Modern Machine-Shop Practice Part 171

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 pa.s.s 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 pa.s.sed through the rod (to take up the journal and bra.s.s 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 pa.s.ses over the bore, while at the ends only the f.l.a.n.g.e of the piston head at one end and the follower at the other pa.s.ses 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.

[Ill.u.s.tration: Fig. 2525.]

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 f.l.a.n.g.e 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 bra.s.s; 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 bra.s.ses 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 bra.s.s 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.

[Ill.u.s.tration: Fig. 2526.]

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 bra.s.s) before it becomes excessive. To find how much the bottom bra.s.s 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.

[Ill.u.s.tration: Fig. 2527.]

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 f.l.a.n.g.es 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 pa.s.s through the centre of bore of pillow-block bra.s.ses, 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 f.l.a.n.g.e 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 pa.s.s the pillow block at the centre of the bore of its bra.s.ses. This is sometimes done by inserting thin pieces of sheet tin, metal, or even paper beneath the cylinder f.l.a.n.g.es 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 bra.s.ses 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 bra.s.ses in the pillow blocks true to be bored (the pillow blocks being bolted in position). The distance of the face P, of the bra.s.s 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 bra.s.s f.l.a.n.g.e.

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 bra.s.ses 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 bra.s.ses 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 bra.s.ses, 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 bra.s.s to leave it of the proper thickness.

[Ill.u.s.tration: Fig. 2528.]

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 bra.s.ses by the flat places they will be bored to stand at a right angle to A. To set the bra.s.ses the other way a circle is struck from D, as a centre, upon the faces of the bra.s.ses as in the end view, Fig. 2528, in which the straight-edge C is shown wedged in the bore of the bra.s.ses, 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 compa.s.s 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 bra.s.ses 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 bra.s.ses 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 bra.s.ses, 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 bra.s.ses may fit a journal perfectly, but that is no a.s.surance that the bra.s.ses 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 princ.i.p.ally 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 pa.s.sed 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.

[Ill.u.s.tration: Fig. 2529.]

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 pa.s.sing through the centre of the pillow block or bearing s.p.a.ces. 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 bra.s.ses, 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 bra.s.ses) 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.

[Ill.u.s.tration: Fig. 2530.]

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 f.l.a.n.g.e of the crank-shaft bra.s.s (which is between the crank face and the frame) is too thick, or the inside f.l.a.n.g.e 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 bra.s.s, if one has been put in.