Note: Descriptions are shown in the official language in which they were submitted.
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MILL LOAD CONDITION DETECTOR
FIELD o~ THE IWVENTIO~
The pre~ent invention relates to a detector
for detec~ing mill load conditions on start-up. More
particularly the present in~ention rela~es to a system
for warning an operator of impending charge drops or
automatically controlling the mill to prevent a charge
drop.
BACKGROUND OF THE INVENTIO~
Grinding millæ for grinding ore and the like
usually are composed of relatively large drum members
mounted at their opposite ends on trunnions and
driven, for example, via a suitable gearing, ~uch as,
a pinion and ring gear drive to rotate the drum and
tumble the charge. In some mills the trunnions are
eliminated and the mill is supported by bearing
sur~aces wrapped around the mill. In some mills
"gearle~s" drives ar~ used where a portion of the mill
orms the rotor of the drive motor. These mills are
gPnerally quite large and may at any one time be
charged with upwards of a million pounds. This load
is tumbled and axially advanced by the rotation o the
mill to reduce the size of the material entering at
one axial end and leaving when reduced to the reyuired
size through the other axial end.
It is not uncommon to stop these mills
sometimes for extended periods of time. Over these
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periods the material contained within the mill
sometimes "freezes" (becomes harder and not easily
~umbled). When such a condition occurs, depending on
the total load in the mill, i.e. if there is only a
relatively small load in the mill, it may no-t be as
important, however, if there is a reasonable sized
load in the mill and the charge is frozen, care must
be taken in star~ing the mill to ensure that no s-ldden
dropping of the charge occurs, i.e. when the frozen
charge is elevated as a unit in the drum and a very
significant portion thereof falls to the bottom of the
drum when the drum has been rotated ~o a position such
that the surface of the charge is at an angle to the
horizontal well beyond its normal angle of repose.
Vnder these circumstances very severe damage may be
inflicted on the mill structure itself and the mill
may be rendered totally inoperative, for example by
fracture of the heads at the end of the drum or
breaking the drum itself. When the charge is frozen a
very time consuming and tedious procedure is
instituted wherein the mill is "inched" to a pre-set
angular position using the inching mechanism ( eparate
from the drive) and then rotated in the opposite
direction thereby gradually freeing the charge. In
some cases it is actually necessary to use jackhammers
or spray water through fire hoses against the charge
to try and free it so that it will tumble in an
acceptable manner.
There is no simple wa~ of detecting whether
a charge i8 frozen nor is there any alternative to the
teaious and time-consuming procedure necessary when
the charge is frozen and ~or these reasons it is not
uncommon for the mill to be started in the
conventional man~er even when the charge is frozen.
Many times the charge drops and only occasionally is
damage done. Because the damage onl~ occurs
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occasionally there is ~urther incentive to risk
attempting the start-up in the normal manner rather
than using the inching technique.
BRIEF DESCRIPTION OF THE INVENTION
It is the object of the present invention to
provide a detecting mechanism to detect a frozen
charge during start up.
Broadly the present invention relates to a
detector for detecting mill load conditions on
start-up comprising a means for providing a
pre-established datum means for sensing changes in at
least one parameter that varies as the mill iB rotated
during start-up and is indicative of mill load
condition thereby to generate a load signal, means for
co-ordinating said lGad signal with said
pre-established datum signal and means ~or triggering
an alarm that may be a warning device or a device for
aborting the start-up by terminating drive to the mill
when said load signal reaches a pre-set value
correlated with pre-set datum signal before said load
signal indicates tumbling of the load has commenced.
In one form of the invention the angle of
repose may be used to set the pre-established datum
and the angular position of the mill used to trigger
the alarm if major shifting of the load has not
commenced or is not detected when the angular position
of the mill is detected.
Detectors for detecting such major shifting
of the load may rely on bearing condition such as
bearing clearances or bearing oil pressure or on noise
generated by the mill or vibration of the mill.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features, objects and advantages
will be evident from the following detailed
description of the preferred embodiments of the
present invention in which:
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Figure 1 is a schematic side elevation view
of one form of mill constructed in accordance with the
present invention.
Figure 2 is a schematic illustration of the
various sensors and con~rol system that may ba used
with the present invention.
Figure 3 is a plan view of a hydrostatic
bearing illustrating the position of the bearing
sensors.
Figure 4 is a section along line 4-4
illustrating the position of the bearing sensors.
Figure 5 shows a plurality of plots on
bearing clearances at ~he locations of the various
proximity sensors in the bearing showing the variation
in clearance at these locations during a normal
start-up.
Figure 6 shows graphs similar to Figure 5
but showing the changes in bearing clearance during a
start-up with a frozen load ~o the point where impacts
commences (the charge drops).
Figure7 shows plots of oil pressure on the
leading and trailing sides of the bearing during a
normal or frozen start.
DETAILED DESCRIPTION OF THE INVENTION
As shown in Figure 1, mill 10 is generally
composed of a drum section 12 closed at its ends by a
pair of heads 14 and mounted on trunnions 16 which in
turn are supported by bearings 18 positioned at
opposite axial ends of the mill 10. The mill in the
illustrated arrangement is driven by a suitable motor
and gearing mechanism 20 through pinion 22 and ring
gear 24. At least one of the bearings 18 will be
equipped with suitable sensors to determine bearing
loads.
As shown in Figures 2, 3 & 4, these sensors
may include proximity sensors (proximitors) 1, 2, 3,
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4, 5 & 6 and oil pressure sensors 7 & 8. It will be
noted from Figures 3 & 4 that proximity sen.sors 2, 4 &
5 are located on the mill side 26 o~ the bearing, i.e.
side of the bearing adjacent the mill and the
proximity sensors 1 t 3 ~ 5 are located at the outboard
side 28 of the mill. The sensors 1 & 2 are located at
the 8 o' clock position as ~iewed facing the mill
sensors 3 & 4 at the 6 o'clock position and sensors 5
& 6 at the 4 o' clock position as shown in Figure 4~
The trunnion rotates in the bearing from the 8 o'clock
position to the 4 o'clock position or counter
clockwise as shown by the arrow 30 in Figure 4. Oil
pressure sensors 7 & 8 are positioned within the oil
cavities or wells which in the particular embodiment
illustrated were located each spaced 35 degress from
thP 6 o' clock postion.
Sensor 7 is in the leading side and the
sensor 8 in the trailing side of the bearing.
It is to be understood that when practicing
the present invention, and sensing bearing load
conditions directly by ~,ensors in the bearing
particularly when bearing oil pressures are sensed as
the main controlling parameter, hydrostatic bearings
will be used whether so specifically specified or
not. Where other parameterR such as angular position,
noi~e generated by the mill, vibration of the mill are
sensed and bearing oil pressure or clearance
measurements are not critical either hydrostatic or
hydromamic bearings ~ay be used.
Depending on how the device is to be
operated, other 6ensors may be used such as torque
sensor 32, which senses the axial thrust of th~ pinion
against the single helical gear, for example~ in the
manner described in detail in applicants
corresponding Canadian Application Serial ~umber
377,578 filed May 14, 1981, and/or an angular
displacement sensor 34 ~ensing the angular
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displacement of the mill as it is rotated and/or an
accoustic or vibration sensor 35 sensing the noise or
vibrations generated by tumbling of the load in the
millO It may also be useful to sens~ the temperature
of the oil being pumped to the bearing using oil
temperature sensor 26. The time from shut down to
start-up may also be recorded in the computer as
"freezing" of the charge is to a degree time dependent.
Not all of the sensors described are
necessary for any given installation but selected ones
may be utilized for given sets of conditions or
control techniques some reflecting the operation more
accurately than others and therefore being more
preferred.
All of the signals generated by the above
sensors are fed to a master computer control 38 that
will be provided with the necessary well known
circuitry to perform the required functions as will be
described in more detail hereinbelow.
It has been found that when a load is frozen
the oil pressure and/or bearing clearances indicative
of bearing load during start-up are significantly
different from the load conditions that would prevail
during start-up if the same charge was not frozen and
that using sensors to detect these difference~ at the
appropriate time one can have advanced warning of an
impending charge drop before such a charge drop occurs.
Figures 5 & 6 show plots based on the
reading of proximitors 1, ~, 3, 4, 5 & 6 representing
bearing clearance against time in seconds for a normal
charge and a frozen charge respectively. Comparing
figures 5 & 6 it will be apparent that for each of the
curves for each of the sensors (numbered to correspond
with the sensor providing the signal) thexe is a
difference in the shape of the curve immediately
before charge drops or begins to tumble when the load
is normal (Figure 5) or Erozen ~Figure 6). Each of
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these graphs 1 to 6 the datum line for each curve is
marked with an arrow and the number of the curve, and
the curves 1, 2, 3~ 4, 5 & 6 correspond to the
readings from the sensor 1, 2, 3, 4, 5 ~ 6
respectively positioned as shown in Figures 3 & 4.
These consistently different patterns before tumbling
of the normal charge or dropping of the Erozen charge
permit these parameters to be used with computer
equipment to detect a frozen load before dropping of
the charge occurs.
Probably the best indication of a frozen
load is given by bearing oil pressure as sensed by the
sensors 7 & 8 respectively. In Figure 7 the curves 7N
and 8N depict oil pressures sensed by sensors 7 & 8
under normal start conditions, i.e. when the charge is
not frozen. Curves 7F & 8F illustrate the change in
pressure at these sensors under frozen load start-up
conditions. It will be noted that the oil pressure
changes (curves in Figure 7) are much more significant
than the variations in the bearing clearances as
depicted by the curves 1-6 inclusive in Figures 5 & 6
and further that changes in oil pressure sensed by the
sensor 8 on the trailing side of the bearing are more
pronounced than the changes in oil pressure on the
leading side of the bearing as sensed by sensor 7 and
depicted by curves 7N and 7F,
In Figure 7 the base lines for curves are
identified by the number of the curve plus an arrow on
the line.
It i8 evident from Figures 5, 6 & 7 that
there are a variety of different parameter~ that may
be sensed to indicate that the load is frozen at
start-up, It i~ important to be able to detect the
frozen charge before the charge drops to provide ample
~5 warning of an impending drop. By far the most
pronounced differences in load conditions are shown
GOK 103-l05
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in the pressure curves given in Figure 7, and in
particular on the trailing side of the bearing shown
by comparing the curves 8~ and 8F. For this reason
the preferred parameter to be sensed ~o determine
load condition using the sensor in the bearing is oil
pressure and preferably on the trailing side of the
baaring.
It will also be apparent that an absolute
value of any of the above bearing load parameters
normally will not be used to trigger the detector but
there must be a comparison with some reference, datum
point or pre-established datum indicative of the
specific load condition at start-up since the curves
generated by the sensors at start-up are dependent
also on the actual start-up conditions such as the oil
temperature, load in the drum, oil pump condition,
etc., all of which affect the shape and absolute
values of the curves for these various parameters.
The clearance and pressure readings are
dependent on the oil temperature and -flow and total
load on the bearing. In view of this dependence on
temperature it may be desirable as shown in Figure 2
to utilize sensor 36 to determine the temperature of
the oil supplied to or alternatively returning from
the bearing and provide this information directly to
the computer 38. Alternatively the effects of oil
temperature variations may be derived indirectly from
condition in the bearing and compensated as required.
In order to permit the computer control 38
to which all of the above signals are fed to determine
if there is or is not a frozen charge in the mill it
may be necessary, depending on the type of controls
selected to establish various datums imperically, in
other words to establish datums for each of a
plurality of different load conditions. This may be
done by generating a plurality of start~up curves with
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different normal charges and oil temperature
conditions for the particular parameter to be
monitored or alternatively for all parameters that may
be used thereby to provide a plurality of
characteristic curves or pre-set datums and to store
this information indicating the safe conditions for
start-up under given load conditions in the computer
38.
Obviously there will be a learning curve and
it will take time to set up the control accurately and
the actual cut-off for any given load condition may
well vary from machine to machine so that for each
machine a separate set of calibration tests may be
required in order to program the computer to
understand when the charge is not frozen and to detect
when the start-up conditions differ significantly
under similar load conditons and to operate
accordingly to activate the trigering means 39 (which
may form part of the computer 38) to warn the operator
and/or abort the start-up. For example if start-up
followq more closely curve 6 of Figure 6 than curve 6
of Figure 5 the computer must be programmed to compare
the curve being generated (Fig. 6) with the
pre-established curve (Fig. 5) and to react as soon a9
the point such as point on curve C of Figure 6 is
reached to terminate drive to the mill as the computer
has recognized that the charge is frozen and may be
about to drop. It is important that the drive to the
mill be cut off or the operator warned as early as
possible and in any event sufficiently early to permit
the mill to be stopped before it reaches the point
where the charge drop~. Comparing curves 3 & 4 of
Fiyure 6 with 3 & 4 of Figure 5 it is obvious that it
would be more difficult to make this prediction based
on proximity reading taken in positions 3 & 4.
Similarly, it would be more difficult using signal~
from proximitor~ at locations 1 ~ 2 to determine the
GOK 103-105
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charge was frozen than using these from proximit,ies 5
& 6. However, if one were to take all of these
proximitors and establish a plurality of curves for
various working conditions and develop the signatures
for the machine at start-up under various load
condiiions all of these proximitors and/or pressure
sensors could be used in concert to more accurately
determine a frozen load condition.
As above indicated the bearing oil pressure,
particularly the pressure on the trailing side of the
bearing is most indicative oE a frozen charge.
Comparing graph 8F with 8N a very significant
difference in the curves is evident with ~he point Y
on curve 8F immediately preceding the drop of the
charge (point Z) indicating significantly lower
pressure than any point on curve 8N thereby providing
a distinct difference in signal,
Instead of using only parameters indicati~e
of the bearing pressure to determine if the charge is
frozen and provide the pre-established datum point it
iQ also possible to use other parameters. For example
the sensor 32 may be used to determine the axial
thrust in the drive to the helical gear (assuming a
single helical gear is used) to determine load
conditions immediately prior to shut down. This
information is stored in the computer to provide the
(datum starting torque is significantly greater than
the torque required to rotate the drum during
operation and thus sensors 32 must be used to
determine by the mill load prior to shutdown). This
datum may be used together with say an oil pressure
~ensor and the pressure signal correlated with the
mill condition prior to shut down (as stored in the
computer) by having the computer programmed to trigger
the alarm if a significant decrease in pressure on the
bearing for the specific load contiion (e.g. as
indicated at Y of curves 7 & 8 is) reached. The
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precise value of Y is determined by repeated use of
the equipment to find a suitable value for Y for each
of a varity of typical load conditions sensed by
sensor 32, i.e., the magnitude of point Y to trigger
the alarm is set based on stored information of the
load conditions on start-up and pre-established
expected conditions for a normal start-up under the
load conditions at shut off.
Another way of determining when the drive to
the mill should be terminated or a warning issued is
by sensing the angular rotation oE the drum during
start-up. ~his angular position of the drum may be
sensed in a variety of different ways, fox example, by
applying a flag to the drum in the 6 o'clock position
and sensing when the flag reaches the angle of repose
of the material in the normal (non-frozen) conditions,
by counting the number of teeth on the ring gear
passing a given point, etc., via sensor 34. 'rhe
start-up is aborted if the angle sensed significantly
exceeds the angle of repose of the material and
tumbling of the load has not commenced. Accordingly,
if pressure point Z has not been reached within a
certain predetermined angular position relative to the
angle of repose, iOe. say 10 degrees beyond the angle
of repose ~this angle may vary from mill to mill and
from charge to charge), the triggering mechanism of
the computer will be actuated to activate the alarm
and/or abort the start-up. It may also be de~irable
to be able to override the control completely, for
example if it is recognized by the computer that there
is a very small charge in the mill start-up may
commence automatically as if the charge was normal
regardless of whether or not the charge is frozen
since the light charge would not damage the mill.
If the charge i~ frozen it i5 sometimes
loosened by turning the mill beyond the angle of
repose and then rocking it back and forth, Care mu~t
GOK 103 105
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be taken to ensure that it is not turned so much past
the angle of repose that an actual drop occurs as this
could damage the mill.
In yet another alternative the sensor 35 may
be used to determine if the load is tumbling by
detecting the amount of noise generated by the mill
(or the vibration of the mill). When the load shifts
and tumbles considerable noise is generated and the
mill is vibrated so that either or both of these
parameters may be sensed to establish tumbling of
the load, if tumbling is not sensed by the time the
mill has been rotated through an angle greater than
the angle of repose for the material in the mill by a
pre-established amount the computer may be programmed
to raise the alarm and/or automatically abort the
start~up.
Here again, depending on the materials being
treated, drum loading, etc. e~perience will determine
just how far it is safe to turn the mill in order to
free the charge. If the frozen charge cannot be
released in thi s manner it is sometimes necessary to
use jackhammers or fire hoses to free the charge so
that the mill can commence operation.
Having described the invention,
modifications will be evident to those skilled in the
art without departing from the spirit of the invention
as defined in the appended claims.
