﻿and Critical Speeds o) Rotors. 149 



in magnitude, the film of oil in the bearings will allow the 

 shaft a certain amount of play ; this will increase the effec- 

 tive length of the shaft and lower the critical speed; with 

 increasing speed the vibration will therefore start up fairly 

 smartly. As, however, the speed increases and the vibration 

 becomes greater, the shaft may bed hard up against the 

 bearing bush, and increased deflexion will decrease the effec- 

 tive length of the shaft, and so raise the critical speed. As 

 tbe speed is further increased a similar state of things is 

 gone through, so that at a certain point the vibration will 

 die down more quickly than if there had been no film of oil. 

 The effect of the oil in the bearings is thus to give an 

 added amount of friction to whirling, and at the same time 

 flatten the maximum peak of the vibration, that is, the 

 vibration will start up and cease fairly smartly and remain 

 more or less constant throughout a fair range of speed. 

 If, however, the film of oil is sufficiently thick or the 

 balance sufficiently good, the vibration may not show itself 

 at all, although it might do so with the same out-of-balance, 

 if the film of oil were thinner. 



Section III. — Oscillatory Vibrations — Second 

 Critical Speed. 



1. Oscillatory vibration may arise in two ways, either 

 through lack of balance or through vibration transferred 

 from the transverse motion. 



2. The lack of balance referred to is of the skew type, that 

 is, is equivalent to a pair of weights at opposite ends of the 

 machine, and 180° apart, giving an out-of-balance couple 

 when the machine rotates ; such an out-of-balance will not 

 show itself when the machine is being statically balanced on 

 knife edges, and can only be corrected through observations 

 when the machine is running. 



3. Vibration can be transferred from the transverse motion 

 only when the machine is unsymmetrical in the sense that a 

 force applied to the centre of gravity at right angles to the 

 shaft gives a displacement which is not parallel to the centre 

 line, that is, in those cases where, on the static deflexion 

 diagram, the shaft in the deflected position is not parallel to 

 the centre line of the bearings. 



4. The form of out-of-balance mentioned will produce a 

 couple rotating with the machine, that is, a couple alter- 

 nating with the frequency corresponding to the running 

 speed. Vibration transferred from the transverse motion 

 ma}-, however, be of the frequency corresponding to the 



