312 Prof. H. H. Jeffcott on Lateral Vibration of Loaded 



with a suitable alteration in scale, the same curves represent 

 both u and F. 



The curves (fig. 4) have not been drawn close to the 

 position co — k, as in that region the damping prevents the 

 values becoming indefinitely great, as would appear to be 

 the case if the formulae used for u and F were fully plotted. 



The curves exemplify clearly how that the critical speed 

 is one that is particularly favourable to vibration, or one at 

 which the shaft is very sensitive to lateral disturbing forces. 



If in the choice of the working speed the critical speed is B 

 avoided by 10 per cent, on either side, i. e. if co does not lie 

 between '9k and l'lk, then the amplitude of vibration will 

 not be more than about five times the eccentricity of the 

 mass centre. 



If the shaft is in good balance this choice of working 

 speed will therefore give good results and sufficiently steady 

 running. 



The better the balance the closer to the critical speed may 

 the shaft be worked. For while keeping the amplitude of 

 vibration u within a definite limit, we may approach k the 

 more closely the smaller the value of a ; and further, the 

 damping will keep the amplitude rather lower than that 

 plotted. 



On the other hand, if the balance is bad the critical speed 

 must be avoided by a large margin, and it may even be 



