Chapter XXIX: Erecting Engines and Machinery (1)

In engines having suspended guide bars, it is not uncommon to set those bars by the working parts of the engine, instead of by lines. This is an advantage when the parts of the engines are not taken down, and, if care is taken, will make a true and smooth working job; but otherwise, it is likely to introduce errors in the lining of the engine, which throw it out of proper line, and cause a great deal of friction.

The proper method of setting the bars depends upon the condition of the engine as to wear. Suppose, for example, that a new piston head has been put in, then, if the gland is new also, or is a good fit to both the piston rod and the bar of the stuffing box, the bars may be set as follows:--

Place the piston at the back end of the cylinder, and put the cross head and guide blocks in proper place on the rod. Put up the bottom guide bars so that they just touch the cross-head guides. Now, in adjusting these bottom bars there are two essential points: first, that the plane of their upper surfaces shall stand parallel with the axial line of the main shaft, and secondly, to place the upper surface parallel with the axial line of the cylinder (it being of course assumed that the cylinder and crank shaft are in proper line). The first of these essential points will be attained when a spirit-level, placed truly along the bore of the cylinder, shows the bubble to stand in the same position in the tube, as it does when placed upon and along the bar. The second will be attained when a spirit-level, placed across the bars, as in Fig. 2518, at A, shows the bubble to stand the same as it does when the level is placed on a parallel part of the shaft, as in the figure at B. When the bars are thus temporarily set, the liners, or pieces of iron, may be fitted to the proper thickness, so that the gland will pass in and out of the stuffing box easily by hand, no matter in what position the piston may be in the cylinder.

To get the thickness of these liners, take wedges made of iron, wood, or lead, and insert the thin end between the faces of the bars and those of the supports, forcing the wedges in sufficiently to leave a mark upon them. By chalking the faces of the wedges they will exhibit the marks more plainly. The wedges should be inserted at each end and on both sides of the bar, for one bar at a time, the liners being got out a trifle too thick so as to allow some for fitting.

If the liners require to be very thin and are difficult to hold in the vice without springing, take a piece of soft wood faced true, and grip it in the vice, and fasten the liner on it by means of brads driven in around the edge of the liner.

When the four liners are ready place them in position between the bars and their seatings. Bolt the bars firmly in position, wipe them clean and test them lengthwise with the spirit-level to ascertain if they are parallel with the cylinder bore, and place the level across the bars at each end to test parallelism with the engine shaft, as in Fig. 2518, and, having noted where further adjustment is necessary, put marking upon the bars and move the cross head back and forth to ascertain how much the respective liners require reducing. If the gland is a fit upon the piston rod and in the stuffing box, moving the gland in and out of the stuffing box will be an admirable test of the guide-bar adjustment.

The straight-edge should also be applied to test if the surfaces of the bars lead true one to the other; thus, in Fig. 2519, A and B are the bars and E the straight-edge, which by being pressed firmly to the surface of A discloses that the surface of A is not in line with B, because if it were so the straight-edge would meet the face of B as in Fig. 2519, where the straight-edge F pressed to the surface of C leads true to the surface of the bar D. All four of the bars require testing in this manner. If the seatings for the bars or the liners are not made flat and of equal thickness, or if from any other cause the bars do not bed properly upon the liners, then bolting up the bars will spring them as shown in Fig. 2520, in which, at A, is shown a bar sprung in the bolting up, because the liners fit at the ends B C only; while at E is shown a bar sprung or bent because the liners fit at the ends D D only. In either case the cross head would be forced to travel in a curve, bending the piston rod, and inducing much friction. The way to test the bars in this respect is, after the above operations, and before loosening the bolts, place a long straight-edge lengthwise along each bar and move it laterally at one end. If it swings from the centre the bar is rounding, while if it shuffles across first at one end and then at the other the bar is hollow in its length and we must find at which end of the bar this spring exists. To do this, slightly slacken the bolt or bolts at one end and again apply the straight-edge, if the spring is removed the error lies in the bedding of the liner at that end. If not removed, retighten the bolts at that end and slacken those or that at the other end, and again apply the straight-edge, and thus may it be determined how much of the spring is due to each of the liners, and this must be remembered and allowed for in filing the liner to its final adjustment. Before putting the liners in a second time it is better to give them a light coat of marking to show where they bear. At each trial of the bars the spirit-level and straight-edge should be applied and the cross head should be moved back and forth to show by the bearing marks how the cross-head guides fit to the bars. These marks are a great deal finer test than any spirit-level adjustment, hence the last part of the fitting should be performed with strict reference to the bearing marks upon both the bars and the cross-head guides as well as upon the liner, the cross-head flanges being adjusted and fitted at the same time as the face fitting.

To set the top bars place the cross head in the middle of its stroke, and place them upon the cross-head guides. Then, with the wedges applied as before, ascertain the required thickness of the respective liners one at a time, leaving them, as previously, a trifle too thick, and testing them while fitting by marking placed upon their faces. The top bars may be entirely adjusted from the contact marks left by the cross-head guides when moved along the bars, thus dispensing with the use of the straight-edge and spirit-level.

As the bolts supporting the bottom bars often require to be loosened to get the top bars off, pieces of wood may be placed beneath the bottom bars to retain them in position when the bolts are loosened. These pieces must be removed during the testing, for if left so as to wedge the bars they may spring them, and thus mislead in the adjustment. After the top bars are adjusted the whole bearing surfaces should be oiled, and the cross head pulled back and forth by hand without the use of a lever, providing the size of the piston does not exceed about eighteen inches diameter. The bars when set true should be clamped to their seatings and the holes reamed out to receive the proper bolts, and, finally, mark each bolt, bar, and liner to its place.

When the bars, tested with the straight-edge and spirit-level as described, show true, if the gland will pass freely in and out of the stuffing box with the cross head at any part of its stroke, the guide bars are set.

In this operation let it be noted that the close fit of the piston to the cylinder bore and of the gland to the stuffing box is alone depended upon as a guide whereby to so set the guide bars that the axial line of the piston rod and its plane of motion shall be in line with the centre of the crank shaft.

Suppose, however, that the piston head is a new one, and the gland is worn a loose fit to the stuffing box, then setting the bar to the gland would produce the result shown in Fig. 2521, in which the dotted line A A is a line or cord stretched axially true with the cylinder bore, and coincident with the centre of the pillow block at B. The gland being a loose fit permits the piston rod to fall below its proper level, and the surface of the bars, if set by the gland only, without using the spirit-level, would be set true to the full line C C, and therefore out of true line. If the bars are set by spirit-level true to the length of the cylinder bore, the gland becomes useless as a guide to set the bars by. It is a not uncommon practice, when the gland has play, to insert in the stuffing-box bore, at the bottom, a piece of tin or sheet brass, equal in thickness to one-half the amount to which the gland is too small, or to insert a similar piece beneath the piston head if it is too small. As a rule, however, there will be at least as much play between the piston rod and the gland bore as between the gland and the stuffing-box bore; hence, if there is any play, it is better to discard the use of the gland altogether.

The proper method of setting guide bars by a stretched line is as follows:--

The cord or line is set true to the cylinder bore, and coincident with the centre of the pillow block, as at A A in Fig. 2521, and the two bottom bars are put up in line horizontally with the axial line of the crank shaft, and at a distance below the stretched line equal to one-half the height of the guides for the cross head, as in Fig. 2522, in which A represents the stretched line, B, B the bottom bars, C C a straight-edge, and D a piece of wire whose length from point to point is equal to one-half the height or thickness of the guide blocks. The width apart of the bars is set to suit the width apart of the flanges of the guide blocks on the cross head, by means of a square. The square is applied in the following manner: On a straight-edge mark two lines A and D, Fig. 2523, a distance apart equal to the distance between the flange edges of the cross-head guides. Midway between A and D mark the line C; place the straight-edge across the bars as shown, and when the edge of a square, placed on the straight-edge, coincides with C and the stretched line, and the marks A and D coincide with the edges of the bars, the latter are set true, and will come right for distance apart, and distance from the centre line, supposing the flange edges of the cross-head guides to be equidistant from the centre of the length of the cross-head journal. If, however, such is not the case, the width from A to C and from C to D must be made to suit, C representing the centre of the length of the cross head journal, D the flange on one guide and A the flange on the other guide. Here it may also be remarked that, if the thicknesses of the cross-head guides vary, or if they are not central to the axial line of the cross-head journal, the bars must be set for distance from A in Fig. 2523, to suit the error, because in that figure the straight-edge is supposed to stand parallel to the axial line of the shaft, as it is also in Fig. 2522, the aim in both cases being to so set the bars that the cross-head journal shall stand parallel with the crank shaft.

It is the liability of variation in the thickness of the guide blocks, and of their not being central to the cross-head journal, that constitutes the disadvantage of setting the bars by lines, it being obvious that the bars must be either set to suit any errors in the guides, or those errors must be eliminated before setting the bars. The top bars must be set parallel to the bottom ones, at a distance from the bottom ones equal to the thickness of the guide blocks, and parallel to one another. It is preferable to set the top ones with the cross head and guides in place, observing all the precautions as to springing them given in the case of the bottom bars.

The bars thus set will be in line with the crank axle, but unless the piston accurately fits the cylinder bore, they will not long remain in line with the line of motion of the piston rod. For example, Fig. 2524 shows a piston head too small for the cylinder bore, the guides fitting the bars properly, and the gland and stuffing box fitting the piston rod; the piston will be suspended in the cylinder, its overhanging weight being supported by the guides B, the gland, and packing ring. This would cause friction and rapid wear of the gland bore and guide surfaces in a direction parallel to the line C, which would gradually let the piston fall to the bottom of the cylinder bore, touching at the end of D first. In some engines provision is made to adjust the piston to take up its wear, which is, it will be seen, a great advantage.

THE HEATING AND POUNDING, OR KNOCKING, OF ENGINES.--The heating of any part of an engine occurs from one of two causes, viz., either the fit of the parts is too close, inducing undue friction, or the parts are not in line.

When the former is the cause the remedy is to ease the fit. If the parts are not in line, the heating may also be remedied by loosening the fit of the parts; but this will often induce a pound or knock, hence the true remedy is to properly align the parts.

The location of a pound may be discovered by placing a piece of metal wire between the teeth, and resting the other end of the wire upon each end of the cylinder, guide bars, and bearings of the main shaft, repeating the operation in each place, and the sense of feeling will distinctly indicate the location of the knock, by imparting a more severe shock to the teeth when the vicinity of the knock is approached.

The most prominent location of the causes of a pound are, first, in the crank pin, from causes to be hereafter explained, and from its wearing oval at the cross-head journal; and second, at the ends of the cylinders, or the ends of the guide bars, because of a ridge forming there as the wear proceeds.

A crank pin cannot wear oval if the brasses are kept adjusted to fit it, because in that case the brass bore must wear it round; but if there is any play it wears oval, because the pressure of contact between the journal and the brass bores is least when the pin is at and near the points of dead centre, and the most when it is at and near half stroke.

The cross-head pin wears oval because the pressure between the pin and its bearing is in a line with the connecting rod, and there is but little wear on the pin in a direction at right angles to the rod.

Ridges form at the ends of the cylinder bore and at the ends of the guides for the following reasons:--

Referring to the cylinder, the location of the piston stroke in the cylinder bore alters as the connecting-rod keys pass through the rod, because that alters the length of the connecting rod, and therefore the path of the cross-head guides on the guide bars, and also that of the piston in the cylinder.

As the piston rod is shortened there is less wear at the extreme end of the cylinder bore farthest from the crank, and the same remark applies to the guide bars.

If the piston head travels past the end of the cylinder bore and into the counterbore at each end, a distance equal to the amount of taper on the connecting-rod keys, or equal to the amount the connecting-rod length will alter while those keys are passed through the rod (to take up the journal and brass wear), the piston head will (if the rod is kept to its original length within that amount) always travel to the end of the cylinder bore, and no ridge should form on account of the length of the rod altering; but even then a slight ridge may form because the wear is naturally less at the ends. Thus in the middle of the cylinder length the whole thickness of the piston head, piston rings, and of the follower passes over the bore, while at the ends only the flange of the piston head at one end and the follower at the other passes over the metal of the bore; hence the friction and wear are less.

The ordinary cause of pounding and heating is a want of truth in the alignment of the crank pin, or in that of the cylinder, main shaft, or guide bars.

The method to be employed to line an engine, or to discover if it is out of line, depends upon the design of the engine and its condition; thus an engine having a Corliss frame has the slides to receive the cross head made at a true right angle to the end face which meets the cylinder end; equidistant from the centre of the gland hole or axis of the piston rod, and the end of the frame fitting either the bore of the piston or the turned flange of the cylinder cover; hence the guide bars must be true if the frame is got up true, the fit of the frame end to the cylinder end insuring truth in the guide or cross-head slides, providing that the centre line of the frame, during the turning and planing operations, leads from the centre of the cylinder end of the frame to the centre of the crank-shaft brass; or, in other words, the planing and boring of the frame must be true with a line running from the centre of the cylinder end of frame to the centre of location for the crank shaft. This will not only cause the outside of the frame casting to stand at its proper level when the cylinder bore stands horizontally level; but it will insure that the crank-shaft bearing brasses both be of equal and of a proper thickness through the crown.

The engine being properly lined at first will not be liable to get out of line, excepting so far as affected by the wear of the crank-shaft bearing, which will cause the crank shaft to drop, as shown in Fig. 2525, where A A represents the true centre line of the cylinder and guide bars, which, when the crank is in the position shown in the cut, should be coincident with the centre line of the connecting rod and the crank, but the crank brass having worn below the centre line of the connecting rod and crank, the crank will get out of line as denoted by the line B B.

As a result, a portion of the piston movement and pressure which should be exerted on the crank after leaving the dead centre, will be exerted on it before it reaches the dead centre, thus causing a back pressure, involving a loss of power. Furthermore, the relative position of the eccentric to the valve gear will be altered, impairing the proper set of the valves; hence it follows that the wear of the crank bearing in this direction should be taken up (by raising the lower brass) before it becomes excessive. To find how much the bottom brass requires raising, or whether it requires raising or not, find the centre of the crank shaft, and from this centre strike the circle B, in Fig. 2526, whose diameter must equal that of the crank pin A, and place the edge of a spirit-level coincident with the perimeters of the crank pin and circle, as shown in the cut. When the bubble of the spirit-level stands in the same position as it does when the level is placed upon the bore of the cylinder or along the piston rod, the crank will be in line with the cylinder bore.

As a rule, the cylinder bore of a horizontal engine stands horizontally true, and the crank centre line should also stand so when the crank is on its dead centre, but if such is not the case the crank centre line must nevertheless stand true with the axial line of the cylinder, when the crank is on the dead centre.

If, instead of having a Corliss frame and fixed guide bars, the engine has a flat bed and adjustable guide bars, as shown in Fig. 2527, the operation is as follows:--

In setting up a new engine it is obvious that if the flanges of the cylinder are planed parallel with its bore and at the proper distance from its axial line, and the pillow block is made of the proper height, a line stretched axially true with the cylinder bore will pass through the centre of bore of pillow-block brasses, or be equal in height from the engine bed; but the length of the cylinder being only about one-fifth of the distance from the cylinder to the centre of pillow block, any error in the planing of the cylinder flange true to the cylinder bore becomes magnified five times at the pillow block; hence it is necessary to stretch a line through the cylinder bore and set the cylinder so that the line, being axially true with its bore, will pass the pillow block at the centre of the bore of its brasses. This is sometimes done by inserting thin pieces of sheet tin, metal, or even paper beneath the cylinder flanges and the bed, and in the requisite positions. The method of stretching the line is shown in Fig. 2527. F is a device for holding the line at that end. It consists of a frame in the form of a cross, with adjusting screws at the end of each arm, and a small hole at its centre to receive the line. The other end of the line A must be secured, under as much tension as the line will safely bear, to a piece of wood clamped to the engine frame at R. The adjustment of the line is made by measuring its distance from the walls of the bore of the cylinder at one end and of the bore of the gland hole at the other end, using a pair of inside calipers or a wire gauge. The latter should be bent in its length to admit of adjusting the same by straightening to increase, or still further bending to diminish, its length to suit the requirements.

The wire, when applied, should only just meet or touch the line and not bear the least hard, or it will spring the line, causing an error of adjustment that will be serious when multiplied by the length of the line to the pillow block as compared with the length of the cylinder bore.

If the pillow block is planed on its bottom face and has its brasses fitted, the latter may be marked off for boring from the line A, Fig. 2527, when stretched to set the cylinder, thus avoiding a second adjustment of the line A A.

Suppose now that it is required to line the brasses in the pillow blocks true to be bored (the pillow blocks being bolted in position). The distance of the face P, of the brass from the stretched line A, in Fig. 2527, must equal the distance from the centre of the length of the crank-pin journal, to the face of the large crank hub, and this distance may be shown by a line marked on the edge of the brass flange.

Place a straight-edge C, Fig. 2527, having a line D parallel with its edge E, so that this line will be in the centre of the width of the pillow-block jaws, and at a right angle to the line A. The line D will then represent the axial line of the crank shaft, and may be used as the centre from which to mark the lines on the brasses used to set them by for boring. To test if A and D are at a right angle, or to set D to A, a large square should be used. If the side face P P of the pillow block stands parallel to A, as it should do when it is true, it will serve to chuck the pillow block by, thus boring the brasses in their places in the pillow block, with the centre line of the bore at a right angle to P. If otherwise, two flat places should be filed on the brasses, as shown in Fig. 2528, in which C is the straight-edge, and A the stretched line as before, H and I representing the flat places whose distance from A, as shown at J J, may be made to represent the thickness of the crank from its large hub face to the centre of length of crank-pin journal; hence the depth of the flat places will show how much to take off the face of the brass to leave it of the proper thickness.

A straight-edge placed across these flat places, or true to the lines H I, must stand at a right angle to the line D, so that by setting the brasses by the flat places they will be bored to stand at a right angle to A. To set the brasses the other way a circle is struck from D, as a centre, upon the faces of the brasses as in the end view, Fig. 2528, in which the straight-edge C is shown wedged in the bore of the brasses, which is the most convenient way when it can be done.

The line D is carried down on the end face of the straight-edge, and the latter is used as a support for the compass points while striking the circle M, which is defined more clearly by indenting it with fine centre-punch marks. The height of the centre for bore of brasses may be carried from the centre line of the cylinder A A to the end of the straight-edge C, by placing another straight-edge across the engine bed and measuring from the end of C to A.

Suppose now that the brasses are bored, and the position of the pillow block is to be set, and the process is the same, the line D being marked true from the bore of the brasses, and the pillow blocks adjusted until D is at a right angle to line A A.

Though in a new engine every part may be made as true as possible in the details of manufacture, yet when the parts come to be put together errors of alignment will generally be found to exist. These errors may be too minute for discovery in the separate piece, and yet form important defects in the finished engine.

In rough practice these defects are left to remove themselves by abrasion and wear, the process being to allow the parts to be somewhat loose (wherever possible) in their adjustment, and adjust them closer as the abrasion proceeds.

This is termed letting the parts _wear down to a bearing_. But the very process of wearing _down to a bearing_ attests that the parts have not been properly fitted to a bearing, whereas to attain the best possible results the parts should be fitted to a bearing, because in wearing down to a bearing, undue abrasion, and to some extent or in some degree, roughness of the wearing surfaces, must ensue, because the strain intended to be distributed over the whole _intended_ bearing area is limited to the _actual_ bearing area. It is necessary, therefore, that, in putting an engine together, each part be properly fitted to its place, and that it be subsequently adjusted in its fit and position with relation to the other parts to which it is connected.

The fitting of the single piece is a test of its individual or disconnected truth; the subsequent or second adjustment is a test of its truth with relation to the others. Thus a pair of brasses may fit a journal perfectly, but that is no assurance that the brasses are so bored as to bring the rod holding them in proper line to enable connection at the other end without springing or bending the rod.

Furthermore, it often happens that the frame work of an engine does not form a base for the whole of the parts, thus in a large stationary engine, the end of the main shaft or crank shaft farthest from the crank (generally called the _outboard_ bearing) is generally supported by a bearing having an independent foundation, and as this foundation does not exist until the engine comes to be permanently fixed for operation, its alignment must be performed when setting the engine. In an old engine this foundation may settle, or the wear itself may throw the engine out of line, so that the lining of an engine becomes periodically a necessity.

As a general rule a want of alignment induced by wear or incurred from repairs to the parts principally affects the main shaft, the cross head remaining more nearly true; and, with the exception of the crank pin, the same holds good with reference to a new engine.

Now while an error of alignment may exist in any direction, it is true, nevertheless, that an error in any direction will be discoverable if the parts be tested at four equidistant parts of the stroke or revolution, as, for instance, on the two dead centres of the crank and at the highest and lowest points of the path of rotation of the crank pin; hence attention may be confined to those four points.

Suppose then an engine already put together requires to be tested for being in line, and we have to test--

1st. The alignment of the main or crank shaft vertically.

2nd. The alignment of the main shaft horizontally.

3rd. The axial truth of the crank pin with the main or crank shaft.

4th. The adjustment of the crank shaft for vertical height, with relation to the cross-head journal.

Referring to this last, it may be necessary to remark that the axial line of the main shaft may be parallel when viewed either vertically or horizontally with the cross-head journal, and yet if a line be passed through the centre of the cylinder bore, and prolonged past the crank centre, the latter may fall above or below that line, but it will generally be below, because from the weight of the crank shaft its bottom bearings wear the most; and, further, to whatever extent those bearings wear after being in proper line, the crank shaft will fall too low.

We may now subdivide the errors of alignment of a crank shaft thus:--

1st. Its axial line, when viewed vertically, may form an acute angle to the axial line of the cross-head journal.

2nd. It may form an obtuse angle with the cross-head journal when so viewed.

3rd. It may, when viewed from the crank-pin end of the engine on about a horizontal position, be too high or too low at the crank-pin end only.

4th. It may be too high or too low at the outboard end only.

5th. It may be too high or too low at both ends, although parallel to the cross-head journals.

It will be found on consideration that with the exception of the last-named case, the connecting rod forms the best test whereby to discover an error in any of these directions, because it magnifies the error and makes it more plainly discernible. It will further be found upon careful observation, that although a combination of these errors may exist, the connecting rod will serve to discover each error separately, as well as the collective error, because, although in some respects two distinct errors may have the same general result, yet the result will be different if taken in detail, and it follows, therefore, that the testing must be taken or made in detail first.

To test the parallelism of the axial line of the crank shaft with that of the cross-head journal, when viewed vertically: In Fig. 2529, let A A represent a line true axially with the bore of the cylinder, and B B a line at a right angle to A A, and passing through the centre of the pillow block or bearing spaces. If the engine were in line, B B would be coincident with the axial line of the crank. Suppose, however, that line B C represents the actual centre line of the crank, not then being at right angles to A, the end E of the connecting rod, if connected to the crank pin as shown, and made a good working fit so that there is no play of the pin in the brasses, will not come fair laterally with the bearing in the cross head. The amount of the error is the amount it is out of true in the length of the crank-pin journal, multiplied by the product of the length of the connecting rod (from centre to centre of the bores of the brasses) divided by the length of the crank-pin journal. It is apparent, however, that if the crank shaft be set to have its axial line at B B instead of at B C, the error at E D will be corrected, and thus we may employ the connecting rod to set the crank shaft in line.

It is, however, not sufficient to try the crank on one dead centre only (as will be seen presently), hence we place it on the other, and move the cross head to the other end of its stroke, and again try the end E of the connecting rod with the cross-head journal, and if it falls to one side, and _on the same side as before, but to a less amount_, it demonstrates that the axial line of the crank forms with the line A A an acute angle. If, however, instead of falling too much laterally towards the side F of the cross head, it fell too much towards D, but more so when tried with the crank on the dead centre nearest to the cylinder than when tried with the crank on the other dead centre, then it is proof that the axial line of the crank shaft forms with A A an obtuse angle.

The reason that the error will be more plainly shown with the crank on one dead centre than when on the other is shown in Fig. 2530, in which A A is a line coincident with the axial line of the cylinder bore, and B B the axial line of the crank shaft, from C to D is the plane of revolution of the crank pin, while G represents the crank centre. The points at C and F denote points central to the length and diameter of the crank-pin journal. Now, the centre line of the connecting rod for one dead centre is represented by E D, and for the other by F C, and it will be seen that the point at E is farther from A than is the point at F. It will be observed that the point D falls _outside_, while the point C falls _inside_ of A A, and yet the centre line of the connecting rod stands, in both cases, at the same angle to the centre line A A of the engine, and in both cases throwing the end of the connecting rod, represented by the points at E F, _outside_ the line A A.

If the connecting rod does not, when connected to the engine, as in Fig. 2529, fall true into the cross-head bearing, the error is the same _in amount_ and comes on the outside of the cross-head journal with the crank placed on each respective dead centre, it is proof that either the flange of the crank-shaft brass (which is between the crank face and the frame) is too thick, or the inside flange of the connecting-rod on the crank pin is too thick, or else the crank is too thick, measured from the crank-pin journal to its inside hub face, the error being in the new crank or new brass, if one has been put in.

It may here be remarked that if the bore of the crank-pin brasses of the connecting rod is not at right angles to the centre line of the rod itself, the end E, Fig. 2529, might fall either inside or outside, laterally, of the cross-head bearing, but in this case the error will show more at one end of the stroke than at the other, for reasons which are explained with reference to Fig. 2530; hence it follows that the connecting-rod brasses should be properly fitted to their journals, and made to lead true before using the rod to line the engine by. In some cases it is more convenient to connect the rod at the cross-head end, and try the other end with the crank-pin journal, as shown in Fig. 2531. In this case, however, the connecting rod will (whenever the axial line of the crank shaft is out of square, forming an acute angle with the centre line A A, as in Figs. 2529 and 2530), fall laterally inside the crank-pin journal when on one dead centre, as in Fig. 2531, and outside when on the other dead centre, as in Fig. 2532, the respective amounts of error being in this case equal for the two positions. The reason for this is that the plane of revolution of the crank pin falls outside of the centre line in one case, and inside for the other, as shown in Fig. 2530 at D C.

If the axis of the crank axle formed an obtuse angle to the engine centre line A A in Fig. 2529, the connecting-rod end tried with the crank pin, as shown in Fig. 2531, would fall outside of the crank-pin journal when the latter was on the dead centre nearest to the cylinder, as shown in Fig. 2534, and inside of the crank-pin journal when on the other dead centre, as in Fig. 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 is _sure_, 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.

In Fig. 2536, let A represent the cross-head journal, and B B a line parallel to it. Let B C represent the axial line of the crank shaft (being out of parallel because the crank end is too high or the other end too low). Let F F represent the centre line of the crank pin when at the top, and G G when 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 journal is less on one side than on the other; thus in the figure distance D is less than E. 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 in Fig. 2537, in which the distance from the centre of the crank pin at two opposite points in its path is represented by dots shown at E F, and from E to H measures less than from F to H, H representing the centre of the cross-head journal.

In Fig. 2537, let A represent the axial line of the cross-head journal, B a vertical line at a right angle to A; C representing the crank shaft extended by a dotted line, so as to enable comparison with A; D the crank, E and F the centre of the crank-pin journal, and G G a line at a right angle to cross-head journal A.

Now G, being at a right angle to A, represents what should be the plane of rotation of the crank pin, whereas C, being out of parallel with A, causes the path of rotation to be in the path from E to F, or as D compared to B; supposing then that the bores of the connecting-rod brasses to be axially parallel one to the other, and keyed up properly, and when at E one bore of those brasses will stand parallel to E while the other is parallel to A, or when at the bottom of the crank rotation, one bore will be parallel to F and the other parallel to A. 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) with A (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 of G G, whereas it falls outside as at E, and inside as at F, while at H it is coincident; hence it appears that starting from a dead centre H, the rod bends, passing at that end outward to E (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 at F, and ceasing at H. This bending also causes undue friction and the consequent heating of the journals; furthermore, if there be any _end_ play 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, but E would fall on the inside instead of the outside of G, while F would fall outside instead of inside.

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 in Fig. 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 in Fig. 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 toward B, Fig. 2540, when the crank pin is at the top, and toward C, Fig. 2541, when it is at the bottom of its path of rotation.

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 be of 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 towards A, Fig. 2542, and when at the top, it would fall in the opposite direction--that is, towards D, 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.

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. In Fig. 2543 let A represent the end of the crank shaft; B the key, and C the crank shown partly in section: suppose the crank bore (whether made taper or parallel) has a slightly easier fit on the side D than on the side E, 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 line F, 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 at E E than at D, 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 at D, 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.

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, in Fig. 2544 is a key binding most at the opposite corners, as denoted by the dotted surfaces A B, and the result will be that the key pressure would tend to twist the crank in the direction of D E, having C as a centre of motion, providing that the error was equal at A and B; but in proportion as the error was greatest and the fit tightest at A, or at B, 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 and is likely to give most in the process of contraction, so that if one part (as F) of the crank be made hotter than another (as G) 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.