US2623508A - Locomotive firebox and bracing means therefor - Google Patents

Description

Dec. 30, 1952 A. TROSS 2,623,508

LOCOMOTIVE FIREBOX AND BRACING MEANS THEREFOR Filed Nov. 23, 1949 INVENTOR ARA/OLD TROSS AGENTS Patented Dec. 30, 1952 LOCOMOTIVE FIREBOX AND BRACING MEANS THEREFOR Arnold Tross, Munich, Germany Application November 23, 1949, Serial No. 128,952 In France September 10, 1948 Claims.

The invention relates to the firebox portion of locomotive boilers, particularly of the Stephenson design. In such a firebox, the outer and the inner shells are connected by crown stays and cross stays on the upper side, by staybolts on the lateral sides, and by the foundation ring at the bottom. The task of the stays and foundation ring is to preserve the shape of the firebox under the stress of high steam pressure and high temperature, and to prevent leakages within the various joints as well as fractures of the various structural elements. This task is difficult to perform, since the inner shell tends to expand more than does the outer shell, because of the much higher, and also less uniform, temperature of the former.

Until recently, the staybolts used for connecting the side sheets were mostly of the rigid type, within the peripheral zones also of the flexible type, with their rigid ends screwed into the sheets and sealed by over-size threads, expanding, and riveting. Sometimes, riveting was later replaced by sealwelding the protruding end of the bolt head to the surface of the sheet, particularly in the case of steel fireboxes. More recently, the thread was abandoned in order to simplify installation and to save tools; plain cylindrical bars or bolts with cylindrical heads were used instead, with their protruding ends fastened to the firebox sheets by welding. The clearance between the staybolt and the corresponding hole, which is necessary for the installation, was kept as small as possible.

The known methods of securing the staybolts to the firebox sheets have not proved satisfactory. The main defects are that, after a short period of service the staybolts first begin to show incipient fissures, then to corrode, and at last to break; the side sheets of the firebox begin to bulge in an outward direction, and, starting from the edges of the drill holes and extending radially into the sheets, cracks begin to appear in the most heavily strained portions of the firebox, particularly in the so-called fire zone. The cracks and fractures necessitate frequent replacement of the damaged staybolts and wall sections, and keep the engine out of service for a considerable length of time.

The present invention is based on the knowledge of the cause of the failures and defects described, and provides meanstopreventthese either completely or very nearly so. Based on thorough preliminary calculation and practical observation, by the inventor, of the mutual interaction of the connected structural elements, it was rec- 2 ognized that the design and the connection of the structural elements of the firebox as hitherto used are not sufficient to prevent permanent deformations which rapidly lead to fatigue failures.

The main cause of the progressing permanent deformation consists in the following process: the lower parts of the side sheets of the inner firebox are particularly exposed to the heat. They are prevented, however, from expanding according to the rise in temperature by being firmly connected to parts of considerably lower temperature; i. e. to the solid foundation ring by a rivet seam; to the outer side sheets by means of the staybolts; and to door sheet and tube sheet by their flanges.

By thus preventing the thermal expansion, a state of stress is caused, which the applicant has termed unrelieved thermal expansion stress. This stress leads to plastic deformation of the individual areas between the staybolts, as well as'of the entire firebox unit-a deformation'termed 'as bulgingwhich also affects the foundation ring and mostly works in an outward direction. The forces caused by the boiler pressure on the outer and inner shells result, as the area of the outer shell is usually larger, in an additional outwardly directed component.

In view of these considerations, the invention shows a new way of construction of a firebox which is fundamentally new. Measures of difierent kind are provided as follows:

(a) The thermal expansion stress is eliminated as the chief reason for the described constant deformations of the construction elements at the points of the boiler connected to each other.

(b) The construction parts connected to each other and their connections are so shaped that the remaining expansions are taken up elastically.

The result is that in spite of the stresses by the boiler pressure and heat acting during long periods of time the service life of the corresponding walls and of the staybolts is materially increased.

By these measures, the economy and, above all, the safety of locomotive operation are decisively increased, and the hitherto inevitable waste of material is prevented. Besides, they result in considerable savings in labor, tools, and other means for conservation and repair work.

The means applied by the invention to prevent permanent deformation of the structural elements of the firebox are exclusively means of design. Some of them refer to the walls, some to the foundation ring, most of them, however, to

the staybolts and the way of fixing these to the wall sheets. They supplement each other and may be applied singly or simultaneously.

Accordingly, the invention aims at an immediate reduction of the stresses within the staybolts by improving their design as well as the method of their installation. This is based on the perception'that failures of 'staybolts may cause heavy strains to the corresponding areas of the shells.

The accompanying drawing shows a staybolt, characterized by cylindrical heads l, 2 and a diameter decreasing towards the cylindrical-middle portion of the shaft, which is rigidly fixed into the firebox by welding. According to one means of the invention, the difference between the diameter 3 of the bolt head and the diameter 4 of the corresponding hole ought to be not 'less than 0.5 mm. and preferably not larger than 2 mm. at least on the side of the inner shell 9. This space takes up that amount of thermal expansion by which the thermal expansion within the fire zone of the inner shell 9 exceedsthe-expansion'of the-outer shell It. Disturbing effects between boltlhead and plate as caused by unrelieved thermal expansion stresses and leading to permanent deformations or even, when the firebox cools down, to cracks at the edges of the holes, are thus safely prevented.

'This measure represents a fully intentional deviation from the common practice of fitting the bolt as tightly into the hole as the installation permits. Relieving the thermalexpansion stress also prevents bulging of the single stay-bolted areas, and reduces bulging of the side sheet a a whole. This, in turn, considerably reduces the S-shaped deflection of the staybolts which results from thelateralshifting of the stay ends, .relative :to each other, in the peripheral zones of the bulgingsheets. Both effects substantially reduce the bending stresses in the sheets and in the stays, and prevent incipient cracks and fractures;

Preferably according to the invention, the shape of the tapered portions of the staybolt shaft is :such that their maximum diameter 5 differs from the minimum diameter tby 4 to 8 mm., and that their length i, contrary to common practice, increases with the length 8 of the shaft. The value of the difference between the diameters dependson the circumstances, 1. e. material used for and diameter of the head of the staybolts. The length of one taper ranges from 0.25 to 0.45 of the total length 8 of the shaft,with the greater length ofthe taper referring to the greater-diameter difierences. staybolts .most commonly used have cylindrical shafts, whiohare sometimes even threaded over thelength. .As regards the relation between the length of the tapered sections and the entire staybolt, it must be emphasized that, with the fewtapered staybolts hitherto known, the tapered length is independent of the total length, and is the same for the whole set of staybolts, of which the lengths vary between approximately 4 and 16 inches for the same boiler. In the design accord- .ing to the inventionthe higher of the given values refer to the greater diameter differences. For a given difference between maximum :and minimum diameters of the tapered portion, there is an approximately constant proportionality be- 1 tween the lengthof the tapered portion and the total length of the shaft, independent of the length-of theentire staybolt.

'The new design simultaneously increases the resilience of thestaybolt, i. e. the strain energy As compared with this type,

4 the staybolt can absorb without exceeding the yield point at operation temperatures. This is extremely important in order to preserve the shape of the firebox as calculation and observation have revealed that the deformation of the firebox begins with a permanent deformation of the staybolts. These form, in a way, the backbone of the firebox.

In order'to 'compensate for the loss in strength caused by the welding heat in the vicinity of the weld seam, and thus to prevent permanent dedesign, of the tapered section at its largest diameter.

If Tb is'the bending tension, Mb the bending momentum and W the momentum of resistance, the following formula must be applied:

Of the various means suited to achieve, ac-

cording to the invention, the effect of inten- W being equal tionally influencing, and particularly of reducing, the permanent deformation for" the structural elements, only some which maybe considered, the most effective have been mentioned, without the invention being in any way'confined to these.

I-claim:

1. In a locomotive boiler having inner and outer firebox shells, staybolts with heads rigidly fastened in the shells :of the .firebox and with a shaft tapering in diameter fromizheirigidly'fastenediheadsito .th'e cylindricalmiddle portion in such a waythat theratio 'Lr/Ls between the tapering length 'Leand the total length of the shaft Ls is, for all staybolt lengths. occurring in the firebox, approximately constant for-a given difference (Dmax-Dmin) between the maximum diameter (Dm'ax)-'a,1'1d. the minimum diameter '(Dmm) o'f thetapered portion,*the numerical value ofthe ratio Lt/Ls being at least 0.25 and at the uttermost 0.45 and the difference between Dmax and Dmin at least 4 mm.and at the uttermost -9 mm.

2. Staybolts according to claim '1, the ratio Li/Lsfor all staybolt lengthsoccurring in the fire-- box being mill-.475 Dmax' 0.035 Balm-0.137), m being a factor the numerical value of which may be between 0.9 and.1.1 forrigidly fixed staybolts.

.3..In a locomotive boiler having inner and outershells, staybolts with heads weld-fixed at .least'to the inner .shelland larger'in diameter than the cylindrical;middle portion of the shaft, wherein the length of the 'one'head connected to the inner shell is about that needed for rigidly fastening the staybolt to the inner shell, whilst the length of the other head connected to the outer-shellis greater'than that of the one head as far-as is necessary, having-regard to any relative inclination between the inner and outer shells and to theeconomical limitation in the number of different staybolt sizes to be provided, said shaft tapering in diameter from the rigidly 'fastened heads to "the cylindrical middle portion in such a way that the ratio lit/ Ls'between the tapering length L1; and the total length of the shaft Ls is for'all staybolt lengths :occurring in the firebox, approximately constant for a given difference (Dinar-Darin) between 'themaximumdiameter (Daisey-and the minimum diameter (Dmln) of the tapered portion, the numerical value of the ratio LIE/LS being at least 0.25 and at the uttermost 0.45 and the difference between Dmax and Dmin at least 4 mm. and at the uttermost 9 mm.

4. In a locomotive boiler having inner and outer shells, staybolts with a shaft and heads, said heads and adjacent portions of the shaft being larger in diameter than the cylindrical middle portion of the shaft tapering in diameter from the rigidly fastened heads to the cylindrical middle portion in such a way that the ratio Lt/Ls between the tapering length Lt and the total length of the shaft Ls is, for all staybolt lengths occurring in the firebox, approximately constant for a given difference (Dma.x-Dmln) between the maximum diameter (Dmax) and the minimum diameter (Dmln) of the tapered portion, the numerical value of the ratio Lt/Ls being at least 0.25 and at the uttermost 0.45 and the difference between Dmax and Dmln at least 4 mm. and at the uttermost 9 mm, at least the heads secured to the inner shell being Welded thereto, the welded heads being of such shape and dimensions in relation to those of the staybolt shaft as to have a momentum of resistance to bending at least 30% greater than that of the shaft at its largest crosssection.

5. In a locomotive boiler having inner and outer shells, staybolts with a shaft and heads secured in holes provided in the shells, the head 6 of each staybolt secured to the inner shell being Weld-fixed and larger in diameter than the cylindrical middle portion of the shaft, the welded heads having an all round clearance at least in the holes of the inner shell, such clearance being provided with a difference in diameter between the heads and the appertaining holes of not less than substantially 0.5 mm.

ARNOLD TROSS.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 502,827 Nixon Aug. 8, 1893 813,120 Tate Feb. 20, 1906 853,099 Livingstone May 7, 1907 876,912 Pike Jan. 14, 1908 984,429 Jacobs Feb. 14, 1911 1,333,372 Barrow Mar. 9, 1920 1,359,587 Flannery Nov. 23, 1920 FOREIGN PATENTS Number Country Date 108,799 Great Britain Aug. 23, 1917 157,776 Austria Jan. 25, 1940 526,721 Germany June 10, 1931 613,954 Germany May 29, 1935

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