Patent Publication Number: US-7591437-B2

Title: Method and device for controlling a crusher, and a pointer instrument for indication of load on a crusher

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates to a method for controlling a crusher at which material to be crushed is inserted into a gap between a first crushing means and a second crushing means. 
   The present invention also relates to a pointer instrument for indication of the load on a crusher, which is of the kind mentioned above. 
   The present invention also relates to a control system for control of the load on a crusher, which is of the kind mentioned above. 
   TECHNICAL BACKGROUND 
   A crusher of the above-mentioned type may be utilized in order to crush hard material, such as pieces of rock material. It is desirable to be able to crush a large quantity of material in the crusher without risking that the crusher is exposed to such mechanical loads that the frequency of breakdowns increases. 
   WO 87/05828 discloses a method to decrease the risk of increased mechanical load and breakdowns resulting therefrom. The number of pressure surges above a certain predetermined level that arise in the hydraulic fluid that controls the position of the crushing head are counted. If the count of pressure surges exceeds a predetermined amount, the relative position of the crushing shells is changed so that the width of the crushing gap increases. Preferably, the number of times that the gap is increased during a predetermined time is also counted after which alarm is given if said number of times exceeds a predetermined amount. 
   The method disclosed in WO 87/05828 may to a certain extent reducing the risk of the crusher breaking down prematurely, but does not increase the efficiency of the crusher as regards the amount of crushed material per unit of time. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a method for controlling a crusher, which method increases the efficiency of the crusher in respect of accomplished crushing work, which, for instance, may result in increased size reduction of a certain quantity of material or increased quantity of crushed material, in relation to the prior art technique. This object is attained by a method for controlling a crusher, which is of the kind mentioned above, which method is characterized by the following steps: 
   a) that the instantaneous load on the crusher is measured during at least one period of time to obtain a number of measured values, 
   b) that a representative value, which is representative of the highest measured instantaneous load during each such period of time, is calculated, and 
   c) that the representative value is compared to a desired value and that the load on the crusher is controlled depending on said comparison. 
   An advantage of this method is that the control is based on a value that is representative of the highest instantaneous loads, also called the load peaks, on the crusher, i.e., the loads that involve highest risk of mechanical damage on the crusher. Thanks to this, an operator can be sure that the function of the crusher is not risked, irrespective of how the crusher is supplied with material. The operator can, by ensuring that the supply of material to the crusher becomes even as regards, among other things, quantity of material, moisture content, size distribution and hardness, decrease the highest instantaneous loads. Thereby, the crusher can operate at a high average load without increasing the risk of breakdown. In crushes that have an even supply and a material which does not cause high load peaks, the method according to the invention will mean that the crusher operates at a higher average load, which means a higher efficiency, than what previously has been possible. In crushes that have an uneven supply, the method according to the invention will enable incentive to alter the supply so that it becomes more even with the purpose of providing a more efficient crushing. The control of desired value is normally a stable and safe type of control. Thus, the desired value is suitably selected to be the highest load that the crusher can operate at without increased risk of mechanical breakdown. Thus, the crusher can be utilized optimally without increasing the risk of breakdown in cases of uneven supply or unusually hard material. The desired value can be locked by the one delivering the crusher, wherein the operator, which cannot affect the desired value, may make alterations in the supply of material with the purpose of increasing the efficiency of the crusher without, because of this, risking mechanical damage. In certain cases, it may, however, be appropriate to let the operator increase the desired value and consciously accept a calculated increase of the number of mechanical breakdowns in order to increase the efficiency of the crusher further. Also, other ways of choosing and/or controlling the desired value are possible. 
   According to a preferred embodiment, step a) also comprises that a sequence of data is formed, which data consist of determinations of the highest load on the crusher in each one of said periods of time, which consist of a plurality of consecutive periods of time. The formation of a sequence of data, where each data is the highest load during a period of time included in the sequence, gives a control that in an advantageous way represents the highest loads. The division into periods of time makes, among other things, that occasional very high load peaks get a limited influence on said representative value. According to an even more preferred embodiment, said representative value is calculated in step b) as a mean value of data included in said sequence. A mean value gives a relevant picture of the load peaks for the control. 
   Preferably, said periods of time follow immediately upon each other. An advantage of this is that also fast courses of events are recorded quickly and may be handled by the control, for instance a rapidly and heavily increasing load may quickly be compensated for, the risk of mechanical damage decreasing. 
   Suitably, measured values are used continuously during operation of the crusher for forming a plurality of sequences of data. An advantage of this is that the control may be based on an almost continuous inflow of sequences and representative values calculated therefrom. The control may thereby quickly react on alterations in the operation of the crusher. Even more preferred is that, upon calculation of said representative value of a current sequence, at least one data is utilized concerning highest load that already has been utilized in an immediately preceding sequence. In this way, the sequences will overlap each other. An advantage of this is that said representative value will be calculated several times per unit of time. This means that the control more often receives new input data and makes that the control better can monitor the actual course in the crusher. 
   Preferably, all sequences include the same number of data concerning highest load. Preferably, said data amounts to at least five for each sequence. At least five data for each sequence makes that occasional very high or very low load peaks get a limited influence on said value, a desired damping of the control being provided. 
   According to a preferred embodiment, at least the highest and/or the lowest of the data included in the sequence concerning highest load is excluded upon calculation of said representative value of the same sequence. In this way, it is avoided that occasional very high and/or low values, which, for instance, may depend on erroneous measurements or occasional hard objects, get an undesired large influence on the representative value that then is calculated for the current sequence. 
   According to an even more preferred embodiment, at least the highest as well as at least the two lowest values of the data included in the sequence concerning highest load are excluded upon calculation of said value of the same sequence, more of the lowest than of the highest values being excluded. An advantage of this is that it is avoided that the control system “is fooled” to increase the load by virtue of a sequence randomly happening to contain a plurality of periods of time with relatively low highest loads. If these periods of time with low highest loads suddenly are followed by a very high highest load at the same time as the control system already ordered increase of the load, there is a risk of mechanical damage. Thanks to the fact that more of the lowest values in the sequence are excluded, the highest peaks get a greater impact and the system becomes more sensitive to the high peaks and can easier avoid that the load rises much above the desired value. A consequence of this becomes that the desired value can be raised somewhat, with an increased crushing capacity as a consequence, without increased risk of mechanical breakdowns. 
   According to a preferred embodiment, the width of the gap is adjustable by means of a hydraulic adjusting device, in step a) the load being measured as a hydraulic fluid pressure in said device. The hydraulic fluid pressure frequently gives a very quick and relevant indication of the condition in the crusher. Thus, the risk of possible delays or fault indications causing mechanical breakdowns decreases. 
   According to another preferred embodiment, in step a) the load is measured as the power of the crusher driving device. The power of the driving device frequently gives a quick and relevant feedback of the load on the crusher. Control based on the power of the driving device is particularly suitable when the capacity of the driving device is what limits the feasible load on the crusher and also at cases when the adjusting device is not of a hydraulic type. The power of the driving device may, for instance, be measured directly as an electric power, if the driving device is an electric motor, be calculated from a hydraulic pressure, if the driving device is a hydraulic motor, or, if the driving device is a diesel engine, from a developed engine power. 
   According to an additional preferred embodiment, in step a) the load is measured as a mechanical stress on the crusher. An advantage of this is that it is possible to choose the component that is the most critical one for the mechanical strength of the crusher and measure a stress, such as a tension or a strain, which is representative of the stress on the same component. Thereby, a direct control of the load in relation to the load that the crusher withstands mechanically is obtained. It is, as mentioned above, not necessary to measure on the very critical component. On the contrary, it may frequently be appropriate to measure a mechanical stress in a place, the stress of which correlates well against the stress on the most critical component. Another advantage is that the mechanical stress may be utilized as a measure of load also in cases when the adjusting device is not hydraulic and in cases when the driving device is not limiting for the load that the crusher withstands. 
   In a crusher where it is possible to measure the load both as hydraulic fluid pressure, as power developed by the crusher driving device and as a mechanical stress, or at least as two of said parameters, the method may be formed with control on the load parameter of these which currently is highest in relation to the desired value thereof. Thus, during a period the load on the crusher may be controlled depending on measured highest hydraulic pressures, while during another period it may be controlled depending on measured highest powers. In this way, the crusher can always operate efficiently without risking damage on that component, for instance the hydraulic system, driving device or crusher frame, which currently is exposed to the highest load relatively seen. 
   According to a preferred embodiment, in step c) the load is controlled by the fact that at least some of the following steps is carried out that the width of the gap is changed, that the supply of material to the gap is changed, that the rotational speed of the crusher driving device is adjusted, and that the mutual movements of the crushing means are adjusted. Thus, the control of the load may take place in various ways and the method being selected may be adapted to the current operational situation and the load being controlled on. An alteration of the width of the gap, frequently gives a very quick alteration of the load on the crusher. In cases when, for instance, it is desired to keep the width constant, it may instead be of interest to alter the supply of material to the gap. If the driving device is exposed to a very high load, it may be suitable to alter the number of revolutions. It is also possible to combine a plural of alterations and, for instance, to alter the width of the gap and adjust the mutual movement of the crushing means simultaneously. The latter may for instance be an adjustment of how much the crushing means move to-and-fro towards each other during the crushing. One example is adjustment of the horizontal stroke of the shaft in a gyratory crusher. 
   An additional object of the present invention is to provide a pointer instrument for indication of load on a gyratory crusher, which instruments makes it easier to improve the efficiency of the crusher in respect of accomplished crushing work, which, for instance, may result in an increased size reduction of a certain quantity of material or an increased quantity of crushed material, in relation to prior art technique. 
   This object is attained by a pointer instrument, which is of the kind mentioned above and is characterized in that the pointer instrument has 
   a first pointer, which shows a comparative value, and 
   a second pointer, which shows a representative value, which has been determined after the instantaneous load on the crusher in one step a) has been measured during at least one period of time to obtain a number of measured values, said representative value in a step b) having been calculated as being representative of the highest measured instantaneous load during each such period of time, said comparative value being determined depending on the load on the crusher such that a comparison of the position of the first pointer and the position of the second pointer gives an indication as to whether the operation of the crusher is effective. 
   An advantage of this pointer instrument is that it becomes very clear to an operator that operates the crusher if the operation is efficient or not. If the first pointer shows almost equally high a pressure as the second pointer, which shows the representative value that is representative of the highest loads, it means that the operation of the crusher is efficient. If, on the other hand, the first pointer shows a considerably lower load than the second pointer, the operator gets an indication that, for instance, the supply of material to the crusher does not work satisfactory but needs be attended to. Thus, the operator gets an easily comprehensible indication of disturbances in the process. The pointer instrument also gives a clear and quick feedback on measures carried out in order to get the crusher to operate more efficiently, for instance measures in order to alter the moisture content or size distribution of the supplied material or to provide a more even inflow of material. The second pointer also gives a feedback on that the control system is working and that the load does not exceed permitted levels, which could cause mechanical breakdowns. 
   According to a preferred embodiment, the first and the second pointer form sides of a sector, the extension of which indicates the operation conditions of the crusher. The sector, which suitably has another color than the dial of the pointer instrument, gives a very clear visual indication of the difference between the value shown by the first pointer and the representative value representing the highest loads. For the operator, it becomes a clear goal to keep the sector as small as possible since this means an efficiently operating crusher. 
   According to a preferred embodiment, the first pointer shows a comparative value that represents the average load on the crusher. The average load is a good measure of the crushing work that the crusher performs. If the average load is close to the representative value, which is representative of the highest loads, it is a clear indication of the crushing operation being efficient. 
   According to another preferred embodiment, the first pointer shows a comparative value, which has been determined after the instantaneous load on the crusher in a first step having been measured during at least one period of time to obtain a number of measured values, said comparative value in a second step having been calculated as being representative of the lowest measured instantaneous load during each period of time. The lowest measured instantaneous loads give, together with the highest measured instantaneous loads, which are shown by the second pointer, a good picture of how much the load in the crusher varies, “beating” up and down, and give indication if something should be altered in order to decrease the variation. As has been mentioned above, the highest loads are most serious as regards mechanical damage. However, it is also relevant to consider to the lowest loads, since a large difference between the highest and the lowest loads means substantial load shifts on the crusher, which increase the risk of mechanical-damage. 
   An additional object of the present invention is to provide a control system for control of the load in a crusher, which control system improves the efficiency of the crusher in respect of accomplished crushing work, which, for instance, may result in increased size reduction of a certain quantity of material or increased quantity of crushed material, in relation to the prior art technique. 
   This object is attained by a control system, which is of the kind mentioned above and is characterized in that it comprises 
   a measuring device, which is arranged to measure the instantaneous load on the crusher during at least one period of time to obtain a number of measured values, 
   a calculation device, which is arranged to calculate a representative value, which is representative of the highest measured instantaneous load during each such period of time, and 
   a control device, which is arranged to compare said representative value with a desired value and to control the load on the crusher depending on the same comparison. 
   An advantage of said control system is that it increases the load at which a crusher can operate without increasing the risk of breakdown. 
   Additional advantages and features of the invention are evident from the description below and the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will henceforth be described by means of embodiment examples and with reference to the appended drawings. 
       FIG. 1  schematically shows a gyratory crusher having associated driving, adjusting and control devices. 
       FIG. 2  shows a flow table for control of a crusher. 
       FIG. 3  schematically shows a first embodiment of sequences of measurements of highest hydraulic fluid pressures during consecutive periods of time. 
       FIG. 4  schematically shows a second embodiment of a sequence of measurements of highest hydraulic fluid pressures during consecutive periods of time. 
       FIG. 5  shows a typical geometry of a hydraulic fluid pressure curve in an efficiently operating crusher. 
       FIG. 6  shows a typical geometry of a hydraulic fluid pressure curve in a crusher, which does not operate efficiently. 
       FIG. 7  shows a first embodiment of a pointer instrument, which visually shows how efficiently the operation of the crusher is. 
       FIG. 8   a  shows a second embodiment of a pointer instrument, which shows the operation in an efficiently operating crusher. 
       FIG. 8   b  shows a pointer instrument which is of the same type as the one shown in  FIG. 8   a , but which shows the operation in an inefficiently operating crusher. 
       FIG. 9  shows a gyratory crusher having mechanical adjusting of the width of the gap. 
       FIG. 10  shows a jaw crusher and associated driving, adjusting and controlling devices. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1 , a gyratory crusher is shown schematically, which has a shaft  1 . At the lower end  2  thereof, the shaft  1  is eccentrically mounted. At the upper end thereof, the shaft  1  carries a crushing head  3 . A first crushing means in the form of a first, inner crushing shell  4  is mounted on the outside of the crushing head  3 . In a machine frame  16 , a second crushing means in the form of a second, outer, crushing shell  5  has been mounted in such a way that it surrounds the inner crushing shell  4 . Between the inner crushing shell  4  and the outer crushing shell  5 , a crushing gap  6  is formed, which in axial section, as is shown in  FIG. 1 , has a decreasing width in the direction downwards. The shaft  1 , and thereby the crushing head  3  and the inner crushing shell  4 , is vertically movable by means of a hydraulic adjusting device, which comprises a tank  7  for hydraulic fluid, a hydraulic pump  8 , a gas-filled container  9  and a hydraulic piston  15 . Furthermore, a motor  10  is connected to the crusher, which motor during operation is arranged to bring the shaft  1 , and thereby the crushing head  37  to execute a gyratory movement, i.e., a movement during which the two crushing shells  4 ,  5  approach each other along a rotary generatrix and distance from each other at a diametrically opposite generatrix. 
   In operation, the crusher is controlled by a control device  11 , which via an input  12 ′ receives input signals from a transducer  12  arranged at the motor  10 , which transducer measures the load on the motor, via an input  13 ′ receives input signals from a pressure transducer  13 , which measure the pressure in the hydraulic fluid in the adjusting device  7 ,  8 ,  9 ,  15  and via an input  14 ′ receives signals from a level transducer  14 , which measures the position of the shaft  1  in the vertical direction in relation to the machine frame  16 . The control device  11  comprises, among other things, a data processor and controls, on the basis of received input signals, among other things, the hydraulic fluid pressure in the adjusting device. 
   When the crusher is to be started, a calibration is first carried out without feeding of material. The motor  10  is started and brings the crushing head  3  to execute a gyratory pendulum movement. Then, the pump  8  increases the hydraulic fluid pressure so that the shaft  1 , and thereby the inner shell  4 , is raised until the inner crushing shell  4  comes to abutment against the outer crushing shell  5 . When the inner shell  4  contacts the outer shell  5 , a pressure increase arises in the hydraulic fluid, which is recorded by the pressure transducer  13 . The inner shell  4  is lowered somewhat in order to avoid that it “sticks” against the outer shell  5 , and then the motor  10  is stopped and a so-called A measure, which is the vertical distance from a fixed point on the shaft  1  to a fixed point on the machine frame  16 , is measured manually and fed into the control device  11  to represent the corresponding signal from the level transducer  14 . Next, the motor  10  is restarted and the pump  8  then pumps hydraulic fluid to the tank  7  until the shaft  1  reaches the lowermost position thereof. The corresponding signal from the level transducer  14  for said lower position is then read by the control device  11 . Knowing the gap angle between the inner crushing shell  4  and the outer crushing shell  5 , the width of the gap  6  may be calculated at any position of the shaft  1  as measured by the level transducer  14 . Usually, the width of the gap  6  is calculated in the position where the gap  6  is as most narrow, i.e. in the position where the inner shell  4  gets in contact with the outer shell  5  during the above-mentioned calibration. However, it is also possible to calculate the width at another position in the gap  6  in stead. 
   When the calibration is finished, a suitable width of the gap  6  is set and supply of material to the crushing gap  6  of the crusher is commenced. The supplied material is crushed in the gap  6  and may then be collected vertically below the same. 
   According to the present invention, a representative value is calculated, which is representative of the highest measured instantaneous loads on the crusher. As used in the present application, “load” relates to the stress that the crusher is exposed to on a certain occasion. The load may, according to the present invention, for instance, be expressed in the form of a mean peak pressure, which is calculated from hydraulic fluid pressures as measured by the pressure transducer  13 . The load may also be expressed as a mean peak motor power that is calculated from motor powers as measured by the transducer  12 , or as a mean peak tension that is calculated from mechanical tensions in the crusher as measured by a tension sensor, for instance a strain gauge. 
     FIG. 2  schematically shows a method for controlling the operation of the crusher depending on the hydraulic fluid pressure. The crushing process results in a varying pressure arising in the hydraulic fluid. At a certain quantity of supplied material of a certain hardness and size, a narrow gap  6  will mean a high hydraulic fluid pressure and a wide gap  6  will mean a low hydraulic fluid pressure. A high mean hydraulic fluid pressure means that the crusher is utilized efficiently in order to crush the supplied material. Thus, it is desirable that for a certain quantity of supplied material keep as high mean pressure as possible without the crusher risking to be damaged mechanically. In the step  20  shown in  FIG. 2 , measurement is commenced of the instantaneous hydraulic fluid pressure in the adjusting device  7 ,  8 ,  9 ,  15  by means of the pressure transducer  13 . The measurement of the instantaneous hydraulic fluid pressure started in step  20  continues as long as the crusher is in operation. The signal from the pressure transducer  13  is received by the control device  11 . In step  22 , the supply of material to the crusher is commenced. In step  24 , the highest hydraulic fluid pressure that has been recorded during a period of time of 0.2 s is stored in the control device  11 . The highest hydraulic fluid pressure measured in step  24  forms, together with the corresponding values for the four closest preceding periods of time, a sequence of repeated measurements of highest hydraulic fluid pressures. In stop  26 , a representative value is calculated in the form of a mean peak pressure as a mean value of the highest hydraulic fluid pressures included in said sequence, which thus have been measured during each one of the five periods of time which are contained within the latest 1.0 s. Said mean peak pressure is thereby a value that is representative of the highest measured instantaneous hydraulic fluid pressures. The calculated mean peak pressure is compared with a desired value in step  28 , the difference between the mean peak pressure and the desired value being calculated. The difference between the desired value and the calculated mean peak pressure obtained in step  28  is utilized in step  30  in order to determine if the pump  8  should reduce or increase the hydraulic fluid pressure in the adjusting device, the period of time the pump should be in operation and if any time should pass before a pressure alteration should be started. In step  32 , the control device  11  emits a control signal to the pump  8 , if the conditions for such a control signal are met, and a new sequence of measurements is initiated by step  24  again being commenced. When the hydraulic fluid pressure is increased or reduced, the shaft  1 , and thereby the inner shell  4 , will be raised or lowered, the gap  6  becoming more slender or wider, respectively. Thus, the hydraulic pressure alteration will affect the width of the gap  6  and thereby the load on the crusher. 
   The occasions when the pump  8  should be taken into operation, “pump”, and how long it should pump hydraulic fluid to or from the piston  15 , is thus controlled by the control device  11 . The pumping tales place during a certain space of time, the length of which is proportional in steps to the difference between the current mean peak pressure and the desired value, i.e., if the current mean peak pressure is within a certain interval at a certain distance from the desired value, pumping is effected during a certain time, while if the current mean peak pressure is in an interval which is closer to the desired value, the pumping is effected during a shorter space of time. 
     FIG. 3  schematically shows a curve P of measured hydraulic fluid pressure during a period of 2 s. Within each period of time of 0.2 s, the highest hydraulic fluid pressure is recorded during that period of time. In  FIG. 3 , the periods of time have been numbered from 1 to 10 and the highest hydraulic fluid pressure in each period of time, which hydraulic fluid pressure is stored in the control device  11  in step  24 , has for period of time 1 to 5 been marked with an arrow. The mean peak pressure mentioned in step  26  is calculated as a mean value of the highest hydraulic fluid pressures from the respective period of time 1 to 5, which are included in a first sequence S 1  of repeated measurements of highest hydraulic fluid pressures. In the iteration following next, i.e., when step  24  again has been commenced, the highest hydraulic fluid pressure in period of time no. 6 will be stored in the control device  11 , a new mean peak pressure being calculated from the highest hydraulic fluid pressures from the respective period of time 2 to 6, which are included in a second sequence S 2  and so on. Thus, a new mean peak pressure will be calculated five times per second and said mean peak pressure will be based on the respective highest hydraulic fluid pressures which have been measured during the five latest periods of time. 
     FIG. 4  schematically shows an even more preferred embodiment, wherein a sifting of the respective highest hydraulic fluid pressures is made before a mean peak pressure is calculated. In this even more preferred method, step  26  has been configured according to the following. The respective highest hydraulic fluid pressures from the latest 10 periods of time are compared, the two highest values and the five lowest values being sifted away. A mean peak pressure is then calculated as a mean value of the remaining 3 periods of time and is utilized in step  28 .  FIG. 4  shows a schematic illustration of how the sifting has taken place in a sequence S 3  of repeated measurements of highest hydraulic fluid pressures. The periods of time which have the two highest and the five lowest values, respectively, of highest hydraulic fluid pressure have been sifted away, which is symbolized by they having been crossed over in  FIG. 4 . Thanks to the fact that more of the lowest than of the highest values of highest hydraulic fluid pressure are excluded, the mean peak pressure, which later is correlated to the desired value, will be more sensitive to the highest pressures. Thus, the control system will react faster on pressure increases than on pressure reductions, which decreases the risk of mechanical breakdowns caused by too high pressures. Thus, the mean peak pressure is calculated as a mean value of the highest hydraulic fluid pressures during those periods of time of the periods of time 1 to 10 that have not been sifted away. Table 1 below indicates how the analysis, which takes place in the control device  11 , may look like: 
   
     
       
         
             
           
             
               TABLE 1 
             
           
          
             
                 
             
             
               Example of calculation of mean peak pressure 
             
          
         
         
             
             
          
             
               Measure 
               Values after measure 
             
             
                 
             
             
               Measure instantaneous hydraulic fluid 
               2.5 2.8 4.3 4.1 4.5 4.4 
             
             
               pressures 
               etc. 
             
             
               Form sequence of highest pressure in each 
               4.5 3.4 6.5 5.4 5.6 3.3 
             
             
               one of ten periods of time 
               5.7 6.2 4.9 5.8 
             
             
               Take away the five lowest pressures in the 
               6.5 5.6 5.7 6.2 5.8 
             
             
               sequence 
             
             
               Take away the two highest pressures in the 
               5.6 5.7 5.8 
             
             
               sequence 
             
             
               Calculate mean value of the three remaining 
               5.70 
             
             
               pressures in the sequence 
             
             
                 
             
          
         
       
     
   
   The control device  11  suitably also measures the mean hydraulic fluid pressure. The mean hydraulic fluid pressure is a measure of the load of the crusher and should be as high as possible. In  FIG. 3  and  FIG. 4 , the mean hydraulic fluid pressure has been marked with a dashed curve A. Thus, the mean hydraulic fluid pressure is an average of all measured instantaneous hydraulic fluid pressures during the preceding 2.0 s. In efficient operation of the crusher, the mean hydraulic fluid pressure, i.e., curve A, should be close to the calculated mean peak pressure, i.e., the mean value of the highest hydraulic fluid pressures measured during respective period of time. Accordingly it is desirable to keep the hydraulic fluid pressure on an even and high level. In such an operation, the crusher will be utilized maximally for crushing without the risk of increasing mechanical breakdowns. 
     FIG. 5  shows a typical geometry of a hydraulic pressure curve P in an efficiently operating crusher. In this case, the desired value of mean peak pressure was predetermined to 5.0 MPa. The mean peak pressure M varies between approx. 4.5 and 5.5 MPa. As is seen in  FIG. 5 , the mean hydraulic fluid pressure A is approx. 4 MPa, i.e., only somewhat below the calculated mean peak pressure M. This is provided by the fact that the supply of material to the crusher is handled in such a way that the flow of material is even and contains material having approximately the same size distribution, moisture content and hardness. 
     FIG. 6  shows a typical geometry of a hydraulic fluid pressure curve P for a crusher of the same type as above but at substantially varying load, which, for instance, may depend on the amount of material and/or the size distribution of the material varying relatively much. The desired value of mean peak pressure was also in this case 5.0 MPa. The mean peak pressure M varies between approx. 4.5 and 5.5 MPa. Thus, the control device  11  can, also on substantially varying load, keep the mean peak pressure M within narrow margins, wherein mechanical breakdowns may be avoided also, for instance, upon uneven supply and operational disturbances. As is seen in  FIG. 6 , the mean hydraulic fluid pressure A is on approx. 3.2 MPa, which is considerably below the mean peak pressure M and, therefore, the crusher operates with relatively low efficiency. 
     FIG. 7  shows a pointer instrument  40 , which visually shows how efficiently the operation of the crusher is. The pointer instrument  40  has a dial  42  and two pointers in the form of needles  44 ,  46 . A first needle  44  shows a comparative value in the form of the mean hydraulic fluid pressure in the crusher. A second needle  46  shows a representative value in the form of the mean peak pressure, i.e., the mean value of the highest hydraulic fluid pressures that have been measured during a number of periods of time, which accordingly is a value which is representative of the highest measured instantaneous hydraulic fluid pressures and which has been calculated according to the above. The distance between the first needle  44  and the second needle  46  is an indication of how efficiently the crusher operates. The desired value of the mean peak pressure has been marked with a line  48  on the dial  42  of the pointer instrument. In the position that is shown in  FIG. 7 , the mean peak pressure, which is shown by the second needle  46 , is incidentally lower than the desired value. Thus, the control device  11  will instruct the pump  8  to pump in more hydraulic fluid so that the crushing head  3  is raised and the hydraulic fluid pressure increases again. 
   In  FIG. 7 , a third pointer is also shown in the form of a dashed third needle  50 , which is included in an alternative design of the pointer instrument  40 . The needle  50  is utilized in order to show a difference calculated by the control device  11  between the mean hydraulic fluid pressure and the mean peak pressure with the purpose of more clearly illustrating how efficiently the crusher operates. 
   In  FIG. 7 , also a fourth pointer is shown in the form of a dashed and dotted fourth needle  52 , which is included in an additional alternative design of the pointer instrument  40 . The needle  52  is utilized in order to show a comparative value calculated by the control device  11  in the form of a mean bottom pressure. The mean bottom pressure is calculated according to the same principle as has been described above for the mean peak pressure, but is instead based on the lowest measured hydraulic fluid pressures. Thus, the mean bottom pressure is calculated as a mean value of the lowest hydraulic fluid pressures that have been measured during a number of consecutive periods of time, and thereby represents the lowest loads on the crusher. The distance between the fourth needle  52 , which shows the mean bottom pressure, and the second needle  46 , which shows the mean peak pressure, thus illustrates how large the variation in load on the crusher is. The fourth needle  52  may be used together with the first needle  44 , which shows the mean pressure, or replace the same, wherein the needle  52  will work as a first pointer that then, together with the second needle  46 , which shows the mean peak pressure, illustrates the operation condition in the crusher. It is also possible to calculate the difference between the mean peak pressure and the mean bottom pressure and let a fifth pointer, not shown in  FIG. 7 , show this difference. 
     FIG. 8   a  shows another embodiment in the form of a pointer instrument  140 . The same pointer instrument  140  is formed as a virtual window, which is shown on a display device, for instance a display device included in the control device  11 . The pointer instrument has a dial  142 , a first pointer  144 , which shows the mean hydraulic fluid pressure, and a second pointer  146 , which shows the mean peak pressure, i.e., the mean value of the highest hydraulic fluid pressures which have been measured during a number of periods of time. The first and the second pointer  144  and  146 , respectively, form between themselves a sector  150  that has another color, for instance black, than the dial  142  and therefore is clearly seen. Thus, the extension of the sector  150  on the dial  142  becomes a visually easy-to-read measure of how efficiently the crusher operates. The position of the first pointer  144  is updated each time a new mean hydraulic fluid pressure has been calculated and the position of the second pointer  146  is updated each time a new mean peak pressure has been calculated. The pointer instrument  140  shown in  FIG. 8   a  illustrates the condition in the crusher, the hydraulic fluid pressure curve P of which is shown in  FIG. 5 , i.e., an efficiently operating crusher. 
   The pointer instrument  140  has also a virtual display  152  that, for instance, may display the current mean peak pressure, mean hydraulic fluid pressure or the difference between these pressures. 
   In  FIG. 8   b , a pointer instrument  140  is shown of the same type as the one shown in  FIG. 8   a . However, the pointer instrument  140  shown in  FIG. 8   b  illustrates the condition in the crusher, the hydraulic fluid pressure curve P of which is shown in  FIG. 6 , i.e., a crusher which does riot operate efficiently by virtue of substantially varying load. As is seen in  FIG. 8   b , the sector  150  has a large extension on the dial  142  since the mean hydraulic fluid pressure, which is shown by the pointer  144 , is considerably lower than the mean peak pressure, which is shown by the pointer  146 , which clearly indicates to the operator that measures needs to be taken in order to increase the efficiency of the crusher. 
     FIG. 9  schematically shows a gyratory crusher that is of another type than the crusher shown in  FIG. 1 . The crusher shown in  FIG. 9  has a shaft  201 , which carries a crushing head  203  having a first crushing means in the form of an inner crushing shell  204  mounted thereon. Between the inner shell  204  and a second crushing means in the form of an outer crushing shell  205 , a crushing gap  206  is formed. The outer crushing shell  205  is attached to a case  207  that has a trapezoid thread  208 . The thread  208  mates with a corresponding thread  209  in a crusher frame  216 . Furthermore, a motor  210  is connected to the crusher, which is arranged to bring the shaft  201 , and thereby the crushing head  203 , to execute a gyratory movement during operation. When the case  207  is turned by an adjustment motor  215  around the symmetry axis thereof, the outer crushing shell  205  will be moved vertically, the width of the gap  206  being changed. In this type of gyratory crusher, accordingly the case  207 , the threads  208 ,  209  as well as the adjustment motor  215  constitute a adjusting device for adjusting of the width of the gap  206 . Upon control of the load on a crusher of this type by means of a control device  211 , it is according to the invention possible to utilize a transducer  212 , which measures the instantaneous power generated by the motor  210 . From the highest measured powers during a number of periods of time, subsequently a mean peak power may be calculated and compared with a desired value. Depending on said comparison, the load on the crusher is controlled. The same control may, for instance, consist of the adjustment motor  215  being instructed to turn the case  207  in order to alter the width of the gap  206 . It is also possible to alter the supply of material, the number of revolutions of the motor  210  and/or the stroke of the shaft  201  in the horizontal direction. 
   An alternative method to measure the load, which method works both in crushes having hydraulic adjusting devices and crushes of the type which is shown in  FIG. 9 , is to measure a mechanical stress or tension in the proper crusher. As is seen in  FIG. 9 , a strain gauge  213  has been placed on the crusher frame  216 . The strain gauge  213 , which measures the instantaneous strain in the part of the frame  216  to which it is attached, is suitably placed on a location on the frame  216  which gives a representative picture of the mechanical load on the crusher. From the highest measured strains, possibly converted to corresponding tensions, during a number of periods of time, a mean peak strain or tension may then be calculated and utilized in order to control the load on the crusher. 
     FIG. 10  schematically shows a jaw crusher. The jaw crusher has a frame  316  and a movable jaw  303  movably mounted therein. The movable jaw  303  carries a first crushing means in the form of a first crushing plate  304 . The frame  316  carries a second crushing means in the form of a second crushing plate  305 . A crushing gap  306  is formed between the first crushing plate  304  and the second crushing plate  305 . The jaw  303  is rotatably and eccentrically secured at its upper end to a flywheel  301 . The flywheel  301  is driven via a belt  302  by a driving device in the form of a motor  310  and thereby gets the upper portion of the jaw  303  to describe a substantially elliptical movement which causes material fed into the gap  306  to be crushed by the crushing plates  304 ,  305 . The lower end of the jaw  303  is supported by a toggle plate  307 . The toggle plate  307  has a hydraulic cylinder  308 , which makes it possible to adjust the width of the gap  306 . At this type of crusher the toggle plate  307  and the hydraulic cylinder  308  an adjusting device for adjustment of the width of the gap  306 . At control of the load on a crusher of this type by means of the control device  311  it is according to the present invention possible to use a gauge  312  that measures the instantaneous power that develops at the motor  310  and sends a signal to the control device  311 . A mean peak power can then be calculated from the highest measured powers during a number of periods of time in accordance with what has been described above and be compared to a desired value. The load on the crusher is controlled depending of this comparison. This control may for example consist in the control device  311  orders the hydraulic cylinder  308  to change the width of the gap  306 . It is also possible to order change of feed of material to the crusher or of the rotational speed of the motor  310 . 
   It is also possible to measure e mechanical load or tension in the crusher itself. As is apparent from  FIG. 10  a strain gauge  313  has been positioned on the crusher frame  316 . The strain gauge  313  that measures the instantaneous strain in the portion of the frame  316  on which it is secured, can be used in a similar way as described above regarding the gauge  213 . Another possibility is to position a strain gauge  314  on the toggle plate  307  for measuring the instantaneous load on the toggle plate  307  and to send a signal to the control device  311  that uses that signal for controlling the crusher. It is also possible to measure the hydraulic fluid pressure in the hydraulic cylinder  308  of the toggle plate  307  and to use said pressure as a measure on the load on the crusher. It is understood that the toggle plate  307  is schematically shown and that other devices and other types of toggle plates may be used for adjusting the width of the gap  306 . 
   It will be appreciated that a number of modifications of the above-described embodiments are feasible within the scope of the invention, such as it is defined by the appended claims. 
   The representative value that is representative of the highest measured instantaneous loads may, for instance, be calculated as a mean peak pressure according to what has been described above. There are, however, a plurality of other methods to calculate said representative value. For instance, a standard deviation from the mean load may be calculated and utilized as said value. A small standard deviation is then an indication of the crusher operating efficiently. An additional alternative is to take both the height and duration of the respective load peak into consideration. For instance, the extension of the peaks in time and height may be calculated by integration, said value being calculated as a mean value of a number of integrated peaks. 
   Two consecutive sequences of data may either partly overlap each other, such as has been described above, or follow immediately upon each other instead of partly utilizing the same data. 
   It will be appreciated that a person skilled in the art by experiments can derive lengths of the periods of time suitable for certain specific operation conditions, how many periods of time that should be included in a sequence, how many data in a sequence that should be retrieved from a preceding sequence and if any data should be sifted away before calculation of mean values and that the above-described statements constitute a preferred example. For instance, a suitable length of each period of time has turned out to be 0.05 to 1 s. 
   Above is described how the control device  11  controls the hydraulic fluid pressure depending on a comparison of said representative value, which, for instance, may be a mean peak pressure, with a desired value of the pressure. However, the control device  11  may also be arranged to take the load of the motor into consideration. If the signal from the transducer  12 , which measures the load of the motor  10 , indicates that the load on the motor  10  exceeds an allowed load value, the control device  11  will instruct the pump  8  to decrease the hydraulic fluid pressure, also if the mean peak pressure does not exceed the desired value of pressure, in order to avoid overload of the motor  10 . 
   Above a method for controlling the crusher is described where it is desirable to keep highest feasible load and size reduce the material as much as possible. The control device  11  aims, in that connection, at keeping a high hydraulic fluid pressure and makes this by continuously keeping the gap  6  as narrow as possible, the supplied material being exposed to a maximum size reduction. In certain cases, it is instead of interest to keep a fixed width of the gap  6  in order to provide a certain size of the crushed product. In such a case, the control device  11  can instead be utilized as a safety function that incidentally increases the gap somewhat in order to reduce the hydraulic fluid pressure when the calculated mean peak pressure during any shorter period exceeds the desired value of pressure. Therefore, in this way, a larger quantity of supplied material can be crushed to a certain desired size without risk of mechanical breakdown. It also becomes considerably simpler to maximize the quantity of material that can be crushed to the desired size. An additional possibility is to let the crusher alternatingly operate in control towards maximum load and in control to a fixed gap. It is also possible to keep the width of the gap  6  constant and instead control the load on the crusher by means of some other parameter, for instance the amount of supplied material. 
   It is understood the width of the crushing gap  6 ,  206 ,  306  can be adjusted in different ways and that the above-described methods, reference being had to  FIGS. 1 ,  9  and  10 , are non-limiting examples. 
   The above described pointer instruments  40 ;  140  are provided with needles  44 ,  46  and pointers  144 ,  146 , respectively, which may be mechanical or be shown on a display device. It is however also possible instead to utilize digital display of the actual numbers concerning the mean hydraulic fluid pressure and mean peak pressure, which have been calculated. Thus, in this case, the pointer of the pointer instrument will consist of displays that suitably digitally, show calculated numbers. It is, as is mentioned above, also possible to calculate the difference between the mean hydraulic fluid pressure and the mean peak pressure and let a third pointer, which may be a needle  50  or a display showing the number in question, show said difference. The difference between mean hydraulic fluid pressure and mean peak pressure may thereby be used for following-up of the operation of the crusher, a small difference meaning, as mentioned above, that the crusher operates efficiently. It is also possible to combine display with needles and display of numbers in question and to in that connection utilize needles in order to show mean hydraulic fluid pressure and mean peak pressure and a display in order to show the calculated difference between the same. 
   It is also possible to form a pointer instrument having a sector that is formed between a second pointer, which shows the mean peak pressure, and a fourth pointer, which shows the mean bottom pressure. A first pointer, which shows the mean pressure, may be imparted another color than the sector and is placed on top of the same in order to also show the mean pressure in the adjusting device.