The practical methods of handling the theoretical shaft equations have reference to the 6t of the shaft within the several pieces upon it. The running fit of a shaft in a bearing is usually considered to be so loose that the shaft could freely deflect to the center of the beariug. This is doubtless an extreme view of the case, but it is the only safe assumption. Hence a shaft running in bearings (see Fig. 31) is supposed to be supported at the centers of those bearings, and its theoretical strength is based on this supposition.
For a tight or driving fit upon the shaft, a safe assumption to make is that there is looseness enough at the ends of the fit to permit the shaft to be stressed by the load a short distance within the faces of the hub, say from ½ inch to 1 inch. For example, referring to Fig. 31, suppose P to be the transverse load, exerted through a hub fast upon the part of the shaft d3. Taking moments about the center of one bearing, and solving for the reaction at the center of the other, we have:
P1u = R1 K; or, R1 = P1 u / k (55)
Also, P1 t = R2 K; or, R2 = p1 t / k (56)
Now, as far as the part of abaft d3 is concerned, it may depend for its size on the bending moment R, b, or on R1 a. The reason the lever arm is not taken to the point directly under the load P1, is because it is not practically possible to break the shaft at that point, on account of the reinforcement of the hub, which is tightly fitted upon it. Trying these moments to see which is the greater, we shall find that the greater moment always occurs in connection with the longer lever arm. Hence R, b will be greater than R1 a. We then write the equation of external moment = internal mo-iHtnt: r2 b = Sd 33 / 10.2 ; or, d3 = ∛ 10.2 R2b / S. (57)
For the size of bearing A we have the maximum bending mo-ment:
R1 L1 / 2 = Sd43 / 10.2; or, d4 = ∛ 10.2 R1 L1 / 2 S. (58)
For the size of bearing B we have the maximum moment:
R2 L2 / 2 = S d23 / 10.2; or, d2 = ∛10.2 R2 L2 / 2 S. (59)
The above calculations are, of course, on the assumption that no torsion is transmitted either way through this axle. We should in that case have combined torsion and bending. This has been made sufficiently clear in preceding paragraphs and in Part I, to require no further illustration.
The dotted line in Fig. 31 shows the theoretical shape the axle should take under the assumed conditions. The practical modification of this shape is obvious. At the shoulders of the shaft the corners should not be sharp, but carefully filleted, to avoid the possible starting of a crack at those points.
Often the diameter of certain parts of a shaft may be larger than strength actually calls for. For example, in Fig. 31, the part d3 need only be as large as the dotted line; but it is obvious that unless the key is sunk in the body of the shaft, the hub could not be slipped into place over the part d€. If, however, the diameter d3 be made large enough so that the bottom of the key will clear d4 the rotary cutter which forms the key way in d3 will also clear d„ and the key way can be more easily produced.
In cases where fits are not required to be snug, a straight shaft of cold-rolled steel is commonly used. Here any parts fastened on the middle of the shaft have to be driven over a considerable length of the shaft before they reach their final position. Moreover, there is no definite shoulder to stop against, and measurement has to be resorted to in locating them.
GENERAL ARRANGEMENT OF JONVAL TURBINE. Central Engineering Works. Oldham, Eng.
It does not pay to turn any portion of a cold-rolled shaft, unless it be the very ends, for relieving the "skin tension" in such material is sure to throw the shaft out of line and necessitate subsequent straightening.
Turned-steel shafts for machines may with advantage be slightly varied in diameter wherever the fit changes; and although the production of shoulders costs something, yet it assists greatly in bringing the parts to their exact location, and enables the workman to concentrate his best skill on the fine bearing fits, and to save time by rough-turning the parts that have no fits.
Hollow shaft are practicable only for large sizes. The advantages of removing the inner core of metal, aside from some specific requirement of the machine, are that it eliminates all possibility of cracks starting from the checks that may exist at the center, permits inspection of the material of a shaft, and, in case of hollow-forged shafts, gives an opening for the forging mandrel. In the last case, the material is improved by a rolling process.
The material most common for use in machine shafting is the ordinary "Machinery Steel," made by the Bessemer process. This steel is apt to be "seamy," and often contains checks and flaws that are detected only upon sudden and unexpected breakage of a part apparently sound. This characteristic is a result of the process employed in the manufacture of the steel, and thus far has never been wholly eliminated. Bessemer steel is, nevertheless, a very useful material, and the above weakness is not so serious but that this kind of steel can be used with success in the great majority of cases.
When a more homogeneous shaft is desired, open-hearth steel is available. This is a more reliable material to use than the Bessemer, and costs somewhat more. It makes a stiff, true, fine-surfaced shaft, high-grade in every respect. It is usually specified for armature shafts of dynamos and motors.
Steels of special strength, toughness, and elasticity are made under numerous processes. Nickel steel is perhaps the most conspicuous example. While for this steel a high price has to be paid, yet its great strength, in connection with other valuable qualities, makes it a material extremely valuable for service where light weight is essential, or where contracted space demands small size.
The range of strength of these various steels is so great that it is well-nigh useless to go into a discussion of it here. Reference should be had to the extended discussions of the handbooks, and to special trade pamphlets. A study of the possibilities of steel in its various forms for use in shafting, is very valuable as a basis for design, as it can almost be said that a machine consists chiefly of a "collection of shafts with a structure built round them." The shafts are like a core, and evidently the size of the core determines the shell about it.