Modern Machine-Shop Practice Part 172

[Ill.u.s.tration: Fig. 2531.]

It may here be remarked that if the bore of the crank-pin bra.s.ses 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 bra.s.ses 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.

[Ill.u.s.tration: Fig. 2532.]

[Ill.u.s.tration: Fig. 2533.]

[Ill.u.s.tration: Fig. 2534.]

[Ill.u.s.tration: Fig. 2535.]

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 bra.s.ses 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 bra.s.ses 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.

a.s.suming 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.

[Ill.u.s.tration: Fig. 2536.]

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.

[Ill.u.s.tration: Fig. 2537.]

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 bra.s.ses to be axially parallel one to the other, and keyed up properly, and when at E one bore of those bra.s.ses 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, pa.s.sing 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 pa.s.ses 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 bra.s.ses and the journals, there will be a pound, as the bra.s.ses 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.

[Ill.u.s.tration: Fig. 2538.]

[Ill.u.s.tration: Fig. 2539.]

[Ill.u.s.tration: Fig. 2540.]

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 bra.s.ses up to a close working fit. At the other end of the connecting rod put the strap keys and bra.s.ses 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 bra.s.s f.l.a.n.g.es 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 f.l.a.n.g.es of the bra.s.ses 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.

[Ill.u.s.tration: Fig. 2541.]

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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. 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.

[Ill.u.s.tration: 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, 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.

[Ill.u.s.tration: 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 in Fig. 2545, in which A A represents the axial line of the cylinder bore prolonged, and B B the axial line of the crank shaft (the two being parallel or in proper line). Let E E represent the centre line of the connecting rod when the crank is on one dead centre, the axial line of the crank pin being at C C. Then the bra.s.ses being keyed up to fit the crank pin, the centre or axial line of the connecting rod would stand as denoted by E E. But the bra.s.ses 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 line A A, we have that the rod is at that end endeavored to be held parallel to A A; hence, keying up the connecting-rod bra.s.ses on the crank pin would tend to bind the rod, one end standing parallel to A A, and the other parallel to E 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 in Fig. 2545 (G G representing its axial line, and F F the centre line of the connecting rod at a right angle to G 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 error _on the same side_ of 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 lines E and F, in Fig.

2545; E being at a right angle to C, and F at a right angle to G.

[Ill.u.s.tration: Fig. 2546.]

Again, it has been shown that when the shaft was out of line, a point on the crank-pin journal pa.s.sed 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 in Fig. 2546, the point being taken at the intersection of E and C C. A A represents the path of rotation of the same, which is parallel to the true face B of 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 bra.s.ses cannot wear to suit the error, which, therefore, only diminishes by the wear of the crank pin. Suppose the error to be 1/64 inch 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 to 3/8 inch.

[Ill.u.s.tration: Fig. 2547.]

In Fig. 2546 the 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 line E, which is on the opposite side of A A to what it is in Fig. 2545. Again, when on the other dead centre, the line F F, in Fig. 2546, falls _outside_, while F F, in Fig.

2545, falls _inside_ of A 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 bra.s.ses 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 at D, 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, at C, Fig. 2547. Now, suppose that the line D shows the rod to fall 3/8 too much on that side, and line C shows it to fall (when on the other dead centre) 3/8 too 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 the 3/8, the rod end falling out of true at C and D, Fig. 2547, gives us 1/64-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 in Fig. 2548 by the lines C D, and then file on the top and the bottom of the crank pin a flat place B, 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 by 1/64 inch, 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 in Fig. 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.