluid control synchronizing arrangement is smaller in size, simpler in construction, and hence can be maintained with more ease than mechanical synchronizing structures, and is more resistant to vibrations, heat, and other adverse environmental conditions than electronic controls. The fluid rate synchronizer of the invention can achieve a higher rate control accuracy than possible with a fluid control arrangement using a flow dividing valve. The earth-working machine also includes a pair of first and second normally closed solenoid-operated valves connected across the third and fourth fluid motors, respectively, for providing bypass passages respectively across the third and fourth fluid motors when the first and second normally closed solenoid-operated valves are actuated. 
     According to another embodiment of the present invention, the earth-working machine also includes a pair of first and second normally open solenoid-operated valves, the first normally open solenoid-operated valve being operatively connected between the first and third fluid motors, the second normally open solenoid-operated valve being operatively connected between the second and fourth fluid motors, and an electrical control circuit including a first switch for simultaneously actuating the first normally closed solenoid-operated valve and the second normally open solenoid-operated valve, and a second switch for simultaneously actuating the second normally closed solenoid-operated valve and the first normally open solenoid-operated valve. The electric control circuit includes a first position detector for detecting a first angular position of the turntable with respect to the mobile chassis, and a second position detector for detecting a second angular position of the carriage with respect to the turntable, the first position detector including a third switch connected parallel to the first switch for simultaneously actuating the first normally closed solenoid-operated valve and the second normally open solenoid-operated valve when the first angular position is detected, and the second position detector including a fourth switch connected parallel to the second switch for simultaneously actuating the first normally closed solenoid-operated valve and the second normally open solenoid-operated valve when the second angular position is detected. When the turntable and the carriage are subjected to an angular positional error, they may be corrected into desired synchronized angular positions by automatically stopping the turntable and the carriage at their first and second angular positions. 
     The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an excavator according to the present invention; 
     FIG. 2 is a side elevational view of the excavator shown in FIG. 1; 
     FIG. 3 is a front elevational view of the excavator of FIG. 1; 
     FIG. 4 is a plan view of the excavator of FIG. 1; 
     FIG. 5 is an enlarged cross-sectional view taken line V--V of FIG. 4; 
     FIG. 6 is an exploded perspective view of a turning mechanism on the excavator shown in FIG. 1; 
     FIG. 7 is a plan view of the turning mechanism, as assembled, of FIG. 6; 
     FIG. 8 is a circuit diagram of a hydraulic control system for controlling the turning mechanism of FIG. 6; 
     FIGS. 9A through 9C are plan views showing successive angular positions of a turntable and a carriage, as they are in synchronism, of the excavator shown in FIG. 1; 
     FIG. 10 is a plan view of the excavator of FIG. 1, showing the turntable as angularly moved with respect to a chassis; 
     FIG. 11 is a plan view of the excavator of FIG. 1, showing the carriage as angularly moved with respect to the turntable; 
     FIG. 12 is a side elevational view of an excavator according to another embodiment of the present invention; 
     FIG. 13 is an exploded perspective view of a turning mechanism on the excavator shown in FIG. 12; 
     FIG. 14 is a plan view of the turning mechanism, as assembled, of FIG. 13; 
     FIG. 15 is a circuit diagram of a hydraulic control system for controlling the turning mechanism of FIG. 13; and 
     FIG. 16 is a circuit diagram of an electric control circuit for controlling the hydraulic control system shown in FIG. 15. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is particularly useful when embodied in an earth-working machine such as an excavator or trenching machine as shown in the drawings. Like or corresponding parts are denoted by like or corresponding reference characters throughout the views. 
     As shown in FIGS. 1 through 5, the excavator is of the self-propelled type having a flat mobile chassis 10 supporting four wheels 11 with an endless track 12 trained around each pair of wheels 11. The mobile chassis 10 includes a central support base 13 (FIGS. 2, 3 and 5) mounted thereon and having an upper annular flange on which a horizontal turntable 14 of an octagonal configuration is rotatably mounted. As better shown in FIG. 4, the turntable 14 supports thereon an engine 15, a fuel tank 16, and a hydraulic oil tank 17 arranged along a rear edge of the turntable 14. A first hydraulic motor 18 is also mounted on the turntable 14 adjacent to the fuel tank 16 and has a drive shaft 37 (FIG. 5) directed downwardly of the turntable 14. As illustrated in FIGS. 2 and 3, an annular horizontal holder base 19 is fixedly mounted on the turntable 14 at a front edge thereof. The annular holder case 19 has an axis held in horizontally eccentric and parallel relation to the axis of the support base 13 and hence the turntable 14. A circular carriage 20 is rotatably mounted coaxially on the holder base 19. 
     As shown in FIGS. 3 and 5, the carriage 20 includes a vertical support 21 to which a pair of vertically spaced legs 22 is secured. A bracket 26 is pivotably mounted on the legs 22 and supports thereon a bent boom 27 which is vertically angularly movable about a pivot on the bracket 26. The boom 27 supports on its distal end a bucket arm 28 having a bucket 29 pivotably mounted on a distal end of the bucket arm 28. Hydraulic cylinders 30, 31, 32 are coupled respectively between the bracket 26 and a central portion of the boom 27, between a central portion of the boom 27 and an end of the bucket arm 28, and between the bucket arm 28 and the bucket 29. The boom 27, the bucket arm 28, the bucket 29, and the hydraulic cylinders 30, 31, 32 jointly constitute an excavating mechanism 33. The bracket 26 also supports a seat base 23 on which there are mounted an operator seat 24 and a hydraulic control box 25 supporting a plurality of pivotable control levers. 
     As illustrated in FIGS. 5, 6 and 7, the excavator includes a mechanism for turning the turntable 14 and the carriage 20, the mechanism having a first annular internal gear 34 fixedly mounted substantially concentrically on the annular flange of the support base 13. The turntable 14 has a slider ring 36 disposed securely therebelow and rotatably fitted over the internal gear 34 with ball bearings 35 rotatably interposed therebetween. Accordingly, the turntable 14 is rotatable coaxially on the first internal gear 34. A pinion 38 is fixed to the drive shaft 37 of the hydraulic motor 18 and held in driving mesh with the internal gear 34. The holder base 19 supports thereon an annular holder 39 affixed coaxially thereto. The carriage 20 has a second annular internal gear 40 fixed to the underside thereof and rotatably fitted in the annular holder 39 with ball bearings 41 rotatably interposed therebetween. Therefore, the carriage 20 is rotatable coaxially with the annular holder 39. A second hydraulic motor 42 is mounted on the turntable 14 and located at a front end portion thereof within the holder base 19, and has an upwardly extending drive shaft 43 on which there is mounted a pinion 44 held in driving mesh with the second internal gear 40. 
     FIG. 8 shows a hydraulic control system for controlling the operation of the turning mechanism shown in FIGS. 5 through 7. The hydraulic control system includes a hydraulic pump 52 driven by an engine 51. The hydraulic pump 52 has a suction port connected to a tank 53 of a working fluid or oil and a discharge port connected through a check valve 54 to a manually operable directional control valve 55 and a relief valve 56 having a port communicating with the tank 53. The directional control valve 55 can be shifted between three selectable positions and has three blocks, namely, a neutral block A, a normal rotation block B, and a reverse rotation block C. A fluid rate synchronizer 57 is composed of a pair of synchronizer motors (hydraulic motors) 58, 59 interconnected by a shaft 60 and communicating with a first outlet port of the directional control valve 55. The synchronizer motors 58, 59 rotate in synchronism with each other for discharging amounts of fluid at a desired accurate ratio at all times. The synchronizer motors 58, 59 have displacement volumes which are selected to be at a ratio of 1 : 2, respectively. The synchronizer motor 58 has inlet and outlet ports coupled to a normally-closed solenoid-operated valve 61, while the synchronizer motor 59 has inlet and outlet ports coupled to a normally-closed solenoid-operated valve 62. The synchronizer motor 58 is connected to a normally-closed loading valve 63 connected in series to the hydraulic motor 18 for imposing a load or back pressure on the hydraulic motor 18 and another normally-closed loading valve 64 which is coupled to a second outlet port of the directional control valve 55. The normally-closed loading valves 63, 64 can be opened alternatively by fluid inputs applied in opposite directions to the hydraulic motor 18. The loading valves 63, 64 are shunted respectively by check valves 65, 66. The loading valves 63, 64 and the check valves 65, 66 thus jointly constitute a counterbalancing valve assembly 67. The hydraulic motor 18 is shunted by a pair of parallel relief valves 68, 69 directed in opposite directions and jointly constituting a brake 70. Likewise, the synchronizer motor 59 is connected to a normally-closed loading valve 71 connected in series to the hydraulic motor 42 for imposing a load or back pressure on the hydraulic motor 59 and another normally-closed loading valve 72 which is coupled to the second outlet port of the directional control valve 55. The normally-closed loading valves 71, 72 can be opened alternatively by fluid inputs applied in opposite directions to the hydraulic motor 42. The loading valves 71, 72 are shunted respectively by check valves 73, 74. The loading valves 71, 72 and the check valves 73, 74 thus jointly constitute a counterbalancing valve assembly 75. The hydraulic motor 42 is shunted by a pair of parallel relief valves 76, 77 directed in opposite directions and jointly constituting a brake circuit 78. The hydraulic motors 18, 42 have displacement volumes which are equal to each other or at a ratio of 1 : 1. 
     Operation of the excavator thus constructed will now be described. 
     The operator sitting on the operator seat 24 operates on the control box 25 to actuate the hydraulic cylinders 30, 31, 32 for thereby moving the bucket 29 upwardly and downwardly to dig a trench in the well known manner. The material scooped up by the bucket 29 can be transferred to a truck or the like behind the excavator by lifting the bucket 29 to a horizontal position, as shown in FIG. 3, with the lower end of the bucket 29 slightly above the parts on the turntable 14 and then turning the bucket 29 rearwardly of the chassis 10. 
     Turning of the turntable 14 and the carriage 20 by the hydraulic motors 18, 42 will be described with respect to three modes of operation: 
     (1) Synchronous Rotation of the Turntable 14 and the 
     Carriage 20 
     The solenoid-operated valves 61, 62 are inactivated to provide the synchronizer motors 58, 59 with no bypass passages, and the directional control valve 55 is shifted to put the normal-rotation block B in operative position. Oil under pressure discharged from the hydraulic pump 52 is supplied through the directional control valve 55 to the synchronizer motors 58, 59 which rotate in synchronism to discharge oil under pressure at rates having the ratio of 1 : 2. The oil under pressure from the synchronizer motor 58 goes through the check valve 65 to the hydraulic motor 18. The oil under pressure having passed through the hydraulic motor 18 is delivered through the two-way valve 64 and the directional control valve 55 back to the tank 53. The oil under pressure from the synchronizer motor 59 is delivered through the check valve 73, the hydraulic motor 42, the two-way valve 72, and the directional control valve 55 back to the tank 53. Since the amount of oil discharged by the synchronizer motor 59 is twice that of oil discharged by the synchronizer motor 58, and the hydraulic motors 18, 42 have the same displacement volume, the hydraulic motor 42 is rotated at a speed which is twice that of rotation of the hydraulic motor 18. When the hydraulic motor 18 is thus rotated, the output shaft 37 the pinion 38 of the hydraulic motor 18 are rotated to enable the slider ring 36 to turn along the internal gear 34, whereupon the turntable 14 is angularly moved with respect to the chassis 10. When the hydraulic motor 42 is simultaneously rotated, the output shaft 43 and the pinion 44 of the hydraulic motor 42 are rotated to enable the the internal gear 40 to rotate along the annular holder 39. Therefore, the carriage 20 mounted on the internal gear 40, the support 21, and the excavating mechanism 33 are rotated with respect to the turntable 14. The hydraulic motors 18, 42 are arranged such that they rotate in opposite directions. Therefore, the turntable 14 and the carriage 20 rotate in opposite directions, allowing the excavating mechanism 33 on the carriage 20 to pass over the turntable 14. 
     The relative angular displacement of the turntable 14 and the carriage 20 rotated by the hydraulic motors 18, 42 will be described with reference to FIGS. 9A through 9C. The carriage 20 starts to be rotated by the hydraulic motor 42 in the direction of the arrow X, and the turntable 14 starts to be rotated by the hydraulic motor 18 in the direction of the arrow Y (FIG. 9A). As described above, the carriage 20 and the turntable 14 are controlled to turn at an angular displacement ratio of 1:2. Therefore, the carriage 20 rotates at a speed twice higher than the speed of rotation of the turntable 14. When the turntable 14 rotates through 90 degrees, the carriage 20 rotates through 180 degrees. Since the turntable 14 and the carriage 20 rotate in the opposite directions, they relatively rotate through 90 degrees. The excavating mechanism 33 is positioned at a right angle to the longitudinal axis of the chassis 10 as shown in FIG. 9B. At this time, the carriage 20 is displaced on one side of the chassis 10 to a maximum extent, with the excavating mechanism 33 moving over the turntable 14 without projecting sideways from the other side of the chassis 10. When the turntable 14 is further rotated through another 90 degrees, the carriage 20 rotates through 180 degrees for a total of 360° to the opposite end of the chassis 10, at which time the excavating mechanism 33 projects from the end of the chassis 10 in a position shown in FIG. 9C which is 180 degrees inverted from the position of FIG. 9A. When the turntable 14 and the carriage 20 reach the position of FIG. 9C, the directional control valve 55 is returned to its neutral position A to stop the operation of the hydraulic motors 18, 42 thus stopping the rotation of the turntable 14 and the carriage 20. Accordingly, the excavating mechanism 33 is turned on the basis of the turning movement of the turntable 14 on the chassis 10 and the opposite turning movement of the carriage 20 on the turntable 14, so that the excavating mechanism 33 will move from a forward position to a rearward position across and over the turntable 14 while rotating in a range in which the excavating mechanism 33 will not project laterally of the chassis 10. When it is necessary to turn the excavating mechanism 33 back from the position of FIG. 9C to the position of FIG. 9A, the directional control valve 55 is shifted to select the reverse-rotation block C to cause the turntable 14 to turn 180 degrees and the carriage 20 to rotate at a certain ratio to the rotation of the turntable 14 in the foregoing manner. The turntable 14 and the carriage 20 are now caused to turn at the predetermined ratio back to the starting position. 
     (2) Rotation of the Turntable 14 Only 
     The solenoid-operated valve 61 is actuated to provide a bypass passage across the synchronizer motor 58. The directional control valve 55 is shifted to select the normal-rotation block B. Oil under pressure from the hydraulic pump 52 is supplied to the synchronizer motors 58, 59. Since there is the bypass passage through the solenoid-operated valve 61, the oil flows through the bypass passage of smaller friction, and the synchronizer motors 58, 59 are not rotated. Only the hydraulic motor 18 is supplied with the oil under pressure, and is operated. The output shaft 37 and the pinion 38 are rotated to rotate the slider ring 36 along the internal gear 34. Therefore, only the turntable 14 is turned in the direction of the arrow Z (FIG. 10) with respect to the chassis 10. As the turntable 14 is thus turned, the carriage 20 and the excavating mechanism 33 project laterally of the chassis 10 as shown in FIG. 10. The excavating mechanism 33 can now be moved up and down to effect digging operation in a position laterally of the chassis 10. 
     (3) Rotation of the Carriage 20 Only 
     The solenoid-operated valve 62 is actuated to provide a bypass passage across the synchronizer motor 59. The directional control valve 55 is shifted to select the normal-rotation block B. Oil under pressure from the hydraulic pump 52 is supplied through the bypass passage to only the hydraulic motor 42. Therefore, the output shaft and the pinion 44 are rotated to turn the internal gear 40 along the annular holder 39. The carriage 20 on the internal gear 40, the support 21, and the excavating mechanism 33 on the carriage 20 are now turned in the direction of the arrow W (FIG. 11) with respect to the turntable 14. Since the hydraulic motor 18 is not in operation, the turntable 14 retains at rest. Therefore, the excavating mechanism 33 is angularly moved through the angular interval through which the carriage 20 is turned with respect to the turntable 14, as shown in FIG. 11. In the position of FIG. 11, only the carriage 20 is angularly moved to enable the excavating mechanism 33 to swing in a sectorial zone in front of the chassis 10, so that the road can be dug by the excavating mechanism in such a sectorial zone. 
     The speed of rotation of the hydraulic motor 42 may be kept twice that of rotation of the hydraulic motor 18 by selecting the ratio of displacement volumes of the synchronizer motors 58, 59 to be 1 : 1 and also selecting the ratio of displacement volumes of the hydraulic motors 8, 42 to be 2 : 1. 
     FIGS. 12 through 16 show an excavator according to another embodiment. As shown in FIG. 12, a first position detector 45 is mounted on the lower side of the turntable 14 at its front portion thereof. The first position detector 45 is composed of a first limit switch 46 which can be actuated by a first contacting member 47 mounted on the chassis 13 and projecting in a forward direction thereof (see also FIGS. 13 and 14). A second position detector 48 is mounted on the front side of the annular holder 19 and comprises a second limit switch 49 which can be actuated by a second contact member 50 mounted on the front side of the carriage 20 and projecting in a forward direction thereof (see also FIGS. 13 and 14). 
     FIG. 15 shows a hydraulic control system for the turning mechanism shown in FIGS. 12 through 14. The hydraulic control system of FIG. 15 is similar to that shown in FIG. 8 except that normally open solenoid-operated valves 79, 80 are connected between the synchronizer motor 58 and the loading valve 63 and between the synchronizer motor 59 and the loading valve 71, respectively. 
     FIG. 16 illustrates an electric control circuit for controlling the hydraulic control system shown in FIG. 15. The electric control circuit includes a battery 81 coupled to parallel manually operable switches 82, 83. The switch 82 is connected to the solenoid-operated valves 61, 80 which are coupled parallel to each other, and the switch 83 is connected to the solenoid-operated valves 62, 79 which are coupled parallel to each other. The battery 81 is also connected in series to a relay 84 and a correction switch 85. The relay 84 can actuate normally open switches 86, 87, 88. The switch 86 is connected to a junction between the relay 84 and the correction switch 85. The switches 87, 88 are connected parallel to the switches 82, 83, respectively. The limit switch 49 has a normally open switch 89 and a normally closed switch 90 which are ganged together, while the limit switch 46 has a normally open switch 91 and a normally closed switch 92 which are ganged together. The normally open switch 89 is connected in series to the switch 87 in parallel relation to the switch 82, and the normally open switch 91 is connected in series to the switch 88 in parallel relation to the switch 83. The switch 86 is also connected to the junction between the normally closed switches 90, 92 which are connected parallel to each other and to ground. 
     Operation of the excavator shown in FIGS. 12 through 16 will be described primarily with respect to those components which have been added in the embodiment of FIGS. 12 through 16. 
     (1a) Synchronous Rotation of the Turntable 14 and the 
     Carriage 20 
     The switches 82, 83 are kept open to leave the solenoid-operated valves 61, 62, 79, 80 de-energized. Therefore, no bypass passage is formed across the synchronizer motors 58, 59. The directional control valve 55 is shifted to select the normal-rotation block B for operation. 
     Oil under pressure flows from the synchronizer motor 58 through the solenoid-operated valve 79 to the hydraulic motor 18, and also flows from the synchronizer motor 59 through the solenoid-operated valve 80 to the hydraulic motor 42. Therefore, the turntable 14 and the carriage 20 operate in the same manner as in the mode (1) described above. 
     (2a) Rotation of the Turntable 14 Only 
     The switch 82 is manually closed to actuate the solenoid-operated valves 61, 80. A bypass passage is formed across the synchronizer motor 58, and the solenoid-operated valve 80 is closed. When the directional control valve 55 is shifted to select the normal-rotation block B, oil from the hydraulic pump 52 flows only to the hydraulic motor 18 through the solenoid-operated valves 61, 79. The turntable 14 operates in the same manner as in the previous mode (2). 
     (3a) Rotation of the Carriage 20 Only 
     The switch 83 is manually closed to actuate the solenoid-operated valves 62, 79. A bypass passage is formed across the synchronizer motor 59, and the solenoid-operated valve 79 is closed. When the directional control valve 55 is shifted to select the normal-rotation block B, oil from the hydraulic pump 52 flows only to the hydraulic motor 42 through the solenoid-operated valves 62, 80. The carriage 20 operates in the same manner as in the previous mode (3). 
     (4) Correction of Asynchronous Angular Positions of the 
     Turntable 14 and the Carriage 20 
     When the turntable 14 and the carriage 20 are subjected to a relative angular positional error or after they have been turned independently of each other, it is necessary to correct their angular positions for synchronous operation. 
     For such correction, the switches 82, 83 are kept open and the correction switch 85 is closed. The relay 84 is energized to close the switches 86, 87, 88. When the switch 86 is closed, the relay 84 is held actuated and a current flows from the relay 84 through the switches 90, 92. The directional control valve 55 is shifted to select the normal-rotation block B of the reverse-rotation block C, whereupon the turntable 14 and the carriage 20 start rotating. When the turntable 14 is turned through a certain angular interval until its front end coincides with the front end of the chassis 10, the limit switch 46 is engaged by the contacting member 47. The switch 91 is now closed and the switch 92 is opened. The solenoid-operated valves 62, 79 are now energized through the switches 88, 91 to stop the supply of oil to the hydraulic motor 18, whereupon the turntable 14 is stopped. When the carriage 20 is turned until its front end coincides with the front end of the turntable 14, the limit switch 49 is engaged by the contacting member 50. The switch 89 is now closed and the switch 90 is opened. The solenoid-operated valves 61, 80 are now energized through the switches 87, 89 to stop the supply of oil to the hydraulic motor 42, whereupon the carriage 20 is stopped. When the switches 90, 92 are opened, no current flows through the relay 84 causing the switches 86, 87, 88 to be opened, and the relay 84 is released. The turntable 14 and the carriage 20 are then brought into synchronized angular positions for starting synchronous operation thereof. 
     Although certain preferred embodiments have been shown and described, it should be understood that many changes and modifications may be made therein without departing from the scope of the appended claims.