Note: Descriptions are shown in the official language in which they were submitted.
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GOK-103-094
MILL LOAD SENSING SYSTEM
The present invention relates to a mill load
sensing system for sensing and controlling the charge
load in a mill during operation of the mill.
In order to achieve the optimum grinding e~ficiency
for a grinding mill, the mass of the charge load in the
mill must be of a predetermined value which is dependent
upon the run-of-mine for an autogenous mill (i.e., the
; material entering the mill) or the grinding media for
a ball, rod or pebble mill. Presently, no system has
been developed which can reliably determine the charge
load in the mill.
Attemps have been made in the past to relate the
mill charge load to the bearing pressure of the
trunnion bearing supporting the trunnion of the mill.
One such attempt is disclosed in Canadian Patent No.
;791,424, issued August 6, 1968 to Harries et al. These
attempts have not been reliable, however, because the
liner in the mill is continuously wearing, whéreby the
20~ mill weight is constantly varying. Also, it should be
understood that 40 to 60 percent of the loading, due to
ravity, placed on thé turnnion bearing, is due to the
wei~ht of the grinding mill. Hence, any reading taken of
~loading on the trunnion bearing, cannot be accurately
measured and related to charge load in the mill, because
fluctuations in the grinding mill weight are largely
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GOK 103-094
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responsible for trunnion bearing load changes.
It is therefore a feature of the present invention
to provide a mill load sensing system for sensing the
charge load in a mill.
Briefly, a mill load sensing system is provided
that includes a helical gear means assembled in secured
relation with a rotatable drum of the mill. The helical
gear means is driven by the pinion of a prime mover for
the mill. The helical gear means during mill operation,
develops react'ion axial thrust forces, the values of which
are substantially a function of the charge load in the
mill. At least one sensing means is located in a region
of the mill that is subjected to the axial thrust forces.
The sensing means senses the thrust forces and provides
output signals which are representative of the value of
these thrust forces. The system further includes a
load control means responsive to the output signals for
determining the charge load in the mill and for providing
control signals to the mill so as to control entry of
materials into the mill.
By sensing directly or indirectly axial thrust
forces developed during mill operation, the system
of the present invention senses a force which is propor-
- tional to the charge load of the mill. It should be
understood that there is some predetermined error in
this system which is due to the friction of the mill.
It is to be expected that the mill will provide for about
; 2%~of the total value of the developed axial forces,
~; ~ as this value is the amount of axial force that is
produced during operation of the mill under no load
condition. This measurement of charge load, however,
is of improved accuracy over previously unreliable methods
of sensing charge load.
In a system of the present invention/ it is
envisaged that the sensing means need only be located in
the region of the mill that is subjected to
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GOK-103-094
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a~xial thrust forces. Such regions exist in various areas
of the mill such as, for example, a trunnion bearing
pocket and a bearing pocket of the pinion and/or the
prime mover. Also, it should be understood that the
sensing means may be located in more than one region.
As can be appreciated, most large grinding mills are
driven by two prime movers which operate in synchronism.
Thus, the sensing means may be located at bearing pockets
for each of these prime movers. The sensing means may
further comprise a load cell such as, for example, a
hydraulic jack or a ~ et~-clec*-r-i-e load cell.
Regarding gear drivers for such grinding mills, some
gear drives may include a gear reduction unit between the
pinion and prime mover. In this case and in the case
where no gear reduction unit is employed, the coupling
of the pinion to prime mover does not normally transmit
axial forces related to the developed unbalanccd axial
thrust forces. Rather, one of the pinion bearings
would be arranged to accept the axial related forces and
provide through the sensing means the output slgnals.
Although the present invention is described with
regards to its application with a grinding mill, it
should be understood that such has been done for the
purposes of illustration only; and that, without departing
~from either the~scope or spirit of the invention, it
may be applied to other types of machines in which the
charge load of the machine develops _ axial
; thrust forces proportional thereto. Accordingly,
reference hereafter in the disclosure and claims to the
term "mill" shall mean a machine as described hereinabove.
Therefore, in accordance with a broad aspect of the
present invention, there is provided a mill load sensing
system for sensing the charge load in a mill. The system
comprises a hel'ical gear means operably with a rotatable
~rum o the mill and beî'ng driven by a pinion of a prime
mover of the mill. The he'lical gear means develops
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-Y~4~d axial thrust forces, the values of which are
~` substantially a function of the charge load in the mill.
The system further comprises at leas-t one sensing means
located in a reyion of the mill that is subjected to
the-unbalana~d axial thrust forces. The sensing means
senses the thrust forces.
For a better understanding of the nature and
objects of the present invention, reference may be had,
by way of example, to the accompanying diagramatic
drawings in which:
Figure 1 is a schematic representation of a
grinding mill and the mill load sensing system therefore;
Figure 2 is a graph showing the relationship between
the mill charge load and-unbalanccd axial thrust forces;
and,
Figure 3 is a partial schematic representation of an
alternate mill load sensing system for a grinding mill.
Referring to Figure I the preferred embodiment of
the present invention is described. A grinding mill 10
is provided with a rotatable drum structure 12. Drum
structure 12 is rotatable about trunnions 14 which are
mounted by trunnion bearings 16 above floor 18. On~
of the trunnions 14 is provided with shoulder 20 and 22
so as to limit the axial displacement of the drum structure
12. Secured to the drum structure 12 is a single helical
gear 24 which is driven by pinion 26 having its helical
teeth in accurate meshing relation with the teeth of the
helical gear 24. Pinion 26 is shown attached via a shaft
to an electric prime mover 28 mounted above the floor 18
by foundation support 30.
During mill operation material, and possible water,
is transported by conveyor belt 32 to one of the trunnions
14 of mill 10. Friable material entering drum structure
~ 12 is ground within the mill and discharge through the
other trunnion 14 to another conveyor belt (not shown).
In order to control the charge load within the drum
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structure 12, a sensing means schematically indicated at
34 is provided in a trunnion bearing pocket between shoulder
20 and one of the trunnion bearings 16. The sensing means
34 may comprise a hydraulic load cell which senses the axial
thrust forces developed by the mill 10 during operation.
Arrow 36 depicts the axial direction in which the axial
forces will be developed.
Line 38 is a schematic representation o an output
path from the hydraulic load cell 34 which runs into a
load control meand 40. The hydraulic load cell 34 senses
the thrust forces and transmits output signals representa-
tive of the values of these thrust forces along output
path 38. The load control means 40 is responsive to the
output signals for determining the charge load in the
mill and providing control signals which are representative
of the charge, to control entry of materials into
the mill. The control of materials entering the mill
by control means 40 is shown schematically by line 42
terminating in arrow 44 at conveyor 32. This may involve
either automatically or manually controlling the rate
of movement of the conveyor 32.
The control means 40 in response to the output
signals determines the charge load by averaging a
predetermined numberof output signals from the sensing
means. The control means compares the average of the
predetermined number of output signals with a predeter-
mined reference signal which provides a control signal
that is indicative of whether the load in the mill i5
increasing or decreasing. The control means may
further be provided with a memory storage means so that
each previous average of the output signals may be
stored while the next series of output signals is being
averaged. The stored output signals may comprise the
predetermined reference signal with which the newly
determined average signals will be compared. As the
electronic components for this function are well known
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in the art, these components are not shbwn.
Referring to the graph of Figure 2, there is shown
a curve 100 which illustrates the functional relationship
~3 between the-~ba~e~ axial thrust forces and the mill
change load from figure 2 the relationship is shown to
be one to one over the operating range of the mill;
however, this relationship ceases after the charge load
in the mill exceeds a predetermined value indicated at
"A" on the graph. As shown, the predetermined value "A"
is just beyond the upper limit of the preferred operating
range and in some instances may define the upper operating
limit. It should be understood that this predetermined
value is normally a known value for which the machine
control system can readily monitor through the sensing
means. In the case of the predetermined value not being
a known parameter, it can be detected after it is exceeded
when the values of ~ L~ee~ axial thrust forces decrease
as a result of an increase in the feed rate of material
entering the mill. Thus the control means 40 in the
2^0 preferred embodiment further provides a warning indication
when the resultant forces decrease as a result of an
increase in the feed rate of material entering the mill.
Referring to Figure 3 an alternate embodiment for
the present invention is shown. In Figure 3 the hydraulic
load cell 34 is shown located in a pocket 102 between
prime mover 28 and flange portion 104 of foundation 106.
This embodiment is illustrated to show the positioning
of the hydraulic load cell in a region other than that
shown in Fiyuxe 1. It is to be understood that load
cell 34 indirectly senses the unba-~anccd axially thrust
forces.
It should be understood that alternate embodiments
may be readily apparent to a man skilled in the art in
view of the foregoing description of the present invention.
For example, the conveyor 28 illustrated in Figure 1 may
~ not be employed. In this instance, the mill may be loaded
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GOK-103-094
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by other means and the control signal indicates that the
mill is to be charged. Accordingly, the scope of the
present invention should only be limited to that which
is claimed in the accompanying claims.
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