Roof bolt anchor

A mine roof bolt expansion anchor utilizes a plurality of transversely bent elongated leaves which are compressed within a bail by the rotation of the roof bolt. The compression results in movement of the leaves toward a flattened position whereby the outer edges of the leaves engage the bore hole walls to anchor the bolt. Downwardly extending flanges on each end of the leaves increase the bearing surface which engages the bore hole wall, while spacers near the center of the leaves act as a spine to evenly distribute the compression to all the leaves so that they all engage the bore hole wall simultaneously and with the same pressure.

BACKGROUND OF THE INVENTION 
This invention relates to mine roof suspension bolt anchors and may be 
described as an expansion anchor for a mine roof bolt. More particularly 
the present invention relates to mine roof bolt anchors which employ 
deformable transverse members to engage an anchor bolt bore hole. 
Mine roof suspension bolts have generally replaced timbers as the means for 
supporting the roof of a mine. These bolts are anchored within bore holes 
drilled into the rock strata overlaying the mine. The bolt is urged 
tightly against a steel roof bearing plate and the stratified rock is 
thereby compressed and bound together into a self-supporting laminated 
beam that is strong enough to support material above and around the 
excavation. 
The obvious problem with mine roof bolts is the stability of the anchor. If 
the anchor slips, the self-supporting beam can be weakened and sag or 
fall. Therefore many attempts have been made to devise anchors which are 
impervious to slippages. Representative of anchors which are in some 
manner similar in appearance to the present invention are those shown in 
U.S. Pat. Nos. 1,367,080; 1,352,201; 2,950,602; 4,100,748; and 4,147,458, 
although some of these are not suitable for use as mine roof bolt anchors. 
The '080 patent and '201 patent are directed to general anchoring means 
and, as will be seen, are not adaptable for use as mine roof anchors. The 
'602, '748 and '458 patents are directed specifically to mine roof 
supports. 
The '080 patent shows a bolt anchorage which utilizes a set of flat hard 
metal washers which are stacked on a bolt in alternating sequence with a 
plurality of transversely bent washers of soft or deformable material. The 
washers are compressed to distend the bent washers into engagement with 
the bore hole walls. This compression is achieved by either: inserting the 
bolt head into the bore hole in a non-rotatable manner and compressing the 
washers by means of a nut threaded onto the bolt, thereby leaving a 
portion of the bolt protruding from the hole; by impaction of the washers 
into the hole and then threadably engaging the washers onto the bolt; or 
by placing a nut in the bottom of the hole in a non-rotating manner, 
filling the hole with washers and inserting a bolt to compress the 
washers. Each instance appears to require a bore hole full of washers and 
does not allow further travel of the bolt; furthermore, it appears that 
the bent washers on the ends of the alternating series would flatten 
first, thereby requiring motion of the flattened washer to enable 
flattening of the other washers. It is readily apparent that motion of a 
seated washer in an expansion anchor has deleterious effects. 
The '201 patent shows an expansion anchor using a plurality of curved 
deformable washers mounted in facing pairs with the curvature of the 
adjacent washers being in opposition. These washers are compressed by 
means such as described with regard to the '080 patent. Again, it should 
be noted that the end washers are apparently going to engage the bore hole 
walls first and must slide along the hole in order to flatten the 
remaining washers. 
The '602 patent shows an expansion anchor which uses split rings 
circumscribing the roof bolt. The split rings are of two different 
diameters and are designed to be compressed with one size within the other 
to engage both the bore hole and the roof bolt. A plurality of such anchor 
rings may be used with each set of anchor rings separated by a metal tube 
which transmits forces between the upper and lower rings. This device 
relies on an upper nut which must non-rotatably engage the bore hole 
through the use of outward projections and a lower nut or stop which must 
advance with the bolt to compress the split rings vertically and thereby 
cause their lateral expansion. When the rings are fully seated the bolt 
cannot be advanced further due to the lower stop engaging the rings. 
The '748 patent shows an expansion anchor which uses a wedge and bail 
supported sleeve which move responsive to the rotation of a bolt such that 
the sleeve is compressed between the downwardly traveling wedge and the 
wall of the bore hole. The primary gripping force in this type anchor has 
been found to exist only in the region of the sleeve that is first 
expanded into engagement with the wall of the bore hole. 
The '458 patent shows a roof pin anchor which utilizes a plurality of 
resilient curved locking discs which are spaced apart by intermediate 
reinforcing spacers therebetween. The assemblage is forced upwardly into 
the bore hole. The discs are not compressed and engage the walls of the 
bore hole solely due to weight pulling downward on the bolt. The spacers 
provide reinforcement for the subjacent disc and provide a locus for the 
pivoting motion of the periphery of the subjacent and superjacent discs. 
More than one spacer element may separate the discs. 
Despite the achievements and efforts involved in developing these and other 
anchors, the problem of slippage remains. The problem may be 
simplistically termed a failure to provide adequate force on the bearing 
surfaces engaging the walls of the bore hole. As noted with respect to the 
'748 and '080 patents this is primarily due to the uneven engagement of 
the anchor mechanism with the bore hole walls which results in decreased 
useful bearing surface. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an expansion anchor for 
a mine roof bolt that has an evenly engaging bearing surface. 
Another object of the invention is to provide an expansion anchor whose 
total bearing surface may be increased or decreased depending on the 
strata within which the bolt is to be anchored. 
Yet another object of the present invention is to provide an expansion 
anchor wherein the force exerted by its bearing surfaces is greater than 
the tensile strength of the associated bolt. 
These and other objects of my invention are accomplished through the use of 
a plurality of formed leaves which extend transversely of the bore hole 
and which are inclined downwardly from their midpoint toward each end. At 
each end of the leaves a downwardly extending flange forms a bearing 
surface for engagement with the bore hole wall. The leaves have a central 
aperture through which the roof bolt extends and adjacent this aperture 
are a pair of vertical tabs. The leaves are held in line by a bail which 
extends downwardly beneath the lowermost leaf mounted on the bolt. A nut 
is threaded onto the bolt inside the bail and is restrained from rotation 
thereby such that rotation of the bolt causes the nut to descend and 
compress the leaves against the bail whereby they are flattened into 
engagement with the bore hole wall to provide anchoring for the roof bolt.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring to FIG. 1, a bore hole 11 is shown with a roof suspension bolt 12 
inserted therein. Normally the shortest bolts in use are about two feet in 
length; however such bolts may be as long as eight to ten feet depending 
on the nature of the strata encountered in attempting to secure the over 
burden. As previously mentioned such a bolt 12 is anchored in the hole 11 
to form a compressed laminated beam of rock above an excavated area. 
Should the anchor and bolt 12 slip under the weight of the compressed 
rock, the laminated beam may fall in what is termed a "local" fall which 
indicates a collapse of the roof coextensive with or shallower than the 
depth of the anchor. Unfortunately, if the anchor is slipping within the 
bore hole there is no perceptible motion of the roof which can be detected 
by unaided eye prior to the fall. Thus, a section of rock five feet square 
and two or more feet thick may fall from the roof without warning. More 
than likely if this occurs, more than one section will fall. 
The bolt 12 extends upwardly and supports a roof bearing plate 13 and in 
some instances a wooden header beam 14. At the end of the bolt 12 inside 
the bore hole 11 is the anchor assembly 16 comprising my invention. The 
anchor assembly 16 utilizes a bail 17 which engages the end of the bolt 12 
and fits snugly within the bore hole 11. The bail 17 has downwardly 
extending arms 18 which carry a pair of inwardly facing retainer members 
19 at their lower extremities. Held between the arms 18 are a plurality of 
bent anchor leaves 21 each of which is an elongated metallic member, bent 
along a transverse line near its center. The leaves 21 are bent downwardly 
as much as 30.degree. on each side. These anchor leaves 21 have 
down-turned flanges 22 at the outer ends thereof which define bearing 
surfaces 25 that are urged against the bore hole walls as hereinafter 
described. A central aperture 23 in each of the leaves 21 permits the 
anchor bolt 12 to pass therethrough. This aperture 23 is slightly 
oval-shaped having a longer dimension extending longitudinally of its 
anchor leaf 21. A pair of tabs 24 extend downwardly from each anchor leaf 
21 proximal the central aperture 23. The tabs 24 and flanges 22 are of 
equal length such that each leaf 21 is supported on the next lower leaf on 
the tabs 24. 
A threaded nut 26 engages the roof bolt 12 within the bail 17 so that the 
bail holds the nut 26 within the confines of the arms 18 and keepers 27 
carried by the bail 17 to prevent the nut 26 from rotating when the bolt 
12 is turned. The nut 26 is free to travel vertically along the bolt 12 in 
response to relative rotational motion therebetween. 
In operation, a bore hole 11 is drilled in accordance with Federal safety 
tolerances and to a depth greater than the roof bolt 12 to be used. The 
bail 17, the leaves 21, and the nut 26 are positioned on the end of the 
bolt 12 and are urged upwards into the hole where the assemblage is held 
by the snug fit of the bail 17 within the bore hole 11. A hydraulic wrench 
or other suitable tool is then used to tighten the roof bolt 12. As the 
roof bolt rotates nut 26 is drawn down onto the upper leaf 21. The stack 
of leaves 21 is restrained from downward motion by the retainer members 19 
and thus are compressed between the nut 26 and the retainer members 19. 
The nut 26 first contacts the upper leaf 21 near the center aperture 23 
and forces it downward; however the tabs 24 are aligned above one another 
to act as a rigid spine to transfer this compressive force downwards so 
that all of the leaves 21 are constrained to move in unison. That is to 
say, no individual leaf 21 can be moved toward a flattened position 
without a corresponding movement of the remaining leaves 21. Thus, since 
the leaves 21 are identical they must be moved toward a flattened position 
to the same degree which causes all of the bearing surfaces 25 to be urged 
into contact with the walls of the bore hole 11 simultaneously thereby 
seating all of the bearing surfaces 25 of the anchor against the walls of 
the bore hole 11 at the same time and with the same pressure. 
It should be noted that the anchor leaves 21 are not completely flattened, 
but rather remain slightly bent to further resist downward forces exerted 
by the weight of the rock or the tensioning of the bolt 12. If the leaves 
21 were completely flattened, as shown in the prior art, they would be 
subject to further downward bending which could result in slippage. 
As the leaves 21 are moved toward a flattened position and engage the walls 
of bore hole 11, increasingly greater force is required to urge the nut 26 
downward to further flatten the leaves 21, thus greater force is exerted 
on the bail 17. The outward pressure exerted by the anchor leaves 21 on 
the surrounding strata results in an anchor strength which is several 
times greater than the strength of the bolt 12. For example, at 182 " roof 
bolt is normally loaded to approximately 9000 pound linear tension, and 
will begin to yield or elongate at about 18,000 pounds of tension. The 
instant anchor is capable of placing a horizontal force on the bore hole 
walls which is thirty times the magnitude of the yield tension when two or 
more leaves are used. Thus, it is seen that the anchor assembly 16 will 
remain in place even though loaded with a burden greater than the strength 
of the bolt 12. Therefore the bolt 12 will stretch if over-stressed; 
however a stretched bolt may be detected within the mine, as by 
deformation of the bearing plate 13 or the header beam 14 whereby some 
warning of the dangerous condition is available. 
The bail 17 is designed to yield or open when the desired anchor forces are 
reached. This means simply that at a predetermined point more force is 
required to urge the nut 26 and leaves 21 downwardly than is required for 
the bolt 12 to penetrate the upper portion 15 of the bail 17. Thus, 
further rotation of the bolt 12 does not further compress the leaves 21 
but rather allows the bolt to travel upwardly within the bore hole 11 and 
be tensioned against the bearing plate 13 at the roof sufface. The bail 17 
may be formed from flat metal, as shown in FIGS. 1 and 2, or may be in the 
form of a bail 17' having a wire-like structure, as shown in FIG. 5. The 
flat metal bail 17 may be stamped from stock sheet metal and thereafter 
formed about the leaves 21 and nut 26. The top of the bail 17 is partially 
cut through as at 28 to define a disc 29 which will yield when sufficient 
force is applied thereto by bolt 12. The wire-like bail 17' is formed with 
a metallic flap 35 therebetween such that the upper wires 31 engaging the 
end of bolt 12 will separate upon the application of a predetermined force 
to allow bolt 12 to travel upwardly in the bore hole 11. 
The leaves 21 may also be stamped from suitable sheetmetal such that the 
tabs 24 are formed integrally with the leaves 21. The flanges 22 and tabs 
24 are turned downward and the leaves 21 are bent about a central 
transverse line as described above. When so bent the leaves 21 are 
slightly shorter longitudinally than the bore hole 11 is wide. However 
upon moving the leaves toward a flattened position, the leaves 21 are 
longer than the transverse width of the bore hole 11; thus the flanges 22 
are urged against the walls of the bore hole 11. By way of illustration, 
the sheet metal from which the leaves 21 are formed may be from 1/16" to 
5/16" thick; the tabs 24 and flanges 22 may extend downwardly from 1/8" to 
3/4"; and each lateral surface of the leaves may slope downwardly as much 
as 30.degree. from the horizontal prior to movement toward a flattened 
position. The number and size of the leaves 21 utilized is dependent upon 
the strata within which the anchor must seat and the total bearing surface 
desired. 
While I have shown my invention in several forms, it will be obvious to 
those skilled in the art that it is not so limited, but is susceptible of 
various other changes and modifications without departing from the spirit 
thereof.