Note: Descriptions are shown in the official language in which they were submitted.
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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 ~
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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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` ~03~986
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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
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; penetrate into the cylindrical outer faces, thus effectively ~'
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increasing the area of contact. This penetration into the soft ;;
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shell surface by rock projections minimizes concentration of
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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.
., ~ .
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:
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Figure 1 is an exploded perspective view of an embodi-
ment of the rock bolt assembly of the invention;-
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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 ~
. . .
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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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
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--, 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
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~` 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
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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` ~;
. :1 i . . .
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.
: :~ ;,.
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
- 12 -
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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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1~399~
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 `~
;~, :
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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