A novel mechanical design of broken rope protection device for enhancing the safety performances of overhead manned equipment in coal mine

Abstract

A novel mechanical design of the broken rope protection device is proposed to enhance the safety performances of the overhead manned equipment. According to the operating characteristics and functional requirements of the overhead manned equipment, a three-dimensional mechanical model of the broken rope protection device was redesigned. Based on the known parameters of the mechanical model, the stress and strength of the main components are readjusted using the statics characteristics of finite element analysis. To ensure the reliability of the control system of the broken rope protection device, the process of people’s falling, the response performance of the tension sensor, and the signal extraction of the broken rope are analyzed under different loading and unloading speeds. The working principle of the broken rope protection device is expounded in detail. The experimental results showed that better effect is obtained by the new broken rope protection device, which is characterized by good durability, low investment, and high reliability.

Introduction

The overhead manned equipment is widely used for conveying of workers, reducing workers’ physical strength consumption and improving the working efficiency of the workers. The overhead manned equipment has become the first choice of professional transport equipment, because of its simple structure, large volume, and low energy consumption.^1–3 However, if the accident of the broken rope occurs during the operation, it will bring a great deal of personal injury and economic loss to the coal mine. The reliability and completeness of the safety protection device of the overhead manned equipment are vital to the personal safety of the workers.^4,5 With many years of development, its safety devices have also been gradually improved; however, there are some security issues such as the cases of speeding and broken rope which need to be resolved to avoid the occurrence of similar incidents.^6–8 The broken rope protection device will reduce a large amount of time and costs. Therefore, it is of great significance to study a new broken rope protection device, which can improve the safety performances of the overhead manned equipment.

The overhead manned device is composed of driving wheel, driven wheel, supporting wheel, a heavy hammer or hydraulic tensioning device, flame-proof motor, reducer, brake, rope clips and hanging chair, sound and light alarm system, mechanical safety protection devices, electrical protection devices, and so on.^9–12 Although the overhead manned device has built up a complete security protection device after years of development in China, there are still many problems that remain unsolved, such as how to prevent personnel swing sharply along the direction of wire rope caused by sudden acceleration or deceleration, give a timely capture after breaking rope, and avoid personnel casualty accidents caused by hanging chair and damage of the gear reducer, brake effectiveness, even sliding, and so on.^13–15 Wire rope focuses on the force transfer and personnel transportation which is one of the most important parts of the overhead manned equipments.^16,17

In order to improve the safety protection level of the overhead manned equipment, and guarantee the safety of mine workers, some scientific research and technical personnel are keeping quicker growth at home and abroad. The broken rope protection device of the overhead manned equipment mainly includes the following points: offside protection technology, rapid speed protection technology, multiply people spacing protection technology, low-speed protection technology, heavy hammer floor protection technology, and life-limit protection technology. For broken rope protection technology of the overhead manned equipment, the research is still in the theory research stage.^18,19 The safety protection devices of the overhead manned equipment are divided into two kinds, a kind of mechanical protection device; including the offside against speed protection device, mechanical protection device, the speed protection device, mechanical catching rope device, and so on; and a kind of electrical protection device, including the electro-static discharge protection devices, the over-speed protection, low-speed protection, and so on.^16,20 Given the circumstances for the troubles of the broken rope capture device, it is the urgent need for good accuracy, stability, and immediacy of rapidity of the broken rope capture device to protect the safety of the workers in coal mine. This article will present a novel mechanical design of the broken rope protection device to improve the safety performances of the overhead manned equipment.

Design and analysis of mechanical structure

Mechanical principle of broken rope protection device

According to the operating characteristics and functional requirements of the overhead manned equipment, a new mechanical design of the broken rope protection device is redesigned based on Pro/Engineer. Figure 1 shows the schematic structure of the broken rope protection device. The broken rope protection device is mainly composed of supporting wheel, hanger, wedge block, slide rail, traction rope, pneumatic draw stem, air supply, and so on.

Figure 1.

Schematic structure of broken rope protection device: (a) the main view and (b) the rear view.

The wedge-shaped block and pneumatic draw stem are arranged in pairs on both the sides of the wire rope; the push bar and wedge-shaped block are fixed together. Connecting rod mechanism is composed of push rod, back bar, and wedge-shaped block, and overall, they can slide along the direction of the sliding rail. In order to realize better capture of wire rope, we add a spring between the wedge-shaped block and the sliding rail. The spring can exert a force which can make accelerated decline of the wedge when the wedge-shaped block slides along with the sliding rail. The action execution part of the rope breaking protection device is realized by the expansion of pneumatic draw stem. When the overhead manned equipment is in normal operation, the pneumatic pusher sticks out and pushes the wedge-shaped block to a certain position. Once the wedge-shaped block is up to the position, it will maintain a certain gap between the wire rope which does not affect the normal traffic of the overhead passenger device. When the wire rope of the overhead manned equipment suddenly breaks, pneumatic pusher shrinks fast, replies, and drives connecting rod mechanism to the slide, so that the wedge-shaped block holds effect on the wire rope.

Finite element analysis of broken rope protection device

Once the element properties and material properties of the components have been defined and the method of components is meshed, it can fill up the constraint conditions and load according to the actual loading conditions and eventually do the finite element analysis for the wedge block, connecting plate, bar, and sliding rail and a combination of mounting plate. Meanwhile, through the form of node equivalent stress and the contour of total deformation, we can get the distribution of stress and strain vividly when the components are under load. The results of stress analysis are shown in Figures 2 and 3.

Figure 2.

Contour map of equivalent stress for catching parts: (a) connecting plate, (b) sliding rail and mounting plate, and (c) wedge clamping mechanism.

Figure 3.

Contour map of equivalent stress for bar group (pusher, back bar, pneumatic draw bar).

Stress analysis

As shown in Figure 2(a), the most equivalent stress values of the connecting plate are in the range of 0–57.49 MPa and the maximum value is 151.86 MPa, which appears in the middle of the extension of the connecting plate and is mainly caused by stress concentration. Figure 2(b) shows that equivalent stress of the integration composed of mounting plate and sliding rails is in the range of 0–13.94 MPa. The maximum value of 31.37 MPa appears at the bottom of the integration and the right side of the sliding rails which is caused by stress concentration. It can be seen that the maximum stress on the wedge clamping block appears at the bottom and near the end with the value of 20.60 MPa in Figure 3, and the most equivalent stress values are in the range of 0–11.45 MPa. Just as Figure 3 shows, we can observe that the most equivalent stress values are on a scale of 0–49.02 MPa. In order to analyze the stress changes of the bar at an instant of broken rope, the stress of the bar is analyzed by ANSYS. Result shows that the maximum stress is 73.53MPa, which appears in the end of the pusher rod and is close to the position of the connecting rods. From the material properties and the results of the analysis of the main components, the safety factor of the connecting plate is K₁ = 1.55, the overall safety factor of mounting plate and sliding rail is K₂ = 7.49, the safety factor of the wedge is K₃ = 11.41, and the safety factor of the bar is K₃ = 3.20. From the above analysis results, the safety factor of the key parts of the broken rope protection devices is above 3.0, which meets the design requirements, except for the connecting plate. Therefore, we can increase the thickness of the connecting plate appropriately in order to improve the safety and reliability of the design.

Strength analysis

In the analysis object, the deformation has a greater influence on the connecting plate and bars; therefore, we need to make an analysis of strength of the connecting plate and bar. As a result, their strain distribution maps of deformation are shown in Figures 4 and 5. From Figure 4, the overall deformation of the connecting plate is in the range of 0–0.34 mm; the maximum value of 0.736 mm appears in the central part of the connecting plate; the connecting plate is under its own gravity and the effect of braking force, resulting in deformation, which has no effect on the broken rope protection device. In order to improve the safety performance, we can increase the thickness and width of the plate. From Figure 5, the overall deformation of the bar is in the range of 0–0.24 mm, a maximum of 0.36 mm located close to the link connection. From the point of its structural features, when the broken rope protection device is reinstated, the broken rope protection device is mainly subjected to the action of thrust from the pusher, resulting in deformation; however, deformation of its term meets the design requirements.

Figure 4.

Strain distribution map of connecting plate.

Figure 5.

Strain distribution map of bar group.

Performance analysis of tension sensor

To ensure the reliability of the control system of the broken rope protection device, it is very important to research the process of people’s falling, the response performance test of the tension sensor, and the signal extraction of the broken rope. In addition, the analysis of the time response of the control system contributes to the rapid, accurate, and stable catch wire rope in fault state of the overhead manned equipment. The tension sensor of the wire rope is used to obtain the signal of the broken rope during accident. Because the response ability of the tension sensor of the wire rope directly affects the performance of the capture device, the performance test of the response time of the tension sensor is necessary.

This section analyzes design and working principle of the tension sensor. Figure 6 shows the working principle of the tension sensor. According to the vertical and horizontal bending theory of the wire rope, we fix the tension sensor on the wire rope which is fully tensioned making these three points of tension sensor contact with the tested surface; in the meantime making this wire rope generate a deformation that is similar to three-point bending. After the deformation is generated, it produces the displacement signal and pressure signal. It also generates deformation of the strain gauge which is fixed on the surface of measure point; with the process of deformation, the resistance will also change. After the transformation of process circuit, these changes in resistance will become the current signal. After the process of these circuit signals, we can obtain the tension of the wire rope at the circumstances that we did not consider the partial bending moment occurred at the contact point of the tension sensor. The tension sensor of the wire rope is composed of tension scaling system and force-electric sensor. The structure and principle of tension sensor are shown in Figure 6, in which A is the force bearing point, p is the pressure that the force-electric sensor takes, F is the tension of the steel wire rope, and α is the intersection angle between the direction of tension and the vertical direction of the force P. Under the stress condition of the steel wire rope, if a displacement Δh is given between the guiding device and the component that is under stress, the stress P that tension F acts on the force-electric sensor through the guiding device can be ascertained. This sensor has great advantages of high reliability and great exchangeability, and it is very easy to operate. It can be set up at the proper place at the tail pulley without changing the original structure and the structure of this sensor can never have the circumstances of bend, fold, and block according to the non-standard operation of the wire rope.

Figure 6.

Working principle diagram of tension sensor.

A new experimental apparatus is built to test the response time of the wire rope’s sudden breaking. Figure 7 shows the experimental apparatus for breaking of the wire rope. The main test content includes the sensor for quick response when slowly loading and slowly unloading, slowly loading and quickly unloading, and sudden loss of load for the reliable application of the broken rope capture device. In order to simulate the breaking process of wire rope aerial, this experiment combines tensile test bench with a combination of signal acquisition system to finish the work. Specific experimental methods are as follows: (1) installing wire rope in tensile test bench; (2) clamping tension sensor of the wire rope in the rope; (3) enabling the valve of hydraulic system to operate the tensile of loading. The loading value of this experiment is, respectively, designed for 2, 4, 6, 8, and 10. (4) Open the oil return valve of the hydraulic system for unloading quickly; this change process will be sent to the signal acquisition system through the tension sensor of the wire rope.

Figure 7.

Experimental apparatus of wire rope sudden breaking.

To observe the responding situation of the tension sensor under different loading and unloading speeds, the experiment is divided into two parts. One of them is conducted at 2, 4, 6, 8, and 10 slow loading and fast unloading. Another is conducted at 2, 4, 6, 8, and 10 T fast loading and slow unloading (Figure 8). It can more clearly be expressed in the form of pictures, because the experiment records a substantial amount of data. We collected 643 data points in the situation of 2 T fast unloading. In the data table, we can see that the time from keeping load to sudden unloading is about 0.02 s. In all, 756 data points are collected in slow unloading. It can be observed from the figure that when the oil return valve opened slowly, wire rope tension will also decline slowly. When it is carried on 4 T fast unloading, 532 data points are collected. In the data table, we can see that the time from keeping load to sudden unloading is about 0.015 s. We collected a total of 431 data points in the slow unloading, and the graph indicates that wire rope tension sensor response is basically good. We gathered 782 data points in 6 T fast unloading, and from the graph, we can see that the time from keeping load to sudden unloading is about 0.015 s; wire rope tension sensor has a quicker response; in slow unloading, we collected a total of 556 data points and the trend of these points is smooth. We gathered 607 data points when it is carried on 8 T fast unloading. When keeping the load, the tension decreases with the leakage of hydraulic oil. This change process can be obviously felt by the sensors. From the graph, we can see that the time from keeping load to sudden unloading is about 0.01 s. This shows that the steel wire rope tension sensor has a fast response. We collected 677 data points in the 8 T slow unloading. When the load achieves 10 T, the chuck that is installed on the test rig suddenly looses due to the large load (Figure 9). We all gather 458 data points, and from the graph, we can see that the time from keeping load to sudden unloading is about 0.005 s. The change trend of wire rope tension sensor’s response performance is obvious.

Figure 8.

Response time of tension sensor on 2, 4, 6, and 8 T: (a) load slow load and (b) fast unload.

Figure 9.

Sensor response on 10 T all of a sudden loss of load.

Working principle of broken rope protection device

The broken rope protection device is divided into catching part, action execution part, and signal acquisition part. The catching part is mainly realized by the wedge-shaped block. The action execution mechanism is realized by a pneumatic draw stem. The signal acquisition part of the rope protection device is realized mainly through the tension sensors. They are installed in the tail end of the overhead manned equipment which are used to tension the wire rope. Under normal circumstances, the tail tensioning device is used to adjust the tension of the traction wire rope. The tension of the wire rope changes within a certain scale. Once the traction wire rope fractures, the tension of tail tensioning wire rope rapidly becomes smaller and beyond the normal limits. The tension sensor will receive the signal and then it will be rapidly transmitted to the control system of the overhead manned equipment. The Programmable Logic Controller (PLC) turns off the switch of the main power supply circuit and also disconnects the power supply of the electromagnetic valve. Based on the need of the overhead manned equipment, the quantity of the broken rope protection device can be determined. When the rope breaks, all the rope breaking protection devices enable the capture action at the same time. This method has improved the reliability of the capture. The pneumatic pusher contracts the working state of the wedge-shaped block. Eventually, the device realizes the capture of the wire rope. The whole schematic assembly of the rope broken rope protection device is shown in Figure 10.

Figure 10.

Picture of the broken rope protection device: (a) front view and (b) rear view.

The working principle of the broken rope protection device is as follows: (1) when the overhead manned equipment is in normal working state, the pneumatic pusher pushes the back connecting rod, enables the wedge-shaped block slide along the sliding rail to a certain position, and keeps enough gap between the wire ropes. At this time, the traction wire rope cannot be captured by the device. It ensures the overhead system works regularly. (2) When the traction wire rope fatigue strength exceeds the limit value or other sudden fault causes the fracture of wire rope, signal acquisition part quickly receives the signal of the breaking rope. This makes the main circuit of the overhead manned system switch off, so that the electromagnetic valve will have no electricity. (3) Pneumatic pusher contracts quickly to reply and pushes back the connecting rod, so as to drive the wedge-shaped block to decline rapidly. The wedge-shaped block contacts with the wire rope traction and the frictional force generates between the wedge block and steel rope. Under the effect of the wedge-shaped block gravity, pneumatic pusher thrust, and the friction between the wire rope and wedge-shaped block, the wedge-shaped block will continue to slide along the direction of the sliding rail. Finally, it will lock the wire rope and make wire rope stop moving. (4) The pneumatic rod is installed at the back of the mounting plate and is mainly controlled by the electromagnetic valve, so as to realize the capture of the wire rope. After the broken rope accident releases, the broken rope protection device can also achieve automatic reset, which makes the control system prepared for the next state monitoring.

The tension sensor enables the broken rope protection device to check the tension change in the wire rope. When the wire rope is broken, the change in the tension signal will eventually turn into the action signal. It is the best available method to acquire accurate signal and reduce the chance of false action. Based on the signal, the broken rope protection device is used to perform the catch. At the same time, when the broken rope accident occurs, the main circuit of the overhead system is switched off timely. This can avoid the related equipment of the overhead manned device operating in the non-normal state sequentially. On one hand, this is beneficial to avoid the damage of the equipment. On the other hand, it can also relieve and prevent other accidents caused by these equipment failures. The device can reduce the injuries and losses to a minimum.

Conclusion

According to the operating characteristics and functional requirements of the overhead manned equipment, the structure design for the broken rope protection device was finished. Based on the wedge clamping principle, the forthcoming implementation of the protection action in the broken rope accident can be realized so as to ensure the safety of the passengers. Based on the finite element analysis for the main components of the broken rope protection device, the improved design is to obtain better effects. By analyzing the response time of the tension sensor, the reliability of the control system of the broken rope protection device improved greatly. The broken rope protection device has a compact structure and reasonable cost. It is flexible and reliable to perform the capture action with a pneumatic pusher. It can achieve automatic reset after the maintenance of the broken rope and shorten the time from failure to restore production. It also saves the manpower and the material resources for the manual reset.