With the increasing number of infrastructure projects in China, concrete mixing station plays an increasingly important role in the concrete construction production in the construction industry. The concrete mixing stations used in the construction site are simple, modular, standard and special. The manufacturers, production capacity, product quality and performance reliability of the concrete mixing stations vary in different levels, but their aggregate lifting forms are no more than two kinds, i.e. bucket-lift mixing stations and belt conveyor mixing stations. Based on years of practical experience, the author introduces some problems existing in belt conveyor mixing station and corresponding solutions, and discusses with users and manufacturers of mixing station in order to ensure reliable use and clean production of equipment.

Existing Problems of Belt Conveyor in Mixing Station

At the end of April 2010, we purchased three sets of 2 m3 belt conveyor HZS120 mixing station from a domestic manufacturer for the construction of Daxi Railway Passenger Dedicated Line. Installation and commissioning are very smooth, and the overall performance of the mixing station is reliable. However, a few stones were found scattered on the climbing ladder of the ramp belt conveyor in the mixing station. A month later, it was found that the climbing ladder on both sides of the ramp belt conveyor in the three mixing stations was basically full of stones from bottom to top. There were also many stones scattered and accumulated in the pit. Every shift had to be cleaned up, which increased the labor intensity of the workers. It brings great inconvenience and some hidden safety hazards to the upper and lower passage of equipment maintenance personnel, and causes great waste, which affects the modern production goal of green environmental protection and energy saving of mixing station. Therefore, we organize personnel to carry out cause investigation, analyze and rectify, and determine the transformation plan and measures, so as to fundamentally solve the problem of spraying materials on the belt conveyor.

2 reasons analysis

Through several cycles of concrete production process, we carefully observed the operation of gravel aggregate on horizontal belt conveyor and inclined belt conveyor, and clarified the control mode of mixing station program. Comprehensive analysis shows that the main causes are the following two aspects.

2.1 Discharge Mode and Sequence of Sandstone Aggregate

Generally, the unloading sequence is to unload stones first and then sand. The unloading time interval requires that the tail of the first aggregate falling on the belt coincides with the head of the last aggregate. If the time interval is too short, the two aggregates will overlap, and excessive stacking will cause bulk materials; if the time interval is too large, there will be vacancies in the middle of the two aggregates, resulting in discontinuous aggregate flow. If the first aggregate is stones, it will roll and jump on the belt and fall off. The unloading sequence of the proportioner is to unload sand first, then unload stones of various specifications at intervals by delaying, and the unloading time intervals are set for a long time. There are vacancies in the middle of each two aggregates, and each material is not continuous in the horizontal belt conveyor, which forms an unstable and continuous material flow state; when conveying on the inclined belt conveyor, each kind of material will be unloaded at intervals of time. At the end of the stone flow, a small number of stones will be rolling and jumping along with the belt moving along along with its own gravity under the action of its own gravity, which will cause the stones of the former specifications to jump and converge to the stones of the next specifications, and some of them will eventually jump out of the inclined skin. Outside the belt conveyor, the wake of the last stone will all jump out of the inclined belt conveyor.

2.2 Material accumulation on hopper at transition point of two kinds of belt conveyors

Concrete mixing station is intermittent production mode, aggregate conveying on inclined belt conveyor is intermittent circulation type, but inclined belt conveyor is continuous operation in the whole production process, the material accumulated in hopper will fall from side plate to inclined belt under the continuous operation and vibration of inclined belt conveyor, and can not form a connection. Continuous material flow, and frequent beating, is difficult to be conveyed to the temporary bucket above the mixing station, only with the operation of the inclined belt conveyor constantly jumping up and down on the belt, eventually all jumped out of the inclined belt conveyor, scattered to both sides of the inclined belt conveyor climbing ladder and pit.

3. Definition and implementation of transformation scheme

3.1 Reform Scheme of Control Procedure

In view of the problem of aggregate discharging sequence, by looking at the control software of the mixing station, it is found that the setting of aggregate discharging sequence and delay in the control procedure is open and can be adjusted by itself. By consulting with the manufacturer and obtaining consent, the aggregate can be adjusted completely by adjusting the discharging sequence and delay setting. The unloading sequence ensures that the final unloading is sand and a stable continuous flow of material is formed.

Aiming at the problem of aggregate unloading mode, the control software of mixing station realizes sequential unloading by delaying the unloading between materials. If the formula of concrete changes, different formula will have different weight of material. In order to achieve continuous material flow, unloading delay must be changed frequently according to the formula changes, which is not conducive to production arrangements. The raw material index of high-speed concrete is strictly controlled. The proportion and raw material can not be adjusted and changed at will after confirmation. The proportion is relatively stable. Therefore, the weight of aggregates basically does not change. Through reasonable delay setting, a stable continuous material flow can be formed on the conveyor belt. So we determined the reform plan of the control program:

Change the unloading sequence of aggregate, unload stone first and then sand.

(2) Reasonable unloading delay between materials is reset so that each material overlaps partially on the belt conveyor to form a stable continuous material flow. In this way, the material will no longer stop flowing and tailings jumping, thus avoiding stones scattering into the ramp and pit on both sides of the belt conveyor.

3.2 hopper modification scheme

We believe that the hopper is only a guide device for changing the direction of material flow, and should not have any storage. The angle of the side plate of the hopper is relatively flat, and the material can not slide down in time after falling from the horizontal belt conveyor to the side plate, resulting in the accumulation of some materials. Based on this, the hopper modification scheme is determined:

(1) One baffle is welded at the side plate position of the hopper which is easy to accumulate material, so as to increase the angle of the side plate of the hopper body, so that the material can not be accumulated here;

(2) Install a replaceable buffer grille device on the upper part of the hopper, so that the material can change the direction of operation in advance and concentrate in the middle of the belt as far as possible;

(3) Replaced the rubber strip of the sealing material at the hopper baffle to prevent the aggregate from extruding outside the belt conveyor. It is believed that through these modifications of the hopper body, the phenomenon of stones and bulk materials caused by the accumulation of materials in the hopper will be eliminated.

3.3 Implementing Reform Program

According to the established reform plan, our staff completed the control program and hopper transformation in one day, and readjusted the tensioning device of the inclined belt conveyor to prevent and correct the deviation of the belt, and debugged successfully in one fell swoop. Through several cycles of production, the effect of transformation is immediate, the material flow is continuous, there is no material accumulation in the hopper, there is no stone bouncing phenomenon on the inclined belt conveyor, and there is no stone accumulation in the ladder and pit on both sides of the belt conveyor. From the end of equipment renovation till now, after long-term use of equipment, there is no stone accumulation in the ladder and pit on both sides of the belt conveyor. Aiming at the material distribution renovation of the belt conveyor, it is very successful. It solves the practical problems in the production process and realizes the modern production goal of green environmental protection and energy saving in the mixing station.

4 Conclusion

Spreading material is a common problem of belt conveyor, and the reasons are various. It needs to be dealt with separately according to different reasons, but the key point is to strengthen daily maintenance and correct use, so as to prolong the service life of the conveyor and save costs. In view of the successful transformation of the belt conveyor in the mixing station, I hope that through the experience of the equipment transformation, it can play a certain reference role for the users and manufacturers of the concrete mixing station, solve the actual problems of the equipment, and play a certain role in the use and technical development of the concrete mixing station.