in the direction of the arrow l in fig7 the above mentioned mechanism is conventional the bracket 33 which defines part of the frame includes a middle part fixed to the top end 32 and a base end part having its upper surface fixed to the top end 32 a forward end portion of the bracket 33 is curved upwardly and rotatably supports the base end 18b of the lever 18 a shaft 29 is rotatably mounted to a base end portion of the bracket 33 a base end of a lever 25 is fixed to one end of the shaft 29 and a top end of the lever 25 is supported rotatably and slidably by a pin 26 and by a slot 27b formed at a widened end 27a of a connecting rod 27 the other end of the connecting rod 27 is rotatably mounted to the lever 18 at a center part of its base end portion 18b by a pin 19 a compressible spring 23 is interposed between spring holders 22 24 beneath the center part of the lever 18 one end of the spring holder 22 is rotatably mounted to the lever 18 by a pin 21 and one end of the other spring holder 24 is rotatably mounted to the middle part of the lever 25 by a pin 28 due to the expansion force of the compressible spring 23 spring holders 22 24 are forced apart in the direction of the arrow f in fig4 and accordingly bias the lever 18 to rotate clockwise in the direction of the arrow g in fig4 with the pin 20 as a fulcrum reference numeral 35 designates a rotation transmission means such as a flexible pipe a spring or the like this rotation.