The steel wire rope core conveyor belt has small deformation,high tensile strength, good adhesion between rubber and steel wire rope, uniform strength of steel wire rope, grooving naturally, available for X-ray inspection and other characteristics, which are widely used in the main transportation line of coal mine. Due to many factors,such as operating environment, vulcanizing process, and daily maintenance, belt breaking accidents occur from time to time. Therefore, it is of great significance to find hidden dangers and prevent belt breaking accidents in time for the safety production of coal mines. The effective measures to prevent belt breaking accidents is to improve vulcanizing process and on-site operation and maintenance level.

1. Analysis of vulcanization production process

1.1 principle of vulcanized joint

The strength of vulcanized joint is mainly realized by the adhesion of steel wire rope to rubber. The adhesion is expressed by the pull-out force, which is the force required to pull out the unit length of steel wire rope embedded in the rubber. The extraction force is a direct proportion to the length of the steel wire rope embedded in the rubber. The longer the embedded length, the greater the force required to extract the steel wire rope. When the embedded length increases to a certain value, the pulling force will exceed the breaking strength of the steel rope itself, and the steel rope will not be pulled out, so as to ensure the connection strength of the joint and realize the connection of the steel rope core tape.

1.2 Vulcanized joint structure

The structure of vulcanized joint directly affects the strength retention rate of the joint, the average effective spacing of the joint and the length of the joint. The selection should be comprehensively considered according to the diameter of the steel wire rope and the center distance of the steel wire rope of the tape. The joint structure of lap and dislocation lap is mainly adopted on site, and the more commonly used are primary lap method, secondary lap method, and tertiary lap method,etc. The commonly used two-stage lap joint structure is shown in Figure 1. The gray area represents the rubber block, which bears the shear stress in the opposite direction, and the white unmarked area represents the rubber block as the negative shear stress area. The steel wire ropes are bonded together in the same direction, and the shear stress is also transmitted due to the different elongation of the steel wire ropes.

The analysis of the joint structure shows that its strength is related to the following factors: ①:The characteristics of rubber and steel wire rope. ② Center rubber gap. The central rubber of the joint and the gap between them are the main factors affecting the strength of the joint, which bear axial shear, torsion and tension. ③ Joint order. Higher order will increase the stress concentration of rubber and steel wire rope. ④ Joint form. The tension acting on rubber and steel wire rope is also different with different joint sequences. ⑤ Joint length. The extraction force is transmitted in the steel wire rope through a shear stress block, and the joint length depends on the length of the step, which is determined according to the diameter, structure, arrangement order of the steps, rubber characteristics and field practical experience of the steel wire rope.

1.3 Vulcanization process

The vulcanized joint uses two kinds of rubber materials: covering rubber and core rubber. The covering rubber has high strength, wear resistance and impact resistance. The core rubber has good adhesion to the steel wire rope to ensure the extraction strength of the steel wire rope. The rubber compound must be selected according to the tape parameters. Before vulcanization, prepare the rubber compound, slurry and various equipment and tools, and check the vulcanization equipment and workplace environment. The vulcanization procedure and key points are as follows: ① Mark the center line of the tape. ② Draw the joint line according to the joint form. ③ According to the marking size, cut the covering rubber and core rubber along the edge of the steel wire rope from the edge, in order to pull out the steel wire and retain the core glue of the transition layer. ④ Cut the wire rope.According to the specified joint form, cut off the marked steel wire rope .⑤ Roughen the adhesive layer on the surface of the steel wire rope and evenly brush the mortar. ⑥ Joint forming and vulcanization. Pay attention to clean with gasoline and ensure the quality of the glue, align with the center line of the tape, arrange the steel wire rope according to the joint form, fill the core rubber film, eliminate the air in the joint, and then close the hot plate for vulcanization. ⑦ The thinning temperature should be controlled at 14515 ℃, the vulcanization pressure should be 1.0 ~ 1.5 MPa, and the vulcanization time should be 40 ~ 50 min when the temperature reaches 145.

1.4 Precautions for vulcanization operation

(1) For underground operation, the air inlet place without drenching water and floating dust shall be selected, with good ventilation conditions, sufficient lighting, management of inflammables and formulation of fire safety measures.

(2) Attention shall be paid to the storage of various rubber materials separately. It is strictly prohibited to mix them and keep them clean.

(3) The steel wire rope and polished surface must be painted with gasoline in time, and the mortar shall be painted twice after drying.

(4) The steel wire ropes shall be arranged evenly and stretched uniformly, and the effective spacing shall be maintained to make the core glue fully penetrate the surface of the steel wire rope. The warped steel wire rope head shall be bound with fine copper wire and filled with core glue to prevent its relative movement.

(5) The surface of the upper and lower hot plates shall be flat without depression or protrusion, and the inspection clearance after pressurization shall not be greater than 0 5 mm。

(6) The surface temperature of the upper and lower hot plates shall be uniform, and the temperature difference at each point on the surface and the temperature fluctuation in the whole vulcanization process shall not exceed + 3 ℃.

(7) During pressurization, the unit pressure of the hot plate shall not be less than 1.2 MPa, the vulcanization temperature shall be 145 15 ℃, and the positive vulcanization time shall be 40 ~ 50 min to prevent under sulfur and over sulfur.

(8) The width of the hot plate shall be more than 100 mm of the bandwidth, and the length shall be more than 200 mm of the joint length.

(9) Fully eliminate the air. First pressurize and cold press for 5 min, and then increase and reduce the pressure for 1 ~ 2 times to help eliminate the air in the joint. Then release the pressure, open the upper hot plate and check whether there is glue. If there is any, repair it, and then close the hot plate for vulcanization.

(10) When the heating temperature reaches 50, tighten the compression screw again. Before heating to 70 ~ 75 ℃, it is strictly prohibited to inject high-pressure water into the water pressure plate to prevent damage to the water pressure plate. During vulcanization, it is strictly prohibited to relieve pressure and prevent “boiling”.

(11) After vulcanization at the specified temperature and time, the hot plate temperature is reduced to below 100 ℃ and the mold is opened. It is strictly prohibited to bear force and start up before the vulcanization head is completely cooled.

(12) When the steel wire ropes are arranged, according to the running direction of the tape, two steel wire ropes covering the rubber edge shall be placed on the outermost two sides in the direction of the tape.

2. Daily operation and maintenance

(1) According to the operation condition of the belt,the timely adjust to prevent the deviation of the belt from aggravating the wear of the belt edge and damaging the upper and lower covering glue, resulting in the exposure and falling off of the steel wire rope.

(2) Strengthen the technical management of belt, and regularly replace the conveyor belt that is beyond the service life.

(3) If the edge glue and covering glue of the tape fall off and the steel wire rope is exposed locally, cold bonding or vulcanization and hot repair shall be carried out in time.

(4) Vulcanization joint is a weak link in the strength of adhesive tape, and its damage is related to factors such as vulcanization process, material, service environment and so on. The initial manifestation of joint opening is surface glue cracking, and water is immersed into the steel wire rope through the gap of core glue, resulting in steel wire rope corrosion, wire breakage, jerking, etc. the change of joint elongation and abnormal change of surface shall be checked regularly, and preventive inspection shall be done.

(5) The attachments on the surface of the transmission drum shall be removed in time to prevent the uneven stress of the joint caused by the irregular shape of the drum surface, resulting in bubbling, wire rope pulling, vulcanization and joint elongation, which affect the strength of the joint.

(6) The drum in contact with the working surface of the tape must be equipped with a scraper or other cleaning device to remove the adhesion on the surface of the roll and the adhesion of the lower support rod.

(7) Keep a daily maintenance and management, and replace the damaged supporting spokes in time to prevent longitudinal tearing accidents.

(8) Scraper and deflector shall be installed in front of the tail drum to prevent coal falling debris from entering between the belt and the drum.

(9) The height of the chute at the loading point shall be properly selected according to the materials and the maximum block size, so as to reduce the drop of the chute as much as possible and reduce the impact of materials on the vulcanization joint.

(10) The full hydraulic automatic tensioning device is adopted to improve the dynamic characteristics during startup, make the conveyor belt start smoothly and reduce the impact dynamic stress on the vulcanization joint.

(11) According to the local defect of the tape, different repair methods such as cold repair, hot vulcanization and filling steel wire rope shall be selected to keep the surface of the tape intact.

(12) Strengthen the technical monitoring and management of vulcanized joints, adopt the method of combining the daily inspection of responsibility to people with the regular inspection of X-ray, and timely repair the defective vulcanized joints.

Conclusion

The damage of vulcanization joint is related to many factors,such as vulcanization process, conveyor belt hot vulcanization materials, service environment . Comprehensive management measures such as improving the structure and joint manufacturing technology of steel wire rope core conveyor belt and improving the operation and maintenance level must be taken to accumulate work experience, so as to effectively prevent the occurrence of belt breaking accident.