CA1188807A

CA1188807A - Mill load condition detector

Info

Publication number: CA1188807A
Application number: CA000406039A
Authority: CA
Inventors: Rhual L. Guerguerian; Marvin B. Shaver
Original assignee: Dominion Engineering Works Ltd
Current assignee: Dominion Engineering Works Ltd
Priority date: 1982-06-25
Filing date: 1982-06-25
Publication date: 1985-06-11

Abstract

MILL LOAD CONDITION DETECTOR
ABSTRACT OF THE DISCLOSURE

A detector for detecting mill load conditions on start-up provided by sensing at least one parameter that varies on start-up and is indicative of mill load conditions to generate a signal, co-ordinating this signal with a pre-established datum signal for that load condition and triggering an alerting means when the load signal reaches a pre-set amount correlated with the pre-established datum signal in the absence of a detector detecting a major shifting of the load.

Description

{~'7 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 .~
. .,

2--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

3~
GOK 103~105 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:

31)7 ~4--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, 3'7

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 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 L~

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 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 3~

_g _ 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 --10~
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 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 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.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A detector for detecting mill load conditions during start-up comprising, means for providing a pre-established datum signal, means for sensing at least one parameter indicative of a mill bearing load condition that varies with rotation of the mill as the mill is started to provide a load signal, computer means adapted to compare said load signal with said pre-established datum signal and actuate means for triggering an alarm means when said load signal reaches a pre-set value correlated with said pre-established datum signal, in the absence of tumbling of the load in said mill.

2. A detector as defined in Claim 1 wherein said at least one parameter comprises one or more parameters selected from a group consisting of bearing oil pressure and bearing clearance.

3. A detector defined in Claim 1 wherein said parameter is bearing oil pressure.

4. A detector as defined in Claim 3 wherein said parameter is bearing oil pressure at the trailing side of a hydrostatic bearing.

5. Detector as found in Claim 1 wherein said means for providing a pre-establishsd datum signal comprises a plurality of pre-recordings of said load signals generated under a plurality of different load conditions and stored by the computer to provide pre-established datums for such load conditions.

6. A detector as defined in Claims 1, 2 or 3 wherein said means for providing pre-established datum signal comprises means for sensing the axial thrust of the drive of the helical gearing driving the mill to determine the load conditions in said mill immediately prior to shut-down.

7. A detector for detecting mill load conditions during start-up comprising, means for sensing the angular position of the mill during start-up to provide a first signal, means for sensing at least one parameter indicative of mill bearing load conditions that vary with rotation of the mill as the mill is started to provide a load signal to detect a major shift in the load indicative of tumbling of load within the mill, and means for triggering an alarm means if said mill turns through a pre-set angular position without detecting a major shift in the load.

8. A detector as defined in claim 1 or 7 wherein said alarm means comprises means to abort start-up of the mill.

9. A detector as defined in claim 7 wherein said parameter comprises at least one of bearing oil pressure, bearing clearance, noise generated in the mill and vibration of the mill.

10. A detector as defined in claim 1, 2 or 3 wherein tumbling of said load is detected by said at least one parameter.

11. A detector as defined in claim 1, 2 or 3 wherein tumbling of said load is detected by detector means detecting at least one of noise generated by operation of said mill and vibration of the mill.