Sealing piston

In an apparatus for clamping and uniformly tensioning an inside diameter cutting blade, hydraulic fluid is contained within an annular fluid channel which includes an opening confronting the inside diameter blade. The hydraulic fluid is prevented from contacting the blade by a pressure-transmitting deformable sealing piston mounted within the fluid channel. The sealing piston is generally U-shaped in cross section and includes, in addition to a relatively axially thick main body part, at the sides of the "U", two outwardly directed arms when the piston is not in place within the fluid channel which it seals. When the arms of the piston are brought toward one another for insertion of the piston within the fluid channel, the piston material, which is resilient, urges the arms radially outward thereby forming a good seal with the inner and outer walls of the fluid channel. The inner and outer walls of the fluid channel are generally unbroken as are the radially inner and outer surfaces of the piston thereby permitting relative axial movement between the piston and the fluid channel.

The present invention relates generally to article tensioning devices, and 
in particular, to an improved combination sealing and 
pressure-transmitting member for use in an hydraulically operated article 
tensioning device. 
Recently, there has been an increasing need for slicing extremely thin 
sections or wafers from large crystals. For example, in the electronics 
industry extremely thin slices of silicon, on the order of 0.007 inches in 
axial dimension, have been used, usually cut from larger silicon crystals 
which can be economically grown, and then sawed into wafers of the 
required thickness. 
Naturally, in view of great expense of the raw silicon material, 
minimization of waste of the silicon is a prime consideration as is 
uniformity of size and surface configuration of the wafers which often are 
as large as 3 inches in diameter. In order to provide the wafers of the 
required size and surface characteristics, while also minimizing waste, 
special cutting blades have been devised for slicing silicon wafers from 
larger ingots. 
One of the required characteristics of the cutting blades is that they must 
be thin enought to produce the required thin wafer and minimize waste and 
must also have a rigidity sufficient to slice through a 3 to 5 inch 
diameter silicon crystal. This combination of thinness and rigidity is 
generally not attainable with the usual cutting blade where the cutting 
surface is on the outside circumference thereof. 
As noted in U.S. Pat. No. 3,827,421 of Aug. 6, 1974, assigned to the 
assignee of the subject application and as noted in U.S. Pat. application 
Ser. No. 605,856, filed Aug. 19, 1975, which has also been assigned to the 
assignee of the present application and the teachings of which are 
incorporated herein by reference, a cutting blade which provides the 
required thinness and rigidity, coupled with the required cutting depth 
capability is a thin, circular saw blade clamped about its outside 
diameter including an opening centrally thereof forming an inside diameter 
blade, hereinafter sometimes referred to as an "ID" blade. 
Typically, an ID blade is utilized in a slicing machine wherein the blade 
is clamped in a housing about its circumference with the surface of the 
blade being uniformily tensioned to provide the cutting surface of the 
inside diameter opening with the required rigidity to produce uniform 
results. 
Since it is well known that fluid transmits pressure applied thereto 
equally in all directions, an hydraulic fluid channel including a means of 
ingress and egress of hydraulic fluid therefrom was formed in a clamping 
ring with an opening of the channel confronting a clamped saw blade. A 
recess channel was radially aligned with the fluid channel on one side of 
the ID blade clamping and tensioning apparatus, and hydraulic fluid was 
made to impinge upon the clamped blade thereby uniformly deforming the 
blade and resulting in a uniformly tensioned and clamped blade. 
While the apparatus described was satisfactory in some respects and 
superior to mechanically clamping and tensioning the ID blade, there was 
often the problem of leakage of the hydraulic fluid with consequent loss 
of tensioning pressure, and consequent disruption of the slicing process. 
When the ID blade broke, virtually all of the fluid would leak out of the 
tensioning fluid channel. 
In order to solve some of the disadvantages of the hydraulic tensioning 
apparatus described hereinbefore, a gasket was devised for retaining the 
hydraulic fluid within the fluid channel, thereby preventing the direct 
contact of the fluid with the saw blade. The gasket was retained within 
the blade clamping apparatus by radially inner and outer projecting ears 
which mated with similarily configured radially inner and outer recess 
channels within the clamping and tensioning apparatus. Such a gasket is 
shown in the 382,421 patent identified hereinbefore. 
The gasket shown in the 382,421 patent was relatively thin in cross section 
at is main body part; and, consequently, the fluid channel thereof 
required a great deal of tensioning fluid therein which fluid is 
relatively expensive and which therefore increased the cost of operation 
of the blade tensioning and clamping apparatus, if the fluid was lost 
during operation of the machine. This loss of fluid occurred whenever the 
gasket ruptured, which was not infrequently due to the thin axial 
dimension of its main body part. In addition, fluid loss occurred due to 
the difficulty of properly inserting the "ears" in the channels. 
In addition, at least partially in view of the thin axial dimension of the 
main body part of the gasket of the 382,421 patent, if the gasket 
developed a permanent deformation or "set" as a result of tension applied 
to the gasket over a long period of time, when the blade required 
replacing either because it began to dull or because the blade broke, the 
"set" of the gasket rendered it very diffucult if not impossible to use 
the same gasket with a new blade thereby often requiring replacement of 
the gasket each time a blade had to be replaced. 
This requirement for replacement of a gasket each time a blade was replaced 
not only increased the expense of operating the cutting machine but also 
increased the inactive or "down" time of the machine thereby resulting in 
costly production delays. 
Solving some of the problems inherent in the single piece gasket shown in 
the 382,421 patent, is a two-piece gasket such as shown and described in 
the 605,856 application. The two-piece gasket was comprised of an outer 
sleeve of a given durometer which was secured within the annular fluid 
channel by radially inner and outer projecting ears located within 
like-configured radially inner and outer annular recesses in a clamping 
apparatus. The outer sleeve was generally U-shaped in cross section and 
was constructed and arranged with the open end of the "U" confronting the 
saw blade and with the bottom of the "U" sealing the fluid channel. A plug 
of different durometer was located within the outer sleeve, was deformed 
by pressure on the outer sleeve caused by the pressure on the fluid and 
was the part of the gasket which actually impinged upon and deformed the 
saw blade. the inner plug was reversible once it attained a permanent 
deformation or "set"; and, its presence within the annular fluid channel 
reduced the amount of tensioning fluid required for proper operation of 
the apparatus. 
Despite the advantages of the one and two-piece gaskets described 
hereinbefore and the problems which they solved, they each had 
disadvantages, difficulties and problems. 
For example, both gaskets noted hereinbefore required that special radially 
inner and outer receiving recesses be machined within the fluid channel to 
receive the radially inner and outer ears necessary for maintaining the 
single piece or the outer sleeve of the two-piece gasket in position 
within the fluid channel. 
In addition, in installing the single piece gasket or the outer sleeve of 
the two-piece gasket, it was often very difficult and time consuming to 
position the radially inner and outer annular ears of the gasket with the 
corresponding radially inner and outer receiving recesses. 
Further, when either the single-piece or the two-piece gasket developed a 
permanent deformation or "set", despite the fact that at least in the 
two-piece gasket the inner filler ring could be reversed for re-use, 
removing the inner filler ring and reversing the same was time consuming; 
and, it was necessary to reverse the inner filler ring (in the two-piece 
piston) or to replace the entire gasket (in the single-piece gasket) 
before a new saw blade could be properly positioned for clamping and 
subsequent tensioning. 
In addition, in view of the space occupied by the projecting ears of the 
seals described hereinbefore, the radially extending working surface for a 
seal of given radial extent was somewhat limited. 
Finally, while the gaskets of the prior art sealed well at relatively high 
hydraulic pressures, before the hydraulic pressure increased, there was a 
tendency for some of the hydraulic fluid to move around the seal; and, the 
same tendency at greatly increased hydraulic pressures when the seal 
effectiveness would tend to break down. 
It is an object of the present invention to provide an improved fluid 
pressure sealing apparatus and pressure transmittal member which 
facilitates uniform hydraulic tensioning of an ID saw blade. 
It is a more particular object of the present invention to provide a 
single-piece sealing member which effectively seals fluid within an 
hydraulic channel over the full range of operating pressures. 
It is a still more particular object of the present invention to provide a 
single-piece, slidable hydraulic seal and pressure-transmitting device for 
use in a blade mounting and tensioning apparatus wherein the sealing and 
pressure-transmitting member is slidable between a first, blade 
insertion-removal position and a second, axially extended blade tensioning 
position. 
It is a further object of the present invention to provide an improvement 
in a blade mounting and tensioning apparatus wherein a slidable fluid 
sealing piston is provided which functions as an effective sealing member 
over an entire range of operating pressures and which is economical to 
produce, install and operate. 
In accordance with a specific embodiment of the present invention, an 
improvement in a blade mounting and tensioning apparatus is provided 
wherein the apparatus includes first and second annular clamping members. 
Means are provided for clamping the periphery of the blade between the 
clamping members. An annular fluid channel exists within the first 
clamping member and is located radially inward of the clamping means and 
has an opening confronting one side of the blade. An annular recess 
channel is located within the second clamping member radially aligned with 
the annular fluid channel and has an opening confronting the other side of 
the blade. A deformable sealing piston is located within the annular fluid 
channel opening and fluid is, in turn, located within the annular fluid 
channel. The sealing piston is constructed and arranged to seal the fluid 
within the annular fluid channel against contact with the blade and is 
further constructed and arranged to bear against a clamped blade upon 
pressure being exerted on the fluid whereby the pressure uniformly deforms 
the sealing piston against the blade therebyuniformly deforming the blade 
into the recess channel and uniformly tensioning the blade. The 
improvement comprises the sealing piston being generally U-shaped in cross 
section, having a main body part and two outwardly directed arms. The main 
body part is axially relatively thick being of a thickness which is on the 
order of one-third of the radial extent of the main body part of the 
sealing piston. The sealing piston and the fluid channel are constructed 
and arranged for the sealing piston to be capable of axial movement within 
the fluid channel in response to pressurization of the fluid.

Referring now specifically to the drawing and first to FIGS. 1 and 2, in 
accordance with an illustrative embodiment demonstrating objects and 
features of the present invention, there is provided a saw blade housing, 
generally designated by the reference numeral 10, which is attached, by a 
hub 12 to the drive shaft 14 of a crystal ingot slicing machine (not 
shown). 
Naturally, while the blade tensioning device of the present invention is 
described as being usable in the preferred embodiment with an ID cutting 
blade for slicing thin sections of silicon, such a use is merely an 
illustrative embodiment of the present invention and other applications 
thereof are possible and are considered to be within the contemplation and 
scope thereof. 
In the preferred embodiment, a spindle plate 16 is shown attached to the 
hub 12 by any conventional means (not shown). A generally cylindrical, 
annular tensioning ring 18 is attached to the spindle plate 16 by a 
plurality of tensioning ring bolts 20 which are equally spaced about the 
spindle plate 16. The tensioning ring bolts 20 pass through smooth-bored 
openings within the spindle plate 16 and engage a series of threaded 
openings within the tensioning ring 18 thereby providing for secure 
attachment of the tensioning ring to the spindle plate. 
The tensioning ring 18 includes a radially outer blade clamping surface 22 
(FIG. 3), to be more fully described hereinafter, which extends axially a 
predetermined distance from the axially outer leftmost surface of the 
spindle plate 16. An annular tensioning ring recess 24, which is generally 
partially circular in cross section, is located within the tensioning ring 
18 radially inward of the clamping surface 22 for a purpose to be 
described hereinafter. 
A tensioning ring abutment surface 26 appears within the tensioning ring 18 
radially inward of the tensioning ring recess 24 and is located 
approximately the same axial distance from the axially outermost surface 
of the spindle plate 16 as is the tensioning ring recess. 
An annular clamping ring 28, of substantially, the same diameter as the 
tensioning ring 18, is removably fixed thereto by means of a plurality of 
clamping ring bolts 30. The claming ring bolts 30 are located within a 
plurality of smooth-bored openings equally spaced within the circumference 
of the clamping ring 28 and which are seated within correspondingly 
threaded openings within the tensioning ring 18. The clamping ring 28 
includes, on its radially outermost axial surface, a clamping ring 
clamping surface 32, to be described more fully hereinafter. 
Located radially inward of the clamping ring clamping surface 32, within 
the clamping ring 28, is an annular fluid channel 34 which is generally 
U-shaped in cross section with the open part of the "U" confronting the 
tensioning ring 18. The annular fluid channel 34 includes an unbroken 
lower surface 26 which is plane in cross section and radially inner and 
outer unbroken surfaces 38, 40 which are also plane in cross section. 
Located radially inwardly of the annular fluid channel 34 is a clamping 
ring clearance surface 42 which is at a greater axial distance from the 
axially outermost surface of the spindle plate 16 than is the surface of 
the clamping ring clamping surface 32, for a purpose to be described 
hereinafter. 
Access to the interior of the annular fluid channel 34 is through an 
hydraulic fluid inlet check valve 44 which is of conventional design. The 
inlet valve 44 is threaded into an opening within the clamping ring 28 and 
is recessed therein. The inlet valve 44 permits fluid to enter the annular 
fluid channel 34 through a conduit 46 which communicates therewith and 
does not permit the passage of fluid out of the channel 34. 
Spaced 180 degrees about the clamping ring 28 is an hydraulic fluid outlet 
valve 48 of conventional design which is likewise threaded within an 
opening in the clamping ring 28. The outlet valve 48 communicates with the 
interior of the annular fluid channel 34 through a conduit and is used to 
ensure that there is no trapped air within the annular fluid channel when 
hydraulic tensioning fluid 50 is placed therein, in a manner to be 
described hereinafter. 
An annular, single-piece, fluid sealing piston 52 is located within the 
annular fluid channel 34 and is generally U-shaped in cross section. The 
sealing piston 52 which functions to some degree as a gasket, and is shown 
in FIG. 6 in cross section. The piston 52 may be formed of any deformable 
resilient material capable of withstanding the operating pressures of the 
subject apparatus in the range of 1000 to 3000 psi, such as polyurethane 
or the like. 
As may be seen by reference to FIG. 6, the piston 52, in cross section, 
includes a relatively thick and generally rectangular main body part 54 
which has two outwardly directed, generally triangular, radially inner and 
outer arms 56, 58 attached to the main body part 54. 
The piston 52 includes an axially innermost blade contact surface 60 which 
is plane in cross section when the piston 52 has not been subjected to 
stress (either current or residual) and has not been deformed. As will be 
discussed in detail hereinafter, when the piston 52 is placed within the 
fluid channel 34, the blade contact surface 60 assumes a curvilinear shape 
in cross section before it is subjected to the stress of the hydraulic 
fluid 50 (see FIG. 3); assumes a more curved shape in cross section when 
it is deformed under the action of pressurized hydraulic fluid 50 (see 
FIG. 4); and tends to retain that greater curvilinear shape (i.e., retains 
a "set") after the pressure on the hydraulic fluid 50 has been removed 
(see FIG. 5). 
An annular saw blade 62 is positioned with locating pin holes therein (not 
shown) over locating pins (also not shown) which project axially outward 
from the tensioning ring 18. The clamping ring 28 includes openings 
therein (not shown) which correspond to the locating pins and which 
openings are aligned with the locating pins. Each of the plurality of 
clamping ring bolts 30 are then inserted, tightened and torqued to the 
same value. 
As may be seen by reference to FIGS. 1 and 2, the annular saw blade 62 
includes an inside diameter opening 64 which is ringed by diamond bort 66 
fixed to the blade to increase the life of the cutting edge thereof, in a 
conventional manner. 
As noted hereinabove, and as may be seen by reference to FIGS. 3, 4 and 5, 
the axial distance of the clamping ring clamping surface 32 and that of 
the clamping ring clearance surface 42 from the axially outermost surface 
of the spindle plate 16 differ. The clearance surface 42 is spaced further 
from the axially outermost surface of the spindle plate than is the 
clamping surface 32. Also as may be noted by reference to FIGS. 3, 4 annd 
5, the tensioning ring clamping surface 22 and the tensioning ring 
abutment surface 26 are substantially the same axial distance from the 
axially outermost face of the spindle plate 16. The reason for these axial 
spacings of the surfaces 22, 26, 32, 42 shall become apparent during the 
discussion of the operation of the subject apparatus discussed 
hereinafter. 
In order to protect the tensioning ring clamping and abutment surfaces 22, 
26 from wear, they may be plated with a tough, corrosion-resistant 
material such as nickel or the like. However, since they are the same 
axial distance from the axially outermost surface of the spindle plate 16, 
they most both be plated with approximately the same amount of material. 
On the other hand, the greater axial protrusion of the clamping ring 
clamping surface 32 (which is axially closer to the axially outermost 
surface of the spindle plate 16 than is the clamping ring clearance 
surface 42) can be obtained by plating only the surface 32 (and not the 
surface 42) with a corrosion-resistant material such as nickel or the 
like. 
In assembly of the apparatus 10, before placing the clamping ring 28 with 
the locating pin openings therein in registry with the locating pins 
protruding axially from the tensioning ring after the ID blade 62 has been 
placed thereover, the sealing piston 52 is placed within the fluid channel 
34 by depressing the radially inner and outer arms 56, 58 inward and 
sliding the piston 52 along the radially inner and outer polished surfaces 
38, 40 of the channel 34. The piston 52 is slid into the fluid channel 34 
an axial distance sufficient to place the rightmost, axially innermost, 
part of the normally plane blade contact surface 60 more axially inward 
within the annular fluid channel 34 than the clamping ring clamping 
surface 32 (see FIG. 3). Naturally, by virtue of the arms 56, 58 being 
moved toward one another, necessary to place the piston 52 within the 
fluid channel 34, the blade contact surface 60 of the piston assumes a 
gently convex outward curved configuration and is no longer plane (again, 
see FIG. 3). 
The channel 34 and the piston 52 are constructed and arranged so that their 
relative axial extent is such that even after deformation of the piston 
(as in FIG. 4) it may be slid into the channel a sufficient distance to 
prevent protrusion of the blade contact surface 60 beyond the clamping 
ring clamping surface 32 (see FIG. 5). 
As a result of the piston 52 being formed of a material of sufficient 
durometer to withstand the operating pressures of 1000 to 2000 psi placed 
on the hydraulic fluid 50 (in a manner to be described hereinafter), the 
material of the piston urges the arms 56, 58 outwardly against the 
polished inner and outer walls 38, 40 of the channel 34 thereby aiding, in 
conjunction with the shape of the inner, fluid contact surface 68, the 
piston 52 in sealing the hydraulic fluid 50 within the channel 34. 
After the apparatus is assembled with the saw blade 62 clamped in position 
and the piston 52 located within the channel 34, hydraulic fluid 50 is 
placed into the fluid channel 34 through the inlet value 44, entering 
through the fluid inlet conduit 46. The hydraulic tensioning fluid bleed 
valve 48 is opened to permit the escape of any air trapped in the channel 
34. When, after a period of time only hydraulic tensioning fluid 50 and no 
air exits from the bleed valve 48, located in the upright position as 
shown in FIG. 1, the bleed valve 48 is closed. 
Sufficient hydraulic fluid 50 is placed under pressure of from 1000 to 3000 
psi and according to well-known laws of physics, the hydraulic fluid 
exerts that pressure equally in all directions on the chamber 34, the 
conduit 46 and the fluid contact surface 68 of the piston 52. 
In view of the piston 52 being the only deformable part of the sealed 
compartment comprised of the channel 34 and the piston 52, and in view of 
the piston being deformable and slidable, the piston slides and deforms 
under the pressure of the hydraulic fluid, sliding axially inward to the 
right as shown in the drawing (compare FIG. 3 with FIG. 4) until at least 
part of the piston including the blade contact surface 60 slides axially 
outward to the right beyond the axially inner limit of the clamping ring 
28. 
As may be noted by reference to FIG. 3, when the saw blade 62 is clamped 
but not tensioned, the radially outer part of the saw blade is held 
between clamping and tensioning ring clamping surfaces 32, 22 and the 
axially inner or rightmost surface of the blade 62 abuts the tensioning 
ring abutting surface 26. By virtue of the axial position of the clamping 
ring clearance surface 42 noted hereinabove, the saw blade 62 does not 
touch the surface 42; and, radially inward of the clamping surfaces 32, 
22, the saw blade is free to move axially. 
When pressure is applied to the hydraulic fluid 50, deforming the piston 52 
and moving it to the right, the piston deforms and moves to the right 
until it contacts the saw blade 62 and it deforms the saw blade into the 
tensioning ring recess 24 thereby radially tensioning the saw blade 62. In 
virtue of the equal pressure which exists throughout the entire 
circumference of the fluid channel 34, the saw blade 62 is equally 
deformed and equally tensioned in all radially outward directions. 
In virtue of the specific configuration of the fluid contact surface 68 of 
the piston 52, and in virtue of the radially outward tension which the 
radially inward-forced arms 56, 58 exert on the surfaces 38, 40, the 
piston 52 exerts a proper seal on the surfaces of the fluid channel 34 to 
maintain the fluid 50 within the channel from commencement of 
pressurization of the fluid until the desired degree of deflection of the 
saw blade 62 into the tensioning ring recess 24 and the desired tensioning 
of the saw blade is attained. 
The fact that the piston 52 does not include mounting projections, such as 
contained in prior art seals, not only permits sliding movement of the 
piston 52 under pressure (which results in less actual deformation of the 
sealing piston and which, in turn, tends to prevent bursting of the 
sealing piston under pressure), but also permits the use of a larger blade 
contact surface 60 for the same radial size opening in the clamping ring 
28 thereby permitting a larger surface 60 to contact the blade 62. Such 
larger blade contact is necessary with larger diameter blades which must 
be deflected more than smaller diameter blades to achieve the required 
tensioning thereof. 
In addition to the slidability of the piston 52 within the channel 34 
permitting less deformation of the piston to achieve the same blade 
deflection contributing to increased piston life, the relative thickness 
of the main body part 64 of the piston 52 (being on the order of 
approximately one-third in axial extent relative to the radial extent of 
the seal, in the preferred embodiment), further contributes to the 
elimination of piston failure. 
When, either through failure of the blade 62 or dulling of the cutting edge 
64 thereof, it is necessary to change the blade 62, the bleed valve 48 is 
opened to relieve the pressure on the hydraulic fluid 50, the tensioning 
ring bolts 30 are loosened and removed and the tensioning ring 28 is 
removed from the apparatus 10. The fragmented or dulled saw blade 62 is 
removed from the locating pins and a new blade placed thereon. 
In prior art apparatuses it was necessary, in order to correctly position 
the new saw blade 62 during tightening of the clamping ring 28 once it was 
replaced on the apparatus 10, to either replace the fluid gasket if it had 
developed a permanent set and outward bow (making it protrude beyond the 
clamping ring clamping surface 32), or if it was a two-piece seal having a 
filler ring, reverse the filler ring within the outer relatively 
stationary part of the gasket. 
However, with the piston 52 of the present invention, it is only necessary 
to slide the piston 52 axially outward to the left so that the axially 
outermost part of the blade contact surface 60 is located axially inward 
with respect to the clamping ring clamping surface 32 (compare FIG. 4 with 
FIG. 5). In view of the particular construction and arrangement of the 
piston 52, the aforementioned sliding can be accomplished relatively 
easily despite the piston 52 having retained a permanent deformation or 
"set". 
The clamping ring bolts 30 are then re-torqued to the desired value and 
sufficient pressure is placed on the hydraulic tensioning fluid 50 in the 
manner discussed hereinbefore to uniformly deform the sliding piston 52 
against the new saw blade 62 and tension the new saw blade the desired 
amount. 
In view of the construction and arrangement of the piston 52 just 
described, as may be noted, longer life is provided a fluid retaining and 
pressure transmitting member, changing of blades is greatly simplified 
thereby and consequently occurs much more rapidly. 
In addition, when it is necessary to finally replace a sealing piston 52, 
in view of it either acquiring a permanent "set" which is unacceptable or, 
as a result of fatigue from repeated cycling, when the piston bursts, 
replacement of the piston is greatly simplified as well. 
All that is required to replace the piston 52 is to remove the old piston 
by merely sliding it out of the polished surfaces 38, 40. The arms 56, 58 
of a new piston are urged toward one another and the piston is slid with 
the compressed arms traveling along the polished surfaces 38, 40 until the 
blade contact surface 60 of the piston is moved axially outward to the 
left a distance sufficient to place the now-crowned surface 60 radially 
outward and to the left with respect to the clamping ring clamping surface 
32. 
Consequently, the construction and arrangement of the piston 52 and the 
fluid channel 34 which permits the slidability of the piston 52 between a 
first, normal, unpressurized or rest position when it is not under tension 
(as shown in FIGS. 3 and 5) and a second, extended, pressurized or tension 
position when it is slid to the right under the action of pressurized 
hydraulic fluid 50 to tension a clamped saw blade (as shown in FIG. 4) 
also aids in insertion and removal of the piston 52 when that is 
necessary. 
As will be readily apparent to those skilled in the art, the invention may 
be used in other specific forms and for other purposes without departing 
from its spirit or essential characteristics. The present embodiment is, 
therefore, to be considered as illustrative and not restrictive, the scope 
of the invention being indicated by the claims rather than by the 
foregoing description, and all changes which come within the meaning and 
range of equivalents of the claims are therefore intended to be embraced 
therein.