1903.] Professor W. E, Dalhy on Vibration Problems. 235 



WEEKLY EVENING MEETING, 



Friday, January 30, 1903. 



The Right Hon. Sir James Stirling, M.A. LL.D. F.R.S., 

 Vice-President, in the Chair. 



Professor W. E. Dalby, M.A. B.Sc. M.Inst.C.E. 



Vibration Problems in Engineering Science. 



Vibration Problems in Engineering Science arise from the fact that 

 the different parts forming engines and machinery are always chang- 

 ing their motion relatively to the frame, the change being in general 

 continuous and periodic. 



To change the motion of a body requires the action of a force, and 

 it is the equal and opposite aspects of the accelerating forces acting 

 on the moving parts of an engine or machine which re-acting on the 

 frame tend to set it in motion and with it the foundations to which 

 it is attached. In this way vibration in the surroundings may be 

 set up. 



My first object is to make this principle clear to you, partly by 

 the aid of diagrams, partly by simple experiments. 



From Newton's Laws of Motion we know that : — 



The natural mode of motion of a body is in a straight line wich 

 uniform speed. 



To change the speed in the line of motion requires the action of 

 a force. 



To turn the body out of its straight path and to compel it to 

 move in a curved path requires the action of a force, although the 

 speed in the curved path may be uniform. 



The word force as usually understood does not convey the idea 

 that a force acting one way must necessarily be accompanied by an 

 equal force acting in the opposite way. Yet such is the case. A push 

 must always be accompanied by an equal and opposite push, a pull 

 by an equal and opposite pull, a twist by an equal and opposite twist. 



A very cursory examination of a machine or engine will show 

 that none of the parts are moving in the natural mode. None of 

 them are passing over equal intervals in equal times in a straight 

 line. The forces which must act to produce these changes are neces- 

 sarily accompanied by equal and opposite forces acting on the frame. 

 These forces try to pull the frame first in this direction then in that, 

 they try to twist it first in one way then in another, and the conse- 

 quent unrest of the frame is communicated to its supports and 

 foundations. 



