# Analysis and theory of the pretightening force of

2022-10-24
• Detail

Analysis and theoretical derivation of the pre tightening force of the buffer bolt of the small anti attachment device

1 proposal of the problem

in the shaft hoisting system of small coal mines in China, most of the small anti falling devices are used as the broken rope protection safety device. After the catching mechanism of the small fall arrester acts, the impact force acting on the buffer rope pulls the steel wire rope out of the buffer (as shown in Figure 1). The final bolt pre tightening force of the buffer and the impact force on the rope must conform to a certain mathematical relationship to make the buffer buffer play a buffering role. Therefore, when designing and installing each fall arrester, the bolt preload of the buffer is an important parameter that must be clarified. Now, bolts with sufficient strength are generally selected after the design and before the installation of each buffer to realize the sharing of resources and data in the whole city, which is obtained through the experimental method. This is to ask the manufacturer to have a certain test ability. At the same time, the test method is not conducive to mass production and will increase the production cost. Therefore, it is necessary to theoretically study the calculation method of the pretightening force of buffer bolts, which can provide an accurate and reliable basis for the selection of bolts in design and the setting of pretightening force in installation

2 analysis and theoretical derivation "" the current popular color trend is the black white high gloss surface

the working process of the buffer is also the average impact force on the buffer wire rope (expressed in FN) The process of overcoming the resistance of the buffer to do work and releasing energy on the buffer. FN is easy to determine, and it is only related to the parameters of lifting unit weight and buffer distance. How to establish the mathematical relationship between bolt preload and FN

first of all, from the buffer structure (as shown in Figure 1), under the action of pre tightening bolts, the fixed wheel group and the movable wheel group squeeze the buffer steel wire rope mutually, causing it to bend and deform, resulting in a certain wrap angle a for the wheel group. At the same time, the extrusion force is mainly concentrated on the extrusion tangent point of the wheel and rope (as shown in Figure 2, point a and point B). If the movable wheel is composed of N wheels, and two pre tightening bolts are installed on both sides of each wheel, there are 2n extrusion tangent points and two pre tightening bolts, so the extrusion force at each tangent point can be regarded as the pre tightening force of each bolt (QI)

for the convenience of research, take any wheel of the movable wheel group and the arc section ab of the local buffer steel wire rope, as shown in Figure 2, and analyze its stress. When FN reaches the maximum static resistance of the buffer, AB is in a critical state of equilibrium stress. By analyzing that at this moment, the tension of the rope before point a is F1, the tension after point a is F0, the tension before point B is F1, and the tension after point B is F2 (assuming that the static friction coefficient between the rope and the wheel is f), it can be obtained that

intercepts a micro arc segment DL, and the corresponding wrap angle is da. Let the tension at both ends of the micro arc section be f and f+df respectively, the positive pressure of the wheel to the micro arc section be DFN, and the limit friction between the rope and the wheel surface be fdfn. From the balance of forces in the normal and tangential directions,

it can be seen from the above formula that the tension of the buffer rope after the buffer movable wheel group has several n extrusion tangent points from top to bottom is

when n=1, F0 represents the force on the buffer rope after the first extrusion point, so F0 is recursively solved and sorted out by formula (4) and formula (1). The above formula is the calculation formula of the buffer bolt tension. It should also be noted that the friction coefficient in the formula, in the buffer process, the rope and the wheel slide relatively, and the sliding friction resistance of the buffer wire rope is smaller than the maximum friction negative force. Therefore, the use of the maximum static friction coefficient is mainly to ensure that the cushioning effect is completed within the design distance

3 conclusion

in view of the particularity of coal mines with high requirements for safety factors, it is also necessary to continue to compare the results calculated by the formula with the test results, and then correct the calculation formula of the pretightening force of buffer bolts derived from the theory, so as to obtain more reliable calculation results

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