Vertical band saw machine with a speed ratio varying device

A vertical band saw machine comprising:

• a machine frame including upper and lower supports spaced apart from each other in an upright direction to define a working space therebetween;
• a worktable disposed to extend into said working space for placement of a workpiece to be sawn thereon;
• a motor for supplying a drive force;
• upper and lower blade mounting pulleys respectively mounted on and rotatable relative to said upper and lower supports;
• an adjusting member disposed to adjust said upper blade mounting pulley relative to said lower blade mounting pulley in the upright direction;
• an endless band saw blade trained on said upper and lower blade mounting pulleys, and having a working segment which extends in the upright direction and which passes through said worktable;
• a first transmitting shaft driven by the drive force of said motor to be rotatable about a first axis;
• a second transmitting shaft disposed on said lower support, rotatable about a second axis parallel to the first axis, and coupled to and rotating said lower blade mounting pulley about the second axis;
• and
• a speed ratio varying device disposed to couple said first transmitting shaft to said second transmitting shaft so as to transmit the drive force of said motor to said second transmitting shaft, thereby driving said second transmitting shaft to rotate said lower blade mounting pulley about the second axis, said speed ratio varying device including
• a first pulley having a first fixed disc which is mounted securely on said first transmitting shaft, and a first movable disc which has a spline connection with said first transmitting shaft so as to be movable relative to said first fixed disc along the first axis, said first fixed and movable discs respectively having first belt-engaging surfaces which cooperatively define therebetween a first belt receiving space,
• a second pulley having a second fixed disc which is mounted securely on said second transmitting shaft, and a second movable disc which has a spline connection with said second transmitting shaft so as to be movable relative to said second fixed disc along the second axis, said second fixed and movable discs respectively having second belt-engaging surfaces which cooperatively define therebetween a second belt receiving space,
• a control unit mounted on said lower support, and having an operable member which is disposed to actuate said second movable disc to move along the second axis so as to be close to or remote from said second fixed disc,
• a biasing member disposed to bias said first movable disc toward said first fixed disc, and
• a belt trained on said first and second pulleys, and frictionally slidable on said first and second belt-engaging surfaces, said belt having a first lengthwise end which cooperates with said first transmitting shaft to define a first radial distance therebetween, and a second lengthwise end which is opposite to said first lengthwise end and which cooperates with said second transmitting shaft to define a second radial distance therebetween such that, when said second movable disc is actuated by said operable member, said second movable disc is movable along the second axis between
• a first position, where said second movable disc is remote from said second fixed disc so that said second lengthwise end is near said second transmitting shaft so as to result in a smaller length of the second radial distance, and where said first movable disc is held close to said first fixed disc by the biasing action of said biasing member, thereby resulting in a greater speed ratio of said second transmitting shaft to said first transmitting shaft so as to rotate said band saw blade at a higher speed, and
• a second position, where said second movable disc is close to said second fixed disc so that said second lengthwise end is remote from said second transmitting shaft so as to result in a greater length of the second radial distance, and where said first movable disc is kept apart from said first fixed disc by a tension force which is generated as a result of displacement of said first lengthwise end toward said first transmitting shaft and which acts against the biasing action of said biasing member, thereby resulting in a smaller speed ratio of said second transmitting shaft to said first transmitting shaft so as to rotate said band saw blade at a lower speed.