To ensure the safe operation of belt conveyors, installing safety detection and protection devices in the conveyor lines is a mandatory requirement both domestically and internationally. GB 50431-2020 The mandatory clauses in stipulate that the following safety detection and protection devices should be installed on belt conveyors.

(1) The following detection and protection devices must be installed in the conveyor lines of belt conveyors :

1) Rope protection device on the walkway side of the belt conveyor ;

2) Longitudinal tear protection device for steel cord conveyor belts conveying large or hard materials ;

3) Belt conveyor misalignment detection device ;

4) Belt conveyor slippage detection device ;

5) Limit protection device for the tensioning device of long-distance and complex belt conveyors.

The purpose of installing detection and protection devices is to improve the safety and reliability of belt conveyor engineering systems. Installing rope switch protection devices, belt slippage detection devices, belt anti-deviation devices, and longitudinal tear protection devices for steel cord conveyor belts on belt conveyors are the minimum required safety device settings. For conveyors transporting hard materials that are prone to damage and tearing of the conveyor belt, fabric core conveyor belts should also be equipped with longitudinal tear protection devices.

Emergency switches are safety devices that manually shut down the equipment in case of accidents or emergency shutdowns. Emergency switches include emergency stop switches and pull-cord switches. Emergency stop switch Generally installed in important locations such as the driver's cab, head end, and drive station where personnel can easily operate them. Pull-cord switch Fixed to the conveyor frame on the walkway side. When an emergency shutdown is needed, the operator pulls the pull cord of the protection device, and the pull-cord switch sends an emergency shutdown signal to the belt conveyor control system, achieving an emergency shutdown of the belt conveyor. Sometimes during conveyor maintenance, pulling the pull-cord switch puts the belt conveyor in a fault state, preventing abnormal startup and protecting the safety of maintenance personnel.

 

Pull-cord switches can be classified into unidirectional or bidirectional lever types, unidirectional or bidirectional rotary types, etc., according to their operating structure. According to their function, they are divided into standard types, types with light indication, or types with address codes. According to the usage environment, they are divided into general types and explosion-proof types. According to the shell material, they are divided into steel, aluminum alloy, and plastic types.

 

Lever-type pull-cord switch





 

Lever-type pull-cord switches are divided into unidirectional and bidirectional lever types.

A. Bidirectional lever-type pull-cord switch

Commonly used bidirectional lever types usually use a dual-switch, dual-moving cam mechanism. Pulling the lever actuates the switch through the cam structure.

Bidirectional levers are widely used and flexible in layout. The lever is pre-tensioned and positioned before the product leaves the factory to ensure that the switch is in the normal working position in non-fault conditions.










 

B. Unidirectional lever-type pull-cord switch

Unidirectional levers use one or two sets of switches. The middle position of the lever shaft inside the device is a variable-diameter concave area. Normally, the contacts of the built-in switch are located in the concave area of the lever shaft. When the lever is pulled, the switch actuates. This pull-cord protection device is generally placed at the end and requires a large pulling force.










 

 

 

 

Rotary pull-cord switch





 

The rotary pull-cord switch actuates the switch by rotating the rotary arm to drive the cam on the shaft. The rotary arm can be connected to the steel wire rope in two directions, so that personnel can conveniently pull the steel wire rope to control the pull-cord switch in two directions. The structure of the rotary arm is divided into a symmetrical arm structure and a single-arm structure. The symmetrical arm structure can connect steel wire ropes in different directions at both ends of the rotation center. Regardless of the direction in which the steel wire rope is pulled, the rotary arm rotates in one direction. The single-arm structure connects the steel wire ropes in two directions to one end of the rotary arm. The direction of the steel wire rope pulling force is different, and the rotation direction of the rotary arm is also different.

 

 

Explosion-proof pull-cord switch





 

For belt conveyors in environments with explosive gases, explosion-proof pull-cord switches should be used, with explosion-proof enclosures.

 

 

Main performance of pull-cord switch





 

The main performance of the pull-cord switch includes reset method, light indication, address coding, pulling force, etc.

A Reset method of pull-cord protection switch

The reset method of the pull-cord switch usually includes automatic reset and manual reset. Automatic reset means that after the fault is cleared and the steel wire rope is tightened and reset, the pull-cord switch automatically resets to the normal state. Manual reset means that after the fault is cleared, the reset handle must be manually turned on before the pull-cord switch protection device can be reset to the normal state.

The choice between manual reset and automatic reset depends on the user's requirements. For safety reasons, manual reset is often used, which means that the fault is confirmed to be eliminated before manual reset. Automatic reset is easily automatically released after misoperation or abnormal pulling of the pull-cord switch, making it difficult to determine the cause of the fault.

B Fault address display function of pull-cord switch

When the pull-cord switch is in a fault state, the fault display function can quickly and easily determine the location of the pull-cord switch that issued the fault signal, so that it can be quickly processed to reduce downtime. The initial device used a small red flag on the rotary arm. When people saw the flag tilted, it indicated that the pull-cord switch protection device was activated. Later, a sound and light alarm device was installed at the top of the pull-cord switch protection device. When the pull-cord switch was activated, it emitted a sound and light alarm signal, which could be heard and seen within a certain distance. However, the above measures can only be discovered when personnel are on site, which is not ideal for long-distance belt conveyors of thousands of kilometers.

Currently, intelligent address coding systems are widely used in long-distance belt conveyor systems. The address coding system consists of a coding controller and coding switches. The coding controller collects information from the coding switches via communication. Modules are installed in the protection device, which can identify, display the fault location, and issue a fault signal through the host monitoring system, realizing automatic safety control and rapid fault processing of the system.

 

GB50431-2020 Regulations The functions of the conveyor belt emergency switch should comply with the following regulations:

1. The drive station of the belt conveyor should be equipped with an emergency stop button.

2. The rope protection device of the belt conveyor should have a manual reset function. The spacing of the rope protection device should not exceed 60m, and a set of rope rings should be provided every 3m~6m.