CA1039986A

CA1039986A - Rock bolt assembly

Info

Publication number: CA1039986A
Application number: CA255,302A
Authority: CA
Inventors: Donald B. Haslam; John W. Reynolds
Original assignee: NATIONAL HARDWARE SPECIALTIES Ltd
Current assignee: NATIONAL HARDWARE SPECIALTIES Ltd
Priority date: 1976-06-21
Filing date: 1976-06-21
Publication date: 1978-10-10

Abstract

ABSTRACT

An expansible shell formed of a deformable material and rock bolt assembly incorporating said shell is described.
The shell comprises a one-piece casting formed of zinc, zinc alloy, aluminum, iron or plastics by die or gravity casting or by injection moulding. Each shell consists of a plurality of outwardly diverging leaves each with a transversely convex curved outer rock engaging face for defining a substantially cylindrical external surface when the shell is compressed to its operative shape. A planar face is formed on the inner side of each leaf inclined at a small angle of about 8° to the longitudinal centerline of the outer convex face whereby the leaf has a thin inner end and a thick outer end. The shell inner ends each have a thin, resilient arm formed as an inner extension which interconnect to form a central bail-like structure.
In a preferred embodiment, the shell is adapted to fit over and engage a nut having a plurality of tapered planar faces corresponding with the planar inner faces of the leaves for expanding the leaves radially upon longitudinal axial movement of the nut relative to the shell.
The shell and rock bolt assembly of the invention provide a simple structural combination wherein the shells can be nested for storage and shipment, easily adapted for use, and the rock bolt assembly readily installed to provide improved surface area contact with a drill hole wall and improved resistance to sleeve slippage.

Description

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1039~6 The present invention relates to rock bolt assemblies and, in particular, is directed to rock bolt assemblies in which a novel expansible shell has a plurality of deformable leaves connected in a one-piece casting by resilient arms which form a ~
::.,. :., bail-like structure. `-In rock bolting to provide stability in a mine roof, `
rock bolt shells engage drill hole walls in a wedging action as tightening of a bolt moves a shell relative to an expansion wedge.
The rock bolt shell in most common use is a two-piece, two or four-leaf malleable iron expansion shell having serrated outer faces which engage the rock wall and substantially conical inner `~-faces which engage outer conical faces of an expansion nut ... ..
threaded onto a bolt. The two pieces comprising the shell are tied together with a spring steel bail which loops over the end of the assembly which first enters a drill hole. On expansion ~' of a shell which has been inserted into a drill hole, edges of `
the shell serrations contact the rock face in a limited area.
For example, in a drill hole having a nominal l 3/8 inch diameter, , . .
anchorage per leaf face of a shell is only about 0.1 square inch, `'~
i.e. an area about 1~8 inch wide by about 3/4 inch long. -Concentrated compressive force applied aga~nst the rock face through the edges of the serrations and~shear stress resulting ;
from tightening of the bolt impose excessive stresses on these small rock areas. Spalling of the rock occurs and the leaves of the shell plow the rock surface to move into new positions. The , resulting slippage is accompanied by bleed-off of bolt tension and loss of effectiveness of the rock bolt assembly.

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~)39~86 It is desirable to have a rock bolt assembly which gives good shell-to-rock contact in a hole which may vary in diameter. For example, a nominally 1 3/8 inch diameter hole may vary in size between 1 1/~ inch and 1 1/2 inch. Since a drill hole is not perfectly uniform in diameter throughout its length, and since this diameter also varies with bit wear, it is difficult to match the external shape of the shell with the shape of the hole, The leaves of a shell in an oversize portion of a drill hole will abut the rock along the central portion of the face of each leaf as the leaf expands to touch the rock surface while the sides of the leaf face will be spaced from the ;
hole wall. Conversely, in an undersize portion of a drill hole, the edges of the face of each leaf will abut the rock while the central part of the leaf face is spaced from the hole wall. -~
This difflculty is most apparent with a shell having only two leaves. As the number of leaves is increased, the arc portions of the shell become shorter and the contact area is increased.
The shortcomings of two-piece, malleable iron expansion shells having serrated outer faces and substantially conical ;, inner faces have been overcome in part by the rock bolt assemblies of tbe following patents.
In Canadian Patent No. 575,401, R. J. Letourneau describes a rock bolting unit in which the innex end of a rod has six flat tapered faces which merge with the body af the rod.
Three tapered leaves generally triangular in shape and having serrations on their outer faces are secured against alternate rod faces by a retaining ring. A compression spring biases the leaves to slide up the tapered faces of the rod to provide automatic expansion of the sleeve. This provides initial engagement with the hole wall which permits movement of the rod faces relative to the leaf faces during tightening of the rod.

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An anchoring device described by G. Allimann in Canadian Patent No. 5~8,337 comprises an elongated rod having , at one end a plurality of outwardly divergent wedge surfaces -~
and a plurality of clamping jaws which co-operate with the wedge surfaces of the rod and which are suspended from an elastic ''~
hood carried by the end of the rod. Each clamping jaw moves ~-independently of all' the others both axially and radially. ,~
The one-piece expansion shell described in "`~
F. P. Dickow's United States Patent No. 3,200,693 is formed of a malleable iron casting. The upper end of the shell is divided into four segments and at least two of these segments have - , ' inwardly extending tabs which engage the upper end of a bolt as `''' the bolt is tightened and which are deformed to pass over the `,` ~ ,' bolt threads as the bolt pulls the shell segments against a , tapered wedge nut. The tapered wedge nut has a cylindrical .,; ,, upper portion and a concave toroidal lower portion. ~i , A mine roof expansion shell assembly having a one-piece shell which may be a malleable iron casting is described in Canadian Patent No. 937,792 by F. P. Dickow et al. The ,' shell comprises four like fingers projecting from a ring which `' engages a bolt. The components of the shell assembly are maintained in an assembled relation by a U-shaped strap which extends over the end of a wedge, between adjacent opposite ' -' pairs o'f shell fingers, and hooks onto the ring at the outer end ~
of the shell to retain the wedge wi'thin the space enclosed by r'' '~ ' "
the fingers.
We have found that disadvantages inherent in the rock ~ ; , bolt assemblies described in the foregoing patents can be ' obviated or minimized by the novel expansible shell and rock ,~j bolt assembly of the present invention. Our shell comprises, ' '-'.: '.
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in general, a one-piece casting having a plurality of outwardly diverqing leaves each with a transversely convex curved outer, - rock-engaging face, said leaves collectively capable of defining a substantially cylindrical external surface, and a planar, ~-.
: expansion means-engaging face formed on the inner side of each leaf, ~
:, i said planar inner face inclined at a small angle to the longitudi-nal axis of the corresponding outer face, said angle opening outwardly from the leaf inner end towards the lea outer end :
whereby said leaf has a thin inner end and a thicker outer end, 10 and a plurality of thin, resilient arms formed as inner extensions -~ of the thin inner end of said leaves interconnected to form a :~ central bail-like structure.
:~ More particularly, the rock bolt assembly of our invention comprises, in combination, a shell formed of a one-.` piece casting having a plurality of outwardly diverging leaves each with a transversely convex curved outer, rock-engaging face, .`
said leaves collectively capable of defining a substantially cylindrical surface, and a planar, expansion means-engaging face formed on the inner side of each leaf, said face inclined at a . 20 small angle to the longitudinal axis of the corresponding outer face, said angle openlng outwardly from the leaf inner end towards the leaf outer end whereby said leaf has a thin inner end and a thicker outer end, and a plurality of thin, resilient arms formed as inner extensions of the thin inner end of said ` leaves interconnected to form a central bail-like structure; i expansion means having planar faces corresponding with the planar inner faces of the leaves and oppositely inclined at the same small angle to its principal axis adapted to fit within said shell, said expansion means operatively connected to a bolt; :~
and means for moving said ~xpansion means longitudinally relative ,. .
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to said shell whereby said shell leaves expand by movement of said inclined planar inner faces on said oppositely inclined planar faces of said expansion means to bring the shell convex `
outer faces into engagement with said drill hole wall while ~-retaining the shell planar inner faces in large area contact ~
with the planar faces of the e~pansion means. -: .
The shell of the assembly of the present invention has -~
.
a plurality of cast leaves, preferably six leaves, which have substantially smooth cylindrically contoured outer faces to engage the rock wall and which have inclined planar inner faces to engage and abut equal but oppositely inclined planar faces of expansion means such as an expansion nut. On expansion of the shell, the leaves move radially outwardly to provide a large `~
area of contact between their cylindrically contoured outer faces and the rock wall while retaining their planar inner faces in ~;
large area contact with the planar outer faces of the expansion -`
nut. These wide area contacts provide solid back-up for the shell leaves thereby restraining the leaves against movement.
Conventional rock bolt expansion shells are generally formed of malleable cast iron, steel or ductile iron castings, `
e.g. Designation ES 3-75 of the American Society for Testing and Materials section 5.3. The shells of the present invention are preferably made of zinc or zinc alloy and may be gravity or die cast. Other materials such as aluminum, iron or synthetic plastics, shells of the latter material formed by injection moulding, may be used. The shell material is softer than the rock and as the relatively soft leaves of the shell are pressed , against a rock surface on the drill hole wall, rock projections ! ' . , .
; penetrate into the cylindrical outer faces, thus effectively ~' . , .
increasing the area of contact. This penetration into the soft ;;
. -:~ , shell surface by rock projections minimizes concentration of _ 5 _ .`. ' .
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rn~rcssive forces and shear stresses normally encountered with hard serrated shell faces and substantially reduces rock failure by spalling with improved astening of the shell to the rock.
With this drill hole wall engagement, the bleed-off of bolt tension which normally occurs after installation is substantially controlled.
It is a principal object of the present invention to provide a novel expansible shell having deformable leaves with surfaces which can be deformed by uneven rock surfaces under pressure less than that at which rock failure occurs.
It is another object of the present invention to provide - a simple, inexpensive rock bolt assembly having an expansible shell which on expansion makes large area contact with a drill hole wall backed up by large area of contact between the leaves of the shell and expansion means.
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It is a further object of the invention to provide a shell having a plurality of deformable leaves, preferably six to eight, in a one-piece casting, wherein the inner ends of the .: . . .
leaves are connected by resilient arrns which unite to form a bail-like structure which permits both nesting of shells in storage and simple engagement of a shell with an expansion plug and bolt for facile insertion of the assembly into a drill hole without the need for separate retaining devices.
These and other objects of the invention and the manner in which they can be attained will become apparent from the following detailed description, reference being made to the following drawings, in which:

.: ,- , Figure 1 is an exploded perspective view of an embodi-ment of the rock bolt assembly of the invention;-; , ~,. ...
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~ ~ ~03~s~6 Figure 2 is an enlarged perspective view of asingle leaf of the shell shown in Figure l;
Figure 3 is a perspective view of an assembly ::
of the invention with the shell loosely ~:.
. engaging an expansion nut mounted on a ; -threaded rod; ~`. - ~-Figure 4 is an elevation, partially in section, :~ ---~ along the principal axis of the assembly showing the assembly in an unexpanded initial arrangement within a drill hole; ..
~ Figure 5 is an elevation, partially in section, of .. the assembly shown in Figure 4 in an ~ expanded operative position illustrating . engagement of leaves with the drill hole wall; 6 '`! ` ..
Figure 6 is a transverse sectional view on line 6-6 .
of Figure 4 showing engagement of six leaves 'l with the expansion nut, and illustrating .
-~ certain leaf shape embodiments; .
- Figure 7 is an elevation, partially in section, along ~............................ the principal axis of the assembly of another ;.:~ preferred embodiment of our invention in an unexpanded position within a drill hole; and Figure 8 is a perspective view of the embodiment of ~: the shell of the invention shown in Figure 7. v.
With particular reference to Figures l and 3 of the ~
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i ~drawings, rock bolt assembly lO generally comprises threaded bolt 12, facèted expansion nut 14 threaded onto bolt 12, and ` .
~ shell 16 loosely mounted on nu-t 14. Shell 16, and shell 16' .~ 30 illustrated in Figures 7 and 8 wherein corresponding parts of ~- :
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~,' ;' ~: ' ' ' r '1~3~J9~i6 the shell are desiynated by a prime ('), preerably have six out-wardly diverging leaves 18 connected together at their inner ends -~ by resilient arms 20 of reduced thickness which unite to form a bail-like structure 22 of sufficient diameter to span the inner end 24 of bolt 12. It will be understood that the following descrlption of the shell 16 of the invention relative to Figures 1 through 6 herein will generally refer also to the corresponding ' ~; components of the embodiment designated 16' in Figures 7 and 8. ' ~ Expansion nut 14 has a set o symmetrical, outwardly diverging ' ,~ 10 flat faces 26 which form a hexagon as illustrated in Figure 6 to ~, ' correspond with and engage inner flat faces 28 of leaves 18 ,~
,' during expansion of shell 16. A castellated structure may be , formed at the large forward end, i.e. upper end shown in the -~
'~ drawings, comprising a set of radial slots 30 enclosed by lug `~
~' extensions 32 which serve to locate and retain interconnecting ', ., .
arms 20 of shell 16, as shown most clearly in Figure 3, by ~' insertion of lug extensions 32 into openings 23 formed between ',~
'~, arms 20. With reference to Figure 4, the bases of radial slots 30 ' and faces 26 intersect at juncture edges 36, upon which edges leaves 18 pivot during expansion of the shell. Outermost portions ~' ' 34 of lug extensions 32 can be bevelled so that extensions 32 do ~,r?'~' ";' ~ ,' not extend radially beyond outer faces 38 of leaves 18 as the ,~
, shell engages the nut. ','-'~' ,`, The angle 6C defined between flat faces 26 of nut 14 and j. ,,.; , , .
--, a line parallel to its longitudinal principal axis 40, and the ''~'~''''' ' angle ~ defined between flat faces 28 of leaves 18 and imaginary ,~

,~ straight line 42 extending longitudinally on leaf outer face 38, - as indicated in,Figures 3 and 4, preferably are equal to each ,, other, being about 8, for reasons which will become apparent as c,~,,,~ , '30 the description proceeds. Outer faces 38 of leaves 18 ~` ',' ,~

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each have a transversely convex, longitudinally linear shape capable of defining a substantially cylindrical external surface . .~ ` ':
~` when the leaves of the shell are collected together in their operative rock-engaging position.
Bail-like structure 22 may be cylindrically shaped with ;~
arms 20 interconnecting to form an inner face 44 which is flat as : shown in Figure 4. Arms 20 preferably are sufficiently long and have sufficient resilience to permit loose engagement of leaves 18`
with nut faces 26 without breakage and to permit adaptation of `
shell 16 readily to nuts 14 of different sizes. The rearward , extremities, i.e. thicker outer ends, of planar inner faces 28 ~`
of leaves 18 may have central concave recesses 46, as shown in Figures 1 and 3, which are sufficiently deep to preclude contact of the leaves 18 with bolt 12 when leaf faces 28 are pressed ,:, . .~`~ inwardly to pivot about edges 36 to engage nut faces 26. This `
.~ , recessing permits inner faces 28 of an unexpanded shell to extend `~ further beyond the end of nut 14 before contacting bolt 12 than . .
` otherwise possible. Thus, longer faces 28 are permissible ~`
providing greater surface contact during sleeve-to-nut engagement on expansion.
~;~ The embodiment of the shell of the invention illustrated in Figures 7 and 8 differs from the embodiment of Figures 1 y- through 6 only in that arms 20' of bail-like structure 22' of shell 16' may be shortened and may beincurved in section, i.e.
outwardly concave, with the inner face 44' adapated to resiliently . `;~6 '.. .
engage bolt end 24.
For an assembly designated for a 1 1/4 inch to 1 1/2 ..
inch diameter drill hole formed in rock 56, transversely convex .. .
curved, longitudinally straight outer faces 38 of six leaves 18 ~-~ 30 are portions of a 1 3/8 inch diameter cylinder. The transverse .,' `, , _ g _ : .~.
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Q39~t36 confiyuration of the lcaves is illustrated in Fiyure 6 which shows a cross section of leaves 18 arranged about nut 14. Side edge portions 48 of leaves 18 are bevelled parallel with radii of the cylinder such that a space 49 is defined between adjacent edges 48 to avoid interference of leaves 18 with each other on closure of the leaves.
Referring now to Figures 4 and 5, it is evident that `;
as shell 16 is expanded to engage the wall of a arill hole larger ` than the diameter of the shell 16, faces 28 of shell 16 will advance along faces 26 of nut 14 by relative longitudinal movement of the nut 14 to sleeve 16. It is desirable, when the shell ~`;
loosely engages the bolt as shown in Figure 4, that the outer ; ends of leaf faces 28 extend as far as possible beyond the end ;
; of nut 14 without interference with bolt 12. Recesses 46 permit this extension, thus ensuring greater shell-to-nut engagement on y expansion of the shell.
~ Leaves 18 of shell 16 are shown spread apart in Figures ; 1 and 8. This permits packaging of a number of shells in a nested stack, shell 16' being somewhat more suitahle for nesting `
in a compact stack. As indicated in Figure 3, the leaves are pressed closer together prior to use to permit entry of the shell into a drill hole. This closure may be effected by compressing `"~
~, the leaves within the elastic limit of resilient arms 20. Alter- ; -natively, the shell may be heated to a temperature at which it i` ~;
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softens and pressed into its expansion nut-engaging position. On i ;
cooling, the shell will loosely envelope the expansion nut. ~e ` ~
; have found, particu`larly with zinc or zinc alloy shell heated to ~ -about 95C., this heat treatment and shaping of the shell permits `!
~ -.,: ' yood engagement of the leaves with the planar faces of nuts of 30 different sizes. Thus, for example, use of a smaller or larger .. ~ . ' ','''" ', ",: .

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, are deormed and penetrated by rock surfaces to provide greater contact area and an irregular surface interface which provides ;~
improved fastening of the shell to the rock. sail arms 20 may ~ `
be broken during the tightening. ~
Rock bolt assemblies are generally used in holes made ~ -with a rock drill. Spalling of the bore wall during the drilling operation normally leaves surface irregularities and high spots of these irregularities penetrate the surface of a zinc or a -zinc alloy leaf which enables the metal to come into improved contact with low spots. As discussed hereinabove, a multi-leaf shell having cylindrically contoured leaves provides maximum contact with these surface irregularities. Since several short ~-. " .
circular arcs will fit more closely to a circle of different -` diameter than will a pair of semicircles, and since the cylind-rical curvature of each leaf is intermediate those of maximum and minimum diameter holes in which the assembly is to be used, no . .
portion of a cross section circle of a leaf is far from the drill hole wall when another portion makes contact. Maintenance of a substantially cylindrical rock engaging surface throughout radial 20 expansion of an assembly provides best assurance of shell-to-roak ~ . .f`
contact in a drill hole which, although varying in diameter through its total length, is substantially cylindrical through the ~;' short length which a rock bolt shell engages. This radial expansion is assured by the equally inclined flat faces of the leaves and the nut which guide leaf movement during expansion.
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For example, in a six-leaf assembly designed for use in a ' ` nominally 1 3/8 inch diameter hole, each leaf has a minimum , ~ :
, expansion nut contacting surface which is about 0.38 inch by ~;
0.6 inch or 0.228 square inches when fully expanded to a 1 1/2 ,~
30 inch diameter. Total contact area of six leaves with an .. , ' . ., ~:

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diameter nut may cause the same shell to expand to cngage a hole varying betwcen 1 1/4 to 2 inches in diameter. With heat pretreat- :
ment and shaping, the castellated structure of the nut to provide slots 30 may not be required. To achieve sufficient elasticity -- ~
for closure from a packaging to a hole entry leaf arrangement, - -long resilient arms 20 of shell 16 may be preferred. ; -- Initial placement within the drill hole is shown in ` ~ ~
Figures 4 and 7. Even though pressed while heated to a nominal ~ -. , , .. , . ,.~
; diameter which is about that of the drill hole, both embodiments -!;, - `.
of the shell illustrated provide good initial leaf-to-rock engagement which keeps the shell from rotating during initial rotation of the bolt. The method of shaping does not precisely enclose the outer extremities of all the leaves within the same G' ` circle. This permits some leaves to extend, resiliently beyond . . ~ .
the others, thus facilitating initial engagement. Rotation of expansion nut 14 wlth bolt 12 is restrained by engagement of `~
. . i.,.:
resilient arms 20 of the leaves within radial slots 30 of nut 14. `-During tightening of the bolt, leaves 16 pivot on nut edges 36 until leaf faces 28 are in full contact with opposed nut ~`~
~20 faces 26 and cylindrically shaped outer leaf faces 38 are - substantially parallel to bolt axis 40 and in full contact with the hole wall. Some movement of nut 14 within shell 16 to a position intermediate those shown in Figures 4 and 5 occurs. On !~:,-, .
further tightening of the bolt, movement between equally but i ~-oppositely inclined flat faces 26 and 28 causes leaves 18 to , j . ~- . . .
move radially from the bolt axis into an expanded arrangement, ~, shown in Figure 5, in which there is large area pressure contact ~-of outer cylindrically shaped leaf faces 38 with the drill hole ;;

wall. As shell securement force is applied to the bolt, the ~` ;`
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, 30 surfaces of faces 38 of, for example, zinc or zinc alloy leaves , ., ',`';
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10399~36 expansion nut is therefore 1.37 square inches. This is illustrated in Figure S which shows part of the surfaces of the expansion nut, -~
i.e. the 0.6 inch portion at the thicker end, in contact with the leaves. Good positive back-up by the expansion nut behind the -~
area of contact with the rock is achieved.
Although the zinc leaves of the present invention -- -preferably have smooth rock-engaging faces which provide large area contact, a slightly rough, sand cast type surface may ba formed on the leaf surfaces without detracting from the ~ ~-advantages of being cylindrical. Also the tips of outer leaf ;-faces 38 can be knurled, as indicated in Figure 2, to enhance frictional engagement with the rock wall in a smooth drill hole for initial seating of the shell. In modifications of the invention, particularly applicable to assemblies designated for .
use in drill holes having a diameter greater or less than that `~ ;
of the cylindrically curved outer faces o the shell leaves, the leaves may be cast with faces 38 having flat centràl portions 58 or having central portions 60 having a slight reverse curvature ., as shown in Figure 6. Initial contact of each leaf with the rock ` 20 will then be on two curved face portions 62 or on two curved face ~- portions 64. Formation of slight bevels 66 between faces 38 and 48 of the leaves will decrease the chance of edge contact when . ~ .
the shell engages a hole portion having a diameter less than that of the curvature of faces 38. These modifications ensure use of ~` two wide area contact zones extending through the length of each `~
; leaf face 38, whether the hole diameter at the area of contact is ~ greater or less than the diameter of the curvature of faces 38.
.
~ Zinc or zinc alloy castings have sufficient tensile . ,.i .. ~ . .
strength and are sufficiently creep reslstant to serve as rock bolt shells. The shells are preferably die cast. -~
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Intricacy of design of the expansion nuts of the `~
invention dbes not permit economic machine fabrication of nuts.
However, nuts with the required surface configuration and having adequate tensile strength and creep resistance can be made by a process involving powder metallurgy. Iron or aluminum may be used.
It may be desirable to form nut 14 integral with rod 12 by upsetting the rod end and forging a plurality of flat tapered i~

. . . .
- faces symmetrically thereon. For example, a hexagonal plug 10having the shape and dimensions of nut 14 can be formed on a `~
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threaded rod to receive a shell 16 and longitudinal movement of `
the plug relative to the sleeve provided by axially retracting the rod, without rotation of the rod, from a drill hole by tightening a nut threaded on the projecting end of the rod and seated against a stationary bearing plate. Seating of shell 16 :: ~
^` within a drill hole would occur as has been described hereinabove :!, ~`~ relative to the embodiments illustrated.
Although dissimilar metals in a wet environment are ~ subject to corrosion by galvanic action, corrosion of zinc shells `; 20 is not an important factor in potential rock bolt failure because most holes are drillea in mine roofs permitting drainage of ~ ~`
' moisture.
It will be understood, of course, that modifications can be made in the embodiment of the invention illustrated and described herein without departing from the scope and purview of the invention as defined by the appended claims.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A shell for use in a bolt assembly for anchoring in a drill hole, comprising: a one-piece casting having at least six symmetrically disposed outwardly diverging deformable leaves each with a substantially transversely convex curved outer rock engaging face, said leaves collectively capable of defining a substantially cylindrical external surface having a large area of contact, and a planar face formed on the inner side of each leaf, said planar inner face inclined at a small angle to the longitudinal axis of the corresponding outer face, said angle opening outwardly from the leaf inner end towards the leaf outer end whereby said leaf has a thin inner end and a thicker outer end, and a plurality of thin, resilient arms formed as inner extensions of the thin inner end of said leaves interconnected to form a central breakable bail-like structure, said casting formed of a material deformable by the rock in which it is anchored whereby rock projections can be embedded in the shell leaves.

2. A shell as claimed in Claim 1, wherein said casting has a plurality of outwardly diverging leaves symmetrically disposed about a central bail-like structure whereby said shells can be stacked by nesting.

3. A shell as claimed in Claim 1, wherein said one-piece casting is formed of zinc, zinc alloy, iron or aluminum by gravity or die casting or of plastics by injection moulding.

4. A shell as claimed in Claim 1, 2 or 3, wherein said thin resilient arms are of reduced width relative to the leaves whereby radial openings are defined between adjacent resilient arms about the centre of the bail-like structure.

5. A shell as claimed in Claim 1, 2 or 3, wherein the small angle is about 8 degrees.

6. A shell as claimed in Claim 1, 2 or 3, wherein the central bail-like structure is cylindrically shaped.

7. A shell as claimed in Claim 1, 2 or 3, wherein the opposite side edges of the leaves are bevelled parallel with radii of the cylindrical surface definable by the leaves whereby the leaves are spaced from each other upon closure of the shell.

8. A shell as claimed in Claim 1, 2 or 3, wherein the planar inner face of each leaf has a concave central recess formed therein at the thicker outer end.

9. A shell as claimed in Claim 1, 2 or 3, wherein each leaf outer face has a flat central portion extending the length thereof providing two wide-area control zones on each said leaf outer face.

10. A shell as claimed in Claim 1, 2 or 3, wherein each leaf outer face has a concave central portion extending the length thereof providing two wide-area contact zones on each said leaf outer face.

11. A rock bolt assembly for anchoring in a drill hole comprising, in combination: a shell comprising a one-piece casting having at least six symmmetrically disposed outwardly diverging deformable leaves each with a substantially transversely convex curved outer, rock-engaging face, said leaves collectively capable of defining a substantially cylindrical external surface having a large area of contact, and a planar face for receiving expansion means formed on the inner side of each leaf, said face inclined at a small angle to the longitudinal axis of the corresponding outer face, said angle opening outwardly from the leaf inner end towards the leaf outer end whereby said leaf has a thin inner end and a thicker outer end, and a plurality of thin, resilient arms formed as inner extensions of the thin inner end of said leaves interconnected to form a central breakable bail-like structure; expansion means having planar faces corresponding with the planar inner faces of the leaves and oppositely inclined at the same small angle to its principal axis adapted to fit within said shell, said expansion means operatively connected to a bolt; means for moving said expansion means longitudinally relative to said shell whereby said shell leaves expand by movement of the shell inclined planar faces on said oppositely inclined planar faces of said expansion means to bring the shell convex outer faces into engagement with said drill hole wall while retaining the shell planar inner faces in large area contact with the planar faces of the expansion means, said shell casting formed of a material deformable by the rock in which it is anchored whereby rock projections can be embedded in the shell leaves.

12. A rock bolt assembly as claimed in Claim 11, wherein said expansion means comprises a nut threaded onto said bolt for longitudinal movement thereon, said nut having planar faces formed symmetrically thereon corresponding to planar inner faces of the leaves for engaging said leaves.

13. A rock bolt assembly as claimed in Claim 12, wherein said nut has a castellated end defining recesses and lugs and said shell thin resilient arms are of reduced width relative to the leaves defining radial openings between adjacent resilient arms about the centre of the bail-like structure whereby the nut lugs can project into the shell radial openings to locate and retain said arms.

14. A rock bolt assembly as claimed in Claim 13, wherein outer edges of said lugs are bevelled such that the lugs will not project beyond the outer face of the shell when said lugs engage the shell.

15. A rock bolt assembly as claimed in Claim 12 or 13, wherein said shell has six leaves symmetrically disposed about the central bail-like structure and said nut has a correspond-ing hexagonal shape.

16. A rock bolt assembly as claimed in Claim 11, said expansion means comprising a plug formed integral with said rod, said plug having a plurality of flat tapered faces formed symmetrically thereon corresponding to planar inner faces of the leaves.

17. A rock bolt assembly having a shell as claimed in Claim 11, 12 or 13, wherein said one-piece shell casting is formed of zinc, zinc alloy, iron or aluminum by gravity or die casting or of plastics by injection moulding.

18. A rock bolt assembly as claimed in Claim 12 or 13, wherein the small angle of inclination of the shell planar faces and of the nut planar faces is about 8 degrees.

19. A rock bolt assembly having a shell as claimed in Claim 11, 12 or 13, wherein the central bail-like structure is cylindrically shaped.

20. A rock bolt assembly having a shell as claimed in Claim 11, 12 or 19, wherein said arms of said bail-like structure permit initial resilient engagement of the leaves with the faces of the expansion means without breakage and can break during tightening of the bolt.

21. A rock bolt assembly having a shell as claimed in Claim 11, 12 or 13, wherein the opposite side edges of the leaves are bevelled parallel with radii of the cylindrical surface definable by the leaves whereby the leaves are spaced from each other upon closure of the shell.

22. A rock bolt assembly having a shell as claimed in Claim 11, 12 or 13, wherein the planar inner face of each leaf has a concave central recess formed therein at the thicker outer end to receive the bolt therein.

23. A rock bolt assembly having a shell as claimed in Claim 11, 12 or 13, wherein each leaf outer face has a flat central portion extending the length thereof providing two wide-area control zones on each said leaf outer face.

24. A rock bolt assembly having a shell as claimed in Claim 11, 12 or 13, wherein each leaf outer face has a concave central portion extending the length thereof providing two wide-area contact zones on each said leaf outer face.