Patent Publication Number: US-3874305-A

Title: Robot locomotive for classifying rolling stocks

Description:
United States Patent [1 1 [111 3,874,305 Sema et al. 5] Apr. 1, 1975 1 ROBOT LOCOMOTlVE FOR CLASSIFYING ROLLING STOCKS [75] Inventors: Katsutosh Sema, Omiya; Mitsuru Wakao, lwatsuki; Yoshinori Kobayashi, Sagamihara; Koichi Hara, Tokyo, all of Japan [73] Assignee: Japanese National Railways and Kayabakogyo Kabushikikaisha, Tokyo, Japan [22] Filed: Oct. 24, 1973 121] Appl. No.: 409,140  
 [30] Foreign Application Priority Data Dec. 29, 1972 Japan 48-3787 [52] US. Cl. 105/26 R, 105/64 [51] Int. Cl. B6lc l/00, Bole 17/00 [58] Field of Search 104/147 R, 154, 155, 88; 105/26 R, 63, 64  
 [56] References Cited UNITED STATES PATENTS 3.709.153 H1973 Herscovitch 105/26 R Primary Examiner-M. Henson Wood, Jr. Assistant E.raminerD. W. Keen Attorney, Agent, or Firm-Saul Jecies [57] ABSTRACT A robot locomotive is disclosed which is adapted to be releasably coupled to a freight car which is approaching at a relatively lower speed to the head of a classifying network in a yard or which is stopped at the head so as to accelerate and push it off on a desired track. Upon a main frame supported by wheels riding on guide rails laid inside a railway track are mounted a hydraulically operated pusher assembly adapted to forcibly press against the flanges of the first wheel of a freight car and a hydraulic brake system. In response to the external signals an electronic controller controls various hydraulic control valves in a hydraulic controller so as to control the hydraulically operated pusher assembly and brake system.  
 7 Claims, 7 Drawing Figures PATENTEDAPR 1 I975 874.305  
 sum 3 0F 5 mm Om mm 0v mm mm mm WQE ROBOT LOCOMOTIVE FOR CLASSIFYING ROLLING STOCKS BACKGROUND OF THE INVENTION The present invention relates to a robot locomotive adapted to classify freight cars or the like in a railway yard and to push a rolling stock on a slope.  
  In general the railroad yards may be classified into level and slope yards. In the level yard a freight car to be classified must be pushed to the proper track by a switch engine while in the slope yard with a built-up hump at the head of the classifying network, freight cars must be pushed up to the hump by a switch engine. Therefore, both the level and slope yards require switch engines so that the efficiency of yard operation is low. Especially in the level yard, because of the nature of the switch engines the freight car trains must be switched from one track to another many times so that the efficiency of yard operation is further decreased. Therefore there has long been a need for an inexpensive system for classifying the freight cars of one classified train which is pushed by a switch engine to a predetermined track or position in a yard.  
 There has long been also a need for a robot locomotive which pushes a rolling stock when the latter starts on a sloped track and which is adapted to accelerate over a short distance a freight or passengc car or cars at rest into a desired direction.  
  In view of the above there has been recently devised and demonstrated a robot locomotive driven by a linear motor, but there is a distinct defect in that the linear motors cannot provide sufficient thrust or tractive force and starting torque when the linear motor system is installed in a relatively limited space between the rails of a railroad track. Therefore the linear motor robot locomotives may accelerate a freight car which is approaching at slow speed, but cannot start a relatively heavy freight car at rest. Furthermore the linear motor robot locomotive must travel over a long distance before it can accelerate a freight ear to a desired speed and then push it off. Therefore a large installation space is required. Moreover a predetermined spacing between the stationary and moving members of a linear motor system must be maintained with a higher degree of accuracy in order to produce sufficient torque. Thus the installation and operation costs of a linear motor robot locomotive system are prohibitively expensive at present.  
 SUMMARY OF THE INVENTION In view of the above, one of the objects ofthe present invention is to provide a novel robot locomotive which is compact in size yet capable of producing greater thrust or tractive force and starting torque and whose installation and operation costs are inexpensive.  
  Briefly stated, according to the present invention there are mounted upon a main frame supported by wheels riding on a guide track, a hydraulically operated pusher assembly adapted to releasably couple to or catch a freight car or the like and a hydraulically operated brake system. Hydraulic liquid under pressure is used to drive a robot locomotive of the present invention because the conversion of the energy of hydraulic liquid under pressure into mechanical energy is very efficient and the control of hydraulic liquid under pressure is easy. Therefore the robot locomotive of the present invention may produce greater thrust or tractive force and starting torque even though it is compact in size. In response to external signals an electronic controller controls various hydraulic control valves in a hydraulic controller, thereby controlling the operations of the hydraulic systems mounted upon the robot locomotive. Therefore the robot locomotive can accelerate over a short distance not only a freight car or the like approaching at a relatively slow speed but also a freight car or the like completely at rest. Furthermore the robot locomotive of the present invention is adapted to push up a rolling stock on a sloped track.  
  The above and other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawing thereof.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIGS. I and 2 are views used for the explanation of the mode of operation of a robot locomotive of the present invention when it is used to classify freight cars approaching at a relatively slow speed and at rest, respectively;  
  FIG. 3 is a top view of a robot locomotive ofthe present invention.  
 FIG. 4 is a front end view thereof;  
  FIG. 5 is a detailed view partly in section of a collision preventive means thereof;  
 FIG. 6 is a hydraulic circuit thereof; and  
  FIG. 7 is a view illustrating another embodiment of a hydraulic circuit thereof.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS:  
  The robot locomotive in accordance with the present invention will be described hereinafter as being used in a railroad yard to classify freight cars by destination, but it is to be understood that it may be also used to push freight cars up to hump or push a locomotive or the like on the sloped track when the electronic and bydraulic control systems are suitably modified.  
  FIGS. I and 2 illustrate the modes of yard operation by a robot locomotive generally indicated by 10 of the present invention. In case of the yard operation shown in FIG. 1, the robot locomotive 10 is coupled to a freight car 11 which is approaching at a relatively slow speed to a predetermined position such as the head of a classifying network. It accelerates the freight car 11 to a predetermined speed, releases it to a predetermined track, and then returns to its initial position. In like manner the robot locomotive I0 shunts the incoming freight cars 11 one by one and combines them by destination into new trains.  
  In case of the yard operation shown in FIG. 2, the robot locomotive 10 starts the freight car which is at rest, accelerates it to a predetermined speed to release it to a predetermined track, and returns to the second freight car I] to repeat the same operation. According to the present invention the above two yard operations may be carried out by the robot locomotive I0 only by the slight modifications of detecting means to be described in more detail hereinafter.  
  Referring to FIG. 3, the robot locomotive 10 of the present invention comprises a carriage or main frame I5 supported by driving wheels 12 and provided with guide wheels 13 whose axes are perpendicular to those of the driving wheels 12; a power source generally indicated by 18 and including a hydraulic pump 16, which is driven by a prime mover 15 such as an electric motor or internal combustion engine, and a reservoir 17 for hydraulic liquid or working oil; a driving mechanism or system generally indicated by 22 and comprising a pair of hydraulic motors 19a and 19b which are driven in parallel or series by working oil under pressure supplied from the power source 18, and driving pinions 21a and 21b drivingly coupled to the hydraulic motors 19a and 19h through reduction gears a and 2012, respectively; a hydraulic controller 23 for controlling the hydraulic motors 19a and 19b and other hydraulic systems to be described hereinafter, the hydraulic controller 23 comprising various hydraulic control valves to be described hereinafter; an electronic controller 24 for controlling the hydraulic control valves in the hydraulic controller 23; an accumulator 25 for temporarily storing working oil under pressure discharged from the hydraulic pump 16; a hydraulically operated pusher assembly 26; and a hydraulically operated brake system.  
  Next referring to FIG. 4, the robot locomotive 10 is adapted to travel in a pit 28 between a pair of railroad rails 27. The driving wheels 12 ride on guide rails 31 securely fixed through supporting arms 30 to supporting members 39 which also serve to support the rails 27. The driving pinions 21a and 21h are in mesh with racks 32 securely fixed to supporting arms 30 immediately above the guide rails 31 so that when the hydraulic motors 19a and 19b are driven, the robot locomotive 10 may travel forwardly or backwardly on the guide rails 31 along the railroad rails 27. The lateral vibration of the robot locomotive may be prevented as the guide wheels 13 move along the supporting brackets 33.  
  Working liquid under pressure supplied from the hydraulic pump 16 or the accumulator 25 to the hydraulic motors 19a and 19/; is controlled by the hydraulic controller 23, and the driving forces are transmitted from the hydraulic motors 19a and 1911 through the reduction gears 20a and 20b to the driving pinions 21a and 21/). Therefore, the robot locomotive I0 is driven. To reverse the robot locomotive 10, the flow of working liquid to the hydraulic motors 19a and 19b is reversed.  
  Since the power source utilizes liquid whose energy conversion is advantageous and whose velocity control is easy and the driving system comprising the driving pinions 21a and 21b in mesh with the racks 32 is employed, greater thrust or tractive force and starting force may be produced when the robot locomotive I0 is started and the wheel slippage may be prevented.  
  Referring back to HO. 3, the pusher assembly 26 comprises a pair of right and left hydraulic cylinders 34, which are substantially similar in construction so that the construction of only one of them will be described hereinafter. The hydraulic cylinder 34 comprises a pusher arm 35 having a roller 36 rotatably fixed to the free end thereof. When the pusher arm 35 is extended, the roller 36 is pressed against the flange 38 of a wheel 37 of the freight car ll (See FIG. 4), thereby retarding the speed thereof. On the other hand when the pusher arm 35 is retracted, the roller 36 is moved away from the flange 38 of the wheel (See FIG. 4, right) so that the freight car 11 is free to roll.  
  The pair of hydraulic cylinders 34 are mounted on a slider 39 and joined to each other by a pair of front and rear side plates 40. The slider 39 is supported by rollers 41 which in turn ride on guide rails (not shown) laid upon the main frame 14 so that the hydraulic cylinders 34 may be moved back and forth in the longitudinal direction as will be described in more detail hereinafter. The free end ofa piston rod 43 of a hydraulic shock absorber 42 mounted upon the main frame 14 is normally pressed against the rear plate of the slider 39 under the force of a spring 44 loaded between the hydraulic shock absorber 42 and the free end of the pisiton rod 43. When the hydraulic cylinder 34 is actuated so as to press the roller 36 at the free end of the pusher arm 35 against the flange 38 of the wheel 37 of the freight car 11, the hydraulic cylinder 34 and hence the slider 39 are pushed backwardly, but the impact is relieved by the hydraulic shock absorber 42 so that the wheel 37 of the freight car 11 may be prevented from floating away from the rail 27.  
  The hydraulic cylinders 34 are actuated by working liquid under pressure which is supplied from the hydraulic pump 16 or the accumulator 25 and whose flow is controlled by the hydraulic controller 23 as will be described in more detail hereinafter.  
  In the instant embodiment, the rollers 36 of the by draulic cylinder 34 are adapted to be retracted as soon as the freight car 11 is pushed forwardly, but if the rollers 36 would not be retracted away from the flange 38 of the wheel 37, they would collide against the flanges&#39; of the next wheels so that the robot locomotive 10 would be forced to run together with the freight car 11.  
 In order to prevent such accident the present invention provides collision preventive means.  
  Referring to FIG. 5, the collision preventive means comprises a lever 46 whose one end is pivoted with a pivot pin 47 to a stay fixed to the end portion of the pusher arm 35 and a collision preventive valve generally indicated by 49. The other end of the lever 46 is slidably interposed between a roller 48 fixed to the main frame 14 and a roller 50 fixed to the main frame 14 and a roller 50 fixed to the top of the collision preventive valve 49.  
  An annular groove 52 is formed around the front end portion of a spool 51 slidably fitted into a cylindrical portion 54 of a valve body 53, and a plurality of thorough holes 55 are equiangularly drilled through the wall of the cylindrical portion 54. Steel balls 56 are fitted into the holes to partly extend into the annular groove 52 of the spool 51. A slide cap 57 is slidably fitted over the cylindrical portion 54, and an annular groove 58 is formed in the inner wall of the slide cap 57. A spring 59 is loaded between the bottom of the slide cap 57 and the end face of the spool 51. The slide cap 57 serves to retain the balls 56 in position, that is in the holes 55 and the annular groove 52 of the spool 51 under the normal condition, thereby preventing an erratic operation of the spool 51.  
  When the pusher arm 35 fails to retract in response to a control signal, the flange 38 of the next wheel 37 collides against the lever 46 so that the latter is caused to rotate about the pivot pin 47 in the clockwise direction as indicated by the arrow, As a result the slide cap 57 is pushed downwardly against the spring 59. Since the balls are pushed radially outwardly by the spool 51 which is pushed downwardly under the force of the spring 59 as the slide cap 57 compresses the spring 59. the balls 56 are immediately forced into the annular groove 58 of the slide cap 57 as soon as the annular groove 58 coincides with the holes 55. As a consequence the spool 51 is now permitted to be retracted into the cylindrical valve portion 54 under the force of the spring 59. ln response to the retraction of the spool 51 of the collision preventive valve 49. the hydraulic cylinder 34 is actuated to rapidly retract its pusher arm 35 so that the collision of the flange 38 of the wheel 37 against the roller 36 can be prevented.  
  Next the mode of operation of the robot locomotive with the above construction will be described with reference to FIG. 6 illustrating the hydraulic circuit thereof.  
 1. Stand By The hydraulic pump 16 pumps working liquid in the reservoir 17 into the accumulator 25, the pressure of working liquid charged into the accumulator being controlled by a pressure switch 60. When the pressure of working liquid in the accumulator 25 reaches a pre determined level, a vent valve 62 of an unloader relieve valve 51 is opened to return working liquid under pressure discharged from the hydraulic pump 16 to the reservoir 17 so that the hydraulic pump 16 is operated under the unloaded condition. The flow of working oil under pressure from the accumulator 25 to the vent valve 62 is prevented by a check valve 63. In response to a signal from the electronic controller 24, a main and auxiliary control valves 64 and 65 and a brake operating valve 66 in the hydraulic controller 23 are energized whereas a forward-reverse control valve 67. a series-parallel switching valve 68, a pusher arm operating valve 69. an acceleration control valve 70, and a brake valve 71 are deenergized. Therefore the robot locomotive I0 is at rest with the hydraulic circuit component parts being in the position shown in FIG. 6.  
 ll. Step of Following Freight Car When a freight car I] to be classified approaches to the robot locomotive l0 and the flange 38 of the first wheel 37 is about to beyond the roller 36, a detector 72 (See FIG. 3) mounted on the main frame 14 forwardly of the pusher assembly 26 generates a signal which is transmitted through the electronic controller 24 to the vent valve 62. the main control valve 64, the forwardreverse control valve 67. the parallel-series switching valve 68 and the pusher arm operating valve 69. The vent valve 62 is energized to switch to the upper or closed position, the main control valve 64 is deenergized to switch to the lower or open position, the forward-reverse control valve 67 is energized to switch to the forward position. the parallel-series switching valve 68 is energized to switch to the parallel position, and the pusher arm operating valve 69 is energized to the right position to extend the pusher arms 35.  
  Working oil under pressure from the accumulator 25 is admitted through the pusher arm operating valve 69 into the hydraulic cylinders 34 behind the pistons to extend the pusher arms 35 so that the rollers 36 are pressed against the flanges of the first wheels 37 of the freight car 1]. Working oil under pressure from the hydraulic pump 16 flows through the main control valve 64. the forward-reverse control valve and the seriesparallel switching valve 68 into the hydraulic motors 19a and 19h so that the latter are driven in the forward direction in parallel. The rotation of the hydraulic motors 19a and 19/1 are transmitted to the driving pinions 21a and 2111 through the reduction gears 20a and 2011 so that the robot locomotive 10 starts to follow the freight car 11 with the pusher arms 35 and hence the rollers 36 being extended.  
 Ill. Acceleration When the robot locomotive [10 has caught up to the freight car 11 and the rollers 36 are pressed against the flanges 38 of the first wheels 37 of the freight car 1], the pusher assembly 26 is pushed backwardly as described hereinbefore and the impact is relieved by the hydraulic shockrabsorbers 42 so that the freight car 11 is prevented from floating away from the rails 27. When the rods 43 of the hydraulic shock absorbers 42 are completely retracted, a detector (See FIG. 3) mounted on the main frame 14 generates a control signal which is transmitted through the electronic controller 24 to the acceleration control valve so that the latter is energized to switch to the left position.  
  Then a pilot check valve 74 is opened so that working oil under pressure flows from the accumulator 25 through the pilot check valve 74 and a check valve and joins with the flow of working oil under pressure discharged from the hydraulic pump 16 so that the flow rate of working oil under pressure charged into the hydraulic motors 19a and 19b is increased. As a result the rotational speed of the hydraulic motors 19a and 19b is accelerated so that the freight car 1 I may be accelerated. Because of the parallel operation of the hydraulic motors 19a and 1917 the robot locomotive 10 may push the freight car 11 with the greater force.  
 lV. Braking in case of forward travel When the robot locomotive 10 has accelerated the freight car 11 to a predetermined speed which is detected by a tachometer generator (not shown) coupled for example, to the output shaft of the hydraulic motor 190, the tachometer generator generates a signal which is transmitted through the electronic controller 24 to the vent valve 62. the main control valve 64, the brake operating valve 66, the forward-reverse control valve 67, the pusher arm operating valve 69 and the acceleration control valve 70. The vent valve 62 is de-energized to be opened so that the hydraulic pump 16 is unloaded. The vent valve 62 is disconnected from the electronic controller 24 and is operatively coupled to the pressure switch 60. The main control valve 64 is en ergized to be closed. and the acceleration control valve 70 is dc-energized to close the pilot check valve 74 so that the flow of working oil under pressure from the hydraulic pump 16 and the accumulator 25 to the hydraulic motors 19a and 19b is interrupted.  
  The forward-reverse control valve 67 is switched to the upper position, that is the reverse position so that the pump operation of the hydraulic motors 19a and 1912 is started as the robot locomotive 10 is still rolling under the force of inertia. Working oil under pressure discharged from the hydraulic motors 19a and 19b is returned to the suction ports thereof through the brake valve 71, a relief valve 76 and a check valve 77 so that the load whose pressure is determined by the relief valve 76 is applied to the hydraulic motors 19a and 1%. As a result the dynamic retarding forces are produced. The operating valve 66 is energized in response to the signal from the tachometer generator to be switched to the right position so that working oil under pressure from the accumulator 25 is admitted through the brake operating valve 66 and a pressure reducing valave 79 into a brake cylinder 80 so that the mechanical retarding forces are applied to the robot locomotive 10. Thus. the robot locomotive 10 is rapidly decelerated by the dynamic and mechanical braking so that the freight car I I may be released away from the robot locomotive 10 at a predetermined speed.  
  The pusher arm control valve 69 is dc-energized in response to the signal from the electronic controller 24 to communicate the chambers behind the pistons of the hydraulic cylinders 34 with the reservoir 17 so that the pusher arms 35 and hence the rollers 36 are retracted away from the flanges 38 of the first wheels 37 of the freight car I]. Thereafter the pusher assembly 26 is returned to its initial position by the hydraulic shock absorbers 42 and their springs 44.  
 V. Reverse and Acceleration When the robot locomotive 10 is stopped after it has pushed off the freight car II. a control signal is generated in the electronic controller 24 in response to the signal from the tachometer generator and is applied to the brake operating valve 66. the series-parallel switching valve 68. the acceleration control valve 70 and the brake valve 71. The brake operating valve 66 is energized to switch to the left position, that is the valve 66 is opened so that working oil under pressure in the brake cylinder 80 is returned to the reservoir 17 through a check valve 81 so that brake is released. The series parallel switching valve 68 is de-cnergized to switch to the lower or series position. and the acceleration control valve 70 is energized to switch to the left position. The brake valve 7] is switched to the upper or reverse position.  
  Therefore the pilot check valve 74 is opened so that working oil under pressure flows from the accumulator 25 through the pilot check valve 74, the check valve 85. the forward-reverse control valve 67. the hydraulic motor 19h. the series-parallel switching valve 68 and the hydraulic motor 19a to the reservoir 17. Thus, the hydraulic motors 19a and I9]; are coupled in series and driven in the reverse direction so that the robot locomotive I is reversed and is gradually accelerated.  
  When the pressure of working oil accumulated in the accumulator 25 drops. the speed of the robot locomotiive would be decreased. However. as described hereinheforc. the vent valve 62 is disconnected from the electronic controller 24 and is operatively coupled to the pressure switch 60 as soon as the brake is applied so that when the pressure in the accumulator drops a predetermined level. the pressure switch 60 is closed to close the vent valve. As a result the hydraulic pump 16 is driven into the on-load state so that working oil under pressure discharged thereform is charged into the accumulator through the check valve 63. Thus the pressure in the accumulator 25 may be always maintained at a predetermined level. In return travel only the robot locomotive 10 rolls so that ifthe hydraulie motors 19a and 19/2 are driven in parallel, the acceleration would become prohibitive. Therefore the hydraulic motors 19a and 19b are driven in series as described above so as to prevent the robot locomotive 10 from being excessively accelerated.  
 Vl. Free rolling When the robot locomotive 10 is accelerated to a predetermined speed. the electronic controller 24 generates a control signal in response to the signal from the tachometer generator. The control signal is transmitted to the auxiliary control valve 65. the forward-reverse control valve 67 and the acceleration control valve 70. The acceleration control valve is de-cnergized to close the pilot check valve 74. and the auxiliary control valve and the forward-reverse control valve 67 are also tie-energized so that the former is switched to the lower or open position, whereas the latter is switched to the center or free rolling position.  
  Therefore a hydraulic circuit through the hydraulic motors 19a and 19b. the forward-reverse control valve 67 and the series-parallel switching valve 68 is established so that the robot locomotive 10 is now free to roll under the force of inertia. In this case, however. there is a danger that cavitation occurs because of the negative pressure generated in the closed hydraulic circuit. To overcome this problem. according to the present invention the auxiliary control valve 65 is opened in response to a control signal from the electronic controller 24. which in turn is generated in response to the signal from the tachometer generator, to flow working oil under pressure from the accumulator 25 through the auxiliary control valve 65. a pressure reducing valve 82 and check valves 83 and 84 into the hydraulic motors I90 and 19h so as to maintain the positive pressure in the closed hydraulic circuit. Thus the cavitation When the robot locomotive 10 is returned to a predetermined position. a sensor 85 (See FIG. 3) mounted upon the main frame 14 is actuated by an actuator 86 (See FIG. 3) on the side of the railroad track so that a control signal is transmitted from the electronic controller 24 to the auxiliary control valve 65. the forwardreverse control valve and the parallel-series switching valve 68. The auxiliary control valve 65 is energized to switch to the upper or closed position. the forwardreverse control valve 67 is switched to the lower or forward position. and the series-parallel switching valve 68 is energized to switch to the upper or parallel position. Therefore the hydraulic motors 19a and 19h start the pump operation and discharged working oil under pressure is returned through the brake valve 71. the relief valve 76 and the check valve 78 to the suction ports. The load whose pressure is determined by the relief valve 76 is applied to the hydraulic motors 19a and [9!) so that the dynamic retardation forces are applied to the robot locomotive it). Thus the robot locomotive I0 is gradually decelerated.  
 Vlll. Travel in&#39;reverse direction at a predetermined speed When the robot locomotive 10 is decelerated to a predetermined speed. a control signal is transmitted from the electronic controller 24 to the vent valve 62, the main control valve 64 and the forward-reverse control valve 67. The vent valve 62 is disconnected from the pressure switch 60 and is energized to switch to the upper or closed position so that the hydraulic pump 16 is driven into the onload state. The main control valve 64 is de-energized to switch to the lower or open position. and the forward-reverse control valve 67 is switched to the upper or reverse position.  
  Therefore working oil under a predetermined pressure discharged from the hydraulic pump 16 flows through the main control valve 64 and the forwardrcverse control valve 67 into the hydraulic motors 19a and 19h so that the latter are driven in reverse direction in parallel. As a consequence the robot locomotive I travels at a predetermined speed in the reverse direction under any conditionv IX. Second braking When a sensor 87 mounted upon the main frame I4 is actuated by an actuator 86 on the side of the track (See FIG. 3), a control signal is transmitted from the electronic controller 24 to the vent valve 62. The vent valve 62 is de-energized to switch to the lower or open position so that the hydraulic pump I6 is driven into the unloaded state. The control signal is also transmitted to the main control valve 64. the brake operating valve 66 and the forward-reverse control valve. The main control valve 64 is energized to switch to the upper or closed position, the brake operating valve 66 is de-energized to the right or operative position, and the forward-reverse control valve 67 is switched to the lower or forward position.  
  The working oil under pressure discharged out of the hydraulic motors I90 and 19/) is returned to their suction ports through the brake valve 71, the relief valve 76 and the check valve 78 so that the dynamic retardation force is applied to the robot locomotive I0 as in the case of the first braking. In this case the working oil under pressure flows from the accumulator through the brake operating valve 66 and the pressure reducing valve 79 into the brake cylinder 80 so that the mechanical retarding forces are also applied to the robot locomotive I0.  
  Therefore the robot locomotive is rapidly dcceler&#39; ated and comes to stop at a position spaced apart by a predetermined distance from the position at which the sensor 87 is actuated. That is, the robot locomotive I0 is returned to its initial position.  
 X. Reset and standby When the robot locomotive I0 is stopped at its initial position. the reset signal is generated by the electronic controller 24 and is transmitted to the brake operating valve 66. The brake operating valve 66 is energized to switch to the left or open position so that working oil under pressure in the brake cylinder 80 is returned to the reservoir I7 through the check valve 81. Asa result the mechanical retardation forces are released. The forward-reverse control valve 67 is tie-energized to switch the center position. and the series-parallel switching valve 68 and the brake valve 71 are also deenergized to switch to the lower positions. Thus all of the components are returned to the initial positions, and the robot locomotive I0 is ready to classify the next freight car II.  
  In like manner the freight cars I] may be classified one by one by the robot locomotive 10.  
  So far the present invention has been described with reference to the case in which a train of freight cars I] is approaching at a relatively slow speed to the head of a classifying network and the robot locomotive I0 accelerates and pushes them one by one to classify them by destination as shown in FIG. I, but the robot locomotive 10 of the present invention may also automatically classify freight cars II which are at rest at the head of a classifying network as shown in FIG. 2. In this case detector 72 may be eliminated. and the first braking operation is started in response to the signal from the detector 85 which is so modified as to be actuated by the first wheel 37 of the freight car. The robot locomotive I0 which is travelling at a predetermined constant speed is subjected to the second braking in response to the signal generated by the detector 87 which is also so modified as to be actuated by the lirst wheel 37 of the freight car 11, whereby the robot locomotive 10 is stopped. In this case, the positions ofthe detectors and 87 are so selected that when the robot locomotive I0 is stopped, the extended rollers 36 of the pusher assembly 26 may be correctly pressed against the flanges of the first wheels of the freight car 1 l, and the robot locomotive I0 is so arranged as to automatically repeat the operation of classifying the next freight car I I when the robot locomotive I0 is stopped. Then only the start and stop signals are required to have the robot locomotive automatically classified the freight cars as shown in FIG. 2.  
  Referring to FIG. 7. hydraulic motors 19c and 19d having a servo cylinder 88 and a servo valve 89 are of the variable displacement type. When the brake is applied in the forward and backward or reverse travels, a control valve 90 is closed so that working oil under pressure flows from the accumulator 25 through the brake operating valve 66 and a check valve 91 and is admitted into a pilot chamber of the servo motor 89 and then into a chamber behind the piston of the servo cylinder, thereby increasing the displacements of the hydraulic motors 19c and 19d to the maximum. Therefore, the maximum dynamic brake force is applied. In case of forward or reverse starting, the above described brake force is applied by having the displacements of the hydraulic motors 19c and Il9d increased to the max imum so that the maximum starting torque may be obtained.  
  In case of operations other than those described above, the hydraulic motors 19c and 19d are driven with a predetermined hydraullic pressure which is determined by the unloader relief valve 61 until the whole working oil under pressure discharged from the hydraulic pump 16 flows into the hydraulic motors 19(- and 194/. Therefore the thrust. or tractive force of the robot locomotive 10 may be maintained constant. When the whole workinng oil under pressure discharged from the hydraulic pump 16 flows into the hydraulic motors 19c and 1941. working oil under pressure in a high pressure circuit is admitted into the pilot chamber of the servo valve 89 through a shuttle valve 92 and the control valve 90&#39;. thereby actuating the servo cylinder 88 to vary eccentricity of the hydraulic motors 19c and 1911. Therefore even when the speeds of the hydraulic motors 19c and 19d are increased, the pressure of working oil may be maintained always at a predetermined constant level so that the outputs of the hydraulic motors [9c and 19! may be also be maintained at a predetermined constant magnitude. In this case, however, an open center type four-port directional control valve 670 must be used as the forwardreverse control valve in order to permit the free travel or rolling under the force of inertia in the reverse direc tion.  
  As described above the present invention utilizes hydraulic liquid whose energy conversion is efficient and advantageous and whose velocity control is easy so that the robot locomotive may produce the greater starting torque and thrust or tractive force even though it is relatively compact in size. Therefore the robot locomotive of the present invention can accelerate over a relatively short distance and push off not only freight cars approaching at a relatively slow speed to the head of a classifying network, but also freight cars which are at rest at the head. Since the pinion and rack drive system is employed, wheel slippage may be prevented and the control of the position of the robot locomotive may be attained in a simple manner. The high pressure required may be controlled in a simple manner and the breakdown due to the overload may be prevented. Furthermore since the accumulator or accumulators are provided, the maximum instantaneous output may be increased by a few times to tens of times the output of the hydraulic motors so that even a heavy freight car may be accelerated over a short distance to a required push-off speed. The robot locomotive is simple in construction and compact in size, and the ground installation is simple so that the initial cost as well as the operating expenses may be considerably reduced.  
  The robot locomotive of the present invention may be used not only in a yard with a built-in hump but also to push a railroad car at a steep slope. lf required, two hydraulic cylinders similar to the hydraulic cylinder 34 may be disposed in parallel on each side of the robot locomotive so that two pusher arms and hence tow rollers may be pressed against the flange of the same wheel of a freight car, thereby pushing the wheel in a desired direction or stopping the freight car at a desired position.  
 What is claimed is:  
  l. A hydraulically operated robot locomotive for pushing rolling stocks comprising a main frame sup ported on wheels so as to travel along a guide track; a power source mounted upon said main frame and comprising a hydraulic pump, means for driving said hydraulic pump, and a reservoir for working hydraulic liquid; a traction system comprising a plurality ofhydraulic motors which are driven by hydraulic liquid under pressure supplied from said power source; a hydraulic control system including hydraulic control valves for controlling said power source and said traction system, an electronic control system for controlling said bydraulic control valves in said hydraulic control system in response to control signals applied from the exterior of said robot locomotive; a hydraulic brake system for applying mechanical brake to said wheels of said main frame; and a hydraulic rolling stock pusher assembly adapted to relcasably couple said robot locomotive to a rolling stock, whereby said robot locomotive may push the rolling stock.  
 2. A robot locomotive as defined in claim I further (ill comprising an accumulator mounted on said main frame for accumulating therein working liquid under pressure and adapted, in response to a control signal, to supply working liquid under pressure to said hydraulic motors, said hydraulic brake system, and said hydraulic rolling stock pusher assembly through said hydraulic control system.  
  3. A robot locomotive as defined in claim 2 wherein said traction system further comprises driving pinions which are drivingly coupled to said hydraulic motors and are adapted to engage with racks laid on the side of a track of rolling stock, whereby said robot locomotive may be driven without slippage.  
  4. A robot locomotive as defined in claim 3 wherein said hydraulic rolling stock pusher assembly comprises collision preventive means each of which comprises a collision preventive valve, and a lever operatively coupled to said collision preventive valve and said hydraulic rolling stock pusher assembly, whereby a wheel of a rolling stock pushes said lever in case of an erratic operation of said pusher assembly to cause the same to retract to the inoperative position.  
  5. A robot locomotive as defined in claim 1, wherein said hydraulic motors are of the variable-displacement type.  
  6. A fluid-operated robot locomotive for pushing rolling stock, comprising a frame supported on wheels for travel along a guide track;  
 a power source mounted on said frame;  
 a traction system coupled with said wheels and driven by said power source;  
 an operations control system for controlling said power source and said traction system;  
 an electronic control system, including detecting means for detecting the proximity of rolling stock, for controlling said operations control system in response to control signals applied from the exterior of said robot locomotive;  
 a brake system for applying mechanical braking force to said wheels of said frame; and  
 a rolling stock pusher assembly adapted to releasably couple said robot locomotive to an item of rolling stock, whereby said robot locomotive may push the rolling stock.  
  7. A robot locomotive as defined in claim 6, wherein said rolling stock pusher assembly comprises collision preventive means for preventing collision between said robot locomotive and rolling stock.  
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