US2412887A - Staybolt - Google Patents

Description

Patented Dec. 17,1946

fr osi-lcs surnom Frederick P. Huston, Fanwood, and Lawton A. Burrows,v Woodbury. NJ., and Walter E. Lawson, Wilmington, Del.; The International Ni York, N. Y., Burrows and said Huston assignor to ckel Company, Inc., New a corporation of Delaware; said said Lawson assignors to E.

I. du

VPont de Nemours & Company, Wilmington, Del., a corporation of Delaware Original application February 2, 1943, Serial No.

Divided and this applcation Decembei' 19, 1944, -Serial No. 568,882. n @aliada `December 16,` 1942 l e claims. d(ci. ss-Lsl lThe present invention relates to heat exchange apparatus and, more particularly, to the staybolts used for securing the fire sheets and Wrapped sheets in the ilre boxes of such heat exchange apparatus as boilers.

Conventional staybolts for use in the assembly of the nre sheets and wrapper sheets of boiler lire boxes may be described as the screwed-` through ends-riveted type of construction. In fitting rigid staybolts in this ltype of construction the nre sheets and wrapper sheets were provided with aligned threaded openings through which the staybolt was screwed so that the ends exltended beyond the cuter faces of the fire sheet and wrapper sheet a distance `at least equal to two threads on the bolt. Flexible staybolts extended through the nre sheet in a similar manner. 'I'hese extending ends were then upset so as `o form a spread rounded head. Considerable diiliculty has been experienced with `this type of construction due to leakage of steam and/or water between the male and female threads where the bolt passes through thesheet. This difficulty has been particularly 4acute on .the lire side of the re box. y

Difllculty was experienced not only in obtaining a tight fit between the staybolts and the re sheets but also, particularly with staybolts of 4the rigid type. it had been found that numerous bolts broke in service. It has been almost universal practice to construct nre boxes with wrapper sheets of heavier gauge than the fire sheets, and it was found that the staybolts which brolre I in service almost always fractured substantially ir tle plane of the inner surface of the wrapper s ee We have invented a staybolt and a method for expanding the ends thereof in the sheets or plates of heat exchange apparatus, boilers, or .the like, which provides uid tight joints at any elevated pressure normally encountered in such equipment, and we also have provided staybolts which are substantially free from breakage during use.

It is an object of the present invention to provide a staybolt with means for insuring a fluidtight joint with a sheet through which it passes throughout the entire areas of adjacency between said staybolt and sheet.'

Another object of the present invention is to provide a means for expanding staybolts into fluid-tight engagement with the sheets with which they are usedby the use of explosive force.

A further object of the present invention is the provision of staybolts in which concentration oi stresses in the plane of a lire sheet and/or 2 wrapper sheet due to notch eiiect in the threads is largely eliminated. l

Other objects and advantages of the present invention will become apparent from the foli lowing detailed description of the invention taken view of a reduced body staybolt screwed into a.

.tapped hole in the ilre sheet, the bolt having a charge of explosive within a bore provided in .this end; and

Fig. e is a fragmentary sectional view showing abolt similar to that of Fig. 3 in expanded condition following detonation of the explosive' charge.

Referring now more particularly to Fig. l, reference character 2 represents the found-ation ring of a iire box having an outside nre box sheet or wrapper sheet and an inside iire box sheet or lire sheet 8. The lire sheet and wrapper sheet are provided with aligned threaded holes` 8 in which staybolts represented generally by th-e reference character S are litted. Two such staybolts are illustrated in Fig. 1. One ofthese, designated by the reference numeral l0, is a straight body, rigid type of stay, whilefthe other, designated by reference character I2, isa rigid stay of the reduced body type.

Straight body rigid stays ordinarily are pro- I vided with threads throughout theiriength, The

reduced body rigid stays, on thecontraiyare threaded only at the ends and the center region of the stay has a body of reduced diameter with respect to the threaded ends. Both types of rigid stays may be provided with a tell-tale hole, such as that illustratedat I4, or they may be hollow .throughout their entire length as illustrated at it.

,Moreoven in order to reduce the notch eect of the threads, the threads may be machined off, as indicated at 22, .to provide a bolt having the lowest practical value of localized stress in the plane of the inner face of the Wrapper sheet and re sheet. The machining should extend only slightly under the root diameter of the vthreads so as not to reduce the net diameter or net larea of .the bolt appreciably.

The 'nre ysheet and wrapper sheet ordinarily will be made of rolled steel plates or sheets. The staybolts may likewise be made of steel but we prefer to make them of a nickel-copper alloy of the type sold commercially under the trade-mark Monel" which contains approximately two parts of nickel to onepart of copper.

`Great difliculty had been experienced in fitting staybolts sufficiently tightly within the holes in the nre sheet and wrapper sheet that leakage was prevented.' Attempts were made to .prevent leakage by extending thestays beyond the plate not less than two threads when installed and thereamasar after upsetting the end to provide a rounded head f such as that illustrated in Fig. 1 by reference character 24. This expedient did not solve the dimculty since leakage still occurred lnboilers having the staybolts headed in this manner.

Moreover, in certain areas of the fire box, the rounded heads of the staybolts were rapidly cut away by the flow of cinders until the bolt was lsubstantially flush with the exposed face of the e re sheet, thus destroying the sealing eiect that the rounded heads were designed toproduce.

l We have discovered I'that completely uidf tight joints may be easily and inexpensively pro- 4 l velocity explosive extending slightly beyond both edges ofthe fire wall. A compressed primer other Words, one that possesses a normal velocity of detonation above 1000 meters per second when shot under the usual conditions of determining velocity. The primary detonating compounds I may be employed, for example, mercury-fulminate, lead azide, diazodinitrophenol. Preferably,

y however, weuse explosives such at tetryl, nitroduced by expanding the ends of the 'staybolts through the use of explosive charges. For this purpose the end of the staybolt which is to be expanded is provided with a bore 26 adapted to receive an explosive charge. This bore should extend at least to the plane of the inner face of the re sheet or wrapper sheetand preferably extends at least about '/s to 1% .inch beyond. In straight body staybolts the bore 26 may bea part of the-tell-tale hole, but in reduced body staybolts the bore 26 has a larger diameter than the tell-tale hole. The bore 26 must not extend beyond the change of section since the net area of the cross section of the metal at the end should not be less than the net area in the reduced section. The staybolt -is screwed into the threaded holes in the rlre and/or wrapper sheet in the usual manner and it may extend onlyiiush with the exposed face as shown at 21 in Fig. 1, or it may extend any-desired distance beyond, as illusvtrated in Figs. 2, 3 and 4. In Figs. 2 and 3 the play between the bolt S and the sheet 6 prior to expanding the end of the bolt has been exag-v gerated forpurposes of illustration.

Either before or after the bolt is in position,

an explosive charge 28 is placed inthe bore 26.

'.Ehe explosive charge 28 may be loaded directly into the bore hole, if desired, as illustrated in Fig. 3. In such case it` may be desirable to insert a disc'or closing device at the base of the hole to keep the explosive from extending into the narrow tell-tale hole. Since controlled compression of the charge is advantageous, it will ordinarily be preferred to precharge the explosive into a cylindrical container adapted to f it into the hole. This will obviate the hazards present in handling and pressing exposed explosive charges under shop conditions. Various methods of bringing about the explosion are suitable, such as the application of heat, percussion, the name from a fuse or other source, electrical methods, etc.

mannite, pentaerythritol tetranitrate, and the like, which detonate at high velocity and at the same time develop great' strength because of the larger gas volumes. The amount of explosive may depend on the diameter of bolt, the characl 'teristics of the metal of the'bolt and other factors. It will be understood that the diameter of the bore hole for containing the explosive should lbe suinciently large to accommodate a charge of the proper amount. At the same time, the thick-- ness of metal betweeny the center -hole and the outer bolt wall must be enough to give.the neeessary strength when the metal has vbeen expand-v ed. An advantage of expanding. the staybolt in accordance with our invention over mechanical expanding methods'lies in the uniformity of expansion in all directions. 'l

. The following examples illustrate specific embodiments of steybqlts Joined to metal wens where the ends of said .bolts have been expanded by means of explosive charges. 1

Example No. 1 Y

' A threaded bolt of inch diameter made of@ inserted a cylindrical metal shell, adapted lto ilt said bore and containing a compressed charge of 10 grains of pentaerythritol tetranitrate at `its base, a primer charge of 2 grains of compressed lead azide adjacent to the base charge, and an l' ignition charge of a suitable mixture. The base In Fig. 2, an assembly is shown similar to that of Fig. 3, with the preferred method of loading and firing illustrated more'in detail. After the bolt S is screwed into the re wall 6 so that the former extends slightly beyond the outer edge of said wall, ametal shell 30 is inserted into the bore hole 26 in the bolt and contains a compressed base charge 28 Vof a high strength, high charge had been loaded under a pressure of about 5000 pounds per square inch and extended slightly beyond both faces of the wall. The charges were brought to explosion by means of a fuse. A closefitting junction, impervious to highpressures, resulted from the expansion of thebolt'metal.

Example No. 2

Similarly, a threaded 7A; inch metal bolt was screwed into another V2 inch steel plate. The

bore hole in the bolt end wasof inch depth and 0.235 inch in diameter. A cylindrical metal container was inserted into thebore hole, said container being loaded with a. 10 grain base charge of tetryl, and suitable primer'and ignition com positions. Again the base charge was such that explosion was brought about by fuse, and a tight joint resulted between the staybolt and the surrounding metal.

Example No. 3

Additional joints were made between staybolts and metal plates in which the tightness ofthe junctures were tested. A length of 4" diameter extra heavy steel pipe was first taken and closed at -both ends by welding thereon pieces of inch boiler plate. A tube was likewise welded onto the pipe-to allow the application ofhydraulic pressure and its measurement by a gauge. Two diiierent bolts made of the` above mentioned nickel-copper alloy were used with such a pressure container, being screwed into opposite openings in the two plates. Longitudinal holes were drilled into the bolts, 0.235 inch in diameter to depths varying from 1% inch to 1/2 inch. Charges of pentaerythritol tetranitrate were used, varying from 5.75 grains to 9.0 grains. In some cases the explosive charge was co-extensive with the thickness of the plate, while in others it extended slightly beyond the edge or just to said edge. It

vwas found, generally, that the tightest joints resulted when the explosive charge overlapped both edges of the plate slightly. In these cases it was found that the metal of the vstaybolt surrounding the bore not only had expanded tightly into contact with the wall of the perforation in the sheet but that it had even bulged slightly on either side of the sheet, resulting in avery strong pressure-tight joint. Somewhat weaker joints were obtained when the explosive charge did not extend past the edge although these joints were also satisfactorily pressure-tight. Under conditions where the charge extended slightly beyond both edges of the plate, tests indicated that no leaking occurred even under a pressure of 4800 pounds per square inch. In no case did even a slight leak occur at a pressure less than 2200 pounds per square inch. This appears very striking and significant when it is realized that steam boilers ordinarily operate at pressures around about 225 pounds to about 250 pounds per square inch.

Using a similar test and an explosive charge of 10.5 grains of pentaerythritol tetranitrate, an unthreaded nickel-copper alloy bolt was used. After the explosive charge had been detonated, the joint was found to withstand a pressure cf 3200 pounds per square inch without any leakiness, which was the maximum pressure that could be exerted on this. particular assembly.

An additional test of an annealed 18-8 chromium-nickel stainless steel bolt under similar conditions showed that leakiness occurred in no case at a pressure below 4600 pounds per square inch. v

In the immediately foregoing examples, bolts have been used consisting of stainless steel and Vthe nickel-copper alloy sold under the trade-mark Monel. It will be understood that various other metals having satisfactory mechanical, metallurgical and chemical propertiesv for staybolts may be fitted bythe process embodying the present invention. Such metals as aluminum, nickel and copper; various nickel, aluminum and copper alloys; steel and certain steel alloys; others may be applicable. It is preferable that a metal for expansion in accordance with our invention have a potential elongation value of at least 20%. A

and many it extended slightly beyond both wall edges. 'Ihe narily operate at a pressure between 6 Y Example No. 4

Heat exchange apparatus was constructed by means of explosive staybolts. A cylindrical catalyst chamber was spaced within a cylindricalv `liuid being treated in the catalyst chamber. The

connections between the bolts and the walls were effected by forming bore holes 0.235 inch in diameter in each end ofthe bolt, and about 5/8 inch in depth. The metal defining these-holes was expanded into contact with the walls to be spaced apart, by means of .an explosive capsule loaded with a 10 grain base charge of tetryl and conventional primer and ignition compositions. Explosion was brought about by means of a fuse to cause the pressure-tight joints already described.

It is not essential in all cases that the staybolts and perforations be threaded as pressuretight joints can be produced-by the process of the present invention between unthreaded bolts and smooth walledl perforations or between staybolts and perforations'which are threadedfor only a portion of the thickness of thepllate. Nor

is it essential that the base chargeof the explosive extend beyond both faces of the re or wrapper sheet. Satisfactory fluid-tight joints are also obtained when the bolt terminates at the plane of the exposed face of the sheet,` as illustrated at 2l in Fig. 1. This has the advantage of providing a smooth surface which is not cut away to any appreciable extent by the now of the cinders therealong. The bolt may also exl tend beyond the face of the're sheet, as shown in Figs. 2, 3, and 4 and be upset after expansion to provide a, rounded head 26 as illustrated in staybolt I2 of Fig. 1.

The present invention provides an extremely simple and rapid method of tting staybolts 'so tightly in the boiler plates that the joints are fluid tight at pressures 1000% to more than 2000% greater than the ordinary working steam pressure. In the United States the boilers ordithe ranges of about 200 to 300 lbs. per square inch, with an average of about 225 to about 250 lbs. per square inch. The present invention provides a high safety factor even for boilers operating at very much higher working pressures such as have been contemplated and to a certain extent used abroad with Working pressures in excess of 1000 lbs. per square inch. These joints are not only fluid-tight initially but they ice.` As those skilled in the art are aware, staybolts are exposed in service to tending not only to loosen the joint between the staybolts and the boiler plates but even to fracture the bolt. Many boilers operate at temperatures up to 550 F. and even higher. These high tem peratures impose stresses on the staybolt and the joint due to thermal expansions of the metals of the staybolts and boiler plates. Moreover, at least once each month the locomotive must be washed out by emptying the hot water and relling with fresh water which sometimes is cold. This invariably results in uneven cooling of the boiler producing very high stresses known in the art as wash-out stresses. The staybolts and boll- The bolts were expanded exploslvely will remain fluid-tight in serv-- severe stresses I er plates are also under static stress due to the l steam pressure. Moreover, particularly in boilers of locomotives and marine vessels, the stay.

bolts also are exposed to dynamic stresses, mechanical shocks, and vibrations resulting from the movement of the boiler. The'movement of the boiler may also impose sheer stresses from lwarping or wrenching oi.' the boiler assembly.

Thus, in locomotive boilers the weaving and warping of the locomotive frame is transmitted to the boiler assembly and causes it also to weave and warp. The Association of American Railroads and various engineering societies, including the American Society of Mechanical Engineers and the American-Society for Testing Materials,

as a result of many' years of experience and re' search, have developed codes and specications,

-deslgned to reduce staybolt breakage and leakage to a minimum. See for example, the A. S.

Boiler Construction Code, 1940 edition, and the specifications lof the .Association ofy American Railroads, 1939 revision.` Some of-:tlie more imcontribution to the boiler art in greatly reducing such breakage and leakage. Thev ends of the bolts that are explosively expanded by the methodv embodying the present invention contact the."wa11s of the holes in the boiler sheets so tightly that the joints will remain fluid tight in service despite thermal stresses due to heating, cooling, wash-outs, etc., and mechanical stresses resulting from shocks, vibration, warping, and the like.

The present application is a division of our copending application Serial No. 474,480, led Feb ruary 2, `1943. l y

Although the invention has been described and illustrated in connection with certain-specic embodiments, it will be appreciated that many modiiications maybe made therein without departing from the scope of the invention as deilnedin portant provisions relating to staybolts appearing in these codes and specifications are as follows:

,Physical and mechanical properties v Tensile strength: 47,000 lbs. per sq. inch minimum Yield point 60% oil the tensile strength as a. min- Y Elongatlon in 8 inches: 28% minimum.

Reduction in area: 48% minimum.

Maximum allowable stress on staybolts: 7,500 lbs.

per sq.,inch and 8,000 lbs. per square inch', depending upon the staybolt length, design and construction.

Permissible variation in size: The staybolt must be truly round within 0.01 inch and the diameter shall not be less than 0.005 inch over nor mtcre than 0.020 inch over the specified diame r.

month and after every hydrostatic test.

The hydrostaticy test shall be made at least every twelve months and it must `be carried out at greater pressure than the working steam the staybolts iitted in accordance with the present invention will stand over pressures from 8 to i6 times that specified for the hydrostatic test.

Even vwith the precautions taken in these speciilcations and codes breakageof the staybolt and leakage `between the staybolts and the boiler the following claims.

We claim: j 1. A metallic staybolt having circular ends adapted for insertion into circular holes in re- 'spective plates, whereby said plates are to. be

joined and maintained at a predetermined distance apart, bores provided in each of said ends Tests: steybeits shan be tested et least every i pressure. In this'connectionit is signicant that plates have occurredin normal operation of boil ers. The' present invention makes an important sule containing a charge of high velocity explosive substantially illling at 'least one of said bores, said bores being open for fescape of the products of explosion, and means for detonating said charge.

`3. A metallic staybolt having circular threaded ends adapted for insertion into circular threaded holes vin respective plates, whereby said plates are to be joined and maintained at a predetermined distance apart, each of said ends beingprovided with a concentric bore of suiiicient depth to extend at least to the inner face of the respective plate, a capsule containing a charge of high velocity explosive substantially filling each of said bores, said bores being open for escape oi the products of explosion, and means for detonating said charges. i

- FREDERICK P. HUSTON.

LAWTON A. BURROWS.

WALTER E; LAWSON.

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