one of either drive sprocket 50 or 51 and one of either driven sprocket 56 or 57 to attain the desired speed of movement of sawing band 12 roller 22 then is engaged with drive belt 40 this engagement of roller 22 with drive belt 40 is facilitated by manual use of finger tab 25 to rotate bracket 20 about fastener 24 as bracket 20 is rotated about fastener 24 spring 30 passes over the axis of rotation of bracket 20 thereby exerting a force tending to bias roller 22 into engagement with drive belt 40 such engagement has the effect of exerting a tension force on drive belt 40 so as to maintain drive belt 40 in efficient contact with the selected driving sprocket 50 or 51 and the selected driven sprocket 56 or 57 during operation the area of contact is made by roller 22 which bears against the outer surface of drive belt 40 roller 22 should be of a suitable material and mounted in a manner suitable to minimize the frictional wear on drive belt 40 fig2 illustrates the belt tensioning device in its operable or engaged position for applying appropriate tension to the drive belt 40 it will be seen that the spring 30 is in an overcenter relationship with respect to the pivot axis defined by the fastener 24 thus the spring 30.