Modern Machine-Shop Practice Part 169

Since the babbitt metal in a bearing is apt to close across the bore when cooling after being poured, a mandrel of slightly larger diameter than the diameter of the journal should be used in place of the working journal to run the bearing on. Some effect the same purpose by wrapping writing paper around the journal; but it is wrong to use the journal, for the following reasons: To get a good, sound, well-fitting babbitt metal box, the metal should be poured as cool as possible, for if made red hot it contracts so much in cooling that it does not fit well in the box or frame of the machine. On the other hand unless the metal be well hot it is apt to cool and set too soon and be unsound. To remedy this the journal, or whatever represents it, must be heated. The heating is very apt to bend it. It is obvious then that instead of the journal a temporary bar of iron of slightly larger diameter than the working journal should be used, heating it to a good black hot heat, so that the babbitt metal may be poured less hot than would otherwise be permissible, and the contraction of the babbitt in the box reduced to a minimum. A little powdered resin sprinkled in the box will help the babbitt to flow easily and make a sound casting.

The temporary spindle, or journal, should also be oiled, and as soon as the metal has well set, the temporary journal should be revolved to free it. Babbitt bearings cast in two halves should be fitted to the journal as already described for bra.s.ses, which will well repay the cost and trouble.

To prevent the metal from running out of the bearing, its ends are closed by means of either clay or putty closely packed against the bearing ends and the shaft, and in pouring in the melted metal it is best to pour it on the top of the shaft, and let it run down its sides into the cavity of the bearing. This heats the shaft equally, and prevents it bending from unequal expansion, as it would do if it met the heated metal on its lower half only, it being obvious that if the shaft bends the bore of the bearing will not be cast in line; hence, the shaft will bear at the end only, and will require to wear the babbitt down to a bearing.

Babbitting is sometimes employed to refit parts that have worn loose, or to bush the bores of work. Suppose, for example, that in a case of emergency a pulley of a certain diameter is required, and that the only one at hand has too large a bore, then we may take a mandrel or arbor of the diameter of the shaft the pulley is required for, and drive on it two thin washers and turn them to fit the bore of the pulley, and cut a recess in each to enable the metal to be poured through. We may then put the arbor and washers in the pulley (the washers serving to hold the arbor true), and fill in the bore with babbitt metal, leaving the pulley set-screw in place and set to just touch the arbor, so as to cast the thread in the babbitt bushing, and thus save drilling and tapping.

PROPORTIONS OF JOURNALS.--It follows from what has been already said that under a given amount of duty the friction will be less and the durability greater in proportion as the bearing area of a journal is increased. But it is an important consideration whether such area shall be obtained in the diameter or in the length of the journal, or, in other words, what shall be the proportions between the diameter and length of a journal. It is found in practice that a journal wears better in proportion as its length exceeds its diameter, providing that the stress is not sufficient to cause sensible flexure, because in that case the pressure is reduced at that part of the journal where the most flexure occurs, and increased where the journal is most rigid. As a result, the abrasion increasing with the pressure becomes locally excessive, the glossy smoothness is lost and increased friction ensues.

If, however, the length of a journal is limited by the exigencies of its location or the design of the machine, the diameter of journal must be increased if necessary in order to obtain sufficient bearing area to withstand the stress without causing undue abrasion.

Referring to the bearing area in proportion to the load, Prof. R. H.

Thurston writes, in an article in the _Railroad Gazette_ of January 18th, 1878, as follows:--

"A pressure of 800 pounds to the square inch can rarely be attained on wrought iron at even low speeds, while 1,200 pounds is not infrequently adopted on the steel crank-pins of steamboat engines. I have known of several thousand pounds pressure per inch being reached on the slow-working and rarely moved pivots of swing bridges. In my own practice, I never, if I can avoid it, use higher pressures than 600 and 1,000 on iron and on steel, and, for general practice, make the pressure less as the speed is greater."

W. Sellers and Co. state that under a pressure of 50 lbs. per square inch, and with oil well distributed over the surface of the box, the metal of the journal will not touch that of the bearing box bore.

In practice bearings are made with a length varying from that equal to the diameter of the journal to about four times that diameter, and but few cases occur in which these limits are exceeded in either direction.

It is to be observed, however, that diminishing the length is apt to increase the abrasion unless the duty is very light indeed, while increasing it increases the durability while not affecting the friction, unless the shaft bends.

There are special cases in which within certain limits the proportions of journals are nearly uniform in practice; thus the length of engine crank-pin bearings rarely exceeds once and a half times the diameter, while the main shaft bearings are often similarly limited in width from the exigencies of designing the engine so that the eccentric shall come in line with the slide-valve spindle. In the case of crank-pins the pin cannot be held sufficiently rigidly to prevent spring or flexure; hence it is desirable to limit its length so that its pressure shall be as short a leverage as possible to the crank. The solid bearings in the framing of machines usually admit of room enough to make their lengths three or four times the diameter. Again, in the case of line shafting, boxes having a length equal to three or four times the diameter may be employed, providing that the alignment be made correct, or that the boxes are self-adjusting. But in all cases the longer the bearings the greater the necessity for correct alignment, so that the axis of the bearing bore may be in line with the axis of the shaft, the error manifestly increasing with the length of the bearing.

PLACING TWO CRANKS ON A SHAFT SO THAT THEIR CENTRE LINES SHALL STAND AT A RIGHT ANGLE.--It is obvious that the keyways in both the crank and the shaft must be cut accurately in their proper positions, because it is a tedious operation to file out the sides of the keyways when the cranks are placed upon the shaft. To mark the keyways in the absence of any tools or appliances specially designed for the purpose we proceed as follows: Placing the shaft upon a marking-off table, we plug up the centres upon which the shaft has been turned by driving a piece of lead in them, leaving the surface level with those of the shaft; and then from the perimeter of the shaft we carefully mark, upon the lead plugs, the centres of the shaft. From this centre we describe a circle whose diameter will be equal to the required widths of the keyway, and then taking a square we place its stock upon the face of the marking-table, and bringing the edge of the blade even with the edge of the circle, we mark a perpendicular line upwards from the circle to the perimeter of the shaft, and then draw a similar line on the other side of the circle, as shown in Fig. 2504, in which A represents the shaft and B the circle, C the perpendicular line struck on one side of the circle, and D the square placed upon the marking-table E, in position to mark the line on the other side of the circle, F and G being wedges to keep the shaft A from moving its position upon the table. We next mark with a scribing-block or surface gauge the depth of the keyway as denoted by the line H, and the marking at that end of the shaft is completed.

Pa.s.sing to the other end of the shaft we find the centre of the shaft, and describe around it a circle equal in diameter to the required width of keyway, and from the edges of the circle to the perimeter of the shaft draw two lines with a scribing-block, as shown in Fig. 2505, A representing the shaft, B the circle, C D the breadth of the keyway, E the marking-off table, F and G the wedges, and H the depth of the keyway, which must, in this case, be marked with a square resting on the table.

[Ill.u.s.tration: Fig. 2504.]

[Ill.u.s.tration: Fig. 2505.]

If, however, the shaft is too heavy or large to be placed on a marking-off table, we may proceed as follows: Strike as before the circle B, Fig. 2504, equal in diameter to the required width of keyway, and adjust a straight-edge held firmly against the end face of the shaft, so that its upper edge is coincident with the perimeter of this circle, while the straight is horizontally level-tested by a spirit-level. Draw a line along the shaft face, using the straight-edge as a guide. This will give us the line C in Fig. 2505. By a similar process the line D, Fig. 2505, may be drawn. At the other end of the shaft similar lines, but standing vertical, may be marked, which will give the positions of the keyways.

[Ill.u.s.tration: Fig. 2506.]

We have now marked off on the end faces of the shaft a keyway at each end, one standing at a right angle to the other; but it must be borne in mind that we have paid no attention as to which crank shall lead; that is to say, suppose in Fig. 2506 A and B represent cranks placed upon the shaft C, and running in the direction indicated by arrow D, it is evident that the crank B leads in the direction in which the engine is to run, and hence the keyway E stands in advance of the keyway F; and therefore, as shown in the figure, the right-hand crank leads. To have made the left-hand crank lead, when the engine runs in the direction of the arrow D, we should, supposing the keyway F to be already cut, have to cut the keyway E on the directly opposite side of the shaft; or, what is the same thing, supposing the keyway E to be already cut, the keyway F would require to be cut on the diametrally opposite side of the shaft.

It is obvious that if the engine ran in the direction of the arrow G, the left-hand crank would lead, supposing in each case the cylinders to stand at H. Here it may be necessary to explain the manner of determining which is the right-hand and which the left-hand crank.

Suppose then that the figure represents a locomotive crank, the cylinders being at H, then as the engineer stands in the cab, facing his engine, A will be the left-hand and B the right-hand crank. It is usual in locomotives to make the left-hand crank lead when the engine is running forward, the practical difference being, that if the workman were by mistake to make the right-hand crank lead, the engine would run forward when the reversing lever was placed to run backward, and _vice versa_. It makes no difference whether the shaft can be turned end for end or not: if the right or left crank is required to lead when the crank is required to revolve in a given direction the keyways in the shaft must be marked off in the relative positions on the shaft necessary to obtain that result.

The keyways may be carried along the circ.u.mference of the shaft by a square applied to its end face, or if that face is not flat by the ordinary keyway marking tool.

[Ill.u.s.tration: Fig. 2507.]

To mark off the keyways in the cranks, we place a centre-piece in the bore of the crank, as shown in Fig. 2507, in which A represents a crank having a centre-piece of sheet iron B placed in the bore. On the face of this centre-piece we mark the centre of the hole into which it fits, and from that centre we describe the circle C, which must be of exactly same diameter as the crank-pin if it is in its place, or otherwise of the crank-pin hole. We then draw the lines D and E, using as a guide a straight-edge placed one end upon the crank-pin journal, or even with the edge of the crank-pin hole, as the case may be, and the other end (of the same edge of the straight-edge) exactly even with the circ.u.mference of the circle C. From D and E we find the centre of the circle F, which must be central between D and E, and whose diameter must be exactly equal to the required width of keyway; and we then mark the circle G, describing it from the centre of the hole, and therefore of the circle C. By drawing the lines H and I, which must be even with the circ.u.mference of the circles F and G, using a straight-edge as a guide, we shall obtain the correct position for the keyway K, and the whole of the keyways may be cut, care being taken to cut them quite true with the lines, and of an exact equal width.

To put the cranks on the shaft, first provide a temporary key, a close fit on the sides, but clear top and bottom, so that it will bind just easily on the sides of the keyways in both the shaft and the crank. The shaft must be placed and wedged with its keyway downwards, so that in putting the crank on, the pin end may hang downwards, which will render it more easy both to put on, handle, and adjust. As soon as the shaft has entered the crank, say a quarter of an inch, we must insert the temporary key (which may have its end edges well tapered off to a.s.sist the operation of entering it) sufficiently far into the keyway of the shaft that it will not fall out, and we may then proceed to put the crank on the shaft to the necessary distance, keeping the temporary key sufficiently far in the keyway to enable it to act as a guide--that is to say, up to at least half the length of the keyway.

[Ill.u.s.tration: Fig. 2508.]

[Ill.u.s.tration: Fig. 2509.]

To put on the second crank, we first place the shaft so that the crank already on stands exactly horizontal, setting it by placing a spirit-level, as shown in Fig. 2508, in which A represents either the crank-pin journal or the crank-pin hole in the crank, and B a circle struck on the end face of the shaft and from its centre, the diameter of the circle B being exactly the same as that of A. If then we so adjust the position of the crank that a spirit-level applied to the exact circ.u.mferences of the circles A and B stands level, the crank will stand level, and we have only to put the second crank on with its centre-line standing perpendicular, and the two cranks will be at a right angle one to the other. We now proceed to put on the second crank, pursuing the same method employed in putting on the first one, save that the temporary key need not be inserted so far into the keyway, because, if the keyways have been cut the least out of true, it will make a great difference at the crank-pin, because of the increased distance of the latter from the centre of the crank-shaft. As soon as the second crank is placed to its position on the shaft we must ascertain if it stands vertical, which we may do by applying the spirit-level as shown in Fig.

2509, bringing its edges exactly fair with the edges of the circles A and B, and moving the crank until the bubble of the level stands true, and taking out the temporary key if it is necessary to adjust the crank at all.

If, however, the crank is to be forced on by hydraulic pressure, this latter adjustment should be made when the crank is just sufficiently far on the crank shaft to enable it to bind enough to well support its own weight, to facilitate which the end of the shaft is sometimes slightly tapered for a very short distance--a practice which is sometimes rendered unnecessary by reason of there being attachments fitted to the hydraulic presses which of themselves adjust the position of the cranks, and insure their being at a right angle one to the other.

After the cranks are on their places the keys may be fitted, care being taken that, if the crank last put on had to be moved to adjust it, the sides of the keyways be filed even, otherwise driving the key will tend to move the crank.

FITTING ENGINE CYLINDERS.[35]--When engine cylinders are made in quant.i.ties, as in locomotive building shops, a great deal of the fitting work is saved by the machine work; but when a single cylinder or a pair of cylinders only are to be fitted up it will not pay to make jigs and appliances; hence, they are usually fitted up entirely by hand. The first thing to do is to mark off all the holes requiring to be drilled, and have the drilling done.

[35] From the "Complete Practical Machinist."

In marking the holes in the cylinder covers it is to be noted whether that part of the cylinder cover which fits into the cylinder has a portion cut away to give room for the steam to enter (as is usually the case), and if so, first mark a line across the inside f.l.a.n.g.e of the cover, parallel to the part cut away, and then scribe each end of the line across the edge of the f.l.a.n.g.e. Then mark a similar line across the cylinder end, parallel to the steam port where it enters the cylinder, and scribe each end of this line across the cylinder f.l.a.n.g.e, so that, when the cylinder cover is placed into the cylinder and the lines on the f.l.a.n.g.es of the cylinder and the cover are placed parallel to each other, the piece cut away on the cover will stand exactly opposite to the steam port, as it is intended to do. The cover may then be clamped to the cylinder, and holes of the requisite size for the tap (the tapping holes, as they are commonly called) may be drilled through the cover and the requisite depth into the cylinder at the same time.

[Ill.u.s.tration: Fig. 2510.]

The cylinder covers must, after being drilled, as above, be taken from the cylinder, and the clearing drill put through the holes already drilled so that they will admit the bolts or studs, the clearing holes being made 1/16 inch larger than the diameter of the bolts or studs. The steam chest may be either clamped to the cylinder, and tapping holes drilled through it and the cylinder (the same as done in the case of the covers), or it may have its clearing holes drilled in it while so clamped, care being taken to let the point of the drill enter deep enough to pa.s.s completely through the steam chest, and into the cylinder deep enough to cut or drill a countersink nearly or quite equal to the diameter of the drill. If, however, the steam chest is already drilled, it may be set upon the cylinder, and the holes marked on the cylinder face by a scriber or by the end of a piece of wood or of a bolt, which end may be made either conical or flat for the purpose, marking being placed upon it; so that, by putting it through the hole of the chest, permitting it to rest upon the cylinder face (which may be chalked so as to show the marks plainly), and then revolving it with the hand, it will mark the cylinder face. This plan is generally resorted to when the holes in the chest are too deep to permit of being scribed. To true the back face, round a hole against which face the bolt head or the face of the nut may bed, in cases where such facing cannot be done by a pin countersink or a cutter used in a machine, the tool shown in Fig. 2510 may be employed, _a_ being a pin provided with a slot at one end to admit the cutter B, which is held fast by the key C, and is also provided with a square end _f_, by which it may be turned or revolved by means of a wrench, and with a thread to receive the nut E, _d_ being a washer; so that, by s.c.r.e.w.i.n.g up the nut E, the cutting-edges of the cutter are forced against the cylinder _g_, and will, when revolved, cut the face, against which they are forced, true with the hole in the cylinder through which the pin _a_ is pa.s.sed.

After the drilling the cylinder should be placed on end and all the holes that can be got at should be tapped. Then the cover joint, supposing it to be a ground joint, should be made according to the directions given for making ground joints, when the cylinder may be turned upside down and the other cover fitted. Then the holes for the cylinder c.o.c.ks and for the steam and exhaust pipe should be tapped, and the faces for these pipe joints fitted as required.

The steam-chest holes should then be tapped and the ports marked out and chipped and filed to the lines, such lines being marked as described in the remarks on lining out work.

The face for the steam-chest seat and the steam-chest cover may then be prepared by filing, sc.r.a.ping, or grinding, as may be required, and simultaneously the valve seat and valve face may be fitted. If the cylinders are to be bolted together as in a locomotive, the holes for holding them together should be drilled about 1/64 inch smaller than the bolts, so that they may be reamed out together after the cylinder bores are aligned.

One cylinder face should be marked and drilled first, and the two cylinder bores being set to align true the other cylinder should be marked from the other, or if there is a saddle between the two cylinders both cylinders may be marked and drilled, and also the holes on one side of the saddle. Temporary bolts may then be put through the holes that are drilled in the cylinder and saddle and clamps used to hold the undrilled cylinder to the saddle, when the cylinder bores may be set true one to the other, and the holes on the remaining side of the saddle marked through those already drilled in the cylinder. These latter holes being drilled, temporary bolts of smaller diameter than the holes (so as to give room to move the cylinders to align their bores) may be used to bolt the cylinders together while their bores are accurately aligned, which alignment may be effected as follows:--

[Ill.u.s.tration: Fig. 2511.]

The bores should be set as near true as possible, tested by a spirit-level rested on the bore and placed as near true as can be judged with the length of the bore, and a plumb rule may be applied to the end faces where the cover joint comes. Then a straight-edge should be applied, as in Fig. 2511, in which S is the straight-edge, and C and D the two cylinder ends.

The method of testing is shown in Fig. 2511, where the straight-edge S is shown in three positions, marked respectively 1, 2, and 3 at one end, and F, G, and H at the other.

[Ill.u.s.tration: Fig. 2512.]

[Ill.u.s.tration: Fig. 2513.]

The first test should be made by simply placing the straight-edge across the two cylinder faces, as at G 3; and when the cylinders are set apparently true and the spirit-level applied to the respective bores shows them true, greater accuracy may be secured by placing the straight-edge in position 1 H, being pressed firmly to its cylinder face with end 1 above the other cylinder face. Then, while end H is held firmly to its cylinder, let end 1 lower until it pa.s.ses entirely over the face of cylinder C, whose face it should just touch; if on meeting C the straight-edge strikes it or does not meet it, further adjustment of the cylinder positions is necessary. Next place the straight-edge in position 2, pressing end F firmly against cylinder D, and pa.s.sing the other end entirely over the end of cylinder C, which it should just touch, and no more. It will then be necessary to repeat this process, pressing the straight-edge against cylinder C and testing the other end with cylinder D, and the cylinders thus set will be (if the end faces are true, as they should be, and usually are) more truly aligned than is possible by the use of the spirit-level. This method also brings the end faces of the cylinders in the same plane, so that each piston head will travel central in the length of the cylinder bore, approaching the cylinder covers equally, and therefore keeps the clearance equal.

Incidentally, also, this secures accuracy in the cross-head traverse on the guide bars (supposing these bars to be bolted to the cylinder cover). The holes for bolting the cylinders together may then be reamed and the bolts driven in and screwed up.

To guide the tap when tapping the cylinder cover and steam-chest holes the guide stand S, shown in Fig. 2512, should be employed. It is bolted to the cylinder face by the bolt B, which pa.s.ses through a slot in the stand.

The tap T is inserted through the two arms of the stand and its end inserted in the hole to be tapped when bolt B is tightened up.

The stem of the tap should be of slightly larger diameter than the tap thread, so as to fit in the holes of the guide or stand.

When, however, one end of the guide bars is carried on the cylinder cover, it is necessary when setting that cover to be marked for the drilling, to so set it that the seats for the guide bar ends shall be horizontally level when the cylinder is on the engine; and when setting the bores of the cylinder in line to mark the holes for bolting the cylinders together or to the saddle, this point should also be looked to, as if these seats are not in line the faces of the guide bars will not be in line, and will not, therefore, bed fair to the cross-head guide unless the error is in some way corrected.

It is desirable that these seatings be quite true and in line one with the other on both cylinders, so that if liners require to be made, or if the ends of the bars require to be filed to let the bars together at any time, the surfaces may be filed true to the face of the bar, and thus be set true and to fit the cross-head guides without requiring to put the bars on and off to fit them true by trial.

[Ill.u.s.tration: Fig. 2514.]