US2764300A - Friction shock absorbing mechanisms for railway draft riggings - Google Patents

Description

A. P. WITHALL 2,764,300 FRICTION SHOCK ABSORBING MECHANISMS FOR RAILWAY DRAFT RIGGINGS 4 Sheets-Sheet 2 MM m v\ 5 li m 21 a Sept. 25, 1956 Filed March 25, 1954 Sept. 25, 1956 A. P. WlTHALL 2,764,300

FRICTION SHO" ABSORBING CH SMS FOR RAI Y DRAFT RI NG Filed March 25, 1954 4 Sheets-Sheet 3 FOR RA AFT RIGG GS 4 Sheets-Sheet 4 Sept. 25, 1956 A. P. WITHALL 2,764,300

FRICTION SHOCK ABSORBING ME NISMS ILWAY DR Filed March 25, 1954 JP.6 39 4/ .4424 4,3 J W/y/ /V 27 I z Inven im United States Patent FRICTION SHOCK ABSORBING MECHANISMS FOR RAILWAY DRAFT RIGGINGS Albert P. Withall, Chicago, 111., assignor to W. H. Miner, Inc., Chicago, 111., a corporation of Delaware Application March 25, 1954, Serial No. 418,649

5 Claims. (Cl. 213-32) This invention relates to improvements in friction shock absorbing mechanisms for railway draft riggings.

This application is a continuation in part of my copending application Serial No. 352,921, filed May 4, 1953, and now abandoned.

One object of the invention is to provide a high capacity shock absorbing mechanism for railway draft riggings, comprising relatively movable friction elements and yielding means opposing relative movement of the friction elements, wherein the yielding means is particularly designed to afford relatively high resistance against compression.

A further object of the invention is to provide a shock absorbing mechanism as set forth in the preceding paragraph, wherein the yielding means is in the form of a rubber cushion composed of a plurality of units in the form of a column, each unit comprising a pair of metal plates and a fiat rubber pad interposed between the plates and bonded thereto. By bonding the rubber to the plates, greater resistance to movementand hence the capacity achieved is greater than it would be if the rubber were merely set between the plates without being bonded thereto.

A still further object of the invention is to provide a yielding resistance means for the relatively slidable friction elements of a friction shock absorbing mechanism, comprising a column of units, each composed of a pair of metal plates and an interposed rubber pad, which is bonded to the plates, wherein the column of units is stabilized by interengaging centering means on the pads and plates of each unit and on the plates of adjacent units of said column.

Still another object of the invention is to provide a friction shock absorbing mechanism of high capacity, comprising a friction casing, friction shoes slidingly telescoped within the casing, a wedge in wedging engagement with the shoes, and yielding means within the casing opposing movement of the shoes inwardly of said casing, wherein the yielding means comprises a column of units, each composed of a pair of metal plates and a homogeneous rubber pad interposed between and bonded to said plates.

Another object of the invention is to provide a shock absorbing mechanism including a casing and yielding resistance means within the casing, which is in the form of a column of units, each composed of a pair of metal plates and a homogeneous rubber pad interposed between and bonded to the plates, wherein the column of units is stabilized by a plurality of centering or aligning followers which divide the column of units into groups, and wherein the aligning followers are slidingly guided within the casing and have projecting aligning flanges which overhang the groups of units.

Still another object of the invention is to provide a mechanism as set forth in the preceding paragraph, wherein the rubber pads of the units comprising the column are made to conform in size and shape with the plates and have their peripheral edges formed concavely "ice in such a manner as to cause the rubber to flow equally outwardly in all directions responsive to pressures exerted on the plate. The concavities thus formed at the edges of the plate are of such volume that when the unit is compressed to'a predetermined extent, the rubber :will flow only to the edges of the plates and will not project beyond those edges where it would be subject to abrasive damage by coming in' contact with metallic members.

Other objects of the invention will more clearly appear from the description and claims hereinafter following.

In the accompanying drawings forming a part of this specification,

Figure 1 is a side elevational view of my improved friction shock absorbing mechanism.

Figure 2 is a top plan view of Figure 1, with the wedge and shoes omitted.

Figure 3 is a front elevational view, looking from left to right in Figure 1.

Figure 4 is a longitudinal, vertical sectional view, corresponding substantially to the line 4-4 of Figure 3, with the rubber cushioning units shown in elevation.

Figure 5 is a view similar to Figure 4, illustrating the mechanism compressed to the full extent of its normal operating range.

Figure 6 is a broken, horizontal sectional view, corresponding substantially to the line 66 of Figure 3.

Figure 7 is a view similar to Figure 3, with the wedge and friction shoes omitted.

Figure 8 is a view similar to Figure 4, illustrating the manner of applying the rear wall to the casing, the section at the rear end of the case portion of the friction casing and the rearmost follower being taken on the line 8--8 of Figure 7.

Figure 9 is a front elevational view of the follower at the front end of the column of rubber cushioning units.

Figure 10- is a broken rear elevational view of the follower shown in Figure 9 and the associated cushioning unit.

Figure 11 is an elevational view of one of the groups of cushioning units employed in my improved mechanism.

Figure 12 is a part side elevational and part transverse,

vertical sectional view, on an enlarged scale, corresponding substantially to the line 1212 of Figure 11.

Shock absorbers or draft gears by which the draft rigging of the car or locomotive are connected to the structure of the vehicle protect the vehicle and lading from shocks incurred during coupling and train operation. The dimensions of the pocket and yoke into which the shock absorber or draft gear must be installed and the distance the gear may travel have been standardized by the Association of American Railroads in its Specification M901.

At the time these standards were set, train speeds were lower than at present, and coupling was effected, during switching, at speeds much lower than is the practice today. The need to protect cars and lading at the higher coupling speeds presents to manufacturers the problem of designing draft gears capable of withstanding far greater impacts than formerly, since the effect of the impact increases with the square of the speed at which coupling is effected.

The present invention presents a solution to this problem by providing a draft gear consisting of the combination of a friction clutch resisted by a stabilized rubber column and is of such dimension that it can be applied within the standard space or pocket between the sills of the vehicle.

The column consists of units of known design consisting of a pair of plates spaced apart and a solid mass of rubber interposed between and bonded to the plates. Each unit has centering bosses projecting from one of its faces and centering sockets recessed into its other face,

which bosses and sockets position and hold the units with respect to each other when stacked in the column. Each unit has edge configuration providing flow space into which the rubber is displaced as the unit is compressed. The units in the column are disposed at right angles to the axis of the column, and the column is divided into sections by rigid separators interspersed between the units, each section containing a plurality of units. The separators are provided with bosses and sockets registered with sockets and bosses respectively of the adjacent units to position the units with respect to the separators.

The separators are disposed at right angles to the axis of the column and are maintained in such relation to the axis by rigid flange means at the edges of the separators, which flange means have sliding engagement with the walls of the casing. Through this arrangement, the column is stabilized.

A flanged rear follower engages the rear end of the column and a removable rear casing wall completes the entering or aligning followers E, F, G, and H.

The casing A comprises a friction shell section at the front end thereof and a case portion 11 rearwardly of the shell section 10. The case portion 11 of the casing A is in the form of a rectangular boxlike member having longitudinally extending, vertically spaced, top and bottom walls 12 and 13, and longitudinally extending,

laterally spaced, vertically disposed side walls 1414.

The rear end of the case portion 11 of the casing A is closed by a removable end wall 15. The top and bottom walls 12 and 13 of the case portion of the casing A are indented to provide longitudinally extending, interior, top

and bottom guides 16 and 17, the top guide 16 being in The guides 16 and 17 present substantially fiat, longitudinally extending guide surfaces, which are parallel. These flat surfaces of the guides 16 and 17 extend from the front end to within a short distance of the rear end of the case portion 11 of the casing and are of substantial width. The vertical side walls 1414 of the case portion 11 present substantially flat, longitudinally extending, vertically disposed, interior surfaces fromend to end of said case portion, forming parallel opposed side guides 1818, for a purpose hereinafter pointed out. At the rear end of the case portion 11, the top and bottom Walls are provided with inturned, transversely extending, top and bottom retaining flanges 1919 for the wall 15.

The wall 15 is in the form of a thick plate having an outer portion of reduced height, thereby providing an inner section which protrudes above and below the re duced portion to present top and bottom stop flanges 20-20, which are engaged in back of the retaining flanges 19-19 of the case 11 to lock said wall against outward movement. The total overall height of the wall 15 is such that the same, when in the inclined position shown in Figure 8, may be readily passed between the flanges 19-19 into the rear end of the case portion 11 of the casing.

The top and bottom walls 12 and 13 of the case portion 11, inwardly of the rear end thereof, have pairs of continuous with the case portion 11, is of lesser width than the latter and of hexagonal transverse cross section. The shell section 10 presents three interior friction surfaces 222222, which are of V-shaped, transverse cross section, each V-shaped surface being formed by two adjacent inner faces of the hexagonal wall of the shell section. These friction surfaces 222222 converge inwardly of the shell section of the casing A, thereby presenting a tapered formation.

At the forward end of the casing A, which end is open, are provided three inturned stop lugs 232323, which are equally spaced circumferentially at said open end. These stop lugs provide means with which the wedge block B cooperates for limiting outward movement of the latter and holding the mechanism assembled.

The wedge block B is of generally cylindrical shape, having a flat front face 24 adapted to bear on the usual front follower (not shown) of a railway draft rigging, and receive the actuating force. The inner end of the wedge block B is provided with three wedge faces 25 2525, which are arranged symmetrically about the central longitudinal axis of the mechanism and converge rearwardly. Each wedge face 25 is of V-shaped, transverse cross section and cooperates with one of the fric tion shoes C. At the rear end thereof, the block B has three radially projecting lugs 262626, which extend between adjacent shoes CC and are engaged in back of the lugs 232323 of the friction shell section 10 of the casing A.

The friction shoes CCC surround the wedge block B and are slidingly telescoped within the friction shell section 10 of the casing. Each shoe C has a longitudinally extending friction surface 27 on the outer side thereof, which is of V-shaped, transverse cross section and engages with one of the V-shaped friction surfaces 222222 of the friction shell section 10. On the inner side, that is, the side of the shoe which faces the wedge block B, each shoe C has a wedge face 28 of V- shaped, transverse cross section, correspondingly inclined to and engaged with one of the wedge faces 25 of the wedge block B. At the rear end, each shoe C presents a transverse face 29 which forms an abutment for the follower E.

The rubber cushioning units DDD are disposed within the case portion 11 of the casing A in back of the friction shoes CCC and form a resilient column providing yielding means for opposing inward movement of said shoes.

Each cushioning unit D comprises a pair of substantially rectangular metal plates 36 and 31 and an interposed flat rubber pad 32, which is vulcanized and bonded to the inner sides of the plates. The rubber pad 32 of each unit D corresponds in surface area and outline to the plates 30 and 31, and has its edge faces formed concavely or grooved, as indicated at 33, to accommodate the flow of the material of the pad, as the same is compressed, thereby preventing the material of the pad from being squeezed outwardly to an extent to bulge beyond the edges of the spacing plates and be thus subjected to the danger of being damaged. As shown in Figure 5, which illustrates the mechanism compressed, the edges of the pads Bill-32 of the units DD lie substantially flush with the edges of the metal plates 3t? and 31 of the same, at that stage of the operation of the mechanism, the depth of the concavities or grooves 33 of the pads, in the expanded condition of the same bei. g gauged to produce the result shown in Figure 5. As will be evident, to attain the desired result it is not absolutely necessary that the edges of the pads 32 lie exactly flush with the edges of the plates 30 and 31, when the units D are compressed to the extent illustrated in Figure 5, but may be inset or recessed slightly so long as they do not bulge or protrude. The plates 30 and 31 of each unit D are of similar design, each having centering projections 34-34 on one side andcentering seats 35-35 g on the opposite side, the same being arranged in. aligned sets, the projection 34 and the aligned seat 35 of each set being provided by an embossed hollow projection on said plate. The interposed pad 32 of each unit has centering projections 36 on one side engaged in the seats 35 of the plate at the corresponding side of the unit and indentations or seats 37 on the opposite side engaged by the projections 34 of the other plate of said unit. v As shown in Figures 4, 5, 6, and 8, the resilient rubber column, which is made up of a plurality of units DD, has the units thereof arranged in groups which are separated by the followers F and G, the groups being three in number and each group comprising a set of threeunits DDD. The three units of each set are held in aligned condition by having the centering projections 34 and seats 35 of the plates 30' and 31 of adjacent units engaged with each other.

It will be observed that in each set of three units D, the only surfaces on which the rubber bears against metal are the surfaces on which the rubber is bonded to the plates 30 and 31. The plate 30 of each unit engages the plates 31 of the adjacent unit and movement between the thus engaging plates is prevented by the centering projections and seats. There is no movement of rubber against metal or against rubber other than internal movement within the rubber pads 32 themselves and abrasion of the rubber is thereby totally eliminated. V v

The centering or aligning follower E is interposed between the front group of units and the friction shoes (DC-C, the follower F is interposed between this group and the second or the next group to the rear thereof, the follower G is interposed between the last named group and the third and rearmost group of units, and the fol lower H is interposed between the last named group and the end wall 15 of the case portion 11 of the casing A.

The follower E'eomprises a thick rectangular plate portion, having a forwardly projecting, hollow, cylindrical, central boss" 38, which extends into the rear end of the friction shell section of the casing A andbears on the transverse end faces 29-29-29 of the shoes CC-C. The follower E has a rearwafdly projecting, peripheral flange 39 which extends from the plate portion of said fOllOWCT and overhangs the adjacent unit D of the front group of units, as shown in Figure 4. The follower E is slidingly guided for movement lengthwise of the casing A, having asliding fit between the top and bottom guides 16 and 17 and the side guides 18-1 8 presented by the side walls 1414 of the case portion 11 of the casing A, thus being held against both vertical and lateral displacement with respect to the case portionl'l. The flange 33 of the follower E embraces. the outermost unit D of the front group on all four sides. The opening defined by the walls of the peripheral flange 39 is preferably slightly tapered to facilitate seating of the'unit D in said follower. The rear side of the plate. portion of the follower E is also provided with a plurality of centering seats lli-49 within which the centering projections 4r3"-t of the plate 30 at the front end of the front group of units D--D-D are engaged.

The followers F, G, and H are of similar design, ex cept as hereinafter pointed out, each comprising a relatively thick, platclike main body portion of substantially rectangular outline having a forwardly projecting, pe riphcral flange extending therefrom, the flanges of the followers F, G, and H being respectively indicated by'4'1,

2, and 43. These followers are all slidingly guided for movement lengthwise of the casing A, each having a sliding fit between the top and bottom guides 16 and 17 and the side guides 18-18 of the case portion 11 of the casing, thus being held against both vertical andlateral displacement with respect to the case portion 11. The flanges 41, 42, and 43 of the followers embrace the rearrnost units DDD of the adjacent groups 'onall four sides thereof. The openings defined by the walls of the peripheral flanges of these followers are preferably slightly tapered to facilitate seating of the units D in. said followers. The front side of the platelike main body portion of each of the followers G, and H is provided with centering projections 4444 which are engaged in the seats 3535 of the plate 31 at the rear end of the associated group of units D. As shown in Figure 4, the flange 41 of the follower F is of such a length that it completely overlaps the unit D immediately in front of it and also the rear plate 31 of the next unit D. The flanges 42 and 42 of the followers G and H are somewhat longer than the flange 41 of the follower F, overhanging, in addition to the units immediately in front of these two followers G and H, the greater portions of the units immediately in front of these last named units. The rear side of the platelike main body portion of the follower H is substantially flat and bears truly on the fiat front side of the end wall 15 of the case portion 11 of the casing A, while the rear sides of the platelike main body portions of the followers F and G are recessed to form depressed areas to accommodate the plates 30-30 of the units DD immediately in back 'of these followers. The surrounding walls of these depressed areas provide, in effect, shallow flanges 4545. The depressed rear sides of the followers F and G are also provided with centering seats in which are engaged the projections 34-34 of the plates 30-30 of the adjacent units DD.

The portions of the flange 43 of the follower H, which are located at the top and bottom sides of said follower, are provided with vertical key receiving openings 46-46 which extend therethrough and are adapted to receive locking keys for holding the follower against movement while the end wall 15 is applied to the casing A, as shown in Figure 8, the follower H being moved inwardly to align the openings 46-46 with the openings 2121 of the case portion 11 to receive the keys, which are indicated by 47--47.

In assembling the mechanism, the casing A is placed in upright position with its front end, that is, the friction shell section end resting on any suitable support, such as an anvil having an opening to accommodate the outer ends of the wedge block B and shoes C-CC, which normally project from the casing. With the casing thus positioned, the wedge block B, shoes CCC, follower E, the front group of three cushioning units DDD, the follower F, the second group of units DDD, the follower G, the third or rear group of units DD-D, and the follower H are placed within the casing in succession. The mechanism is then compressed by any suitable tool engaged with the follower H and the latter forced inwardly of the case portion 11 of the casing A until the key receiving openings 4646 of the follower H are aligned with the openings 21-21 of the case portion 11. The follower H is then locked to the casing by engaging the keys 4747 through the openings 2121 and 46-46, as shown in Figure 8. The tool is then withdrawn and the end wall 15 applied to the casing by inserting the same into the rear end of the case portion 11 while tilted sufficiently to clear the top and bottom flanges 1919 of said case portion, as indicated in Figure 8. After the bottom flange 20 of the end wall 15 has been brought inwardly clear of the bottom flange 19 of the case portion 11, the end wall 15 is permitted to drop onto the follower H and assume a horizontal position. Inward or down-ward pressure is then applied to the follower H, thereby relieving the pressure on the keys 4747, which are then removed. After the follower H has been. thus unlocked, the downward pressure is gradually withdrawn from the wall 15 permitting the latter to be forced outwardly by the expansive action of. the rubber units DD of the cushioning means, until the flanges 2020 of the wall 15 engage the flanges 1919 of the case member 11 to lock the wall 15 to the case member against outward movement.

The operation of my improved dnaft gear is as follows: When an impact is imposed upon the gear, wedge block B is forced inwardly of the casing. The wedge faces 25 bearing against the inclined faces 28 on the fritcion. shoes 7 C resolve pressures into two components, one forcing the shoes inwardlyof the casing and the other forcing the shoes outwardly into tighter frictional engagement with the surfaces 22 of the shell. Inward movement of the shoes is opposed by the cushioning unit through the end follower E. Since the pads D of the cushioning unit are disposed at right angles to this movement of the end follower E, the rubber in the cushioning unit is subjected to forces in compression and only in compression. Since the rubber is a solid homogeneous mass bonded to the metal plates, displacement of the rubber under the forces of compression is outwardly and only outwardly into the flow space provided around the periphery of the units.

In Figure 5, the gear is shown compressed to a travel of two and one-half (2 /2) inches. It will be noted that the face 24 of the wedge block B is still approximately one-fourth A) of an inch out of the plane through the end of the casing. Thus it will be seen that further compression of the cushioning unit is possible before the gear goes .solid. It will also be noted that in Figure 5, with the gear compressed in a travel of two and one-half (2 /2) inches, the edges of the rubber in the units D are still slightly concave, with the result that even under overload impacts causing the gear to go solid, this rubber does not project beyond the edges of the plates to which it is bonded, and as a result does not come in contact with metal surfaces, and abrasion of the rubber is thereby avoided.

As will be understood by those skilled in the art, impacts imposed upon any column of rubber units, such as employed in the gear of the present invention, will tend to cause movement of the column at right angles to its axis and distortion thereof in one direction exceeds the distortion in an opposite direction. This lateral movement is opposed by the interengagement of the centering bosses and lugs of the plates 30 and 31, and the interengagement of this centering means with the followers. The followers have flanges 41, 42, and 43, which engage the walls of the casing to limit this lateral movement. By arranging the rubber units D in stacks of three in the packs, the flanges on the followers can be made wide enough to present a large area of contact with the casing Walls and thereby insure that the webs of the followers will remain parallel at all times, thus further insuring that the column will not be moved laterally by impacts imposed upon it. The spacing of the followers is such that even though the column be compressed sufliciently to cause the gear to go solid, the adjacent edges of adjacent flanges will not engage each other but rather will be spaced apart, as shown in Figure 5.

Specification M-901 provides an endurance test in which the gear is struck by a series of free fall blows of a 27,000 pound hammer, the test being continued until not less than 25,000,000 foot pounds of energy has been delivered to the gear with not more than 100,000 foot pounds of work delivered in any half hour period to prevent objectionable heating of the friction sur faces. Satisfactory performance requires freedom from damaged parts and loss of capacity. The gear of the present invention has passed the AAR endurance test thus specified, without damage or capacity loss, thus exceeding the requirements of that test. Continuation of the endurance input to more than 50,000,000 foot pounds energy input was effected without any loss in capacity of the gear. Long life of the gear at full efliciency is thus assured.

Specification M-901 further provides a sturdiness test in which the gear is subjected to a series of over solid blows. The gear of the present invention has successfully met this requirement of specification M-901.

From the foregoing, it will be apparent that by cornbining the friction shoe and wedge arrangement with the rubber cushion, in which the rubber in the cushion is stressed in compression and only in compression, I have succeeded in producing a gear of such dimensions as to enable it to be applied in standard pockets, which gear is capable of cushioning greater impacts than any of the prior art devices of which I am aware. With a maximum travel of two and one-half (2 /2) inches, the maximum reaction upon the sills of the vehicle is well within the range that the vehicle is capable of withstanding. The gear develops in excess of one-fourth A) of its rated capacity during the first half of its travel and has been shown to have long life without any loss of capacity.

Having thus complied with the statutes, and shown and described a preferred embodiment of my invention, what I consider new and desire to have patented by letters Patent is pointed out in the appended claims.

I claim:

1. A draft gear for railway draft rigging comprising a metallic casing; a resilient column in said casing consisting of a plurality of individual units, each of which comprises a spaced apart pair of metal plates disposed at right angles to the axis of the column and spring means disposed therebetween; rigid separators interspersed in the column and disposed at right angles to the axis thereof for dividing the column into sections, each of which contains a plurality of units; means for applying pressure to said column to compress the same; and rigid flange means on each of said separators fixed with respect thereto and having sliding engagement with the casing for maintaining the separators at right angles to the axis of the column thereby to stabilize the column.

2. A draft gear for railway draft rigging comprising a metallic casing; a resilient column in said casing consisting of a plurality of individual units, each of which comprises a pair of metal plates disposed at right angles to the axis of the column and rubber spring means disposed between and bonded to the plates; rigid separators interspersed in the column and disposed at right angles to the axis thereof for dividing the column into sections, each of which contains a plurality of units; means for applying pressure to said column to compress the same; and rigid flange means on each of said separators fixed with respect thereto and having sliding engagement with the casing for maintaining the separators at right angles to the axis of the column, thereby to stabilize the column.

3. A draft gear for railway draft rigging comprising a metallic casing; a resilient column in said casing consisting of a plurality of individual units, each of which comprises a pair of spaced apart metal plates disposed at right angles to the axis of the column and spring means disposed between the plates; rigid separators interspersed in the column and disposed at right angles to the axis thereof for dividing the column into sections, each of which contains a plurality of units; bosses projecting from and sockets recessed into each unit and interengaged with the sockets and bosses, respectively, of the adjacent unit for maintaining the units aligned in the sections; bosses projecting from and sockets recessed into each separator and interengaged with the sockets and bosses, respectively, of the adjacent units for maintaining the sections aligned with respect to the separators; means for applying pressure to the column to compress the same; and rigid flange means on each of said separators fixed with respect thereto and having sliding engagement with the casing for maintaining the separators at right angles to the axis of the column, thereby to stabilize the column.

4. A draft gear as specified in claim 3 in which the spring means consists of a solid pad of rubber disposed between and bonded to the spaced apart metal plates.

5. A draft gear for railway draft rigging comprising a metallic casing having a rear portion of rectangular section defined by spaced apart top, bottom and side walls, said top and bottom walls being offset outwardly adjacent the rear end of the casing to provide cavities opening interiorly of the casing; flange means at the rear ends of said top and bottom walls defining the rear ends of said cavities and with the side Walls defining a rectangular rear opening for the casing which has the same dimensions as said rear portion of the casing; a

resilient column within the casing consisting of a p1urality of metal faced resilient units; rigid separators interspersed in the column and disposed at right angles to the axis thereof for dividing the column into sections, each of which contains a plurality of units; rigid flange means on each of said separators fixed with respect thereto and having sliding engagement with the casing for maintaining the separators at right angles to the axis of the column, thereby to stabilize the column; and a rear follower having forwardly extending flanges engaging said bottom, top and side walls, said rear follower flanges and top and bottom walls having openings registrable together to receive keys for holding the follower forwardly of said cavities; a planar rear wall for said casing fitting in said rectangular rear opening; and flange means on the top and bottom edges of said rear wall projecting therebeyond into said cavities and abutting against the flange means at the rear ends of said top and bottom walls of the casing for holding said rear wall in said rear opening to close said rear portion of the casing.

References Cited in the file of this patent UNITED STATES PATENTS Graham Nov. 15, 1904 Moore Feb. 14, 1905 Harty Mar. 29, 1927 Woernley Jan. 13, 1931 Tucker July 5, 1932 Whitmore Feb. 14, 1933 Spencer May 21, 1935 Gabriel Nov. 1, 1949 Brennan July 18, 1950 Thornhill Aug. 29, 1950 Spence et a1. Oct. 31, 1950 Dath Dec. 26, 1950 Johnson Feb. 6, 1951 Dath Mar. 11, 1952 Danielson et a1 Oct. 27, 1953 Tillou July 19, 1955 Tillou July 19, 1955

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