U.S. patent number 3,893,631 [Application Number 05/489,416] was granted by the patent office on 1975-07-08 for twin refiner with double floating discs.
This patent grant is currently assigned to Sprout, Waldron & Company, Inc.. Invention is credited to Chester Donald Fisher, Beatrice Pearl Fuller.
United States Patent |
3,893,631 |
Fisher , et al. |
July 8, 1975 |
Twin refiner with double floating discs
Abstract
A high speed ground wood disc refiner having a pair of
oppositely facing fixed refiner plates and twin rotating discs each
carrying refining plates juxtaposed the fixed plates. The rotating
discs are axially adjusted on the drive shaft by a parallel
piston-cylinder hydraulic system which insures that an equal and
opposite thrust force is produced by each rotating head. Since the
thrust forces are absorbed by the drive shaft, this arrangement
permits the use of very light bearings on the drive shaft,
resulting in economies of manufacture and maintenance.
Inventors: |
Fisher; Chester Donald (Muncy,
PA), Fuller; Beatrice Pearl (Muncy, PA) |
Assignee: |
Sprout, Waldron & Company,
Inc. (Muncy, PA)
|
Family
ID: |
23943769 |
Appl.
No.: |
05/489,416 |
Filed: |
July 17, 1974 |
Current U.S.
Class: |
241/37;
241/259.2; 241/146; 241/247; 241/285.2 |
Current CPC
Class: |
D21D
1/303 (20130101); D21B 1/063 (20130101) |
Current International
Class: |
D21B
1/06 (20060101); D21D 1/00 (20060101); D21B
1/00 (20060101); D21D 1/30 (20060101); B02C
007/14 () |
Field of
Search: |
;241/37,146,188A,244,245-247,259.1,259.2,285A |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Custer, Jr.; Granville Y.
Assistant Examiner: Goldberg; Howard N.
Attorney, Agent or Firm: Howson and Howson
Claims
We claim:
1. A disc type refiner comprising a base, a casing supported by
said base, a drive shaft extending through said casing, means on
said casing for rotatably supporting said drive shaft, a pair of
stationary heads mounted within said casing in spaced opposed
relation, a refining plate assembly radially disposed on each said
stationary head, a pair of rotating heads mounted on said drive
shaft, a refining plate assembly on each said rotating head in
parallel juxtaposed relation to the refining plate assembly of the
adjacent stationary head, means connecting said drive shaft and
said rotating heads to provide rotation of said heads with said
shaft while permitting axial movement thereof along said shaft,
conveying means for directing stock to be refined into said casing
between said juxtaposed refining plate assemblies, and means for
adjusting the axial position of said rotating heads on said drive
shaft, said latter means comprising cylinder-piston means on said
shaft connected with each said rotating head, and a parallel
hydraulic circuit operatively connecting said piston-cylinder means
to equalize the positioning force on each said rotating head
thereby permitting said rotating heads to float axially to balance
the hydraulic cylinder-piston force against the axial force
developed between the refining plate assemblies.
2. A disc type refiner comprising a base, a casing supported by
said base, a drive shaft extending through said casing, means on
said casing for rotatably supporting said drive shaft, a pair of
stationary heads mounted within said casing in spaced opposed
relation, a refining plate assembly radially disposed on each said
stationary head, a driving disc secured to said drive shaft
centrally between said stationary heads, a pair of rotating heads
mounted on said drive shaft, a refining plate assembly on each said
rotating head in parallel juxtaposed relation to the refining plate
assembly of the adjacent stationary head, said rotating heads being
disposed one on each side of said driving disc, means connecting
said driving disc and said rotating heads to provide rotation of
said heads with said shaft while permitting axial movement thereof
along said shaft, conveying means for directing stock to be refined
into said casing between said juxtaposed refining plate assemblies,
and means for adjusting the axial position of said rotating heads
on said drive shaft, said latter means comprising cylinder-piston
means on said shaft connected with each said rotating head, and a
parallel hydraulic circuit operatively connecting said
piston-cylinder means to equalize the positioning force on each
said rotating head thereby permitting said rotating heads to float
axially to balance the hydraulic cylinder-piston force against the
axial force developed between the refining plate assemblies.
3. The invention as claimed in claim 2 wherein said rotating heads
are each mounted on a sleeve disposed in concentric axially
slidable relation with said drive shaft.
4. The invention as claimed in claim 3 wherein said cylinder-piston
means connected with each said rotating head comprises a piston
secured to said drive shaft, and a cylinder cooperatively disposed
around said piston connected with the sleeve of said rotating
head.
5. The invention as claimed in claim 3 including hydraulic fluid
conduits in said drive shaft communicating with the cylinder
chambers on either side of said piston, said hydraulic fluid
conduits being arranged in parallel whereby a common fluid pressure
will be simultaneously applied to the corresponding chamber of each
said cylinder-piston means.
6. The invention as claimed in claim 3 including means for limiting
the axial movement of each said rotating head.
7. The invention as claimed in claim 6 wherein said means for
limiting the aixal movement of each said rotating head toward the
adjacent stationary head comprises a stop ring secured to said
shaft adapted for engagement by said rotating head sleeve.
8. The invention as claimed in claim 7 including a flanged stop nut
connected to each said sleeve for engagement with the corresponding
stop ring to limit axial movement of the rotary head away from the
adjacent stationary head.
9. The invention as claimed in claim 2 wherein said conveying means
for directing stock into said casing adjacent each said refining
plate assembly comprises a cylindrical stock conduit concentric
with said drive shaft and overlying each said sleeve, conveying
elements mounted on each said sleeve, and a screw conveyor for
introducing stock into said stock conduit.
10. The invention as claimed in claim 9 wherein said sleeve
conveyor elements comprise a plurality of radially extending lugs,
said lugs each having a pair of intersecting faces oppositely
angularly inclined with respect to the drive shaft axis, the lug
edge formed by said intersecting faces being directed toward the
adjacent rotating head.
11. The invention as claimed in claim 9 including a steam discharge
conduit for removing steam from each said stock conduit, said steam
discharge conduit being axially outwardly spaced from said screw
conveyor.
12. The invention as claimed in claim 2 wherein said means
connecting said driving disc and said rotating heads comprises a
plurality of axially extending lugs on each side of said driving
disc, and a plurality of lugs on each said rotating head engaged
with the lugs of said driving disc.
13. The invention as claimed in claim 2 wherein said casing
comprises a plurality of casing sections, and means for opening
said casing sections to provide access to said refining plate
assemblies.
Description
The present invention relates generally to disc type ground wood
refiners and relates more particularly to a high speed twin refiner
having double rotating floating discs.
Disc type refiners are well known, particularly in the paper making
field, and essentially comprise juxtaposed annular refining
surfaces which are arranged for relative rotation so that material
to be refined introduced at the inner edge of the surfaces is
transported by centrifugal force between the surface and
peripherally discharged in a refined condition. The refiners
commonly comprise either a single pair of refining surfaces or, in
the twin refiners, a double pair of refining surfaces. One type of
single refiner is characterized by a rotationally and axially fixed
refiner head which cooperates with an axially adjustable rotatable
head. In another type of single refiner, the heads are
counter-rotating with one being axially fixed and the other being
axially adjustable. A further type is characterized by a rotating
head which is axially fixed and a nonrotating head having means for
axial adjustment to change the refining plate clearance.
The twin refiners typically include a central rotating head having
attrition plates on each side thereof cooperating with oppositely
disposed rotationally fixed plates. One or both of the heads
supporting the fixed plates may be axially adjustable to vary the
plate spacing, and the shaft carrying the rotating head may be
either axially adjustable or freely floating. An example of the
freely floating type twin disc refiner having one of the fixed
heads axially adjustable is shown in U.S. Pat. No. 3,276,701,
assigned with the present application to a common assignee.
As the refiners increase in size, the thrust forces produced by the
refining action become quite large. Refiners are currently being
built with plate diameters of over four feet and horsepower
requirements exceeding 1,000 horsepower. The conventional twin disc
refiners have to a large extent eliminated the problem of axial
thrust forces on the rotating shaft, and this is particularly true
of the floating disc refiners of the type shown in U.S. Pat. No.
3,276,701. A difficulty, however, with the floating type twin
refiner is the danger of uneven stock feed resulting in a drift of
the floating head and the possibility of plate clashing.
Furthermore, such an imbalance could produce at least momentary
high thrust forces on the shaft and suitable bearings must be
provided for such a contingency.
In the present invention, a twin refiner is provided having a pair
of non-rotating axially fixed stationary heads and a pair of
axially adjustable rotating heads disposed therebetween. Refining
plates mounted on the stationary and rotating heads cooperate in
the conventional manner to carry out an attrition action on
materials passing therebetween. The rotating heads are each mounted
on sleeves axially slidable on the drive shaft and are driven in
rotation by a central driving disc on the drive shaft having
driving lugs extending from each side thereof engaging similar lugs
on the rotating heads. The sleeves supporting the rotating heads
are axially positioned by cylinder-piston means connected to a
parallel hydraulic circuit whereby thrust forces developed by the
rotating heads are equalized. Means associated with the
cylinder-piston means restrict the axial movement of the rotating
heads and prevent the possibility of plate clashing, thereby
allowing operation of the refiner under no-load conditions with a
predetermined clearance between the refining plates.
It is accordingly a first object of the present invention to
provide a high speed twin disc refiner for refining ground wood and
the like of a novel improved design.
A further object of the invention is to provide a twin disc refiner
as described having double rotating floating discs which minimize
thrust forces transmitted by the refiner shaft thereby permitting
the use of very light shaft bearings.
A further object of the invention is to provide a twin disc refiner
as described having means for limiting the floating movement of the
rotating discs to prevent plate clashing and permit running of the
refiner even under no-load conditions.
Still another object of the invention is to provide a twin disc
refiner as described which compared to conventional refiners is
substantially more economical to build and to maintain.
Further objects and advantages of the invention will be more
readily apparent from the following detailed description of an
embodiment thereof when taken together with the accompanying
drawings wherein:
FIG. 1 is a side elevational view of a refiner constructed in
accordance with the present invention;
FIG. 2 is an end elevational view looking into the righthand end of
the refiner shown in FIG. 1;
FIG. 3 is an enlarged sectional view taken along line 3--3 of FIG.
2 and showing the interior details of the refiner;
FIG. 4 is an enlarged partial sectional view with a portion broken
away and in section of the righthand cylinderpiston arrangement
shown in FIG. 3 for positioning the righthand rotating disc;
FIG. 5 is a sectional view taken along line 5--5 of FIG. 3; and
FIG. 6 is a partial plan view taken along line 6--6 of FIG. 3
showing the manner in which the lugs of the driving disc cooperate
with the lugs of the rotating discs.
Referring to the drawings and particularly FIGS. 1-3 thereof, a
refiner generally designated 10 in accordance with the present
invention comprises a base 12 supporting spaced separable casing
sections 14 and 16. A central casing section 18 is removably
secured between the sections 14 and 16 to provide access to the
refiner plates. The casing section 18 may comprise a pair of
arcuate portions which are clamped between the casing sections 14
and 16 by the bolts 20 joining the sections.
Bearing support portions 22 and 24 respectively extend outwardly
from the casing sections 14 and 16 to rotatably support the refiner
drive shaft 26 which extends through the casing sections. With
reference to FIG. 3, the lefthand end of the drive shaft 26 has a
stepped configuration journalled in bearing assembly 27 secured
within the bearing support portion 22. The drive shaft is axially
located by the bearing assembly 27 which abuts against a shoulder
28 of the drive shaft and is maintained thereagainst by the lock
ring 30 threadedly mounted on the drive shaft. The bearing assembly
27, which is seated within a shouldered bore 32 of the cylindrical
support member 34, is secured in place therein by the annular
locking member 36 and the lock ring 38 fastened by screws 40. Seal
means 42 and 44 at opposite ends of the bearing assembly 27 prevent
foreign matter from reaching the bearings. The bearings as
illustrated are of the angular contact type which will absorb light
thrust forces although as described below, the refiner under normal
operation should not generate any substantial thrust forces which
need be absorbed by the refiner shaft bearings.
The drive shaft at the opposite end of the refiner is supported by
the bearing support portion 24 within which the cylindrical support
member 46 is secured. A radial bearing assembly 48 is mounted in
the shouldered bore 50 of the member 46 and secured in position by
the annular ring 52. Seal assemblies 54 and 56 are provided on
either side of the bearing assembly 48.
The casing sections 14, 16 and 18 describe surfaces of revolution
around the axis of the shaft 26 which is considered the refiner
axis. Circular openings 58 and 60 respectively in the ends of
casing sections 14 and 16 permit the passage of the shaft 26
therethrough. As shown particularly in FIG. 2, the base 12 is
provided with an arcuate contoured surface 62 on which the casing
sections 14 and 16 are slidably supported. Although the casing
sections are secured together by the bolts 20 during operation of
the refiner, for plate inspection or replacement, the casing
sections 14 and 16 may be axially moved to the positions 14' and
16' illustrated in dot and dash lines in FIG. 3. To move the casing
sections to the opened position, piston-cylinder assemblies 64 and
66 secured by brackets 64a and 66a to the base are respectively
connected with the casing sections 14 and 16 to provide the axial
force required to slide the casing sections on the arcuate surface
62 of the base.
The casing sections 14 and 16, which are frusto-conical in shape as
shown in FIG. 3, respectively include non-rotating axially fixed
heads 68 and 70 disposed perpendicularly to the drive shaft axis.
The heads are of an annular shape, being secured to the casing
sections at their outer radial edge and being secured at their
inner edge to the cylindrical stock conduit members 72 and 74
disposed concentrically in spaced relation about the drive shaft
26. Refiner plate assemblies 76 and 78 are respectively mounted on
the opposed radial faces of the heads 68 and 70. The refining plate
assemblies each comprise a conventional array of refining plate
elements such as those shown in U.S. Pat. No. 3,473,745, assigned
with the present application to a common assignee.
A driving disc 82 is secured to the drive shaft 26 in a radially
aligned central position equidistant the stationary heads 68 and
70. The driving disc 82 includes a plurality of uniformly spaced
lugs 84 extending axially from each side of the periphery thereof
as shown in FIGS. 3 and 6.
Rotating heads (also known as discs) 86 and 88 are disposed on
opposite sides of the driving disc 82 respectively juxtaposed the
non-rotating heads 68 and 70. The rotating heads 86 and 88 are
generally annular in shape and are respectively mounted on the
sleeves 90 and 92 which overlie the drive shaft 26 in close fitting
but axially slidable relation thereto. The rotating heads 86 and 88
and hence the attached sleeves 90 and 92 are driven in rotation by
the driving disc 82 by means of lugs 94 and 96 extending
respectively from the heads 86 and 88 which engage the lugs 84 of
the driving disc. This lug arrangement effects a rotation of the
rotating heads with the drive shaft but permits the axial movement
thereof since the lugs of the rotating heads may slide axially
along the lugs of the driving disc.
Refining plate assemblies 98 and 100 are respectively secured to
the rotating heads 86 and 88 in juxtaposed relation to the refining
plate assemblies 76 and 78 of the stationary heads 68 and 70. The
refining plate assemblies 98 and 100 in the normal operating
position of the rotating heads are spaced a small fraction of an
inch from the stationary refining plate assemblies 76 and 78 to
effect an attrition effect on the stock passing therebetween. The
spacing of the plates is controlled by adjusting the position of
the sleeves 90 and 92 as described below.
Stock feed conveyor assemblies 102 and 104 are respectively mounted
on the casing sections 14 and 16. Since the feed conveyors 102 and
104 are essentially identical aside from the direction of rotation
of the conveyor screws, only the conveyor assembly 104 will be
described in detail. With reference to FIGS. 2 and 3, the conveyor
assembly 104 includes a horizontal cylindrical conveyor casing 106
which extends from a vertical flanged input conduit 108 exteriorly
of the casing section 16 to a downwardly directed discharge chute
110 within the casing section vertically above the refiner axis.
The discharge chute 110 connects with opening 112 in the
cylindrical casing member 74. A conveyor screw 114 mounted within
the conveyor casing 106 is driven in rotation by a motor 116
connected thereto by chain drive assembly 118 as shown in FIG. 2.
Stock directed into the input conduit 108 will accordingly be
advanced by the conveyor screw 114 to the discharge chute 110 and
pass through opening 112 in the member 74 into the annular
sectioned chamber 120 formed between the sleeve 92 and the member
74. A similar chamber 122 is formed at the opposite end of the
refiner and stock is fed thereto by the conveyor assembly 102 in
the identical manner to that described with respect to conveyor
assembly 104. The rotation of the conveyor screws is such as to
direct the stock toward the rotating heads 86 and 88 and thus as
viewed in FIG. 3 the conveyor screw 114 is rotated in a clockwise
direction while the screw of the conveyor assembly 102 will rotate
in a counterclockwise direction.
The feed of the stock from the chambers 120 and 122 respectively
into the refiner throat areas 124 and 126 adjacent the refining
plate assemblies is effected by conveying means comprising a
plurality of triangular lugs 128 rotating with the drive shaft and
extending into the chambers 120 and 122. The triangular lugs 128
are disposed so that angularly oriented faces thereof will direct
the stock toward the throat regions 124 and 126 regardless of the
direction of rotation of the drive shaft. The reversibility of the
drive shaft is desirable to permit a uniform wearing of the
attrition elements of the refiner plates. The lugs 128 are
preferably arranged in evenly spaced relation in axially aligned
rows with the lugs of each row being axially staggered with respect
to the lugs of the adjacent rows. The lugs 128 are secured to
sleeve extensions 90a and 92a which overlie and extend axially
outwardly beyond the sleeves 90 and 92.
The lugs 128 permit the passage of steam generated in the refining
process back through the incoming stock to steam discharge conduits
130 and 132 which pass respectively vertically thorugh the casing
sections 14 and 16 at the outermost ends thereof. The reversible
lug conveyor construction illustrated is in accordance with the
disclosure of copending application Ser. No. 296,564, now U.S. Pat.
No. 3,853,276, assigned with the present application to a common
assignee. Although this is the preferable stock conveyor
construction in view of its reversibility, a ribbon screw refiner
feeder could also be utilized such as that shown in U.S. Pat. No.
3,441,227 also assigned with the present application to a common
assignee. Either of these conveyors permits the desirable reverse
flow of steam which is generated in the refining of high
consistency stock.
The refined stock passes from between the sets of refining plates
into an annular chamber defined by the casing intermediate section
18, the stationary heads 68 and 70, and the rotating heads 86 and
88. Some refined stock may find its way between the driving disc 82
and the rotating discs 86 and 88 although the bulk of the refined
stock will be thrown by centrifugal force into the peripheral
regions of the chamber 134. A discharge conduit 136 communicating
with the annular chamber 134 passes through the base 12 for
discharge of the refined stock.
The hydraulic system for axially positioning the rotating heads 86
and 88 comprises a coaxial bore 138 in the drive shaft 26 extending
from the lefthand end thereof as viewed in FIG. 3 to a point
approximately even with the outer end of the casing section 16. The
bore 138 includes a pair of cylindrical plugs 140 and 142 disposed
proximate the outer ends of the sleeves 90 and 92 respectively.
Since the construction and function of each of the plugs 140 and
142 as well as the passages in the drive shaft adjacent the plugs
is essentially the same, only the plug 142 and the cooperative
hydraulic passages as shown in the enlarged view of FIG. 4 will be
described.
The bore 138 is threaded at its open end 144 for connection to a
source of pressurized fluid (not shown). The hydraulic fluid in the
bore 138 passes through passages 146 in the plugs 140 and 142, thus
filling the bore 138 throughout its length. Radial passages 148 and
150 connect the bore 138 with chambers 152 and 154 respectively
formed adjacent the drive shaft 26 by the pistons 156 and 158 which
are threadedly secured to the drive shaft, and the flanged ends 160
and 162 of the sleeve extensions 90a and 92a. The introduction of
pressurized hydraulic fluid into the chambers 152 and 154 acts to
move the rotating heads axially toward the driving disc 82 and
hence away from the stationary heads to increase the plate
clearance.
Chambers 164 and 166 are formed on the outer sides of the pistons
156 and 158 by the flanged end cover members 168 and 170 which are
secured to the flanged ends 160 and 162 of the sleeve extensions
90a and 92a respectively. The end cover members 168 and 170 are in
sealing engagement with the drive shaft 26 and accordingly, the
introduction of pressurized hydraulic fluid into the chambers 164
and 166 will result in the outward movement of the floating heads
to decrease the plate clearance. Pressurized hydraulic fluid is
introduced to the chambers 164 and 166 through diagonal passages
172 and 174 respectively which intercept the bore 138 at the plugs
140 and 142, communicating with an annulus 176 therein. As shown in
FIG. 4, each annulus 176 communicates in turn with radial passage
178 leading to axial passage 180 of the plug. A conduit 182 passing
axially through the bore 138 is threadedly connected to the plugs
to communicate with the axial bores therein. The conduit 182
terminates in a threaded connector 184 for connection with a
pressurized hydraulic source (not shown).
The described hydraulic arrangement for controlling the axial
position of the rotating plates is essentially a piston-cylinder
arrangement wherein the pistons are fixed and the cylinders move
with respect to the fixed pistons. The travel of the rotating heads
86 and 88 is respectively limited in the closing direction by stop
rings 186 and 188 which are threadedly adjustable on the drive
shaft and locked in the desired position by lock screws. Flanged
stop nuts 190 and 192 threadedly attached to the end covers 168 and
170 limit the opening movement of the rotatable heads 86 and 88
respectively.
For operation, the refiner drive shaft 26 is connected by means of
extending shaft end 26a to a power source, normally an electric
motor of substantial size, for example 1,000 horsepower. The stock
input conduits are connected with a source of stock to be refined
which typically flows under gravity force from an elevated location
into the feed conveyors 102 and 104. The drive shaft bore 138 and
the conduit 182 are connected with appropriate sources of
pressurized hydraulic fluid through control and valve means which
will connect one of the channels to drain while the other is
connected to the pressurized fluid source.
The feed conveyors 102 and 104 are driven in rotation at a
predetermined speed to introduce a uniform flow rate of stock into
the chambers 120 and 122. The stock is directed into the throat
regions 124 and 126 by the triangular lugs 128 and is centrifugally
advanced between the refiner plates in a well known manner. The
refined stock is discharged from the chamber 134 through the
discharge conduit 136. Steam which may develop during the refining
process passes from the throat regions through the chambers 120 and
122 and is discharged through the discharge conduits 130 and 132.
Although the steam is travelling against the infeed of stock, the
stock will be centrifugally thrown against the outer walls of the
chambers 120 and 122, permitting the steam to pass along the inner
chamber walls without interfering with the stock flow.
The control of the plate spacing is accomplished by means of the
cylinder-piston assemblies acting in parallel to axially adjust the
position of the rotating heads 86 and 88. The application of
pressurized hydraulic fluid to the chambers 152 and 154 while
opening the chambers 164 and 166 to drain will result in an
increase in the plate spacing. Conversely, an application of
pressurized hydraulic fluid to the chambers 164 and 166 and a
draining of the chambers 152 and 154 will decrease the plate
spacing. Since the recited chambers are connected in parallel with
the sources of hydraulic pressure, the rotating heads 86 and 88 are
freely floating on the drive shaft 26 and will become positioned so
that the forces developed between the two sets of refining plates
are equal inasmuch as the opposing forces provided by each of the
piston-cylinder assemblies is equal.
The use of a parallel hydraulic system to govern the plate spacing
results in an equal and opposite thrust being exerted on the piston
rings 156 and 158 during the operation of the refiner. Since this
thrust is absorbed by the refiner shaft, there is no need for any
substantial shaft thrust bearings other than those necessary to
support incidental loads. Considering the enormous horsepower input
commonly utilized in twin refiners, the elimination of sizable
thrust bearings and their maintenance comprises a substantial
saving in contrast to conventional refiner construction.
An advantageous feature of the cylinder-piston control of the
rotating heads is the stop arrangement which prevents any
possibility of plate clashing, even if the refiner is run without
stock flow. The stop rings 186 and 188 are threadedly adjusted in
position and locked on the drive shaft so that the end covers 168
and 170 respectively will engage the stop rings before the refining
plates clash. Similarly, the flanged stop nuts 190 and 192 limit
the degree to which the refining plates can be separated since the
flanges of the stop nuts will engage the stop rings at a
predetermined point.
Although in the preferred embodiment of the invention illustrated
the rotary heads 86 and 88 are driven in rotation by the driving
disc 82 secured to drive shaft 26, it will be obvious that other
driving means could be employed for this purpose. For example, keys
or splines could be provided on the drive shaft for cooperation
with appropriate slots or grooves on the rotating heads. The
present arrangement is preferred, however, since the driving force
between the driving disc lugs and rotating heads is minimized by
establishing their contact at a maximum radius from the drive
shaft. The minimal contact force between the lugs reduces the axial
force required to move the rotating heads, thus improving the
"floating" action of the rotating heads.
Manifestly, changes in details of construction can be effected by
those skilled in the art without departing from the spirit and the
scope of the invention.
* * * * *