Modern Machine-Shop Practice Part 166

[Ill.u.s.tration: Fig. 2475.]

FITTING BRa.s.sES TO THEIR JOURNALS.--Bra.s.s bores always require fitting to their journals after having been bored, because the finished hole is not a true circle, but too narrow across the joint face, as at F in Fig.

2475, in which the full lines represent the form of the bra.s.s before, and the dotted line its form after being bored and released from the pressure of the devices or chuck that held it while it was being bored.

This almost always occurs to a greater or less degree, and it arises from local strains induced from the unequal cooling of the casting in the mould, which strains are released as the metal is removed (in the process of boring) from the surface of the bore. It would appear, however, that if the finishing cut taken by the boring cutter be a very fine one it should leave the hole true and round, but the pressure which is placed upon the bearing to hold it against the force of the cut prevents the bearing from a.s.suming its natural form until released from that pressure.

If a bearing be bored to very nearly its finished size and first released altogether from the pressure of the holding chuck, or other device, and then re-chucked, it is probable that the finished bore would be practically quite round and true, but such re-chucking is not the usual practice.

Suppose, however, that the bearing shown in Fig. 2475, be properly fitted to a journal, still improper conditions arise from wear, because the area of the surface D becomes from the weight and from vibration condensed, and finally it stretches, causing the bore at F to close upon the journal and bind it with undue friction.

[Ill.u.s.tration: Fig. 2476.]

If the shape of the bedding part of the bra.s.s, or bearing, be such as shown in Fig. 2476, the surfaces A B and C will condense and stretch, closing the diameter of the bore at E and making the sides G G fit loosely in their places. It is to be observed that a similar condensation of the metal occurs to some extent around the bore of the bearing; but this surface is being continuously worn away by the journal, and it is, therefore, at all times less stretched and condensed than that on the bedding surface.

[Ill.u.s.tration: Fig. 2477.]

There is, therefore, a constant action causing the bra.s.s to bind unduly hard at and near its joint face E, Fig. 2476, and thus to cause heating and undue abrasion and wear. To prevent this it is necessary to ease away that part of the bra.s.s bore, as is shown in Fig. 2477 from J to K, clear of the journal.

But in the case of bearings receiving thrust, as in engine main bearings, the line of pressure is in a horizontal direction; and hence the most effective bore area to resist that pressure has been removed.

Furthermore, the bearing area of the bra.s.s bore has been reduced, thus increasing the pressure per square inch on the remaining area.

[Ill.u.s.tration: Fig. 2478.]

The methods employed to avoid this evil are as follows:--In the form shown in Fig. 2478 the joint faces are at an angle instead of being horizontal and parallel to the line of the thrust, or the joint faces may be made to stand at a right angle to the line of journal thrust, so that the crown of the bra.s.s will receive the thrust. But the bra.s.ses will still close across the joint faces (as already described) as the wear proceeds, and the areas from J to K in Fig. 2477, must still be eased away, requiring frequent attention and giving a reduced bearing area. Furthermore, in proportion as the line of the joint faces of the bra.s.ses is at an angle to the line of thrust, the strain on the top or cap bra.s.s will fall on the bolts, so that if those joint faces be at a right angle to the line of thrust, the whole strain of that thrust will fall on the bolts that hold the cap and cap bra.s.s.

[Ill.u.s.tration: Fig. 2479.]

Another plan is to make the bearing in parts, as in Fig. 2479, in which the top and bottom parts of the bearing extend to the joint face on one side, but admit a chock or gib, A in the figure, which may be adjusted by a set-screw as shown. By this means the bearing area may extend all around the bore. In some cases two of such chocks and set-screws, one on each side of the journal, are employed.

[Ill.u.s.tration: Fig. 2480.]

In place of the set-screws, whose ends, from receiving the pressure of the thrust, are apt to imbed themselves into the chock and to thus loosen the adjustment, wedges lifted by bolts pa.s.sing up through the cap, as shown in Fig. 2480, are employed, being preferable to the screws.

In the Porter Allen engine the wedges pa.s.s clear through the bearing, as in Fig. 2481, so that they may be pushed up after the manner of a key and their pressure against the side chocks judged independently of the nuts at the top.

In some designs the top and bottom parts of the bearing are free to move in the line of the thrust, and the side chocks or blocks alone are relied on to resist the thrust.

When the bra.s.ses are in two halves, they may be fitted so as to have a known degree of bearing pressure upon the journal, and the fit may thus be accurately adjusted, in which case they will wear a long time before requiring re-adjustment. On the other hand when the side chocks are used the wear in the line of the thrust may be taken up as it proceeds. In one case the attending engineer cannot alter the fit of the bearing nor the alignment of the shaft, while in the other he can do both. Thus the facilities that enable him to make these adjustments properly also enable him to make them improperly. But this would be of no consequence, providing it could be determined whether the adjustment were improving the conditions without first making it. With an engine at rest it is easy to determine, by means of the connecting rod, whether the chock adjustment is correct, so far as the adjustment of the shaft is concerned, but it is not easy so to determine the pressure of the chock on the journal; nor when each chock has two adjusting screws is it easy to determine when they both bear alike.

[Ill.u.s.tration: Fig. 2481.]

When the bearing is in four pieces, and three of them have two screws each, it is still more difficult to operate all so as to have the bearing equal on the journal. The fit to the journal can only be determined by the results: if too easy, the bearing pounds; if too tight, the bearing heats and wears.

But undue wear may take place without heating, and this is one of the greatest objections to this method of adjustment.

[Ill.u.s.tration: Fig. 2482.]

A design of bearing used in American locomotive practice is shown in Fig. 2482. Here the joint faces C, B of the bra.s.s is bevelled, fitting into a corresponding bevel in the box, which prevents the bra.s.s from closing across the joint face; hence, the bearing on the journal may extend all around the bra.s.s bore from the oil cavity A to the edges B C.

The bra.s.s is, in this case, forced to its place in the axle box under hydraulic pressure, and this pressure springs the box open at H, making it wider; but when the box is put to work the bra.s.s compresses somewhat, and its surfaces conform more closely to the bedding surface of the box than when first put in, and this causes the box to close slightly at H.

To prevent this closure from carrying the bra.s.s with it and close it across the joint face (as in the case of the bra.s.s shown in Fig. 2476) the following plan is adopted. The bra.s.ses, after having been turned in the lathe, are filed along the entire surface (on each side) for a distance of about 1-1/2 or 2 inches, so as to clear the bore of the box near the bevels B, C. When the box is put into the hydraulic press, to have the bra.s.s forced in, a centre-punch mark J is made, and part of a circle L L is struck; when the bra.s.s is home in the box the arc of circle K is made, the distance between K and L showing how much the box has been sprung open by the bra.s.s; the amount allowed is about 1/32 of an inch. If, as the bra.s.s is pressed in, it is found that this will be exceeded, it is taken out and eased. When the engine is running and the boxes spring to some extent they do not carry the bra.s.s with them, because the sides being eased away gives liberty to the box to come and go slightly; the bevels also tend to keep the bra.s.s bore open.

Here, then, the bra.s.ses may be fitted to align the axle perfectly, and it is not permitted to the engineer to alter that alignment, while at the same time the fit of the bra.s.s to the journal being made correct, the engineer cannot alter it. Under these conditions the whole area of the bra.s.s is effective in holding the journal, which increases the durability of the bra.s.s by keeping the pressure per square inch on the bra.s.s bore at a minimum.

If side chocks are used, however, it is better to set them up by wedges than by screw bolts, because from the tightness of the fit of such screws in the tapped holes, it is difficult to determine, with precision, with what degree of pressure the chocks are forced against the journal. Furthermore, the screws may not fit with an equal degree of tightness; hence, when screwed up with an equal degree of pressure, one end of the same chock may be set tighter to the journal than the other end, and any undue pressure of fit at either end tends to throw the shaft out of line as well as inducing undue wear. But when wedges are used to set up the side chocks the nuts operating those wedges may be an easy fit without fear of their becoming loosened (as set-screws in the line of thrust are apt to do).

On the fast engines of the Pennsylvania Railroad solid bronze boxes, without bra.s.ses, are used, and when the boxes require truing from having cut or from having worn oval they close them under a steam hammer, closing the bore across and enabling it to be trued out in the lathe without taking much metal out of the crown of the bore. The wedges and adjusting shoes are thickened when this becomes necessary by reason of the box closure or width.

If a bra.s.s bore does not bed fully and equally over the entire intended bearing area the part not fitting will at first perform no duty as bearing area, and the whole strain will be thrown upon a less area than is intended by the construction, causing undue abrasion until the bra.s.s bore has what is termed worn down to a bearing. The amount of this wearing down to a bearing may be so small as to be scarcely perceptible to ordinary observation, but if the oil that has pa.s.sed through the journal be smeared upon stiff white paper, as writing paper, it will be found to contain the particles of abraded metal, which will be plainly distinguishable. Under these conditions the journal will have a dull, though perhaps a smooth appearance, and will not have that mirror-like surface which is characteristic of a properly fitted and smooth working bearing, while under a magnifying gla.s.s the journal will show a series of fine rings or wearing marks. It is necessary, therefore, that each bra.s.s be properly fitted to its journal so that it shall bed fairly and evenly over all the area of its bore that is intended to bear upon the journal.

The most expeditious method of fitting a new bearing box or bra.s.s to its journal is to first file the bore until it fits the journal when simply placed thereon by hand, and without going to the trouble to put the bra.s.s or the journal in position in the frame which holds them. So soon, however, as the crown of the bra.s.s beds to the journal along its whole length, the bra.s.s should be placed in its box, or in the frame, and the journal adjusted in its place and rotated so as to leave its bearing marks upon the bra.s.s bore, to a.s.sist which it may have a faint coat of red marking on its surface. The fitting should be continued both with file and sc.r.a.per until the whole area of the part intended to bed fits well and is smooth and polished. To produce this result the finishing should be done with a very smooth half-round file, draw-filing so as to leave the marks in a line with the circ.u.mference of the bore, and finally with a half round sc.r.a.per, which will remove the file marks. The degree of contact should be such that, when the bearing is bolted up, bra.s.s and bra.s.s, as it is termed (which means that the joint faces of the bra.s.ses are held firmly together), the journal will rotate as freely as when the top bra.s.s is removed, while the contact marks on the top bra.s.s have been removed by sc.r.a.ping. By this means the fit will be just sufficient to permit the lubricating oil to pa.s.s between the journal and the bearing, and the journal will work freely and easily without any play, knock, or pound. If, when the top bra.s.s or bearing is bolted home and the shaft is rotated by hand, that bra.s.s on removal shows contact marks on its bore, although it may rotate comparatively easily it will be so tight a fit that the oil cannot pa.s.s, and as a result the wear, instead of producing a glossy surface, will produce a dull one, and undue abrasion will ensue even though no rings appear.

When bra.s.ses are held in rods that connect two journals together the fitting of the bra.s.s bore must be conducted with a view to have the bra.s.ses fit their journals all over the intended bearing area of their bores, which can only be accomplished by trying the bra.s.s bores to their journals while in the rod, in the manner to be hereafter described with reference to connecting rods and to lining engines.

When a journal is worn in rings, or so rough as to cause destructive abrasion and undue friction, it may be refitted as follows:--First, with a smooth file draw-file the journal in the direction of its length, taking off all the projecting rings. Then sweep a very smooth file that is somewhat worn (which will cut smoother than a sharp file) around the circ.u.mference of the journal so that the file marks will be in the plane of revolution. Then wrap a piece of fine and somewhat worn emery paper around the journal, and wrap around it (say twice around) a piece of coa.r.s.e string, leaving the two ends about two feet long. Take one end of the string in each hand and pull first one end and then the other, causing the emery paper to revolve around the journal and smooth it.

To refit the bearings, first with a smooth half-round file remove the rings or rough surface, and then fit the surface with the file, so that when in its place the journal is rotated the contact marks show a proper bearing. Then draw-file the bore with a smooth half-round file and finish with a half-round sc.r.a.per, easing away the high spots until the bore shows proper contact and is smooth. A piece of fine emery paper may then be wrapped around a half-round file and the surface smoothed with the emery paper moved across the bore and not in the direction of the circ.u.mference of the same. The emery paper should be well worn for the finishing and of a fine grade number, so as to leave a bright polish and not dull marks.

In some practice the bores of bra.s.ses are left rough-filed, the file marks being lengthways of the bearing of bore. If the journal requires smoothing it is draw-filed lengthways of the journal. The philosophy of this is, that the file marks will hold the oil and afford unusually free lubrication while the bearing and journal are wearing down to a bearing.

But where the framework holding the bearings and journals are rigid, these bearings and journals may, with care, be fitted to a polished and equal bearing, leaving a smoother surface than that produced by wearing down to a bearing. But if, as in the case of a locomotive, the framework is subject to torsion, rough surfaces left to adjust themselves are possibly better than those accurately fitted, because the whole framework holding the bearings changes its form when the full load is upon it and after put to work, and the fitting done when there was no load upon the parts is no longer quite correct. The lubrication of the bearing, however, should be very free, and the effort appears at present to be to afford more ample oil ways than hitherto even at some sacrifice of bearing area.

LEAD-LINED JOURNAL BEARINGS.--If a journal is worn in grooves or undulations it becomes impracticable to properly fit the bra.s.s to it without reducing its diameter to remove the rings, and to obviate the cutting and heating which necessarily follow, as well as to obviate the necessity of fitting the bra.s.ses at all, Mr. D. A. Hopkins introduced lead-lined bearings; the lead lining being merely auxiliary to the bearing proper, which is made preferably of hard bronze, and to which the thin layer or facing of lead is firmly attached by a soldering process, so that the two metals are virtually one piece. Into this lead facing the journal, under the pressure of the car, moulds or imbeds itself from the start, and afterwards gradually wears its way through it into the hard metal. The perfect fit thus secured under all conditions of the journal, aided with proper lubrication, not only prevents heating, but secures the full wear of the bra.s.ses, and makes them at all times perfect counterparts of the journal surfaces.

[Ill.u.s.tration: Fig. 2483.]

Fig. 2483 shows at the top an unfitted bearing without the lead lining, resting upon a worn and badly-cut journal, the only points of contact being near the ends. For obvious reasons such a journal is sure to run hot.

The engraving below shows the application of the lead lining to the same journal, the dark shading between the journal and bearing representing the lead which has been pressed into the worn and hollow surface of the journal, forming a complete bearing and distributing the weight equally upon its surface.

[Ill.u.s.tration: Fig. 2484.]

Fig. 2484 represents an end view of an unfitted journal and the same lead lined.

The lead compresses until the bra.s.s meets the journal and thus permits between the two contact over the area that does fit or touch; while the lead fills the remaining area of the bra.s.s bore, giving it a bearing on the journal, thus relieving the touching points from receiving the whole weight of the load, and preventing the cutting or abrasion that would otherwise occur. As, however, the wear takes place the lead compresses, permitting the journal and bra.s.s to come into bearing over its full area, being obviously effective providing the bearing be kept free from grit, which would imbed in the bearing and cause it to unduly wear the journal.

If a bra.s.s is too tight a fit upon its journal, heating and abrasion, or "cutting" as it is termed, ensues. But if a bra.s.s or box does not fit close to its journal, lost motion and sometimes knocking or pounding ensues. When the joint faces of bra.s.ses abut, or come bra.s.s and bra.s.s as it is termed, they should be a proper fit to the journal when they are keyed, or otherwise set up close together; hence there is no danger of either having a pound in the bra.s.s, or of heating and cutting. The objection to this plan is that the bra.s.ses must be removed from their boxes and the joint faces filed away to let the bra.s.ses together, to take up the wear; hence, in positions in which it is difficult to get the bra.s.ses out, the joints should be left open, while in all cases where they can be readily removed they should be made bra.s.s and bra.s.s.

It is to be observed that bra.s.ses that come bra.s.s and bra.s.s require less adjusting and last longer than those left open, because the latter often suffer from the abrasion due to an improper adjustment.

In bra.s.ses that are left open, it is not an uncommon practice to adjust the fit as follows: Between the bra.s.s joint faces at each of the four corners a piece of lead wire is inserted; the bra.s.ses are keyed as close home as can be upon the journals, which compresses the lead wire; the top bra.s.s is then released until the piece of lead wire can be moved freely between the bra.s.s joints.

[Ill.u.s.tration: Fig. 2485.]

[Ill.u.s.tration: Fig. 2486.]

A compromise between the bra.s.s and bra.s.s and the open joint is sometimes effected by the insertion of slips, as shown in Fig. 2485 at A, B. These slips may be taken out by simply removing the top bra.s.s, while their reduction of thickness lets the bra.s.ses together to take up the wear.

The thickness for these strips may be readily obtained by means of the pieces of lead wire used as already described.

In the case of large bra.s.ses which come bra.s.s and bra.s.s, much of the filing on the joint faces to let them together may be saved by reducing their thickness and area by cutting away part of the metal, as at A A in Fig. 2486.