Quarried material is often processed by means of crushing plant, for the production of aggregate, for example. There are various known forms of crushing plant for the comminution of rock material and the like, one of which is referred to as ajaw crusher. One conventional jaw crusher consists of a frame having side walls and a pair of jaws, a fixed jaw and a swing jaw, disposed therebetween. The fixed jaw and a swing jaw each have a crushing face, the crushing faces being arranged in a spacedapart relationship to define a crushing chamber for receiving material to be crushed. The swing jaw is movable between a first position in which the crushing face of the swing jaw is inclined to the crushing face of the fixed jaw, and a second positionin which the crushing face of the swing jaw is brought substantially parallel to the crushing face of the fixed jaw, at a predetermined spacing therefrom. The upper end of the swing jaw is connected to an eccentric shaft, which is located in a rotatable bearing. In use, as the bearing is rotated, the shaft is caused to proscribe a circle, which in turn causes the upper end of the swing jaw toproscribe a circle in the direction of the fixed jaw. Hence, the crushing face of the swing jaw moves in a crush cycle between the first and second positions, up and down, as well as towards and away from the crushing face of the fixed jaw. Movement ofthe swing jaw in this manner causes impelling forces for crushing material present in the crushing chamber. Typically, a jaw crusher as described above will include a toggle plate located behind the swing jaw, adjacent the lower end of the swing jaw, for supporting the lower end of the swing jaw during the crush cycle. In a known type of jaw crusher,one end of the toggle plate reacts against the rear face of the swing jaw, and the other end of the toggle plate reacts against a cross beam provided behind the swing jaw and extending between the side walls of the jaw crusher frame. To enable a predetermined maximum product size to be produced during the crush cycle, the spacing between the pair of jaws at their lower ends, i.e. where the crushed material is discharged during the crush cycle, can be adjusted. It is known toinsert or remove shim packs or other adjustment means between the toggle plate and the cross beam, thus reducing or increasing the distance between the lower ends of the two jaws. It will be understood that larger pieces of crushed material are producedusing a greater jaw spacing than would be produced by using a smaller jaw spacing. If an uncrushable object enters the crushing chamber, during the crushing cycle, substantial forces are generated as the swing jaw acts to complete its cyclic motion against the uncrushable object. The forces generated can make the removal ofthe uncrushable object a dangerous exercise. Moreover, the generation of these forces can cause damage to the jaw crusher. In some cases, the substantial forces generated will cause the toggle plate to yield, which renders the jaw crusher inoperativeuntil the toggle plate is replaced, therefore effecting productivity. GB812507 describes a jaw crusher substantially as described above which teaches a solution to these problems. In this case, the cross beam is slidably received in the side walls of the jaw crusher frame, whereby the ends of the cross beam extendoutside the walls of the jaw crusher frame. The ends of the cross beam carry bearing blocks and a tie-rod is attached to each bearing block, each of which extend away from the bearing blocks in the direction of the fixed jaw. The other end the tie-rodsare each secured to a crosshead located on the outside of the respective wall of the jaw crusher frame. A pair of pressure cylinders, in parallel, is mounted on either side of the jaw crusher frame, in line with the tie rods and between an associatedcrosshead and bearing block. Each cylinder includes a piston rod which is attached to a respective crosshead. Under normal operating conditions, the cylinders act to push the crossheads forwards, i.e. in the direction of the fixed jaw, thereby pulling the tie-rods in a direction away from the bearing blocks. Hence, the tie-rods are put in tension, whichbiasses the cross beam in its slidable mounting in the direction of the fixed jaw, to bias the toggle plate against the swing jaw. When excessive pressure is generated in the crushing chamber, for example when an uncrushable object enters the crushing chamber, forces act to move the swing jaw backwards, i.e. away from the fixed jaw, against the toggle plate, to urge thecross beam to slide backwards in the side walls. This movement acts against the biassing action of the cylinders transmitted through the tie rods and crossheads, as described above, which can cause a further build up of pressure in the crushing chamberleading to an overload situation. However, both sides of the cylinders are in communication with an hydraulic control system, for providing an hydraulic buffer for the crossbeam and toggle plate against overload during the crushing cycle. In the event of an excessive build up ofpressure during the crush cycle, the control system communicates with the cylinders to allow backwards movement of the cross beam, thus avoiding an inertial yield of the toggle plate. The arrangement of GB812507 has the disadvantage that, since the tie rods and associated cylinders are outside the walls of the jaw crusher frame, the action of the cylinders puts the cross beam into bending, under normal operating conditions. If excessive pressures are generated during the crush cycle, as described above, the action of the toggle plate against the cross beam causes further bending stresses in the crossbeam, which significantly magnifies the bending effect of the tie rods onthe cross beam. Given the immense bending stresses which are associated with an uncrushable object entering the crushing chamber, this arrangement is not considered to be satisfactorily practical or safe, and does not effectively absorb the magnitude ofthe generated forces. In addition, the magnitude of the forces involved dictates that the cylinders must, in practice, be of a very large diameter, which increases the offset distance of the line of action of the cylinders from the side walls, thus increasing thebending stresses still further. U.S. Pat. No. 4,927,089 describes a jaw crusher which teaches an alternative solution to the problems of known jaw crushers referred to above. In this case, a plurality of parallel hydraulic cylinders are provided between the cross beam andthe toggle plate, in communication with an hydraulic circuit having a pressure relief device. Once a pre-determined pressure is reached in the cylinders, due to an uncrushable object being present in the crushing chamber, for example, hydraulic fluid isreleased from each cylinder via the relief device, which allows the swing jaw to be moved away from the fixed jaw, to enable the uncrushable object to be passed through the chamber. However, there are problems associated with the jaw crusher of U.S. Pat. No. 4,927,089. For instance, due to the substantial pressures generated in the cylinders during the crushing process, typically from zero to a maximum pressure with everycycle of the swing jaw, seal life within the cylinders can be compromised. Furthermore, hydraulic fluid is compressible to a degree, and therefore crushing efficiency can be compromised, as the cylinders compress the fluid during the crushing cycle, forexample. According to the present invention, there is provided a jaw crusher comprising a frame having a fixed jaw and a swing jaw, which define a crushing chamber for receiving material to be crushed, the swing jaw being mounted for cyclic movement inthe direction of the fixed jaw; a cross beam adjustably disposed in the transverse axis of the frame; a toggle plate for operative communication between a rear portion of the swing jaw and a first face of the cross beam, characterised in that anhydraulic cylinder arrangement is provided in operative communication with an opposite, second face of the cross beam, and in which, in use, the hydraulic cylinder arrangement is pressurised to a predetermined value to provide an adjustable, pre-loadedreaction against the toggle plate. Since the cylinders are mounted in the apertures in the walls, the load from the cylinders is transferred directly on to, and in line with, the walls, which greatly reduces the stresses within the crusher frame for any given weight of structure. This enables a substantially compact design of jaw crusher, which has particular advantage for use on a mobile crusher plant. For example, the mobile chassis can be significantly narrower than is conventionally required, thus reducing the weight of theplant and increasing the movability of the plant.
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