Machinery placed on Upper Floors of Buildings. 399 



which is a, which is the force in dynes or poundals required 

 to deflect it unit distance, either 1 cm. or 1 inch. If pads 

 are being used — say six ^ inch pads each 6 inch square — 

 a has reference to the whole area of 216 sq. in. (1340 cm.) ; 

 a would be 96 g times the force in lb. required to compress 



the pads J 



n ' /n b J2L 



. njorn. 

 n 2 /n s i> 



1-3 16 



inch. 



Fig-. 3. 



1-0 10 



For 



A 













\3 



<^, 









V N^N 







\a 





X£ \ 





\r 





7 













^x 



















•6 6 -4 -2 



Rstio of weights, m/M, with n a - n b i 



" speeds, n a /n b , for various m/M. 



Taking a particular case, let the motor run normally at 

 1000 r.p.m. ; let the floor have a natural frequency of 

 vibration, 7^ = 800 ; let ???/M = 0*l ; to avoid resonance we 

 need to have n x and n 2 well below 1000 r.p.m. : suppose we 

 decide to have n 2 less than 900 r.p.m., k 2 < 900/800 < 1*125 ; 

 then fig. 3 gives <£<0*9 so n a <7'20 r.p.m. ; the deflexion of 

 the motor support under the influence of the motor weight 

 should therefore exceed 1*8 mm. (fig. 2). If this deflexion 

 can be obtained by using 3 inch square cork pads each 2 inch 

 thick instead of using 6 inch pads ^ inch thick, the obnoxious 



2 E 2 



