US2423682A - Sheet metal structure - Google Patents

Description

y 1947; J/B. CASTLE 2,423,682

SHEET METAL STRUCTURE FiledMay so, 1944 2 Sheets-Sheet 1 Join B. Ca slle IN V EN TOR.

ATTORNEY July 8, 1947. J. B. CASTLE SHEET METAL STRUCTURE Filed'May 30, 1944 2 Sheets-Sheet 2 Ja/m 5 @5276 INVENTOR.

Y v BY 7 g '55 ATTQRNEK Patented July 8, 1947 SHEET METAL STRUCTURE John B. Castle, Los Angeles, Calif., assignor to Douglas Aircraft Company, Inc., Santa Monica,

Calif.

Application May 30, 1944, Serial No. 538,023

' 2 Claims. 1

My invention relates to a method and means of stifiening thin sheets of metal or other comparatively rigid material, and specifically to the stiffening of beams having thin webs of the type commonly used in aircraft construction, as for instance to constitute wing ribs, and particularly relates to the strengthening of such thin metal sheets when provided with flanged lightening holes.

As is well known, beams having webs of thin sheet material, particularly when the web is rela-' tively deep, are liable to fail under heavy loads by buckling of the web. For this reason wing ribs formed as beams of thin sheet metal are commonly stiffened by the addition of vertical stiffening members extending across the web. While beams having thin webs provided with flanged lightening holes are much stiffer, weight for weight, than beams not so constructed, nevertheless failure will also occur when the calculated maximum load is approached or passed by buckling of the web to the side to which the flanges are carried.

It is the general object of the invention to provide a method and means of increasing the stiffness of a sheet metal member without increasing the weight of the member.

An object of the invention is to provide a method and means of construction to increase the strength of beams with thin metal webs provided with flanged lightening holes without increasing the weight of the beam by pressing stiffening elements from the material of the web between adjacent flanged lightening holes or cutouts.

It is a further object of the invention to provide beams of the kind described in the form of Wing ribs having lightening holes and strengthening elements without increase in the weight of the ribs because of the presence of the strengthening elements, said strengthening elementsbeing pressed out of the material of the Web on the opposite side to that to which the flanges of the lightening holes are carried. An additional object is toprovide a wing rib in which the flanges of the lightening holes and the stifiening elements extend in opposite directions, so that loadings can be carried well in excess of those that can be carried by beams not so constructed.

Further objects and features of the invention will hereafter appear in the following description and accompanying drawings in which a preferred embodiment of the invention is described by way of example, but it is to be understood that this illustrative showing is not in any way limitative of the scope of the invention which is defined by the appended claims.

In the drawings, in which identical reference numerals indicate identical parts in the several figures:

Figure 1 is a side elevation of a beam having a thin sheet web provided with flanged lightening holes and incorporating the construction of my invention.

Figure 2 is a cross section taken on the line 2-2 of Figure 1.

Figure 3 is a broken cross section on the line 3-3 of Figure 2.

Figure 4 is a side elevation of another form of beam having a thin sheet metal Web provided with flanged lightening holes, one of which is circular and another of which comprises straight sides joined by arcs of a circle, and also including the construction of my invention.

Figure 5 is a cross section on the line 55 of Figure 4.

Figures 6 and '7 are diagrammatic views illustrating the deflection of a beam with a thin metal web provided with lightening holes, but not including the construction of my invention.

Figures 8 and 9 are diagrammatic views similar to Figures 6 and 7 of a beam construction including the construction of my invention.

In the drawings HI indicates a beam of relatively light sheet metal of the type used to form the rib members of airplane wings. The thin web portions of such ribs are commonly provided with lightening holes formed by punching out material and forming the material surrounding the hole to provide a flange having a generally angular cross section with one element of the flange parallel to the face of the web. These flanges of the hole are made as deep as possible without unduly stretching the material of the web. The beam comprises flange portions l2 and web portion M, the beams being provided with flanged lightening holes Hi, the flanges l8 of which are carried to the same side as the flanges l2. Such a beam, while stiff for its weight, will nevertheless fail if loading in the face plane 26 0f the web is carried to a high enough value, by buckling toward the side to which the flanges of the lightening holes are carried out. I have discovered and verified by numerous tests that the beam can be strengthened so as to have almost twice the strength of a beam of the known construction by providing stiifening elements between adjacent lightening holes as indicated at 22, these stiffening elements being pressed out of the web to rise from the opposite side to that to which the flanges of the lightening holes are carried. The strengthening elements are formed so that they extend from a root portion 24 merging with th web portions of the heel or bend line tangent to top and bottom of adjacent lightening holes to a crown portion 25 which is positioned at a point intermediate the adjacent lightening holes in a zone through which passes the line joining the centers of the circles defining circular lightening holes. The construction appropriate to lightening holes not formed as true circles is described withreference to Figure 4. It will be noted that the strengthening element increases continuously in height from the root portion 2A to the crown portion 26, this increase of height of the strengthening element preferably following a straight line. The effect of this construction is to provide crowned trengthening columns in the web of the beam. Loads W applied .as indicated by the arrows in Figure 2 tend to cause the flanges of the lightening holes to move away from the plane of-the face of the web 20 and to cause the formed web intermediate the lightening holes to move in the opposite direction and thereby to increase the depth of the beam resulting in an increase in the moment of inertia of the beam, thus increasing the resistance to deflection of the beam. The comparative behavior of a beam formed as a rib of sheet metal having flanged lightening holes of usual construction is shown in Figures 6 and 7, the deflection of the beam under load W being indicated by dotted lines. It will be noted that the cross section of the beam has become more shallow under load, reducing its moment of inertia, and therefore its stability has been decreased which will result in failure by buckling under heavy load, whereas in a beam construction according to my invention, as diagrammatically illustrated in Figures 8 and 9, the deflection of the beam under load W is such that while the web may bend to deflect to the side of the flanged edges of the lightening holes, such deflection is resisted by the crowned column, while if the web tends to deflect to the side on which the crowned column is positioned, this tendency is resisted by the flanges of the lightening holes.

Figure 4 shows the manner of forming the strengthening elements arranged between lightening holes having straight side portions or between lightening holes of circular form and such holes having straight side portions. The end of the root portion of the strengthening element as before described lies along a heel line tangent to the upper and lower arcs defining the lightening holes. The increase of depth of the strengthening element however, should not be carried past the point at which the straight portions of the lightening holes become tangent to the arcs joining the straight portions. The crown of the strengthening member is therefore flat toward the lightening hol 28 which comprises straight side portions'Bll joined by arcs 32, this straight portion extending from point 34 to 39. On the side toward the circular lightening hole 33 the crown of the stiffening member will lie approximately at the point 40.

The height of the crown portion of the stiffening element above the plan of the sheet is preferably approximately half the depth of the lightening hole flange.

A generally satisfactory spacing of the lightening holes in the web of a thin sheet metal rib is such that adjacent hole are apart by slightly more than th depth of the flanged lightening hole, the width of the crown of the stiffening column occupying the whole distance between adjacent holes.

Where the width of the stiffening elements toward the heel lines tangent to the periphery of adjacent lightening holes is such'as to leave relatively large areas of sheet metal, it is advantageous to press out or otherwise form a pillow or stiffening depression as is standard practice instiffening such areas.

It has been found that where a structural member of the kind to which the invention relates is subjected to exceptionally high shear stresses, stiffening may be effected according to my invention although material is not removed from dished out stiffening depressions to form lightening holes, said additional material serving to increase the shear strength of the member.

It is pointed out that it is immaterial whether the beam is formed with a flange integral with the web as in the rib construction described by way of illustration, or formed with separate flange members between which a thin web formed according to my invention is positioned, as for instance by riveting it to top and bottom flanges. It has been found as a result of test, that the portion of the beam of my invention most liable to failure is at the point of connection of the flange and web and not in the web.

Various embodiments of the invention other than that described will be evident to those skilled in the art and Various changes may be made in the form and arrangement of my invention for stiffening thin sheets of rigid material Without departing from the scope of my invention as defined by the appended claims.

I claim:

1. A beam having a web of sheet material provided with flanged lightening holes arranged in a juxtaposed series in the web, the flanges of said holes projecting angularly from said web and terminating in an inwardly directed annular member substantially parallel to said Web; and columns, arranged between adjacent flanged lightening holes, projecting to the side opposite to that to which the flanges of said lightening holes project, said columns rising from the face plane of said web from lines tangent to the top and bottom peripheries of adjacent flanged lightening holes to a crown intermediate the length of said columns.

2. A beam having a web of sheet material provid-ed with flanged lightening holes arranged in a juxtaposed series in the web, the flanges of said holes projecting angularly from said web and terminating in an inwardly directed annular member substantially parallel to said web; and columns, arranged between adjacent flanged lightening holes, projecting to the side opposite to that to which the flanges of said lightening holes project, said columns rising from the face plane of said web from lines tangent to the top and bottom peripheries of .adjacentflanged lightening holes to a crown intermediate the length of'said columns, the height of each column at the crown portion thereof being equal to substantially-half the distance between the inwardly directed-annular member and said web.

JOHN B. CASTLE.

REFERENCES CITED The following references are of record in the file of this patent:

Great Britain Aug. 28,1919

Images