Chain tension control mechanism

A tension control mechanism for a flexible drive member for a self-propelled power tool having a chassis, a power source mounted on the chassis, and a propulsion wheel comprising:

• a drive wheel rotatably mounted with respect to the chassis and driven by the power source;
• a jack wheel rotatably mounted with respect to the chassis and driven by the drive wheel, the jack wheel being operatively connected to the propulsion wheel;
• a flexible drive member operatively connecting with and entraining the drive wheel and the jack wheel;
• an idler arm pivotally mounted to the chassis;
• an idler wheel rotatably mounted to the idler arm and driven by the flexible drive member;
• a first bias member operatively connected with the idler arm and biasing the idler arm in a first rotational direction so that the idler wheel tends to displace the flexible drive member to increase tension in the flexible drive member;
• a second bias member operatively connected with the idler arm and biasing the idler arm in a second rotational direction so that the idler wheel tends to displace the flexible drive member to decrease tension in the flexible drive member;
• a ratchet arm having a proximal end portion and a distal end portion, the proximal end portion being operatively attached to the idler arm, and the distal end portion having a hook;
• a ratchet head fixed to the chassis, the ratchet head having a plurality of ratchet teeth for engaging the hook of the ratchet arm;
• wherein the hook and the ratchet teeth permit relative motion between the hook and the ratchet teeth in a first direction and limit relative motion between the hook and the ratchet teeth in a second direction, the relative motion in the first direction corresponding to movement of the idler arm in the first rotational direction, and the relative motion in the second direction corresponding to movement of the idler arm in the second rotational direction;
• and
• wherein the first bias member and the second bias member maintain the tension in the flexible drive member within a predetermined operating range, and the ratchet arm and the ratchet head cooperate with the first bias member and the second bias member to allow the idler arm to rotate in the first rotational direction to increase the tension in the flexible drive member and, once the idler arm has been rotated to increase the tension in the flexible drive member, to limit the rotation of the idler arm in the second rotational direction.