Figs. No. 1 and No. 2 show the design of the old and new main bearings, and, I think, require but little explanation. Most of you present will remember your feelings when, after a hot bearing, the brasses were found to be cracked at top and bottom, and the trouble you had afterward to keep these brasses in position. When a smoking hot bearing occurred, say in the heating of a crank pin, it had the effect of damaging the material of the shaft more or less, according to its original soundness, generally at the fillets in the angles of the cranks. For when the outer surface of the iron got hot, cold water, often of a low temperature, was suddenly poured on, and the hot iron, previously expanded, was suddenly contracted, setting up strains which in my opinion made a small tear transversely where the metal was solid; and where what is termed lamination flaws, due to construction, existed, these were extended in their natural direction, and by a repetition of this treatment these flaws became of such a serious character that the shafts had to be condemned, or actually gave way at sea. The introduction of the triple expansion engine, with the three cranks, gave better balance to the shaft, and the forces acting in the path of the crank pin, being better divided, caused more regular motion on the shaft, and so to the propeller. This is specially noticeable in screw steamers, and is taken advantage of by placing the cabins further aft, nearer the propeller, the stern having but little vibration; the dull and heavy surging sound, due to unequal motions of the shaft in the two-crank engines, is exchanged for a more regular sound of less extent, and the power formerly wasted in vibrating the stern is utilized in propelling the vessel. In spite of all these improvements I have mentioned, there remains the serious question of defects in the material, due to variety of quality and the extreme care that has to be exercised in all the stages during construction of crank or other shafts built of iron. Many shafts have given out at sea and been condemned, through no other cause than original defects in their construction and material.
The process of welding and forging a crank shaft of large diameter now is to make it up of so many small pieces, the best shafts being made of what is termed scrap, representing thousands of small pieces of selected iron, such as cuttings of old iron boiler plates, cuttings off forgings, old bolts, horseshoes, angle iron, etc., all welded together, forged into billets, reheated, and rolled into bars. It is then cut into lengths, piled, and formed into slabs of suitable size for welding up into the shafts. No doubt this method is preferable to the old method of "fagoting," so called, as the iron bars were placed side by side, resembling a bundle of fagots of about 18 or 20 inches square.
The result was that while the outside bars would be welded, the inside would be improperly welded, or, the hammer being weak, the blow would be insufficient to secure the proper weld, and it was no uncommon thing for a shaft to break and expose the internal bars, showing them to be quite separate, or only partially united. This danger has been much lessened in late years by careful selection of the materials, improved methods of cleaning the scrap, better furnaces, the use of the most suitable fuels, and more powerful steam hammers. Still, with all this care, I think I may say there is not a shaft without flaws or defects, more or less, and when these flaws are situated in line of the greatest strains, and though you may not have a hot bearing, they often extend until the shaft becomes unseaworthy.
[Diagrams shown illustrated the various forms of flaws.] These flaws were not observable when the shafts were new, although carefully inspected. They gradually increased under strain, came to the outside, and were detected. Considerable loss fell upon the owners of these vessels, who were in no way to blame; nor could they recover any money from the makers of the shafts, who were alone to blame. I am pleased to state, and some of the members here present know, that considerable improvement has been effected in the use of better material than iron for crank shafts, by the introduction of a special mild steel, by Messrs. Vickers, Sons & Co., of Sheffield, and that instead of having to record the old familiar defects found in iron shafts, I can safely say no flaws have been observed, when new or during eight years running, and there are now twenty-two shafts of this mild steel in the company's service.
I may here state that steel was used for crank shafts in this service in 1863, as then manufactured in Prussia by Messrs. Krupp, and generally known as Krupp's steel, the tensile strength of which was about 40 tons per square inch, and though free from flaws, it was unable to stand the fatigue, and broke, giving little warning. It was of too brittle a nature, more resembling chisel steel. It was broken again under a falling weight of 10 cwt. with a 10 ft. drop = 12½ tons.
The mild steel now used was first tried in 1880. It possessed tensile strength of 24 to 25 tons per square inch. It was then considered advisable not to exceed this, and err rather on the safe side. This shaft has been in use eight years, and no sign of any flaw has been observed. Since then the tensile strength of mild steel has gradually been increased by Messrs. Vickers, the steel still retaining the elasticity and toughness to endure fatigue. This has only been arrived at by improvements in the manufacture and more powerful and better adapted hammers to forge it down from the large ingots to the size required. The amount of work they are now able to subject the steel to renders it more fit to sustain the fatigue such as that to be endured by a crank shaft. These ingots of steel can be cast up to 100 tons weight, and require powerful machines to deal with them. For shafts say of 20 inches diameter, the diameter of the ingot would be about 52 inches. This allows sufficient work to be put on the couplings, as well as the shaft. To make solid crank shafts of this material, say of 19 inches diameter, the ingot would weigh 42 tons, the forging, when completed, 17 tons, and the finished shaft 11¾ tons; so that you see there is 25 tons wasted before any machining is done, and 5¼ tons between the forging and finished shaft. This makes it very expensive for solid shafts of large size, and it is found better to make what is termed a built shaft; the cranks are a little heavier, and engine framings necessarily a little wider, a matter comparatively of little moment. I give you a rough drawing of the hydraulic hammer, or strictly speaking a press, used by Messrs. Vickers in forging down the ingots in shafts, guns, or other large work. This hammer can give a squeeze of 3,000 tons. The steel seems to yield under it like tough putty, and, unlike the steam hammer, there is no jarring on the material, and it is manipulated with the same ease as a small hammer by hydraulics.
The tensile strength of steel used for shafts having increased from 24 to 30 tons, and in some cases 31 tons, considering that this was 2 tons above that specified, and that we were approaching what may be termed hard steel, I proposed to the makers to test this material beyond the usual tests, viz., tensile, extension, and cold bending test. The latter, I considered, was much too easy for this fine material, as a piece of fair iron will bend cold to a radius of 1½ times its diameter or thickness, without fracture; and I proposed a test more resembling the fatigue that a crank shaft has sometimes to stand, and more worthy of this material; and in the event of its standing this successfully, I would pass the material of 30 or 31 tons tensile strength. Specimens of steel used in the shafts were cut off different parts—crank pins and main bearings—(the shafts being built shafts) and roughly planed to 1½ inches square, and about 12 inches long. They were laid on the block as shown, and a cast iron block, fitted with a hammer head ½ ton weight, let suddenly fall 12 inches, the block striking the bar with a blow of about 4 tons. The steel bar was then turned upside down, and the blow repeated, reversing the piece every time until fracture was observed, and the bar ultimately broken. The results were that this steel stood 58 blows before showing signs of fracture, and was only broken after 77 blows. It is noticeable how many blows it stood after fracture. A bar of good wrought iron, undressed, of same dimensions, was tried, and broke the first blow. A bar cut from a piece of iron to form a large chain, afterward forged down and only filed to same dimensions, broke at 25 blows. I was well satisfied with the results, and considered this material, though possessing a high tensile strength, was in every way suitable for the construction and endurance required in crank shafts.
Sheet No. 1 shows you some particulars of these tests:
| Tensile Tons. | Elong. in 5" | Bend. | Fractured Blows. | Broke Blows. | Fall In. | |
| A = | 30.5 | 28 p. c. | Good | 61 | 78 | 12 |
In order to test the comparative value of steel of 24¾ up to 35 tons tensile strength, I had several specimens taken from shafts tested in the manner described, which may be called a fatigue test. The results are shown on the same sheet: