Mine hoist is mainly used for lifting and lowering personnel, materials and equipment in coal mines, metal mines and non-metallic mines. It is an important transportation tool to connect the underground and underground, and is the throat equipment of the mine.

1. As the key component of the safety braking system of the mine hoist, the working reliability of the disc brake is directly related to the safety performance of the mine hoist and affects the safety production of the mine. Once the disc brake fails, it may affect the production progress, or even lead to malignant accidents. The common disc brake failure is mainly manifested as the failure to brake or open the brake, resulting in a variety of reasons for its failure. From the angle of disc brake working principle, we analyze the causes of failures, propose corresponding preventive measures and future maintenance measures, and enumerate typical cases for analysis.

2 Common fault types of disc brake and causes of brake failure

Most of the failures of disc brakes are that they can not complete the braking task according to the regulations, or the braking effect can not meet the requirements. Generally, disc brake failures are mainly divided into two categories: brake failure and brake release failure.

2.1 Brake failure of disc brake

The main reason for disc brake failure is insufficient braking torque. The following two main factors affect the braking torque.

(1) The insufficient positive pressure output by the brake is mainly caused by: ① The disc spring is not rigid enough due to fatigue or fracture due to repeated stretching and compression; ② Excessive deflection of the brake disc or excessive wear of the brake shoe results in excessive clearance between the brake shoe and the brake disc. The disc spring pre compression decreases when fully braking, resulting in insufficient positive pressure output; ③ The aging of piston seal ring and the pollution of hydraulic oil cause the increase of piston motion resistance, resulting in insufficient positive pressure; ④ The residual pressure of the brake system is too high due to oil pollution or oil circuit blockage, resulting in insufficient brake output positive pressure; ⑤ The large bolt connecting the liner cylinder and the piston is loose, which reduces the pre compression of the disc spring, resulting in insufficient positive pressure.

(2) The reduction of friction coefficient between brake shoe and brake disc According to relevant literature and field test data, the main factors causing the reduction of friction coefficient are: ① The brake disc or brake shoe surface is polluted by oil contamination, mainly due to oil leakage of brake hydraulic cylinder, oil pipe open welding, loose joint oil leakage and oil leakage caused by aging of piston seal ring in hydraulic cylinder; ② The deformation of the contact surface of the brake shoe is mainly due to the overheating of the brake shoe due to the overheating of the working temperature, resulting in burning or zooming, thus affecting the friction coefficient; ③ The friction factor is related to the moving speed of the object. When the elevator runs at a high speed, the friction factor will be reduced.

2.2 Failure of disc brake release

The main factors causing brake release failure are:

(1) The oil pressure of the hydraulic station is insufficient to overcome the spring force and piston motion resistance;

(2) The tight fitting of the hydraulic cylinder or the cylinder clamping causes the piston movement resistance to increase dramatically, making the brake oil pressure unable to overcome the movement resistance and open the brake.

3 Preventive measures

The maintenance of the equipment is to eliminate the failure in the bud through scientific and reasonable maintenance before the failure through the clues that appear during the operation of the equipment. By analyzing the common failure types and causes of disc brake, the author proposes the following measures to prevent disc brake failure during daily maintenance:

(1) In accordance with the provisions of the Safety Regulations for Metal and Non metal Mines, the working performance of the working brake and the safety brake shall be regularly tested, the braking torque shall be checked, and the speed of the safety brake shall be measured.

(2) There should be no oil stain on both sides of the brake disc of the disc brake, which should be checked at least once per shift. If oil stain is found, it should be stopped for treatment in time to avoid affecting the friction coefficient between the brake shoe and the brake disc.

(3) When the hoist operates normally, if the brake hydraulic cylinder leaks oil, replace the seal ring in time.

(4) In the process of lowering heavy objects, power braking shall be put into use, and braking cannot be done solely by the brake disc, otherwise, the brake shoe will heat up, which will easily cause thermal deformation of the brake shoe, and reduce the friction coefficient of the brake shoe, thus affecting the braking force.

(5) The brake clearance shall be kept at about 1 mm and shall not be greater than 2 mm. The old equipment without brake clearance protection device shall be configured in time. When the brake clearance exceeds the specified value, it shall be able to automatically alarm or power off. After the brake clearance is adjusted, the positioning bolt must be locked.

(6) Filter the hydraulic oil once half a year to ensure the oil quality is clean; The hydraulic station shall be thoroughly cleaned once a year; Check the oil quantity of hydraulic oil once per shift to ensure that the oil quantity is sufficient.

(7) The service time of the belleville spring shall not exceed the provisions of the national standard, and the belleville spring shall be replaced when the number of actions exceeds 500000.

(8) The automatic detection device for brake disc deflection is installed. When the brake disc deflection is detected to be greater than 1 mm, the flame method or turning method shall be used to re calibrate the brake disc to make its deflection ≤ 1 mm, meeting the relevant requirements of the regulations.

(9) Regularly check the idling time of the brake. If the idling time is greater than 0.3 s, repair it immediately. For the sake of safety, it shall be tested every half a year. During maintenance, first, remove the rust on the surface of each component, second, check whether the surface of the disc spring has cracks and deformation, third, check the wear of the hydraulic cylinder, and replace it immediately in case of any abnormality.

(10) According to the workload of the hoist, check the remaining thickness of the brake shoe regularly, and replace it immediately when it is less than 6 mm.

For the disc brake device that can not work normally after reaching the service life or long-term maintenance, it is necessary to timely carry out the overall transformation, replace the new brake system to meet all aspects of the requirements of the regulations, and ensure the safe operation of the mine hoist.

4 Typical case analysis

4.1 Accident process

In January 2016, a copper mine in Anhui had a tank falling accident. The equipment at the accident site was JKM-2.25X4PI multi rope friction hoist. When the materials lifted by the equipment were 5 m away from the wellhead, the lifting container slipped. The driver immediately applied emergency braking when he found it. The emergency braking failed. The lifting container at the heavy load side continued to slide and fell to the bottom of the shaft, eventually causing damage to the wire rope cage, the head rope, the tail rope and the two lifting containers. The accident caused huge losses and seriously affected the production progress of the mine.

4.2 Field observation

It was observed on site that there were traces of oil contamination along the circumference of the brake disc, as shown in Figure 2. After removing the disc brake in use, it is found that all brake shoes are seriously worn, and the wear of each brake shoe is uneven, even some brake shoes have broken, as shown in Figure 3. It was also found that the large bolts connecting the lining plate cylinder and the piston inside the brake were loose, some disc springs in the disc spring group were broken, and the sealing ring at the piston was also aged and damaged.

Contaminated brake discs on site                      Broken brake shoe on site

The professional department inspected the accident site and found that the hoist is equipped with 4 pairs of TP1-63 brake heads, with positive output pressures of 5, 15, 15 and 20 kN respectively; The brake disc deflection measured on site reached 3 mm, which greatly exceeded the specification requirements by 1 mm; The maximum oil pressure of the hydraulic station is 3.3 MPa; The residual pressure of hydraulic system reaches 1 MPa during full braking.

4.3 Accident causes

According to the field observation, the main cause of the accident is the brake failure of the disc brake.

(1) The disc spring has insufficient stiffness due to fatigue or fracture, which reduces the pre compression force of the spring; The large bolt connecting the liner cylinder and the piston is loose, which reduces the pre compression of the disc spring and the pre compression force of the spring; The residual pressure of hydraulic system is too large, causing the pressure of working chamber to increase; The aging and damage of the sealing ring cause the increase of the piston movement resistance. Therefore, the positive pressure output by the disc brake is greatly reduced under full braking.

(2) The aging of the piston seal ring leads to oil leakage of the brake head, which pollutes the brake disc and reduces the friction coefficient between the brake shoe and the brake disc.

(3) The excessive deflection of the brake disc leads to uneven wear of each brake shoe. The contact area between the brake shoe and the brake disc is too small during braking, which leads to uneven force on the brake shoe and fracture, resulting in deformation of the contact surface between the brake shoe and the brake disc, further reducing the friction factor between them.

It can be seen that the above factors finally lead to a serious shortage of braking torque, and the emergency braking can not stop it, which ultimately leads to the occurrence of tank falling accident.

4.4 Solutions

(1) Calibrate the brake disc by flame method or turning method to make its deflection ≤ 1 mm, so as to meet the requirements of the regulations;

(2) Replace all disc springs, sealing ring components and brake shoes in the brake head;

(3) Tighten the large bolts connecting the lining plate cylinder and piston inside the brake according to the specified tightening torque;

(4) Clean the brake disc, brake piston, hydraulic cylinder, hydraulic oil circuit and hydraulic station, and replace with new hydraulic oil.

If the above measures are taken to maintain the disc brake, it is still unable to stably implement the working braking and safe braking for a long time, it is recommended to replace the entire hydraulic braking system, that is, to conduct an overall transformation of the braking system. At the same time, a series of detection devices, such as brake clearance detection, brake disc deflection detection and oil pressure detection, are configured on the newly replaced braking system to achieve real-time online monitoring of the braking system and ensure its safety and reliability.

5 Conclusion

Although the failures of the disk brake of the mine hoist are various, as long as it is used reasonably, and the daily management and maintenance are strengthened according to the above measures, the use status of the disk brake and the problems in its use can be known in time, and hidden dangers can be eliminated timely and accurately, so as to ensure the safe and reliable operation of the mine hoist, prevent accidents, and ensure the safe production of the mine.