Abstract:
An electro-hydraulic servomotor includes: an electric motor ( 41 ) which rotates a drive shaft ( 51 ) in response to an inputted signal; a hydraulic motor ( 60 ) which rotates an output shaft ( 61 ) using hydraulic pressure of operation oil; a first geared shaft ( 53 ) rotatable along with the output shaft ( 61 ); a second geared shaft ( 52 ) threadingly engaged with the drive shaft ( 51 ) and meshed with the first geared shaft ( 53 ); and a spool ( 71 ) axially movable along with the second geared shaft ( 52 ) depending on a rotational difference between the drive shaft ( 51 ) and the first geared shaft ( 53 ), to control supply and discharge of the operation oil to and from the hydraulic motor. ( 60 ).

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an electro-hydraulic servomotor used for hydraulic shovels, cranes, asphalt finishers and machine tools (those machines will be referred to simply as external machines). 
     In this type of the electro-hydraulic servomotor, as shown in FIGS. 13 and 14, an output shaft  2  is rotatably supported on a casing  1  by bearings  3  and  4 . A valve plate  9  is fastened to the inner wall of the casing  1 , and a cylinder block  7  is fastened to the circumferential portion of the output shaft  2 . A plurality of pressure chambers  7   a  is formed in the cylinder block  7 . Pistons  8  are disposed within those pressure chambers  7   a , and the pistons  8  are reciprocally moved in their axial direction by a hydraulic pressure of an operation oil introduced into the pistons  8 . 
     A slanted plate, which is slanted at a given angle with respect to the valve plate  9 , is fastened to a portion of the inner wall of the casing  1  which is closer to the top end of the output shaft  2 . The top ends of the pistons  8  slidably push the slanted plate  6 , and the cylinder block  7  slides to the valve plate  9 , whereby the output shaft  2  and the cylinder block  7  are rotated together. 
     A spool valve  11 , which moves in the axial direction, is provided in the casing  1 . A screw member  12  and a gear  13  are fastened to the top end and the base end of the spool valve  11 , respectively. A pulse motor  14  is mounted on the casing  1 . A motor shaft  15  of he pulse motor  14  is rotatably supported on the casing  1 . A rotational force of the motor shaft  15  is transmitted to the spool valve  11  via gears  16  and  13 . A rotational force of the output shaft  2  is transmitted to the spool valve  11  via screw members  10  and  12 . When the spool valve  11  is turned, an oil discharging passage  1 , an oil supplying passage  1   b , and communicating passages  1   d  and  1   d  communicate with one another. In the electro-hydraulic servomotor, the output shaft  2 , the spool valve  11  and the pulse motor  14  are disposed on the same axial line. 
     Since in the thus constructed electro-hydraulic servomotor, the output shaft  2 , spool valve  11  and the pulse motor  14  are disposed on the same axial line, the entire length of it is long. For this reason, it is difficult to neatly assemble the electro-hydraulic servomotor into another machine. A speed ratio of the screw members  10  and  12  is 1:1. Because of this, to increase the spindle speed of the output shaft  2 , it is necessary to increase a capacity of the pulse motor  14  and to drive the pulse motor  14  at high speed. The spool valve  11  rotates together with the screw member  12 . Therefore, a sliding surface of the casing  1 , which is in contact with the spool valve  11 , will be worn because of presence of its friction resistance. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide an electro-hydraulic servomotor which is small in size. 
     Another object of the present invention is to provide an electro-hydraulic servomotor which enables the capacity of it to be reduced, and is free from wearing of the spool valve and the casing. 
     Another object of the invention is to provide a small electro-hydraulic servomotor which reliably controls a spool position of the spool in the axial line direction independently of temperature of the operation oil. 
     As a preferred embodiment of the present invention, an electro-hydraulic servomotor is provided, which includes: an electric motor which rotates a drive shaft in response to an inputted signal; a hydraulic motor which rotates an output shaft using hydraulic pressure of operation oil; a first geared shaft rotatable along with the output shaft; a second geared shaft threadingly engaged with the drive shaft and meshed with the first geared shaft; a spool axially movable along with the second geared shaft depending on a rotational difference between the drive shaft and the first geared shaft to control supply and discharge of the operation oil to and from the hydraulic motor. According to the servomotor can be made small in size. 
     In the electro-hydraulic servomotor, the spool may be constructed as a single integral member, maybe divided into first and second discrete spool members. The first and second spool members are preferably urged toward one another. 
     The electro-hydraulic servomotor may further include: a displacement sensor which detects an axial position of the spool. 
     The electro-hydraulic servomotor may further include: a rotary sensor which detects number of rotation of the first geared shaft. 
     The present disclosure relates to the subject matter contained in Japanese patent application Nos. Hei. 11-13633 (filed on Jan. 21, 1999), Hei. 11-291477 (filed on Oct. 13, 1999), Hei. 11-291478 (filed on Oct. 13, 1999) and Hei. 11-348927 (filed on Dec. 8, 1999), which are expressly incorporated herein by reference in their entireties. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional side view showing an electro-hydraulic servomotor according to a first embodiment of the present invention. 
     FIG. 2 is a sectional view taken along a line B—B of FIG.  1 . 
     FIG. 3 is a schematic view showing an arrangement of the electro-hydraulic servomotor shown in FIG.  1 . 
     FIG. 4 is a perspective view showing major parts of the electro-hydraulic-servomotor shown in FIG.  1 . 
     FIG. 5 is a front view showing an electric motor and the vicinities thereof in the electro-hydraulic motor shown in FIG.  1  . 
     FIG. 6 is a sectional view showing an electro-hydraulic servomotor according to a second embodiment of the present invention. 
     FIG. 7 is a sectional view taken along a line B—B of FIG.  6 . 
     FIG. 8 is a sectional view showing an electro-hydraulic servomotor according to a third embodiment of the present invention, which is taken along a line corresponding to the line B—B of FIG. 1 or  6 . 
     FIG. 9 is a sectional side view showing spool position detecting means and vicinities thereof shown in FIG.  8 . 
     FIG. 10 is a side view showing the spool position detecting means. 
     FIG. 11 is a sectional side view showing an electro-hydraulic servomotor according to a fourth embodiment of the present invention. 
     FIG. 12 is a sectional view taken along a line A—A of FIG.  11 . 
     FIG. 13 is a sectional side view showing a related. electro-hydraulic servomotor. 
     FIG. 14 is a sectional view taken along a line A—A of FIG.  13 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
     &lt;1st Embodiment&gt; 
     A construction of an electro-hydraulic servomotor according to an embodiment of the present invention will be described. 
     In FIGS. 1 through 4, an electro-hydraulic servomotor  100  includes a first casing  30  shaped like a cup, and a second casing  31  fastened to the first casing  30  by a bolt  32 . The first casing  30  includes a bolt hole  33  bored therein into which a bolt is screwed when the electro-hydraulic servomotor  100  is firmly fixed to an external machine, not shown. An oil supplying passage  31   a,  communicating passages  31   b  and  31   c,  and an oil discharging passage  31   d  are formed in the second casing  31 . 
     A pulse motor  40  as an electric motor for rotating a rotary shaft  41  in accordance with a signal input thereto is mounted on the outer wall of the second casing  31 . A drive shaft  51 , as a first shaft, having a male screw  51   a  formed in the outer circumferential surface is integrally coupled to the rotary shaft  41  of the pulse motor  40  such that those shafts will rotate in the same directions. In the embodiment, the rotary shaft  41  and the drive shaft  51  are formed in a one-piece construction. If required, those drive shafts  41  and  51  may separately be formed. Reference numeral  37  designates a cap cover for preventing the operation oil from flowing into a pulse motor body  42 . 
     A first helical gear  52 , as a second shaft, is cylindrical in shape, and includes a female screw  52   a  formed on the inner circumferential surface thereof and an external gear  52   b  formed on the outer circumferential surface thereof. The first helical gear  52  is coupled to the drive shaft  51  such that the male screw  51   a  of the drive shaft  51  is screwed into the female screw  52   a  of the first helical gear  52 . A second helical gear  53 , as a third shaft, which includes an external gear  53   a  formed on the outer circumferential surface thereof, is coupled to the first helical gear  52  such that the external gear  52   b  of the first helical gear  52  intermeshes with the external gear  53   a  of the second helical gear  53 , while those helical gears  52  and  53  are oriented such that the axial lines of those helical gears are perpendicular to each other. 
     One end of a hydraulic pressure motor  60  as hydraulic pressure driving means to be described later is integrally coupled to one end of the second helical gear  53  with the aid of a coupling member  54  such that the motor and the gear rotate in the same directions. The other end of the second helical gear  53  is rotatably supported on a cap cover  34  applied to the second casing  31 . In the embodiment, the second helical gear  53  and an output shaft  61  are separately formed. If necessary, those component parts  53  and  61  may be formed in one-piece construction. 
     The male screw  51   a,  female screw  52   a,  external gear  52   b  and external gear  53   a  are configured such that when the number of revolutions of the drive shaft  51  is different from that of the second helical gear  53 , the first helical gear  52  moves in the axial line direction while rotating about its axis in accordance with the number-of-revolutions difference. 
     The hydraulic pressure motor  60  is rotatably supported on the first and second casings  30  and  31  with the aid of gears  68  and  69 . The hydraulic pressure motor  60  is made up of the output shaft  61 , a valve plate  62 , a cylinder block  63 , pistons  64 , shoe members  65 , and a slanted plate  66 . The output shaft  61  is urged toward the other end thereof by an urging force of a spring  67 . The valve plate  62 , fastened to the side wall of the second casing  31 , includes a plurality of arcuate holes  62   a.  Those holes are arranged equidistantly in the circumferential direction on the valve plate, and communicate with the communicating passage  31   b  and the communicating passage  31   c.  The cylinder block  63  is brought into slidable contact with the valve plate  62  by an urging force of the  67 . The cylinder block  63  is fixed to the outer circumference of the output shaft  61  such that the block and the shaft rotate in the same directions. The cylinder block  63  includes a plurality of pressure chambers  63   a . Those pressure chambers  63   a  are arranged equidistantly arranged on the cylinder block in a state that their axial lines are parallel to the axial line of the output shaft  61 . A plurality of pistons  64  include spherical ends  64   a  formed at the top ends, respectively. And those are located within the pressure chambers  63   a  of the cylinder block  63  such that those are slidable in the axial line directions. The shoe members  65  engage the spherical ends  64   a  of the pistons  64  while rollable thereon. The slanted plate  66  is secured to the inner wall of the first casing  30 . It slidably engages the shoe members  65 . It includes a slanted surface  66   a  slanted at a given angle with respect to the output shaft  61 . 
     The output shaft  61  protruded out of the first casing  30  is coupled to a drive section of the external machine (not shown) so that its rotational force is transmitted to the drive section. 
     A spool valve  70  is formed with a spool  71  and the second casing  31 . 
     A spool  71  is coupled to the first helical gear  52  through gears  55  and  56  as a pair of gear means. The spool  71  slidably engages a cap cover  36  mounted on the second casing  31 , while a key  35  as spool-rotation preventing means interposed therebetween. Therefore, the spool  71  does not rotate about its axis. 
     The gears  55  and  56  consist of thrust bushes, respectively. 
     An elongated groove  71   c,  while extending in the axial line direction, is formed in the mid portion of the spool  71  as viewed in the axial line direction. The first helical gear  52  is inserted into the elongated groove  71   c,  and held by the spool  71  such that the axial line of the spool  71  is parallel to that of the first helical gear  52 . The spool  71  slidably engages the cap cover  36 , which is mounted on the second casing  31  with the aid of the key  35 . With this structure, the spool  71  does not turn about its axis. 
     Annular grooves  71   a  and  71   b  are formed in the outer circumferential surface of the spool  71 . Those grooves allow the oil supplying passage  31   a  and the oil discharging passage  31   d  of the second casing  31  to communicate with the communicating passage  31   b  or  31   c.    
     An operation of the thus constructed electro-hydraulic servomotor  100  will be described. 
     When the number of revolutions of the rotary shaft  41  is different from that of the output shaft  61 , the electro-hydraulic servomotor  100  rotates the output shaft  61  in accordance with a number-of-revolutions difference between those shafts  41  and  61 . 
     An operation description will be given hereunder about a case where when the number of revolutions of the rotary shaft  41  is different from that of the output shaft  61 , the electro-hydraulic servomotor  100  rotates the output shaft  61  in accordance with the number-of-revolutions difference between those shafts  41  and  61 . 
     Since the drive shaft  51  is integrally coupled to the rotary shaft  41  such that those shafts rotate in the same directions, the number of revolutions of the rotary shaft  41  is equal to that of the drive shaft  51 . Since the second helical gear  53  is integrally coupled to the output shaft  61  through the coupling member  54  such that those components rotate in the same direction, the number of revolutions of the output shaft  61  is equal to that of the second helical gear  53 . 
     Therefore, when a difference is produced between the numbers of revolutions of the rotary shaft  41  and the output shaft  61 , a difference is produced also between the numbers of revolutions of the drive shaft  51  and the second helical gear  53 . 
     When the number of revolutions of the drive shaft  51  is different from that of the second helical gear  53 , the first helical gear  52  moves in the axial direction while rotating about its axis in accordance with the difference of the number of revolutions between the drive shaft  51  and the second helical gear  53 , as described above. 
     When the first helical gear  52  moves in the axial direction while rotating about its axis, the spool  71  is coupled to the first helical gear  52  through the gears  55  and  56 , and the spool  71  also moves in the axial line direction while linking with a motion of the first helical gear  52 . When the spool  71  moves in the axial direction with the motion of the first helical gear  52 , the operation oil flowing through the oil supplying passage  31   a,  communicating passage  31   b,  communicating passage  31   c  and oil discharging passage  31   d  varies in its flow rate since the annular grooves  71   a  and  71   b,  which communicate the oil supplying passage  31   a  of the second casing  31  with the communicating passage  31   b  or  31   c  thereof, are formed in the outer circumferential surface of the spool  71 . 
     When the operation oil flowing through the oil supplying passage  31   a,  communicating passage  31   b,  communicating passage  31   c  and oil discharging passage  31   d  varies in its flow rate, a flow rate of the operation oil flowing out into the plurality of the pressure chambers  63   a  since the communicating passages  31   b  and  31   c  communicate with the plurality of the pressure chambers  63   a,  which are formed in the cylinder block  63 , via the plurality of the arcuate holes  62   a  formed in the valve plate  62 . When the operation oil flowing out to the plurality of the pressure chambers  63   a  varies in its flow rate, The pistons  64  slides in the axial direction in accordance with a pressure of the operation oil flowing out into the plurality of the pressure chambers  63   a  since the pistons  64  are slidably located within the pressure chambers  63   a  of the cylinder block  63 . When the pistons  64  slide in the axial direction, the pistons  64  press the slanted surface  66   a  of the slanted plate  66  with the aid of the shoe members  65  since the spherical ends  64   a  of the pistons  64  engage the shoe members  65  in a rollable fashion, and the shoe members  65  slidably engage the slanted surface  66   a  of the slanted plate  66 . When the pistons  64  press the slanted surface  66   a  of the slanted plate  66  through the shoe members  65 , the cylinder block  63  is rotated about its axis by a counter force to the force by the pistons  64  which presses the slanted surface  66   a  of the slanted plate  66 . 
     When the cylinder block  63  rotates about its axis, the pressure chambers  63   a,  which are formed in the cylinder block  63  and communicate with the communicating passages  31   b  and  31   c  through the plurality of the arcuate holes  62   a  formed in the valve plate  62 , vary in pressure. When the pressure chambers  63   a,  which are formed in the cylinder block  63  and communicate with the communicating passages  31   b  and  31   c  through the plurality of the arcuate holes  62   a  formed in the valve plate  62 , vary in pressure, a flow rate of the operation oil flowing into the plurality of the pressure chambers  63   a  varies. When a flow rate of the operation oil flowing into the plurality of the pressure chambers  63   a  varies, the cylinder block  63  rotates again about its axis, as described above. 
     Accordingly, when the operation oil flowing through the oil supplying passage  31   a,  communicating passages  31   b  and  31   c  and oil discharging passage  31   d  varies in flow rate, the cylinder block  63  rotates about its axis in a rotational direction and at a spindle speed, which depend on a flow rate of the operation oil flowing through the oil supplying passage  31   a,  communicating passages  31   b  and  31   c  and oil discharging passage  31   d.    
     When the cylinder block  63  rotates about its axis in a rotational direction and at a spindle speed, which depend on a flow rate of the operation oil flowing through the oil supplying passage  31   a,  communicating passages  31   b  and  31   c  and oil discharging passage  31   d,  the output shaft  61  also rotates about its axis in a rotational direction and at a spindle speed, which depend on a flow rate of the operation oil flowing through the oil supplying passage  31   a,  communicating passages  31   b  and  31   c  and oil discharging passage  31   d  since the cylinder block  63  is fastened to the peripheral outer surface of the output shaft  61  such that the block and the shaft rotate in the same rotational directions. 
     A direction in which the first helical gear  52  axially moves while rotating about its axis when a difference of the number of revolutions between the drive shaft  51  and the second helical gear  53  is produced, may be determined by the configurations of the male screw  51   a,  female screw  52   a,  external gear  53   a  and external gear  52   b.  That is, when a difference of the number of revolutions is produced between the drive shaft  51  and the second helical gear  53  by the configurations of the male screw  51   a , female screw  52   a,  and external gears  53   a  and  52   b,  the rotational direction and the spindle speed in and at which the output shaft  61  rotates may be determined depending on the number-of-revolutions difference between the drive shaft  51  and the second helical gear  53 . 
     Accordingly, when the configurations of the male screw  51   a , female screw  52   a,  and external gears  53   a  and  52   b  are determined and as a result, a number-of-revolutions difference is produced between the drive shaft  51  and the second helical gear  53 , that is, a number-of-revolutions difference is produced between the rotary shaft  41  and the output shaft  61 , the output shaft  61  may be rotated so as to reduce the number-of-revolutions difference that is produced between the rotary shaft  41  and the output shaft  61 . 
     Thus, when the number-of-revolutions difference is produced between the rotary shaft  41  and the output shaft  61 , the electro-hydraulic servomotor  100  rotates the output shaft  61  in accordance with the number-of-revolutions difference between the rotary shaft  41  and the output shaft  61 . 
     The key  35  prevents the spool  71  from turning about its axis. Accordingly, it prevents such an unwanted situation that the spool  71  turns about its axis and collides with the second helical gear  53 , thereby damaging the spool  71  or the second helical gear  53 . 
     While in the embodiment described above, the second and third shafts are the helical gears, it is evident that those may be constructed with other suitable components than the helical gears. A given velocity ratio may be set up between the second and third shafts by use of another transmission gear, worm gear and worm wheel or the like. When the given velocity ratio may be set up between the second and third shafts, the number of revolutions of the output shaft  61  is reduced by the second and third shafts. Accordingly, the number of revolutions of the second shaft may be smaller than that of the output shaft  61 . As a result, the pulse motor  40  may be reduced in capacity, and hence the electro-hydraulic servomotor  100  is reduced in size. 
     In the embodiment, the gears  55  and  56  are constructed with thrust bushes. It is evident that any other components than the thrust bushes may be used if the following requirement is satisfied: when the first helical gear  52  moves in the axial line direction, the spool  71  is moved in the axial line direction, and when the first helical gear  52  rotates about its axis, the spool  71  is prevented from being turned about its axis. 
     In the embodiment, the first helical gear  52  is coupled to the second helical gear  53  such that the axial lines of those gears are perpendicular to each other. Accordingly, the axial line of the rotary shaft  41  is perpendicular to that of the output shaft  61 . If required, the rotary shaft  41  and the output shaft  61  may be arranged so that the prolongation of the axial line of the rotary shaft  41  is oriented at another angle with respect to the prolongation of the axial line of the output shaft  61 . 
     In the embodiment, the spool  71  is coupled to the first helical gear  52  through the gears  55  and  56 . If necessary, the spool  71  may be coupled to the first helical gear  52  through a spring. 
     &lt;2nd Embodiment&gt; 
     A second embodiment of the present invention will be described with reference to FIGS. 6 and 7. One of the features of the second embodiment resides in that the spool  71  in the first embodiment is divided into a couple of spools  71 A and  71 B. 
     A couple of spools  71 A and  71 B, respectively, are rotatably coupled to both ends of a helical gear  52 , while bearing  55  and  56  are interposed therebetween, respectively. The spools  71 A and  71 B are respectively urged by a couple of springs  153  so that those spools approach to each other. A backlash of a screw drive portion of the helical gear  52 , which will be caused by the drive shaft  151 , may be removed in a manner that the spring loads of the springs  153  are selected to have a proper difference therebetween. 
     The annular grooves  71 A a  and  71 B b,  while extending in the circumferential directions, are formed in the outer surfaces of the annular grooves  71 A a  and  71 B b,  respectively. When those spools are moved in the axial directions, the annular grooves  71 A a  and  71 B b  communicate with an oil discharging passage  31   d,  an oil supplying passage  31   a  and communicating passages  31   b  and  31   c,  which are formed in a second casing  31 , whereby the annular grooves  71 A a  and  71 B b  are controlled in their opening percentage. To be more specific, in FIG. 7, when the helical gear  52  is moved to the right, the oil discharging passage  31   d  communicates with the communicating passage  31   b,  and the communicating passage  31   c  communicates with the oil supplying passage  31   a,  and an operation oil is supplied to and discharged from an arcuate hole  62   a  of a valve plate  62 . When the helical gear  52  is moved to the left, the oil supplying passage  31   a  communicates with the communicating passage  31   b,  and the communicating passage  31   c  communicates with the oil discharging passage  31   d,  and the operation oil is supplied to and discharged from the arcuate hole  62   a  of the valve plate  62 . 
     An electric motor, e.g., a pulse motor  40 , is mounted on an outer wall of the second casing  31 . A drive shaft  151  is coupled to the motor shaft  41  of the pulse motor  40 . The drive shaft  151  is inserted into the helical gear  52 , and coupled to the same by means of screws. The pulse motor  40  is movable in either of the axial directions with rotation of the motor shaft  41  of the pulse motor  40 . 
     An operation of the invention will be described. 
     In the electro-hydraulic servomotor described above, when the drive shaft  151  is rotated, the helical gear  52  is moved to either of the axial directions, and the number of revolutions of the output shaft  61  is controlled following up the number of revolutions of the pulse motor  40 . The operation oil is supplied to the pressure chamber  63   a  of the cylinder block, and a counter force, which is generated when a top end  64   a  of a piston  64  presses a slanted plate  66 , causes the output shaft  61  to rotate together with the cylinder block  63 , whereby an external machine is driven. Selection of the supplying or discharging of the operation oil to and from the pressure chamber  63   a  is carried out by the cylinder block  63  and the arcuate hole  62   a  of the valve plate  62 . 
     When a load acts on the external machine by some reason, and the number of revolutions of the output shaft  61  decreases, the number of revolutions of the helical gear  53  decreases, so that a difference is produced between the number of revolutions of the helical gear  53  and that of the drive shaft  151 . The helical gear  52  helically moves with respect to the drive shaft  151 , and moves in its direction. 
     With the movement of the helical gear  52 , the couple of the spools  71 A and  71 B move in their axial direction, and the annular grooves  71 A a  and  71 B b  are increased in their opening percentage. For this reason, the operation oil that is introduced through the oil supplying passage  31   a  is supplied to one of the arcuate holes  62   a  and the pressure chamber  63   a  of the piston  64 , through the annular groove  71 A a  of the spool  71 A of those spools and the communicating passage  31   b.  In this case, an amount of the operation oil supplied to the arcuate holes  62   a  is larger than that of the operation oil supplied to the pressure chamber  63   a.  Accordingly, the piston  64  strongly presses the slanted plate  66 , and at the same time the operation oil in the compressed side pressure chamber  63   a  of the piston  64  is discharged in large amount through the oil discharging passage  31   d  from the other arcuate holes  62   a  of the valve plate  62 , via the communicating passage  31   c  and the annular groove  71 B b  of the other spool  71 B. As a result, the number of revolutions of the output shaft  61  increases. 
     In this way, with the movement of the spools  71 A and  71 B, the number of revolutions of the output shaft  61  is increased up to a predetermined number of revolutions, and the former is fairly accurately controlled so as to follow up the number of revolutions of the pulse motor  40 . 
     &lt;3rd Embodiment&gt; 
     One of the features of a third embodiment shown in FIGS. 8 through 10 resides in that a displacement sensor  80  is added to the mechanical arrangement of the first embodiment. 
     Reference numeral  80  designates a displacement sensor  80  as signal detecting means which detects a position of the spool  71  as viewed in the axial line direction, and outputs a spool signal in accordance with the spool position. The displacement sensor  80  includes a sensor shaft  81  and is fixed to the cap cover  36 . A male screw is formed at the top end  81   a  of the sensor shaft  81 . A female screw is formed in the sensor shaft coupling portion  71   c  of the spool  71 . Therefore, the sensor shaft  81  is coupled to the spool  71  by screwing the male screw of the top end  81   a  into the female screw of the sensor shaft coupling portion  71   c.    
     Reference numeral  90  designates a central processing unit (referred simply to as CPU) as input signal processing means which processes a signal to be input to the pulse motor  40  and a spool position signal so that a position of the spool  71  as viewed in the axial line direction is within a predetermined range, and outputs the resultant signal to the pulse motor  40 . 
     Reference numerals  91 ,  92  and  93  are signal transmission paths, respectively. 
     The pulse motor  40  is located at one end of the spool  71 , and the displacement sensor  80  is located at the other end of the spool  71 . 
     The electro-hydraulic servomotor  100  is capable of preventing the spool  71  from colliding with the cap cover  36  or the cap cover  37  by use of the displacement sensor  80 . 
     An operation of the displacement sensor  80  will be described. 
     As described above, the sensor shaft  81  is coupled to the spool  71 , so that when the spool  71  moves in the axial line direction, the sensor shaft  81  also moves in the axial line direction. Accordingly, the displacement sensor  80  detects a spool position of the spool valve  70  in the axial line direction by detecting a distance of the sensor shaft  81  measured from its initial position. 
     The displacement sensor  80  outputs a spool position signal which depends on the detected spool position of the spool valve  70  in the axial line direction. 
     Next, the function of the electro-hydraulic servomotor  100  which prevents the spool  71  from colliding with the cap cover  36  or  37  by use of the displacement sensor  80  will be described. 
     For some reason, for example, the reason that a great difference of the number of revolutions occurs between the rotary shaft  41  and the output shaft  61 , the spool  71  greatly moves in the axial line direction while linking with a motion of the first helical gear  52 , and approaches a position located within a predetermined distance from the cap cover  36  or cap cover  37 . 
     Then, the spool  71  approaches a position within a predetermined distance from the cap cover  36  or  37 , and then the CPU  90  judges that the spool  71  has approached a position within the predetermined distance from the cap cover  36  or  37 , from a spool signal output through the signal transmission path  93  from the displacement sensor  80 . 
     When the CPU  90  judges that the spool  71  has approached a position within the predetermined distance from the cap cover  36  or  37 , the CPU  90  processes a signal which comes in through a signal transmission path  91  and is to be input to the pulse motor  40  so that the spool  71  approaches a position within the predetermined distance, viz., a position of the spool  71  in the axial line direction, is put within a predetermined range, and outputs the processing result to the pulse motor  40 . 
     Finally, the pulse motor  40 , which has received the processed signal through a signal transmission path  92  from the CPU  90 , rotates the rotary shaft  41  in accordance with the signal coming in through the signal transmission path  92  from the CPU  90 . 
     Let us consider the following case: The signal to be input to the pulse motor  40  is input through the signal transmission path  91  to the CPU  90  from outside, and the CPU  90  outputs the signal, which comes from outside through the signal transmission path  91  and is to be input to the pulse motor  40 , to the pulse motor  40  through the signal transmission path  92 . As a result, a great difference of the number of revolutions is produced between the rotary shaft  41  and the output shaft  61 . The spool  71  greatly moves in the axial line direction while linking with a motion of the first helical gear  52 , and approaches a position within a predetermined distance from the cap cover  36  or the cap cover  37 . 
     In this case, the CPU  90  first judges that the spool  71  has reached a position within the predetermined distance from the cap cover  36  or cap cover  37 , by use of a spool signal output through the signal transmission path  93  from the displacement sensor  80 . 
     Then, the CPU  90  processes a signal to be input to the pulse motor  40  from outside via the signal transmission path  91  so that the spool  71  does not reach a position within the predetermined distance from the cap cover  36  or cap cover  37 , and the rotary shaft  41  rotates at the number of revolutions closest to that at which the rotary shaft rotates in accordance with the signal input to the pulse motor  40  from outside via the signal transmission path  91 , and outputs the processed signal to the pulse motor  40  by way of the signal transmission path  92 . 
     Let us consider the following case: The output shaft  61  receives a large load from an external machine. A great difference of the number of revolutions is produced between the rotary shaft  41  and the output shaft  61 . The spool  71  greatly moves in the axial line direction while linking with a motion of the first helical gear  52 , and reaches a position within the predetermined distance from the cap cover  36  or the cap cover  37 . 
     In this case, the CPU  90  first judges that the spool  71  has reached a position within the predetermined distance measured from the cap cover  36  or cap cover  37 , by use of the spool signal output from the displacement sensor  80  via the signal transmission path  93 . 
     Then, the CPU  90  processes a signal to be input to the pulse motor  40  from outside via the signal transmission path  91  so that the spool  71  does not reach a position within the predetermined distance from the cap cover  36  or cap cover  37 , and the rotary shaft  41  rotates at the number of revolutions closest to that at which the rotary shaft rotates in accordance with the signal input to the pulse motor  40  from outside via the signal transmission path  91 , and outputs the processed signal to the pulse motor  40  by way of the signal transmission path  92 . 
     While the embodiment is arranged so as to prevent the spool  71  from colliding with the cap cover  36  or cap cover  37 , the cap cover  36  or cap cover  37  may be substituted by any member if it will collide with the spool  71 . 
     The displacement sensor  80  is not limited to the those sensors employed in the embodiments, but may be any other sensor if it is capable of a spool position as viewed in the axial line direction of the spool valve  70 . 
     &lt;4th Embodiment&gt; 
     One of the features of a fourth Embodiment shown in FIGS. 11 and 12 resides in that a number-of-revolutions detector  180  is added to the mechanical arrangement of the first embodiment. 
     A detected shaft  181  as a fourth shaft is coupled at one end at the other and of the second helical gear  53 . The detected shaft  181  is accommodated in the a detector first housing  184  and a second housing a detector second housing  185 , which are mounted on the second casing  31 , and is rotatably supported on the detector second housing  185  by means of a bearing  183 . The number-of-revolutions detector  180  as a number-of-revolutions detecting means is installed in the detector first housing  184 . The number-of-revolutions detector  180  detects the number of revolutions of the detected shaft  181  at the other end of the detected shaft  181 , and outputs a number-of-revolutions signal in accordance with the number of revolutions of the detected shaft. A seal  182  is disposed in a space defined by the detector first housing  184  an the detected shaft  181 . The seal blocks a flow of the operation oil from the second casing  31  into the number-of-revolutions detector  180 . 
     Reference numeral  190  designates a central processing unit (CPU) as signal processing means. The CPU  190  receives a signal to be input to the pulse motor  40  and the number-of-revolutions signal. The CPU  190  processes the input signal by use of the number of revolutions of the rotary shaft  41  and the number-of-revolutions signal so that a position of the spool  71  as viewed in the spool  71  is located within a predetermined range, and outputs the processed one to the pulse motor  40 . In the figures,  191 ,  192  and  193  designate signal transmission paths, respectively. 
     Description will be given about the operation of the electro-hydraulic servomotor  100  to prevent the spool  71  from colliding with the cap cover  36  or  37 . 
     When the spool  71  greatly moves in the axial line direction while linking with a motion of the first helical gear  52 , and approaches a position within a predetermined distance measured from the cap cover  36  or  37 , the number of revolutions of the drive shaft  51  or the second helical gear  53  varies since a position of the first helical gear  52  in the axial line direction is determined by the number of revolutions of the drive shaft  51  and the second helical gear  5 . 
     Since the number of revolutions of the drive shaft  51 , i.e., the number of revolutions of the rotary shaft  41  is determined by the signal output from the CPU  190 , the CPU  190  always provides the number of revolutions of the drive shaft  51 . Since the number of revolutions of the second helical gear  53 , i.e., the number of revolutions of the detected shaft  181 , is applied, in the form of a number-of-revolutions signal, to the CPU  190  from the number-of-revolutions detector  180  by way of the signal transmission path  193 , the CPU  190  always obtains the number of revolutions of the second helical gear  53  from the number-of-revolutions signal output from the number-of-revolutions detector  180 . 
     When the number of revolutions of the drive shaft  51  or the second helical gear  53  varies, the CPU  190  judges that the spool  71  has reached a position within a predetermined distance from the cap cover  36  or the cap cover  37 . 
     When the CPU  190  judges that the spool  71  has reached a position within a predetermined distance from the cap cover  36  or the cap cover  37 , the CPU  190  processes a signal to be input to the pulse motor  40 , which comes in through the signal transmission path  191 , by use of the number-of-revolutions signal and the number of revolutions the rotary shaft  41  so that the spool  71  does no reach a position within a predetermined distance from the cap cover  36  or the cap cover  37 , viz., a position of the spool  71  as viewed in the axial line direction is within a predetermined range. Then, the CPU  190  outputs the processed one to the pulse motor  40  by way of the a 192 . 
     When the CPU  190  outputs the signal to the pulse motor  40  via the signal transmission path  192 , the pulse motor  40 , the pulse motor  40  rotates the rotary shaft  41  in accordance with the output signal of the CPU  190 , thereby locating a position of the spool  71  within the predetermined range. 
     In this way, the electro-hydraulic servomotor  100  prevents the spool  71  from colliding with the cap cover  36  or the cap cover  37 . 
     Exemplar cases where the spool  71  approaches a position within the predetermined distance from the cap cover  36  or the cap cover  37  follow. In a fist case, the CPU  190  outputs a signal to the pulse motor  40  via the signal transmission path  192 . As a result, a great difference of the number of revolutions is produced between the rotary shaft  41  and the output shaft  61 . The spool  71  greatly moves in the axial line direction while linking with a motion of the first helical gear  52 , and approaches a position within the predetermined distance from the cap cover  36  or cap cover  37 . In another case, the output shaft  61  receives a load from an external machine. As a result, a great difference of the number of revolutions is produced between the rotary shaft  41  and the output shaft  61 , and the spool  71  greatly moves in the axial line direction while linking with the first helical gear  52  and approaches a position within the predetermined distance from the cap cover  36  or cap cover  37 . 
     The number-of-revolutions detector  180  is not limited to the illustrated one, but may be any detector if it is capable of the number of revolutions of the detected shaft  181 .