the rivet by the sum of these two quantities gives the diagonal pitch. This rule may be formulated as follows: (Pitch )+(diameter of rivet X)= diagonal pitch. A more simple form of this rule, however, is as follows: Rule. To six times the pitch add four times the diameter of the rivet, and divide by 10, or placed in formula: This rule does not, however, apply to joints having rows of rivets that are not all spaced alike; thus the diagonal pitch of the joint, in Fig. 21, in which every other rivet is omitted in the outer row, is found by the following rule. For the diagonal pitch in inches between the inner and middle row of rivets, the formula is: Wide pitch × 3 + (diameter of rivet X 4 divided by 10 diagonal pitch between inner and middle row of rivets. As the wide pitch is in this case 8 inches, and the rivet diameter 1.25 or 1 inches in diameter, we have: The diagonal pitch of the outer row of rivets is obviously the same, being worked from the middle row. -0:0 PART III. CALCULATING THE WORKING PRESSURES FOR BOILER SHELLS. The rules of the Board of Trade for calculating the working pressure (W. P.) of the cylindrical shell of a boiler, are based upon the following: For the highest class of boiler (or in other words, for a boiler proportioned in what the Board consider the best possible manner, and built under the best description of workmanship), they require a factor, or margin of safety of 5, or in other words, for such a boiler they will grant a certificate allowing the use of the pressure at which the boiler would, (according to calculation), burst. For every deviation from this quality of design or workmanship, which they consider inferior, an increased factor of safety is required, and this correspondingly reduces the W. P. (working pressure) allowed. The highest class of boiler is deemed to be one in which all the rivet holes are drilled after the shell plate has been bent into its circular form. The plates are then taken apart and all the burrs removed from around the edges of the drilled holes. All the seams must be butt jointed, and covered by double butt straps at least double rivetted. The boiler must be constructed under the inspetion of one of the Board's surveyors, and tested to twice the W. P. by hydraulic pressure, in his presence. For every departure from this highest class of construction and work manship an addition is made to the factor of safety of 5. Thus if the holes were punched instead of being drilled, if the plates were drilled before being bent, if single instead of double butt straps were employed, or if any other element of weakness were introduced in either the design or workmanship of the boiler, the factor of safety of 5 would be increased in a proportion deemed to be in accordance with the deterioration caused by the departure from the rules, the proportionate increase being laid down in the rules, as follows: Af .15 B C .3 .3 D .5 E* F .75 .1 Gt 15 Η It J* K .15 .2 .2 To be added when all the holes are fair and good in the longitudinal seams, but drilled out of place after bending. To be added when all the holes are fair and good in the longitudinal seams, but drilled out of place before bending. To be added when all the holes are fair and good in the longitudinal seams, but punched after bending instead of drilled. To be added when all the holes are fair and good in the longitudinal seams, but punched before bending. To be added when all the holes are not fair and good in the To be added if the holes are all fair and good in the circum- To be added if the holes are fair and good in the circumfer- To be added if the holes are fair and good in the circumferential seams, but punched before bending. To be added if the holes are not fair and good in the circumferential seams. .2 To be added if double butt straps are not fitted to the longitudinal seams, and the said seams are lap and double rivetted. L .1 M N O P .3 .15 1. P 1 Q To be added if double butt straps are not fitted to the longi- To be added if the circumferential seams are fitted with .2 To be added if the circumferential seams are fitted with single butt straps, and are single rivetted. R 1 To be added if the circumferential seams are fitted with double butt straps and are single rivetted. S .1 To be added if the circumferential seams are lap joints and are double rivetted. T .2 To be added if the circumferential seams are lap joints and W* .25 are single rivetted. To be added when the circumferential seams are lap and the strakes of plates are not entirely under or over. .3 To be added when the boiler is of such a length as to fire from both ends, or is of unusual length, as in the case of flue boilers; and the circumferential seams fitted as described opposite P, R, and S, but of course when the circumferential seams are as described opposite Q and T, V .3 will become V .4. .4 X* .4 To be added if the seams are not properly crossed. To be added when the iron is in any way doubtful, and the Surveyor is not satisfied that it is of the best quality. Ytt 1.65 To be added if the boiler is not open to inspection during the whole period of its construction. Where marked the allowance may be increased still further if the workmanship or material is very doubtful or very unastisfactory. When the holes are to be rimered or bored out in place, the case should be submitted to the Board as to the reduction or omission of A, B, G, and I as heretofore. When boilers are comparatively short the cases should be submitted to the Board for consideration as to the omission of V as heretofore. When the circumferential seams are double strapped and double rivetted, or lapped and treble rivetted, and the calculated strength not less than 70 per cent. of the solid plate, V .3 may be omitted as heretofore. When surveying boilers that have not been open to inspection dur ing construction, the case should be sudmitted to the Board as to the factors to be used. EXAMPLES IN CALCULATING THE WORKING Example 1. Steel Plates. Steel Rivets. Double Rivetted Joints; Longitudinal Seams Fitted with The first example is of a boiler meeting all the Board's highest require. ments, except that the circumferential seams are double rivetted lap joints instead of having double butt straps double rivetted, and for this departure. 1 is added to the factor of safety, making it in this case, 5.1 The particulars of the shell are as follows: Steel shell... Thickness of shell plate. Diameter of steel rivets. Pitch of rivets..... tional square inch. Strength of material allowed as being 28 tons (of 2240 lbs. each) per sec The strength of the rivetted joint is found as follows: Pitch of rivets. = 3.625) 268.7500 (74: = percentage of plate section as 25375 500 compared to the solid plate. We have next to find the percentage of rivet section as compared to the solid plate thus: |