BELT CREEP 33 



at 0, and at or before the point p it feels the effect of the 

 increased tension and stretches to i+s inches. It travels 

 from p to m and goes on pulley A while stretched. At or 

 before reaching point n, as the tension decreases it contracts 

 to one inch and so completes the cycle. With a light load 

 the belt creeps ahead of the pulley B at or near the point p. 

 If the load is heavy the creep works toward the point o and 

 the belt may slip. This also takes place when the belt 

 tensions are too light, even with small loads. On the pulley 

 A the belt creeps back relative to the pulley surface at m, 

 and as in B may extend to the point m where regular slip will 

 occur. 



Prof. Bird derived the following formulas as the result 



p 



-1 



PIG. 5. DIAGRAMMATIC REPRESENTATION OF THE PHENOMENON OF BELT 



CREEP. 



of his experiments, x being the percentage of slip, v and 1)2 

 respectively the pitch velocities of pulleys A and B, p the 

 effective tension (Ti Tz), pound per square inch, E the 

 modulus of elasticity of the leather, and 5 the stretch per 

 inch due to p. 



/y _ /v * 



~"~ j y ' J """" ~n 



The table on page 34 shows the percentage of creep for 

 different values of p and a range of values of E. 



The value of E = 20,000 was stated to be a fair value for 

 ordinary working conditions. Prof. Bird concluded that 



