Fig. 2534Fig. 2534.
Fig. 2534.
Fig. 2535Fig. 2535.
Fig. 2535.
If the axis of the crank axle formed an obtuse angle to the engine centre linea ainFig. 2529, the connecting-rod end tried with the crank pin, as shown inFig. 2531, would fall outside of the crank-pin journal when the latter was on the dead centre nearest to the cylinder, as shown inFig. 2534, and inside of the crank-pin journal when on the other dead centre, as inFig. 2535.
Now, suppose either of the errors to exist, and the alignment be neglected, then if the brasses at each end be keyed up to fit their respective journals, then the body of the rod must be bent into a bow shape, and the strain of forcing or springing it into this shape will fall upon the journals, which will heat and pound in consequence.
It is now to be explained how to test if the axial line of the crank shaft is at a right angle to that of the cross-head journal, when viewed from the crank-shaft end and horizontally.
From a want of parallelism in this direction, heating of the crank pin and cross-head journals issure, and a pound or thump is, to some extent, liable to occur, and the cause, if the error is slight, is difficult to discover, save by using the connecting rod to test it with.
When a thump occurs at the end of the stroke (when the crank is on a dead centre), it may arise from a ridge at the cylinder, or at the guide-bar end, or from the connecting-rod brasses being insufficiently keyed up; but when it occurs while the crank is at half stroke these causes are eliminated, and the cause must be looked for in either a crank pin not parallel to the crank shaft, or, as in the case now under consideration, because of one or the other of the crank-shaft journals being too low.
Assuming the crank pin and crank shaft to be axially true, one with the other, we may proceed to show separately the cause of the heating and that of the pounding, if the crank journal is too low at either end.
Fig. 2536Fig. 2536.
Fig. 2536.
InFig. 2536, letarepresent the cross-head journal, andb ba line parallel to it. Letb crepresent the axial line of the crank shaft (being out of parallel because the crank end is too high or the other end too low). Letf frepresent the centre line of the crank pin when at the top, andg gwhen at the bottom of its path of rotation, and it will be observed that the vertical distance between the crank pin and the axial line of the cross-head journalis less on one side than on the other; thus in the figure distancedis less thane. We have in this case measured these distances on a plane at a right angle to the cross-head journal, but it will make no difference if we measure them on a plane with the path of rotation of the crank pin, as will be seen inFig. 2537, in which the distance from the centre of the crank pin at two opposite points in its path is represented by dots shown ate f, and frometohmeasures less than fromftoh,hrepresenting the centre of the cross-head journal.
Fig. 2537Fig. 2537.
Fig. 2537.
InFig. 2537, letarepresent the axial line of the cross-head journal,ba vertical line at a right angle toa;crepresenting the crank shaft extended by a dotted line, so as to enable comparison witha;dthe crank,eandfthe centre of the crank-pin journal, andg ga line at a right angle to cross-head journala.
Nowg, being at a right angle toa, represents what should be the plane of rotation of the crank pin, whereasc, being out of parallel witha, causes the path of rotation to be in the path frometof, or asdcompared tob; supposing then that the bores of the connecting-rod brasses to be axially parallel one to the other, and keyed up properly, and when ateone bore of those brasses will stand parallel toewhile the other is parallel toa, or when at the bottom of the crank rotation, one bore will be parallel tofand the other parallel toa. Thus the rod will be twisted, and the strain due to this twist will cause the bearings to heat. That this twisting is continuous throughout the whole revolution may be seen by the want of parallelism of the dotted line (representing the crank pin when on the dead centre) witha(representing the cross-head journal).
It is now to be observed that if the plane of the crank rotation were at a right angle to the axis of the cross head, as it should be, the path of the centre of the crank-pin journal would be in the plane ofg g, whereas it falls outside as ate, and inside as atf, while athit is coincident; hence it appears that starting from a dead centreh, the rod bends, passing at that end outward toe(when the crank has made a quarter revolution), where it attains its maximum bend, thence diminishing until finally ceasing, when the crank reaches the other dead centre. As soon, however, as it passes the last dead centre a bend in the opposite direction takes place, attaining its maximum atf, and ceasing ath. This bending also causes undue friction and the consequent heating of the journals; furthermore, if there be anyendplay between the brasses and the journals, there will be a pound, as the brasses jump from one end of the journal to the other at different parts of the stroke. It is obvious that if the crank end of the crank shaft was too high instead of too low, as in our example, then the effects would be the same, butewould fall on the inside instead of the outside ofg, whilefwould fall outside instead of inside.
Fig. 2538Fig. 2538.
Fig. 2538.
To discover if the crank shaft is out of parallel in the direction here referred to, connect the connecting rod to the cross-head journal, setting the brasses up to a close working fit. At the other end of the connecting rod put the strap keys and brasses in their places, but not on the crank-pin journal. Place the crank in its highest position, and lower the end of the rod down to the crank-pin journal, as shown inFig. 2538, and if the crank shaft is parallel (in the respect here referred to) to the cross-head journal, the brass flanges will just meet the faces of the crank-pin journal, as shown inFig. 2539. If, however, the crank end of the crank shaft is too low, as in our example, the flanges of the brasses will fall to one side of the crank-pin journal, and that side will be towardb,Fig. 2540, when the crank pin is at the top, and towardc,Fig. 2541, when it is at the bottom of its path of rotation.
Fig. 2539Fig. 2539.
Fig. 2539.
Fig. 2540Fig. 2540.
Fig. 2540.
Fig. 2541Fig. 2541.
Fig. 2541.
Fig. 2542Fig. 2542.
Fig. 2542.
The effects will be precisely the same, and in the same direction with relation to the various parts of the crank’s revolution, if the crank-pin end of the shaft was of correct height; but the other end was too high, hence, in correcting the error, it is desirable to place the engine on the dead centre, so as to determine which end of the shaft to operate on—that is to say, whether to raise the crank-pin end or lower the other end. But suppose the error to be that the crank-pin end of the shaft was too high instead of too low, then, the testing being continued as before, the effects will beof the same general character, but altered with relation to the specific parts of the revolution. Thus, when the crank is at the bottom, the rod would fall towardsa,Fig. 2542, and when at the top, it would fall in the opposite direction—that is, towardsd,Fig. 2542.
We now come to one of the most common errors in the alignment of the parts of an engine, and to the one that it is the most difficult to locate or discover, namely, a want of parallelism between the axial line of the crank pin and that of the crank shaft.
This generally arises from improper methods in the chucking of the crank to bore it, or from errors induced in fastening the crank to its shaft. The results are precisely alike in both cases, supposing, of course, the errors to exist in the same direction in the two cases.
The error in chucking usually consists in planing one surface of the crank, and bolting the planed surface against the chuck to bore both crank holes. In this case the crank holes will be out of true to twice the amount the lathe face plate may be out of true, and to whatever amount the crank may alter its form from having its surface metal removed.
To avoid these errors the large bore and its hub face should be turned at one chucking, and this hub face should be bolted to the face plate for the second chucking, the small end swinging free, except in so far as the ends of the plates may touch against it to steady it.
Fig. 2543Fig. 2543.
Fig. 2543.
The error in putting the crank on may occur from the key springing the crank out of true, and if the crank is shrunk on from too great an allowance for shrinkage or improper heating for the shrinkage or contraction, as it is sometimes termed. Referring to the error in keying, it is more liable to occur when the crank bore and its seat upon the shaft are made taper, than when made parallel, because it is a difficult matter to insure accuracy in the fit of the taper, and the key pressure will spring the crank over on the side at which it is the easiest fit. InFig. 2543letarepresent the end of the crank shaft;bthe key, andcthe crank shown partly in section: suppose the crank bore (whether made taper or parallel) has a slightly easier fit on the sidedthan on the sidee, and the pressure of the key (supposing it to fit properly top and bottom) would spring the crank over in the direction shown in the figure, the axial line of the crank pin standing at the angle denoted by the linef, instead of parallel to the axial line of the shaft. Suppose the crank to be put on by hydraulic pressure, and the key to fit on the sides and not on the top and bottom, then its fit to its seat on the shaft would depend on the truth and smoothness of its bore and seat on the shaft, the amount allowed for the forcing fit and the amount of the error. If the latter amount was so small that the crank would fit at both ends, but simply fit tighter ate ethan atd, the crank would remain true, but might possibly get loose in time. This would be especially liable to occur if the tool marks on the bore and seat were so deep that the contact was mainly at the tops of those marks or ridges which would be apt to compress. But if the surfaces were cylindrically true and smooth, and the amount allowed for forcing was sufficient as stated to give the bore and seat contact atd, with a key fitting sideways, the crank would probably remain tight and true.
Were the bore and its seat parallel the crank would remain true, no matter whether the key fitted on the sides or at the top and bottom, providing the key fitting top and bottom were bedded fairly from end to end.
When the surfaces are not smooth, but contain tool marks or ridges, an unequal pressure of the key at one end, as compared to the other, sets the crank over, as shown in the figure, because the key pressure compresses the ridges and lets the crank move over.
Fig. 2544Fig. 2544.
Fig. 2544.
Supposing the strain of the key, or keys, to be depended upon to hold the crank, they must fit top and bottom, and their accurate fit becomes of the first importance; because not only is it necessary that they fit equally at each end, but they must also fit equally across the width of the key at each end. For example, inFig. 2544is a key binding most at the opposite corners, as denoted by the dotted surfacesa b, and the result will be that the key pressure would tend to twist the crank in the direction ofd e, havingcas a centre of motion, providing that the error was equal ataandb; but in proportion as the error was greatest and the fit tightest ata, or atb, would the centre of motion be moved nearer to either point.
Supposing now that the crank is to be shrunk, or contracted on, then the points of consideration are (supposing the crank to fit properly to its seat, whether the same be either parallel or taper) that the hub of the crank opposite to the throw is the weakest andis likely to give most in the process of contraction, so that if one part (asf) of the crank be made hotter than another (asg) it will give way more, and this will twist the crank. This is specially liable to occur if an excessive amount of difference in the bore and seat diameters has been allowed for contraction.
Fig. 2545Fig. 2545.
Fig. 2545.
It may not happen that a crank pin is out of truth in a direction in which the error will show plainest when the crank is on its dead centres, or at half-stroke; but if a crank pin, tested in those four positions, fails to show any error when tested by the connecting rod, it will be true enough for all practical purposes, and true enough to avoid heating and pounding, both of which evils accompany an untrue crank pin. Suppose, now, that a crank pin stands out of true in the direction shown inFig. 2545, in whicha arepresents the axial line of the cylinder bore prolonged, andb bthe axial line of the crank shaft (the two being parallel or in proper line). Lete erepresent the centre line of the connecting rod when the crank is on one dead centre, the axial line of the crank pin being atc c. Then the brasses being keyed up to fit the crank pin, the centre or axial line of the connecting rod would stand as denoted bye e. But the brasses at the other end of the rod being keyed up to fit the cross-head journal, and their lines being at a right angle to the linea a, we have that the rod is at that end endeavored to be held parallel toa a; hence, keying up the connecting-rod brasses on the crank pin would tend to bind the rod, one end standing parallel toa a, and the other parallel toe e.
This would place great strain on the outer radial face of the cross-head journal, as well as on the cylindrical body of the journal.
When, however, the crank pin arrives at the opposite dead centre, as denoted by the dotted lines inFig. 2545(g grepresenting its axial line, andf fthe centre line of the connecting rod at a right angle tog g), the want of truth in the pin throws the cross-head end of the connecting rod against the inside face of the cross-head journal. Hence, twice in each revolution is the connecting rod bent, and twice does it jam from side to side of the cross-head journal.
It may now be pointed out that if we take either dead centre singly, and connecting the rod at the crank-pin end, try it at the cross-head end, and it will be a difficult matter to determine whether any want of truth at the latter end is caused by the crank pin being out of axial truth, or whether it is the crank shaft itself that is out of line. But there is this difference between the two cases. When the error is due to want of alignment in the crank shaft, the connecting rod will show the erroron the same sideof the cross head, no matter on which dead centre the crank pin stands; but when it is due to the crank pin, the rod will fall inside the cross head on one dead centre, and outside when tried on the other dead centre, as is shown by the respective lineseandf, inFig. 2545;ebeing at a right angle toc, andfat a right angle tog.
Fig. 2546Fig. 2546.
Fig. 2546.
Again, it has been shown that when the shaft was out of line, a point on the crank-pin journal passed outside of the cylinder centre line at one dead centre and inside at the other; but when the pin is axially out of parallel, the path of a point on its journal will remain in the true plane, as is shown inFig. 2546, the point being taken at the intersection ofeandc c.a arepresents the path of rotation of the same, which is parallel to the true facebof the crank.
From the angle of the axial line of the pin being in opposite directions, when on opposite dead centres to the axial line of the crank shaft, the bore of the brasses cannot wear to suit the error, which, therefore, only diminishes by the wear of the crank pin. Suppose the error to be1⁄64inch in a crank-pin journal 3 inches long, and that the connecting rod is 6 feet long, the error at the cross-head end of the rod will amount to3⁄8inch.
Fig. 2547Fig. 2547.
Fig. 2547.
InFig. 2546the error is shown to exist in an opposite direction, throwing the rod to the other side of the cross-head journal. But, in this case, the crank, when on the dead centre nearest to the engine cylinder, throws the connecting-rod end against the inside face of the cross-head journal, as denoted by the linee, which is on the opposite side ofa ato what it is inFig. 2545. Again, when on the other dead centre, the linef f, inFig. 2546, fallsoutside, whilef f, inFig. 2545, fallsinsideofa a, and it is by this difference that we are enabled to know in which direction the crank pin is out of true. To find the amount to which it is out of true in the length of its journal, place the crank on one dead centre, and with the connecting-rod brasses keyed up firmly home on the crank pin, and the other end of the connecting rod entirely disconnected from the cross head, mark on the latter a line coincident with the side face of the rod end, as atd,Fig. 2547. Then, with the crank pin placed on the other dead centre, mark another line on the cross head, coincident with the other side face of the rod, atc,Fig. 2547. Now, suppose that the linedshows the rod to fall3⁄8too much on that side, and linecshows it to fall (when on the other dead centre)3⁄8too much on the other side of the journal, and that the length of the rod is 6 feet, while that of the crank-pin journal is 3 inches, then the latter, divided into the former, gives 24, and this sum divided into the3⁄8, the rod end falling out of true atcandd,Fig. 2547, gives us1⁄64-inch as the amount the crank pin stands out of true in its length; hence, to correct the error, we may file on the crank pin a flat place at each end, as shown inFig. 2548by the linesc d, and then file on the top and the bottom of the crank pin a flat placeb,1⁄128-inch deep, and of equal depth all along the journal; by then filing the crank pin round and bringing the flat places just up to a circle, we shall have reduced the diameter of the crank pin by1⁄64inch, and have made it axially true with the cross-head journal. It is important, however, to bear in mind that, in this case, the crank pin is supposed to be out of true in the direction shown inFig. 2545, and to stand axially true with the cross-head journal, when the crank is placed at half stroke, top and bottom, the crank shaft being in proper line.
Fig. 2548Fig. 2548.
Fig. 2548.
If the axial line of the cross-head journal stands truly horizontal, the flat places on the crank pin may be filed horizontally level, with the crank placed on the corresponding and respective dead centres. But as the length of the cross-head journal is so short, it is difficult to gauge, if it does stand axially exactly horizontal, hence it is better to try the rod, or follow the above directions; especially as the cross-head journal and crank shaft may be in line without being axially horizontal.
Suppose now that the axial line of the crank pin stands true with that of the cross-head journal when the crank is on either dead centre, but out of true when at the top and bottom half stroke. The connecting rod, connected as before, and tried with the cross head, will fall first to one and then to the other side of the cross-head journal, and the direction in which the crank is out of true may be known from the position of the crank pin when the error shows itself.
Fig. 2549Fig. 2549.
Fig. 2549.
Fig. 2550Fig. 2550.
Fig. 2550.
If the error exists to an extent that is practically measurable, a pound in the journals, as well as their heating, is the inevitable result. InFig. 2549, for example, the rod end is shown in section, and it will be noted that the error being in the direction there shown, and the crank pin in the respective positions there shown, the brass bore only contacts with the journal at each end, and that the diameter of the bore of the brasses is greater than the diameter of the crank pin journal totwicethe amount the crank pin is out of line. Now let us place the crank at the top of its revolution, as inFig. 2550, and as its axial line then stands parallel to that of the cross-head journal, the brass bore is too large to fit the crank pin journal and there is lost motion.
From the time the crank pin passes the dead centre this lost motion increases in amount until it becomes sufficiently great to slam the rod over against the side of the cross-head journal, while at the same instant the crank pin pounds in the connecting-rod brasses. At what precise part of each quarter crank revolution this action will occur, depends upon the amount the crank pin is out of line; but the more it is out the nearer to the dead centre it will be, and, conversely, the nearer true it is the nearer the crank will approach its highest and lowest positions before the pound takes place. If it is attempted to key up the brasses so as to spring the rod and let them close along the journal, the brasses will heat in proportion to the amount of error; hence when the crank pin pounds with the brass properly adjusted, and heats while keyed up enough to stop the pound, the crank pin is out of true.
To test the alignment of an engine with stretched lines take out the piston and rod, and take off the connecting rod, then fasten a piece of iron at the open end of the cylinder so that it will hold a stretched line true with the axis of the cylinder bore. Provide at the crank end of the engine bed a fixed piece of wood to hold the other end of the line, and then with a piece of wire as a gauge set this line (tightly stretched) true with the cylinder bore. Then place the crank pin at the top of its path of rotation and drop a plumb line from the centre of its journal length, and this line should, if the crank shaft is horizontally level, just meet the stretched line. If it does not do so place a spirit level on a parallel part of the crank shaft, and if the shaft is not level it should be made so, and so adjusted that the line from the centre of the length of the crank pin journal just meets the stretched line from the cylinder bore.
To test if the axial line of the crank shaft is at a right angle to the cylinder bore axis move the crank pin nearly to its dead centre, and measure the distance from the middle of its length to the stretched line. Then move the crank pin over to nearly the opposite dead centre, and (by means of the plumb line) measure the distance of the plumb line from the stretched line. To be correct the plumb line from the crank pin will during this movement just touch the stretched line.
To test if the stretched line is fair with the centre of the crank shaft place a square on the end of that shaft and even with its centre, and the blade should then just meet the stretched line.
The edges of the guide bars may also be tested with the stretched line, and the top and bottom of the guide-bar flanges may be tested to prove if the bars are of the correct height.
To further test the bars place a spirit-level across them and lengthwise on them.
If the piston rod and connecting rod are in place the alignment may be tested as follows; Let the piston rod be as far out of the cylinder as possible, and stretch a line to one side of it, just far enough off to clear the guide bars, &c. Set this line as follows: Let it be in line with the rod as sighted by the eye when standing some few feet away from it but horizontally level with the centre of the rod, set it parallel to the rod with a rule or its equivalent.Then the centre of the crank-pin journal should measure from the stretched line, the distance of the line from the piston rod added to half the diameter of that rod. This test, however, is not very accurate on account of the difficulty in setting the line, and because the piston rod may not have worn equally on each side.
Setting Slide-Valves—An engine slide-valve may be so set as to accomplish either one of three objects. First, to give equal lead for each stroke; second, to cause the live steam to be cut off and expansion to begin at an equal point in each stroke; and third, for the exhaust to begin at an equal point in each stroke.
If we, set the eccentric so that the exhaust will begin at corresponding points for the two strokes, the valve lead will not be equal, and the exhaust opening will be greater when the piston is at one end of the cylinder than it will be when the piston is at the other end.
If the eccentric be set to cut off the steam at corresponding points for the two strokes, then the lead, the admission, and the exhaust of the steam at one port will differ (with relation to the piston movement) from that at the other. It is generally preferred to set the eccentric so as to give equal lead for the two ports when the piston is at the respective ends of its stroke, which gives an equal amount of exhaust opening when the piston is at the respective ends of its stroke.
The only operations properly belonging to the setting of a slide-valve are those of finding the true dead centres of the crank pin, and setting the eccentric to give the valve the desired amount of lead. It is generally found, however, that the length of the eccentric rod requires a little correction, and as this must be done before the eccentric can be set, the setting operations should be conducted with a view to making the correction as early as possible.
In many of the instructions given by various writers it is directed to first square the valve, which is to attach the parts and move the engine crank, or fly-wheel, through one revolution, to ascertain if the valve moves an equal distance on each side of the centre of the cylinder ports, correcting the length of the eccentric rod until this is the case. This is an error, because on account of the angle of the eccentric rod the valve does not, when set to have equal lead at each end of the stroke, move an equal distance on each side of the cylinder ports, but travels farther over the port nearest than it does over that farthest from the crank.
When the travel of the valve is equal to twice the width of the steam port, added to twice the amount of steam lap, the valve does not fully open the farthest port from the crank. When the valve-travel is more than this amount both ports may open fully, but the error due to the unequal valve-travel from the angularity of the eccentric rod is increased. That the amount of error induced by squaring the valve is appreciable, may be seen from the fact that with 11⁄4inch steam ports,3⁄4inch steam lap, and 41⁄2inches of valve-travel, it amounts to about1⁄8inch with an eccentric rod 4 feet long. As the eccentric rod has (if a solid one, as in the case of a locomotive) to be operated upon by the blacksmith to alter its length, and requires some accurate setting for alignment after having its length corrected, it is obviously preferable to obtain its exact length at once. This may be done with less work than by the squaring process, which is entirely superfluous.
Fig. 2551Fig. 2551.
Fig. 2551.
Assuming, then, that all the parts are properly connected and oiled, the valve is set as follows: Upon the face or edge of the fly-wheel an arc, true with the centre of the wheel, should be drawn, as ata b, inFig. 2551, marking it on opposite sides of its diameter and opposite to the crank pinp. The engine should then be moved in the opposite direction to that in which it is to run, until the guide blockiis very near its full travel. A straight-edge must then be placed to bear against, or be coincident with, the end face of blocki, and held firmly while a line is drawn across the edge of the guide bars, as shown atc. There should then be fastened to the floor (which must be firm, and not give under the engineer’s weight), a piece of ironw, having a deep centre-punch mark, or its equivalent. A steel tram-rodt, pointed at each end, is then set in the centre-punch mark atw, and with the upper endda line made across the wheel edge or face. The fly-wheel must then be moved so that the crank passes the dead centre, the guide block moves back and away from the linec, and then approaches it again. When the end of the guide block is again coincident with the linec, the tram should be set as before and a second line,f, marked on the fly-wheel rim, and from these two lines,dandf, the crank may be placed upon its true dead centre asfollows:—
Fig. 2552Fig. 2552.
Fig. 2552.
InFig. 2552a section of the fly-wheel rim is shown (enlarged for clearness of illustration); from the linesd,fthe centreeis found, and marked with a centre punch dot to define it. It will be obvious, then, that if the fly-wheel be moved until this line and dot come fair with the upper edge of the tramt, the guide block will be at the exact end of its travel, and the crank, therefore, on its dead centre. By a similar operation performed with the guide block at the other end of the guide bars, and with lines on the other side of the wheel rim (as shown atb,j,k), the other centrelmay be found. In obtaining these centres, however, a question arises as to the direction in which the wheel should be moved for bringing the guide block up to the lines atc, and for marking thelinesd fandj k, or for bringingeorltrue with the tram point. If the fly-wheel be moved in the opposite direction to that in which the engine is to run, the cross-head journal and crank pin will bear against the boxes of their brasses in the direction in which they will have contact when the engine is running. Suppose, for example, that the top of the fly-wheel when the engine is in motion moves from the cylinder, then the cross-head and crank-pin journals, driven by the piston, will bear against the half-brass nearest to the cylinder, which,when the force-producing motion is applied to the fly-wheel instead of to the pistonwill be the case when the fly-wheel is moved in the opposite direction. By moving the fly-wheel in an opposite direction to that in which the engine is to run, the lost motion in the journals and bearings is therefore taken up in the proper direction so far as the connecting-rod brasses are concerned, and any lost motion between them and their journals will not impair the set of the valve, as would be the case were the fly-wheel moved in the direction in which it is to run.
But by moving the fly-wheel backwards the play in the eccentric and in all the joints between it and the valve spindle is up in the wrong direction, because the power to move the rods is being applied in the opposite direction to that in which it will be applied when the engine is running, and, therefore, the play motion of the jointed or working parts will cause a lost motion impairing the set of the valve.
Now there are generally more working parts between the eccentric and the valve than between the crank pin and the piston, and hence more liability for lost motion to exist, and it follows that in such case it is better to move the engine in the direction in which it is to run.
It may be remarked, however, that the play may be taken up in the proper direction in both cases, and the engine be brought upon its dead centre, by moving it in the opposite direction to that in which it is to run, and that in setting the eccentrics they be moved on the shaft in the direction in which the engine is to run, as forward for the forward eccentric, and backward for the backward one (assuming the engine to have a link motion, and, therefore, two eccentrics).
It is obvious that any other resting place may be used instead of the floor for the tram; thus in a locomotive the wheel guard may be used, the tramtbeing used to mark lines on the upper part of the wheel rim, instead of opposite the crank. To set the valve, place the fly-wheel on its dead centre, moving the fly-wheel as directed until one of the points (eorl, saye) comes fair with the point of the tram; then move the eccentric on the shaft until the steam port is open to the required amount of lead, and fasten the eccentric to the main shaft. Next move the fly-wheel around until on the opposite dead centre, and if the lead is the same in amount for both ports the valve is set. Suppose, however, that in this last case the lead is too great; then it shows that the eccentric rod is too long, and it must be shortened to an amount equal to half the difference in the lead. Or suppose that the lead when the wheel was tried on the last dead centrel, was less than for the other port; then the eccentric rod must be lengthened to half the amount of the difference. Assuming that the rod was too long by1⁄32of an inch, then it may very often be shortened by simply heating about six inches of its length to a low red heat, and quenching it in water. If the rod has a foot which bolts on a corresponding foot on the eccentric, then to lengthen it a liner of the requisite thickness may be placed between the two feet.
Fig. 2553Fig. 2553.
Fig. 2553.
Suppose there is an equal amount of lead at each end but the amount is not sufficient or is too great: then the eccentric must be moved on the shaft until the proper amount of lead appears at the port. The lead must then be again tried at the other dead centre. In moving the eccentric, however, it must, under all conditions, be moved in the direction in which it will rotate, for reasons already given. The best method of measuring the lead where the lines on a rule cannot be seen is with a lead wedgep, as shown inFig. 2553; this, if slightly forced in, will mark itself, showing how far it entered.
Fig. 2554Fig. 2554.
Fig. 2554.
In some practice the position of the valve is transferred to the valve stem outside of the stuffing box or gland, as shown inFig. 2554, sectional view. The valve stem being disconnected from the rod or arm that drives it, the valve is moved by hand to have the proper lead, as ata; a centre-punch mark is then made outside the stuffing box and a trambrested thereon; with the other end of the tram a markcis made on the valve stem. A similar mark is made on the stem when the crank is on the other dead centre, and the tram and marks, applied as shown, are employed instead of measuring the lead at the ports themselves. This involves extra work, but gives no more correct results. It involves marking lines on the valve stem, which is objectionable. If several trials have to be made there is a confusion of lines on the valve stem, and the wrong one is apt to be taken. On the other hand it affords a facility for setting the valve without having the steam chest open, which may in some cases be desirable. If this plan be adopted the lines on the valve rod should not be defined by centre-punch marks, for they will cut the packing in the stuffing box.
When the eccentrics are secured to the shaft by a set-screw only, and not by a feather, it is an excellent plan, after they are finally set, to mark their positions on the shaft, so that if they should move they may be set to these marks without moving the engine around.
For this purpose take a chisel with the cutting end ground to the form of a fiddle drill, one cutting edge being at a right angle to the other. The chisel must be held so that while one edge rests upon the axle, the other edge will bear against the radial face of the eccentric. A sharp blow with a hammer upon the chisel head will make a clean indented cut upon the axle and the eccentric, the two cuts exactly meeting in a point where the eccentric bore meets the axle circumference, so that when they coincide the eccentric is in its proper position.
If the eccentrics of a locomotive should slip when the engine is upon the road, and there are no marks whereby to readjust them, it may be done approximately as follows:—Put the reverse lever in the end notch of the forward gear, then place the crank as nearly on a dead centre as the eye will direct, and open both the cylinder cocks, then disconnect the slide-valve spindle from the rocker arm, and move the valve spindle until the opening of the port corresponding to the dead centre on which the crank stands will be shown by steam blowing through the cylinder cock, the throttle valve being opened a trifle. The position of the valve being thus determined, the eccentric must be moved upon the shaft until the valve spindle will connect with the rocker arm without being moved at all. The throttle valve should be very slightly opened, otherwise so much steam will be admitted into the cylinder that it will pass through any leak in the piston and blow through both cylinder cocks before there is time to ascertain which cock first gives exit to the steam.