Abstract:
A bendable portion control device for an endoscope including a control portion and an insertion portion extending therefrom, the insertion portion having a bendable portion at a distal end thereof, the bendable portion control device includes a control wire fixed thereto and extending to the control portion through the insertion portion; a control knob disposed on the control portion; an operating force transmission mechanism having a pair of relatively rotatable members connected to the control knob and a proximal end of the control wire, respectively; and a torque limiting mechanism for allowing torque to be transmitted to the bendable portion via the control wire when the torque is equal to or less than a predetermined torque, and for preventing the torque from being transmitted to the bendable portion via the control wire by disengaging the relatively rotatable members from each other when exceeding the predetermined torque.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a bendable portion control device (bendable portion steering device) for manipulating and steering the steerable bendable portion of a flexible insertion portion of an endoscope. 
     2. Description of the Related Art 
     In typical endoscopes, the steerable bendable portion provided in the vicinity of the distal end of the insertion portion of the endoscope can be bent freely in any direction at any angle (i.e., the orientation of the tip of the distal end can be freely adjusted) by pulling and extending control wires from a control portion coupled to the proximal end of the insertion portion of the endoscope, and the control portion is provided thereon with a freely-rotatable control knob (bendable portion control knob) for manually pulling and extending the control wires from the proximal end side thereof. 
     If a control wire is accidentally snapped (broken) by being acted upon by an excessive tensile force, an extremely troublesome overhaul is required; moreover, the bendable portion remains bent, which may make it difficult to remove the insertion portion of the endoscope from a body cavity safely. To prevent this problem from occurring, an endoscope in which each control wire is provided at some midpoint therein with a reduced-strength portion which is weaker in strength than the control wire has been proposed. In this endoscope, the reduced-strength portion breaks upon an excessive tensile force acts on the control wire. This structure is disclosed in, e.g., Japanese unexamined patent publication 2003-339630. 
     On the other hand, in conventional endoscopes in which the control wires are pulled and extended by a motor(s), an endoscope which is provided between the motor and the proximal end of an associated control wire with a friction clutch which slips upon a load over a predetermined load being exerted so that the bendable portion may not push a parietal (somatic wall) excessively hard and so that an excessive load is not exerted on the motor is known in the art. This type of endoscope is disclosed in, e.g., Japanese examined utility-model publication S55-54481. 
     However, as disclosed in JUPP 2003-339630, even if the aforementioned reduced-strength portion that is weaker in strength than the control wire is installed at some midpoint therein, the reduced-strength portion breaks upon the bendable portion control knob being rotated with an excessive torque. Once the reduced-strength portion breaks, the broken part needs to be fixed, so that the endoscopic operation cannot continue to be performed at any rate. 
     As disclosed in JEUMP S55-54481, in the configuration in which the aforementioned friction clutch that slips upon a load over a predetermined load being exerted is provided, a cork disk serving as a frictional member needs to be sandwiched between two metal disks, and additionally, a disk spring (belleville spring) or the like for setting an appropriate frictional force needs to be provided to be overlaid on the metal disks having the cork disk therebetween. Due to this structure, even if the friction clutch can be installed in a portion of the endoscope which does not have to be directly manually operated like a motor drive mechanism, the bendable portion control knob increases in size to thereby deteriorate the operability of the endoscope if one intends to install the friction clutch into, e.g., the internal space of the bendable portion control knob that is manually operated to manipulate the bendable portion. This increase in size of the bendable portion control knob results in a serious loss in the operability of the endoscope. 
     SUMMARY OF THE INVENTION 
     The present invention provides a bendable portion control device of an endoscope which makes it possible for the endoscope to continue to be used without being damaged even if an excessive torque is exerted on the bendable portion control knob during use of the endoscope, and further makes it possible to achieve this bendable portion control device by a mechanism which is sufficiently small and thin so as to be capable of being installed in, e.g., the internal space of the bendable portion control knob. 
     According to an aspect of the present invention, a bendable portion control device is provided, for an endoscope including a control portion and an insertion portion extending from the control portion, the insertion portion having a bendable portion at a distal end thereof, the bendable portion control device including a control wire, a distal end of which is fixed to the bendable portion and which is extended to the control portion through the insertion portion; a manually-rotatable control knob disposed on the control portion for pulling the control wire to bend the bendable portion; an operating force transmission mechanism for transmitting a torque exerted on the control knob to the bendable portion via the control wire, the operating force transmission mechanism having a pair of relatively rotatable members one and the other of which are connected to the control knob and a proximal end of the control wire, respectively; and a torque limiting mechanism for allowing the torque to be transmitted to the bendable portion via the control wire by holding the pair of relatively rotatable members engaged with each other when the torque is one of equal to and less than a predetermined torque, and for preventing the torque from being transmitted to the bendable portion via the control wire by disengaging the pair of relatively rotatable members from each other when the torque exceeds the predetermined torque. 
     It is desirable for the torque limiting mechanism to be accommodated and arranged in an internal space of the control knob. Hence, the torque limiting mechanism can be easily made slim. 
     It is desirable for one and the other of the pair of relatively rotatable members to include a grooved rotational plate rotatable about an axis thereof, at least one engaging groove being formed in one of an outer peripheral surface and an inner peripheral surface of the grooved rotational plate; and at least one spring engaging arm made of a resilient material which is positioned along the one of the outer peripheral surface and the inner peripheral surface of the grooved rotational plate, in which the engaging groove is formed, to be freely rotatable relative to the grooved rotational plate. At least one engaging lug is provided on the spring engaging arm so as to project therefrom to be capable of being engaged in and disengaged from the engaging groove. 
     It is desirable for the engaging lug to project from the spring engaging arm at a free end thereof, and for the spring engaging arm to be resiliently deformed to make the engaging lug disengaged from the engaging groove upon the torque exceeding the predetermined torque. 
     It is desirable for the grooved rotational plate to be provided on a member to which the control wire is connected, for the engaging groove to be formed on an outer peripheral surface of the grooved rotational plate, and for the spring engaging arm to be provided on another member connected to the control knob. 
     It is desirable for the grooved rotational plate to be provided on a member connected to the control knob, for the engaging groove to be formed on an inner peripheral surface of the grooved rotational plate, and for the spring engaging arm to be provided on another member to which the control wire is connected. 
     It is desirable for the bendable portion control device to include a second spring engaging arm which is positioned along the one of the outer peripheral surface and the inner peripheral surface of the grooved rotational plate to be resiliently pressed against the one of the outer peripheral surface and the inner peripheral surface of the grooved rotational plate. 
     In a state where the engaging lug is disengaged from the engaging groove, it is desirable for the spring engaging arm to be in sliding contact with the grooved rotational plate to produce a frictional resistance between the spring engaging arm and the grooved rotational plate due to resiliency of the spring engaging arm when a relative rotation occurs between the grooved rotational plate and the spring engaging arm. 
     It is desirable for the spring engaging arm and the grooved rotational plate to not be in contact with each other in a state where the engaging lug is disengaged from the engaging groove. 
     It is desirable for the torque limiting mechanism to lie in a plane orthogonal to an axis of rotation of the control knob. 
     It is desirable for the operating force transmission mechanism to includes a cylindrical shaft fixed to a stationary member of the control portion to extend upright coaxially with an axis of the control knob; a tubular drive shaft positioned around the cylindrical shaft to be freely rotatable on an axis of the cylindrical shaft; and a pulley fixed to the tubular drive shaft, the proximal end of the control wire being fixed to the pulley. The grooved rotational plate is fixed to the tubular drive shaft. 
     It is desirable for the spring engaging arm and the second spring engaging arm to be integral to be formed as a substantially C-shaped ring. 
     According to the present invention, since the bendable portion control device is provided with a torque limiting mechanism, positioned in the operating force transmission mechanism, for allowing a torque which is exerted on the control knob to be transmitted to the bendable portion via the control wire when the torque is at or below a predetermined torque and for preventing the torque from being transmitted to the bendable portion via the control wire when the torque is over the predetermined torque, the endoscope can continue to be used without being damaged even if an excessive torque is exerted on the bendable portion control knob during use of the endoscope; moreover, this bendable portion control device can be achieved by the small and thin torque limiting mechanism which is sufficiently small and thin so as to be capable of being installed in, e.g., the internal space of the bendable portion control knob that is manually operated to manipulate the bendable portion. 
     The present disclosure relates to subject matter contained in Japanese Patent Application No. 2006-121596 (filed on Apr. 26, 2006) which is expressly incorporated herein in its entirety. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described below in detail with reference to the accompanying drawings in which: 
         FIG. 1  is a cross sectional view of a torque limiting mechanism provided in a bendable portion control device of a first embodiment of an endoscope according to the present invention (taken along I-I line in  FIG. 4 ); 
         FIG. 2  is a cross sectional view of the torque limiting mechanism shown in  FIG. 1  in an operational state; 
         FIG. 3  shows the outward appearance of the first embodiment of the endoscope, showing the overall structure thereof; 
         FIG. 4  is a longitudinal cross sectional view of an operating force transmission mechanism of the bendable portion control device of the first embodiment of the endoscope; 
         FIG. 5  is a cross sectional view of a torque limiting mechanism provided in the bendable portion control device of a second embodiment of the endoscope according to the present invention (taken along a section corresponding to the section shown by I-I line in  FIG. 4 ); 
         FIG. 6  is a cross sectional view of a torque limiting mechanism provided in the bendable portion control device of a third embodiment of the endoscope according to the present invention (taken along a section corresponding to the section shown by I-I line in  FIG. 4 ); 
         FIG. 7  is a cross sectional view of a torque limiting mechanism provided in the bendable portion control device of a fourth embodiment of the endoscope according to the present invention (taken along a section corresponding to the section shown by I-I line in  FIG. 4 ); 
         FIG. 8  is a cross sectional view of a torque limiting mechanism provided in the bendable portion control device of a fifth embodiment of the endoscope according to the present invention (taken along a section corresponding to the section shown by I-I line in  FIG. 4 ); and 
         FIG. 9  is a cross sectional view of a torque limiting mechanism provided in the bendable portion control device of a sixth embodiment of the endoscope according to the present invention (taken along a section corresponding to the section shown by I-I line in  FIG. 4 ). 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 3  shows the general configuration of a first embodiment of the endoscope. The endoscope is provided with a flexible insertion portion  1  and a control portion  4  coupled to the proximal end of the insertion portion  1 . The insertion portion  1  is provided in the vicinity of the distal end (free end) thereof with a remote-controllable bendable portion  2 , and is further provided with an end body  3  fixed to the end (free end) of the bendable portion  2 . An objective window and others (not shown) are provided in the end body  3 . 
     The control portion  4 , which is coupled to the proximal end of the insertion portion  1 , is provided thereon with a U-D control knob  5 UD for bending the bendable portion  2  upward and downward in a controlled manner, and an R-L control knob  5 RL for bending the bendable portion  2  rightward and leftward in a controlled manner. The U-D control knob  5 UD and the R-L control knob  5 RL are overlaid on each other and coaxially arranged so that each of the U-D control knob  5 UD and the R-L control knob  5 RL can be freely rotated on a common axis. 
     An upward-direction control wire  6 U and a downward-direction control wire  6 D are inserted into the insertion portion  1  and distal ends thereof and are fixed to the end body  3 . Turning the U-D control knob  5 UD counterclockwise causes the upward-direction control wire  6 U to be pulled toward the control portion  4  to thereby cause the bendable portion  2  to bend upward (e.g., upward direction of a monitoring screen which corresponds to forward direction of the control portion  4 ) as shown by two-dot chain lines in  FIG. 3 . Turning the U-D control knob  5 UD clockwise causes the downward-direction control wire  6 D to be pulled toward the control portion  4  to thereby cause the bendable portion  2  to bend downward. 
     Additionally, turning the R-L control knob  5 RL counterclockwise causes a leftward-direction control wire  6 L ( FIG. 4 ), which is inserted into the insertion portion  1  to be installed therein, to be pulled toward the control portion  4  to thereby cause the bendable portion  2  to bend leftward. Turning the R-L control knob  5 RL clockwise causes a rightward-direction control wire  6 R ( FIG. 4 ), which is installed in the insertion portion  1 , to be pulled toward the control portion  4  to thereby cause the bendable portion  2  to bend rightward. In this manner, the bendable portion  2  can be remote-controlled to be bent freely in any directions at any angle from the control portion  4  side. 
     The endoscope is provided, at midpositions of the upward-direction control wire  6 U and the downward-direction control wire  6 D inside of the control portion  4 , with two slack removing devices  7 U and  7 D which move with the upward-direction control wire  6 U and the downward-direction control wire  6 D to absorb slack thereof which occur when the upward-direction control wire  6 U and the downward-direction control wire  6 D are extended toward the bendable portion  2  from the U-D control knob  5 UD (in a direction opposite to the direction in which the upward-direction control wire  6 U and the downward-direction control wire  6 D are pulled), respectively, and the endoscope is further provided inside of the control portion  4  with two stationary stoppers  8 U and  8 D against which the two slack removing devices  7 U and  7 D abut when the upward-direction control wire  6 U and the downward-direction control wire  6 D are pulled to respective predetermined positions to prevent the upward-direction control wire  6 U and the downward-direction control wire  6 D from being pulled therebeyond, respectively. The control portion  4  is provided with a U-D brake control knob  15 UD and an R-L brake control knob  15 RL which are coaxially arranged with the U-D control knob  5 UD and the R-L control knob  5 RL. The U-D brake control knob  15 UD is operated to actuate a U-D brake mechanism  14  (see  FIG. 4 ) to lock the upward control wire  6 D and the downward-direction control wire  6 D, and the R-L brake control knob  15 RL is operated to actuate an R-L brake mechanism (not shown) to lock the leftward-direction control wire  6 L and the rightward-direction control wire  6 R. 
       FIG. 4  shows an operating force transmission mechanism of a bendable portion control device (bendable portion steering device) provided between the U-D control knob  5 UD and the proximal ends of the upward-direction control wire  6 U and the downward-direction control wire  6 D. The U-D control knob  5 UD is formed so that a ring-shaped control portion thereof has a large internal space, and a supporting column (cylindrical shaft)  10  of the operating force transmission mechanism is fixed to an internal main frame  50  of the control portion  4  to extend upright coaxially with the axis of the U-D control knob  5 UD. 
     As shown in  FIG. 4 , the operating force transmission mechanism is provided with a U-D pulley  9 UD having two outer peripheral grooves in which the upward-direction control wire  6 U and the downward-direction control wire  6 D are engaged and wound around the U-D pulley  9 UD by a half to one turn and extend therefrom toward the bendable portion  2 . The operating force transmission mechanism is also provided with an R-L pulley  9 RL, similar in structure to the U-D pulley  9 UD, having two outer peripheral grooves in which the leftward-direction control wire  6 L and the rightward-direction control wire  6 R are engaged to be wound around the R-L pulley  9 RL by a half to one turn and extend therefrom toward the bendable portion  2 . 
     An R-L tubular drive shaft  11  fixed at one end thereof to the R-L pulley  9 RL is rotatably fitted on the supporting column  10  and a U-D tubular drive shaft  12  fixed at one end thereof to the U-D pulley  9 UD is rotatably fitted on the R-L tubular drive shaft  11 . In order to transmit rotations of the R-L control knob  5 RL and the U-D control knob  5 UD to the R-L pulley  9 RL and the U-D pulley  9 UD, the R-L tubular drive shaft  11  and the U-D tubular drive shaft  12  project outwards (upwards as viewed in  FIG. 4 ) from the control portion  4  so that the other ends thereof are positioned outside of the control portion  4  to be associated with the R-L control knob  5 RL and the U-D control knob  5 UD, respectively. 
     A cylindrical U-D bearing  13  in which the U-D tubular drive shaft  12  is fitted to be freely rotatable on the axis thereof is fixed at the base end of the U-D bearing  13  to a base of the supporting column  10  which is fixed to the internal main frame  50  of the control portion  4 . The U-D bearing  13  also serves as a pulley cover which covers the R-L pulley  9 RL and the U-D pulley  9 UD to prevent the control wires  6 U and  6 D and the control wires  6 R and  6 L from being disengaged from the U-D pulley  9 UD and the R-L pulley  9 RL, respectively. 
     Rotating the U-D tubular drive shaft  12  about the axis thereof (on the axis of the supporting column  10 ) causes the U-D pulley  9 UD to rotate to thereby pull one of the control wires  6 U and  6 D in accordance the direction of rotation of the U-D tubular drive shaft  12 . 
     The U-D brake mechanism  14  gives frictional resistance to the rotating operation of the U-D control knob  5 UD about the axis thereof so that the bendable portion  2  remains stationary in a bent state. The U-D brake mechanism  14  is installed and accommodated in an internal space of the U-D control knob  5 UD and operated to give frictional resistance to the rotating operation of the U-D control knob  5 UD and release the same frictional resistance selectively via operation of the U-D brake control knob  15 UD. 
     A substantially disk-shaped seating plate  16  that is integral with the U-D control knob  5 UD is positioned thereon to close an outer open end of the internal space of the U-D control knob  5 UD, and an outer end surface of a grooved rotational plate (relatively rotatable member)  17  made of metal which is formed integral with the U-D tubular drive shaft  12  is in sliding contact with an inner surface of the seating plate  16  to be freely rotatable on the axis of the grooved rotational plate  17 . 
     As also shown in  FIG. 1 , that shows a cross sectional view taken along I-I line in  FIG. 4 , the grooved rotational plate  17 , the outer edge of which is formed in a circular shape as a whole, is provided on a part of an outer peripheral surface thereof with an engaging groove (notch)  21 . An arc-shaped spring engaging arm (relatively rotatable member)  18  made of a resilient material is fixed at a base end portion thereof to the seating plate  16  by three set crews  19  to be positioned around an outer periphery of the grooved rotational plate  17 . The spring engaging arm  18  is provided at a free end thereof with an engaging lug  22  which projects radially inwards to be freely capable of being engaged in and disengaged from the engaging groove  21  of the grooved rotational plate  17 . 
     When no external force is applied to the spring engaging arm  18 , the spring engaging arm  18  is set in a state shown in  FIG. 1  where the engaging lug  22  is engaged in the engaging groove  21  of the grooved rotational plate  17 . The spring engaging arm  18  in this state can freely rotate integrally with the seating plate  16  about the axis of the thereof, and the engaging lug  22  remains engaged in the engaging groove  21  of the grooved rotational plate  17  when a torque exerted on the U-D control knob  5 UD is equal to or less than a predetermined torque. 
     Accordingly, when the U-D control knob  5 UD is manually rotated, the spring engaging arm  18  rotates with the seating plate  16  to thereby cause the grooved rotational plate  17  to rotate, so that the rotational control force exerted on the U-D control knob  5 UD is transmitted to the control wires  6 U and  6 D via the U-D tubular drive shaft  12  and others. 
     Upon an excessive control force exceeding the aforementioned predetermined torque being exerted on the U-D control knob  5 UD, the spring engaging arm  18  is resiliently deformed to make the engaging lug  22  disengaged from the engaging groove  21  of the grooved rotational plate  17  as shown in  FIG. 2 , so that the grooved rotational plate  17  does not rotate with the seating plate  16  even if the seating plate  16  rotates, and consequently, a rotational force exerted on the U-D control knob  5 UD is not transmitted to the control wires  6 U and  6 D. Accordingly, even if an excessive torque is exerted on the U-D control knob  5 UD, the endoscope (control wires  6 U and  6 D) is not damaged. 
     If the engagement of the engaging groove  21  of the grooved rotational plate  17  with the engaging lug  22  is released in the above-described manner, the bendable portion control device shown in  FIGS. 1 and 2  can be brought back to a state where the engaging lug  22  is engaged in the engaging groove  21  of the grooved rotational plate  17  if the U-D control knob  5 UD is manually rotated after the grooved rotational plate  17  has been locked so that it cannot freely rotate about the axis thereof by actuating the aforementioned brake mechanism  14 , which makes it possible to allow the endoscopic operation to continue to be performed normally. 
     In this manner, the grooved rotational plate  17  (in which the engaging groove  21  is made) and the spring engaging arm  18  (on which the engaging lug  22  is formed) constitute a torque limiting mechanism for allowing a torque exerted on the U-D control knob  5 UD to be transmitted to the bendable portion  2  via the control wires  6 U and  6 D when the torque is equal to or less than a predetermined torque and for preventing a torque exerted on the U-D control knob  5 UD from being transmitted to the bendable portion  2  via the control wires  6 U and  6 D when the torque exceeds the predetermined torque. As shown in  FIG. 4 , this torque limiting mechanism is sufficiently thin and small so as to be capable of being installed in a small space in the U-D control knob  5 UD. The torque limiting mechanism lies in a plane orthogonal to the common axis of rotation of the U-D control knob  5 UD and the R-L control knob  5 RL. 
     In this particular embodiment of the endoscope, a second spring engaging arm  18 ′ extends from the base end portion of the spring engaging arm  18  to be positioned around an outer periphery of the grooved rotational plate  17 , similar to the spring engaging arm  18 . The spring engaging arm  18  and the second spring engaging arm  18 ′ are formed integral with each other so as to form a substantially C-shaped ring. The second spring engaging arm  18 ′ is provided at a free end thereof with a pressure projection  23  which projects radially inwards to be resiliently brought into pressing and sliding contact with an outer peripheral surface of the grooved rotational plate  17 . The second spring engaging arm  18 ′ is formed to be shorter in length than the spring engaging arm  18  that is provided with the engaging lug  22 . 
     Due to this structure, frictional resistance occurs between the grooved rotational plate  17  and an outer peripheral surface of the pressure projection  23  not only in a state as shown in  FIG. 1  where the engaging lug  22  is engaged in the engaging groove  21  of the grooved rotational plate  17  but also in a state as shown in  FIG. 2  where the engaging lug  22  is disengaged from the engaging groove  21 . Due to this frictional resistance, as long as either of the control wires  6 U and  6 D is not acted upon by a repulsive force from the bendable portion  2  or the like, manually rotating the U-D control knob  5 UD causes the grooved rotational plate  17  to rotate to some extent, thus causing the upward-direction control wire  6 U or the downward-direction control wire  6 D to be pulled so that the bendable portion  2  is bent to a certain extent. 
       FIG. 5  shows a cross sectional view of the torque limiting mechanism provided in the bendable portion control device of a second embodiment of the endoscope according to the present invention, taken along a section corresponding to the section shown by I-I line in  FIG. 4 . The torque limiting mechanism shown in  FIG. 5  is constructed so that the outer diameter of the grooved rotational plate  17  is reduced to a degree that the outer peripheral surface of the grooved rotational plate  17  does not come in contact with the engaging lug  22  to allow the U-D control knob  5 UD to rotate freely with no resistance upon the engaging lug  22  being disengaged from the engaging groove  21 . This structure facilitates the operation of bringing the engaging lug  22  back into the engaging groove  21 . 
       FIG. 6  shows the torque limiting mechanism provided in the bendable portion control device of a third embodiment of the endoscope according to the present invention. In this embodiment, the grooved rotational plate  17  is provided with a plurality of engaging grooves  21 , each into which the engaging lug  22  is engageable. According to this structure, even when the engaging lug  22  is disengaged from one engaging groove  21 , the engaging lug  22  is immediately re-engaged in another engaging groove  21  so that the bendable portion  2  returns to a controllable state thereof without delay. 
       FIG. 7  shows the torque limiting mechanism provided in the bendable portion control device of a fourth embodiment of the endoscope according to the present invention. In this embodiment, not only the grooved rotational plate  17  is provided with a plurality of engaging grooves  21  in each of which the engaging lug  22  is engageable just like the grooved rotational plate  17  shown in  FIG. 6  but also the second spring engaging arm  18 ′ is provided at a free end thereof with an engaging lug  22  which projects radially inwards to be freely capable of engaging in and disengaged from each engaging groove  21  of the grooved rotational plate  17 . With this structure, the engaging force between the engaging lug  22  and the engaging groove  21  in which the engaging lug  22  is engaged (i.e., the limit value of the torque at which the engaging lug  22  is disengaged from the engaging groove  21 ) can be set at a large force. 
       FIG. 8  shows the torque limiting mechanism provided in the bendable portion control device of a fifth embodiment of the endoscope according to the present invention. In this embodiment, the grooved rotational plate  17  is provided, on the outer peripheral surface thereof at substantially equi-angular intervals with a plurality of engaging grooves (notches)  21  (specifically three grooves in this particular embodiment), while a corresponding plurality of spring engaging arms  18  (specifically three spring engaging arms in this particular embodiment), each of which is provided at the free end thereof with an engaging lug  22  which projects radially inwards to be freely capable of being engaged in and disengaged from each engaging groove  21  are provided around the grooved rotational plate  17 . The torque limiting mechanism can be constructed in this manner to achieve a great engaging force between the engaging lugs  22  and the engaging grooves  21 . 
       FIG. 9  shows the torque limiting mechanism provided in the bendable portion control device of a sixth embodiment of the endoscope according to the present invention. In this embodiment, a grooved rotational plate  117  which is provided on an inner peripheral surface thereof with an engaging groove (notch)  21  is configured to rotate with the U-D control knob  5 UD, and a seating plate  116  which is provided on an outer peripheral surface thereof with an engaging lug  22  which projects radially outwards to be freely engageable in the engaging groove  21  is formed integral with the U-D tubular drive shaft  12  to which the upward-direction control wire  6 U and the downward-direction control wire  6 D are connected via the U-D direction pulley  9 UD. It is possible that the relative position between the engaging groove  21  and the engaging lug  22  be reversed in this manner in each of the above described embodiments. 
     The present invention is not limited solely to each of the above described particular embodiments. For instance, the torque limiting mechanism provided in each of the above described particular embodiments can also be applied to the other bendable portion control device (bendable portion steering device) provided between the R-L control knob  5 RL and the proximal ends of the rightward-direction control wire  6 R and the leftward-direction control wire  6 L. 
     Obvious changes may be made in the specific embodiments of the present invention described herein, such modifications being within the spirit and scope of the invention claimed. It is indicated that all matter contained herein is illustrative and does not limit the scope of the present invention.