U.S. patent number 3,804,346 [Application Number 05/287,779] was granted by the patent office on 1974-04-16 for liner for grinding mills.
Invention is credited to Telfer E. Norman.
United States Patent |
3,804,346 |
Norman |
April 16, 1974 |
LINER FOR GRINDING MILLS
Abstract
Liners for grinding mills having alternating bars of alloy steel
and strips of an elastomer, such as rubber, bonded together and to
a backing plate which is sufficiently flexible that the liner may
be manufactured flat, but drawn against an arcuate portion of the
cylindrical side wall of the mill by attaching bolts. Converging
slots formed in the center bar of the liner are engaged by bolt
heads having converging sides. For a ball mill, the bars of each
liner are of the same height and the strips are of the same height,
with the bars initially slightly higher than the strips. The liners
are installed so that the bars and strips extend longitudinally of
the mill, the effect of the balls and material being ground being
to produce a corrugated or ripple wave surface on each liner which
provides a lifting effect to the grinding media and causes the
balls to bounce or oscillate. For rod mills, each liner has several
center bars and strips of a greater height, with a series of one
bar and one strip of successively lesser heights on each side of
the center bars and strips. Wear during use produces not only a
ripple wave contour of the bars and strips but also a higher
portion of the liner toward the direction of rotation from the
initial highest portion, which accentuates the lifting effect on
the grinding media. A liner for a segment of the end wall of the
mill includes alternating bars and strips of alloy steel and
rubber, with the center bar being disposed along a radius of the
end wall and the remaining bars and strips parallel thereto.
Inventors: |
Norman; Telfer E. (Denver,
CO) |
Family
ID: |
23104314 |
Appl.
No.: |
05/287,779 |
Filed: |
September 11, 1972 |
Current U.S.
Class: |
241/182;
241/300 |
Current CPC
Class: |
B02C
17/225 (20130101) |
Current International
Class: |
B02C
17/00 (20060101); B02C 17/22 (20060101); B02c
017/22 () |
Field of
Search: |
;241/294,295,298,299,300,182,183,DIG.30 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
999,521 |
|
Jul 1965 |
|
GB |
|
399,990 |
|
Aug 1924 |
|
DD |
|
Other References
"No. 178,351 Lining for Tube Mills" The Canadian Patent Office
Record, Vol. 45, July, 1917, Page 2,180..
|
Primary Examiner: Custer, Jr.; Granville Y.
Attorney, Agent or Firm: Van Valkenburgh; Horace B. Lowe;
Frank C.
Claims
1. A liner for at least a portion of the interior of a grinding
mill adapted to utilize iron balls, steel balls, iron rods, steel
rods, pebbles, rocks and the like as the grinding media,
comprising:
bars formed of a relatively high abrasion resistant material;
strips formed of less abrasion resistant material and alternating
with said bars; and
means for maintaining said bars and strips in alternating relation,
whereby when installed in said mill, said bars and strips will
engage the material to be ground and will wear differentially to
produce a corrugated or
2. A liner as defined in claim 1, wherein:
said bars have the abrasion resistant qualities of alloy steel;
and
said strips are formed of an elastomer having substantially the
abrasion
3. A liner as defined in claim 1, wherein:
said bars are formed of an elastomer having the abrasion resistant
qualities of rubber; and
4. A liner as defined in claim 2, including:
a backing plate extending beneath said bars and strips, said bars
and
5. A liner as defined in claim 4, wherein:
said backing plate is sufficiently flexible to permit said backing
plate to be manufactured with said backing plate flat, but said
backing plate conforming to an arcuate portion of a cylindrical
side wall of said grinding mill when installed with said bars and
strips extending longitudinally of said mill through the
compression of said elastomer
6. A liner as defined in claim 1, wherein:
said liner has a configuration conforming to a segment of an end
wall of said mill, with a center bar or strip extending in a
direction corresponding to a radius of said end wall and the
remaining bars and
7. In a liner as defined in claim 1, wherein:
each of said bars is initially of substantially the same height;
and
8. A liner as defined in claim 1, including:
a series of centrally disposed bars and strips of a predetermined
height; and
a plurality of series of at least one bar and one strip disposed
laterally at each side of said centrally disposed bars and strips,
with each series
9. A liner as defined in claim 1, wherein:
the composite assembly has sufficient lateral elasticity to permit
it to be manufactured with a flat back but said flat back will
conform to an arcuate portion of a side wall of said mill, with
said bars and strips
10. A liner as defined in claim 1, for installation on a
cylindrical side wall of a mill, wherein:
a central bar of said liner is divided into sections, with the ends
of adjacent sections formed to provide a slot, at least a portion
of the sides of which converge toward said side wall; and
a series of bolts for attaching said liners to said side wall of
said mill, said bolts having heads provided with converging side
surfaces to engage the converging side portions of the
corresponding slots, whereby the bars may extend longitudinally of
said mill.
Description
This invention relates to liners for grinding mills, such as ball
mills and rod mills used in grinding or comminuting ore and the
like.
Grinding mills are used for the purpose of reducing the size of
lumps or other pieces of ore or the like for various purposes. In
the case of ore, the normal function of the grinding mill is to
reduce the size of the ore to particles within a fine sieve range
for flotation, in which through the use of suitable additives, the
ore bearing particles are separated from the gangue. Both ball
mills and rod mills have been used for grinding ores or the like,
such a mill normally being a rotating cylinder containing a number
of balls or rods formed of steel, usually special alloy steels,
which crush and comminute the ore between them or against the side
of the mill, or perhaps the end thereof. In addition to balls or
rods, pebbles or natural rock have also been used as grinding
media. A grinding mill thus normally consists of a cylindrical
shell rotated about a central horizontal axis, with ends which
close the ends of the shell. Various diameters and lengths of
shells have been utilized, varying considerably but normally in
proportion to the capacity of the mill. In view of the abrasive
character of the material being ground, the wear on the inside of a
grinding mill has been a serious problem. Normally, grinding mills
have been lined with cast or wrought abrasion resistant ferrous
alloy liners several inches thick, or in some cases, with rubber or
ceramic liners. These liners have been made in sections, with
various configurations, usually being held in place by bolts which
hold the liner tightly against the interior of the mill shell or
the mill end. Cast ferrous alloy liners require molds for the
production thereof, while liners manufactured from wrought steel
require special rolls for forming to shape.
During service, the liners tend to be worn easily by the abrasive
action of the grinding media and the minerals being ground in the
mill. The cost of periodic replacement of liners is thus a major
item of cost in the operation of mills or plants which grind ore or
other minerals. In addition, the configuration of the surface of
the liner, during its service life, has been found to be an
important consideration affecting both the life of the liner and
the grinding efficiency or grinding rate of the mill. Furthermore,
a configuration which produces the highest grinding efficiency or
grinding rate for a ball mill is not necessarily the configuration
which will produce the highest efficiency or grinding rate for a
rod mill, and vice versa.
Among the objects of this invention are to provide a novel liner
for grinding mills; to provide such a liner which may be adapted
for use as a liner for the shell and also a liner for an end of the
mill; to provide such a liner which has a longer life and therefore
requires replacement at less frequent intervals; to provide such a
liner which is readily modified in construction, to provide a
configuration most suitable for use in a ball mill or pebble mill,
as well as a different configuration which is most suitable for use
in a rod mill or an autogenous grinding mill using large diameter
natural rock; to provide such a liner which may be readily
manufactured in a flat or planar form but will fit tightly against
the mill shell, irrespective of the mill diameter, thereby avoiding
the cost of machining or of molds for casting liners for shells of
different diameters; to provide such liners which may be made in
different lengths and widths without undue expense; to provide such
liners which may be modified for use in grinding fine or soft
materials; and to provide such liners which are economical to
manufacture and are efficient and effective in use.
The foregoing objects are generally accomplished by utilizing a
series of longitudinally extending, alternating strips of highly
abrasion resistant material, such as hardened alloy steel, and
strips of elastomer, such as rubber, which are bonded to each other
and to a backing plate, to form a laminated composite liner. The
backing plate, as of soft steel or the like, will conform to the
inner diameter of the shell when attached thereto, while the
resiliency of the elastomer strips permits compression thereof to
accommodate the curvature of the shell. For use in ball mills or
pebble mills, the height of the more highly abrasion resistant
strips or bars is slightly greater than the height of the
alternating elastomer strips, in order to provide what may be
characterized as a "low wave" ripple configuration. For rod mills
or autogenous grinding mills, with large diameter natural rock as
grinding media, the height of the strips and bars are varied to
provide a greater height at the center of the liner, to produce
what may be characterized as a "high wave" ripple configuration.
For lining the ends of a cylindrical mill, the "low wave"
configuration is conveniently utilized, with the bars and strips
extending radially of each liner, which forms a segment of the
circle of the end of the mill. When fine or soft materials are
ground, a good quality of abrasion resistant rubber or other
elastomer may prove to have a greater abrasion resistance than
steel or iron. In this case, higher bars should be made from the
abrasion resistant rubber or other elastomer and the lesser
abrasion resistant strips may be made of other suitable material,
such as a harder, less abrasion resistant rubber or elastomer, a
suitable plastic material or hard wood.
The attainment of the foregoing and additional objects of this
invention, as well as the novel features thereof, will further be
apparent from the following description of preferred embodiments of
this invention, illustrated in the accompanying drawings, in
which:
FIG. 1 is a perspective view, on a reduced scale, of a grinding
mill in which the liner of this invention is particularly
useful;
FIG. 2 is a fragmentary circumferential section, showing one
segmental liner constructed in accordance with this invention,
portions of two adjacent liners and a portion of the shell of the
grinding mill in which the liners are installed, such liners being
particularly useful for a ball mill;
FIG. 3 is a fragmentary section, on an enlarged scale, taken along
line 3--3 of FIG. 2 and showing particularly the head of an
installation bolt and associated parts;
FIG. 4 is a fragmentary section taken along line 4--4 of FIG.
3;
FIG. 5 is a fragmentary section, similar to FIG. 2 but showing the
condition of the alternating steel and elastomer strips after wear
to what may be characterized as a "low wave" configuration;
FIG. 6 is a transverse section of the grinding mill liner of FIG.
2, prior to installation;
FIG. 7 is a condensed top plan view of the liner of FIG. 6;
FIG. 8 is a fragmentary section, similar to FIG. 2 but showing an
alternative arrangement of alternating steel and rubber strips
particularly useful as a rod mill liner;
FIG. 9 is a fragmentary section, on an enlarged scale and taken
along line 9--9 of FIG. 8;
FIG. 10 is a fragmentary section, taken along line 10--10 of FIG.
9;
FIG. 11 is a fragmentary section, similar to FIG. 8 but showing the
liner segment after wear to what may be characterized as a "high
wave" configuration;
FIG. 12 is a transverse section of the grinding mill liner of FIG.
8, prior to installation;
FIG. 13 is an inside view of one end of a grinding mill, also
provided with liners constructed in accordance with this invention,
the shell and shell liners being omitted for clarity of
illustration; and
FIG. 14 is a planar section, on an enlarged scale, of one of the
liners of FIG. 13.
A rotating cylindrical grinding mill, of a type in which liners
constructed in accordance with this invention may be used, is shown
in FIG. 1 and includes a cylindrical shell 10 having bolts 11 by
which the liners are attached to the inside of the shell, with the
bolts extending therethrough. An end plate 12 is attached to one
end of the shell 10 and a corresponding end plate is attached to
the opposite end of the shell, while a cover 13 for a ring gear 14,
having teeth (not shown), enclosed within the cover 13 is disposed
at one end of the mill. The ring gear is engaged by a gear drive,
which is conventional and therefore not shown. The grinding mill is
mounted for rotation through bearings 15 and 16 at opposite ends of
the mill, with supports 17 and 18 for the respective bearing being
mounted on any suitable type of foundation. The ore or other
material to be ground is introduced into the mill through a feed
tube 19, while the ground material is discharged from the mill
through a discharge tube 20, as indicated by the arrows.
A liner L, constructed in accordance with this invention and
illustrated in FIG. 2, is flanked by portions of additional liner
sections L constructed in the same manner, all mounted on the
inside of the shell 10 by bolts 11. Each liner L may cover a
section of the circumference of the shell 10, with each liner or
two or more liners placed end to end corresponding to the length of
the shell so that the length of each liner L may be such that the
weight thereof permits it to be relatively easy to handle. For a
mill shell which has a 10 foot inside diameter, there may be 20
rows of liners, each occupying an 18.degree. segment of the shell.
Thus, the chord width of an 18.degree. segment for a 10 foot inside
diameter shell would be 18.84 inches. However, a clearance or space
21 between adjacent liners is desirable, so that the liner L, in
the instance under discussion, may be 18-5/16 inches wide. When
more than one liner is placed end to end in the shell, a spacing
between the ends of these two liners, similar to the spacing
between adjacent liners, may be provided.
The liner L includes a series of hardened alloy steel bars 22 which
alternate with a series of elastomer strips 23, such as formed of
rubber, of the same width and length, and a pair of outside
elastomer strips 23' of lesser width, for the liner illustrated.
The alternating bars 22 and strips 23 may have a similar width,
such as between 1/2 inch and 2 inches, with the bars 22 and strips
23, as shown in FIG. 2, being approximately 1-5/16 inches in width
and the outside elastomer strips 23' being 5/8 inch in width. The
upper ends of the steel bars 22, which may be rounded as shown, are
spaced above the elastomer strips 23 a distance less than the width
of the steel bars, such as approximately 1/2 inch, when the bars
and strips have the dimensions indicated above. Of course, for
other widths of bars and strips, or for a liner made for a grinding
mill shell having a different diameter, the outside strips 23' may
be unnecessary.
In accordance with this invention, each of the bars 22 and strips
23 and 23' are bonded to each other and to a backing plate 24,
which may be formed of a low carbon steel, since it is not
subjected to abrasion during use. A rubber strip 25 is conveniently
disposed between the backing plate 24 and the bars and strips, for
bonding purposes. Thus, the steel bars 22 and elastomer strips 23
and 23' may be bonded to each other and the backing plate 24,
through the strip 25, by a suitable rubber cement mixture, followed
by vulcanization, as in an autoclave, to produce a strong bond.
Other suitable ways of attaching the bars 22 and strips 23 to each
other and to the backing plate 24 may, of course, be utilized. The
thickness of backing plate 24 need not be great, so that, when the
liner is mounted against the inside of the shell 10 by a series of
bolts 11, as in FIG. 2, the backing plate 24 will bend to conform
to the inside of the shell. It will be noted that the liner L is
initially formed in a straight or planar condition, as in FIG. 6,
and that the backing plate 24 will be curved, when the backing
plate is drawn into engagement with the inside of the shell, as in
FIG. 2. The elasticity of the strips 23 will permit a slight
compression, principally along the upper portion thereof, by the
steel bars 22, when the liner is installed. Thus, there should be
no difficulty in the liner conforming to the inside curvature of
the shell 10, substantially irrespective of the diameter
thereof.
For bolt attachment purposes, a series of slots 27, as in FIG. 7,
corresponding to the number of bolts 11 utilized in attaching the
liner to the shell, are provided between sections of center bar 22.
Each slot 27 has a rectangular configuration down to an angular
surface 28 at each side and which slants inwardly toward a hole 29
in the backing plate 24, when installed, is in alignment with a
hole 32 in shell 10, through which the shank of a bolt 11 extends.
Each bolt head 30, as in FIGS. 3 and 4, is generally rectangular in
cross section down to opposed angular surfaces 31 which fit against
the corresponding angular surface 28 of the corresponding slot 27,
to permit the bolt to draw the liner tightly against the shell 10.
The shell 10, in appropriate positions, is provided with a series
of holes 32 through which the shank of each bolt 11 extends, as for
attachment of a conventional washer and nut, as in FIG. 2.
Since the direction of impingement of the grinding media and the
minerals on the liners is approximately perpendicular to the joint
lines between the laminated bars and strips, the action of the
grinding media and minerals tends to produce different rates of
wear on the more abrasion resistant bars and less abrasion
resistant strips, respectively. Thus, as the liner wears, it
developes a ripple wave wearing surface, as shown in FIG. 5. This
ripple wave or corrugated contour persists on the liner surface
until it is worn down to the steel backing plate. At that time, the
worn liners may be replaced with a new set of liners. The ripple
wave contour provides the desired lifting effect to the charge of
grinding media, as the mill rotates, and also causes the balls to
bounce or oscillate with a motion generally perpendicular to the
tangential direction of mill rotation indicated by the arrow 33 of
FIG. 5. This action, in turn, assists to increase the grinding rate
of the mill through the production of a more dynamically active
charge of grinding media. This lifting action is also effective in
reducing materially the wear, sometimes quite rapid, which occurs
when the grinding media slides or impinges at a low angle against a
smooth, homogenous surface of a liner, as when each liner is formed
from one material. As will be evident, the use of the liner
construction of this invention reduces the wear of the liner and
therefore increases its useful life, as compared with the types of
liners used herefore.
As indicated previously, the alternating bars and strips, with the
more abrasion resistant bars slightly higher than the less abrasion
resistant strips, but with all of the bars normally of about the
same height and all of the strips normally of about the same
height, provides the "low wave" configuration illustrated in FIG.
5. As indicated previously, this "low wave" configuration is
particularly useful for ball mills and pebble mills.
For rod mills, i.e. mills in which elongated rods are placed in the
mill, rather than balls, or mills using an autogenous grinding
media of relatively large diameter or sized rocks of the material
being ground, the "high wave" configuration shown in FIG. 11,
produced by the configuration of FIG. 8, as it wears, is
particularly useful. As in FIG. 8, a "high wave" liner L' will
include a pair of longitudinally abutting center bars 35 of alloy
steel provided with bolt slots 27', with another pair of bars 36
outside the center bars, but with elastomer strips 37 interposed
therebetween. The central bars 35, 36 and strips 37 are the highest
of the bars or strips utilized in this embodiment, for a purpose
described later. On each side of the center bars 36 is an elastomer
strip 38 and then a hardened alloy steel bar 39, each of lesser
height than the central bars and strips; then strips 40 and bars 41
of lesser height and strips 42 and bars 43 of successively lesser
height. A pair of narrower elastomer strips 44, of the same height
as bars 43, complete the assembly of bars and strips on the
outside. A space 21', between liners L', is provided, as before. As
before, the bars and strips of the liner L' are bonded together and
to a backing plate 24' by a rubber strip 25', as in the manner
described previously. The respective liners L' are attached to the
interior of the shell of the mill by bolts 11', with each liner
occupying a predetermined arcuate portion of the inner
circumference of the shell and the liners having either a length
corresponding to the shell, or two or three liners placed end to
end longitudinally of the shells, for shells with increasing
longitudinal dimensions. Slot 27' of each bolt 11' is again
rectangular in cross section to an inwardly angular surface 28' at
each side but is formed between aligned sections of the two center
bars 35, rather than between sections of a single bar. Head 30' of
bolt 11' is similar in configuration to the bolt heads 30 of FIG.
3, except for the height, being provided with a longer angular
surface 31' adjacent the shank of the bolt, with the angular
surfaces of the bolts being longer than the angular surfaces 30 of
the bolt 11, to compensate for the greater weight of the liner of
the "high wave" configuration. For the "high wave" configuration,
the center bars 35, strips 37 and bars 36 may be on the order of 6
inches high, with the strips 38 and bars 39 being on the order of 5
inches high, the strips 40 and bars 41 on the order of 4 inches
high, and the strips 42, 44 and bars 43 being on the order of 3
inches high. Each of the bars and strips may be on the order of 1
inch wide, except the strips 37 and 44, which may respectively be
on the order of 5/8 inch and 1/2 inch wide, or a total width of
181/4 inches. The width of the liners may, of course, vary
considerably, such as ranging from about 15 inches to about 24
inches in width.
During the use of liners L' of FIG. 8, when rotated in the
direction of the arrow 45 of FIG. 11, the impingement of the
grinding media and material being ground, on the bars and strips,
will tend to wear away to a greater extent the bars 35 and strips
37, as well as the bars and strips on the side opposite the
direction of rotation, so that the generally higher portion of the
liner will become the bars and strips disposed toward the direction
of rotation. Such wear will produce a pronounced peak, as well as a
series of peaks and valleys of a generally lesser height. However,
the differential effect of the differences in height of the
portions of the liner toward and away from the direction of
rotation will accentuate the lifting effect on the rods in the rod
mill, or large diameter rock utilized in an autogenous grinding
mill.
For lining either end of the mill, such as end 12 of FIG. 1 or the
opposite end of the grinding mill, the "low wave" ripple
configuration is preferable. Thus, each end of the grinding mill
may be provided with a series of liners L", each corresponding to a
segment of the annulus between the inner circle 50 and outer circle
51 of the grinding mill end. Spaces 52 may be provided between the
liners L", similar to the spaces 21 of FIG. 2 and 21' of FIG. 8.
Each liner L" or segment may have the construction illustrated in
FIG. 14, such as including a center bar 53 formed of hardened alloy
steel and a pair of elastomer strips 54 on each side thereof and
disposed parallel thereto. It will be noted that the center bar 53
extends radially of the end of the mill. Additional bars 55, 56 and
57 may also extend parallel to the center bar 53, interspersed with
elastomer strips 58, 59 and 60 of the same or different width, and
again extending parallel to the center bar. The bars and strips of
FIGS. 13 and 14 have relative heights similar to the bars and
strips of FIG. 2. During use, the alternating bars and strips will
tend to become worn to a "low wave" ripple configuration, similar
to that shown in FIG. 5. The center bar 53 is provided with bolt
slots 27 between sections, similar to the bolt slots 26 of FIG. 3
and adapted to receive the bolts for attaching the liner L" to the
end plate, such as the end plate 12 of FIG. 1, or the opposite end
plate. With respect to the liner L", it will be noted that these
liners are not only manufactured in a flat form, but also utilized
in a flat form, or they may deform when bolted in the mill to fit
the slightly conical surface which exists at the ends of many
grinding mills. As before, the alternating strips and bars are
bonded to each other and may be bonded to a suitable backing plate
corresponding to backing plate 24 of FIG. 6, with a rubber strip
interposed between the backing plate and the alternating bars and
strips to facilitate bonding, as in the manner described, through a
rubber cement mix and vulcanizing.
The advantages of the liners of grinding mills constructed in
accordance with this invention are numerous. Thus, the liners of
this invention have a longer life, primarily due to the "ripple
wave" contour which persists throughout the life of the liner. A
contributing factor is that unusually hard and abrasion resistant
metal bars can be used in the construction of these liners, since
each bar is well supported by and securely bonded to the tough
rubber bars and to the mild steel backing plate, so that even if a
metal bar cracks in service, it will continue to be held in place
and perform its desired function throughout the life of the liner.
Thus, the liners can be worn down to a very thin section before
replacement is necessary. The liners of this invention also have a
lower first cost than homogeneous liners of equivalent volume or
thickness, due to the fact that the metal bars can be made from
standard and relatively light sections, either rolled or cast,
which can be produced at a lower cost per unit weight than the
special heavier sections required in homogeneous cast or rolled
liners. The alloying elements needed for the lighter section bars
to produce full hardening is also less than for heavy sections. The
rubber or elastomer strips in the composite liner can also be
produced at a substantially lower cost per unit volume than a
homogeneous metal liner of heavy section. For making up composite
liners, it is desirable to use only a few sizes or preferably one
size of steel bars in a standard and therefore readily available
dimension. If necessary, a greater number of sizes of rubber bars
may be used to provide the desired width of the liner. The length
of the individual bars, as supplied to the shop, can be variable,
since they can be cut or trimmed at the shop to a desired length.
Cuts on the ends of the sections of the bars which abut the bolts
can be made accurately with an abrasive cut off wheel, or by any
other suitable means available for cutting hardened tool steel.
While there is an additional assembly cost involved in the
production of composite "ripple wave" liners, this cost is
relatively low and is only a small fraction of the cost of
materials.
The liners of this invention produce an increased grinding rate in
the mill, due to the fact that the grinding media are activated and
oscillated by the "ripple wave" effect. When desired, relatively
thin liners may be used to increase the effective interior diameter
of the grinding mill.
The liners of this invention require less shut down time of the
mill for liner changes, due to the inherently longer life of the
liners and also to their ease of handling. If desired, the number
of pieces to be handled can be reduced by the use of longer liners,
which lend themselves well to production in the "ripple wave"
configuration.
It will be understood that the method or design of bolt attachment
may be changed from the forms shown. For instance, a series of cap
screws extending through the wall of the mill may be utilized to
attach each liner to the inside of the mill. In such an instance,
it may be desirable to use a softer steel strip, such as S.A.E.
1040, which is more readily tapped, for anchoring the cap screws.
Such a softer steel strip may be flanked by thin strips of rubber
or the like and harder steel bars on each side thereof. Such a
construction avoids cutting the softer steel strip and also
eliminates the hole or socket at each bolt head.
Although certain preferred embodiments of this invention have been
illustrated and described, it will be understood that other
embodiments may exist and that various changes may also be made,
such as in methods or designs of bolt attachment, or in widths of
the individual bars and strips forming the laminated composite,
without departing from the spirit and scope of this invention.
* * * * *