Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENTION
The present invention relates to gripper plates, and
more particularly to a push-on gripper plates for securing a mesh
screening means onto the protruding end of rock bolts, such as
are used in mine wall and roof stabilization.
R~ ROUND OF THE INVENTION
It is well known in the mining and shoring arts to
utilize rock bolts to secure a wire mesh over the rock face of
a mine roof or wall so as to stabilize the rock face and reduce
the risk of injury to mine personnel by way of falling rock.
Similar mesh screening means are sometimes used in excavation and
shoring operations where rock or shale faces may be exposed. In
both types of such operations, a suitably sized hole is drilled
into the rock or shale material generally perpendicular to its
face, and the rock bolt is inserted into the hole to a depth at
which its threaded free end protrudes beyond the rock face by
several inches. The rock bolts are retained in the drilled holes
by any conventional means, and such retention means, together
with the rock bolts themselves are well-known and are not part
of the present invention.
After insertion of the rock bolt into a pre-drilled
hole, a retention plate having a centrally positioned hole is
typically placed over the protruding free end portion of the rock
bolt and a conventional washer and nut are threaded onto this
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free end to hold fast the retention plate against the rock face
and thereby stabilize the rock bolt within the rock material.
Conventional forms of retention plates are readily available for
this purpose, and examples thereof can be seen in U.S. Patent
Nos. 3,090,203 (Durget) and 4,740,111 (Gagnon). Once the rock
bolts are installed in this manner, the wire mesh can be applied
over the rock face and held thereagainst by means of secondary
fastening means. Such secondary fastening means may be applied
to the rock face independently of the rock bolts, or more
efficiently, by attachment to the protruding free end of the rock
bolts. The present invention is concerned with the provision of
an improved secondary attachment means of the latter type.
The mesh screening means used is typically a wire mesh
screening means, although other materials can be used. The size
of the openings in the mesh screening means is larger than the
diameter of the rock bolts, so that the rock bolts can pass
freely therethrough. Of course, the mesh openings are
sufficiently small to prevent large pieces of the rock face from
breaking loose and passing through the installed mesh.
SUNMARY OF THE INVENTION
The present invention relates to a push-on gripper
plate for use in securement of a mesh screening means on the
protruding, threaded, free end of a rock bolt inserted into a
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rock face, wherein the gripper plate comprises a generally planar
base portion adapted to overlie the mesh screening means. A
central opening is provided in the base portion having two or
more leg portions positioned on the perimeter of the opening, the
leg portions being inclined downwardly, inwardly toward the
centre of the opening below the plane of the base portion. A
plurality of tab portions are also positioned on the perimeter
of the opening in alternating arrangement with the leg portions,
the tab portions being inclined downwardly, inwardly toward the
centre of the opening below the plane defined by the base
portion. The inclination angle of the tab portions is preferably
steeper than the inclination angle of the leg portions. With
this arrangement, the leg portions are dimensioned and otherwise
adapted to frictionally engage therebetween the threaded end of
the rock bolt to hold the gripper plate on the end of the rock
bolt over the mesh screening means. Moreover, the tab portions
are dimensioned and otherwise adapted to together provide a
gripping surface for overlying engagement by a socket or other
type of wrench, so as to allow the wrench to be used for pushing
the gripper plate on to the free end of the rock bolt, and/or for
screw tightening thereof during installation, should this be
necessary.
The gripper plate described is simpler to manufacture
and less expensive than previously known gripper plates. It will
be appreciated that both the leg and the tab portions can be die
punched in a single punch operation from the material surrounding
the perimeter of the opening. Not only does this make the
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punching dies simpler and cheaper to construct, but there is no
material waste in the production of the gripper plate.
Other prior art gripper plates, such as that disclosed
in U.S. Patent No. 4,740,111 (Gagnon), have side flanges around
the outer perimeter, which side flanges require considerably more
material to construct, at substantially increased cost. Also,
such side flanges, or other protruding tabs spaced from the
central opening make packaging, handling and shipping of the
gripper plates more difficult, as tight nesting of the prior art
plates is not easily achievable.
It should also be appreciated with the gripper plate
of the present invention, that the folding back of the tab
portions away from the edge of the central opening give a greater
freedom of choice in the angling and dimensioning of the leg
portions of the gripper plate, which factors are highly critical
to the yield strength of the gripper plate. Moreover, such
freedom of choice allows the leg portions to be freely
dimensioned and angled to accommodate a wide range of rock bolt
diameters and thread patterns. In a preferred embodiment of
gripper plate according to the invention, each of the leg porions
extending from the perimeter of the central opening has a
different inclination angle, such that each leg portion contacts
the rock bolt thread at a different angle. This difference in
contact angle can be tailored to a specific thread pattern, so
as to dramatically increase the gripping strength of the gripper
plate, to the extent that loads in excess of the failure loads
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of the screening means can be achieved. As such, the gripper
plate ceases to be the weakest link in the support system, as has
been the case with prior art gripper plates. These and other
advantages of the novel gripper plate disclosed herein will
become more apparent from the detailed description of a preferred
embodiment of the invention which follows immediately below.
DE~TT-T~n DESCRIPTION OF A PREFERRED ENBODINENT
Introduction to the Drawings
Figure 1 of the drawings appended hereto is a schematic
view illustrating installation of a wire mesh screening means
onto the end of a rock bolt utilizing a gripper plate according
to the invention;
Figure 2 of the drawings is a perspective view of the
gripper plate and rock bolt of Figure 1;
Figure 3 of the drawings is a top plan view of the
gripper plate of Figures 1 and 2; and,
Figure 4 is a sectional view of the gripper plate of
Figures 1, 2, and 3, taken along line 4-4 of Figure 3.
Referring now to the drawings, there will be seen a
push-on gripper plate 10 for use in securement of a mesh
screening means 12 on the protruding, threaded, end portion 15
of a rock bolt 14 inserted into a rock face 16. The rock bolt
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14 is of well-known construction, and is inserted into a drilled
hole (not shown) in the rock face 16, so that the threaded
portion 15 protrudes several inches beyond the rock face. After
such insertion, but prior to installing the gripper plate 10, a
conventional retention plate 30 is placed over the threaded
portion 15 of the rock bolt 14, followed by a conventional washer
32. A conventional nut 34 is then tightened against the
retention plate 30, so as to hold the retention plate 30 against
the rock face 16, and thereby anchor the rock bolt 14. Once the
rock bolt 14 is installed in this manner, a conventional wire
mesh screening means 12 is placed over the protruding threaded
end portion 15 of the rock bolt 14, with the threaded end 14
passing through a hole in the screening means 12. The gripper
plate 10 is then pushed over the threaded end portion 15, either
by hand, or, if necessary, with the aid of a socket or similar
type of wrench (not shown) in a manner more fully described
below.
The gripper plate 10 comprises a generally planar base
portion 18 adapted to overlie the mesh screening means 12. A
central opening 20 in the base portion 18 has four leg portions
22 positioned on the perimeter of the central opening 20, each
of the leg portions being inclined, in use, downwardly, inwardly
toward the centre of the opening 20 above the plane of the base
portion, as best seen in Figure 1. The inclination angle of the
leg portions 22 is nominally 45 degrees from the plane defined
by the base portion 18, but each of such leg portions may be
several degrees greater or less than 45 degrees. In fact, it is
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preferable that the inclination angle of the four leg portions
22 not be equal, so as to enhance retention of the gripper plate
10 on the screw threads of the threaded portion 15 of the rock
bolt 14. The exact inclination angle of each leg portion 22 can
be routinely calculated in respect of each leg portion 22, with
specific reference to the size of the central opening 20, the
diameter of the threaded end portion 15 and the dimensions and
angling of the particular thread pattern used on the end portion
15. The object of such calculations is to angle the leg portions
22 to generally mimic the thread pattern of the nut 34, (which
thread pattern is complementary to that of the threaded end
portion 15) so as to maximize the axial load bearing
characteristics of the gripper plate 10, while at the same time
allowing the gripper plate 109 to be pushed on to the protruding
end 15 of the rock bolt 14 without the absolute need of full
threading. The free ends 38 of the leg portions 22 are preferably
concavely curved (see especially Figure 4) so as to more firmly
nest in engaged relation between the individual threads of the
threaded end portion 15 (see especially Figure 2).
Four tab portions 28 are also positioned on the
perimeter of the central opening 20 in equidistantly spaced
relation from one another, the tab portions 28 being
preferentially angled at substantially 90 degrees relative to the
plane defined by the base portion 18, so as to facilitate their
being frictionally engaged by the inside diameter of a socket
wrench for ease of installation. In such instance, the gripper
plate 10 is simply placed on the end of the socket wrench (not
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shown) with the four tab portions inserted into the socket
wrench. Then, the gripper plate is pushed over the threaded free
end 15 of the rock bolt 14, until the base portion 18 contacts
the nut 34 or the wire screening means 12, whereupon final
tightening of the gripper plate 10 by means of the socket wrench
can be achieved. Of course, such rotary tightening can be
utilized at any point of travel of the gripper plate 10 along the
threaded end portion 15, should the leg portions 22 bind with the
threading on the rock bolt 14. Moreover, removal of the gripper
plate 10 through use of a socket or similar wrench is facilitated
by the presence of the tab portions 28.
As illustrated, it is preferred to have four leg
portions 22 and four tab portions 28 positioned on the central
opening in alternating fashion, each of the leg 22 and tab 28
portions being equidistantly spaced around said opening. With
the tab portions 28 pulled downwardly to an inclination angle of
substantially 90 degrees, this arrangement provides for maximum
flexibility in design of the leg portions 22.
A raised boss 36 of circular plan outline is preferably
provided on the lower surface (as seen in Figure 1) of the base
portion 18 of the gripper plate 10 adjacent to the outer
perimeter of the base portion 18. This boss 36 adds rigidity to
the base portion 18.
During installation, the friction plate 10 is pushed
on to the rock bolt 14 to secure the mesh screening means 18
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between a standard retention plate 30 and the rock face 16, as
shown in Figure 1. A socket or similar wrench (not shown) may
be used in the installation to assist in pushing on or tightening
of the gripper plate, but this is not necessary in most
5 applications. The installation operation is simpler than with
prior art gripper plates, and may be done by hand without the use
of jacklegs or stoppers in a minimum period of time.
Preliminary pull tests were done on steel gripper
10 plates as illustrated constructed of C1050 spring steel, hardened
and tempered to a Rockwell "C" scale hardness of RC 32 to 40,
with an average hardness of RC 38. The thickness of the plate
metal was 0.060 inches, and the outside diameter was 5 inches.
The threaded section of a 5/8" O.D. rock bolt was inserted into
15 the tensioner and the gripper plate was installed on the rock
bolt as previously described. Loads were then applied to the
gripper plates in three different modes as follows: a) directly
to the hub of the plates; b) as an annulus 2" from the hub of
the plates; and, c) as an annulus 4" from the hub of the plate.
20 In the hub loading mode a), average loading at failure was 3.75
tons. In the 2" annulus loading mode b), average loading at
failure was 2.5 tons, and in the 4" annulus loading, the average
loading at failure was 2.2 tons.
Similar test conducted on the gripper plates using a
3/4" O.D. rock bolt resulted in average failure loads of 2.8
tons.
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Failure during testing was gradual, on a thread by
thread basis, with the gripper plates tending to slide down the
rock bolt a thread at a time. Unlike the prior art gripper
plates, including the plate disclosed in U.S. Patent No.
4,740,111, the threads of the rock bolts were not damaged after
failure. Moreover, the gripper plates tested could, after
failure, still be re-installed on the rock bolt by hand, and
after such re-installation still carried an average residual load
of almost 1 ton for the 5/8 inch bolts and 1 ton for the 3/4 inch
bolts. These factors combine to provide a better chance than
with known gripper plates to contain the mesh screening means
following an instantaneous loading situation. Moreover, these
tests indicate consistent failure loads in excess of the failure
loads of most mesh screening means used in mining operations,
thus confirming that the subject gripper plates are not the weak
link in the supplementary support system described herein.
Although this invention has been disclosed with
reference to a particular preferred embodiment as shown and
described, it is to be understood that it is not to be limited
to such embodiment and that other alternatives are envisaged
within the scope of the following claims.
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