Flexible tubular portion of endoscope and endoscope having this flexible tubular portion

1. An insertion portion of an endoscope, the insertion portion comprising:

a flexible tube having a distal end and a proximal end as defined with respect to a longitudinal axis of the flexible tube, the flexible tube being insertable into a body cavity along a longitudinal axial direction;

a bending portion extending from a distal end of the flexible tube, the bending portion being bendable in a direction different from the longitudinal axial direction in accordance with actuation of an operation knob of the endoscope; and

a spiral tube disposed in the flexible tube, the spiral tube comprising:

a sparsely wound portion in which a wire is spirally wound with adjacent turns of the wire being spaced apart from each other along the longitudinal axial direction; and

a closely wound portion in which the wire is spirally wound with adjacent turns of the wire being in close contact with each other along the longitudinal axial direction;

wherein the sparsely wound portion is provided at respective ends of the closely wound portion and terminates at the distal and proximal ends of the flexible tube, and

wherein in the closely wound portion, in which the adjacent turns of the wire are in close contact with each other along the longitudinal axial direction, the wire is under an initial tension that is a force which keeps the adjacent turns of the wire in close contact in the longitudinal axial direction against a load smaller than or equal to a predetermined value applied along a radial direction of the flexible tube.

2. The insertion portion according to claim 1 , wherein the closely wound portion and the sparsely wound portion are alternately provided along the longitudinal axial direction.

3. The insertion portion according to claim 2 , wherein a number of turns of at least one of the closely wound portion and the sparsely wound portion is configured to change along the longitudinal axial direction.

4. The insertion portion according to claim 1 , wherein an outside diameter of at least one of the closely wound portion and the sparsely wound portion is configured to change along the longitudinal axial direction.

5. The insertion portion according to claim 1 , wherein a magnitude of the initial tension is configured to change along the longitudinal axial direction.

6. The insertion portion according to claim 1 , wherein a cross-sectional shape of the wire of the closely wound portion in an axial direction of the closely wound portion is configured to change in the longitudinal axial direction.

7. The insertion portion according to claim 1 , wherein a cross-sectional shape of the wire of the closely wound portion is at least one of a rectangular shape, an oval shape, a circular shape, and an elliptic shape.

8. An endoscope comprising the insertion portion according to claim 1 .

9. The insertion portion according to claim 1 , wherein when the load is applied to the closely wound portion and the sparsely wound portion along the radial direction of the flexible tube, a flexing amount of the closely wound portion being flexed under the load is smaller than a flexing amount of the sparsely wound portion being flexed under the load.