BIPOLAR DIAGRAM 



161 



diagrammatic end view of this coupling: the point D is the center 

 of the shaft; the line DO is the radius of the driving crank pin and 

 DB that of the driven crank pin; OB is thus the distance between 

 the two crank pins, to which the tension of the elastic band is pro- 

 portional ; the lever arm of the tension about the center of the shafts 

 is DC = DB cos (|>, and the corresponding turning moment trans- 

 mitted from driver to driven is thus proportional to DCXOB. But 

 DC=DO cos fy, and the torque or turning moment is T=OBXDO 

 cos^; or since OB cos fy = OBo, T=OBoXOD; i.e., the torque is pro- 

 portional to the area of the rectangle DOBo. OBo is the tangential 



FIG. 82. 



component of the tension OB, and OD is the corresponding lever 

 arm. 



Consider now the operation of this coupling for given crank pin 

 radii and a given stiffness of the elastic band OB. At no load or 

 zero torque the crank DB will be pulled ahead into line with DO, 

 the tangential component of the tension OB will be zero, the angle 

 will be zero, BO will coincide with 0, and the rectangle DOBo will 

 collapse into a line according to the above-indicated relation between 

 the torque and the area of this rectangle. If now the load torque 

 be increased, the point B will fall back to the right, and the angle 6 

 will increase until OBo, the tangential crank effort, has attained the 

 the value imposed by the load. A further increase in the load torque 

 will be accompanied by a further increase in 6 and in OBo; but it 



