32 LEATHER BELTING 



croft ranged all the way from 0.15 to 1.38. The coefficient 

 of friction was determined in the early experiments by hang- 

 ing a section of a belt over a pulley or drum, suspending 

 weights from the two strands and noting the difference in 

 weight at which the belt would begin to slip. This method 

 is correct enough for determining the coefficient of friction 

 of leather on iron or wood under certain limited conditions, 

 but it manifestly does not represent the conditions under 

 which belts are operated. In the first place the coefficient 

 is determined at practically zero velocity. Secondly, this 

 method takes no account of the alternate lengthening and 

 shortening that takes place in the slack and tight strands 

 of the belt as it passes over the pulleys, giving rise to the 

 phenomenon of belt creep. These factors have a distinct 

 effect on the coefficient of friction and cannot be neglected. 



The experiments of Wilfred Lewis l showed that the value 

 of the coefficient of friction depended in part on the velocity 

 of slip of the belt on the pulleys, and that other conditions 

 influencing it were the condition of the leather, the tempera- 

 ture and the pressure. Mr. Lewis's experiments indicated 

 that a total allowance of 2 per cent for the slip of the belt 

 on a pair of pulleys was good practice; that is, a slip of 2 

 per cent will not be injurious to the belt nor to the service 

 that it is performing. This allowance includes the creep of 

 the belt as described below, but which was not considered 

 separately by him. 



The phenomenon of belt creep was studied by Prof. W. W. 

 Bird, who presented his conclusions, 2 in 1895. Belt creep 

 was explained by Prof. Bird as follows : 



Referring to Fig. 5, A is the driving and B the driven 

 pulley, running in the direction indicated by the arrow. 

 The tensions in the tight and slack sides are Ti and T% 

 respectively. One inch of slack belt goes on the pulley B 



1 Transactions, American Society of Mechanical Engineers, vol. vii, p. 549. 



2 Ibid., vol. xxvi, p. 584. 



