Chapter I: Preface

This work is based upon notes and diagrams which were prepared by the writer with no more ambitious object, originally, than to help his railway students toward the obtainment of clear _general_ notions upon the important subject of the slide valve.

For practical demonstration of the functions and operation of the slide valve, a large sectional model engine, with provision for variation of the operation of its parts, is very valuable; but it seems to the writer to be desirable that each student should have in his own possession a model to cogitate upon, and that his model should give him, graphically, results which should be not merely qualitative, but also, in some degree, _quantitative_, to enable him to institute comparisons between the actions of different valves operated under varying conditions. With this end in view, the writer conceived the idea of using on a base-board a rotary disc to represent a crank-shaft, together with the idea of obtaining concentric circular diagrams of results (see fig. 21, for instance) by using a crank-arm marked on the disc as an index-finger, and recording on the base-board the beginnings and ends of the arcs swept through by the crank in the various distribution-periods. Following upon this, it was thought that the eccentric and its rod could be eliminated by bringing the disc under the valve and fixing upon the valve a rigid slotted arm to extend down across the face of the disc and engage a pin on the latter, so that rotation of the disc would cause reciprocation of the slotted arm and the valve. This promised, however, to be unmechanical and expensive, and was discarded in favor of the model described in Chapter I., which comprises the simple method of correlating (as described in Chapter II.) the movements of valve and crank-shaft by scales adjacent to each, so that the reading given by the valve on the valve-scale at any position of the valve would show what position on the eccentric-scale the eccentric ought correspondingly to occupy. The fact that this involved a step-by-step adjustment of the valve and shaft alternately was found in practice to be advantageous rather than the reverse.

As to the diagrams and sketches, the writer has sought, in some of them, to clear up graphically some of the incidental minor difficulties related to the subject; see, for instance, figs. 12 and 13, illustrating “Order of Cranks,” fig. 22 on “Width of Port”; also compare figs. 23, 24, and 25, “Double-ported Valve,” and see fig. 37, which last is in effect a graphical summary of results from nearly all the different examples which precede it.

Fig. 36, which concerns the reversal of an engine in motion under steam, will probably have a special interest to students of the locomotive.

W. J. T.

------------------------------------------------------------------------

CONTENTS.

PAGE

INTRODUCTION TO AMERICAN EDITION, 1

CHAPTER I.
THE SIMPLE SLIDE, 5

CHAPTER II.
THE ECCENTRIC A CRANK. SPECIAL MODEL TO
GIVE QUANTITATIVE RESULTS, 13

CHAPTER III.
ADVANCE OF THE ECCENTRIC, 20

CHAPTER IV.
DEAD CENTRE. ORDER OF CRANKS. CUSHIONING
AND LEAD, 24

CHAPTER V.
EXPANSION—INSIDE AND OUTSIDE LAP AND
LEAD; ADVANCE AFFECTED THEREBY.
COMPRESSION, 31

CHAPTER VI.
DOUBLE-PORTED, MULTIPLE ADMISSION AND
PISTON VALVES, 43

CHAPTER VII.
THE EFFECT OF ALTERATIONS TO VALVE AND
ECCENTRIC, 57

CHAPTER VIII.
NOTE ON LINK MOTIONS, 66

CHAPTER IX.
NOTE ON VERY EARLY CUT-OFF, AND ON
REVERSING GEARS IN GENERAL, 73

------------------------------------------------------------------------