In a jaw crusher, the reciprocating movement of crusher jaws relative to each other achieves reduction of the infeed raw material to be crushed being fed into the nip between the jaws. The reciprocating movement is accomplished by means of an eccentric shaft actuating an oscillating arm known as the pitman. According to their operating principle, jaw crushers generally fall into two categories: so-called single toggle jaw crushers and so-called double toggle jaw crushers. In a single toggle jaw crusher, the function of the toggle plate is to maintain a constant distance between a pivotal point at the pitman end and a pivotal point at the rear end of the crusher. In a double toggle jaw crusher, the number of distances to be kept constant is two: the distance between the rear-end pivotal point and the connecting rod end and the distance between the connecting rod end and the pivotal point at the pitman end.

When a single toggle jaw crusher is idling, the force imposed by the spring of the return rod and, from the force of gravitation imposed on the pitman, the force com- ponent directed parallel to the toggle plate force of gravitation are the only forces acting on the toggle plate. Thus, the compressive force inflicted on the toggle plate varies vigorously in a running crusher. Having the crusher jaw cavity being filled with rocks, the entire crushing force of the pitman lower part is backed by the toggle plate.

The compressive force inflicted on the toggle plate is strongly dependent on the phy- sical properties of the raw material to be fed into the crusher. For different minerals, these properties are almost invariably predictable. When rocks are to be crushed, for instance, it is relatively easy to estimate in beforehand the force required for reducing

the rock material and, hence, the compressive force imposed on the toggle plate. This information is utilized at the design stage of a crusher for proper strength dimension- ing of the crusher components.

A jaw crusher that conventionally is used mainly for crushing mineral raw material is today also employed for reducing other materials of varying kind. A typical modern application for a jaw crusher is recycle material crushing wherein a jaw crusher is employed for comminution of different demolition waste for reuse of such waste One typical demolition waste material of this kind is concrete used in building con- structions. As to crushing, concrete is a problematic material inasmuch as structures made thereof generally are embedded with different kinds of reinforcing steel mem- bers whose size and shape cannot be determined by visual inspection of a concrete chunk at the demolition site. To perform comminution of these steel members or reduction thereof to an aggregate size smaller than the crusher setting may need a crusher jaw force substantially higher than the normal force needed for crushing the concrete itself. Hence, it is much more difficult to assess the reactive forces imposed on the crusher during reduction of recycle material than the forces that occur during crushing of rock material.

Having a simple and inexpensive construction, the toggle plate is generally dimen- sioned to form the intentionally weakest element of the crusher. The goal of such a design is that when a difficult-to-crush object enters the crusher jaw cavity, the toggle plate yields thus widening the crusher setting and allowing the object to fall uncrushed through the jaw gap without causing damage to the more expensive elements of the crusher, such as crusher frame, eccentric shaft and bearings.

A problem hampering this kind of crusher is that the yielding of the toggle plate is not generally identified instantly after the uncrushable object has traveled through the crusher. Commonly, an uncrushable object passing through the crusher jaw gap initially causes in the toggle plate a buckling failure resulting in a small deformation.

In this case the crusher still can perform crushing up to several hours until the toggle plate material ultimately undergoes fatigue failure, whereby the toggle plate breaks

into a useless condition. After such a long time it may be impossible to trace the ultimate cause of the toggle plate failure: the uncrushable object initiating the damage is either buried deep in the heap of crushed aggregate or it may even have been hauled away from the crushing site. Herein, in spite of the fully planned failed failure of the toggle plate, the actual instant of failure may take the crusher operator by full surprise. Should the operator be warned on an impending risk of toggle plate breakage immediately at the first buckling of the toggle plate, the crusher operator could straightway commence actions to clarify the cause of toggle plate buckling and thus prevent reoccurrence of such an incident. Moreover, the crusher operator could in good time order a spare toggle plate and make preparations to replace the toggle plate at an instant most convenient in regard to the workload.

From patent publication DE 601 522 is known an arrangement having the toggle plate of a jaw crusher equipped with a breakable switch that is adapted connectable to the control circuit of the crusher drive motor. The breakable switch serves to stop the crusher immediately at the instant the switch elongation or flexure exceeds the breaking strength of the switch. Hence, no information is obtained on deformations occurring prior to the failure and, moreover, the switch triggers immediate shutdown of crusher operation. Furthermore, the embodiment according to this reference publi- cation is handicapped by offering only a one-time function of the system, whereby certain embodiments disclosed in the publication involve a potential environmental risk caused by the release of quicksilver to the ground from the breakable switch where it serves as a conductive element.

Patent publications DE 1 276 422 and FR 2 683 462 disclose systems having the monitoring means adapted to operate in conjunction with a pivotal support point of the toggle plate thus facilitating measurement of compressive stress imposed on the toggle plate but not the deformations thereof. The thus obtainable information is not sufficiently accurate for preventing a possibly impending fracture of the toggle plate.

From patent publication FI 105 257 is known an embodiment having conductors mounted in close vicinity to the toggle plate with their insulation removed at the area

of the toggle plate. The conductors and the toggle plate are configured into a loop connected to the crusher control circuit so that flexure of the toggle plate makes a contact to the exposed conductor, whereby the short circuit in the loop acts as a control signal to either an alarm system or an automatic control system of the crusher. The embodiment disclosed in this reference publication, however, gives unreliable function under the vibratory conditions of a crusher plant that readily induce oscillation of conductors mounted in close vicinity of the toggle plate, whereby the failure monitoring system issues false alarms thus hampering the crusher station operation.

Now a novel method has been invented for monitoring the loading status of a jaw crusher and solving the problems associated with the above-described prior-art techniques.

In the method according to the invention, the loading status of a jaw crusher is monitored based on the detection of deformation in the toggle plate. The toggle plate stresses and position are detected with the help of sensor pairs adapted to measure the mutual distance between the sensors. This arrangement gives accurate informa- tion in real time on toggle plate deformations.

More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1 and the jaw crusher according to the invention is characterized by what is stated in the characterizing part of claim 9.

In the following, the invention will be examined in more detail by making reference to the appended drawings in which FIG. 1 is a longitudinally sectional side elevation view of a single toggle jaw crusher according to the invention; and FIG. 2 is a bottom plane view of a single toggle jaw crusher according to the invention.

Referring to FIG. 1, the main components of a jaw crusher shown therein comprise a front end 1, a rear end 2, a sensor pair 3,3', a pitman 4, an eccentric shaft 5, a toggle plate 6, a return rod 7, a movable jaw 8, a stationary jaw 9, a return spring 10 and a crusher control system 11.

According to the invention, the sensor pair 3,3'is located in the crusher so that one of the sensors is mounted on the crusher pitman 4 or, alternatively, on a member secured substantially rigidly thereto such as a backing support surface of the toggle plate, while the other sensor is mounted on the crusher rear end 2 or, alternatively, on a member connected substantially rigidly thereto such as a backing support surface of the toggle plate. Utilizing conventional electrical/electronics circuitry, the sensor pair monitors the mutual distance between the sensors and, thus, indirectly the distance between the pitman and the crusher rear end. A reduction in the distance between the pitman and the crusher rear end detected by the sensor pair is indicative of a deformation of the toggle plate. This in turn tells that through the crusher has been passed uncrushed material which has caused crusher overload and the resultant toggle plate deformation.

Alternatively, the sensor pair may be placed on the toggle plate itself, whereby one of the sensors is located substantially at one end of the toggle plate, while the other sensor is located substantially at the opposite end of the toggle plate.

In addition to monitoring a reduction in the distance between the pitman and the crusher rear end, the layout of the sensor pair location also permits monitoring of the opposite situation. Namely, a sudden increase in the distance between the pitman and the crusher rear end may be indicative of the so-called crusher overthrow situation, wherein forces occurring in the crusher gap or other forces possibly resulting from unfavorable infeed arrangements reduce the thrust force imposed on the crusher toggle plate. The increase in the monitored distance may also result from insufficient tightening of return spring 10. The ultimate outcome of such situations can be a fall- off of the toggle plate from its normal place between the pitman and the crusher rear end.

In the crusher of FIG. 2, the main components are denoted by corresponding reference numerals as those used in FIG. 1, complemented with side plate 12 and sensors 13, 13'.

Also shown in FIG. 2 is a sensor layout for monitoring the mutual distance between the toggle plate and the side plate. Herein, the sensor pair 13,13'is installed so that one sensor 13 is secured to the toggle plate 6 itself, while the other sensor 13'is secured to the crusher side plate 12. This arrangement allows the monitoring of possible lateral shifts of the toggle plate in a crusher.

A lateral shift of the toggle plate may be encountered in a situation in which the crusher is fed unsymmetrically so that the infeed material lodges in the crusher cavity predominantly at either side of the crusher cavity. Lateral shift of the toggle plate may invoke damage to the crusher.

Without departing from the scope and spirit of the invention, the sensor pair need not necessarily comprise two sensors as illustrated in the figures, but rather, some part of the crusher may perform as one half of the sensor pair. Reflection-based sensing, for instance, can be carried out without using a second sensor as the reflective surface, which may as well be formed by any surface of the crusher. Particularly for monitoring the lateral shifts of the toggle plate, the emitting sensor can be, e. g. , an ultrasonic transducer that uses the crusher side plate as the reflective surface.

The sensor pair may be connected in different ways to the crusher control and/or alarm system in order to inform the crusher operator or control the crusher functions.

In the simplest embodiment, the sensor pair is adapted to control a monitoring sys- tem that issues, e. g. , a visual or sonic alarm to the crusher operator, whereupon the crusher operator at his will can stop the crusher in order to identify the cause of the overload situation.

The sensor pair may also be connected to trigger the control system to automatically shut down the crusher equipment. Preferably, this takes place in a controlled shut- down sequence of the crushing process. Herein, all equipment associated with the crushing process upstream and downstream of the crusher is stopped at predeter- mined intervals and in a predetermined order.

Furthermore, the sensor pair may be connected to trigger the control system to stop or slow down the infeed of raw material to be crushed into the crusher.

The invention may as well be utilized in a double toggle type of crusher.

The invention is not limited as to sensors of a given type. The sensors employed therein may be selected from the group of, e. g. , ultrasonic sensors, photoelectric sensors, inductive sensors or, very simply, even mechanical sensors such as limit switches, for instance.

Furthermore, the invention is not limited as to the use of a given type of control or alarm system.

Moreover, the invention is not limited as to any given type of jaw crusher main frame construction. The main frame may comprise separate end and side plate ele- ments dismountably connected to each other or, alternatively, the frame may have a cast or welded construction in which the frame end and side plate elements are not dismountable from each other.