Richard Staiert, Honeywell Sensing and Control
An industrial conveyor belt usually consists of two or more pulleys, with a continuous loop of material (belt) that rotates about them. One or both of the pulleys are powered, moving the belt and the material on the belt forward. There are two main industrial classes of belt conveyors: general material handling (moving boxes along inside a factory) and bulk material handling (transporting industrial/agricultural materials).
Limit switches are utilized to both provide conveyor system monitoring and control as well as safety in case of a problem. A limit switch is a precision snap-action switch that has been encased to protect it from external forces, such as hazardous chemicals, extreme temperatures, water, oil and dirt and is most commonly used to detect presence or absence in areas where physical contact is allowed. In the conveyor belt application as shown in Figure 1, a variety of Limit Switches can be used.
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Figure 1: A variety of limit switches can be used in a conveyor application
Number 1 - Often used on conveyors designed for use in harsh indoor and outdoor environments, heavy-duty limit switches can be used to reliably indicate position to the system’s controls. In this configuration, the limit switch could be used to count the items that are flowing through it. It could also be used to ensure that the materials are corrected positioned and will sound an alarm or stop the belt if there is a problem.
Number 2 - Safety Switches - Cable-pull limit switch are emergency stop switches often control access around the conveyor perimeter or provide emergency-stop cable-pulls along the conveyor. These kinds of limit switches provide a highly reliable, highly visible, safe to-use rope-pull device to protect operators working near conveying systems.
Number 3 - Hazardous Location Switches are very similar to Switch 1 but are housed in completely sealed enclosures to prevent a spark or electrical discharge from setting off an explosion. They are often utilized for door or diverter position detection in outdoor, aboveground, potentially explosive environments, such as grain handling and oil/gas applications.
Number 4 - Safety and Hazardous Location Switches have also been designed with an explosion-proof housing. Often used in outdoor, above-ground, potentially explosive environments, such as grain handling conveyors, or oil and gas applications, these kinds of switches can quickly and reliably stop system operation when the switch is triggered by the operator (see Figure 2).
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Figure 2: Limit switches are available in a wide range of formats, sizes and types including wireless.
Limit switches can also be used in variety of other ways on conveyor belts. For example, a limit switch alongside the belt, could ensure that the conveyor belt is working correctly, that packages and materials are correctly positioned on the conveyor belt. The limit switch signal connects to a controller, which will stop down the belt if there is a problem.
To prevent jamming the belt’s discharge, limit switches could measure the number or height of the products on the conveyor belt. If too much product is going down the belt, it can shut down the system before damage or spillage occurs.