The present invention relates to a method for controlling a gyratory crusher comprising a crushing head provided with a first crushing mantle, and a second crushing mantle which, in combination with said first crushing mantle, defines a crushing gap, the width of which is adjustable by varying the relative axial posi¬ tions of said first and second crushing mantles by means of an adjusting device, the material to be crushed in a crushing operation being introduced into said gap, and a driving device causing said crushing head to perform a gyratory pendulum movement.

In the operation of gyratory crushers of the above- mentioned type, the crushing head is adjusted such that the gap between the first, inner crushing mantle and the second, outer crushing mantle will obtain a given predetermined width which depends on the desired crushing degree. Adjustment is carried out with a certain margin of safety to the maximally permissible load to which the crusher may be subjected. During operation, the load on the crusher will vary, and if the gap width is adjusted to a value which is too low, the crusher may be subjected to overloading and damage. As the crushing operation proceeds, the mantle surfaces are subjected to wear which increases the gap width and detracts from the production efficiency. In view hereof, the position of the crushing head is corrected successive¬ ly, either manually or automatically, such that the gap width will again be the one initially set.

If the crusher is charged incorrectly, or if the charged material does not have the correct composition, for example when it contains too much moisture or too many stones harder than the remaining material, a tendency toward caking may occur, resulting in substantial but short-lived load peaks, and short-lived pressure increases,

2 so-called pressure surges, are encountered in the hy¬ draulic fluid of the adjusting device, if the device is operated by hydraulic means. To cope with such occasional load peaks, most crushers are equipped with a relief system. Such a system which may comprise, for example, an accumulator into which the hydraulic fluid is temporarily injected, or mechanical springs, causes a brief increase of the gap width so that the material which has caused the load peak can pass. However, these relief systems suffer from the dis¬ advantage that they do not change the actual setting of the crushing gap, and as soon as the temporary relief peak has been passed, the crushing head again assumes its adjusted position. If charging is performed incorrectly, or if the composition is not correct, the crusher may thus be subjected for long periods of time to many short¬ lived load peaks which may damage the crusher. Also if the load peaks are so short that the relief system does not have time to respond, pressure surges are obtained which, if they occur too frequently, may damage the crusher.

It therefore is the object of this invention to provide a method for controlling a gyratory crusher in such a manner that the above-mentioned shortcomings are eliminated.

This object is achieved by means of a method which is characterised- in that the number of pressure surges in the hydraulic fluid in the adjusting device exceeding a predetermined pressure level are counted during a predetermined period of time, and that the relative positions of the crushing mantles are changed such that the width of the crushing gap is increased if the counted number of pressure surges exceeds a predetermined sum. Thus, if the frequency of the pressure surges is too high, the width of the crushing gap is increased such that any cakes can come loose, whereby damage to the crusher is prevented.

3 An embodiment of the invention will be described in more detail below, reference being had to the accom¬ panying drawing which illustrates schematically the gyratory crusher and its driving, adjusting and control- ling devices.

In the embodiment, it is assumed that it is the position of the crushing head, i.e. the first crushing mantle, which is changed when the relative positions of the crushing mantles are changed, and that the width of the crushing gap decreases when the crushing head is raised in the axial direction.

The gyratory crusher illustrated in the drawing comprises a shaft 1 which is eccentrically mounted at its lower end 2 and carries a crushing head 3, on the outer side of which a first, inner crushing mantle 4 is mounted. A second, outer annular crushing mantle 5 surrounds the inner crushing mantle 4 and defines, together therewith, a gap 6 which, in the axial section shown in the drawing, has a downwardly decreasing width. The shaft 1 is vertically movable by means of a hydraulic adjusting device comprising a tank 7 for hydraulic fluid, a pump 8, and a gas-filled container 9. Also connected to the crusher is a motor 10 which is adapted, during operation, to cause the shaft 1 and the crushing head 3 to perform a gyratory pendulum movement, i.e. a movement during which the two crushing mantles 4, 5 approach one another along a rotating generatrix and are moved apart at a diametrically located generatrix.

During operation, the crusher is controlled by a control device 11 which at an input 12' receives input signals from a transducer 12 mounted on the motor and measuring the load on the motor. At an input 13', the control device receives signals from a pressure trans¬ ducer 13 measuring the pressure in the hydraulic fluid of the adjusting device, and at an input 14', the device receives signals from a level transducer mounted on the bottom of the crusher and measuring the vertical

4 position of the shaft 1.

The control device 11 strives to maintain the crushing head 3 in a position in which the load on the crusher is 100%, the position of the crushing head 3 being controlled by pumping hydraulic fluid into or out of the tank 7 by means of the pump 8. The load on the crusher is determined by the motor output and the pressure in the hydraulic fluid, and the 100% load is defined as the maximum output level determined for the motor and the maximum pressure level determined for the hydraulic fluid, which is obtained first when the crushing head 3 is being raised. As the mantle surfaces are being worn, a control device will thus change the upward position of the crushing head so that the wear of the mantle surfaces is compensated for.

However, if the charged material contains much moisture, cakes may begin to form in the gap 6. The load on the crusher then increases to a level exceeding the 100% level, and one or more pressure surges occur in the hydraulic fluid. If the load peak is sufficiently long and powerful, the hydraulic fluid will be pressed into the gas-filled container 9. When the cake has come loose, the shaft 1 and the crushing head 3 resume their earlier positions. However, the pressure surges are recorded by the pressure gauge 13 which supplies a signal to the controlling unit which, for a given predetermined period of time, counts the number of pressure surges exceeding a predetermined pressure value stored in the control unit. If the number of pressure surges counted is greater than a sum predetermined for this period, the pump 8 is activated to pump hydraulic fluid into the tank 2 so that the crushing head 3 is lowered and any cakes can come loose. However, the control unit also records the number of times the shaft 1 and the crushing head 3 have been lowered. If the number of these lowerings during a given period of time exceeds a predetermined sum, this indicates that the crusher

5 does not operate as it should, and then an alarm is provided, and charging is stopped. Controlling the crusher in this manner protects it against damage and eliminates unnecessary breakdown. It will be appreciated that the embodiment described above is but one of many conceivable embodiments, and that modifications within the scope of the appended claims are possible.