Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1317~89
This invention relates to a rock bolt and more
particularly, but not exclusively, to a rock bolt
for use in underground mining operations.-
Many different kinds of rock bolts are used to
secure rock strata underground. One kind comprises
a length of reinforcing bar having ribs along its
length. The ribs enhance the anchoring ability of
the bolt. In the case where a settable material
with a catalyst is used to secure the bolt in a
hole in the rock matrix, the bolt is usually
rotated in the hole to mix the settable material
and the ribs also serve the purpose of assisting
the mixing of the settable material. However,
known ribbed formations on rock bolts do not
provide good mixing characteristics.
The end of the bolt which is intended to project
from the rock face has a rolled thread thereon
which receives a nut used to tension the bolt. The
bar from which such a prior art rock bolt is made
is manufactured in a steel mill and is supplied
with the ribs formed on the whole length of the
bar. Thus, in order to provide the rolled thread
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on the bolt, the bolt has to be machined to remove
the ribs on a section thereof and to ensure that
such section has a circular cross-section-. Such a
machining operation adds to the cost of the prior
art rock bolt.
It is an object of the invention to lessen the
problems associated with prior art rock bolts.
In this specification, the term "pitch diameter",
in relation to a straight thread, means the
diameter of an imaginary co-axial cylinder, the
surface of which passes through the thread
profiles at such points as to make the width of the
groove equal to one half of the basic pitch of the
thread. On a perfect thread this occurs at the
point where the width of the thread and groove are
equal.
The term "bar" when used in this specification
includes a pipe and a bar with an axial bore.
According to the invention, a method of making a
rock bolt with a thread on a section thereof,
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comprises the steps of providing a metal bar of
circular or near circular cross-section having a
diameter which is substantially equal to-the pitch
diameter of the thread to be provided thereon,
forming a series of protrusions on a selected
section of the bar, straightening the bar, and,
forming a thread with the said pitch diameter on a
further selected section of the bar.
This method obviates the need for machining the bar
before forming the thread thereon. As will become
evident in the subsequent description, the method
also results in a saving of material without
reducing the yield or tensile load-carrying ability
of the rock bolt.
In one form the metal bar is first cut to a
discrete length, is then advanced through means
forming the series of protrusions on the selected
section of the bar, and is thereafter advanced
through straightening means to straighten it,
whereafter thread is formed on the further selected
section of the bar.
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In another form a continuous length of the metal
bar is first advanced through means forming the
series of protrusions on selected sections of the
bar, the continuous length is thereafter advanced
through straightening means to straighten it,
whereafter the bar is cut to required lengths and
thread is formed on a selected section of each
length.
The protrusions may be formed in the metal bar in a
single pass through a pair of rolls in the cold
condition of the bar.
The protrusions may be formed on the bar by
pinching the metal of the bar at intervals along
the length of the selected section of the bar. The
protrusions which are thus formed may be in the
form of flat lobes provided in staggered formation
along opposite sides of the bar. The sides of each
lobe nearest and furthest the axis of the bar may
be arcuate so that each lobe has a generally
elliptical outline.
The protrusions may be formed on substantially the
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entire length of the bar, save for the selected
section on which the thread is formed.
The thread formed on the further selected section
of the bar may be formed by cold rolling.
According to a further aspect of the invention
there is provided a method of making a rock bolt
from a metal bar comprising the steps of providing
a pair of rolls defining a nip between them and
having formations provided at selected positions
along their peripheries, rotating the rolls in
opposite directions so that a formation on one roll
coincides periodically with a formation on the
other roll in the nip of the rolls, and, feeding
the bar lengthwise through the nip of the rolls so
that the formations pinch the metal of the bar at
selected positions along its length.
The metal bar may be s~raightened in two planes,
preferably by passing it sequentially through two
sets of opposed, staggered rollers.
The invention also provides apparatus for making a
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1317~8~
rock bolt from a metal bar comprising a pair of
rolls, means for driving the rolls about their axes
in opposite directions, each roll having a channel
formed in its surface facing the other roll, the
channels defining a passage for receiving the bar
in lengthwise fashion, each roll having formations
provided along its periphery which align
periodically with similar formations on the other
roll in the nip of the rolls when they are rotated
in opposite directions.
The apparatus may include means for straightening
the bar after it has been passed between the rolls.
The means for straightening the bar may comprise
two sets of opposed staggered rollers through which
the bar is passed, the rollers of one set being
located at right angles to those of the other set.
The formations on the rolls may be in the form of
pegs mounted on the rolls.
According to a further aspect of the invention
there is provided a rock bolt comprising a metal
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bar having a circular or near circular
cross-section of a selected uniform diameter, the
bar having a thread with a selected pitch-diameter
formed on a selected section thereof located at at
least one end of the bar, the pitch diameter of the
thread on the bar being substantially equal to the
diameter of the bar, and a series of protrusions
pinched from the bar formed on a further selected
section of the bar.
The rock bolt of the invention can be of any
required length and the selected section carrying
the protrusions will normally be longer than
metre.
The invention also extends to a rock bolt when made
according to the method of the invention described
above.
A preferred embodiment of the invention will now be
described by way of example with re'erence to the
accompanying drawings in which;
Figures A to C illustrate a prior art method of
making a rock bolt;
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Figures la to lc illustrate steps in making a
rock bolt according to the
method of the invention-;
Figures 2a and 2b are a schematic elevation and
plan view respectively of
apparatus used to carry out the
method of the invention;
Figures 3a to 3e are enlarged views of parts of
the apparatus of Figures 2a and
2b, and
Figures 4a and 4b show two different kinds of rock
bolts made according to the
invention,
With reference to Figure A, a prior art steel bar
(a) is provided, and is cut to the required
length. The bar (a) carries a series of
diametrically opposed inclined ribs (b) separated
by a pair of longitudinal ribs (c) of which only
one is shown. The ribs (b), (c) are formed
conventionally by hot rolling during manufacture of
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the bar (a) in a steel mill. The next step in
making the prior art rock bolt is to machine or
shave the bar (a) as shown in Figure B along a
section (d) of its length to remove the ribs (b),
(c) and to provide the section (d) with a circular
cross-section of diamater (e). Thereafter a thread
(f) is rolled onto the section (d) of the bar as
shown in Figure C to complete the rock bolt. It
will be appreciated that the tensile strength of
the bolt is determined by the minimum diameter of
the thread (f) of the bolt, since that is its
smallest diameter.
With reference to Figures la to lc, a rock bolt
according to the invention is formed from a steel
bar 1 of circular or near circular cross-section.
Thè first step in making a rock bolt from the bar 1
is to form a series of protrusions in the form of
flat lobes 2 in the bar by a method which will be
more fully described hereunder. A section 3 of the
bar which is to be threaded is left free of lobes
2. After the lobes 2 have been formed in the bar
and after the bar has been passed through a
straightening station to straighten it, a thread 4
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1~17~89
is cold rolled on to the section 3 of the bar. The
pitch diameter of the thread 4 is indicated by
numeral 5 and is substantially equal to the
diameter of the bar.
In this embodiment of the invention, as most
clearly shown in Figures lb and lc, the lobes 2 are
arranged in staggered formation along the length of
the bar 1. The sides 2a, 2b of each lobe
respectively nearest and furthest the axis of the
bar are arcuate so that each lobe has a generally
elliptical outline in plan view.
In an alternative method the thread 4 may first be
rolled onto the section 3 of the bar after which
the lobes 2 may be formed on the remainder of the
bar. Thereafter the bar may be straightened.
~he manner of forming the lobes 2 on the bar and
the apparatus used for this purpose is illustrated
in Figures 2 and 3. The apparatus, indicated
generally by numeral 6, comprises a frame 7 having
a pair of rolls 8, 9 mounted thereon for rotation
about horizontal axes. An electric motor 10 is
~317~8~
provided to drive the rolls about their respective
axes in opposite directions. The drive is effected
through a pulley 11 and a suitable gear train (not
shown).
The rolls 8, 9 are mounted in such a manner that
they can selectively be displaced towards and away
from one another. This may conveniently be
effected by hydraulic means (not shown).
The rolls 8, 9 have grooves 12, 13 formed
respectively in their edges facing one another,
which grooves define a passage for receiving the
bar 1 in lengthwise fashion (Figure 3a). Each roll
has a plurality of pegs 14 mounted thereon along
its periphery. The pegs 14 are arranged in such a
manner that corresponding pegs on the rolls
periodically align in the nip of the rolls when
they are rotated. The pegs 14 are preferably of
cirular cross-section but they -ay be of any
required cross-section depending o-. the shape of
the lobes 2 required on the bar 1.
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Alongside the rolls 8, 9 there is a set of driven
rollers 15 followed by a set of straightening
rollers 16 which are arranged in opposed,-staggered
formation (Figures 2a, 2b). Alongside the
straightening rollers 16 there is a further set of
driven rollers 17. The sets of rollers 15, 16 and
17 are mounted for rotation about horizontal axes.
Alongside the driven rollers 17 there is a further
set of straightening rollers 18 arranged in
opposed, staggered formation and mounted for
rotation about vertical axes. Finally, alongside
the straightening rollers 18 there is a set of
driven rollers 19 mounted for rotation about
horizontal axes. The sets of driven rollers 15, 17
and 19 are driven by an electric motor 20.
In use, the bar 1 is cut to a discrete length and
is fed through a pair of guide rollers 21 into the
nip of the rolls 8, 9 while the rolls are driven in
opposite directions. As the bar 1 passes through
the nip of the rolls the pegs 9 periodically pinch
the metal of the bar 1 to form the lobes 2
therein. This is achieved in a single pass of the
bar 1 through the rolls 8, 9 in the cold condition
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1317~8~
of the bar. (Owing to the size of the drawings,
the lobes 2 are not shown on the bar 1 in Figures
2a and 2b. They are, however, shown in Figures 3a
and 3d.)
The rolls 8, 9 are controlled to ensure that the
lobes 2 are formed only on a selected section of
the bar. In this embodiment of the invention this
is achieved by means of a pair of sensing devices
22, 23 mounted ahead of and behind the rolls 8, 9.
The sensing device 23 mounted behind the rolls 8, 9
comprises a support 24 carrying a retractable stop
member 25 and a sensor 26 (Figure 3b). The support
24 also carries two pairs of dependent guides 27,
28 through which the bar 1 moves. The sensing
device 22 mounted ahead of the rolls 8, 9 may be
the same as the sensing device 2, 3 but the
retractable stop member 25 may be omitted.
With the rolls 8, 9 in an open position, the bar 1
is advanced freely therebetween. As the leading
end of the bar enters the sensing device 23 located
behind the rolls 8, 9 the sensor 26 of the device
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1317~89
23 causes the stop member 25 to retract and causes
the rolls 8, 9 to close and to commence rotating.
The bar 1 advances further through the rolls 8, 9
as the lobes 2 are formed thereon and as the
S trailing end of the bar leaves the sensing device
22, the rolls 8, 9 are caused by the sensor of the
device 22 to open and to cease rotation. The
sensing devices 22, 23 are adjustable in the
direction of the path of the bar 1 and hence the
sections of the bar which are free of lobes 2 and
which carry lobes 2 can be selected by forward and
rearward adjustment of the sensing devices 22, 23.
The driven rollers 15, 17 and 19 serve to advance
the bar 1 along its path through the apparatus 6.
To maintain pressure between opposing sets of
driven rollers 15, 17 and 19, pneumatic heads 29
are provided which carry the upper rollers in the
sets. Each head 29 has a dependant rod 30 which
connects to a mounting 31 which carries the
associated roller 16 and which is slidable in a
guide 32 tFigure 3c).
As the bar passes between the straightening rollers
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131748~
16 it is straightened in a vertical plane and as it
passes between the straightening rollers 18 it is
straightened in a horizontal plane. The rollers 18
have central channels 33 formed therein to
accommodate the lobes 2 on the bar as it passes
between the rollers (Figure 3d).
Each of the straightening rollers 16, 18 is
adjustable towards and away from its opposing
rollers to enable its effect on the bar 1 to be
varied. For this purpose each straightening roller
16, 18 is mounted on a slide 34 which is slidable
in a guide 35 and is adjustable by means of a
setting bolt 36 (Figure 3e). The slide 34 is
secured by lock nuts 37, 38. In a preferred form,
the rollers, 16, 18 will be so adjusted that a
flexing of the bar 1 takes place as it passes
between the rollers.
Once the bar 1 has passed through the apparatus 7,
the thread 4 is rolled onto a selected section of
the bar in conventional manner.
A chute 39 is positioned beneath the rolls 8, 9 to
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collect scale generated by the action of the rolls
on the bar 1.
The manner of forming the lobes 2 on the bar 1
described above is relatively simple and
inexpensive and the apparatus 7 can maintain speeds
of the bar 1 passing therethrough of at least
35m/min.
In this embodiment of the invention the lobes 2 are
formed in a single plane but if desired they can be
formed in two or more planes.
In an alternative form, a continuous length of the
bar 1 may be fed through the apparatus shown in
Figures 2a to 2b. In such a case the rolls 8, 9
are periodically displaced away from and towards
one another to ensure that the lobes 2 are formed
only on consecutive selected sections of the bar.
Once the bar has passed through the apparatus, it
is cut into the required lengths and threads are
rolled onto the individual lengths in conventional
manner.
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In Figures 4a and 4b two rock bolts made according
to the invention are shown. The rock bolt 40 shown
in Figure 4a is intended for embedding in a
settable resinous material mixed with a catalyst.
It has thread 4 formed at one end thereof to
receive a nut (not shown). At its other end it has
a chamfered point 41 which is used to rupture a
capsule of the settable material ~not shown)
located in the hole in the rock matrix in which the
bolt is to be embedded.
The rock bolt 42 shown in Figure 4b is similar to
the one shown in Figure 4a but instead of the point
41 it has a thread 43. The thread 43 is used to
connect the rock bolt to a conventional mechanical
anchor (not shown) which expands when the rock bolt
is tensioned to anchor it in a hole. In addition
to the mechanical anchor, settable resinous
material or concrete can be used to grout the rock
bolt 42 in the hole.
Where a rock bolt made according to the invention
is to be embedded in a settable material mixed with
a catalyst, the rock bolt will, in use, be rotated
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about its axis to assist in mi~ ae settable
material. The lobes 2 on the bolt will in such a
case assist considerably in mixing the settable
material by causing lateral displacement of the
settable material during rotation of the bolt. In
this regard, any particle situated adjacent the
bolt between two lobes 2 will be displaced by the
lobe on the opposite side of the bolt when it is
rotated. The improved mixing ability of the bolt
arises from the fact that with the use of the lobes
2, the diameter of the bolt at the apices of
opposed lobes can be as m~ch as 1,5 times the
diameter of the bolt. It will also be appreciated
that the rock bolt described above provides a
continuously changing perimeter along its length
and this in itself enhances its mixing ability in
use. The lobes 2 will also assist in the anchoring
of the bolt in settable material.
Whilst the rock bolt of the invention provides a
continuously changing perimeter along its length,
the provision of the lobes 2 thereon does not
materially alter its cross-section along its
length. This means that the provision of the lobes
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131748~
does not result in a sacrifice of cross-section and
hence does not affect the tensile strength of the
rock bolt.
With the method of making a rock bolt in accordance
with the invention the machining or shaving step in
the prior art method described above, is dispensed
with. In addition, a substantial saving of
material can be obtained with a rock bolt made
according to the invention, when regard is had to
the following. The diameter of the ribbed section
of the prior art rock bolt is larger than the
diameter (e) thereof. Yet this larger diameter
does not increase the tensile strength of the prior
art bolt, since the tensile strength is determined
by the minimum diameter of the thread (f). The
prior art rock bolt therefore carries excess
material over the length of its ribbed section. In
the rock bolt made according to the invention, such
excess material is limited since the diameter of
the bar 1 is in the first place chosen to equal the
pitch diameter of the thread 4. In this way a
saving of material of up to 25% can be achieved
compared with a prior art rock bolt, which results
- 20 -
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in a less expensive roek bolt when made aeeording
to the method of the invention.
It is well known that the load eapaeity of the
anchorage provided by a roek bolt embedded in a
settable material sueh as conerete is proportional
to the perimeter of the boit. To ensure that a
rock bolt according to the invention does not have
a lesser anchorage eapacity eompared with a
conventional roek bolt, the perimeter of the bar 1
may be increased before, during or after the
operation in whieh the lobes 2 are formed in the
bar. This may be done, for example, by changing
the eross-seetion of the bar in the lobe forming
operation from a cireular to an elliptical shape.
Thus, the roek bolt of the invention formed in this
way from a bar of smaller eross-seetion than that
of a conventional rock bolt, can provide the same
anehorage eapacity as the conventional rock bolt.
The roek bolt of the invention ean also be made
from pipe or metal bar with an axial bore, for use
in applieations where settable material is injected
- 21 -
1317489
through the rock bolt itself. Also, any part of
the rock bolt of the invention may be left free of
lobes 2, if required.
The invention has particular application to rock
S bolts which are provided in lengths in which the
lobed section is longer than 1 metre.
Other embodiments of the invention may be made
without departing from the scope of the invention
as defined in the appended claims.
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