1. Field of the Invention
The present invention relates to an endoscope wherein a bending portion provided to an insertion portion is bent by operating a bending operation knob provided to an operation portion.
2. Description of the Related Art
Conventionally, there has been wide use of an endoscope apparatus wherein an elongated endoscope is inserted into a body cavity or the like to allow observation and various treatments of an area to be examined to be performed. In general, an endoscope of which insertion portion is flexible has a bending portion configured by articulating multiple bending pieces provided to the insertion portion. The bending portion is configured to bend in a desired direction by operating operation means. To be more precise, the bending portion is bent at a target angle by turning a bending operation knob provided to an operation portion and thereby moving forward and backward a bending wire fixed on a distal-most bending piece constituting the bending portion. It is thereby possible to perform observation and insertion into the area to be examined with an observation optical system provided at the distal end portion of the insertion portion in a target direction.
For instance, Japanese Patent Laid-Open No. 10-234654 indicates an endoscope which has a small number of parts, is easily adjustable as to assembly and a bending angle and includes an operation portion which securely controls bending exceeding a maximum bending angle. As for the endoscope, a horizontal knob or a vertical knob is turned in a desired direction upon completing connections of connected fixing portions in all the directions, so that a horizontal sprocket or a vertical sprocket corresponding to the operations of the knobs are turned and the corresponding connected fixing portion and bending operation wire are pulled into a user side. In that case, a compressive force generated in the insertion portion is received by an insertion side guide-coil and a user side guide-coil in each direction. Therefore, a pulling force of the corresponding bending operation wire transmits to the bending portion without shrinking the insertion portion so that the bending portion bends in the desired direction.
And when fingers manipulating the knob of the endoscope are released to put the knob in a so-called free state with the bending portion bending in the desired direction as mentioned above, the bending portion in a bending state changed to return from the bending state to a straight state. It was because a bending reaction of built-in components of the endoscope was larger than a sum of an actuating force amount of the operation portion, resistance of the bending portion and a frictional force between the wire and the guide-coils.
In the case of reducing a diameter of the endoscope insertion portion in an endoscope structure of Japanese Patent Laid-Open No. 10-234654, however, the diameters of the built-in components of the endoscope are also reduced so that the bending reaction generated from the built-in components becomes smaller when the built-in components are bent as the bending portion of the endoscope bends. Thus, the bending reaction of the built-in components of the endoscope becomes smaller than the sum of the actuating force amount of the operation portion, the resistance of the bending portion and the frictional force between the wire and the guide-coils. In that case, there is a possibility that, when the knob is put in the free state, the bending state of the bending portion which has been bent may be held as-is without returning to the straight state.
In the case of the endoscope of which maximum bending angle of the bending portion is 210 degrees, the sum of the actuating force amount of the operation portion, the resistance of the bending portion and the frictional force between the wire and the guide-coils becomes very large when in the bending state of bending the bending portion at more than 180 degrees. For that reason, there is a higher possibility that, when the knob is put in the free state, the bending state of the bending portion which has been bent exceeding 180 degrees may be held as-is. In the case where the bending state of the bending portion has been held in the state of bending exceeding 180 degrees, it becomes difficult, even if the knob is in the free state, to return the bending state of the bending portion to an approximately straight state with an external force when removing the insertion portion.
For the purpose of solving the problem, Japanese Patent Laid-Open No. 2002-236260 indicates an endoscope which can naturally return the bending portion to the approximately straight state and improve operability just by putting the operation means in the free state. In the case of the endoscope, bending resistance force as a resultant force of the actuating resistance of the operation knob, the bending resistance of the bending portion alone and the frictional force between a pulling wire and external members is smaller than the sum of the bending reaction generated by the built-in components when bending the bending portion.