A brief discussion on the failure of the disc brakes and preventive countermeasures of mine hoist

　　Mine hoist is mainly used in coal mining, metal and non-metallic mineral raising and lowering personnel, materials and equipment, etc., it is one of the important means of transport link and underground inoue, is the throat of the mine equipment.

　　Disc brake for mine hoist safety brake system of key components,  its working reliability directly related to the safety performance of mine hoist, affects the safety of mine production,  the disc brake in the event of failure,  light affect production progress, or lead to malignant accidents.  The common disk brake failure is mainly manifested in the failure of the brake or open gate, the causes of its failure are various. In this paper,  the author analyzes the causes of the failure from the Angle of the working principle of disc brake, puts forward the corresponding preventive measures and the maintenance measures in the future, and enumerates the typical cases for analysis.

　　The working principle of a disk brake

　　At present most of the new type of mine hoist braking system have used disc brakes, compared with the block brake, disc brake has a compact structure, action quick, quality characteristics such as light and heat faster [1].

　　As shown in figure 1, F1 is oil pressure and F2 is precompression force of spring. It is obvious that the disc brake is based on the oil pressure F1 brake, the spring precompression force F2 brake to realize its function.

　　The oil pressure

　　

 

　　In the formula: p for work oil pressure, Pa; A is the area of the hydraulic cylinder in the disc brake, m2.

　　Spring precompression force

　　

 

　　In the formula: K is the stiffness of disc spring, N/m; Δ 0 for oil pressure p = 0 disc spring precompression, m.

　　

 

　　Fig.1 working principle of disc brakes

　　Fig.1working principle of disc brake

　　1. Brake disc 2. Brake shoe 3. Hydraulic cylinder 4. Piston 5. Disc spring

　　When oil pressure p = 0, namely the F1 = 0, gap between the brake shoe and brake disc Δ = 0, for full braking state at this time.

　　Because of the piston motion resistance and work residual pressure, the full braking state of the disc brake output positive pressure

　　

 

　　In the formula: C is the motion resistance of the piston in the disc brake, N; P0 is the residual pressure in the hydraulic system, Pa.

　　When the oil pressure p increases to a certain value the pn, brake shoe out of the brake disc brake shoe and brake disc gap between Δ > 0, no positive pressure between the two, brake in the completely loose brake state at this time.

　　At this time to meet

　　

 

　　Among them

　　

 

　　The formula (1) and formula (5) are substituted into the formula (4), which can be completely loosened

　　

 

　　In the formula: F2 'is the compression force of disc spring for oil pressure p= pn, N; Δ 'for the oil pressure is p = pn disc spring compression capacity, m.

　　The braking torque of disc brakes is generated by pressing the brake disc along the axial direction from both sides to the brake disc, and the calculation formula is

　　

 

　　In the equation: mu is the friction factor; R is the braking radius, m; N is the number of brake tiles.

　　The common fault type of the 2 disc brake and the reason of brake failure

　　Most failures of disc brakes are not to complete the braking tasks according to the regulations, or the braking effect cannot meet the requirements. Generally, the failure of disc brakes is mainly divided into brake failure and loose brake failure. [2].

　　2.1 disc brake failure

　　Disc brake brake failure, its main reason is the brake torque insufficient. It can be seen from formula (7) that there are two main factors affecting the braking torque.

　　(1) the positive pressure of brake output insufficient FN by type (3) it can be seen that brake output caused by insufficient pressure are the main factors are: (1) because the disc spring repeated tensile and compression fatigue fracture, or cause the disc spring stiffness K deficiency, leading to insufficient pressure FN; (2) the excessive wear of the deflection of brake disc is too big or brake shoe, brake shoe and brake disc gap between Δ is too big, full brake disc spring precompression Δ 0 decreases, and lead to the output of the positive pressure FN; The piston seal ring aging and hydraulic oil pollution caused the piston motion resistance C to increase, resulting in the positive pressure FN deficiency; Oil pollution or oil road blockage causes brake system residual pressure p0, which causes the output of brake to be insufficient. (5) the connecting plate cylinder and piston big bolts to loosen, make the disc spring precompression Δ 0 is reduced, resulting in the positive pressure of FN.

　　(2) the brake shoe and brake disc friction factor between mu decrease according to the relevant literature and field test data shows that the main factors of mu lower friction factor are: (1) contaminated with oil or brake disc brake shoe surface, the main reason is that the brake hydraulic cylinder oil, pipe welding, loose connection within the oil and the hydraulic cylinder piston ring aging caused by oil spill; The deformation of the contact surface of the brake shoe is mainly due to overheating of the brake shoe due to high working temperature, which can affect the friction factor mu [3]. The size of the friction factor is related to the moving speed of the object, which causes the friction factor mu to decrease when the hoist is running at high speed.

　　2.2 disc brake failure

　　According to the formula (6), the main factors that cause brake failure are:

　　(1) the hydraulic pressure p is insufficient to overcome the spring force F2 'and piston motion resistance C;

　　(2) the hydraulic cylinder compacts or the cylinder causes the piston motion resistance C to increase sharply, so that the brake oil pressure F1 cannot overcome the motion resistance C without opening the brake.

　　3 prevention countermeasures

　　The maintenance of the equipment is to make sure that the device can be used in the process of the operation of the device, and the fault is eliminated in the bud through scientific and reasonable maintenance before the failure. By analyzing the common fault types and causes of disc brake, the author suggests that the measures to prevent disc brakes failure in daily maintenance are as follows:

　　(1) according to the regulations of the metal non-metal mine safety regulations, the working performance of braking and safety braking shall be tested regularly, and the braking torque shall be checked and the speed of safety braking shall be determined [4].

　　(2) there should be no oil pollution on both sides of the brake disk of disc brake, and at least once every class should be checked at least once, and the oil pollution should be stopped in time so as not to affect the friction factor [5] between the brake and the brake disc.

　　(3) in the normal operation of the hoist, the sealing ring should be replaced in time if the leakage of the hydraulic cylinder of the brake occurs.

　　(4) in the process of down weight should be put into dynamic braking, cannot only depend on the brake disc brake, or you will make the shoe fever, easy to cause the brake shoe thermal deformation, reduce the brake shoe friction factor and influence braking force.

　　(5) the gap between the locks is kept at about 1 mm and should not be more than 2 mm. The old equipment for the protection of uninstalled gap protection device should be configured in time, and when the gap exceeds the regulation value, it should be able to automatically alarm or automatically cut off power. The positioning bolt must be tightened after the clearance of the brake.

　　(6) filter hydraulic oil every six months to ensure the oil is clean; Once a year, the hydraulic station should be cleaned thoroughly. Check the oil amount of hydraulic oil per shift to ensure sufficient oil.

　　(7) the use of the disc spring should not exceed the national standard, and the general action number should be changed more than 500,000 times.

　　(8) the deflection of brake disc automatic detection equipment, the detected amount of deflection of brake disc is greater than 1 mm, flame or turning method should be adopted to correction of brake disc, the deflection of the quantity to 1 mm or less, to meet the requirements of the relevant procedures.

　　(9) check the air time of the brake regularly, and repair the time of air moving time greater than 0.3 s [6]. For safety, every six months should be tested. During maintenance, the surface rust of all parts should be removed. Check the surface of the disc spring for cracks and deformation, and check the wear of the hydraulic cylinder.

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

　　To achieve useful life and still can't work normally after long-term maintenance of disc brake device, should be in time for the whole renovation, replace the new codes on various aspects of the braking system to meet the requirements, guarantee the safe operation of mine hoist.

　　4. Typical case analysis

　　4.1 accident process

　　In January 2016, an accident occurred at a copper mine in anhui province. The scene of the accident of equipment for JKM type - 2.25 X4PI multi rope friction hoist, wellhead equipment promotion materials distance of 5 m, increase in container, the driver found immediately after the emergency brake, emergency brake failure, ascending vessel overloading side continued to slide, falling to the bottom of the wellbore, eventually leading to guide wire rope, a rope, tail rope and 2 elevated tanks were destroyed. The great loss caused by the accident seriously affected the production schedule of the mine.

　　4.2 field observation

　　On the spot, oil pollution marks are distributed along the disc, as shown in FIG. 2. After removing the disc brake in use, it is found that all the brake shoe wear is serious, and the various brake shoe wear is uneven, even the individual brake shoe has broken, as shown in FIG. 3. It also found that the large bolts of the inner join lining of the brake lining and the piston were loose. In the disk spring group, there were individual disc springs broken, and the sealing ring of the piston was also worn and damaged.

　　

　　Fig. 2　Contaminated braking disc on scene

　FIG. 3 brake shoe of site rupture

　　Fig. 3　Cracked braking shoe on scene

　　According to the detection of the accident site by the professional department, the lifting machine is equipped with 4 tpp-63 brake head, and the positive output pressure is 5, 15, 15 and 20 kN respectively. The deflection of the brake disc in the field is up to 3 mm, which exceeds the requirement of 1 mm. The highest oil pressure in hydraulic station is 3.3 MPa. The residual pressure of hydraulic system reaches 1 MPa when full braking.

　　4.3 cause of accident

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

　　(1) the spring compression force F2 decreases due to the lack of rigidity K due to fatigue or fracture. Join big of cylinder liner and piston is loose, reduce amount of disc spring precompression Δ 0, the spring precompression force F2 decreased; The pressure p0 of the hydraulic system is too large, resulting in the increase of pressure p0A in the working chamber. The sealing ring is worn and damaged, causing the piston motion resistance C to increase. Therefore, equation (2) and (3) can be seen that the positive pressure of the output of the disc brake in full braking mode is greatly reduced.

　　(2) due to the aging of the piston seal ring, the brake head is leaking oil, and the oil pollution is fouling the brake disc, resulting in the decrease of friction factor mu between the brake disc and the brake disc.

　　(3), large amount of deflection of brake disc of the uneven wear of each brake shoe, brake shoe and brake disc joint area is too small, lead to brake shoe uneven force and appear rupture, deformation of the contact surface between the brake shoe and brake disc, further lead to the friction factor between the two streams.

　　According to the formula (7), the above factors will eventually cause the brake torque M to be seriously insufficient, and the emergency braking can not stop the car, resulting in the accident of the falling tank.

　　4.4 solution

　　(1) adjust the braking disk by flame method or cutting method to make it less than 1 mm to meet the requirements of the code;

　　(2) replace all disc springs, sealing rings and brake shoe in the head of the brake;

　　(3) the large bolts of the inner join lining and piston of the brake are tightened by tightening torque;

　　(4) clean the brake disc, brake piston, hydraulic cylinder, hydraulic oil road and hydraulic station, and replace the new hydraulic oil.

　　If the disc brake, the measures to maintain long-term stability is still not able to implement working brake and safety brake, recommend replacing the whole hydraulic braking system, the braking system for the overall transformation, at the same time on the new change of the braking system configuration detection, the deflection of brake disc brake clearance test and inspection equipment of the hydraulic testing and a series of the braking system of real-time online monitoring, ensure the safety and reliability.

　　5 conclusion

　　Mine hoist disc brake failure are varied, but as long as reasonable use, in accordance with the measures to strengthen the daily management of maintenance, can timely understand the use status of disc brake and the problems existing in the use of it, accomplish know fairly well, eliminate hidden danger promptly and accurately, thus ensuring safe and reliable operation of the mine hoist, prevent the occurrence of accidents, ensure the safety of the mine production.