Abstract: The basal end part of a second arm is turnably connected to the leading end part of the first arm. An engagement recess is formed in the leading end face of the first arm. The second arm is provided at its basal end part with a lock member such that the lock member is movable in the longitudinal direction of the second arm, and the lock member is biased toward the basal end of the second arm by a tension coiled spring. The lock member is provided with an engagement pin engageable with the engagement recess through a roller so that the first arm and the second arm are non-turnably locked. A turnable member is turnably disposed on the lock member and abutted with the roller by a tension coiled spring. An abutment surface of the turnable member is abutted with an arcuate surface formed on the leading end part of the first arm. By this, the lock member is brought into a release retaining position where the roller is brought out of the engagement recess.

Abstract: A first bush is attached to each of the opposite end parts of a first cylindrical part of a first hinge member. Second bushes are attached respectively to the inner end parts of second cylindrical parts which are disposed at the opposite end parts of a second hinge member and to the opposite end parts of a second cylindrical part which is disposed at the central part of the second hinge member. The first bush is provided with a protection cylindrical part. This protection cylindrical part is fitted to the outer periphery of the first cylindrical part. The second bush is provided with a second protection cylindrical part. This second protection cylindrical part is fitted to the outer periphery of the second cylindrical part.

Abstract: A first bush is attached to each of the opposite end parts of a first cylindrical part of a first hinge member. Second bushes are attached respectively to the inner end parts of second cylindrical parts which are disposed at the opposite end parts of a second hinge member and to the opposite end parts of a second cylindrical part which is disposed at the central part of the second hinge member. The first bush is provided with a protection cylindrical part. This protection cylindrical part is fitted to the outer periphery of the first cylindrical part. The second bush is provided with a second protection cylindrical part. This second protection cylindrical part is fitted to the outer periphery of the second cylindrical part.

Abstract: The basal end part of a second arm is turnably connected to the leading end part of the first arm. An engagement recess is formed in the leading end face of the first arm. The second arm is provided at its basal end part with a lock member such that the lock member is movable in the longitudinal direction of the second arm, and the lock member is biased toward the basal end of the second arm by a tension coiled spring. The lock member is provided with an engagement pin engageable with the engagement recess through a roller so that the first arm and the second arm are non-turnably locked. A turnable member is turnably disposed on the lock member and abutted with the roller by a tension coiled spring. An abutment surface of the turnable member is abutted with an arcuate surface formed on the leading end part of the first arm. By this, the lock member is brought into a release retaining position where the roller is brought out of the engagement recess.