Tensioner

A tensioner

• comprising
• a tubular member on which multiple locking teeth are formed, and
• one or more locking pieces on which are formed locking teeth that engage with said tubular member's locking teeth;
• and
• including a locking-piece receiver that receives said locking pieces and a shaft disposed inside said tubular member;
• and wherein said tensioner
• either said tubular member or said shaft moves reciprocally in relation to each other due to a biasing force;
• there is provided a ratchet mechanism in which said locking pieces move in the locking pieces' diameter-reduced direction so as to climb over the locking teeth of said tubular member so that said moving member can move in the locking pieces' diameter-reduced direction, and so as to restrain the backward movement of said moving member, with said restraint resulting because said locking pieces move in the locking pieces' diameter-enlarged direction so as to engage with the locking teeth of said tubular member, and
• said locking teeth are screw teeth, each of which has a lead, and the dimensions of the parts and other specifications are set so as to satisfy the relationship of Formula 3.
• R1·tan ρ1+R2·tan(ρ2−θ)<0 Formula 3
• where R1=d1e/2, d1e=effective contact diameter of the locking-piece receiver, R2=d2e/2, d2e=the effective diameter in contact of locking teeth of locking pieces, ρ1=tan−1μ1 (friction angle), μ1=apparent coefficient of friction of locking pieces and locking-piece receiver, ρ2=tan−1ρ2 (friction angle), ρ2=apparent coefficient of friction of the locking teeth of the tubular member and the locking pieces.