APPLICATION OF GRAPHIC METHODS 305 



no more tension on element o than is necessary. In that case, 

 the line of pressure at the point of contact will make the 

 stated angle with the surface of contact. Link 2 is drawn from 

 the intersection of 5 and 1 to the friction circle at the centre of 

 6. Link 4 is drawn from the intersection of 5 and 6 to the 

 friction circle at the centre of c. The intersection of 4 and 1 is 

 then joined by link 3 to the intersection of 6 and 2. The three 

 lines meeting at B show the positions of the three forces under 

 which 6 is in equilibrium, and the arrows show the directions of 

 those forces. The three lines meeting at C show the forces 

 under which the second wheel c is in equilibrium, and the arrows 

 at C show the directions of those forces. The rules given in 

 8 will enable the draughtsman to determine on which side of 

 each friction circle the link is to be tangent. Fig. 256 shows 

 the manner in which the diagram becomes modified when the 

 directions of links 6 and 1 make smaller angles than link 5 

 makes with the normal to the joint be. Fig. 256 also shows the 

 effect of rolling friction at this joint, which, however, may 

 generally be neglected. At the point of contact the material is 

 continually being crushed, and the material is not perfectly 

 elastic. We have, therefore, a resisting couple analogous to 

 that met with in the case of ropes, and the effect is to shift in a 

 disadvantageous direction one part of link 5 by a distance equal 

 to the arm of this couple. If excessive tension is employed in 

 a, the direction of link 5 will be found by compounding that 

 tension with the force transmitted at the periphery. The 

 diagram where one roller is driven by a couple and the other 

 roller resisted by a couple, is easily deduced from that for spur- 

 wheels. 



22. Belt and PuUey. The complete belt and pulley machine 

 is shown in Fig. 26, p. 306. It is composed of the pulleys b and c ; 

 the element a in which their bearings run, the flexible belt d, the 

 driving element e, and the resisting element /. The dynamic 

 frame without friction is given in Fig. 26a. Link 5 is the direc- 

 tion of the resultant of the tensions on the two bands, which 

 may be considered as together forming one split link. "When 

 we assume that no more tension is used than is necessarv, the 

 ratio between the tensions on the tight and slack side of the 

 bands is determined by the arcs round which the belts are in 

 VOL. n. x 



