Patent Publication Number: US-8523288-B2

Title: Bit replacing device for excavating machine

Description:
TECHNICAL FIELD 
     The present invention relates to a bit replacing device provided on the cutter head of an excavating machine, by which a worn bit during excavation can be replaced with a new bit. 
     BACKGROUND ART 
     In Patent Literature 1, a rotary support member is arranged in the rear part of the front surface plate of a cutter head so as to rotate about an axial center vertical to the front surface plate. Further, a mounting hole is formed in a position of the rotary support member facing an opening formed on the front surface plate. A replacement bit is slidably contained in the mounting hole. During the replacement of bits, after the replacement bit is retracted from the opening of the front surface plate and is contained in the mounting hole of the rotary support member, the rotary support member is turned 90 degrees, so that the opening is closed by the closing surface portion of the front surface of the rotary support member. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent No. 3692267 
       
    
     However, according to Patent Literature 1, in a state in which the replacement bit is retracted and held in the mounting hole, the rotary support member is turned. Thus, a large rotary drive force is required, so that a large rotation device is disadvantageously required for the rotary support member. 
     SUMMARY OF INVENTION 
     Technical Problem 
     The present invention has been devised to solve the problem. An object of the present invention is to provide a bit replacing device for an excavating machine, by which the need for a large drive force for closing an opening can be eliminated, a replacement bit can be replaced by being retracted only in a linear direction, and the workability is not impaired. 
     Solution to Problem 
     In order to solve the above-described problem, the invention according to a first aspect is a bit replacing device for an excavating machine, the bit replacing device being provided on a cutter head which is placed in the front part of an excavating machine body to excavate the ground, and the bit replacing device includes: a rear surface frame provided on the rear surface side of the front surface plate of the cutter head to form a workspace for an operator to enter and exit; an opening into and from which at least one replacement bit can be inserted and removed, the opening being formed on the front surface plate; a bit guide body provided on the rear surface side of the opening, the bit guide body having a bit guide hole through which the replacement bit can be inserted and fixed; a gate plate placed between the front surface plate and the bit guide body so as to turn about an opening/closing axial center vertical to the front surface plate, the gate plate having a valve port through which the replacement bit can be inserted and removed; and a gate opening/closing device capable of turning the gate plate between a communication position for communicating the opening with the bit guide hole and a close position where the opening is closed. 
     The invention according to a second aspect is the bit replacing device for an excavating machine having the configuration of the first aspect, wherein the at least one replacement bit comprises a plurality of replacement bits provided at equal angles on the same circle centered about the opening/closing axial center of the gate plate. 
     The invention according to a third aspect is the bit replacing device for an excavating machine having the configuration of the second aspect, wherein the two replacement bits are provided on a radial axis line passing through the rotation axis of the cutter head, and the cross section of the workspace vertical to the radial axis line has a smaller width in the circumferential direction on the rear surface side than that in the circumferential direction on the front surface side. 
     The invention according to a fourth aspect is the bit replacing device for an excavating machine having the configuration of any one of the first to third aspects, wherein gate sealing materials are provided between the gate plate and the front surface plate and between the gate plate and the bit guide body, respectively, for cut off at the outer periphery of the gate plate and the peripheral edge of the valve port, and a bit sealing material for cut off is provided in a gap between the replacement bit and the bit guide hole. 
     The invention according to a fifth aspect is the bit replacing device for an excavating machine having the configuration of any one of the first to fourth aspects, wherein the gate opening/closing device includes a gear rotating mechanism having an external gear formed on the outer periphery of the gate plate in the circumferential direction centered about the opening/closing axial center and a pinion engaged with the external gear. 
     Advantageous Effects of Invention 
     According to the invention of the first aspect, after the replacement bit protruding from the opening is retracted and contained in the bit guide hole, the gate opening/closing device turns the gate plate from the communication position to the close position, and the valve port provided on the gate plate is displaced from the opening, so that the opening can be closed by the gate plate. Thus, the opening can be closed with a smaller drive device than in the prior art in which the rotary support member is turned together with the replacement bit. With this configuration, the bit guide body has only to be placed via the gate plate on the rear surface of the front surface plate, so that the weight and size of the bit replacing device can be reduced and the workspace can be widely obtained. Further, during the replacement of bits, since soil pressure or water pressure acts, a jig is required for pulling out the replacement bit. However, since the replacement bit is simply retracted in the linear direction and is pulled out without being positioned in other postures by a rotation or side shift, it is possible to easily install a jig. Moreover, the gate plate turned to open or close the opening is placed between the front surface plate and the bit guide body and is not exposed, resulting in high safety. Since the replacement bit is pulled out only backward, it is also possible to easily design the rear surface frame such that the workspace behind the replacement bit can be widely obtained beforehand. 
     According to the invention of the second aspect, since the multiple replacement bits can be provided on the same circle centered about the opening/closing axial center of the gate plate, the invention is advantageous in manufacturing cost. Further, since the multiple replacement bits can be closely provided, effective excavation can be achieved. 
     According to the invention of the third aspect, the cross section of the workspace is formed such that a width in the circumferential direction on the rear surface side (excavating machine body side) is smaller than that in the circumferential direction on the front surface side (cutter head side). Thus, it is possible to reduce frictional resistance due to the intake or agitation of excavated soil. Further, the depth of the workspace is smaller on two sides of the workspace in the circumferential direction. In response, the replacement bit can be arranged on the radial axis line, so that the workspace behind the replacement bit can be widely obtained, and the workability can be enhanced. 
     According to the invention of the fourth aspect, the gate sealing material can effectively prevent high-pressure mud water from entering the gap between the gate plate and the front surface plate and the gap between the gate plate and the bit guide body. Further, the bit sealing material can effectively prevent high-pressure mud water from entering through the gap between the replacement bit and the bit guide hole. 
     According to the invention of the fifth aspect, the gate opening/closing device includes the gear rotating mechanism having the external gear and the pinion, so that the gate plate can be easily turned by a manually-operated tool or automatic tool. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a cross-sectional view showing a main spoke member in an excavation use state of a bit in a first embodiment of a bit replacing device of a shield machine according to the present invention. 
         FIG. 1B  is a cross-sectional view showing the main spoke member in a replacement retreat state of the bit. 
         FIG. 2  is a front view showing the overall shield machine. 
         FIG. 3  is a vertical cross-sectional view showing the overall shield machine. 
         FIG. 4  is a partially enlarged front view showing a main cutter spoke. 
         FIG. 5  is a partially enlarged cross-sectional view showing the main cutter spoke as viewed from the rear. 
         FIG. 6  is a vertical cross-sectional view showing a bit replacing unit. 
         FIG. 7A  is a partial cross-sectional view showing a replacement bit as viewed from the bottom. 
         FIG. 7B  is a partial cross-sectional view showing the replacement bit as viewed from the side. 
         FIG. 7C  is a rear view showing the replacement bit. 
         FIG. 8  is a cross-sectional view taken along the line A-A of  FIG. 6 . 
         FIG. 9  is a cross-sectional view taken along the line B-B of  FIG. 6 . 
         FIG. 10A  is a front view showing an outer peripheral bit replacing unit. 
         FIG. 10B  is a vertical center cross-sectional view showing the outer peripheral bit replacing unit. 
         FIG. 11A  is a front view of a bit replacing unit showing a second embodiment of the bit replacing device of the shield machine according to the present invention. 
         FIG. 11B  is a cross-sectional view taken along the line C-C of  FIG. 11A . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Referring to the accompanying drawings, the following will describe embodiments of a bit replacing device for a shield machine which is an excavating machine according to the present invention. 
     First Embodiment 
     A first embodiment will be described with reference to  FIGS. 1A ,  1 B to  10 A, and  10 B. 
     As shown in  FIG. 3 , a pressure bulkhead  12  keeping a face colluvium pressure is provided in the front part of a cylindrically-shaped shield body (excavating machine body)  11 , and a rotary ring body  14  is supported by the pressure bulkhead  12  via a rotary bearing  13  so as to rotate about shield axial center O (excavating machine axial center). A circular cutter head  16  is supported at the front ends of a plurality of support legs  15  projecting forward from the rotary ring body  14 . A cutter drive device  17  for rotationally driving the cutter head  16  is provided behind the pressure bulkhead  12 . The cutter drive device  17  includes a ring gear  17   a  provided on the backside of the rotary ring body  14 , a plurality of drive pinions  17   b  engaged with the ring gear  17   a , and a plurality of rotation drive devices (hydraulic or electric motors)  17   c  for rotationally driving the drive pinions  17   b . Further, a screw conveyor for soil removal  18  is provided on the pressure bulkhead  12  to discharge soil excavated by the cutter head  16  backward in the space kept at an atmospheric pressure while keeping a face colluvum pressure. 
     As shown in  FIG. 2 , the cutter head  16  includes a plurality of main spoke members  21  extended along a radial axis RL from a center member  20  on the shield axial center O, a plurality of auxiliary spoke members  22  extended between the main spoke members  21  in a radial direction, and an intermediate ring member  23  and an outer peripheral ring member  24  which are placed in an arc direction centered about the shield axial center O to connect the main spoke members  21  and the auxiliary spoke members  22 . Soil inlets  25  are formed between the members  21  to  24 . 
     A plurality of bit replacing units (bit replacing devices)  30  of the present invention are arranged on the front part of the main spoke member  21 , and two replacement bits  31  are provided for each bit replacing unit  30 . Outer peripheral bit replacing units  60  are provided on two sides of the outer peripheral end of the main spoke member  21  in a circumferential direction. Further, a plurality of fixed bits  26  are provided on the respective front and side surfaces of the center member  20  and auxiliary spoke members  22  and the respective side surfaces of the main spoke members  21 . 
     As shown in  FIG. 3 , on the rear surface side of the center member  20 , a manhole  27  is provided through which an operator can enter and exit. The manhole  27  passes through the pressure bulkhead  12  and communicates with the backward space such that the operator can enter and exit from the backward space kept at the atmospheric pressure. Further, a rear surface frame  29  is provided on the rear surface side of the main spoke member  21  to form a workspace  28  which the operator can enter and exit through the manhole  27 . In the cross section of the rear surface frame  29  of  FIGS. 1A and 1B , a width on the rear surface side in the circumferential direction (direction of rotation) is smaller than that on the front surface side in the circumferential direction (direction of rotation). In other words, the rear surface frame  29  includes right and left side surface plates  29   a  suspended backward from the right and left side edges of a spoke front surface plate (front surface plate)  21   a , inclined surface plates  29   b  which are inclined inward from the back edges of the right and left side front surface plates  29   a  to the rear surface side, and a rear surface plate  29   c  linking the back edges of the right and left inclined surface plates. The rear surface frame  29  is substantially trapezoidal in cross section such that the width of the rear surface plate  29   c  is small. With this configuration, the frictional resistance due to the intake and agitation of excavated soil is reduced, the strength is ensured, and the workspace  28  has a depth which is tapered backward on two sides of a radial axis line RL ( FIGS. 2 and 4 ) in the circumferential direction. 
     As shown in  FIGS. 2 ,  4 , and  5 , the bit replacing units  30  are arranged on the main spoke member  21  at a predetermined pitch along the radial direction. Two replacement bits  31  are placed for each bit replacing unit  30  on the radial axis line RL serving as the center of the main spoke member  21 , and the widths of the workspace  28  in the depth direction and the circumferential direction behind the replacement bits  31  are sufficiently obtained, so that the workability is improved. 
     (Replacement Bit) 
     The replacement bit  31  has, as shown in  FIGS. 7A to 7C , a cylindrical base cylinder  31   b , an excavating blade portion  31   a  composed of a base  31   h  attached to the front surface of the base cylinder  31   b  and an excavating blade  31   i  protruded forward from the base  31   h , and a flange  31   c  which projects from the back end of the base cylinder  31   b  toward the outer peripheral side and has fixing bolt holes. The excavating blade portion  31   a  is smaller in outer diameter than the base cylinder  31   b  and is retractable through a bit guide hole  38  which will be described later. As shown in  FIGS. 1A ,  1 B, and  6 , the base cylinder  31   b  has a hollow portion  31   d  into which a push pull jack (jig)  55  is inserted, the push pull jack being capable of pushing and pulling the replacement bit  31 . The hollow portion  31   d  communicates with and opens to the workspace  28  via a center hole  31   j  of the flange  31   c . Use position recesses  31   e  for positioning in an excavation use state are formed in the symmetric positions of the flange  31   c . Further, retreat position recesses  31   f  for positioning in a replacement retreat position where the replacement bit  31  is retracted are formed in the front part of the base cylinder  31   b . Moreover, a detent edge  31   g  is provided on the flange  31   c  to notch a face of the flange orthogonally crossing the use position recess  31   e  in a parallel fashion. 
     (Bit Replacing Unit) 
     As shown in  FIG. 4 , the bit replacing unit  30  has continuous mounting holes  21   b  formed on the radial axis line RL on the spoke front surface plate  21   a  of the main spoke member  21 , the mounting holes  21   b  having circles parts of which overlap each other. Unit front surface plates  32  are fitted to the circular portions of the mounting holes  21   b  via pairs of right and left mounting ring pieces  40  and are fixed by fixing bolts. Opening/closing axial centers OC are provided in parallel to the shield axial center O at the center positions of the unit front surface plates  32 . Openings  33  are formed for each unit front surface plate  32  at two intersection points of arcs centered about the opening/closing axial center OC and the radial axis line RL. Further, as shown in  FIGS. 1A ,  1 B, and  6 , a gate plate  34  is arranged on the rear surface side of the unit front surface plate  32  via the mounting ring pieces  40  so as to rotate about the opening/closing axial center OC. The gate plate  34  has two valve ports  35  capable of communicating with and closing the two openings  33 . A gate opening/closing device  36  is provided which can reciprocatingly turn the gate plate  34  90 degrees between a communication position where the opening  33  coincides with the valve port  35  and a close position where the valve port  35  is displaced from the opening  33  to close the opening. A bit guide body  37  is attached on the rear surface side of the gate plate  34  and has two guide holes  38  communicating with the openings  33  via the valve ports  35 . 
     (Gate Opening/Closing Device) 
     The gate opening/closing device  36  includes a gear rotating mechanism having an external gear  36   a , a pinion  36   b , and a drive shaft  36   c . The external gear  36   a  is formed in the range of about 90 degrees in a circumferential direction centered about the opening/closing axial center OC on the outer periphery of the circular gate plate  34 . The pinion  36   b  is rotationally supported between the bit guide body  37  and the mounting ring piece  40  to engage with the external gear  36   a . The drive shaft  36   c  having the pinion  36   b  fixed thereto protrudes to the side of the workspace  28 . A manually-operated tool such as a ratchet wrench or an automatic tool such as an electric or hydraulic torque wrench is mounted on the drive shaft  36   c  to rotate the drive shaft  36   c , drive the gate opening/closing device  36 , turn the gate plate  34 , and open and close the opening  33 . 
     Further, a rotation regulating tool  39  for regulating a turn of the gate plate  34  is provided on the rear surface (or front surface) of the gate plate  34 . The rotation regulating tool  39  includes an arc groove  39   a  and a guide pin  39   b . The arc groove  39   a  is formed over the range of 90 degrees centered about the opening/closing axial center OC on the rear surface of the gate plate  34 . The guide pin  39   b  projects to the inner surface of a face plate portion  41  of the bit guide body  37  and is movably fitted to the arc groove  39   a . Thus, the turning limit of the gate plate  34  can be formed. 
     The gate opening/closing device  36  drives the outer periphery of the gate plate  34  to turn, reducing spatial interference with the workspace  28 . Further, small torque can turn the gate plate  34 , so that opening/closing can be easily performed by simple work equipment such as a manually-operated tool or automatic tool. Moreover, since the gate plate  34  is circularly-formed and the outer periphery is rotationally supported via the mounting ring piece  40 , an axis member serving as the turning center of the gate plate  34  becomes unnecessary. Accordingly, the outer diameter of the gate plate  34  can be reduced, resulting in a size reduction in the bit replacing unit  30 . 
     (Bit Guide Body) 
     The bit guide body  37  includes the face plate portion  41 , two holding cylinder portions  42 , and a linking block  43 . The outer periphery of the face plate portion  41  is fixed to the unit front surface plate  32  via the mounting ring piece  40  by fixing bolts to cover the rear surface of the gate plate  34 . The holding cylinder portions  42  project backward from the face plate portion  41  and have the bit guide holes  38 . The linking block  43  links the holding cylinder portions  42 . A fix block  44  engaged with the detent edge  31   g  of the replacement bit  31  is attached to the rear end surface of the linking block  43 . Further, a bolt hole, through which the flange  31   c  of the replacement bit  31  is fixed via fixing bolts, is formed on the rear end surface of the holding cylinder portion  42 . Moreover, locking claws  45  for positioning, which can be engaged with and disengaged from the use position recesses  31   e  and retreat position recesses  31   f , are placed in the symmetric positions via engaging/disengaging bolts  46 . The locking claws  45  can be engaged with or disengaged from the use position recess  31   e  and retreat position recess  31   f  and be fixed, since the engaging/disengaging bolts  46  are inserted through the long holes of the locking claws  45 . 
     (Cut-Off Sealing Material) 
     In the bit replacing unit  30 , in order to prevent high-pressure mud water from entering from the face side, first and second bit sealing materials  51  and  52  are mounted on the outer peripheries of the base  31   h  and the rear ends of the base cylinder  31   b  of the replacement bit  31  via mounting grooves. In the use position, the first bit sealing material  51  seals a gap between the replacement bit  31  and the opening  33 , and the second bit sealing material  52  seals a gap between the base cylinder  31   b  and the bit guide hole  38 . In the replacement retreat position, the first bit sealing material  51  seals a gap between the base  31   h  of the replacement bit  31  and the bit guide hole  38 . The first and second bit sealing materials  51  and  52  can be replaced during the replacement of the replacement bit  31 , and sufficient sealing properties can be obtained with high reliability. 
     As a matter of course, the first and second bit sealing materials  51  and  52  can be mounted on the slidable contact surface of the unit front surface plate  32  and the face plate portion  41  via the mounting grooves. 
     First gate sealing materials  53  are mounted on the peripheral edge of the valve port  35  via mounting grooves on the front and rear surfaces of the gate plate  34 . Further, second gate sealing materials  54  are mounted on the outer periphery of the gate plate  34  via mounting grooves. The first and second gate sealing materials  53  and  54  on the front surface prevent mud water from entering a gap between the gate plate  34  and the unit front surface plate  32 , and the first and second gate sealing materials  53  and  54  on the rear surface prevent mud water from entering a gap between the gate plate  34  and the face plate portion  41 . 
     The first gate sealing material  53  for sealing the periphery of the valve port  35  passes through the opening  33  and the bit guide hole  38  during a turn of the gate plate  34 . However, the first gate sealing material  53  completely adheres to a sliding surface with a low risk of damage during excavation and replacement, so that sufficient sealing properties can be ensured. Further, the second gate sealing material  54  for sealing the outer periphery of the gate plate  34  closely adheres to the sliding surface at all times without a change in the capacity of sealing space and with a low risk of damage, so that sufficient sealing properties can be ensured. 
     As a matter of course, the first and second gate sealing materials  53  and  54  can be mounted on the side of the unit front surface plate  32  or the face plate portion  41  via mounting grooves. 
     (Push Pull Jack) 
     The push pull jack  55  of  FIGS. 1A ,  1 B, and  6  is a jig for pulling and pushing the replacement bit  31 , and a fix guide member  56  having a dovetail groove-shaped cross-section is attached to the inner surface of the rear surface plate  29   c  of the rear surface frame  29  along the radial axis RO. Meanwhile, a jack body  55   a  of the push pull jack  55  is inserted into the hollow portion  31   d  of the replacement bit  31 , and a connecting ring  57  fixed to the outer periphery of the jack body  55   a  is connected to the flange portion  31   c  via connecting bolts. A piston rod  55   b  of the push pull jack  55  is connected to an engagement block  58  which is adjustably fitted to the fix guide member  56 . 
     Thus, the jack body  55   a  is inserted into the hollow portion  31   d , and the connecting ring  57  is attached to the replacement bit  31 . The piston rod  55   b  is connected to the engagement block  58  fitted to the fix guide member  56 . Further, the push pull jack  55  is shrunk, and the replacement bit  31  is pulled out from the opening  33 , the valve port  35 , and the bit guide hole  38 . 
     With the above-described configuration, bit replacement can be performed similarly to the following bit replacing unit  30 . 
     (Bit Replacement of the Bit Replacing Unit  30 ) 
     In a tunnel excavation state, the replacement bit  31  is fixed in an excavation use position. When the excavation proceeds and the replacement bits  31  are worn, the cutter drive device  17  stops at a position where the main spoke member  21  having the targeted replacement bits  31  is positioned, for example, in a horizontal posture. Further, the operator enters the workspace  28  through the manhole  27  to replace the bits. 
     1) In the workspace  28  of the targeted bit replacing unit  30 , the operator inserts the jack bodies  55   a  of the push pull jacks  55  into the hollow portions  31   d  via the center holes  31   j  of the replacement bits  31  from the rear surfaces of the holding cylinder portions  42 , so that the jack bodies  55   a  are attached to the flange portions  31   c  of the replacement bits  31  via the connecting rings  57 . Further, the engagement blocks  58  are engaged with the fix guide member  56  attached to the rear surface plate  29   c  of the rear surface frame  29 , and the piston rods  55   b  of the push pull jacks  55  are extended, so that the push pull jacks  55  are connected to the engagement blocks  58 . 
     2) The fixing bolts of the flange  31   c  are removed, so that the replacement bit  31  and the bit guide bodies  37  are separated from each other. Further, the engaging/disengaging bolts  46  are loosened, so that the locking claws  45  are retracted from the use position recesses  31   e.    
     3) When the push pull jacks  55  are shrunk, the replacement bits  31  are retracted, and the leading ends of the excavation blade portions  31   a  pass through the valve ports  35  of the gate plates  34  and are contained in the bit guide holes  38 , the locking claws  45  protrude to be engaged with the retreat position recesses  31   f , the engaging/disengaging bolts  46  are tightened, and the replacement bits  31  are fixed in the replacement retreat positions. 
     4) When the two replacement bits  31  are fixed in the replacement retreat positions, a manually-operated tool or automatic tool is used to rotationally drive the drive shaft  36   c  of the gate opening/closing device  36 . When the gate plate  34  is turned 90 degrees by means of an opening/closing gear mechanism, the gate plate  34  closes the two openings  33 . 
     5) The piston rods  55   b  of the push pull jacks  55  and the fix guide member  56  are separated from each other, the connecting rings  57  of the jack bodies  55   a  are removed out from the flanges  31   c  of the replacement bits  31 , the push pull jacks  55  are separated from the replacement bits  31 , and then the push pull jacks  55  are removed out. 
     6) After the engaging/disengaging bolts  46  are loosened and the locking claws  45  are retracted from the retreat position recesses  31   f , the worn replacement bits  31  are removed out from the bit guide holes  38  and are transported. 
     7) New replacement bits  31  transported into the workspace  28  are loaded into the bit guide holes  38 . The locking claws  45  are engaged with the retreat position recesses  31   f , the engaging/disengaging bolts  46  are tightened, and the replacement bits  31  are fixed in the replacement retreat positions. 
     8) The jack bodies  55   a  of the push pull jacks  55  are inserted into the hollow portions  31   d  through the center holes  31   j  of the replacement bits  31 , and the jack bodies  55   a  are fixed to the flanges  31   c  of the replacement bits  31  via the connecting rings  57 . Further, the engagement block  58  engaged with the fix guide member  56  and the piston rod  55   b  of the push pull jack  55  are connected to each other. 
     9) The gate opening/closing device  36  is driven by the manually-operated tool or automatic tool to turn the gate plate  34  90 degrees in the opposite direction, the valve ports  35  coincide with the two openings  33 , and the openings  33  are opened. 
     10) After the engaging/disengaging bolts  46  are loosened and the locking claws  45  are retracted from the retreat position recesses  31   f , the push pull jacks  55  are extended to push the replacement bits  31  into the bit guide holes  38 . The excavation blade portions  31   a  protrude forward from the valve ports  35  via the openings  33 . Further, the locking claws  45  are engaged with the use position recesses  31   e  to be positioned, and the flanges  31   c  and the holding cylinder portions  42  are connected to each other by the fixing bolts, and new replacement bits  31  are fixed to the bit guide bodies  37  in the excavation use position. 
     11) The connecting rings  57  provided on the jack bodies  55   a  of the push pull jacks  55  are detached from the flanges  31   c  of the replacement bits  31 , and the piston rods  55   b  and the fix guide member  56  are separated from each other, and the push pull jacks  55  are removed out from the replacement bits  31 . 
     (Outer Peripheral Bit Replacing Unit) 
     As shown in  FIGS. 10A and 10B , an outer periphery excavation bit (replacement bit)  61  for outer periphery excavation is provided in the outer peripheral bit replacing unit  60 . The outer periphery excavation bit  61  is tilted forward at θ° to the opening/closing axial center, OC in an outer peripheral direction. A circular outer peripheral unit front surface plate  63  is attached to an outer peripheral front plate connected to the main spoke member  21  of the cutter head  16  via a mounting ring  62 , and an opening  64  is formed close to the outer periphery of the outer peripheral unit front surface plate  63 . A gate plate  65  is placed on the rear surface of the outer peripheral unit front surface plate  63  via the mounting ring  62  so as to turn about the opening/closing axial center OC in the range of 90 degrees. A valve port  66  is formed on the gate plate  65  so as to coincide and communicate with the opening  64 . A bit guide body  67  is also attached to the rear surface of the outer peripheral unit front surface plate  63  via the mounting ring  62 . The bit guide body  67  has a bit guide hole  68  communicating with the valve port  66  and the opening  64 , and is tilted at the same angle θ°. Other constituent elements are configured similarly to those in the gate opening/closing device  36  and the bit replacement unit  30  and are indicated by the same reference numerals, and an explanation thereof is omitted. 
     Effects of the First Embodiment 
     In the first embodiment, after the replacement bit  31  is retracted and contained in the bit guide hole  38  of the bit guide body  37 , the gate plate  34  is turned by the gate opening/closing device  36 , the valve port  35  is displaced from the opening  33 , and the opening  33  is closed by the gate plate  34 . 
     Thus, compared to the prior art in which a replacement bit is rotated concurrently with a rotary support member, the opening  33  can be closed with a small drive force since the gate plate  34  having the valve port  35  is turned. Further, the bit guide body  37  has only to be placed on the rear surface of the spoke front surface plate  21   a  via the gate plate  34 , so that the weight and size of the bit replacement unit  30  can be reduced and the workspace  28  can be widely obtained. 
     Further, since soil pressure or water pressure acts during the replacement of bits, the push pull jack  55  is required to pull or push the replacement bit  31 . However, the replacement bit  31  is simply retracted in a linear direction but is not positioned in other postures by a turn or side shift, so that the push pull jack  55  can be easily placed. 
     The gate plate  34  is mounted between the spoke front surface plate  21   a  and the face plate portion  41  of the bit guide body  37 , the outer periphery of the gate plate  34  is surrounded by the mounting ring piece  40 , and the driven portion is not exposed into the workspace  28 . Thus high safety is provided. 
     Moreover, the cross section of the workspace  28  formed by the rear surface frame  29  is formed such that the width in the circumferential direction on the rear surface side (excavating machine body side) is narrower than that on the front surface side (cutter head side). Thus frictional resistance due to the intake or agitation of excavated soil can be reduced while ensuring strength. Furthermore, since the replacement bit  31  is placed on the radial axis line RL and is retracted only backward along the linear direction during replacement, the workspace  28  can be sufficiently ensured in a direction of pulling out the replacement bit  31  during replacement, resulting in an improvement in workability. 
     Since the first and second gate sealing materials  53  and  54  are provided, it is possible to effectively prevent high-pressure mud water from entering through the gap between the gate plate  34  and the unit front surface plate  32  and a gap between the gate plate  34  and the bit guide body  37 . Further, since the first and second bit sealing materials  51  and  52  are provided, it is possible to effectively prevent high-pressure mud water from entering the gap between the replacement bit  31  and the bit guide hole  38 . 
     Moreover, since the gate opening/closing device  36  includes the gear rotating mechanism having the external gear  36   a  and the pinion  36   b , the gate plate  34  can be turned by a manually-operated tool or automatic tool to easily open or close the opening  33 . 
     Second Embodiment 
     Bit Replacing Unit 
     In the bit replacing unit  30  of the first embodiment, the two replacement bits  31  (openings  33 ) are placed on the radial axis line RL. However, as shown in  FIGS. 11A and 11B , in a bit replacing unit  70 , three replacement bits  31  are provided at regular intervals (equal angles) centered about an opening/closing axial center OC. In other words, in the bit replacing unit  70 , openings  33 , valve ports  35 , and bit guide holes  38  are formed 120° apart on the same arc centered about the opening/closing axial center OC, and the three replacement bits  31  are provided. The replacement bits  31  are tilted α° toward the front outer peripheral side, and a gate opening/closing device  36  reciprocatingly turns a gate plate  34  in the range of 120°. Other configurations are the same as those in the first embodiment, the same components are indicated by the same reference numerals, and an explanation thereof is omitted. 
     In the bit replacing unit  70 , the three replacement bits  31  can be provided, advantageously reducing cost, and the three replacement bits  31  can be closely provided, thereby enabling effective excavation. Further, the replacement bits  31  are tilted, so that the workspace  28  can be effectively used.