Note: Descriptions are shown in the official language in which they were submitted.
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A ROCR BOLT ASSEMBLY
The ~reaent invention relates to an assembly of a rock
bolt and a nut.
The ~resent invention relates particularly, although
by no mean~ exclu~ively, to an assembly of a rock bolt and
a nut that i~ ada~ted to form part of a rock bolt system in
which:
(i) the rock bolt is anchored by means of a cement
grout or a chemical resin (hereinafter referred
to as "re~in") in a hole drilled in a rock
formation; and
(ii) the nut tension~ the rock bolt and ~ro~ides the
end restraint on one end of the bolt to retain
bearing plates, strap~, mesh etc.
Rock bolt sy~tems are used to stabilise rock
formations in a wide range of ~ituation~, such a~
underground and surface mines, tunnel~ and cuttings, and
rock bolt~ have achieved a high acce~tance in both the
mining and civil engineering indu~tries.
In any given a~plication, the ~urpo~e of a resin
anchored rock bolt ~y~tem i~ to apply a clam~ing or
confining action to a rock ma~s to control deformation of
the rock mass and to enh~nce the atrength of the rock mass.
In other words, the ~ur~ose of a rock bolt system is to
allow load to be tran~ferred from a rock mass, through the
resin, to the rock bolt.
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A conventional procedure for installing a rock bolt
assembly com~rise~ the following ste~s:
(1) Drill a hole in a rock mas~ to a required de~th.
(2) Pu~h a cartridge of re~in into the drilled hole
with the rock bolt until the cartridge reaches
the end of the hole and the rock bolt ~enetrate~
the cartridge and relea~e~ the re~in.
(3) Rotate the rock bolt to mix the re~in.
(4) After the re~in ~et~, typically within a minute,
rotate the nut on the rock bolt until the nut i8
tight up again t a bearing plate against the rock
mas~ and a tension i~ generated in the rock bolt.
There are a number of known option~ for cau~ing
rotation of the rock bolt in the re~in mixing step (3).
The options are ba~ed on the use of a drilling mach;ne and
on providing some mean~ on or associated with the rock bolt
that can be received in a drive ~ocket of the drilling
mach;ne. The known "means~ include:
(i) a forged nut on the end of the rock bolt;
(ii) a forged drive square on the end of the rock
bolt;
(iii) a crimped section of thread on the end of
the nut, ~uch that a threshold breakout
torque i~ required to break through the
crim~ed section of thread and release the
nut for rotation, and whereby ~o~itioning a
drive aocket to receive the nut and
o~erating at torques les~ than the threshold
torque cau~e~ rotation of the rock bolt with
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the nut;
(iv) a washer tack welded on the end of the nut
which, as with (iii) above, requires a
threshold breakout torque to release the
nut;
(v) a resin or ~lastic plug inserted into the
end of the nut which, as with (iii) and (iv)
above, requires a threshold breakout torque
to release the nut;
(vi) a small steel roll pin inserted into a hole
drilled through a side of the nut and into
the threaded section of the rock bolt which,
as with (iii) to (v) above, requires a
threshold breakout torque to release the
nut;
(vii) a washer located in a recessed section in a
nut and retained by "crimps" at the end of
the recessed section and, as with (iii) and
(iv) above, requires a threshold breakout
torque to release the nut;
(viii) double lock nuts which, as with (iii), (iv)
and (vii) above, requires a threshold
breakout torque to release the nut; and
(ix) an increased diameter of the rock bolt
immediately above the nut which requires a
threshold breakout torque to screw the nut
past the increased diameter section of the
rock bolt.
The above-described known o~tions have a range of
disadvantages including, inconvenience, high cost, residual
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material left in the drive aocket which accumulates with
each rock bolt assembly, and residual tor~ue - the nut~ are
not free r~nn;ng once the breakout torque has been
overcome.
An object of the present invention i~ to provide an
o~tion for causing rotation of a rock bolt in the re~in
mixing ste~ (3) which alleviate~ the disadvantage~ of the
known o~tions described above.
According to the ~re~ent invention there is ~rovided a
method of ~tabili~ing a rock mas~ by means of a rock bolt
as~embly located in a drilled hole in the rock ma~ and
retained in the hole by resin, the rock bolt as~embly
comDrising a rock bolt having a threaded ~ection, a
tensioning nut on the threaded ~ection, and a barrier for
~reventing removal of the nut from a rearward end of the
rock bolt, the method being characterised by:
(i) locating the rock bolt of the rock bolt a~embly
and a cartridge of re~in in the hole;
(ii) rotating the nut of the rock bolt assembly in a
first direction against the barrier and thereby
causing rotation of the rock bolt with the nut
and thereby mixing the re~in in the hole; and
(iii)after the re~in has ~et, rotating the nut in the
op~osite direction to tighten the nut directly or
indirectly against the rock ma~ and thereby
tensioning the rock bolt.
According to the pre~ent invention there is al~o
provided a rock bolt as~embly for tabilising a rock ma~,
which rock bolt as~embly com~ri~es:
(i) a rock bolt having a forward end, a rearward end,
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and a threaded section;
(ii) a nut on the threaded section; and
(iii)a barrier which preventa remo~al of the nut from
the rearward end of the rock bolt.
The rock bolt may be formed from any suitable
material. By way of exam~le, the rock bolt may be formed
from steel. By way of further exam~le, the rock bolt may
be formed from a plastic~ material.
The barrier may be formed from any ~uitable material
and be of any ~uitable shape.
By way of exam~le, the barrier may be a deformation of
the rearward end of the rock bolt which disru~ts the thread
or alter~ the cross-section of the end.
By way of further exam~le, the barrier may be a sto~
de~ice welded, forged, glued or otherwi~e co~nected to the
rearward end of the rock bolt.
Preferably, the main criteria for the ~hape of the
barrier are that:
(a) the barrier be formed to contact a rearward face
of the nut and thereby ~revent the nut being
screwed off the rearward end of the rock bolt;
(b) the nut doe~ not become locked on the barrier; in
other words, the torque required to enable the
nut to be screwed away from the barrier (ie, the
relea~e torque) should be les~ than that a~lied
to rotate the nut and the rock bolt in ste~ (ii)
(ie, the mixing torque); and
.
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(c) the barrier ~hould not damage the thread of the
nut ~o that the nut ia free to rotate during
tightening.
Preferably the barrier i~ a washer welded to the rock
bolt.
It iR ~referred ~articularly that the maximum width
dimension of the barrier be less that that of the nut so
that the barrier doe~ not prevent altogether or restrict a
drive ~ocket of a drill mean~ engaging the nut.
The a~licant has found in a ~eries of laboratory
te~t~ that wa~her~ welded onto rock bolt~ are an effective
barrier. S~ecifically, the a~licant found that the
relea~e torque required to relea~e nuts from contact with
washer~ welded on rock bolt~ waR 23 to 28% less than the
mixing torque a~plied in the fir~t instance to rotate the
nut~ against the wa hers and thereafter to rotate the nutc
and the rock bolts. This mean~ that there is minimal if
any ri~k of locking nut~ on the wa hers after rotating the
- nuts in the first direction de~cribed above.
The rock bolt may be of any suitable configuration.
Preferably the rock bolt com~ri~e~ a continuous or
di~continuou~ rib exten~;ng along at least ~art of the
length of the rock bolt.
The present invention i~ de~cribed further by way of
example with reference to the accompanying drawing which is
a partially ~ectional side elevation of a ~referred
embodiment of a rock bolt a~embly of the ~resent invention
re~in anchored in a drilled hole 11 in a mine roof 13.
The rock bolt a~embly of the ~re~ent invention is
~articularly although by no means exclusively ada~ted for
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u~e in coal or metalliferous mines.
The rock bolt assembly com~ri~e~ a rock bolt 3 having
a forward end 17, a rearward end 19, a generally
cylindrical core 7, a continuous rib 21 exten~;ng along a
~ub tantial part of the length of the core 7, and a single
~tart left hand thread 9 formed on a lower ~ection of the
core 7.
The rock bolt assembly further comprise~ a nut 5
threaded on the rock bolt 3.
The rock bolt as~embly i8 characteri~ed by a barrier
41 connected to the rearward end 19 of the rock bolt 3
which in u~e can contact a rearward face 43 of the nut 5
and thereby prevent removal of the nut 5 from the rearward
end 19.
The barrier 41 may be any ~uitable form of barrier.
Ty~ically, the barrier 41 comprises a ~top device, such a~
a wa~her (a~ shown in the figure), welded or forged onto
the rock bolt 3. Alternatively, the barrier 41 is a
deformed section of the rock bolt 3 formed by hammering or
otherwise a~plying a force to the rearward end 19 of the
rock bolt 3 to di~ru~t the thread on the ~ection of the
rock bolt 3 or to change the cro~ ection of the end.
In u~e of the rock bolt a~sembly, the hole 11 is
drilled in the roof 13 of a mine, a bearing ~late a~embly
15 is pre-a~embled on the rock bolt a~sembly, and a
cartridge (not ~hown) of resin and the rock bolt 3 are
inserted into the hole 11 until the rock bolt 3 penetrate~
the cartridge and relea~e~ the re~in. Thereafter, a drive
~ocket (not ~hown) of a drill meanq i~ po~itioned to
receive the nut 5 and i~ operated to drive the nut 5 in a
clockwi~e direction. Thia rotation ha~ the effect of
moving the nut 5 relative to the rock bolt 3 until the nut
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5 contacts the barrier 41 on the rearward end 19 of the
rock bolt 3. Thereafter, continued rotation of the nut 5
in the clockwi~e direction al~o rotate~ the rock bolt 3,
and this ha~ the effect of mixing the re~in in the hole 11.
After the reain sets, the drilling machine is o~erated to
rotate the nut 5 in the anti-clockwi~e direction, and thi~
rotation screw~ the nut 5 away from the barrier 41 and into
contact with the bearinq ~late assembly to tension the rock
bolt 3.
It is noted that the nut 5 would be rotated in the
op~osite direction to that indicated above in situations
where the thread 9 i~ a right hand thread rather than a
left hand thread as shown in the drawing.
In laboratory te~ts on the above-described rock bolt
as~embly the a~plicant was able to operate with mixing
torques of 280 Nm with hydraulic bolting equi~ment and 120
Nm with hand-held bolting equipment without break-out and
thread damage. Moreover, as indicated above, the applicant
found that the relea~e torque~ were 23 to 28% less than
that of the mixing torque~. In addition, the applicant
applied torque~ of u~ to 500 Nm without severing the
wa~hers and breAk;ng out the nut~ from the rearward end~ of
the rock bolts. The laboratory te~t~ were carried out on
the above-de~cribed rock bolt assembly with rock bolts
having nominal diameters of 21.7mm and 25.8mm and with
wa~her~ having a nominal thickness of 4mm, central hole
diameters of 24mm and 25mm, and outer diameter~ of 36mm and
40mm welded to the rock bolts. The weld~ connecting the
washers to the rock bolt~ extended around a~proximately one
half of the circumference of the central hole of washer on
both faces of each wa~her.
The above-described rock bolt a~sembly ha~ a number of
advantages over conventional o~tion~.
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By way of example, the ~re~ent invention makes it
possible to ensure a high level of control over the extent
to which the nuts 5 are retained on the rock bolts 3 - this
level of consiAtency is im~ortant in mining o~erations.
Furthermore, the ~resent invention ensures that nuts
will never experience ~remature breakout.
Furthermore, the rock bolt 3 and the nut 5 can be
assembled together off-site and thi~ reducea handling
difficulties on-site and the number of component~ that must
be handled on-site.
Furthermore, the present invention enable~ one
breakout ~ystem to be used for all different rock bolt
lengths and for all different drilling m~h;ne~ compared to
having up to 5 different breakout nuts all with different
breakout torque~.
Furthermore, the problem of debri~ accumulating in the
drill ~ocket from the break-out mechanisms in the nut i~
overcome.
The prior art ~roblem of high residual torque required
to turn nut~ after the breakout "mechani~mn ha~ been broken
is avoided by the ~re~ent invention.
Furthermore, the pre~ent invention make~ it ~o~ible
to de~ign the thread form on the rock bolt pecifically to
im~rove the mechanical advantage of the nut - rather than
to trigger the breakout mechanism.
Many modification~ may be made to the ~referred
embodiment of the rock bolt a~embly described above
without de~arting from the s~irit and ~co~e of the ~re~ent
invention.
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- 10 --
By way of exam~le, whilst the preferred embodiment
described above com~rises a rock bolt 3 of a ~articular
configuration, it is noted that the ~resent invention is
not 80 limited and extends to any suitable form of rock
bolt 3.
