Back to Basics: Limit switches for conveyors

Process monitoring and control improves with proper application of limit switches in conveyor applications. See diagram.

10/24/2011


An industrial conveyor belt usually consists of two or more pulleys, with a continuous loop of material (belt) that rotates around them. One or both 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 used to 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, water, oil and dirt, and is used to detect presence or absence in areas where physical contact is allowed.

Discrete sensors, properly applied, can improve usefulness and lower risk of a conveyor application. Courtesy: Honeywell Sensing & Control

In the conveyor belt application shown, a variety of limit switches can be used.

1. Often used on conveyors designed for use in harsh indoor and outdoor environments, heavy-duty limit switches can reliably indicate position for system controls. In this configuration, the limit switch could be used to count the items flowing through it. It could also be used to ensure that materials are correctly positioned and will sound an alarm or stop the belt if there is a problem.

2. Safety switches: Cable-pull limit switches are emergency stop switches that 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.

3. Hazardous location switches are very similar to switch 1 but are housed in sealed enclosures to prevent a spark or electrical discharge from setting off an explosion. They are often used for door or diverter position detection in outdoor, above-ground, potentially explosive environments, such as grain handling and oil and gas applications.

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, hazardous location switches can quickly and reliably stop system operation when the switch is triggered by the operator.

Limit switches can also be used in other ways on conveyor belts. For example, a limit switch alongside the belt could ensure that packages and materials are correctly positioned on the conveyor belt. The limit switch signal connects to a controller that will stop 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, the system will shut down before damage or spillage occurs.

- Richard Staiert is Honeywell applications engineer. Edited by Mark Hoske, CFE Media, Control Engineering, www.controleng.com.

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