US3905557A

US3905557A - Crushing roller mill protective method and system

Info

Publication number: US3905557A
Application number: US471966A
Authority: US
Inventors: Helmut Grommes; Willi Lehr
Original assignee: Deutsche Babcock and Wilcox AG
Current assignee: Deutsche Babcock and Wilcox AG
Priority date: 1973-05-24
Filing date: 1974-05-21
Publication date: 1975-09-16
Anticipated expiration: 1992-09-16
Also published as: GB1423936A; DE2326470C2; DE2326470A1; ZA743303B; IN141212B; YU121074A; YU35716B; AT324813B; CS170108B2; DD114352A1

Abstract

A method and system for protecting crushing roller mills against excessive wear arising from blockage of a crushing roller includes continuous electrical measurement of the temperature and level of oil within the oil sump of the crushing roller bearing. The oil temperature and level indicates whether or not the crushing roller is rotating properly. Signals related to the oil temperature and level may be transmitted to indicators, alarms or other control means. Each crushing roller of a mill may be continuously monitored to ascertain any irregularities in operation.

Description

United States Patent Grommes et al.

[ 51 Sept. 16, 1975 1 1 CRUSHING ROLLER MILL PROTECTIVE METHOD AND SYSTEM [75] Inventors: l-lelmut Grommes, Duisburg; Willi Lehr, Oberhausen, both of Germany [73] Assignee: Deutsche Babcock & Wilcox Aktiengesellschaft, Oberhausen, Germany 122] Filed: May 21, 1974 [21 Appl. No.: 471,966

[30] Foreign Application Priority Data May 24, 1973 Germany 2326470 [52] U.S. Cl. 241/30; 241/36; 241/106 [51 Int. Cl. B02C 25/00 [58] Field of Search 241/33, 36, 103, 105, 106, 241/30; 308/76, 77, 78, 109, 119, 127

[56] References Cited UNITED STATES PATENTS 1,808,914 6/1931 Bennett 241/36 3,138,337 6/1964 Bogot et al. 241/103 3,350,018 10/1967 Harris et a1. 241/36 X 3,471,094 10/ 1969 Kearney et al 241/30 Primary Examiner-Granville Y. Custer, Jr. Attorney, Agent, or Firm-Roland T. Bryan 10 Claims, 2 Drawing Figures i CRUSHING ROLLER MILL PROTECTIVE METHOD AND SYSTEM BACKGROUND of THE INVENTION This invention relates'to'crushing roller mills and more particularly to a security method and system for protecting such mills against excessive 'grinding plate, crushing roller or bearing .wear. Particularly advantageous utility of the protection 'me'thod and system of this invention is found with'rnills'of the type comprising a horizontally rotating"grindingplate' and stationary, that is, nonorbiting, crushing rollers rolling over it and transferring grinding pressure I to the grinding plate while attached to a shaft by means of 'an'oil lubricated bearing. t

ln crushing roller mills of the type described, it occasionally occurs that a single particularly hard fragment or foreign body in the material to be crushed is not rolled over and fragmented but instead" becomes wedged between the crushing roller and grinding plate to block the roller. This bloc kage causes the roller to act somewhat like a drag shoe so that, instead ofgrinding, the blocked crushing roller pushes the softer material that is to be ground away from the grinding strip with a snowplow effect; The roller itself then slides over the empty grinding strip which no material in the grindingpath. lnsuc h cases there is usi ally no appreciable difference in the energy requirements of the mill and hence'recognition of the blockedcondition by observation of the mill energy requirements is not possible. Moreover, until the blocking fragment is finally ground up or the foreign body jumps-away, the sliding of the roller over the ,grinding stripor. plate causes a plane surface to be ground away on the circumference of the roller. This condition of wear. favors a renewed blocking'in the same position and can-lead. to an enlargement of the ground area on the roller. Therefore, sucha condition of .wear on the. rollerincreases the danger that other less significant causes will also lead to a blocking of the crushing roller. in the same position. For example, a normal change inthe feed of the material to be crushed to a harder material; and the application ofa grinding pressure that is too low, may bring about this problem if the roller is already worn in one area'Thus, there arises the disadvantage that further operation of the mill is'with a reduced grinding yield and also leads to mill downtime andexpensive :periods of disuse, as well as the necessity for startu'p -work.

' It is common to observe-the pressure of'the'lubricating'oil'of bearings, or, in 'the case of splash lubrication,

the oil level as well as the-temperatu'reof the bearing,

or of the oil, duringoperat'ion'. A lowering of the oil level indicates an oil'lossfiAn'in'crease in bear-ing'tern- 'perature, which cannot be tolerated, indicates stoppage of lubrication. Thus, an increase in temperature warns of imminent damageto' the'beari'ngi Measuring techniques to check the' temperature of 'a liquidwith the aid of a primary element or the level of a liquid with theaid of a capacitor or condenser element which is partly submerged in the liquid, are known to be-usefulfor'remote control andlor 'monitoringof those conditions. However, a continuous monitoring of the oil temperature and/or of the oil level," through remote control, of the bearings of all the fl crushing rollers of a mill ofthe type described above; has not heretofore been carried SUMMARY OF THE INVENTION The present invention greatly reduces the possibility of grinding of flat areas at the circumferenceof the crushing rollers ina mill of the,type.described above. This wear problem associated with crushing roller mills is solved, according to this invention, by providing remote monitoring and control of the splash lubrication of the bearings of the crushing roller during the normal operation thereof. This remote monitoring shows the oil level and the oil temperature and provides-control that permits stoppage and starting up of the mill when, during operation, the oil temperature drops by a certain amount and/or the oil level rises by a certain amount in one of the crushing rollers.

The protection and security method and system of this invention utilizes the normal start up and stopping devices of the mill. The system includes several temperature measuring primaryelectrical elements, protected by a metal shell, as well as several electrical remote indicating control devices which react to temperature changes of the primary elements or to changes in capacitance. These remote indicating control devices may, if desired, be connected to' alarm devices for warning control.

According to this invention, there is provided a temperature feeler located in the oil sump of the splash lubrication means of the roller bearing and which dips into the oil therein. The temperature feeler is advantageously positioned within a bore in each crushing roller shaft and connected to a remote control measuring device. The temperature feeler is surrounded by a pol' ished metal shield or shell which is electrically insulated from the shaft bore and together with the bore forms two capacitor surfaces opposite one another with an annular space therebetween. The oil of the sump penetrates into the annular space, that is, the oil rises within the annularspace to a height of the oil level in the sump, and functions as a dielectric between the capacitor surfaces so that changes in capacitance occur with changes in oil level. The metal shell and bore are connected to a remote control measuring device which reacts to changes in the capacitance.

Through the special arrangement of the temperature measuring feelers in the bores of the shafts of the crushing rollers, it is possible, according to this invention, to

continuously and simultaneously monitor,through remote-control means, the condition of each crusher roller during the normal operation of the mill for both the temperature of theoil and height of the oil level in the sump of the splash lubrication means.

In bearings that are splash lubricated during operation so that oil film is present betweenthe gliding surfaces, a higher oil temperature than the ambient temperature of the oil is always observed because of the friction of, the oil liquid. If the bearing is stopped, indicating, for example, that a crushing roller is blocked and'is not rotating, then the friction of the oil ceases and the temperature of the oil in the sump of the splash lubrication system drops in a short time through the loss of heat. The drop in temperature may therefore be utilized as an indication showing that the crushing roller is blocked. When themill starts up, the rotation of the crushing rollers draws part of the oil out of the sump-for lubrication, so that the oil level drops. if a crushing roller is blocked, the oil will flow to the sump and be collected therein so that the oil level therein rises.

If during normal operation there is observed a drop in the temperature of the oil in the sump of the splash lubrication system of a bearing, and/or a rise in the oil level therein, then this functions as a warning for the person in charge of operation and maintenance that the crushing roller is blocked and that the mill should be stopped, and then started up again to unblock it. before a larger surface can be ground away at the circumference of the crushing roller and thus cause excessive wear and deterioration.

Accordingly, it is an object of this invention to provide a method and system for the remote continuous monitoring and control of the characteristics of oil temperature and oil level in the oil sump of the bearing of a crushing roller.

Upon receiving an indication that the temperature has fallen by more than a predetermined amount or that the oil level has risen by more than a predetermined amount, the mill is stopped so that the crushing roller may be rolled back over a short distance to remove the particularly hard fragment or foreign body. This is accomplished by turning the rotating grinding plate back to free the fragment so that when the mill is started up again, the fragment or foreign body is pushed out of the grinding strip by the crushing roller or jumps free of it. To ascertain that the crushing roller that had been blocked is freely operating, the oil level in the sump should be observed because this characteristic reacts faster to indicate normal operation then does an increase in temperature after start up. If the sump oil level observed does not drop and hence indicates that the roller is not free, the mill must be stopped again and then once again started up.

Thus, another object of this invention is the provision of a method and system for observing the oil level in the sump of a crushing roller splash lubrication system in order to determine whether or not the crushing roller is operating free from blockage.

A drop in temperature of the oil of the splash lubrication system of the crushing roller can be better ascertained, according to this invention, by advantageously continuously comparing the oil temperature of the bearings of all the crushing rollers of the mill operating under the same conditions. The measured oil temperature of the blocked crushing roller then will show the actual temperature while the other oil'temperature will show how high the oil temperature of the blocked crushing roller should be if it is operating properly. Thus, a yet further object of this invention is the provision of a method and system of a continuously measuring oil temperature conditions of oil in splash lubrication systems of all the rollers of a crushing roller mill in order to determine whether they are all operating the same.

The temperature measuring feeler of this invention assumes the temperature of the oil by transmission of .heat therefrom and this temperature may be visibly indicated on an electric measuring instrument through the electric remote control measuring device of this system. The capacitance of the capacitor formed from the metal shell and the bore changes with the height of the oil level because in this area of the capacitor the dielectric constant of the lubricating oil becomes operative. Thus, in the part of the capacitor containing oil, the capacitance is approximately doubled by the lubrieating oil. The change in oil level causes a change in the capacitance and this may be indicated through measurement of the capacitance by a remote control measuring device on an electric measuring instrument.

Accordingly, another object of this invention is the provision of a protective method and system providing continuous monitoring and control of the oil levels and of the oil temperature in a crushing roller bearing splash lubrication system and yielding early warning signals indicating the need to be on the lookout for imminent damage to the bearing and the crushing roller in order to obtain an improvement in the operation of the mill through a great decrease in the grinding of flat surfaces at the circumference of crushing rollers and thus avoiding excessive wear and costly downtime maintenance.

BRIEF DESCRIPTION OF THE DRAWINGS The above objects, aspects and advantages of this invention as well as others will become more apparent from the following detailed description of a preferred embodiment thereof as shown in the drawings herewith in which: I

FIG. 1 shows a partial cross-sectional view taken through a crushing roller mill showing the crushing roller, the crushing roller shaft and lubrication system and the rotating grinding plate; and

FIG. 2 is a schematic diagram illustrating the electrical system permitting continuous monitoring and control of the oil levels and oil temperatures of the crushing roller bearings.

DETAILED DESCRIPTION OF THE DRAWINGS Referring to FIG. 1 there is shown, in partial crosssection, a single crushing roller, A of a mill comprising three crushing rollers A,, A and A The crushing rollers are non-orbiting," and roll on an annular grinding strip 1 of a horizontally rotating plate 2 which is driven by a motor 3 via a drive 4, as shown diagrammatically in FIG. 1. A switch 5, also shown diagrammatically, is illustrative of a common device for starting and stopping the mill. The crushing roller A is illustrative of the remaining crushing rollers A and A of the mill and they comprise like elements.

The drive of the mill can best be -illustrated on the basis of the attached drawings. These drawings show a plan view and a side view of a crushing roller mill embodying the present invention. The crushing rollers A A and A are stationary in relation to the housing of the mill. They roll on the annular strip 1 of a horizontal revolving plate 2. Plate 2 is driven by the electric motor 3 via gear 4. In the power supply to the electric motor 3 there is a switch 5 by which the starting and stopping of the mill are monitored from a control position.

The crushing roller A is attached to a shaft 7 which permits it to rotate, by a combined roller bearing fi, for

the transmission of the grinding pressure. The hub of the crushing roller seals the bearng 6 and the end of the shaft 7 in a housing which is filled with oil up to the level indicated at B The oil fill forms;a sump, shown at D of a splash lubrication system into which the rollers of the bearings dip and through which they pass upon rotation of the crushing roller. This passage of the roller bearing 6 through the oil in the sump D brings part of the oil therein up for lubrication. so that the oil level during the operation of the crushing roller drops to the level shown at 8' This oil in the sump D rises again to the original oil level B when the rotation stops because the oil no longer is drawn upward for lubrication of the bearings and the oil drains and collects in the sump D During operation, the friction of the oil lubricating the crushing roller bearings during their rotation gives rise to heat which is conveyed to the oil contained in the sump and there transferred to the housing and the shaft journal as a consequence of temperature differences. This heat transfer causes the temperature of the oil fill in the sump to rise and increase above the temperature of the housing and of the shaft journal during operation but to decrease when operation ceases and rotation stops. When rotation ceases, the heat transfer stops and the temperature of the oil adjusts to the temperature of the housing and the shaft journal.

Thus, when the rotation of the crushing roller A stops, meaning that the roller is blocked in some manner, the temperature of the oil fill in the sump drops by a certain amount and the oil level l3' rises to the original oil level B To the contrary, when the crushing roller A rotates but an irregularity in the bearing 6 has occurred, then the temperature of the oil fill in the sump D rises too high when lubrication fails or the oil level B drops further when oil loss occurs. All these conditions can be measured according to this invention as described hereinafter.

To measure the oil temperature, a temperature measuring feeler 8 is provided which is surrounded by a polished bright metal jacket 9 and which dips or penetrates beneath the level of the oil of the sump D The temperature measuring feeler 8 is connected, through electrical wiring, to the measuring device 10 and an indicating instrument E which indicates the oil temperature. For measuring the oil level, the metal jacket is connected to a remote control measuring device 11 and a measuring instrument F which indicates the oil level through an electrical circuit.

For determination of the oil level B or B the metal jacket 9 is positioned within bore 12 of the shaft 7 so that it is electrically insulated from the bore and forms together with the bore two opposite capacitor surfaces with an annular space 13, into which the oil of the sump D penetrates, to the height of the oil level B from below. Above the oil level B the bore 12 is connected with the bearing space by a cross bore 12' for the equalization of pressure therein so that the oil level may rise and fall therein in relationship to its rising and falling in the sump D In the area of the capacitor indicated generally at 14, oil is in the annular space 13 so that the capacitance of this area of the capacitor is doubled due to the dielectric constant of the lubrication mineral oil. When the oil level drops to the height B' then the oil flows out of the space 13 and the capacitance increasing action of the oil disappears. This change in capacitance is utilized for the remote monitoring of the oil level B through a telemetering device 11 reacting to capacitance changes and providing transmission of indication of oil level.

Referring now to FIG. 2, there is shown diagrammatically the temperature measuring and oil level measuring elements together with telemetering circuits utilized for providing continuous remote monitoring and control of the oil level and temperatures of the oil in the splash lubrication system. These are shown corresponding to the three rollers, A A and A of the crushing mill. The temperature measuring feelers 8,

shielded with the polished metal jackets 9, are shown diagrammatically with respect to the bore 12 and and are the same for all crushing rollers A A and A Resistance thermometers are provided for the measuring of the oil temperatures, and the temperature measuring feelers 8 are apart of the Wheatstone bridges 10 in which the indicators E E or E measure the bridge current present and through appropriate calibration provide indication of the temperature at a remote location. Thus, the feelers 8 generate an electrical signal related to the temperature.

The telemetering devices, indicated generally at 11, that react to capacitance changes comprise a high frequency generator, the oscillatory circuit of which, con sisting of an inductive reactance and a high frequency generator, is stimulated by direct current via a coupledback transistor. These elements are shown schematically and are known in the art, as indicated earlier therein. The capacitor formed by the metallic jacket 9 together with the bore 12 is part of the oscillatory circuit. Thus, the oscillation frequency changes when the capacitance changes through a change in the oil level from B to 8' or the reverse thereof. The oscillation frequency of the high frequency generator 15 is transmitted connected by means of a Wien-Robinsons bridge circuit 16, built up from resistors and capacitors, which is coordinated with the oscillation frequency of the high frequency generator 15 to indicate change in the oscillation frequency by a voltage change in the bridge potential. A direct current circuit is controlled with this voltage potential with the aid of a transistor which supplies the measured value for the recording element F Thus, the capacitor formed of the jacket 9 and bore 12 generates an electrical signal related to the oil level. The recording element F indicates the oil level and when it has risen from 8' to B Constant monitoring and operating control is cffected by assuring that the indicators E E and E which can be coupled to warning devices or other cir cuit devices, for control, show that the temperature of the oil in the sumps D D and D does not rise too much, that is, above a predetermined value, dependent upon the operating condition of the particular mill, and further through observing the recording elements F F and F to be sure that the oil level 8' 8' and B;, does not sink too low, that is, below a predetermined level dependent upon the operating condition of the particular mill as a result of oil lossin the sump.

However, if only one of the indicators E E or E;,, for example indicator E ,.indicates that, as compared to the other indicators E and E the oil temperature in the sump D has decreased, this does not necessarily mean that the operating conditions of the bearing have improved, but rather that the crushing roller A may be blocked. in this circumstance, the indication of the measuring instrument F showing a rise in oil level B' to B does not necessarily mean that the oil in the sump D has been filled up but rather than the crushing roller A is blocked. Thus, this indicates that the crushing roller is pushing the unground material out of the grinding strip 1 shown in FIG. 1, and sliding on the empty grinding strip so that the grinding strip wears the crushing roller to produce a flat surface G also shown in FIG. 1, at the circumference of the crushing roller A This condition is harmful and a further enlargement of the surface G may be avoided according to this invention through observations of these conditions of oil temperature and oil level to stop the mill and'start it up as previously described to remove the obstacle.

Thus, if during normal operation the measuring indicators E or F show, directly or via a warning or alarm circuit. that the oil temperature is lowered and/or there has been a rise in the oil level from B to B then, as a precaution, the mill is stopped through operation of the switch shown schematically in FIG. 1 and started up again by switching the switch on. This starting and stopping will cause any particularly hard fragments that may block the crushing roller or foreign bodies to be pushed aside or flung out of the grinding track. As previously described, upon starting up again, care must be taken that the indicator F shows a drop in the oil level from B to B since this is the most responsive characteristic upon start-up to make sure that possible damage to the bearing does not occur in the rotation of the crushing roller. The same protective and safety precautions are taken in a similar manner when the other crushing roller A or A may be blocked.

Thus, there has been described a method and system for continuously monitoring the operation of a crushing roller mill to protect against excessive wear and deterioration of the crushing roller arising from blockage thereof. This is accomplished by monitoring the oil temperature and oil level in the splash lubrication system of the crushing roller bearings and controlling the starting and stopping of the mill in response thereto. Alarm devices or visual indication may be provided for comparison between each roller of the mill. Of course, it is possible to utilize the generated output signals to effect automatic remedial action without human action, by utilizing known per se apparatus to cause the mill to stop when a predetermined set of outputs are monitored. 1

We claim:

1. A method for monitoring and controlling the operation of a crushing roller mill of the type including a rotating horizontal plate having an annular grinding strip over which roll one or more non-orbiting crushing rollers rotatably mounted to shafts by means of bearings splash lubricated by oil contained in a sump within the crushing roller bearing housing comprising continuously measuring at least one of the temperature and the level of the oil in the sump of at least one of the rollers,

' generating a signal related to at least one of the aforesaid measurements and stopping the operation of the mill in response to the signal generated.

2. A method as claimed in claim 1 wherein the measurement taken of the oil is of its temperature and the operation of the mill is stopped if the measured temperature of the oil changes during operation by a predetermined amount.

3. A method as claimed in claim 1 wherein the measurement taken of the oil is of its level in the sump and the operation of the mill is stopped if the measured level of the oil changes during operation by a predetermined amount.

4. A method as claimed in claim 1 wherein the oil temperature and the oil level in the sump are measured and the operation of the mill is stopped if either the temperature or the level of the oil changes, by-prede terminedamounts, during operation of the mill.

5. A method as claimed in claim 1 comprising mea- 'suring at least one of the temperature and the level of foregoing measurements for all of said rollers determining if said compared measurements of one roller differ by a predetermined amount from those of the other rollers, and stopping the operation of the mill in response thereto.

6. In a crushing roller mill of the type including a rotating horizontal plate having an annular grinding strip over which roll one or more non-orbiting crushing rollers rotatably mounted to shafts by means of bearings splash lubricated by oil contained in a sump within the crushing roller bearing housing, the improvement comprising a system for monitoring the operation of the mill to protect it against unnecessary wear or breakdown comprising temperature measuring means positioned in the sump for measuring the temperature of the oil therein, signal generating and transmitting means operatively connected to the temperature measuring means for generating and transmitting a signal related to the temperature measured, control means responsive to the temperature signal for controlling the operation of the mill.

7. In a crushing roller mill of the type including a rotating horizontal plate having an annular grinding strip over which roll one or more non-orbiting crushing rollers rotatably mounted to shafts by means of bearings splash lubricated by oil contained in a sump within the crushing roller bearing housing, the improvement comprising a system for monitoring the operation of the mill to protect it against unnecessary wear or breakdown comprising oil level measuring means positioned in the sump for measuring the level of the oil therein, signal generating and transmitting means operatively connected to the boil level measuring means for generating and transmitting a signal related to the oil level measured, control means responsive to the oil level signal for controlling the operation of the mill.

8. The improvement as claimed in claim 7 wherein the oil level measuring means comprises a capacitor means having opposite capacitor surfaces penetrating below the level of the oil in the sump between which the oil acts as a dielectric so that the capacitance varies with the level of the oil, the signal generating means comprises a telemetering means including an oscillatory circuit means the oscillation frequency of which changes with the capacitance changes brought about by an oil level change, circuit means relating the oscillation frequency to a voltage potential and the control means includes means for acting in response to the voltage potential. I

9. In a crushing roller mill of the type including a rotating horizontal plate having an annular grinding strip over which roll one or more non-orbiting crushing rollers rotatably mounted to shafts by means of bearings splash lubricated by oil contained in a sump within the crushing roller bearing housing, the improvement comprising .a system for monitoring the operation of the mill to protect it against unnecessary wear or breakdown comprising providing a crushing roller shaft having an axially longitudinal bore therein arranged in communication with the sump so that oil in the sump rises in the bore, a resistance-type temperature measuring feeler means positioned in the bore to penetrate beneath the surface of the oil for measuring the temperature of the oil-in the bore, a metal jacket means surrounding the temperature measuring feeler means concentrically positioned within the bore and forming an annular space therebetween, the metal jacket means viding an electrical signal output related to the oil level, indicating means responsive to the temperature and oil level signal means for indicating when the temperature or oil level varies from a predetermined value.

10. The improvement as claimed in claim 9 wherein the mill includes means for continuously comparing the same in each roller and determining whether one differs by a predetermined amount from the other.

Claims

1. A METHOD FOR MONITORING AND CONTROLLING THE OPERATION OF A CRUSHING ROLLER MILL OF THE TYPE INCLUDING A ROTATING HORIZONTAL PLATE HAVING AN ANNULAR GRINDING STRIP OVER WHICH ROL: ONE OR MORE NON-ORBITING CRUSHING ROLLERS ROTATABLY MOUNTED TO SHAFTS BY MEANS OF BEARING SPLASH LUBRICATED BY OIL CONTAINED IN A SUMP WITHIN THE CRUSHING ROLLER BEARING HOUSING COMPRISING CONTINUOUSLY MEASURING AT LEAST ONE OF THE TEMPERTURE AND THE LEVEL OF THE OIL IN THE SUMP OF AT LEAST ONE OF THE ROLLERS, GENERATING A SIGNAL RELATED TO AT LEAST ONE OF THE AFORE

4. A method as claimed in claim 1 wherein the oil temperature and the oil level in the sump are measured and the operation of the mill is stopped if either the temperature or the level of the oil changes, by predetermined amounts, during operation of the mill.

5. A method as claimed in claim 1 comprising measuring at least one of the temperature and the level of the oil in the sump of each of the rollers rolling on the grinding strip of a mill, comparing the same among the foregoing measurements for all of said rollers determining if said compared measurements of one roller differ by a predetermined amount from those of the other rollers, and stopping the operation of the mill in response thereto.

8. The improvement as claimed in claim 7 wherein the oil level measuring means comprises a capacitor means having opposite capacitor surfaces penetrating below the level of the oil in the sump between which the oil acts as a dielectric so that the capacitance varies with the level of the oil, the signal generating means comprises a telemetering means including an oscillatory circuit means the oscillation frequency of which changes with the capacitance changes brought about by an oil level change, circuit means relating the oscillation frequency to a voltage potential and the control means includes means for acting in response to the voltage potential.

9. In a crushing roller mill of the type including a rotating horizontal plate having an annular grinding strip over which roll one or more non-orbiting crushing rollers rotatably mounted to shafts by means of bearings splash lubricated by oil contained in a sump within the crushing roller bearing housing, the improvement comprising a system for monitoring the operation of the mill to protect it against unnecessary wear or breakdown comprising providing a crushing roller shaft having an axially longitudinal bore therein arranged in communication with the sump so that oil in the sump rises in the bore, a resistance-type temperature measuring feeler means positioned in the bore to penetrate beneath the surface of the oil for measuring the temperature of the oil in the bore, a metal jacket means surrounding the temperature measuring feeler means concentrically positioned within the bore and forming an annular space therebetween, the metal jacket means being electrically insulated from the bore, the metal jacket means and the bore wall forming opposite capacitor surfaces with the oil in the bore as a dielectric therebetween, circuit means connected to the temperature measuring feeler for providing an electrical signal output related to the temperature of the oil in the bore, circuit means connected to the metal jacket means and bore wall for determining the capacitance change occurring from change in the oil level in the bore and providing an electrical signal output related to the oil level, indicating means responsive to the temperature and oil level signal means for indicating when the temperature or oil level varies from a predetermined value.