1.59, or an increase in the transverse strength of 59 per cent., as before. (940.) Another important question connected with this subject is to determine the effect of Stirling's process on Cast iron in girders of ordinary sections, Mr. Berkley's experiments (897) having shown that with ordinary iron, the strength of girders is not simply proportional to the transverse strength of small test-bars cast from the same metal. Table 68 gives the result of experiments by Mr. Owen, H.M. Inspector of Metals, on a large girder of Mr. Hodgkinson's form, Fig. 79; there were 13 experiments on common cast iron of different kinds, and 11 with Stirling's iron, or cast iron mixed with different proportions of wrought-iron scrap, varying from 17 to 33 per cent. of the cast iron, the mean of the whole being 22 per cent. The mean breaking weight with common cast iron was 38.3 tons, and with Stirling's iron 52.3 tons, the ratio is 1 to 52.3÷ 38.3 1.366, showing an increase of 36.6 per cent. only, whereas the small test-bars gave, as we have seen in (938), an increase of 60 per cent. (941.) But, analysis of the details of these experiments will show that the effect of Stirling's process varies very much with the strength of the particular cast iron to which it is applied, weak iron being very greatly improved in strength, while very strong irons are scarcely affected at all. Thus, with the Calder, which is a very weak iron, experiments 2 and 4 give 33 and 34 tons respectively, the mean being 33.5 tons :-by experiments 14, 18, 21, and 24, this same iron mixed with 25 per cent. of wrought-iron scrap gave 48, 52, 52, and 60 tons respectively, the mean being 53 25 tons, and we have 53 2533·5 = 1.59, or an increase in strength of 59 per cent., agreeing very nearly with that given for rectangular bars by Table 143, which was 60 per cent., and agreeing exactly with the calculations in (939). But in experiments 12 and 13 we have a strong mixture of irons which gave 47 and 471 tons respectively, the mean being 47.1 tons, whereas Calder iron gave 33.5 tons only. Now this strong iron mixed with 20 per cent. of wrought-iron scrap, gave in experiment 15 a breaking weight of 48 5 tons, and we have 48.547.1 1·03, an increase in strength of 3 per cent. only. In accordance with these facts, Mr. Stirling states that the special object he had in view was to raise the weaker irons to the strength of the strongest; generally speaking, he finds that Scotch iron requires a larger proportion of wrought iron than Staffordshire, and Welsh least of all. For No. 1 Scotch he recommends from 18 to 21 per cent.; No. 2, from 27 to 36 per cent.; No. 3 he does not recommend at all. But it should be observed that the section of girder adopted by Mr. Owen, with bottom and top flanges having areas as 6 to 1, was specially adapted to give maximum results with ordinary cast iron whose crushing and tensile strengths were in that same ratio. But with Stirling's iron, the ratios of those strengths is 55.7 12.46 4.47 to 1·0, and the flanges should have had that ratio in order to obtain a maximum effect. (942.) Experiments have shown that the strength of Stirling's iron is affected by the size or thickness of the casting, as we found to be the case with ordinary cast iron. Thus Calder iron with 42 per cent. of wrought iron and in bars 1 inch, 11⁄2 inch, and 2 inches square, gave as the value of MT, or specific transverse strength, 3514, 2895, and 2754 lbs. respectively; the ratios being 1.0, 824, and 784 respectively. The rule in (934) gives for these same sizes with ordinary cast iron, the ratios 1.0, 838, and ·739; from which it appears that the effect of size or thickness of casting is practically the same for Stirling's as for ordinary cast iron. ON THE STRENGTH OF WHEEL-TEETH. (943.) The tooth of a wheel may be regarded as a simple cantilever, and where the strain upon it is known, as for example with the gearing of a crane, it appears to be a very simple matter to calculate the strength and to adapt it to the strain. But while this may be done satisfactorily for a dead load such as that on the large 1st motion wheel of a crane, it will be found not to apply to the other wheels of the train, for although the motion is a very slow one, the strength seems to be governed by the laws of Impact, or of forces in motion, which differ entirely from those dominating a statical force or dead load. Examples of the proper method of calculating the strength of the teeth in a train of crane-wheels are given in (594) and (598), therefore need not be repeated here; but we will consider the strength of wheels carrying the power of Steamengines or other motors,—an important matter which is fully considered in the Author's Treatise on 'Mill-Gearing.' For iron-and-iron toothed wheels we have the Rules : (944.) (945.) HN = √D x R × p2 × w × ·043. H1 = √D x R x px w x 0645. For Mortise Wheels the Rules become : (946.) (947.) H1 = √ D x R x p2 x w x 075. In which HN Nominal Horse-power; H1 = =net Indicated Horse-power; D = diameter of the wheel at pitch line in feet; p = pitch in inches; w width in inches; R = revolutions per minute; My and M, Multipliers for nominal and Indicated Horse-power respectively. The relations of the Nominal and Indicated powers are explained and illustrated in (572). = = = Thus, an iron toothed wheel 6 feet diameter, 5 inches wide, 2 inches pitch, 24 Revolutions, gives Hy√√6 x 24 × 22 × 5 x 043 10.32 Nominal Horse-power. Again; a spur mortise wheel 4 feet diameter, 24 inches pitch, 7 inches wide, 30 Revolutions, gives by Rule (947), H1 = √4 x 30 x 21 x 7 x ⚫075 36 Net indicated Horse-power, or by Rule (946), HND x R x 22 x 7 x .05 24 Nominal Horsepower, &c. = The width of wheels on the face is to a great extent arbitrary; a good proportion is given by the Rule: = In which Ρ pitch, and w = width in inches: col. 2 of Table 144 has been calculated by that rule: now, multiplying p2 by the width thus found we obtain the ratios of power in col. 3, which shows how rapidly the power rises with the pitch, being in fact proportional to the 3 power of the pitch, or p3.5, and col. 4 has been calculated by that rule. Thus a wheel of any diameter and revolutions which with 1-inch pitch gives 1 Horse-power, would with 4-inch pitch, &c., give 128 Horsepower, &c. TABLE 144.-Of the RATIO of the POWER of TOOTHED Wheels. (949.) "Crank-pins."-The strain on the crank-pin of a Steam-engine may be found with sufficient accuracy from the area of the piston, pressure of steam, &c. Then, regarding the pin as a cantilever, its strength will be directly proportional to d3 and inversely as the length, but inasmuch as the length is usually proportional to the diameter, the strength is reduced to d2 simply, and we have the empirical Rule: |
