CORRESPONDENCE. (To the Editor Progress.)

Progress, Volume IV, Issue 7, 1 May 1909, Page 244

CORRESPONDENCE. (To the Editor Progress.)

bir,— Argument often arises as to whether the top of a locomotive wheel moves at a different speed to the bottom or not. "Will you kindly tell me per medium of your valuable paper, whether there is a difference, and, if so, at what speed would the top, centre, and bottom, of a wheel be moving supposing the locomotive to which the wheel is attached to be travelling at fifty miles per hour?. Also would you kindly say whether engineers generally agree on that point and have they any recognised means of demonstration whether there is or is not a difference?. If so, what are these means ?. Thanking you in anticipation and wishing you all progress to your excellent journal. — Yours, etc., H. W. POOLMAN. The idea that any portion of the circumference of a locomotive wheel travels at a different speed to any other portion is quite a fallacy. If we could neglect the axle and consider the point of contact between the wheel and the rail as the turning point it would appear to the ordinary layman quite possible. Thus, A.B. representing the centre line of a wheel it would appear quite possible to move the centre line of wheel to C. while A. remained in contact with the rail, but this would be rather a hinging movement than a roll. To prove that all portions of the circumference of a locomotive wheel move at the same speed it is only necessary to carry the diagram a little further. Let A.B. represent the centre line of wheel with centre of axle at D. If the upper portion of the centre line be moved to C, the centre of axle B. must also be moved to a point on the line drawn from C. to A., and as the true point of support for any load on the wheel can only be in a direct vertical position under the centre of the axle, it is easy to demonstrate that the movement of the bottom and the top of the wheel is equal; by drawing a vertical line E.F. at right angles to the rail through the altered position of the axle at D' and it will be found that the displacement on the circumference of the wheel at F., A. (bottom) is exactly equal to C.E. Nt the top. — Ed. P.