US3690270A

US3690270A - Railway truck bolster

Info

Publication number: US3690270A
Application number: US19711A
Authority: US
Inventors: Paul F Gutmann Jr; Thomas L Mueller
Original assignee: SCULLIAN STEEL CO
Current assignee: SCULLIAN STEEL CO
Priority date: 1970-03-16
Filing date: 1970-03-16
Publication date: 1972-09-12
Anticipated expiration: 1989-09-12

Abstract

A railway truck has a cast bolster, the ends of which are substantially hollow and project through windows in side frames. The bolster ends are snubbed by wedge-shaped friction castings which are urged upwardly by side springs and have inclined friction surfaces which bear against wear plates attached to the bolster ends. Access to the connecting devices for securing the wear plates to the bolster ends is gained through holes formed in the top walls of the bolster ends. The top walls are further embossed at the outer edges of the holes to provide clearance around the connecting devices and are depressed at the inner ends of the holes so that a fastening tool can be inserted through the hole and advanced into engagement with each connecting device.

Description

United States Patent Gutmann, Jr. et al.

[ Sept. 12, 1972 [54] RAILWAY TRUCK BOLSTER [72] Inventors: Paul F. Gutmann, Jr.; Thomas L. Mueller, both of St. Louis County,

Mo. [73] Assignee: ScullianSteel Company, St. Louis,

[22] Filed: March 16, 1970 [21] Appl.No.: 19,711

[52] U.S.Cl. ..105/197D [51] Int. Cl. ..B61f5/06 [5s] FieldofSearch..... ..l05/19 7R,l97D,

' i 19' M 105/191 D.B,.2 6 f2.24

[56] f References Cited I UNI'I'EDSTATESPATENTS 5 3,559,589 2/1971- Williams ..-..105/197 2,237,953 4/1941 Webbs "1 05/19 7 2,118,006 5/1938 Couch 105/197 Primary Examiner-Kenneth H. Betts Attorney-Gravely, Lieder & Woodruff [57] ABSTRACT A railway truck has a cast bolster, the ends of which are substantially hollow and project through windows in side frames. The bolster ends are snubbed by wedge-shaped friction castings which are urged upwardly by side springs and have inclined friction surfaces which bear against wear plates attached to the bolster ends. Access to the connecting devices for securing the wear plates to the bolster ends is gained through holes formed in the top walls of the bolster ends. The top walls are further embossed at the outer edges of the holes to provide clearance around the connecting devices and are depressed at the inner ends of the holes so that a fastening tool can be in serted through the hole and advanced into engage ment with each connecting device.

, 10 Claims, 5 Drawing Figures PATENTEDSEP 12 191 "saw 2 or z IIIIIIIIIIII BACKGROUND OF THE INVENTION V This invention relates in general to railway trucks and more particularly railway trucks having snubbed bolsters.

Railway trucks having snubbed bolster ends, more commonly known as Barber stabilized trucks, employ a pair of wedge shaped friction castings between the side surfaces of the bolster ends and the opposing surfaces on the side frames. Those castings are urged upwardly by springs so that they fit snugly between bolster ends and the corresponding side frames. The presence of the spring loaded friction blocks or castings prevents the truck suspension system from bottoming out, even when subjected to the severe impacts caused by rough road beds, and furthermore tends to damp oscillations.

- In Barber stabilized trucks of current manufacture, the wedge shaped friction castings fit into friction pockets in the bolster ends and therein bear against inclined wear surfaces which form an integralpart of the truck bolster. Since considerable friction is generated at the engaged wear surfaces on the bolster ends and friction blocks,.both the bolsters and friction blocks in time. wear out at those surfaces. While the friction blocks are relatively small castings and easily replaced at little expense, the truck bolster is not. On the contrary, truck bolsters are large and relatively complex castings which are only replaced at considerable expense.

The foregoing problem is particularly acute where abnormal wear occurs such as on some unit trains and other equipment which is subjected to high mileage service.

SUMMARY OF THE INVENTION wear plates of the type stated which is easily installed on the truck bolster with tools and securing devices currently available in railway maintenance shops. Yet another object is to provide a bolster of the type stated which possesses the strength of conventional truck bolsters. Still another object is to provide a bolster which is suitable for unit trains and other equipment subjected to high mileage service where abnormal wear occurs. These and other objects and advantages willbecome apparent hereinafter.

SUMMARY OF THE INVENTION The present invention resides as a bolster provided at its ends with replaceable wear plates which engage spring loaded friction blocks on the side frames. The bolster ends are substantially hollow and access to the connecting devices which secure to plates thereto is gained through holes in the bolster ends.

The invention also consists in the parts and in the arrangements and combinations of parts hereinafter described and claimed.

DESCRIPTION OF THE DRAWINGS In the accompanying drawings which form part of the specification and wherein like numerals and letters refer to like parts wherever they occur:

FIG. I is a fragmentary side elevational view of a railway truck provided with a bolster constructed in accordance with and embodying the present invention;

FIG. 2 is a sectional view-taken along line 2-2 of FIG. 1;

FIG. 3 is a sectional view taken along line 3-3 of FIG. 2; I

FIG. 4 is a sectional view taken. along line 4-4 of FIG. 3; and

FIG. 5 is a sectional view taken along line 5-5 of FIG, 4.

DETAILED DESCRIPTION Referring now in detail to the drawings 2 designates a railway truck which broadly speaking includes a bolster 4, a pair of side frames 6, spring sets 8 for supporting the bolster 4 at its ends on the side frames 6, and a pair of wheel sets 10 supporting the ends of the side frames 6. 1 I

Each side frame 6 (FIG. 1') is of the conventional truss variety and has pedestal end 12 provided with downwardly extending jaws 14 into which hearing assembles 16 are fitted. The bearing assembles 16 in turn journal the ends of axles forming part of the wheel sets 10. The pedestal ends 12 of each side frame 6 are joined through an upper compression member 20, as well as through a pair of oblique tension members 22 interconnected by a lower tension member 24, the latter being located directly below the center portion of the upper compression member 20. The tension member 24 and compression member 20 are connected by a pair of columns 26, thus forming a bolster' window 28 in the side frame 6.

The sides of the window 28 are defined in part by vertical wear plates 30 (FIG. 3) which are attached to the columns 26 by conventional railway 'lockbolts 32. In particular, the lockbolts 32- extend through the plates 30 and likewise through the columns 26, and beyond the columns 26 they are fitted with collars 34 which are clamped about the shanks of the bolts 32 by a swaging tool designed specifically for that purpose. The swaging tool fits in the triangular opening located on each side of the bolster 4. Swaging tools of this nature are commonly found in railway shops and are known as lockbolt guns. They are available from the Townsend Company or the Huck Manufacturing Company.

Cast integral with the lower tension member 24 is a spring seat 36 (FIGS. 1 and 2) having spring retainer lugs 38 projecting upwardly from it. The spring seat 36 furnishes subjacent support for the spring set 8, the individual springs of which are restrained at their lower ends by the spring retainer lugs 38.

The bolster 4 has an enlarged center portion 40 (FIG. 2) which supports the railway car body, and that center portion tapers to reduced bolster ends 44 which project through the bolster windows 28 of the side frames 6. Each bolster end 44 rests upon a spring set 8 so that bolster 4 is spring suspended on the side frames 6.

Both the central portion 40 and the bolster ends 44 are cast integral to one another and for the most part are hollow. Yet they contain numerous integrally cast ribs, fillets, gussets, and the like for imparting strength to the entire bolster 4. The hollow portion of the bolster 4 is formed in the conventional manner, that is by supporting cores within a mold and pouring the metal in the voids between the cores and mold. The cores are of course held in place by core supports extending from the mold and these supports leave various outwardly opening holes in the final casting, that is in the bolster 4. Two of these holes are in each bolster end 44.

More specifically, each bolster end 44 in transverse cross section (FIG. 4) includes a bottom wall or spring seat 46 which engages and rests upon the top ends of the coil springs in the corresponding spring set 8. Like the spring seat 36, the spring seat 46 has retainer lugs 48 which retain and prevent lateral shifting movement of the top ends of the springs of the spring set 8. The spring seat46 forms the bottom wall of the bolster end 44 and is located below a generally horizontal top wall 50. Interconnecting the top wall 50 and spring seat 46 is a central rib 52 which extends vertically through the interior .ofthe bolster end 44. The top wall 50 and spring seat 46 are further connected by side walls 54, (FIG. 2) which extend inwardly from the inner edges of the wear plates 30 and also outwardly from the outer edges of the wear plates 30. The side walls 54 are furthermore parallel to the exposed surfaces on the wear plates 30, but are set slightly inwardly therefrom so that a slight amount of clearance exists between the walls 54 and the wear plates 30. The side walls 54, are interrupted opposite to the wear plates 30, and there the spring seat 46 and top wall 50 are connected by inclined backing walls 56 which converge toward the spring seat 46. Each backing wall 56 furthermore has a rectangular recess 58 into which a wear plate 60 is fitted.

The wear plate 60 is formed from a hard steel and is rectangular like the recess 58 which receives and positions it. The wear plate 60 is also provided with a centrally disposed hole 62 having beveled sides. When the wear plate 60 is fitted into the recess 58 the hole 62 registers with another hole 64 in the backing wall 56, the latter hole being located considerably closer to the top wall 50 than to the spring seat 46. The holes receive a lockbolt 66 which like the lockbolt 32 is of the conventional construction and is found in most railway repair shops. The lockbolt 66 has a countersunk head which fits into the beveled hole 62 of the wear plate 60 so that is does not protrude beyond the outwardly presented surface of that plate 60. The shank of the lockbolt 66 extends through the hole 64 and projects beyond the inwardly presented surface of the backing wall 56 where it is embraced by a locking collar 68 swaged onto it as will be subsequently described in greater detail.

Inasmuch as the backing walls 56 are set inwardly from the side walls 54, they create a pair of generally triangular pockets within each bolster end 44 and the sides of each pockets are defined by a pair of parallel guide walls 70 (FIGS. 2, 4 and which are perpendicular to the backing wall 56 and the portions of the side wall 54 on each side of the backing wall 56. Each pocket opens outwardly toward one of the wear plates 30 on the columns 26 of the side frame 6.

The pockets within the bolster end 44 contain friction castingsor blocks 72 which possess a wedge shape conforming with the triangular configuration of the pockets and are urged upwardly into the pocket by side springs 74 which seat against the spring seat 36. In particular each friction casting 72 has a pair of angularly related friction surfaces 76 and 78, which abut'against the

wear plates

30 and 60, respectively. Moreover, the width of the friction casting 72 is slightly less than the width of the pocket into which it fits which is the distance between the guide walls 70. Consequently, the friction castings 72 are confined within the pockets and little clearance exists for lateral movement. Inasmuch as the friction castings 72 are urged upwardly between the angularly

related wear plates

30 and 60 by the springs 74 they have a snubbing effect on the bolster 4 as it is depressed in relation to the side frame 6. This enables the truck 2 to withstand extremely severe shocks without having its spring sets 8 bottom out. Furthermore, the friction castings 72 damp oscillations.

Referring again to the top wall 50, immediately rearwardly from each backing wall 56 and between the ends of the corresponding guide walls 70, it flares upwardly in the provision of a curved embossment 80 (FIGS. 35). The thickness of the top wall at the embossments 80 remains unchanged so that the embossments 80 have the effect of providing greater clearance around the collars 68 which embrace the lock bolts 66. Between the two embossments 80 the top wall 50 sinks downwardly below its normal posture to form a single depression 82 (FIGS. 3-5) in the top of each bolster end 44. Again the thickness of the top wall remains constant through this depression 82 so that the depression 82 has the effect of decreasing the height of the hollow interior of the bolster adjacent to the center rib 52. Since the thickness of the top wall 50 remains constant through the embossments 80 and the depression 82 the strength of the bolster end 44 is not significantly altered when compared with conventional bolsters having flat top walls across their bolster ends.

As previously noted the presence of core supports in the mold during casting operation leaves various holes in the bolster 4, and the top wall 50 has a pair of these holes 84 (FIG. 3) to enable the bolster end 44 to be cast with a hollow interior. The holes 84 are elongated and each has its major dimension extending transversely of bolster. Each hole 84 moreover is positioned between the single depression 82 and one of the embossments so that the interior of the bolster end 44 is exposed through them. Moreover, the holes 84 are positioned such that they afford access to the ends of the lockbolts 66 and collars 68 embracing those lockbolts. In this connection, the embossments 80 raise the outer ends of the holes 84 outwardly, whereas the depression 82 brings the inner ends of the holes 84 downwardly (FIGS. 4 and 5). Consequently, the side margins of the holes 84 assume an orientation more closely approaching perpendicular to the axes of the lockbolts 66 than would be the case if the holes 84 were located in a conventional flat top wall. In addition, since the major dimensions of the holes 84 extend transversely of the bolster end 44 instead of longitudinally as is the case with conventional bolsters, they are positioned to compensate for the oblique disposition of the top wall 50 relative to the axes of the The swaging tool 90 is the same swaging tool-used to swage thecollars 34about the shanks of the lockbolts 32 which hold the wear plates 30 on the side frame columns. 26. In particular, the tool 90 includes a body 92 having a reduced cylidrical nose portion 94 projecting from one ,end, and a handle 96extending from its side. The tool 90 is connected to a hydraulic pump and contains a set of hydraulically operated jaws and a swaging anvil precedingthe jaws.'The jaws extend into the .nose portion 94 and when open will receive the shank of a

lockbolt

32 or 36. The anvil possesses an annular configuration, the inner diameter .of (which is smaller than the initial external diameter of the

collars

64 or 68. When the shank of the

lockbolt

32 or 64 is inserted into the open jaws, the anvil will abut against the outer end-of the

corresponding collar

34 or 68. It should be noted thatthe shanks of the

lockbolts

32 and 36 possess closely spaced annular grooves which never open into one another so that no threads or spiral configurations are formed along the shanks. The shank of each lockbolt initially extends completely through and considerably beyond the end of its collar, and just beyond the collar it is provided with an enlarged groove (FIG. 4) which forms a point of weakness in the shank. The portion of the-shank located beyond the groove is known as the tail and isthe part embraced bythe jaws of the tool 90. The collars 34'and 68 are formed from a relatively soft steel or aluminum 50 that they are deformed with relative-case. 3

When the tool90 is actuated, the jaws tightly embracethe shank of the

lockbolt

32 or 66 and draw inwardly. This exerts a pull on the shank and forces the collar tightly against the work. As the pull exerted by the jaws increases the swaging anvil moves over the collar, reducing the diameter of the collar and deforming it into the grooves of the shank. In time the force'exerted by the jaws on the shank o'vercomesthe tensile strength of the shank, and the shank fractures at its weakest point which is at the enlarged annular groove. The tool 90 is then removed, leaving the collar swage'd to the remaining portion of the shank.

The swaging tool 90 is a standard tool used in many railway shops and is more commonly known as a lockbolt gun. It is available from the Townsend Company or the Huck Manufacturing Company.

To assemble the truck 2, the wear plates 60 are positioned in the rectangular recesses 58 of the backing walls 56 at the bolster ends 44, and are oriented such that their holes 62 register with the holes 64 in the backing walls 56. Next a lockbolt 66 is inserted through the registered holes 62 and 64 in the abutting wear plates 60 and backing walls 56 until the countersunk head is positioned within the hole 62. When this occurs the shank of the lockbolt 66 will extend considerably beyond the backing wall 56 and into the interior of the bolster end 44. Thereafter, the collar 68 is fitted through the hole 84 and advanced along the shank of the bolt 66 until it abuts the inwardly presented face of the backing wall 56.

Once the collar 68. is positioned on the bolt 66, the nose 94 of the swaging tool 90 is inserted through the hole 84 providing access to that particular bolt 66. The tool 90 is advanced until the swaging anvil in its nose portion 92 is positioned against the outer end of the collar 68, in which case the shank of the bolt 66 will extend. further into the tool 90 where it will be received by the jaws located therein. It should benoted that the embossment 80' provides sufficient clearance at the top of the collar 68 to enable the nose portion 94 to ad- 'vance still further and-completely around the collar 68 (FIG. 4). Similarly, the depression 82 and the orientation of the hole 84-with its major dimension extending transversely of the bolster end 44, provides sufficient clearance for the cylindrical body 92 of the swaging tool 90 andiprevents the topwall 50 from interfering with that body 90. Thus, the tool 90 has direct access to the collar 68 and no binding or misalignment occurs.

Oncethe swaging anvil of the tool 90 abuts against the collar 68, the tool 90 is activated. This, causes the jaws to grip the extended portion or tail of the shank on the lockbolt 66 and to draw the entire lockbolt 66 inwardly, thereby causing the wear plate 60 to seat snugly against the backing wall 56 within the confines of the recess 58. It also causes the swaging anvil to move along the collar 68 and swage or compress the collar 68 about the shank of the bolt 66. Since the shank of the bolt 66 in this vicinity is provided with annular grooves, the metal of the collar 68 flows into the grooves, firmly attaching the collar 68 to the shank of the bolt 66. In time the axial force exerted on the lockbolt 66 overcomes the tensile strength of the bolt 66, and the bolt 66 fractures at its weakest point, which is the enlarged groove or point of weakness located immediately beyond the end of the collar 68. After this fracture occurs the entire tool 90- is withdrawn from the hole 84 and the bolster 4.

The wear plates 30 are installed on the columns 16. of the side frames 6 in the same manner, and access to the collars 34 of the lockbolts 32 is gained through the triangular cutouts which are on each side of the columns 16.

Once the

wear plates

30 and 60 are secured in place, the bolster ends 44 are fitted through the bolster windows 28 of the side frames 6, and the friction castings 72, their side springs 74, and the spring sets 8 are all installed in the usual manner. Likewise, so are the wheel sets l0.

When the truck 2 encounters an uneven section of track the bolster ends 44 depress relative to the side 1 frame 6, and the coil springs of the spring 8 and likewise the side springs 74 are accordingly compressed. The downward movement of the bolster 4 causes the friction castings 72 to wedge even tighter siderable friction between the friction surfaces 76 and the wear plate 30 and likewise between the friction surface 78 and the wear plate 60. Wear, of course, accompanies the friction and in time the friction castings 72 and the

wear plates

30 and 60 wear out. The friction castings 72, being relatively small and simple castings, are easily replaced in the conventional manner, and likewise so are the wear plates 30.

When the wear plates 60 are no longer serviceable they may be removed from the bolster ends 44 by cutting the collars 68 away from the lock bolts 66. The wear plates 60 are then replaced with new ones in the manner previously described. The wear plates 60 prevent the friction castings 72 from acting directly upon the casting of the bolster 44 and as a result the bolster casting is not worn. Thus, after extended periods of use, the railroad need not replace the entire bolster 4, as is the conventional practice, but need only replace the wear plates 60. This in turn saves railroads considerable expense, since the cost of the four wear plates 60 is considerably less than the cost of an entire bolster 4. Moreover, the wear plates 60, being formed from hardened steel, are not as soft as the steel from which the bolster 4 is cast. Consequently, they last longer, and this provides a considerable saving in labor and tear down expenses.

In lieu of the lockbolts 66 and collar 68, the wear plate 60 may be held againstthe backing wall 56 by bolts having threaded shanks and lock nuts engaging those threaded shanks. Similarly, the holes 84 would provide access to the nuts, while the embossments 80 would afford sufficient clearance around the nuts to enable a socket wrench tobe fitted over them. The depression 82 on the other hand lowers the inner margins of the holes 84 so that the socket wrench may extend into those holes without interferring with the top wall 50.

This invention is intended to cover all changes and modifications of the example of the invention herein chosen for purposes of the disclosure which do not constitute departures from the spirit and scope of the invention.

What is claimed is:

1. In a railway truck including side frames, each provided with a bolster window and having wear surfaces located at the sides of the bolster window, springs extending up through the windows, and spring biased wedge-shaped friction members positioned against the wear surfaces, a bolster extending between the side frames and comprising ends projecting through the windows, the bolster ends resting upon the springs and including backing walls presented opposite to the wear surfaces on the side frames, the bolster ends being substantially hollow at the backing walls and further including top walls extending between the backing walls and overlying the hollow portions of the bolster ends, the backing walls being inclined relative to the wear surfaces on the side frames and to the top walls, the top walls being embossed upwardly next to the backing walls and depressed inwardly from the backing walls, the top walls having holes opening into the hollow interiors of the bolster ends and extending into the depressions thereof, wear plates positioned against the backing walls and engaging the friction members whereby the friction members are fitted between the wear plates on the bolster ends and wear surfaces on the side frames, and connecting means extending from the wear plates and through the backing walls, the connecting means extending into the hollow interiors of the bolster ends below the embossments of the top walls and being accessible through the holes in the top walls thereof, the embossments of the top walls providing additional clearance around the connecting means and the extensions of the holes into the depressions of the top walls affording better access to the connecting means.

2. A structure according to claim 1 wherein the holes are elongated and have their major dimensions extending transversely of the bolster; and wherein the holes extend into the embossments in the top walls.

3. A structure according to claim 1 wherein thickness of each top wall remains substantially constant and does not change through the depressions and embossments therein.

4. A structure according to claim 3 wherein spring seats connect the bottom ends of the backing walls and bear against the upper ends of the springs; and wherein the depressions are positioned centrally between the backing walls.

5. A structure according to claim 4 wherein a rib connects the spring seat and the top wall at each depression.

6. A structure according to claim 1 wherein each connecting means comprises a bolt having a head countersunk in the wear plate and a shank extending through the backing wall into the interior of the bolster end, and a locking element encircling the bolt shank behind the backing wall and connected securely to the bolt shank.

7. A structure according to claim 6 wherein the shank has separate annular grooves; and wherein the element is a collar which is swaged about the shank and deformed into the grooves.

8. A structure according to claim 6 wherein each backing wall has a recess; and wherein the wear plates fit into the recesses.

9. In a railcar truck including a bolster, a side frame having generally vertical columns which partially define a bolster window into which the end of the bolster projects, springs carried by the side frame and supporting the bolster at its end, wedge-shaped friction blocks positioned adjacent to the columns, and additional springs urging the wedge-shaped friction blocks into snug fitting relation between the columns and the bolster end; an improved bolster end comprising: a top wall; a bottom wall spaced from the top wall and forming a seat for the springs which support the bolster; spaced sidewalls interconnecting the top and bottom walls on each side of the bolster end; spaced backing walls also interconnecting the top and bottom walls on each side of the bolster end, the backing walls being located opposite to the columns on the side frame and being set inwardly from the sidewalls to form pockets which open toward the columns for reception of the friction blocks, wear plates against the backing walls and engaged with the friction blocks so that the backing walls do not wear as the friction blocks shift relative to the end portion; bolts engaged with the wear plates and extending through and beyond the backing walls; and locking elements embracing the portions of the bolts located behind the backing walls for holding bolts by aligning a tool axially with respect to the bolt and extending the tool through one of said openings. can

10. The structure according to claim 9 wherein each .bolt has a head engaged with its backing plate and a shank provided with circumferentially extending grooves, and wherein the locking element is a collar which is swaged into the grooves-of the bolt shank.

P t n 3.690.270

lnventofls) Paul F. Gutmann, Jr. and Thomas L. Mueller It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

The Assignee should be: Diversified Industries, Inc. d/b/a Scullin. Steel Company Y Signed and sealed this 17th day of April 1973.

(SEAL) Attest:

EDWARD M.PLETCHER,JR.v

ROBER GOT' SCHALK Attesting Officer Commissioner of Patents FORM PO-105O (10-69) USCOMM-DC GO376-P69 u.s GOVERNMENT PRINTING OFFICE: I969 0-366-334

Claims

9. In a railcar truck including a bolster, a side frame having generally vertical columns which partially define a bolster window into which the end of the bolster projects, springs carried by the side frame and supporting the bolster at its end, wedge-shaped friction blocks positioned adjacent to the columns, and additional springs urging the wedge-shaped friction blocks into snug fitting relation between the columns and the bolster end; an improved bolster end comprising: a top wall; a bottom wall spaced from the top wall and forming a seat for the springs which support the bolster; spaced sidewalls interconnecting the top and bottom walls on each side of the bolster end; spaced backing walls also interconnecting the top and bottom walls on each side of the bolster end, the backing walls being located opposite to the columns on the side frame and being set inwardly from the sidewalls to form pockets which open toward the columns for reception of the friction blocks, wear plates against the backing walls and engaged with the friction blocks so that the backing walls do not wear as the friction blocks shift relative to the end portion; bolts engaged with the wear plates and extending through and beyond the backing walls; and locking elements embracing the portions of the bolts located behind the backing walls for holding the bolts and wear plates in position; the top wall having openings therein substantially aligned with the bolts and sized to receive the locking elements, whereby each locking element cam be installed on one of said bolts by aligning a tool axially with respect to the bolt and extending the tool through one of said openings. can

10. The structuRe according to claim 9 wherein each bolt has a head engaged with its backing plate and a shank provided with circumferentially extending grooves, and wherein the locking element is a collar which is swaged into the grooves of the bolt shank.