400 



Mr. A. B. Eason on Critical Speeds of 



vibrations might be avoided. The effect of increasing the 

 area of the pads will be to increase a and thus increase n a ; 

 to increase the thickness of the pads will reduce a and n a . 

 The question of whether n a should be increased depends 

 altogether upon the relative values of n, n b , n a , or of o>, 



Ob, Ma- 

 Fig. 4 shows the critical speeds of the motor with 

 m/M = 0'l for any two Speeds n a and r? 6 , all the speeds being 



Fig:. 4. 



Rev. 

 per 

 Mm. 



2000 

 1600 











n i / 











//s 3 





m 

 M 



m 

 M~' 2 







1200 





n b 



n i ^^^ 







n b 









Z= 







800 



400 











^a 







M ~ 



- i 





■1 



n a / 



% 









•e i-2 



Value of na/n^ 



referred to n h . The actual speeds will be n b multiplied bv 

 the values of the ordinates for the particular value of <jf> 

 under consideration. It is seen that the critical speeds are 

 always above and below the natural frequencies, (1) of the 

 motor on its support, and (2) of the floor on its supports. 



In order to avoid resonant vibrations in any actual case, 

 one must know n b accurately or approximately. One can 

 find n b experimentally thus : — Place a motor directly on the 

 floor ; attach a small weight to its armature to unbalance it, 

 start the motor running and notice if there are big vibrations 

 in the floor at any speed up to the full speed. If such 

 vibrations exist notice the approximate speed and then run 



