Modern Machine-Shop Practice Part 199

As will be seen by inspection of Fig. 3012, the weld between the cap and the saddle comes about the middle of the wrist, and the cheek pieces support the cap sideways. By means of the piles and welds described, the grain of the iron was so disposed as to offer the most resistance to working strains. This method was devised by Mr. Farrell Dorrity, of the Morgan Iron Works.

FORGING LARGE CRANK SHAFTS.[45]--The following paper describes the method of forging marine crank shafts adopted at the Lancefield Forge, Glasgow. It will be better understood if a short account is first given of the ordinary methods in use for the same purpose.

[45] From a paper read at the Glasgow meeting of the Inst.i.tution of Mechanical Engineers, by W. L. E. MacLean.

"_First Method._--The most common method is technically termed by the forgeman, 'finishing the piece before him.' He begins with a staff or stave, as shown in Fig. 3025, suspended by a chain from the crane, and made round for the convenience of manipulating under the steam hammer; this stave is used over and over again for many forgings, as it is merely the "porter" to carry the piece and enable it to be worked. The forging is begun by two or three slabs being placed on the stave as at S S S, and then inserted in the furnace. These slabs are flat blocks made up of pieces of sc.r.a.p iron, which have been piled and heated, and then welded together. After being brought to a welding heat in the furnace, the slabs are withdrawn, placed under the steam hammer, and beaten down solid. The piece is then turned upside down, and two or three similar slabs placed on the opposite side, as shown at S S. When sufficient iron has been thus added to form the collar of the shaft (a.s.suming it is to have a collar), it is rounded under the hammer, as at C, Fig. 3026, and the body of the shaft next to the collar is roughly formed, as at D.

More slabs, S S S, are added to bring out the body, and afterwards the crank itself is proceeded with, on the same plan. The piece will begin to a.s.sume the appearance of A, Fig. 3026. Then more slabs are welded on the top, as at S S S, till the depth of the crank is obtained, after which the forgeman proceeds to finish the collar and body of the shaft, as shown. The collar on being finished is cut all round, as shown at C D, Fig. 3027, so that it may be more easily detached from the stave when the shaft is completed, leaving only sufficient connection to carry it till then. The forgeman then cuts the gable of the crank as at E G, and rounds up the body and neck as at B N, Fig. 3027.

"This, it will be observed, is a speedy process, and would invariably be adopted if it were not attended with a very serious drawback; it is very hazardous to the solidity of the forging. For it will be easily understood that not above a third of the crank itself can be thus formed, because the iron at the neck N would not carry a greater ma.s.s; if the whole ma.s.s of the crank, or even the half of it, was formed before the body and neck of the shaft were finished, a proper heat could not be taken on the body and neck for finishing, without the neck giving way or rupturing. Indeed, as it is, the undue proportion often causes the shaft to be strained at this part, where most strength should be, so that it is rendered weak, and a flaw is developed which by-and-by causes it to be removed from the steamer as dangerous and useless, if indeed it does not break outright; so that the forgeman, if he adopts this method, must be very careful to proportion the amount of iron he has ma.s.sed in the furnace to the size of the body he is finishing, otherwise the weakening above mentioned will take place. All marine engineers will easily recognise this defect, which frequently occurs, but the cause of which is probably not well understood. Such a flaw will present a similar appearance to that shown at F, Fig. 3033, taken from an actual example.

[Ill.u.s.tration: Fig. 3028.]

[Ill.u.s.tration: Fig. 3029.]

"This difficulty of proportioning the part of the crank first forged to the size of the neck, will be still better understood by the appearance of it in the furnace, as shown in Fig. 3028. Having reached this stage, with one end of the shaft completed, as also that portion of the crank itself which of necessity was completed before the collar was cut, in order that the neck might be finished, no more iron can be added on the top edge, as it is up to the full depth already; it must therefore be added on the flat, as in Fig. 3029, where the piece is shown on its flat side in the furnace, the finished portion being outside the furnace door. A number of slabs S S S are then placed side by side to bring out the width of the crank further; these being welded down, the piece is turned upside down, and the process repeated on the other side.

Afterwards other slabs are similarly placed on both sides, as shown in Fig. 3030, of which one is the flat, and the other is the edge view of the crank at this stage; and this is continued until sufficient iron has been ma.s.sed to allow of the other gable of the crank being cut down, as at A, Fig. 3031, and sufficient also to allow of the other part of the body B being rounded and prepared for further piecing out.

[Ill.u.s.tration: Fig. 3030.]

[Ill.u.s.tration: Fig. 3031.]

[Ill.u.s.tration: Fig. 3032.]

"Now it will be observed that the first gable finished has the slabs all welded on the edge of the crank, and the hammering has all been on the edge; hence the subsequent hammering on the flat has a tendency to open up the weldings, if they have not been thoroughly made. A section taken at A B, Figs. 3028 and 3029, will show as in Fig. 3032, on the left, the weldings being across the web of the crank; the circle indicates the section which the crank pin would present if cut through there. But when the slabs are placed on the flat afterwards, some of the joinings of the ends of the slabs, or "scarf ends," are certain to fall within the crank pin, as seen in Figs. 3028 and 3029; therefore the section through C D, Fig. 3030, will show somewhat like Fig. 3032 on the right, and the crank pin necessarily includes some of these flaws. The flaw thus produced, called 'a scarf end in the pin,' is readily recognizable by all marine engineers; at F, Fig. 3033, is a sketch from an actual occurrence.

"When the second gable is cut, and the other end is rounded, there is only the other collar to be put on (if a double-collared shaft), and the forging is completed.

[Ill.u.s.tration: Fig. 3033.]

This method is so speedy, compared with any other, that it is often resorted to even at the risk of making a bad forging; and too many broken shafts testify to the fact. Besides, it may be observed that in making a double crank shaft, while the one crank may be made in this way, the other must; for, the first crank, A, Fig. 3033, being completed, and the body, B, between the two cranks, also completed, the second crank, C, must of necessity be pieced off this body, even at the risk of the neck N being strained. This may account for the many instances in which one of the cranks of a double crank shaft gives way, rendering the shaft useless; and also for the plan, now almost universal, of making the two cranks separately and coupling them together; a further object being, no doubt, to have the means of replacing a defective half, if need be, without losing the whole shaft.

"At Lancefield, when a double crank shaft is to be made, the after crank, A, is first made by the method afterwards described, so as to insure that this crank, through which, as being next the propeller, all the power of the engine pa.s.ses, is perfectly sound; and in piecing the other crank off the body, it is worked with slabs on the flat instead of on the edge, as afterwards described.

"The writer's own opinion is that the crank is the most important part of the shaft, and, therefore, at all costs, should be made first.

Others, no doubt, may take the same view, and, to avoid the risks just mentioned, may adopt the process described in the second method.

[Ill.u.s.tration: Fig. 3034.]

[Ill.u.s.tration: Fig. 3035.]

"_Second Method._--This method builds the middle first, and is called "turning the shaft end for end." The shaft is begun from a stave, by the addition of slabs, as shown in Figs. 3034 and 3035; Fig. 3034 shows it with iron added in slabs, till a b.u.t.t is formed, as at B, to form the nucleus of the crank; slabs S S S are then piled on it to bring the crank up to the height.

[Ill.u.s.tration: Fig. 3036.]

"These are beaten down and welded, and more are added, as at S S S, Fig.

3035, till the full height of the crank is reached. Should the web (or edgeway of the crank) be thick, two slabs are frequently used to make up the breadth, placed edge to edge, as shown in Fig. 3035 on the right hand of the figure; the widths of these slabs are limited by that at which the shinglers can conveniently work and turn them under the steam hammer. The crank, however, is completed without any "side slabs," for the beating down of the slabs on the edge will broaden out the ma.s.s, and give sufficient material to forge out the crank to the proper height by hammering on the flat. The crank is afterwards cut at the off gable at G, Fig. 3036, the body B pieced out and rounded, the collar welded on, and then a small stave S is drawn upon the end, to enable the forgeman to handle the piece when he "turns it end for end" to complete the other end of the shaft.

[Ill.u.s.tration: Fig. 3037.]

"This method, though better than the last, is also objectionable; for though there is not equal risk of 'scarf ends' in the pin, yet the weldings are all on the edge, as in the lower view, in Fig. 3036, where the section of the crank pin is shown by the dotted circle; and the cheeks of the crank, O O, are thus liable to give way if a heavy strain comes on the crank when at work. The defects arising from this cause are shown in Fig. 3037, and will be readily recognised by all engineers.

"_Third Method._--Considerations such as these have led to the adoption of the third or Lancefield method.

[Ill.u.s.tration: Figs. 3038 and 3039.]

"Fig. 3038 shows the piece begun from the stave in the usual way, with the slabs all welded, however, on the flat, till a basis is formed for the building up of the crank. A portion A is roughly rounded to form the one end of the shaft, and the b.u.t.t of the crank will present the appearance of a slightly elongated square, as shown at B, Fig. 3039. The workman then "scarfs" or hollows it down at one edge all along the side, as indicated in the end view by the dotted line from C to D; it will then present the appearance shown by the end view, Fig. 3040, being somewhat bulged outward at the points E and F. Three long thin slabs, Fig. 3042, shaped for the purpose, are then placed on the hollowed part, the piece lying flat in the furnace. These slabs are tapered a little the broad way, not on the length, and little pieces of iron are interposed between them, to keep the surfaces apart, and allow the flame free access between them. The object of making them thin is that they may be all equally heated, which is not so readily achieved when the slabs are thick; and the object of the tapering is to allow the slag to flow out freely when the uppermost slab is struck by the steam hammer.

The surfaces thus get solidly welded.

[Ill.u.s.tration: Figs. 3040 and 3041.]

[Ill.u.s.tration: Fig. 3042.]

"Fig. 3041 represents the slabs thus placed in elevation, and the figure on the right, in section. The slabs are forged long enough to go right across the whole width of the crank, excepting about 6 inches; this margin is necessary to allow of the lengthening out of the slabs to the whole width under the process of forging. After these slabs are perfectly welded, the piece is turned upside down, and the process is repeated on the other side, as shown in Fig. 3042. When welded down the ma.s.s has increased in depth as well. Another scarfing takes place on the first side, and then another on the second side, as shown in the figure, and so on, till the full size is obtained; and it will be seen, as in the right-hand view in Fig. 3042, that by this process of "scarfing"

equally from, both sides, the iron from the very middle of the body of the shaft is drawn up quite to the crank pin. The location of the pin is indicated by A A, and it will be seen that by no possibility can there be a "scarf end" in the crank pin, as the slabs in all cases go right across the crank, and also that the cheeks of the cranks have no edge weldings crossing them, as in the previous cases; for the tail of a slab may be at R, Fig. 3042, while the other end may be at S. The fibre is also developed by the continuous drawing up of the iron consequent upon the repeated flat scarfings across the whole width of the crank. When the crank has been thus ma.s.sed sufficiently large, it is cut at the gable, with sufficient material left to piece out the other body of the shaft. This is now done, the coupling welded on, and a small stave drawn on the end to enable the forgeman to manipulate it, when it is turned end for end, to complete the other end.

"These proceedings occupy longer time than either of the other two methods, and consequently costs a little more; but the advantage is well worth all the difference, as greater confidence can be entertained that the forging is every way satisfactory. In brief, by making the crank first, is avoided the liability to weakness at the neck, characteristic of the forgeman's making the shaft before him, as in the first method; by the repeated 'side scarfing' is avoided the liability to fracture across the cheeks, consequent upon the edge weldings of both first and second methods; while by having the slabs the whole length of the width of the crank, any 'scarf end' in the length way of the crank pin is impossible (such as may occur in the first method); and the welding of the ma.s.s of the crank being wholly on the flat must tend to form a more solid forging than if hammered otherwise. Thus, if the forging is well heated and properly hammered, the system promises to insure that no weak part will be found in the shaft after it is finished and put to work.

The writer believes, from the success which has already followed in every case the adoption of this method, that it will eventually be found that almost more depends on the mode in which a crank shaft forging is constructed than on the material of which it is made.

"This leads him to some observations regarding the material for such shafts. It is of course well known that in the early days of engineering, before the time when steam navigation had received a great impetus by the invention of the screw propeller, the connecting rods, cranks, shafts, &c., of land engines were all formed of cast iron; except, indeed, where the connecting rods were made of wood, strapped with plates of wrought iron, as frequently was the case with pumping, winding and blowing engines. In fact, all the parts that could be made of cast iron were so made, and the piston rods, bolts, keys, straps, and other smaller parts were alone made of malleable iron, the smaller pieces being made from rolled bars direct, as at present, and the larger made of similar bars, but placed side by side and bound together or 'f.a.goted,' as they were called, from their resemblance to a bundle of f.a.gots. These bars, thus f.a.goted, were either brought to a welding heat in a smith's hearth and welded under the sledge-hammers of the men called 'strikers,' or hammermen; or else heated in a furnace, and welded under the tilt hammer worked by a steam engine. By-and-by it was found necessary to adopt the stronger material, wrought iron, for parts. .h.i.therto confined to cast iron, because the latter was found too deficient in cohesion to stand the strains due to the power of high-pressure steam, which was now almost universally superseding the use of low-pressure steam in the condensing engine. The system of f.a.goting, however, was still carried out, even far into the history of marine engineering; but when the rapid increase in the dimensions of engines, both stationary and marine, called forth the steam hammer, and so rendered the forging of heavy ma.s.ses comparatively easy, the system of f.a.goting fell into disuse, for the following reason: In making up a f.a.got, say, of 18 inches or 20 inches square, it was found, that in the furnace the outside bars would reach a welding heat much sooner than those in the middle; consequently on welding this f.a.got under the steam hammer, though the blow might reach to the centre, yet the interior would not be welded, while the surface was; hence the shaft or other forging would not be welded throughout, and it was no uncommon thing for a shaft to break and expose the internal bars quite loose and separate from each other.

"When it was seen that malleable was so much superior to cast iron, and that the system of f.a.goting was so imperfect, the adoption of 'sc.r.a.p iron,' which was then composed princ.i.p.ally of parings of boiler plates, pieces of cuttings from smiths' shops, old bolts, horseshoes, angle iron, &c., became general. These being piled together in suitable pieces, and in a pile of suitable size, for the convenience of working, were brought to a welding heat, and beaten out into a slab, or oblong-shaped piece, ready for the forgeman; who would build two or three together, adding more when required, and so bring out his piece to a sufficient size to enable him to shape his forging out of it. Then it was that engineers, seeing what an increase of strength they obtained by these means, invariably specified on their drawings (as many of them still do), 'These forgings are to be made of carefully selected sc.r.a.p iron, free from flaws and defects.'

"To meet the requirements of their customers, therefore, forge-masters had now nothing to do but to select and use the best available sc.r.a.p iron; but the universal adoption of iron hulls in place of wooden ones, conjoined with the rapid and unprecedented increase in steam navigation, soon introduced a cla.s.s of sc.r.a.p iron which did not possess the qualifications of good sc.r.a.p, and also called for a very much greater supply of forgings than could be obtained in superior sc.r.a.p iron. The consequence was that shafts of sc.r.a.p iron, when turned and finished, became liable to exhibit streaks and seams, not due alone to imperfect welding in the forging, but likewise to the laminations and imperfections of the original sc.r.a.p iron, which the process of piling and shingling into the slab was not sufficient to obliterate. So constantly does this yet occur that it causes a strong temptation to make such forgings of new iron puddled direct from the pig and then shingled into slabs or blooms, under the idea that these streaks and seams will thus be avoided, and that the iron will be improved almost to the condition of sc.r.a.p iron, while being forged under the steam hammer.

This, however, is found not to be the case. The forging is certainly free from the streaks of the sc.r.a.p iron, but this is obtained at the expense of strength; for the material is too raw; it wants cohesion, and has not had the proper kind or amount of working to bring it to the condition of superior wrought iron. This method is still further tempting, inasmuch as it is far cheaper than the other; the material costs less than sc.r.a.p iron, and, as it welds at a lower temperature, a forging can be much more quickly and easily made. Still, for whatever cla.s.s of machinery it may be fitted, it should certainly be renewed in every case for a crank shaft or propeller shaft.

"From these considerations it has been the custom at Lancefield, in the preparation of the iron for crank shafts, to improve upon the ordinary condition of the sc.r.a.p iron in the following manner: The pile is made up of carefully cleaned and selected sc.r.a.p; it is brought to a welding heat, and then hammered under the steam hammer. But instead of being beaten into a flat slab for the forgeman, it is beaten into a square billet, which is afterwards rolled in the rolling-mill into a flat bar, as if for 'best best' merchant iron. By this additional heating, hammering and rolling, all the different qualities of the sc.r.a.p iron composing the pile are merged into one h.o.m.ogeneous material, having the fibre given to it that was lost in the separated portions of the sc.r.a.p iron; and this, when cut up into proper lengths, and again piled and shingled into the slab, results in a material possessing somewhat the closeness and density of steel, while retaining all the toughness and tenacity of superior malleable iron. The improved method of constructing the forging, previously detailed, is worthy the use of this superior material; and both having been adopted at Lancefield with results which have commended themselves so unmistakably to many engineers, that they now not only specify the material, but stipulate for the mode of manufacture, it is thought the system has only to be more widely known in order to be universally adopted. It is certain to give greater confidence in the endurance of such important parts of the machinery, although this confidence may have to be obtained by a small increase in the cost, due to the extra workmanship both on the material and on the forging.

"When we take into consideration the vastly accelerated speed of the marine engine in late years, and the many disastrous effects which follow the breaking of a shaft at sea--also that the tendency of the age is still towards much higher pressures, and further lengthening of stroke it is not surprising that improvement in such an important part as the crank shaft should be eagerly sought after; but it has. .h.i.therto been sought in the direction of the material alone. Cast steel has been advocated, and brought to some extent into use; but its expense renders such shafts costly out of all proportion to the other parts of the engine; while, in the event of their heating when at work (a very frequent casualty), and having the water-hose directed upon the crank pin or journals, it cannot be expected that the material will behave any better, or even so well, as tough wrought iron. What is termed puddled steel is liable to the same objection, and probably, from its mode of manufacture, in a still greater degree. The so-called mild steel is no doubt proving itself a superior material, and yielding good results when rolled into ship or boiler plates. But thus prepared it is more costly than 'rolled sc.r.a.p bar;' and if not rolled, but cast into an ingot, then it possesses some of the crystalline characteristics of steel, with all the disadvantages attending its manipulation into a forging.

"For extra large crank shafts, the fear of unsoundness, arising from the ordinary mode of forging, has led some engineers to consider the propriety of building the shafts and cranks in separate pieces. This, with engineers generally, has not hitherto been looked upon with favor; as the fewer the pieces the more rigid the shaft. Moreover, the increased weight necessitated by this separate building is viewed as a disadvantage, even although it were not attended with greater cost, as undoubtedly it is.

"The material and mode of manufacture advocated in this paper may tend to dissipate some of these apprehensions. They will not obviate defective construction in the engines themselves, or faulty proportion of their parts, or neglectful supervision of their working, but they will reduce to a minimum the risk of breakage in such untoward circ.u.mstances. If any objection be taken on the score of extra size, the enterprise which a quarter of a century ago engaged in the making of the unusually large shafts necessary for the 'Great Eastern' may still be trusted to meet the advancing requirements of the present day."

Fig. 3043 represents a foot-power hammer or Oliver. The hammer is upon a shaft in bearings, and is held in the position shown by an open coiled spring. On the shaft is a chain pulley, the other end of the chain being connected through a leather strap to the treadle. Means are provided to adjust the height to which the hammer will lift to bring the hammer face fair with the work and to give the required degree of tension to the spring.

Fig. 3044 represents a Standish's foot-power hammer, in which the hammer and the anvil are provided with dovetail seats for receiving dies, swages, &c. The force of the blow is regulated by the height to which the hammer is raised, which may be adjusted by the nuts beneath the spiral springs. The handle on the hammer is for pulling the hammer down by hand when adjusting the lower die fair with the upper one.

What are known as power hammers are those driven by belt and pulley; while those known as trip hammers have their helve lifted through the medium of revolving lugs or cams. Steam hammers are those in which the hammer is lifted by a piston in a steam cylinder; while in hydraulic hammers, the hammer is moved by water pressure.

Fig. 3045 represents a Justice's power hammer, in which the hammer is guided in a slideway and is operated by leather straps attached to the ends of a spring, at the crown of which is attached a connecting rod driven by a crank disk. The stroke is altered by means of placing the crank pin in the required position in the slot in the crank disk. By means of gibs the hammer may be set to match the dies. The pulley is provided with a friction clutch operated by the treadle, shown.

Fig. 3046 represents a Bradley's Cushioned Hammer, in which motion is obtained by a belt pa.s.sing over a pulley on a crank shaft, whose connecting rod R is capable of adjustment for length, so as to govern the distance to which the hammer shall fall, which obviously varies with different sizes of work. The hammer is lifted through the medium of a rubber cushion A, seated in a casting to one end of which is connected the rod R, while the other end is pivoted. The lever to which the hammer is affixed is raised against the compression of the rubber cushion B, and at the top of its stroke also meets the rubber cushion C; hence these two cushions accelerate its motion after the crank has pa.s.sed its highest point of revolution. The cushion D prevents the rebound of the hammer after the blow is struck; hence as a result of these cushions, heavy or light blows may be struck with great rapidity and regularity.

The weight W is on a lever that actuates a break upon the wheel shown at the side, so as to enable the stopping of the hammer quickly. The machine is put in motion by pressing the foot upon the treadle T, which operates a belt tightener, the belt running loose when the treadle is released.

[Ill.u.s.tration: Fig. 3043.]

[Ill.u.s.tration: Fig. 3044.]

[Ill.u.s.tration: Fig. 3045.]