of truth, so that its C. G. is I in. from the centre of rotation. Find what pull this will cause on the shaft at 80 revolutions. Ans. 2036 lbs. This pull, since it always lies along OG, will be continuously bending the shaft in different directions, and will set up very great vibration. Hence the necessity for perfect balance, especially at high speeds. CHAPTER XIII STRESS AND STRAIN-TENSION AND COMPRESSION A MACHINE transmits energy by means of force, which is transmitted along the chain of pieces of which the machine consists. At each contact of a pair we have seen there is a mutual action between the surfaces in contact, which mutual action we have called a Stress (page 106). The connections between energy exerted, work done, velocities, and the forces acting on the various pieces, have been considered in Part I. We now wish to go further, and inquire into the effects which the forces produce on the pieces to which they are applied. For the purposes of Part I we did not require to know the transverse dimensions of the pieces; but for our present purpose these will be of primary importance. The effects produced by forces on a piece of material depend very much on the manner in which they are applied to it; we shall consider various modes of application in order, commencing now with the case in which the forces are applied in the direction of the length of the piece. The forces applied are often called Loads even when not due to the action of gravity, and we are now about to study the effect of Longitudinal Loads. The shape of the piece must be defined, and the case we consider is that of a straight rod of uniform crosssection; in most cases the section will be taken to be circular, this being most common in actual practice. |