What Is a Cantilever?

One end of the cantilever beam is a fixed support that does not produce axial, vertical displacement and rotation, and the other end is a free end (can generate forces parallel to the axis and perpendicular to the axis). In engineering mechanics force analysis, it is a typical simplified model. In the actual engineering analysis, most of the actual engineering stress components can be simplified into cantilever beams.

(1) A prominently elongated portion of a machine or mechanism, which often moves up or down. example:

The end face leakage of the fully mechanized mining face is not only related to the roof lithology, the development of the structure and cracks, and the support conditions, but also closely related to the turning movement of the broken block in the key layer. The overburden first layer of the overburden mining face in a very high mining face is easy to break into the collapse zone and form a "cantilever beam" structure. It is different from the stable and articulated "masonry beam" structure in the key stratum of a low mining height fully mechanized mining face. There is no horizontal lateral restraining force behind the broken block, and it will not be able to form a self-stabilizing load-bearing structure; when the initial support force of the support is insufficient to balance the load of the "cantilever beam" broken block and the overlying collapse zone rock layer It is easy to cause the block to undergo instability and misalignment and cut directly to the top, thereby causing the occurrence of penetrating end-face leakage. ^[1]

Through theoretical analysis and combined with simulation experiments, three types of movements of the "cantilever beam" structure in key strata of large mining heights are proposed, namely: "cantilever beam" direct collapse type, "cantilever beam" two-way swing collapse type, " "Cantilever beam-masonry beam" alternate. Combined with the comparative analysis of the influence of the "cantilever beam" and "masonry beam" structures on the stope pressure in the key layer, the different influences of the three types of movements of the "cantilever beam" structure in the key layer on the stope pressure are obtained, and obtained Verification of the mine compaction measurement data for the 7.0m support fully mechanized mining face in the Bulianta Coal Mine. ^[2]

Analysis of cantilever "cantilever beam" structure

In fact, the key reason for the overburden collapse of the overburden key strata in the large mining stope is due to the rotation angle of the broken block being too large, which caused the rotation deformation instability at the hinge, that is, the larger rotation angle caused the key The immediate collapse of the floor. The key layer 1 is in the collapse zone of the high stope. Its regular block breaks delimit the collapse zone into two areas: "regular collapse zone" and "irregular collapse zone". It can be seen that the amount of sinking of the block to be broken and its fracture step will directly affect the rotation angle of the block, and then the block's motion shape. That is, when the subsidence of the block to be broken is small and the fracture step is large, the block will only touch with a small rotation angle, and this block will also be formed due to the smaller rotation angle. Stable hinged structure. ^[2]

Cantilever "cantilever beam" structure movement type

After simulation experiments, it was found that during the breaking motion of the "cantilever beam" structure in key layer 1, there were three different types of movements due to the falling shape of the collapsed rock blocks at the rear and the block fracture block size.

(1) The broken block of the "cantilever beam" in the key layer directly revolves, and the hinged structure cannot be formed due to the large rotation angle, and finally collapses directly in the goaf, and the key layer forms a new "cantilever beam" structure.

(2) When the cantilever beam in the key layer breaks through the block and rotates at a small angle, it touches the broken block in the rear and stops rotating, and temporarily forms a stable balanced structure. After the working face continues to mine for a certain distance, the block rotates in the reverse direction and It collapsed and finally formed a new "cantilever beam" structure.

(3) The cantilever beam in the key layer touched the broken block in the rear and stopped turning after a small angle of rotation, and formed a stable hinge structure. It continued to move forward with the mining of the working face, but after several broken hinges, finally Collapsed to form a new "cantilever beam" structure. ^[2]

Comparative analysis of influence of cantilever cantilever beam structure on ground pressure

(1) Press to step. Stress analysis of the cantilever beam structure of the key stratum during high mining heights and the block to be broken of the masonry beam structure of the key stratum during normal mining heights, where l ₁ and l ₂ are the fractures of the key strata in two structural states respectively. Steps; Q ₁ and Q ₂ are the dead weights of the two blocks to be broken respectively; M is the restraining moment of the fixed end of the rock mass in front; R is the shear force of the broken block to the broken block behind the "masonry beam" structure; q is the uniform load of the upper rock formation. Therefore, under the "cantilever beam" structure with a large mining height, the fracture step of the key layer will be larger than that of the "masonry beam" structure with a normal mining height. Assume that the stepping pressure of the working face is equal to the fracture step of the key layer. Under the influence of the "cantilever beam" structure of the key layer with large mining height, the stepping pressure of the key layer is larger than that of the ordinary "masonry beam" structure with high mining height.

(2) Compression length. The periodic breaking and slewing movement of the key layer finally caused the periodic pressure of the working face. Therefore, when the broken block of the key layer stopped rotating, the pressing was ended. Because the cantilever beam structure has a large slewing space when it breaks, and it is not easy to form a stable structure, the top beam of the stent needs to be pushed completely beyond the breaking line of the key layer. The pressure on the working surface will stop. For the breaking movement of the "masonry beam" structure of the key layer in the case of general mining height, the block can reach a stable state after a small angle of rotation, that is, the working face can be pushed a little distance and then the pressure can end. Therefore, compared with the general mining face, under the action of the "cantilever beam" structure in the key strata of large mining height, the continuous length of the working face is longer. ^[2]