Designing And Analysis Of A Bolt Axial Force Monitoring Device Of Hammer Acoustic Method

As the frequent reports of collapse of so- and- so mines from all over the world, huge economic losses and casualties have been caused to the society.Roadway safety problems has increasingly arisen people’s attention. Anchor bolt support is served as a main roadway supporting form, its security problem has been become the research focus of many coal mine safety researchers. Existing monitoring devices can be roughly divided into two types. One is supplied by power, which can accurately realize the monitoring of anchor bolt, avoiding collapse accident; but at the same time, higher requirements have been put forward to explosion- proof work in the mine. Another type is a mechanical structural form, which can better eliminate security hidden troubles, but due to its complex structure, high cost and large volume, etc., this type of monitoring device has not been widely used.Targeted at the problems existing in the monitoring devices, this paper proposes a bolt axial force monitoring device of hammer acoustic method. Each structure size of that device has been confirmed through theoretical analysis,ANSYS structure modal simulation analysis and Virtual. lab acoustic sound field simulation analysis, achieving the target of judging anchor bolt axial force through magnitude of sound frequency. Based on structure inherent frequency and linear relations with its internal stress, the monitoring device changes its inherent frequency that is the free vibration frequency of the monitor device through changing load sustained by the monitor device. Therefore, it can judge the magnitude of the anchor bolt axial force via the magnitude of vibration frequency of the monitor device.This paper firstly conducts discussion and analysis based on two vibration structure, that is, radial vibration and axial vibration. It carries out structural strength, modal and vibration response as well as sound field radiation analysis when impacted by a certain magnitude of the impulse force respectively on monitoring devices of these two structures. Monitoring result of vibration body of monitoring device doing axial vibration is better than that of the vibration body doing radial vibration. Therefore, an annular plate is selected as a vibration body; monitoring device consists of two parts, upper cover and lower tray that is constituted by a base and an annular plate.Finally, analysis has been conducted on the property of a monitoring device in two aspects of structure size and monitoring parameter. Analysis results are shown as below:(1) According to analysis of sizes of three structures of monitoring device,it obtains that position of sound frequency loading change interval can be changed through extending length L with unchanged size of interval, and then sound frequency can be controlled within low- frequency area; the magnitude of sound frequency loading change interval can be changed through taper α and axial distance h, so as to realize adjustment of sensitivity of monitoring device, but which will be limited by structural strength of monitoring device.(2) From analysis of the five monitoring parameters, it can conclude that the load change is 17.8t and frequency change of sound heard is 216.92 Hz;impulse force is allowed to change within the scope between 3 and 10 N, which can reach the same monitoring results and user- friendly; stuffs shall stand at the75- degree angle with anchor bolt axial line, the best position of which is around the extending line of axial line along with monitoring device. By doing this, it can obtain the most accurate monitoring effect through hearing the best effect of sound. Damping ratio of structural modal of monitoring device will affect acoustic pressure amplitude of sound, but without no influence on identification of sound frequency, so that it can be omitted from consideration.