Il6 STATICS. [193. 



must pass (as the point C in Fig. 60), its direction is found by drawing 

 through this point a tangent to the friction circle. 



193. If the shaft revolved in the opposite sense, i.e. clockwise 

 (instead of counter-clockwise, as assumed in Fig. 60), the tangent to 

 the friction circle would have to be drawn through C on the other side 

 of the friction circle. 



In the case of axle-friction, i.e. when the journal, or axle, is fixed, 

 while the bearing, or hub, revolves about it, the same considerations 

 would apply, except that the point of application of the total reaction 

 would now be at the top, at D', instead of D. 



194. Pin-friction, as it occurs in link-work and jointed frames that 

 are not absolutely stiff, is not different from journal friction or axle- 

 friction, and can be treated in the same way. Thus, a link connected 

 to other parts of a machine by means of a pin at each end would trans- 

 mit the force along the line joining the centres of the pins if there were 

 no friction. To take account of pin-friction, we have only to draw the 

 friction circles about the centre of each pin; the direction in which 

 the force is transmitted by the link is tangent to both these circles. 



Which one of the four common tangents represents this direction must 

 be decided in each particular case by considering that the reaction 

 exerted by one link on another connected with it by a pin is in the 

 direction of the motion of the former relative to the other. Thus if 

 the link AB (Fig. 61) be subject to tension, and its motion relative to 



Fig. 61. 



the adjoining links at A and B be as indicated by the arrows in the figure, 

 the contact between the link and pin will be on the outside both at A and 

 at B ; the friction is, therefore, directed downwards at A and upwards at 

 B, and the line PQ along which the force is transmitted touches the 

 friction circle at A below, at B above. 



If the link were under compression, with the same relative motions, 

 the line of force would have the direction P' Q'. 



