Modern Machine-Shop Practice Part 158

Furthermore the step may be located so as to come against the rod end when the wear has let the key down, and this will prevent the strap from pa.s.sing too far upon the rod, and, therefore, tend to prevent the rod length from being improperly altered from errors in the thickness of the liners placed behind the bra.s.ses to take up the wear.

FITTING UP CONNECTING RODS.--The method of fitting up a connecting rod depends entirely upon its size. Very small rods to be made in numbers are usually got out by means of special devices which leave the fitter but little to do; indeed, sometimes the machine work is so accurately and finely fitted and finished as to finish the rod without the aid of the vice hand, save to put it into its place upon the engine or machine.

As, however, the dimensions of the rod increase, this method of manipulation is in practice departed from, and the filing, fitting, and adjusting operations increase. In any event, however, the principles to be observed in the manipulation are the same, because the points to be observed in the fitting by hand work must be accomplished by the machine if the rods are to be finished by machine work.

Let Fig. 2360 represent a connecting rod; A representing the centre line in the side, and F the centre line in the edge view, and it is obvious that the axial lines, B and C, of the bra.s.s bores must stand at a right angle to line F, and be parallel to each other, because the journals on which they fit will do so. Furthermore, the faces of the bra.s.ses, as E, must stand their proper distance from the centre line F, this distance being at each end respectively half the whole width D, and the faces E must be in the same plane whatever their widths may be. The centre lines A and F are imaginary lines not worked to (except it be in marking or lining the rod out for the planing operations); but the method employed to fit up the rod must be such as will make all parts true to those lines if they were tested by them.

The process of fitting up a connecting rod may be tersely stated as follows: 1st, the rod is planed; 2nd, the straps are planed; 3rd, the straps are fitted to the rods; 4th, the straps are drilled and bolted to the rod; 5th, the keyways are cut, and the keys and gibs fitted; 6th, the side faces of the rod ends are again planed with the straps on; 7th, the bra.s.ses are fitted and the rods marked off for length and the bra.s.ses bored; and, 8th, the file finis.h.i.+ng and polis.h.i.+ng done.

[Ill.u.s.tration: Fig. 2360.]

[Ill.u.s.tration: Fig. 2361.]

[Ill.u.s.tration: Fig. 2362.]

[Ill.u.s.tration: Fig. 2363.]

[Ill.u.s.tration: Fig. 2364.]

In the case of very large rods the two ends are made and fitted up as separate pieces, and are afterwards welded to the body or stem of the rod, and the setting of the ends true one to the other after the welding affords such an excellent insight into the alignment of rods that it may be well to describe it. First, then, the rod being laid on its side, two straight-edges, or rather winding strips, S and S', Fig. 2361, are placed on the side faces, and the rod will be set in this direction when their ends A, B, C, D, appear parallel when sighted by the eye. If the winding strips are adjusted to stand straight across the rod, and, therefore, parallel one to the other, any twist or wind in the two rod faces will be very plainly discernible by the sighting process. The rod is then stood on edge, as in Fig. 2362, to test the alignment of the side faces. A straight-edge S is pressed firmly against one of the faces, as H in the figure, with the other end elevated as shown. The elevated end is then lowered, the motion serving to keep the end fairly bedded against face H. The distance, I J, Fig. 2363, is then measured.

The straight-edge is then used in the same manner on the other side of the rod as at S in the figure, and the distance K L is measured, the setting in this direction being correct when distances I J and J K are equal. The straight-edge is then applied to the edge faces of end H of the rod, as in Fig. 2364, at M and at N, the distances O, P, are made equal. During these operations a straight-edge is applied along the body of the rod to see where to set it to effect any required adjustment, and if that body is straight the adjustment is made near the end at which the straight-edge is pressed to the rod.

The setting of the small end I is effected in the same manner, but the straight-edge will in this case fall over the face at the larger end, as is shown in Fig. 2365; hence, instead of measuring, lines as G and T are marked coincident with the edge of the straight-edge and the distances T U, I G, are made equal. Winding strips are applied to the edge faces as well as to the side faces, and as making one adjustment or alignment may alter another, the whole process must be repeated until the whole of the tests prove the setting to be true.

[Ill.u.s.tration: Fig. 2365.]

[Ill.u.s.tration: Fig. 2366.]

[Ill.u.s.tration: Fig. 2367.]

[Ill.u.s.tration: Fig. 2368.]

Now suppose the rod to have been forged solid and all these faces to have been made true in the planing, and the first operation is to fit the straps to the rod ends. The strap should be put in place on the rod and moved laterally, when the centre of its motion where it moves the least will be the place where it binds and therefore requires filing. If its side faces come atwist with the side faces of the rod end, as shown in section in Fig. 2366, either the faces of the rod end or the inside faces of the jaw are out of square as denoted by the dotted lines. In any event the face E, Fig. 2367, of the rod end should be surfaced true and made at a right angle to the side face, and if to be made parallel to M, also at a right angle to K, a square and a surface plate are used to test them. If the diameter J is to be smaller than that at H, then the angle of both face E, and its opposite, should be equal with reference to K. These faces should be finished by draw-filing, with the file marks lengthwise of the rod. To fit the strap, proceed as follows: To find where it requires filing, place it on the rod (having previously put red marking on the rod end), and move it endwise and sideways, observing where the least motion takes place when the strap is moved sideways by pressing its crown end, for this point of least motion is always where it fits the tightest. To test the jaw faces for being square apply a straight-edge S, and a square P, Fig. 2368, pressing S against the strap, and P firmly against S.

[Ill.u.s.tration: Fig. 2369.]

When the strap shows to bed well on the rod and its motion is an ambling one (and not a pivoted one), it fits properly, and if both rod and strap have been filed square, their side faces will come fair or even. The keyways being drilled, may then if necessary be filed out, for which purpose it is necessary to bolt the strap to the rod, a process that requires very skilful treatment, because if the tightening of the bolts moves the strap on the rod, or if the strap be moved on the rod after the clamp is tightened, the keyways will not come fair when the clamp is taken off. In Fig. 2369 the strap is shown held to the rod by plates C and bolts B, the rod being shown in position ready to file out the keyway. It is better, however, to let the side face of the rod stand vertical as the strap will stand steadier that way. The strap should be set fair with the outside faces, which will bring the keyway fair if it is properly located. The bolt nuts should be tightened gradually, first one a little and then another, going over all four once or twice before they are fully tightened, and if the strap is not fair when they are all tight, all must be loosened before the strap is adjusted, or the clamp pressure will cause the strap jaws to spring out of true, and the keyways will not come fair when the clamp is removed.

Should the keyways not come fair when the strap sets fair on the rod the strap may be set to accommodate the keyways, and thus save filing, but this must be done before clamping it to the rod end. Care must, however, be taken to see if cutting the strap out to suit the keyway may not leave too little metal on one side of the keyway when the strap is subsequently planed.

The sides of the keyway should be filed true to a surface plate, using a well-bellied file and as stout a one as possible, so that it may not bend under the pressure, and file away the edges of the keyway.

The keyway should be made parallel to the side face of the strap, so that it may be fair with the centre line F in Fig. 2360. It should be made of equal width throughout, a piece of iron being used as a gauge in place of the key, and this same piece of sheet iron will serve as a gauge to plane the keys to thickness.

The corners of the keyway, if to be made square, should be filed out with the corner of a smooth half-round file, because the corners even of safe-edge files do not come up sharp enough.

For filing out the end faces of rectangular keyways, a square file with both edges safe must be used, the safe edges being on opposite sides of the file. For roughing out, a taper square, but for finis.h.i.+ng, a parallel, or equalizing file is preferable.

The next operation is to fit the keys and gibs. The key should first be fitted and should be filed true to a surface plate, for in no other way can a really good reliable gib be obtained, no matter how well the keys may have been planed or milled. It should be filed a tight fit to the keyway so that it may be used (with a light coat of red marking) to show tight places in the keyway, driving the key in for that purpose from first one and then the other end of the keyway. If, however, it is driven too forcibly, it may seize or cut, and it will be difficult to get it out, besides damaging both it and the keyway. When the keys are reduced so that they will drive lightly into the keyway, they should be tried in the rod and in the strap separately, moving the key laterally or edgeways, so that it may mark any high places in the keyway of either of them.

The finished key and gib should be left tight enough, that they will hold themselves in any position in the keyway of the strap or of the rod when standing vertical.

[Ill.u.s.tration: Fig. 2370.]

The head of the gib should be chamfered as in Fig. 2370, so that it may be driven in and out to fit without raising burrs which would prevent it from pa.s.sing into the keyway, and the key should be similarly chamfered and rounded in its width.

[Ill.u.s.tration: Fig. 2371.]

The width of the key and gib should be such as to just fill the key ways, leaving no draw when the key is down in the keyway so that its head is level with the head of the gib, as in Fig. 2371, A equaling the keyway width; and their edges should bed fairly one against the other, and against the edges of the keyway. The strap must then be keyed upon the rod, and the side faces of the rod and strap planed to thickness, placing a bolt and nut in the rod end in place of the bra.s.ses, so that the key may lock the strap and bind it in position. The rod end should be planed to thickness for the bra.s.ses and of equal thickness on each side of the keyway. The bra.s.ses should be planed after the rod end is planed to thickness. The width for the bra.s.ses should be measured while the strap is on the rod end, because the width between the jaws of the strap is greater when the strap is in place on the rod end than when it is off, because in order to make the strap jaws a tight fit to the rod end it is made narrower between the jaws than the width of the rod end, so that the jaws spring open when the strap is pushed on the rod end.

The sizes for the bra.s.ses to be planed to will then be the width of the strap across its edge face, and the width of the strap between the jaws _when it is on the rod_; and for these sizes a wire gauge should be made; or an adjustable gauge may of course be set.

The method to be pursued in planing the bra.s.ses is an important consideration. It is most convenient to plane both the bra.s.ses together, by which means much time is saved. To obtain this end the bra.s.ses are sometimes cast together, as in Fig. 2372, and after planing and before boring are cut in two at the narrow section A. In this case the bra.s.ses are cast sufficiently wide from crown to crown as denoted by B to allow for the length cut away in separating them. In other practice the joint faces of the bra.s.ses are faced first and then soldered together for the planing; but very large bra.s.ses are planed separately. In either case the joint face of the bra.s.s should be made at a right angle to the faces of the bra.s.s that fit the strap.

[Ill.u.s.tration: Fig. 2372.]

The bra.s.ses should be fitted separately to the strap, and hence should, if joined, be separated, being cut in two in a shaper, if of the form shown in Fig. 2372, and split by driving a keen chisel between the corners of the joint faces, if the latter have been soldered. The back or crown bra.s.s, and not the key bra.s.s, should be fitted first. The corners of the ways, in the bra.s.s, for the strap should be eased just clear with the edge of a smooth half-round file, because otherwise they will rub down the sharp edges of the strap, and make the strap jaws appear to be a bad fit when on the rod. The bra.s.s should be driven in and out of the strap to fit, using a block of wood to strike on, otherwise the skin of the bore may become pened, and when the bra.s.ses are bored they will close in at the sides and become loose in the strap.

[Ill.u.s.tration: Fig. 2373.]

As a guide when fitting the bottom bra.s.s in the strap, place the strap on the rod as in Fig. 2373, and take the measure of the strap at A A, the strap overlapping the rod to admit the calipers or gauge. Each time the bra.s.s is driven in the strap to try the fit, the calipers so set should be tried in the strap (the bra.s.s being in the strap), as in the figure, and when the calipers very nearly touch the strap jaws, the strap with the back bra.s.s still in should be tried on the rod end, or in the case of a very heavy strap the caliper measurement minutely taken may be relied on to show that the bra.s.s does not spring the strap jaws too wide open. It is better, however, to leave the bra.s.ses a little too tight in the strap as they close slightly in the boring, becoming easier in the strap.

[Ill.u.s.tration: Fig. 2374.]

[Ill.u.s.tration: Fig. 2375.]

After the bra.s.s has been tried in the strap, and before it is filed again, it should be tried with a square, using a straight-edge also if the square back is too short to cross both faces of the bra.s.s. The method of testing is shown in Fig. 2374, in which B represents the bra.s.s, S the square, and T the straight-edge. The inside face of the f.l.a.n.g.e should also be tried as in Fig. 2375, in which P represents the surface plate, S the square, and B the bra.s.s. This will insure that the bra.s.s face joint is square as it should be, and is further necessary because the bearing marks on the bra.s.s are not to be altogether relied upon.

[Ill.u.s.tration: Fig. 2376.]

In Fig. 2376, for example, the bra.s.s is shown in section in the strap, and the side A of the bra.s.s has a bearing against the jaw B of the strap, and hence would show marks of contact. The succeeding blows in driving the bra.s.s, however, may cause the bra.s.s to have contact on the side C with the jaw D; hence the bearing marks would show the bra.s.s to fit well when such was not the case. This may be detected by striking the bra.s.s on its joint face, and then measuring from E and from F to the end of the strap, and then striking the joint face at F and again measuring both distances, when any canting of the bra.s.s will readily be detected. It is better, however, to also apply the square, as shown in Figs. 2374 and 2375, because by this means the joint faces E F being parallel to the crown face G of the bra.s.s, the bra.s.s will be fitted so that when G meets the crown face H of the strap, the two will be parallel to each other and require but little filing to fit or bed together.

The crown of the bra.s.s should be bedded very finely to the strap, or it will spring the strap jaws away from the rod when the key is driven home.

Suppose, for example, that the crown of the bra.s.s did not bed well at A in Fig. 2377, then keying up the strap would spring its jaws away from the rod end, as shown at B C, the least error in the bedding having this effect notwithstanding the fit of the gib jaws.

[Ill.u.s.tration: Fig. 2377.]

The second bra.s.s must be made to just fit the strap when the back bra.s.s is in its place, and is small enough when the calipers, set as shown in Fig. 2373, and tried as shown in Fig. 2376, just fit the strap. This will insure that both bra.s.ses fit the strap when it is in its place on the rod.

When both pairs of bra.s.ses have been fitted to their straps, the latter should (if held by bolts) be bolted to their places on the rod, and the centre of the respective s.p.a.ces for the bra.s.ses will be the location for the marks G, G, Fig. 2360. A pair of trammels should, however, be set to the proper length of the rod and these marks tested. If the strap is held by gibs and keys, as in the small end in Fig. 2360, the strap should be put on its place with the gibs in, and drawn up the rod by slowly forcing the key in until the mark G at that end stands in its proper distance from G at the other end, at which time the key should come through its proper distance.

The thickness of the bra.s.ses must be measured from these marks G, G to the crowns of the straps and the ends of the rod respectively. If the rod is of its proper length and the straps are in their proper positions, these marks will come in the centre of the s.p.a.ce for the bra.s.ses. If, however, there is any error, as there is apt sometimes to be in very large rods, the course to be pursued depends upon the kind of rod end. If both straps are bolted to the rod end, the error may be divided equally at each end. If one end has a key and gib or gibs, but no bolt, as at the small end in Fig. 2360, the key bra.s.s may be made of such thickness as to b.u.t.t against the end of the rod and meet the mark G.

[Ill.u.s.tration: Fig. 2378.]

For the large end, the thickness of the key bra.s.s, or, in other words, the distance D in Fig. 2378, must be taken after the face of the crown bra.s.s has been squared up, as described with reference to Figs. 2374 and 2375, the connecting rod strap being placed in such position that the key will be up in its proper place.

When the joint faces of bra.s.ses do not meet, but are left open to take up the wear, it is a difficult matter to properly adjust the bra.s.s bore to the journal. If the f.l.a.n.g.es of the bra.s.ses do not quite fit the length of the journal, as is very commonly the case, it is customary to tighten the key until the rod end can just be moved by hand so as to force the bra.s.s f.l.a.n.g.es against first one and then the other end of the journal. This is an approximate adjustment; and if the journal heats at all, the key is slacked back a trifle; whereas if it pounds, the key is set up a little. As a matter of fact, then, nothing is actually known of the precise fit of the bra.s.s to the journal; and while looseness may be detected by the pounding, the bra.s.s may be tight enough to cause undue wear without very sensibly heating the journal, especially if the latter is freely lubricated. If, however, the bra.s.ses fit the length of the journal, and do not b.u.t.t, it is usual to drive the key in till the bra.s.ses bind the journal, and to then slack the key back to the necessary amount. What that amount should be cannot be stated, because it varies with the taper of the key and the force with which it is driven home. As a result, then, the operation is left to the judgment, or, in other words, to guess-work, of men, many of whom are not well experienced in the operation; while under any circ.u.mstances the actual fit is not positively known. A plan not infrequently adopted is to insert a piece of lead wire of small diameter between the bra.s.ses, the key is first driven tightly home, and then slacked back until the lead wire is just freed. It is estimated that the adjustment will then be correct; there is no actual certainty of the fit, however, even in this case.

Another objection is that the oil is apt to flow out of the opening, and the bra.s.s having communication with the oil cup is better lubricated than the other bra.s.s.