,,the ,,slide ,,valve1 ,,simply ,,explained4 ,,by ,w4 ,j4 ,,tennant1 ,,asso4 ,m4,i4,m4,e ,,revised ,,and ,,much ,,enlarged ,,by ,j4 ,h4 ,,kinealy1 ,d4 ,e41 ,professor of ,mechanical ,engineering in ,washington ,university2 ,m4 ,am4 ,soc4 ,mech4 ,engrs4 ,author of 8,steam ,engines and ,boilers20 8,low ,pressure ,steam ,heating ,charts40 ------------------ ,,fully ,,illustrated ,,with ,,original ,,drawings ,,and ,,diagrams4 ------------------ ,,spon ,,chamberlain1 #ab ,,cortlandt ,,street1 ,,new ,,york4 #ahii4 ---------------------------------------- -------------------------------- ,entered according to ,act of ,congress in the year #ahii1 ,,by ,,spon ,,chamberlain1 in the ,office of the ,librarian of ,congress1 ,washington1 ,d4,c4 ---------------------------------------- ----------------- ,,the ,,burr ,,printing ,,house1 ,,new ,,york1 ,u4 ,s4 ,a4 ---------------------------------------- -------------------------------- ,,preface4 ,this work is based upon notes and diagrams which were prepared by the writer with no more ambitious object1 originally1 than to help his railway students toward the obtainment of clear general notions upon the important subject of the slide valve4 ,for practical demonstration of the functions and operation of the slide valve1 a large sectional model engine1 with provision for variation of the operation of its parts1 is very valuable2 but it seems to the writer to be desirable that each student should have in his own possession a model to cogitate upon1 and that his model should give him1 graphically1 results which should be not merely qualitative1 but also1 in some degree1 quantitative1 to enable him to institute comparisons between the actions of different valves operated under varying conditions4 ,with this end in view1 the writer conceived the idea of using on a base-board a rotary disc to represent a crank-shaft1 together with the idea of obtaining concentric circular diagrams of results 7see fig4 #ba1 for instance7 by using a crank-arm marked on the disc as an index-finger1 and recording on the base-board the beginnings and ends of the arcs swept through by the crank in the various distribution-periods4 ,following upon this1 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 latter1 so that rotation of the disc would cause reciprocation of the slotted arm and the valve4 ,this promised1 however1 to be unmechanical and expensive1 and was discarded in favor of the model described in ,chapter ,i41 which comprises the simple method of correlating 7as described in ,chapter ,,ii47 the movements of valve and crank-shaft by scales adjacent to each1 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 occupy4 ,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 reverse4 ,as to the diagrams and sketches1 the writer has sought1 in some of them1 to clear up graphically some of the incidental minor difficulties related to the subject2 see1 for instance1 figs4 #ab and #ac1 illustrating 8,order of ,cranks10 fig4 #bb on 8,width of ,port02 also compare figs4 #bc1 #bd1 and #be1 8,double-ported ,valve10 and see fig4 #cg1 which last is in effect a graphical summary of results from nearly all the different examples which precede it4 ,fig4 #cf1 which concerns the reversal of an engine in motion under steam1 will probably have a special interest to students of the locomotive4 ,w4 ,j4 ,t4 ---------------------------------------- -------------------------------- ,,contents4 ,,page ,,introduction ,,to ,,american ,,edition1 #a ,,chapter ,i4 ,,the ,,simple ,,slide1 #e ,,chapter ,,ii4 ,,the ,,eccentric ,a ,,crank4 ,,special ,,model ,,to ,,give ,,quantitative ,,results1 #ac ,,chapter ,,iii4 ,,advance ,,of ,,the ,,eccentric1 #bj ,,chapter ,,iv4 ,,dead ,,centre4 ,,order ,,of ,,cranks4 ,,cushioning ,,and ,,lead1 #bd ,,chapter ,v4 ,,expansion--,,inside ,,and ,,outside ,,lap ,,and ,,lead2 ,,advance ,,affected ,,thereby4 ,,compression1 #ca ,,chapter ,,vi4 ,,double-,,ported1 ,,multiple ,,admission ,,and ,,piston ,,valves1 #dc ,,chapter ,,vii4 ,,the ,,effect ,,of ,,alterations ,,to ,,valve ,,and ,,eccentric1 #eg ,,chapter ,,viii4 ,,note ,,on ,,link ,,motions1 #ff ,,chapter ,,ix4 ,,note ,,on ,,very ,,early ,,cut-,,off1 ,,and ,,on ,,reversing ,,gears ,,in ,,general1 #gc ---------------------------------------- -------------------------------- ,,introduction ,,to ,,american ,,edition4 ,there is no more important part of the steam-engine than the valve--the part which determines when and for how long the steam shall be admitted to the cylinder1 how long it shall stay there and how long it shall be allowed for leaving4 ,and the principles which should govern the construction of the valve of an engine in order that the steam shall be properly admitted and released are the same for ,american1 ,english1 ,french and ,german engines4 ,the principles of their construction and action are the same1 but the details vary4 ,the action of every steam-engine depends upon its valve1 and hence it is important that every engineer of whatever country should be thoroughly conversant with the principles of the construction of valves of different types2 should know the meaning of 8lap10 8lead10 8advance10 etc42 and should know how a change of one of these will affect the others and the admission and release of the steam4 ,one of the best ways to learn all about valves and valve motions is to study them with the aid of sectional models1 made so that the inside as well as the outside of the parts may be seen1 but such models are difficult to obtain except at considerable cost4 ,diagrams1 such as are so freely used in this work1 rank next to models in utility and as a means of enabling one to thoroughly understand the workings of the different parts of machines4 ,the use of diagrams requires no knowledge of mathematics1 and enables explanations to be presented in a clear1 concise manner4 ,from an educational point of view1 diagrams possess an advantage which models do not1 as they make one think more4 ,their use gets one in the habit of forming mental pictures of the parts of the machines discussed1 and thus enables one to more readily and quickly think out the effect of a change in one part on other parts4 ,and this ability to picture in the mind the various relations of the parts to one another is absolutely necessary in order that these relations may be thoroughly understood4 ,the man who is accustomed to work out his problems from diagrams and drawings1 reasons from cause to effect and from effect to cause2 while the man who must have a model to work with1 works on the 8cut-and-try0 method4 ,this little book is confined strictly to an explanation of the principles which underlie the action of the different types of slide valves4 ,the plain1 simple ,d-valve1 as it is called in this country1 without lap or lead1 is first taken up and discussed2 and gradually lap and lead are introduced1 and the effect of each upon the admission1 the cut-off1 the release1 and the compression is fully worked out and shown4 ,then other types of slide valves are taken up and discussed4 ,the advance is made step by step1 from the most simple form of slide valve to the more complicated forms used for attaining certain results not so easily attained by the use of the simple forms4 ,the form of multi-ported valve shown in fig4 #be is so much like the ,giddings valve used on the ,russell and some other single-valve engines1 that any one reading the explanation given here will have no trouble in understanding the ,giddings valve 7shown in fig4 #bf74 ,the multiple admission valve is used to such an extent in ,america that it is but proper that a few words should be said of it1 and that the most common form of multiple admission valves should be discussed4 ,and hence the ,straight ,line and the ,woodbury valves are shown in figs4 #bg and #bh4 ,there are so many engines in this country in the front rank of automatic1 high-speed engines which use piston-valves1 that a treatise on slide valves would be incomplete without some mention of this type of slide valve4 ,the form of piston-valve shown in fig4 #cj1 in which the steam is admitted at the middle and exhausted at the ends1 is well known to all engineers who have used an ,ide or an ,ideal engine4 ,the valve used on these engines is shown in fig4 #ca4 ,many ,american engineers prefer engines on which the cut-off may be changed without in any way affecting the lead1 release1 or compression1 and1 therefore1 the description of the ,meyer valve will be read with interest4 ,many will recognize at once in the valve used on the ,watertown engine the ,meyer valve in almost its original form1 and in the valve of the ,buckeye engine 7shown in section in fig4 #ci71 the ,meyer valve in a modified form4 ,j4 ,h4 ,,kinealy1 ,d4,e41 ,prof4 of ,mech4 ,eng41 ,washington ,univ41 ,st4 ,louis1 ,mo4 ---------------------------------------- -------------------------------- ,,the ,,slide ,,valve4 ,,chapter ,i4 ,,the ,,simple ,,slide4 ,the slide valve has always been regarded by the writer as the pons asinorum of the student of the steam-engine4 ,his own early attempts at crossing that bridge were greatly facilitated by the simple little device to be presently submitted to the reader2 with the aid of this expedient the student may easily obtain clear notions upon all the functions and operations of the slide valve1 at a nominal cost1 and with but a small expenditure of time and trouble4 ,it is necessary that at the outset the nature of the construction above alluded to should be clearly appreciated1 for the reason that many of the explanations given in the succeeding pages are referred thereto4 ,in passing1 it may be remarked that much of the difficulty which the ordinary student experiences in the study of valves and valve gear arises because he approaches the subject with a false impression that it is of necessity a difficult one2 this idea is generated on the one hand in the atmosphere of mystery which in the shop is made to surround the matter1 and on the other hand by the somewhat formidable aspect of most of the geometrical treatises on the subject4 ,if the beginner can bring himself to believe that the slide valve is nothing more complex than a sliding shutter with a cavity in its face1 travelling backwards and forwards over three ports1 the outermost of which are alternately opened to steam and placed in communication with the central exhaust-port1 he will have made a satisfactory commencement4 ,take a piece of stout white cardboard and upon it set out an enlarged copy of fig4 #a4a ,illustration3 ,,fig4 #a4--;,explanatory ,diagram4 ----- ,footnote #a3 ,see also fig4 #da4 ,the upper portion of this diagram represents a section taken at right angles to the port-face through a set of steam and exhaust-ports as ordinarily arranged2 the graduated ring beneath the section will receive consideration later4 ,postponing for the moment the further investigation of fig4 #a1 reference should here be made to figs4 #b1 #c1 and #d4 ,fig4 #b shows1 in perspective1 a cylinder and valve-chest with parts removed to make clear the manner in which the two steam-ports 7,s#a,s#b7 at the upper ends of the two outer passages 7,p#a,p#b7 afford a communication between the valve-chest 7,,vc7 and the opposite ends 7,c#a;,c#b7 of the cylinder 7,c71 and to show the passage from the central exhaust-port 7,,ep7 to the outlet 7,o7 at which the exhaust steam is discharged4 ,the relationship between this view and the upper part of fig4 #a will be obvious upon a comparison of the two4 ,illustration3 ,,fig4 #b4--,perspective ,section of ,steam-,engine ,cylinder4 ,a typical slide valve is shown in section1 in its place above the ports in fig4 #b1 and separately in figs4 #c and #d4 ,from these latter1 the large ratio of the width to the length of the valve is apparent1 and it will be seen from fig4 #b that the ports are correspondingly made wide and short4 ,this arrangement enables a small linear movement of the valve to effect the uncovering of a large area of port1 and to secure free passage for the steam with but a small amount of valve-travel1 and so to reduce to a minimum the work unavoidably wasted in overcoming friction between the port-face and the valve heavily loaded by the pressure of steam upon its back4 ,illustration3 ,,figs4 #c and #d4--,separate ,views of the ,slide ,valve4 ,let us return now to fig4 #a4 ,draw1 near the edge of a strip of moderately stiff paper1 the section of the elementary form of slide valve illustrated by fig4 #e1 taking its dimensions from the ports of fig4 #a1 so that the two views agree in the manner indicated4 ,by sliding this diagram backwards and forwards across the ports of fig4 #a1 the following explanation of the action of this1 the simplest form of slide valve1 will be readily appreciated4 ,illustration3 ,,fig4 #e4--,section of ,slide ,valve4 ,this valve has first to open one port to steam1 and when the steam thus admitted has forced the piston towards the opposite end of the cylinder1 to put the same port in communication with the exhaust1 while opening the other port to the steam which effects the return movement of the piston4 ,the duration of the admission of steam is the same as that of the exhaust1 the admission1 owing to the dimensions of the valve in relation to the ports1 necessarily taking place on one side of the piston simultaneously with the occurrence of exhaust from the other side 7see fig4 #f74 ,the travel of this rudimentary valve equals twice the amount to which the port is opened to steam4 ,it happens to open each port fully in this case 7although this is rarely the case with modern slide valves71 so that the 8travel0 equals twice the width of steam-port3 twice1 because 7starting with the valve in its central position1 as shown in fig4 #e7 it has to move to the right1 say1 by an amount equal to the width of the left-hand port1 in order to open that port fully to steam1 and then it moves back again and travels to the other side of the central position by an equal amount1 in order to open the equally dimensioned right-hand port to steam4 ,in fig4 #f the valve is shown at the two extremes of its travel1 in full lines at one end1 and in dotted lines at the other end4 ,illustration3 ,,fig4 #f4--,section showing ,position of ,slide ,valve in relation to ,steam and ,exhaust-,ports4 ---------------------------------------- -------------------------------- ,,chapter ,,ii4 ,,the ,,eccentric ,a ,,crank4 ,,special ,,model ,,to ,,give ,,quantitative ,,results4 ,the necessary travel of the valve is given to it by means of an eccentric1 which is keyed upon or formed as part of the crank-shaft4 ,the eccentric is1 in effect1 a crank1 whose throw equals the amount of eccentricity of the sheave4 ,this may not be obvious2 let us investigate a little4 ,suppose that we have a big crank-shaft1 and want to put a little crank in the middle of it 7for this is what happens in the case of a steam-engine1 the travel of the valve being such that a crank of the usual form would be disproportionate to the shaft of which it formed part--like fig4 #g1 perhaps74 ,now1 that such a shaft would be extremely weak at the pin goes without saying4 ,imagine1 that to lessen the weakness the crank-pin of this little crank is made of greater diameter1 as in fig4 #h1 or even more so1 as in fig4 #i2 still we get the same result1 which is that the throw of the crank remains the same1 and equals the distance of the centre of the crank-pin from the centre of the crank-shaft--i4e41 the amount that this crank-pin is 8ex-centric10 out of centre1 and the eccentric sheave being simply an exaggerated crank-pin we get back to our original statement that it is virtually a crank whose throw equals the half-travel of the valve1 which in turn equals the amount of eccentricity of the sheave4 ,illustration3 ,,fig4 #g4--;,half-travel of ,valve4 ,illustration3 ,,fig4 #h4--;,half-travel of ,valve4 ,illustration3 ,,fig4 #i4--;,half-travel of ,valve4 ,being fixed to the crank-shaft and moving with it1 always in a fixed relationship to the main crank1 the eccentric operates the valve in the necessary accordance with the movement of the crank and piston1 as will presently be explained4 ,returning now to fig4 #e1 it must be understood that a valve of the type exemplified therein must be in the middle of its travel whenever the piston is at either end of its stroke4 ,the reason for this must be clearly appreciated for the sake of what follows hereafter2 it may be arrived at very easily with the aid of fig4 #a4 ,put a disc of card in the circle provided in that figure1 and let it turn about a drawing-pin stuck through its centre2 this disc is the equivalent of a crank-shaft4 ,a crank-arm 7,c7 may be permanently marked upon it in ink1 as in fig4 #aj2 another arm1 which may be marked upon it in pencil1 will serve to represent a 8valve-crank10 the equivalent of the eccentric by which the valve is to be operated4 ,illustration3 ,,fig4 #aj4 ,actual connection between that valve-crank and the valve being dispensed with1 some means for making the movement of the valve correspond accurately with that of its crank must be provided4 ,this provision is made by the numbered scales#b in fig4 #a2 the circular scale within which the card disc revolves is numbered in correspondence with scales on the ports1 and to these latter an arrow on the strip of paper which carries the valve is adjusted 7see also fig4 #aj74 ,to find the position in which to mark the arrow on the valve1 put the latter in the centre of its travel and mark the arrow-head just above the 8#j0 on the ,no4 #a scale4 ,when the disc is turned1 the end of the eccentric-arm or valve-crank travels with it around the circular scale4 ,when the eccentric-arm is moved step by step around the circle to the various numbered graduations1 the valve must be moved step by step to bring its arrow-head to the corresponding numbers on the horizontal scale on the ports4 ,thus the eccentric and valve1 although not moved simultaneously1 as they would be if a rod connected them1 are kept in accurate relationship whilst moved independently4 ,if the reader will now read the preface1 which he has probably skipped1 he will the more readily appreciate the principle of this arrangement4 ----- ,footnote #b3 ,the horizontal scales on the port-faces are obtained by projecting up equidistant circumferential marks from various circles having different valve-cranks for their radii4 ,the scale 7,no4 #a7 is for the elementary form of valve1 without lap2 it serves also for link-motion in mid-gear2 ,no4 #b is for a valve having outside lap2 and ,no4 #c is for link-motion 8linked up40 ,the student need not1 unless he pleases1 trouble himself to investigate the construction of the diagram1 which is drawn to scale1 so that if copied accurately it will serve to give him the demonstrations desired4 ,further1 taking the main crank-arm 7,c7 as an index-finger1 the position of its end at the points of cut-off1 release1 etc41 with different valves and eccentrics1 may be marked in pencil on the circular scale1 or better1 on a piece of tracing-paper interposed between the disc and that scale2 by this means the student will obtain circular diagrams enabling him to compare the results due to different settings and proportions of gear1 and he will also discover in what manner those results are affected by individual elements of the mechanism in any given example4 ---------------------------------------- -------------------------------- ,,chapter ,,iii4 ,,advance ,,of ,,the ,,eccentric4 ,assume a horizontal engine1 with the main crank moved into a horizontal position so that the piston would be at1 say1 the left-hand end of its stroke4 ,assume also that the engine is at rest1 and put the valve in the centre of its travel 7as in fig4 #e71 when its faces exactly cover the steam-ports and its cavity covers the bridges and the exhaust-port2 the first movement of the valve is wanted to be to the right1 in order to admit steam to the left-hand end of the cylinder to move the piston also to the right1 no matter whether the crank-shaft is to have negative or positive rotation4 ,now1 a valve of this pattern evidently will not admit steam to the cylinder to start the movement of the piston 7supposing that the engine has not yet been started71 which will therefore have to be helped by some external agency2 admitting this as a point which shall be touched again presently1 place the crank next1 so that the piston would be at the opposite end of its stroke2 the valve1 which had to move to the right as explained1 must have come back into the position whence it started1 and be in readiness to move equally to the left1 afterwards returning once more to the central position by the time that the piston gets back to the left-hand end of the cylinder4 ,hence we see that1 whenever the piston is at either end of its stroke1 this valve must be in the middle of its travel4 ,if this be granted1 we may proceed further3--,having the piston at the right-hand end of its stroke1 for instance1 draw a line on the disc at right angles with the crank 7see fig4 #aj72 this will represent two 8eccentric-arms0 of the necessary throw if each arm equals in length the half-travel of the valve2 either of them at present suits the position of the valve1 which is in the middle of its travel4 ,choose1 now1 the direction in which your crank-shaft shall rotate2 let us suppose1 for example1 that it shall have 8right-handed0 rotation1 and place an arrow on the disc to indicate the direction of rotation4 ,under these conditions the upper eccentric will follow the crank round1 operating the valve to close the right-hand port and open the left-hand one1 admitting steam to stop the engine4 ,cross this upper arm out1 then1 for manifestly we have to use the lower eccentric1 which1 going ahead of the crank1 will cause the valve to open the right-hand port when required4 ,but now conversely1 suppose that the engine is to be run in the opposite direction2 the upper eccentric 7represented by the crossed-out line7 now goes ahead of the crank1 and gives the valve motion in the right direction1 consequently the lower one must now go out of use1 for under the altered conditions of working it tends to move the valve in the wrong direction4 ,the deduction from this reasoning is1 that in whichever direction an engine runs1 the eccentric used for the time being must be set in advance of the crank1 and that advance must be at least #ij4 ,if it were less than #ij the wrong port would be opened--the port at the opposite end of the cylinder to that at which the piston might happen to be4 ,this effect may be shown by purposely giving the eccentric less advance4 ,the amount which the angle between the eccentric and the crank exceeds #ij is the 8angle of advance02 and the distance which the valve is moved from midposition when the piston is at the end of the stroke is the 8linear advance40 ,because of the necessity for different settings of the eccentric for different directions of rotation of the crank1 engines required to be reversible usually have either3 7#a7 ,means for shifting the position of a single eccentric upon the shaft1 that it may always be placed so as to lead the crank4 ,or 7#b7 ,two eccentrics fixed upon the crank-shaft so that one of them is always ahead of the crank in whichever direction it rotates1 mechanism being arranged in connection with the eccentrics1 so that the valve can be driven by the leading eccentric1 or operated by the conjoint action of both eccentrics4 ,or 7#c7 ,radial or other special valve gears4 ,the simple form of slide valve shown in fig4 #aj does not permit of the economical use of steam1 inasmuch as it allows steam at full pressure to follow the piston for the whole of the stroke1 and does not admit of the use of its expansive properties1 for the simple reason that1 at the instant the admission of steam ceases1 the exhaust of the same body of steam must immediately commence1 as may be clearly seen from the circular diagram4 ---------------------------------------- -------------------------------- ,,chapter ,,iv4 ,,dead ,,centre4 ,,order ,,of ,,cranks4 ,,cushioning ,,and ,,lead4 ,it is desirable at this point to clear the ground by touching for a moment upon one or two elementary matters which1 if not now explained1 might cause confusion subsequently4 ,dead ,centres4--,a crank is said to be 8on the centre0 or 8on the dead centre0 when the connecting-rod and crank are in line1 and this occurs twice in every revolution1 as shown in fig4 #aa4 ,illustration3 ,,fig4 #aa4--;,dead ,centre--,connecting ,rod and ,crank in ,line4 ,right-hand ,crank to ,lead4--,the phrases 8right-hand crank to lead10 or 8left-hand crank to lead10 are sometimes used1 and are confusing to beginners2 the 8leading0 crank is that one which leads when the engine is running ahead4 ,in most two-crank engines one of the cranks will be a quarter of a revolution 7or less than half a revolution1 where the cranks are not set at #ij degs47 in advance of its neighbour1 and will1 therefore1 lead it in the direction in which it should go4 ,of course1 that neighbour might be said to lead the other by being three-quarters of a revolution in advance 7or more than half a revolution1 where the cranks are not set at #ij degs471 and this is where a little confusion sometimes arises2 the leading crank must always be taken as that one which is less than half a revolution in advance of its fellow when the engine is running ahead1 and in locomotives and similar engines is 8left-hand0 or 8right-hand10 according as it lies to the left or right of a spectator looking from behind the crank-shaft towards the cylinders4 7,see fig4 #ab1 which shows two arrangements of the driving cranks of an ,english locomotive4 ,the cylinders are supposed to be to the right of the wheels47 ,illustration3 ,,fig4 #ab4--,order of ,cranks4 ,the accompanying diagram 7fig4 #ac7 will perhaps serve to make clear without further explanation the meaning of the expression1 used with reference to three-crank marine engines1 of 8order of cranks1 high1 intermediate1 low10 or 8order of cranks1 high1 low1 intermediate10 as the case may be4 ,illustration3 ,,fig4 #ac4--,order of ,cranks4 ,let us revert here to one of the points previously noted1 which was that the simple valve1 when we had it in connection with an eccentric set #ij degs4 in advance of the crank1 would not admit steam to the cylinder at the exact commencement of the stroke1 i4e41 when the crank is on either of the 8dead centres40 ,keeping this fact in mind1 we will consider the matters of ,cushioning and ,lead4--,it has long been found desirable 7especially in quick-running engines7 that the motion of the returning piston should be opposed as it arrives at either end of its stroke1 so that the moving weights of piston1 piston-rod1 and connecting-rod may be 8cushioned10 to prevent injurious stresses and to conduce to easy running4 ,to use a homely illustration1 one might say that just in the same way is it desirable1 when striking out from the shoulder1 to have one's adversary within range1 that he may provide the necessary 8cushioning10 and so prevent stresses in one's arm and possible dislocations4 ,this 8cushioning0 is provided in part by checking the exit of the exhaust steam1 as will be shown later1 and in part by setting the eccentric a little more than #ij degs4 7fig4 #ad7 in advance of the crank1 so that the valve commences to open the port to admit steam in front of the returning piston before it arrives at the end of its stroke1 continuing to open the port as the piston comes to a rest1 and is started on the return stroke4 ,illustration3 ,,fig4 #ad4 ,the amount to which the port is found to be open when the piston is at the end of its stroke is called the 8lead0 of the valve1 and it would be expressed as 8one-eighth of an inch lead10 or 8three-sixteenths of an inch lead40 etc41 as the case might be4 ,to set this valve for lead1 keep the piston at the end of its stroke1 and set the eccentric back until the valve is opened to the desired lead2 then the eccentric would be fixed in its newly-found position1 as shown in fig4 #ad4 ,the radius of the 8eccentric-arm10 which is the lighter of the two radial lines on the disc in fig4 #ad1 is equal in length to one-half of the travel of the valve1 and therefore does not reach the edge of the disc1 at which1 however1 there is a short guiding-mark in the line of the eccentric-arm 8produced40 ,by using this guiding-mark and moving the arrow-head of the valve over the scale on the ports1 in correspondence with the movement of the guiding-mark on the edge of the disc within the circular scale1 the valve1 the main crank 7,c71 and the eccentric can be operated in concord1 very much as if they were mechanically connected4 ,the explanation just given of the meaning of lead1 and of its effect1 should be borne in mind1 as we shall presently have to consider it in conjunction with other matters1 for1 as has been said1 there is something more than 8lead0 employed in the production of cushioning4 ---------------------------------------- -------------------------------- ,,chapter ,v4 ,,expansion--,,inside ,,and ,,outside ,,lap ,,and ,,lead2 ,,advance ,,affected ,,thereby--,,compression4 ,expansion4--,in order that the expansive properties of steam may be utilised1 it must be cut off before the piston arrives at the end of its stroke1 and on being cut off must not be allowed to commence exhausting immediately1 but must be confined in the cylinder1 in order that by expanding it may force the piston further before it4 ,the old valve1 as we have seen it1 did not allow this1 and to effect it the faces of the valve are lengthened1 so that after the steam is cut off by the edge 7a7 of the valve 7fig4 #ae7 the steam is confined1 and expands in the cylinder until the edge 7b7 arrives at c1 when exhaust immediately commences4 ,now put the piston at the end of its stroke1 and put the new valve in a central position 7fig4 #af74 ,the distance that the valve1 when in this central position1 overlaps the outer edge of the steam-port1 is called the 8outside or steam lap10 or more commonly the 8lap0 of the valve1 that is1 the distance d a 7fig4 #af74 ,illustration3 ,,fig4 #ae4 ,illustration3 ,,fig4 #af4 ,the first thing noticeable is that this valve must be given considerably more travel than the old one1 as1 to open fully each port to steam1 we have to bring the edge a to c1 first for one end of the valve and then for the other1 so that the travel of this valve must be twice the distance from a to c--i4e4 7lap plus port-opening7 #b1 whereas for the old valve it was the distance from d to c which had to be multiplied by two to give the travel4 ,increased travel means increased throw of the eccentric1 and we draw on the disc in fig4 #af two longer eccentric-arms than before 7the bolder dotted lines7 for the forward and backward eccentrics4 ,we want 8lead0 with this valve1 and we must therefore give each eccentric more advance than the #ij degs4 shown4 ,now with the old valve we give each eccentric an advance upon the #ij degs4 which was determined solely by the lead1 no lap being then in question2 but we now have a lap to deal with1 therefore each eccentric must be set on past #ij degs4 until it first has drawn edge a of the lap to the outer edge of the port1 and then still further until the edge a gives the necessary 8lead40 ,hence we see that the linear advance of the eccentric lap plus lead1 so as to draw the valve out of its central position by that amount4 ,travel1 then1 depends upon outside lap and port-opening1 and lead does not affect it4 ,advance depends upon lap and lead4 ,the advance of the eccentric is usually stated by specifying the angle of advance or the number of degrees exceeding #ij degs4 that the eccentric is in advance of the crank--for instance1 8an angle of advance of #aj degs40 means #ij degs4 #aj degs4 #ajj degs4 in advance of the crank4 ,the line of travel of the valve is sometimes inclined to the line of travel of the piston2 in such cases1 one of which is illustrated in fig4 #ag1 the angle of advance must1 of course1 be measured from a line at right angles to the valve-travel line1 and not at right angles to the line of piston-travel1 as we have hitherto assumed4 ,illustration3 ,,fig4 #ag4--;,engine in which the ,line of ,travel of ,valve is ,inclined in ,relation to that of the ,piston4 ,we must now carefully consider the further results obtained by the use of a valve having lap and giving lead1 and whose eccentric of greater throw is advanced accordingly4 ,we should expect cut-off to take place earlier than with the old valve1 for the eccentric is advanced1 and the edge a 7fig4 #ae7 would arrive at e a little earlier than would the edge d of the old valve1 even without advance of the new eccentric4 ,this seen1 we can understand that in causing the edge a 7fig4 #ah7 to reach the edge e earlier in the stroke of the piston than it otherwise would do1 we shall have hastened all the other operations so that they must occur earlier in the stroke 7not necessarily by exactly the same amount for each1 however1 as we shall see further on71 hence the edge b 7fig4 #ah7 arriving earlier at c1 exhaust will commence before the piston completes its stroke1 and also in returning to the right 7fig4 #ai7 the valve is earlier1 and instead of the exhaust lasting during the whole of the return stroke some of it will be shut in by b and compressed before the returning piston until the edge a arrives at e1 whereupon the lead commences coming in on and reinforcing the compressed steam4 ,thus it is that compression and lead together affect the cushioning1 and consequently the piston is eased in stopping1 and the return stroke is commenced without injurious shock or jar4 ,a similar operation1 of course1 takes place at the opposite end of the cylinder when the piston arrives there4 ,illustration3 ,,fig4 #ah4 ,illustration3 ,,fig4 #ai4 ,the distribution-diagram for a valve with lap1 lead1 and a suitable eccentric is given in fig4 #bj1 which should be compared carefully with figs4 #aj and #ad4 ,illustration3 ,,fig4 #bj4 ,inside ,lap1 exhaust lap1 or 8cover0 as it is sometimes called1 is possessed by many slide valves1 and it is the amount that the valve when in its central position overlaps the inner edges of the steam-ports4 ,conversely1 ,inside ,lead is the amount of space left1 in certain cases1 between the inner edge of the valve and the inner edge of the steam-port when the valve is in its central position4 ,the amount of inside lap does not affect the setting of the eccentric1 neither does it necessitate any alteration in the travel of the valve4 ,with a given cut-off1 steam is confined longer in the cylinder by a valve having inside lap because of the lengthening of the valve-face due to the inside lap1 and compression will commence earlier and will be of longer duration from the same cause2 exhaust being shortened in two ways--by the amount added to the duration of expansion at its end1 and by a similar amount added to the compression at the commencement thereof4 ,inside lead may be considered as something taken off the exhaust edge of the valve1 and in effect1 exactly the opposite to exhaust lap1 for1 as may easily be seen1 it shortens expansion and compression by reason of the shortening of the valve-face1 and lengthens the exhaust at commencement and end by the amounts by which the operations of expansion and compression are shortened4 ,the left-hand side of the valve in fig4 #ba has inside lap1 whilst the right-hand side has inside lead1 and the circular distribution-diagram below compares the distribution in the case of valves having inside lap and inside lead with the distribution for an ordinary valve such as was considered with reference to fig4 #bj4 ,by reference to the lower portion of fig4 #ba1 it may be seen that the operations of admission and cut-off are unaffected1 as they are performed by the outer edges of the valve2 but ,illustration3 ,,fig4 #ba4 is shorter and exhaust earlier with inside lead4 ,expansion is longer and exhaust later with inside lap4 is later and shorter with inside lead4 ,compression is earlier and longer with inside lap4 ,from this it will be obvious why many express locomotives are given inside lead1 and why many good engines have inside lap4 ,free ,exhaust4--,in order to give as free a passage as possible to the exhaust steam1 so that it may leave the cylinder easily1 and not have to be forced out by the returning piston and cause back pressure thereupon1 it is customary to make the steam-ports wider than the amount to which they are open to steam4 ,this must on no account be confused with inside lead1 which is quite a distinct matter4 ,the effect of having wide ports is that the exhaust may be opened to any extent up to the half-travel of the valve1 although the steam-opening may be much smaller1 as may be understood from the exaggerated diagram 7fig4 #bb71 whence also should be seen that this widening of the port does not affect the duration of expansion or of any other operation of the valve1 neither does it necessitate any allowance in the setting of the eccentric4 ,it simply provides a free exhaust4 ,illustration3 ,,fig4 #bb4 ---------------------------------------- -------------------------------- ,,chapter ,,vi4 ,,double-,,ported1 ,,multiple ,,admission1 ,,and ,,piston ,,valves4 ,double-ported ,valves4--,in large marine engines using considerable quantities of steam per stroke1 and consequently requiring a large amount of port-opening1 the ordinary slide valve would require an undue amount of travel4 ,to obviate this1 each end of the cylinder may have two steam-ports1 both ports opening into one passage as shown1 and a double-ported valve4 ,the double-ported valve is virtually a combination of two slide valves in a single casting 7fig4 #bc71 one valve for the inner set of steam-ports and the exhaust-port1 the other valve being halved1 as it were1 for each outer steam-port4 ,the exhaust cavity of each of the outer 8half-valves0 having a passage leading to the exhaust cavity of the inner valve1 both the valves exhaust into the central port1 which is widened accordingly4 ,the fig4 #bc is merely diagrammatic1 to enable the principle of this valve to be easily borne in mind2 the next drawing 7fig4 #bd7 shows in section a double-ported valve as actually constructed1 with the exception that the valve-spindle and its chamber are omitted in order that the form of the valve may be more clearly shown4 ,the section is taken in two planes1 one upon the longitudinal centre line of the valve1 and the other 7the left-hand half of the valve7 a little to the near side of the centre line4 ,fig4 #be shows a sectional view of a double-ported valve in perspective4 ,illustration3 ,,fig4 #bc4--;,diagram showing ,flow of ,steam and ,exhaust through a ,double-ported ,slide ,valve4 ,illustration3 ,,fig4 #bd4--,vertical ,section through a ,double-ported ,slide ,valve4 ,illustration3 ,,fig4 #be4--,perspective view 7in ,section7 of a ,double-ported ,slide ,valve4 ,now1 any movement given to the 8inner valve0 is given1 of course1 to the outer one1 so that if the inner valve be moved so as to give1 say1 #b ins4 of port-opening1 the outer valve gives at the same time another #b ins4 of port-opening2 therefore1 with a given movement of a double-ported valve we get double the amount of port-opening that we should have obtained with the ordinary valve4 ,hence the travel of the double-ported valve 7lap half the two port-openings7 #b2 whereas that of the ordinary valve 7lap the port-opening required7 #b4 ,the inner and outer valves of the double-ported construction1 acting simultaneously1 have each the same amount of lap2 but they give the lead between them1 and the eccentric is set with a linear advance equal to one of the laps and one of the lead-openings4 ,the distribution of steam with this valve is similar to that of the ordinary valve whose distribution-diagram was shown in fig4 #bj4 ,the different periods are of the same duration in the example chosen1 but the operations of cutting off steam and of opening exhaust are performed by the edges of the inner and outer valves instead of by a single edge as in the ordinary valve4 ,a valve very similar to the double-ported valve1 shown in fig4 #bc1 is the ,giddings valve used on the ,russell engines and some other automatic high-speed engines in ,america4 ,by its peculiar construction1 this valve obviates the necessity of double steam ports1 yet it is so much like the double-ported valve that it may be noticed here rather than under the head of multiple admission valves1 where it might be said to properly belong4 ,this valve accomplishes exactly what the double-ported valve does1 and in pretty much the same way4 ,the ,giddings valve in fig4 #bf shows steam being admitted to the right-hand end of the cylinder and exhausted from the left-hand end4 ,the travel of this valve is1 as in the case of the double-ported valve 7lap half the port opening required7 #b4 ,the valve should be set with a linear advance equal to the lap plus one-half the required lead4 ,illustration3 ,,fig4 #bf4--,the ,giddings ,valve4 ,,multiple ,,admission ,,valves4 ,in ,america1 the type of engine known as the automatic high-speed engine is very much used4 ,these engines have a wide range of cut-off1 from almost #j to stroke1 and in order to avoid a long valve and a long travel and to secure a quick admission and cut-off of the steam1 recourse is had to multiple admission valves4 ,these valves are so constructed that when one of them is moved from midposition1 a distance equal to1 say1 of an inch more than the steam lap1 the width of the opening for the steam to pass through into the cylinder is multiplied by the number of admissions of the valve4 ,thus1 if the valve is a double admission valve1 and it is moved plus the steam lap from midposition1 the port is opened a distance equal to #b of an inch4 ,double admission valves are quite common and there are also a number of quadruple admission valves used4 ,they are made as flat valves and as piston valves1 but all are made with a common object in view1 that is1 to secure an early cut-off and quick opening and closing of the ports with a valve whose length and travel shall be short4 ,one of the best-known forms of double admission valves is the 8,straight ,line0 or 8,sweet0 valve1 which was invented by ,professor ,john ,e4 ,sweet1 formerly of ,cornell ,university1 and first used by him on the ,straight ,line engines4 ,this valve is now used by a large number of manufacturers of automatic high-speed engines4 ,the valve is shown in section in fig4 #bg1 where a is the 8cover plate10 and b is the valve1 proper4 ,the valve1 b1 moves back and forth between the cover plate1 a1 and the valve seat4 ,the cover plate rests on pieces called 8distance pieces10 placed at the sides of the valve4 ,these distance pieces are made of such a thickness that while the cover plate cannot touch the valve1 it is so close to it as to make a steam tight joint between the plate and the valve4 ,therefore1 the valve acts as a piston of rectangular cross-section1 moving back and forth between the cover plate and the valve seat4 ,the result of its peculiar construction is that a balanced flat valve is secured2 a valve which works with very little friction1 as there is no surface upon which the steam can act so as to press the valve against the valve seat4 ,the cover plate is kept in place by the pressure of the steam against its back1 aided by one or two small springs inserted between the plate and the steam-chest cover4 ,illustration3 ,,fig4 #bg4--,the ,straight ,line ,valve4 ,this valve should be set so that the distance the valve is from midposition when the piston is at the end of its stroke1 or the linear advance1 shall be equal to the steam lap plus one-half the desired lead4 ,the travel of the valve is equal to twice the sum of the steam lap and one-half the maximum opening of the port for steam1 or #b 7steam lap maximum opening of port for steam74 ,in the case of a quadruple admission valve1 such as the ,woodbury valve shown in fig4 #bh1 the travel is equal to twice the sum of the steam lap and one-fourth the maximum opening of the port for steam1 or #b 7steam lap maximum opening of port for steam74 ,illustration3 ,,fig4 #bh4--,the ,woodbury ,valve4 ,the different phases of the movement of a multiple admission valve are just the same as for the ordinary slide valve1 the only difference being in the suddenness with which the ports are opened and closed4 ,piston ,valves4--,it may be readily seen that between the ordinary flat slide valve and the valve-seat considerable friction is set up1 because the steam pressure on the back of the valve is only partly balanced by the varying pressure in the cylinder steam-passages acting upon small portions of the area of the 8rubbing0 or under-side of the valve2 but by making the port-faces cylindrical and making the faces of the valve in the form of pistons1 the pressure is given no surface upon which to press in the direction of the valve-seat2 or1 in the case of hollow piston valves 7an example of which will be presently illustrated71 although the steam has access to the interior of the tube1 pressure on parts thereof is balanced by an equal pressure in an opposite direction upon exactly opposite parts1 and therefore is confined to stressing the valve and has no tendency to force any part of it on to the valve-seat4 ,in the plain piston valve shown 7fig4 #bi7 the faces are formed by the two valve pistons1 which work in a cylinder wherein steam and exhaust openings are made4 ,illustration3 ,,fig4 #bi4 ,sometimes the steam-supply enters between the two pistons1 as in figs4 #cj and #ca1 exhaust taking place into the spaces at the ends of the valve1 which spaces are connected by passages formed in the valve1 which is tubular1 as shown4 ,in such valves the steam-lap must be put upon the inner edges of the valve pistons1 for the steam and exhaust edges have their positions reversed2 and the lead will also be given by the inner edges4 ,illustration3 ,,fig4 #cj4 ,illustration3 ,,fig4 #ca4--,the ,ide ,valve4 ,the eccentrics for a valve with internal steam-supply must be placed #ahj degs4 in advance of the positions which they would occupy if the valve were of similar proportions1 but with external steam-supply1 for the reason that the internally supplied valve must be moving up1 for instance1 when the ordinary valve would be moving down1 and vice versa4 ,if the valve be an externally supplied valve1 but driven from one end of a two-armed rocking-shaft1 the valve gear actuating the other end1 as in ,american locomotive practice1 the eccentrics must1 for obvious reasons1 be set #ahj degs4 in advance of their usual position4 ,fig4 #cj is given by the kind permission of ,messrs4 ,w4 ,denny ,company1 the well-known ,clyde ship-builders1 and it possesses many points of interest1 one of which is that the packing in the valve-spindle stuffing-box below the valve1 and not shown in the drawing1 is subject only to the influence of the temperature and pressure of exhaust steam1 instead of to that of the higher temperature and pressure of the supply steam2 this is a matter of importance1 having in view the high pressures of steam which are now used4 ,further1 it may be seen that the upper piston of the valve is of greater area than the lower one1 and that the high-pressure steam entering between the two pistons will consequently exert a total pressure upward upon the larger piston greater in amount than the downward pressure exerted upon the lower and smaller one2 thus the valve has a tendency to rise1 so long as steam is on4 ,this upward tendency balances wholly or in part the downward tendency due to the weight of the valve1 which has not to be lifted by the valve gear1 which may be of a lighter character than usual1 and give less trouble in working1 owing to diminution of wear and tear4 ,the valve-spindle of the ordinary flat valve is sometimes prolonged 7where the valve works vertically7 to end in a small piston exposed to steam pressure1 and acting so as to balance the weight of the valve4 ,in the 8,joy0 assistant-cylinder1 a balance-piston is provided with special passages which1 in the movement of the valve-spindle1 are brought opposite ports in the assistant-cylinder wall1 so that steam is let in to propel the balance-piston up or down1 and thereby assist the valve gear in each stroke4 ,the balance-piston acts as its own exhaust valve in sliding over exhaust-ports in the assistant-cylinder wall4 ,fig4 #ca shows the piston valve used in ,america on the well-known ,ide and ,ideal engines4 ,it is very similar to the valve shown in fig4 #cj1 and has the same advantages which have been pointed out as belonging to that valve4 ,as1 however1 it is principally used on horizontal engines1 the two pistons are made of the same diameter1 in order to secure a perfectly balanced valve4 ---------------------------------------- -------------------------------- ,,chapter ,,vii4 ,,the ,,effect ,,of ,,alterations ,,to ,,valve ,,and ,,eccentric4 ,advance of the ,eccentric4--,it should here be established that to any given example of valve1 ports1 and eccentric 7leaving the piston out of consideration for a moment7 belongs a certain cycle of operations1 the occurrence of these operations and their duration1 expressed in fractions of a revolution of the rotating eccentric1 being constant for a given case1#c but the fraction of stroke traversed by the piston between or during any of these operations of the valve will depend upon where in the stroke of the piston the cycle of operations is started1 taking any convenient operation as being the commencement of the cycle1 for the speed of the piston rises from nil at the commencement of its stroke to a maximum near the middle1 falling again to nil at the end4 ----- ,footnote #c3 ,to make the idea clear one might plot these operations round a circle representing one revolution of the eccentric1 when it would be seen that the operations of the valve could be considered quite independently of those of the piston--then it would remain to show what the piston might do1 or have done upon it1 by comparing its motion under varying conditions with the unvarying operations of the valve1 in the manner explained later4 ,now we may go on to consider the effect of increasing the advance of an eccentric beyond that which it would ordinarily have 7fig4 #cb74 ,if we suppose our eccentric and valve to give us a cut-off at ,c and an exhaust at ,e1 expansion lasting #j4ag of the stroke 7as shown in full lines1 fig4 #cb71 and that by advancing the eccentric we get a cut-off at ,c#a #j4ad earlier1 we must not expect to find that exhaust also will be #j4ad earlier 7dotted lines1 fig4 #cb71 for although the exhaust edge of the valve always gives exhaust at a certain constant period of time later than steam is cut off by the steam edge1 yet in that interval the speed of the piston slackens less in this second case than in the first example1 and it therefore travels through a greater distance than in the same time-interval taken later in the stroke in the first case4 ,hence1 with increased advance we get earlier cut-off1 with expansion of lengthened duration because exhaust is not earlier in the same degree1 neither are all the other operations performed by the valve1 although they each commence earlier and last longer in the stroke1 lead included1 but admission1 of course1 excepted4 ,the idea may1 perhaps1 be more easily grasped by keeping the eccentric and valve in view at an unaltering standard of reference1 and when it is desired to consider the effect of advancing the eccentric1 to imagine instead that it is the setting of the main crank which is altered--which in effect is the same thing1 and is1 perhaps1 easier to reason out4 ,illustration3 ,,fig4 #cb4 ,lengthening or ,shortening the ,eccentric ,rod--,shifting the ,valve on its ,spindle4--,starting with the valve in its central position1 and lengthening or shortening the eccentric rod or shifting the valve on its spindle1 or combining both operations in the same sense1 the distance between the centre of the crank-shaft and the centre of the valve is altered1 and the travel of the valve is shifted bodily to the right or left1 i4e41 the valve when in the centre of its travel 7the extent of travel remaining of course unaltered7 will be either further from or nearer to the crank-shaft than before1 while the position of the ports remains unaltered2 therefore the arrow upon the valve at its centre 7see fig4 #af7 lies to the right or left of the centre line of the port-face1 and1 that arrow being no longer appropriate as a pointer at mid-travel to the zero of the scale on the ports1 we must adopt as our index-finger the graduation mark made adjacent to the arrow and lying over the said zero1 in order that when the disc may indicate that the valve should move1 say1 from #j to #f the new mark or index-finger1 and not the arrow1 may be given that movement4 ,if the valve had previously been properly set for lead1 and the alteration shifted it in4 to the left1 for example1 we should find that the lead at the left-hand end would be diminished by that amount1 and that subsequently1 at cut-off1 release1 and compression1 for both ends of the cylinder1 the valve would always be found in4 to the left of its proper position4 ,the result would be1 as shown in fig4 #cc1 that3 ,at left-hand of cylinder 7see lines of dashes73-- ,lead decreases earlier1 these operations ,cut-off takes place taking place ,exhaust commences as the valve moves towards the left4 ,compression commences later1 this operation taking place as the valve moves from the left4 ,at right-hand end of cylinder 7see lines of dots73-- ,lead increases later1 because when ,exhaust commences effecting them the ,cut-off takes place valve is moving from the left4 ,illustration3 ,,fig4 #cc4 ,compression commences earlier1 for it occurs whilst the valve is moving to the left4 ,the alterations at opposite ends being in opposite sense4 7,the normal distribution is shown by full lines for comparison47 ,and although the commencement of the operations is advanced and retarded by the same amount of error1 operating in opposite sense on opposite sides of the piston1 yet there is a difference in the work done upon the opposite sides because of the commencement of the two cycles of operation at different times1 whence we infer differences in the speeds of the piston1 in the quantities of steam admitted1 and in the expansion-curves4 ,in some cases such a difference is desirable1 and with the working diagram suggested in ,chapter ,i4 another series of results may be obtained by using the ordinary valve for distribution on one side of the piston1 and for the other side using a differently-proportioned valve with a different index-pointer1 as though opposite ends of one valve were differently proportioned1 as in certain large vertical engines4 ,adding ,extra ,lap to a ,valve4--,in some cases cut-off may be wanted earlier in the stroke1 permanently1 with expansion of longer duration4 ,under such circumstances more lap may be added to the valve4 ,if this be done1 and the valve with its extra lap be re-set by giving the eccentric greater advance in order to get the same lead as before 7travel of valve remaining unaltered71 the steam-ports will not be opened to so great an extent as formerly1 and will close earlier1 partly because of the added lap1#d and partly because of the increased advance4 ,the increased length of the valve-face will1 after cut-off1 confine the steam longer in the cylinder1 and it should be obvious 7the effect of advancing the eccentric having already been shown with reference to fig4 #cb7 that in addition to the earlier commencement and lengthened duration of expansion1 the exhaust and compression will begin somewhat earlier because of the increased advance1 and the compression will be of longer duration because of the increased length of the valve-face4 ----- ,footnote #d3 ,to assure one's self of this1 take an extreme case and imagine an excessive amount of lap to be added4 ,in such a case1 with travel unaltered1 the ports might not be opened to steam at all1 the valve simply sliding backwards and forwards without uncovering them2 now1 taking this lap off piecemeal the port would first be opened to an exceedingly small degree and instantly shut again1 the amount and duration of opening increasing with each successive removal of lap4 ,general4--,it is here seen that in altering either the outside or the inside laps with intent to affect only the steam admission with the former1 or only the exhaust with the latter1 we must perforce affect other things4 ,for instance1 we alter the outside lap1 with the intention1 say1 of simply making cut-off earlier or later as the case may be--but having altered the lengths of the valve-faces1 they will alter the duration of expansion and of compression1 and the alteration necessitated in the advance of the eccentric in order that the lead may remain unaltered has its own effect in addition4 ,this interdependence1 which is often inconvenient1 is characteristic of the slide valve worked by an eccentric1 and has led to the use of valve gears and valves which permit1 more or less1 of the independent regulation of some or all of the various operations in the distribution of steam4 ,some of these gears are used with a slide valve 7as in the case of the ,joy and other well-known valve gears71 while others operate special valves4 ---------------------------------------- -------------------------------- ,,chapter ,,viii4 ,,note ,,on ,,link ,,motions4 ,the ,link ,motion4--,in engines whose duty frequently varies1 as in the familiar case of a locomotive1 permanent alteration of lap would not meet the requirements of the case1 for the amount of expansion should be susceptible of ready variation at any time1 in accordance with alterations in the weight of train1 gradients1 and weather4 ,the link motion1 in addition to being a reversing gear1 is also a 8variable expansion0 gear4 ,a precise analysis of any given case of link motion is a somewhat difficult operation1 but1 speaking broadly1 the link motion may be said to provide what is virtually an eccentric of different throw and advance for each degree of expansion4 ,although1 actually1 the same two eccentrics remain constantly in use1 the link motion may be so adjusted as to have an effect on the valve similar to that which would ensue upon the substitution1 one for another1 of different eccentrics as stated4 ,assuming that the lowest range of expansion is being used1 the valve will have the travel1 cut-off1 and lead due for the most part to the throw of the actual eccentric and to its position with relation to the crank1 almost as in the case of an eccentric whose rod is directly connected to the valve-spindle2 now1 as the link is raised 7an operation sometimes termed 8linking-up07 its centre gets nearer to the block on the end of the valve-spindle than it was before 7i4e41 nearer to 8mid-gear071 and we get in effect3-- ,a series of eccentrics of ,with link motion decreasing throw1 but of having 8open0 rods4 increasing angular and linear advance4 ,a series of eccentrics of decreasing throw and of increasing angular1 but ,with link motion decreasing linear advance2 having 8crossed0 rods4 throw1 therefore1 decreasing more rapidly than in the case of the link motion with 8open0 rods4 ,take the case of a partially linked-up motion with open rods2 we shall find that because the throw of the eccentric is lessened1 the travel of the valve to the right and left of its central position will be diminished1 and the ports will not be opened so wide to steam or exhaust2 because of the increased advance of the eccentric1 steam will be cut off earlier1 compression and lead will also be earlier1 expansion and compression lasting longer1 and exhaust and admission being shortened4 ,the difference between working in full gear and linked up is shown by the diagram in fig4 #cd4 ,illustration3 ,,fig4 #cd4 ,below are diagrams showing the effect of putting the link motion in 8mid-gear0 7fig4 #ce7 and of putting the link motion into 8back gear0 7fig4 #cf7 while the engine is running ahead1 as for a locomotive1 but it will be understood that the effect of the compression shown in the latter will be modified by the lifting of the valve from its face1 with the result that some compressed air and steam will go up the exhaust4 ,illustration3 ,,fig4 #ce4 ,illustration3 ,,fig4 #cf4 ,the set of diagrams forming fig4 #cg is intended to give a comparative view of the effect upon the distribution of steam1 of differently arranged valves and eccentrics4 ,the operations taking place upon only one side of the piston are shown1 except in that section of the diagram which concerns a shifted valve1 in order to avoid confusion4 ,the upper line of each diagram1 being read from left to right1 shows the operations which take place on the forward stroke1 those of the return stroke being read from right to left upon the lower portion of each diagram4 ,illustration3 ,,fig4 #cg4 ---------------------------------------- -------------------------------- ,,chapter ,,ix4 ,,note ,,on ,,very ,,early ,,cut-,,off1 ,,and ,,on ,,reversing ,,gears ,,in ,,general4 ,very early ,cut-off4--,theoretically there is no limit to the range of cut-off which may be obtained from an ordinary slide valve1 but a moment's consideration will show that if we require a valve to cut off steam very early in the stroke of the piston so as to cause it to expand throughout an increased fraction of that stroke1 the lap must be considerably increased beyond that which is usual1 if the valve-face is to be enabled to keep the steam confined in the cylinder for a sufficient period after cut-off is effected4 ,as a consequence of increased lap1 the travel of the valve1 made up of lap plus the required port-opening1 will need to be increased correspondingly2 the linear advance 7the sum of lead plus an increased lap7 will be increased also1 and the valve will therefore come back earlier into its central position 7where compression begins7 than would the normal valve1 so making compression begin earlier and last longer than before4 ,in an unbalanced valve1 this increased travel1 together with the abnormal compression1 cannot be tolerated1 so that for obtaining an early cut-off without too much compression1 with a sliding valve1 gear of the 8separate cut-off slide10 or ,meyer type1 is employed4 ,this is characterised by a cut-off plate or plates on the back of a main slide valve1 which may have the usual exhaust cavity1 and in addition possesses supply steam-conduits leading through the valve from the back to the 8lap-edges0 7fig4 #ch74 ,exhaust and compression are timed by the edges of the central exhaust cavity2 lead is given by the 8lap-edges10 but cut-off is controlled by movement of the cut-off plates on the back of the moving main valve1 so that though a 8lap-edge0 may not at any time have closed one of the steam-ports proper1 a cut-off plate worked by a separate eccentric can close the appropriate steam-conduit above referred to1 and so stop the current of steam flowing past the lap-edge to the still open port4 ,moreover1 by a suitable setting of the eccentrics of the main and cut-off valves1 it is possible to arrange that1 at the instant of cut-off1 the two valves shall be moving in opposite directions1 and so shall effect a sharp cut-off1 whereby wire-drawing is reduced4 ,illustration3 ,,fig4 #ch4--,slide ,valve1 with ,cut-off ,plate on back4 ,with the model referred to in the earlier chapters1 the action of a ,meyer or similar gear can be readily investigated if an extra eccentric-arm be added to the crank-pin disc and an extra travel-scale marked upon the baseplate4 ,in fig4 #ci is shown the 8,buckeye0 valve used in ,america on the ,buckeye engine2 a is the main valve1 which determines the admission1 release and compression of the steam2 and b is the small auxiliary valve which rides on the top of the main valve a1 and which effects the cut-off4 ,this valve is constructed so as to have all the advantages of a perfectly balanced valve as well as those of the ,meyer valve4 ,there are two faces between which the main valve slides steam tight1 and the small valve works on a face inside of the main valve1 as shown in the figure4 ,steam is exhausted past the outer edges of the main valve4 ,in order that steam may enter the cylinder1 the port in the main valve must be in communication with the steam port and at the same time it must not be covered by the auxiliary valve b4 ,in the figure the valves are shown in such a position as to admit steam to the left-hand end of the cylinder and to allow it to exhaust from the right-hand end1 as indicated by the arrows4 ,illustration3 ,,fig4 #ci4--,the ,buckeye ,valve4 ,each valve is moved by its own eccentric1 quite independently of the other4 ,by changing the advance or eccentricity1 or both1 of the auxiliary valve1 the cut-off may be changed as desired1 without in any way affecting the motion of the main valve4 ,and as the main valve determines the admission1 release1 and compression of the steam1 it is possible1 with this valve1 to change the cut-off as desired without affecting these at all4 ,the ,trick or ,allen ,valve4--,in connection with the subject of early cut-offs the ,trick or ,allen valve may be mentioned4 ,it is made with a passage-way through the back of the valve1 as shown in fig4 #dj1 adapted to deliver an extra supply of steam into the port by rendering available an additional proportion of the area thereof1 when the narrow port-opening given by the lap-edge alone1 during linked-up working1 is found to wire-draw the steam4 ,whilst the lap-edge a1 for instance1 uncovers the edge b of the right-hand port1 the left-hand end of the ,trick passage becomes uncovered by moving beyond the edge c of the port-face1 so that a current of steam flows from left to right through that passage to join the stream passing the edge a4 ,the total area of the two supply-streams flowing into the right-hand port being practically twice that which would be available were the ,trick passage absent4 ,illustration3 ,,fig4 #dj4 ,the occurrence and duration of the operations of steam-distribution are exactly the same with this valve as with an ordinary slide-valve of corresponding proportions1 the distinctive feature of the valve having no effect upon them so far as their timing is concerned4 ,reversal4--,it may be of interest to consider the question--8,why do we need reversing gears80 ,the gears in common use are nearly all variable expansion gears1 as well as being reversing gears1 but it is desired to consider now only the question of reversing4 ,are reversing gears really needed8 ,suppose that one could have a transparent cylinder and valve-chest1 so that movement of the piston and valve could be clearly seen4 ,suppose also that the connecting-rod1 crank1 valve gear1 and fly-wheel could not be seen4 ,we look into the cylinder and see valve and piston reciprocating regularly1 and in harmony with one another4 ,we turn away1 and before we look again1 the engine1 let us assume1 has been reversed without our knowledge2 yet when we turn again to the engine there are the piston and valve reciprocating just as they were at first 7although they must have 8changed step10 as we shall presently find71 and1 look as intently as we may1 with nothing else visible we shall be unable to tell that the engine has been reversed4 ,the valve is doing now exactly what it did before1 performing its operations in the same order4 ,why have we had to employ reversing gear8 ,solely because--when the engine ceases to run ahead for instance1 stopping at half-stroke the piston and valve will be in a certain position1 and will have been moving in certain directions relatively to one another4 ,and to reverse the piston's motion1 we want the valve ,firstly--,to take up a new position on the port-face1 so as1 for example1 to give steam where it had previously given exhaust1 and thus we get reversal of the engine2 and ,secondly--,to move away from that new position1 after the engine has started astern1 in the same direction as that in which it was moving when the engine was running ahead4 ,and seeing that the valve gear derives motion from the crank-shaft directly or indirectly1 we cannot give the valve motion in this same direction from an engine now moving reversely1 unless we also reverse the gear4 ,and thus we may conclude that a reversing gear is needed simply for the purpose of regulating the direction of the first motion of the valve after stopping 7i4e41 for 8changing the step071 for after it next reaches the end of its travel its motion will be in all respects just as it was when the engine ran ahead1--so that1 if we had a single-cylinder engine and never stopped it except on the dead centres1 we could drive its valve by a separate valve-engine always running one way1 and our main engine would1 if pushed off by hand1 run equally well either ahead or astern1 whilst served by a valve having lap1 lead1 and travel as is customary4 ,it is hoped that this little speculation may not appear unprofitable1 for it has a bearing on the action of the reversing slides in radial valve gears such as those of ,hackworth1 ,walschaert1 and ,joy4 ,illustration3 ,,fig4 #da4 ,,fig4 #da ,,shows ,,enlarged ,,key-,,view ,,of ,,cardboard ,,crank-,,disc1 ,,which ,,should ,,turn ,,within ,,the ,,circle ,,drawn ,,in ,,the ,,centre ,,of ,,fig4 #a4 ,the arm marked ,c is the crank-arm1 and serves as an index finger in the recording and reading of results4 ,the mark #a is the index-mark for the forward eccentric to operate a valve without lap4#e ,the mark #b is the index for a forward eccentric set for lead1 using a valve without lap4#e ,the mark #c is for a forward eccentric for a valve with lap and lead run in 8full gear40#f ,the mark #d is for a forward eccentric for a valve with lap and lead run with a link motion partially 8linked up40#f ,the mark #e is for a valve with lap and lead run with the link motion in mid-gear4#e ,the mark #f is for a backward eccentric for a valve with lap and lead1 run with a link motion partially linked-up4#f ,the mark #g is a backward eccentric for a valve with lap and lead run in 8full gear40#f ,the mark #h is for a backward eccentric set for lead1 using a valve without any lap4#e ,the mark #i is for a backward eccentric for a valve without lap4#e ----- ,footnote #e3 ,used with ,no4 #a travel-scale1 fig4 #a4 ,footnote #f3 ,used with ,no4 #b travel-scale1 fig4 #a4 ,the radial lines between the centre of the disc and the small circles represent the actual throw assumed for the various eccentrics and their angular position in relation to the crank2 where the actual throw is not sufficient to enable the arms to reach the edge of the disc1 the throw-lines are produced to the edge4 ---------------------------------------- -------------------------------- ,transcriber's ,notes ,missing or obscured punctuation was silently corrected4 ,typographical errors were silently corrected4 ,the 8#j0 in fig4 #a was transcribed as the number zero to represent the center of the scale1 and the 8,o0 in fig4 #b was transcribed as the letter ,o to represent the outlet4 ,text in italics is enclosed by underscores 7italics74 ,superscripts are written using a caret 77 followed by the text enclosed in braces 7superscript74 ,subscripts are written using an underscore 77 followed by the text enclosed in braces 7subscript74