Design and structural features of the Roller Screen in the coal conveying system
Blade design
At the entrance of the Roller Screen, there is an electric three-way damper, which is lined with wear-resistant material. When the equipment is under maintenance, to avoid affecting the coal feeding of the system, the damper can be moved to the bypass position, and the material passes through the bypass. The opening of the damper is operated by an electric push rod (it can also be manually controlled), and there is an internal limit switch.
When the electric push rod makes the damper reach the open or closed position, the limit switch sends a signal to the PLC programmable controller of the local control cabinet, which then transmits the signal through the remote contact point to the control system. The local control cabinet can be linked with the main control system of the coal conveying system and achieve automatic switching. At the entrance of the Roller Screen, there is an electric three-way damper with wear-resistant material. The steel plate thickness of the electric three-way damper is not less than 10mm, and it is lined with HARDOX500 wear-resistant steel plate with a thickness of no less than 12mm.
Roller Screen is driven by a single-axis single motor, that is, each group of screen shafts has a set of drive device. When one group of screen shafts fails, the coal can be pushed by the inertia of the previous screen shaft over the faulty screen shaft and continue to advance. The Roller Screen can still operate normally, and the faulty shaft does not involve other shafts during maintenance. The motor is an integrated roller conveyor electric motor. The drive device of the motor and the reducer uses products from the Flanders company, and the dynamic and static seals use labyrinth seals and frame seals. The sealing uses labyrinth seal.
Screening performance and cleaning device
The Roller Screen can operate normally without sticking or blocking when the moisture content of the coal is 10-15%. Under normal conditions, the screening efficiency is not less than 90%, and in extreme difficult screening conditions, it is not less than 75%, and no coal larger than 30mm can become the screened material. The bonding coal between the screen plates should have a self-cleaning function. The Roller Screen should be equipped with a cleaning device, which can effectively remove the sticky coal and debris on the comb teeth and shafts through the cleaning device. It does not block the sieve holes when dealing with wet sticky coal.
The screen area, inclination angle, and screen shaft rotation speed of the Roller Screen are designed based on the approximate equal thickness screening principle. The screen surface is divided into three sections with an inclination angle of 25°, 15°, and 5° respectively. After the material enters the screen machine, it is rapidly dispersed under the impact force of the coal flow, forming an equal thickness coal layer. All rollers act simultaneously, greatly improving the screening efficiency, completely solving the problem of coal blockage, facilitating the movement of the material, improving the screening efficiency, and making the screen surface less likely to accumulate material. The large hexagonal screen plates with surface mechanical processing are inserted interlacedly between the adjacent two shafts to form sieve holes.
The screen plates have the function of automatically cleaning the sticky coal on the screen shafts, greatly reducing the possibility of material blockage on the screen surface. The surfaces of all the screen plates are machined, and the surface finish is improved, reducing the possibility of sticky coal even when the surface moisture of the coal is high. The six screen shafts at the front of the Roller Screen have cleaning devices below them. The material is made of 45# steel, and the scraping surface is quenched, with a blade-shaped structure, ensuring sufficient strength and no deformation, and can remove the sticky coal and tangled debris on the screen shafts when the moisture of the material is too high.
Start-up and overload protection design
The Roller Screen should be able to start under load, and each screen shaft has an overload protection device. The Roller Screen can achieve load start-up. When there is material on the screen surface, the PLC programmable controller is first made to start the tail cloth screen shaft in sequence to separate a small amount of coal, and then gradually start the middle and head screen shafts in sequence to complete the load start-up.
There is a coupling between the reducer and the screen shaft, and the coupling has a safety pin. When the screen shaft is overloaded, the safety pin will be cut off, providing mechanical protection. When the screen shaft is overloaded, the current increases, the thermal relay disconnects, cutting off the motor, thus providing electrical protection. At the same time, necessary protection measures such as overcurrent, overload, short circuit, and phase failure are provided.
The part of the screen that comes into contact with the material should take anti-abrasion measures. The screen shaft should be able to withstand the impact of large pieces of coal and large stones. The screen shaft is made of 42CrMo material, which has high strength, rigidity, and good heat treatment performance. All internal parts that are subject to severe wear, such as the screen plates, are made of ZG40Mn2, which is resistant to impact and wear; the two sides of the body and the inlet baffle both have wear-resistant protective plates made of 16Mn and easy to replace.
Roller Screen should have measures to prevent blockages, effectively suppressing blockage phenomena, ensuring smooth material flow and no coal blockage (coal blockage signal).
Vibration and noise control
Measures should be taken in the design and manufacturing process to reduce the vibration and noise during the machine’s operation. The side plates and upper cover of the upper box of the Roller Screen can be easily disassembled, facilitating equipment maintenance and repair. After the overall welding of the bearing seat plates on both sides of the box, the flatness of the two installation surfaces and the position accuracy of the holes are fully guaranteed, thereby improving the reliability of the entire machine’s operation. The noise is less than 85 dB(A).
The outer cover of the entire machine should be tight to prevent the leakage of coal dust and should be equipped with the opening conditions for dust removal equipment.
Equal-thickness screening and parameter design
Using equal-thickness Roller Screen enables the screened raw coal to be quickly and uniformly transported and evenly distributed on the entire screen surface, improving its screening efficiency. The screen shaft and screen plates should have sufficient mechanical strength and rigidity, and meet the requirements for starting and running under load.
The material of the screen shaft should also have good wear resistance. The width of the screen surface is 1600mm, the effective flow area of the screen holes on the screen surface is 2.3 square meters, and the rotational speed of the screen shaft is 100 revolutions. The area of the screen surface is 4823 X 1600mm.
The sliding part has good sealing, no oil leakage, the temperature rise of the rolling bearings should not exceed 40℃, and the maximum temperature of the bearings should not exceed 80℃. Safety pins or other types of mechanical protection devices should be set. The exposed parts of the Roller Screen are equipped with non-rotating protective covers to ensure personal safety.
A coupling is set between the Roller Screen reducer and the screen shaft, and the coupling has a safety pin. When the screen shaft is overloaded, the safety pin will be cut off, thus providing mechanical protection. Using a roller conveyor motor also has grounding protection.
