Introduction to Safety Protection Devices for Belt Conveyors

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A belt conveyor is a material conveying device widely used in manufacturing enterprises, and it mainly consists of a conveyor belt, a drive motor, as well as supporting equipment and control and protection devices. Typically, a complete conveying system is formed by several conveyor belts intersecting with each other, enabling the long-distance transport of materials. The operating condition of belt conveyors is directly related to the safety of conveying equipment and personnel during the production process. It is of great practical significance to determine how to enable the belt conveyor protection devices to function efficiently, reliably, and stably during production operations. There are mainly two types of protective devices commonly used in belt conveyors: one is to prevent fire accidents involving the belt conveyor, and the other is to protect the conveyor belt. Below is a brief introduction to several common types of protective devices for conveyors along with their working principles. 1. Slip detector: During the operation of a belt conveyor, if, for some reason, the speed of the drive pulley becomes asynchronous with the speed of the conveyor belt, relative sliding occurs between them, resulting in a slip phenomenon. Slipping comes in two types: low-speed slipping and high-speed slipping. The reasons for low-speed slippage include reduced frictional traction on the drum, the conveyor belt getting stuck, or insufficient tension; such situations are common ； High-speed slipping generally occurs when the motor of a downward-conveying belt conveyor is generating electricity; high-speed operation of the belt conveyor itself is less common in such cases. The working principle of the slip protection system is to detect changes in the speed of the conveyor belt and compare it with the speed of the drive drum; under normal operation, there is no difference between the two speeds. If slipping occurs, a difference appears between them. When the belt speed exceeds 10% of its normal value or falls below 25%–30% of its normal value, a slip alarm signal is generated. At this point, the user can stop the machine via the control network, thereby preventing production accidents caused by belt slipping as well as avoiding unnecessary frequent braking. II. Vertical tear protection device: During operation, belt conveyors can suffer from vertical tears as a result of large foreign objects such as sharp rocks, coal pieces, or long metal strips getting stuck on the conveyor belt and causing damage. These tears usually occur at the feeding point; in particular, belt conveyors with wire rope cores are prone to tearing due to the weak lateral strength between the steel wires in their core, so protection and monitoring measures are necessary. Longitudinal tearing of conveyor belts not only results in losses for the conveying equipment but also severely affects production. The main solution currently is to install longitudinal tearing protection devices at the feeding points. The longitudinal tear protection device consists of tear sensors and a control box; one control box can be used in conjunction with 4 to 6 sensors. The sensors are typically installed between the buffer idlers beneath the conveyor belt in the receiving section of the belt conveyor, while the control box is placed on a vibration-free wall in a nearby location. III. Bidirectional pull-cord protection devices
Bidirectional pull-cord switches are generally installed on the intermediate frames or legs of belt conveyors, at a height below the conveyor belt surface (this height ensures easy access for on-site personnel). They are arranged along one side of pedestrian walkways. When there are pedestrian walkways on both sides of the belt conveyor, pull-cord protection devices must be installed along both sides of the conveyor. Typically, they are placed at intervals of 30–50 meters. The pull cords on both sides should be laid parallel to the conveyor belt, and any sagging of the cords must be minimized to ensure the proper functioning of the switches. IV. Deviation prevention protection device. Belt deviation is a common problem during the operation of belt conveyors. The main risks associated with belt deviation include early tearing and damage to the edges of the belt, improper material transfer along the conveyor, and in severe cases, fires caused by excessive friction. To prevent the conveyor belt from drifting, in addition to improving the quality of installation, forward-leaning idlers and self-aligning idlers can be installed on the conveyor. However, to avoid accidents caused by belt drift, deviation protection and monitoring devices must also be installed along the conveyor, so that it can stop automatically when persistent drifting occurs. V. Check valve: Under certain conditions, when the inclination angle of the conveyor belt exceeds a specific value, this device must be installed to prevent the belt from reversing and the material from flowing back in case of a power outage, thus avoiding material spillage and potential safety accidents. It can be installed on the drum shaft, or the reducer comes equipped with a check valve. To prevent reversal or slipping when an inclined belt conveyor stops while under load, a check is performed on the braking torque, and a check valve or braking device is added as appropriate depending on the specific circumstances. The interior of the check valve contains several irregular-shaped blocks distributed within the raceways formed by the inner and outer rings; when the inner ring rotates in the forward direction, it drives these irregular-shaped blocks to rotate as well. For safety or functional reasons, many machines and devices allow rotation in only one direction under normal conditions. A check valve primarily serves to prevent the reducer from rotating in reverse; reducers equipped with check valves are generally required on belt conveyors with an angle as well as elevators (such as bucket elevators). The main purpose of a one-way clutch in a brake is to regulate rotational motion in a direction opposite to the normal operating direction ； The overdrive clutch is mainly installed at the connection point between the auxiliary transmission and the main reducer, serving to link the two ； Roller check valves are suitable for use with head drums; they utilize reducers for power transmission, have a high power capacity, and feature large drum diameters. These valves are appropriate for belt conveyors of the ZQ65 model or higher, and are well suited for upward transportation on belt conveyors with an inclination angle of ≤18 degrees. Its advantages include rapid reversal and a short reversal distance, making it the preferred brake for medium and large inclined conveyor systems. It is widely used in applications such as reducers, coking plants and transportation machinery, conveyors, elevators, electric drums, as well as other mechanical devices that require a check function. VI. Chute clogging prevention switch: It is located at the head of the funnel. Characterized by toothed baffles installed on the inner walls of the three main material flow channels within the funnel, these baffles are used to direct the material discharged from the conveyor head into subsequent equipment, storage bins, or another conveyor, thereby preventing material from splashing and dust from escaping. The head funnel is equipped with a guide plate below the feed inlet; at the feed inlet end, this guide plate has a flexible scraper attached to it. The upper end of this scraper comes into contact with the belt in the returning section at the lower edge of the head drum, and its width is not less than that of the belt. A flexible scraper is used to maintain close contact with the tape at the lower edge of the drum, thereby preventing residual material on the tape from falling onto the ground along its path as it moves, which avoids environmental pollution. This approach also extends the lifespan of the tape, reduces the need for manual maintenance, and improves work efficiency. The chute clogging prevention switch is installed on the side of the discharge hopper (the header funnel) that cannot be reached by the material, and it is used to detect whether the material in the header funnel has accumulated to an excessive level; when this happens, the pressure from the material causes the switch to send a signal. In summary, although there are many types of protection devices for belt conveyors, their basic working principles are quite similar. With the rapid development and use of belt conveyors in various industries, those with large capacity and high belt speeds are being used on an increasingly widespread scale. Due to the rather complex and harsh working environment and conditions of belt conveyors, various unexpected accidents are inevitable. How to reduce or prevent such accidents, and how to respond promptly in the event of an accident to minimize losses, are very important issues in the operation of belt conveyors. The engineering design specifications for belt conveyors stipulate that, in the conveying system of a belt conveyor, it is necessary to install various monitoring and protection devices, including devices to prevent the belt from drifting out of alignment, devices to protect against longitudinal tearing of the belt, devices to detect belt slippage, and tensioning rope protection devices. These protection devices provide a reliable guarantee for the efficient and safe operation of the conveyor.